The authors would like to acknowledge UC Riverside&#8217;s Center for Glial-Neuronal Interactions for valuable discussion of the scaling protocols and data. The authors would also like to give a sincere thank you to Abcam for sponsoring the publication of their&#160;work.

<ol>
	<li>Araque, A., Martin, E. D., Perea, G., Arellano, J. I., Buno, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synaptically+released+acetylcholine+evokes+Ca2++elevations+in+astrocytes+in+hippocampal+slices.">Synaptically released acetylcholine evokes Ca2+ elevations in astrocytes in hippocampal slices.</a> J. Neurosci. 22, 2443-2450 (2002).</li><li>Navarrete, M., Araque, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Endocannabinoids+mediate+neuron-astrocyte+communication.">Endocannabinoids mediate neuron-astrocyte communication.</a> Neuron. 57, 883-893 (2008).</li><li>Bekar, L. K., He, W., Nedergaard, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Locus+coeruleus+alpha-adrenergic-mediated+activation+of+cortical+astrocytes+in+vivo.">Locus coeruleus alpha-adrenergic-mediated activation of cortical astrocytes in vivo.</a> Cereb. Cortex. 18, 2789-2795 (2008).</li><li>Hertz, L., Lovatt, D., Goldman, S. A., Nedergaard, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Adrenoceptors+in+brain:+cellular+gene+expression+and+effects+on+astrocytic+metabolism+and+Ca2.">Adrenoceptors in brain: cellular gene expression and effects on astrocytic metabolism and Ca2.</a> Neurochem. Int. 57, 411-420 (2010).</li><li>Porter, J. T., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hippocampal+astrocytes+in+situ+respond+to+glutamate+released+from+synaptic+terminals.">Hippocampal astrocytes in situ respond to glutamate released from synaptic terminals.</a> J. Neurosci. 16, 5073-5081 (1996).</li><li>Bernardinelli, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Astrocytes+display+complex+and+localized+calcium+responses+to+single-neuron+stimulation+in+the+hippocampus.">Astrocytes display complex and localized calcium responses to single-neuron stimulation in the hippocampus.</a> J. Neurosci. 31, 8905-8919 (2011).</li><li>Panatier, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Astrocytes+are+endogenous+regulators+of+basal+transmission+at+central+synapses.">Astrocytes are endogenous regulators of basal transmission at central synapses.</a> Cell. 146, 785-798 (2011).</li><li>Wang, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Astrocytic+Ca2++signaling+evoked+by+sensory+stimulation+in+vivo.">Astrocytic Ca2+ signaling evoked by sensory stimulation in vivo.</a> Nat. Neurosci. 9, 816-823 (2006).</li><li>Fiacco, T. A., Agulhon, C., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sorting+out+astrocyte+physiology+from+pharmacology.">Sorting out astrocyte physiology from pharmacology.</a> Annu. Rev. Pharmacol. Toxicol. 49, 151-174 (2009).</li><li>Agulhon, C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Calcium+Signaling+and+Gliotransmission+in+Normal+vs+Reactive+Astrocytes.">Calcium Signaling and Gliotransmission in Normal vs Reactive Astrocytes.</a> Front. Pharmacol. 3, 139 (2012).</li><li>Nedergaard, M., Verkhratsky, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Artifact+versus+reality--how+astrocytes+contribute+to+synaptic+events.">Artifact versus reality--how astrocytes contribute to synaptic events.</a> Glia. 60, 1013-1023 (2012).</li><li>Nizar, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vivo+stimulus-induced+vasodilation+occurs+without+IP3+receptor+activation+and+may+precede+astrocytic+calcium+increase.">In vivo stimulus-induced vasodilation occurs without IP3 receptor activation and may precede astrocytic calcium increase.</a> J. Neurosci. 33, 8411-8422 (2013).</li><li>Sutton, M. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Miniature+neurotransmission+stabilizes+synaptic+function+via+tonic+suppression+of+local+dendritic+protein+synthesis.">Miniature neurotransmission stabilizes synaptic function via tonic suppression of local dendritic protein synthesis.</a> Cell. 125, 785-799 (2006).</li><li>Ibata, K., Sun, Q., Turrigiano, G. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+synaptic+scaling+induced+by+changes+in+postsynaptic+firing.">Rapid synaptic scaling induced by changes in postsynaptic firing.</a> Neuron. 57, 819-826 (2008).</li><li>Xie, A. X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Bidirectional+scaling+of+astrocytic+metabotropic+glutamate+receptor+signaling+following+long-term+changes+in+neuronal+firing+rates.">Bidirectional scaling of astrocytic metabotropic glutamate receptor signaling following long-term changes in neuronal firing rates.</a> PLoS ONE. 7, (2012).</li><li>Takahashi, A., Camacho, P., Lechleiter, J. D., Herman, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+intracellular+calcium.">Measurement of intracellular calcium.</a> Physiol. Rev. 79, 1089-1125 (1999).</li><li>Petravicz, J., Fiacco, T. A., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Loss+of+IP3+receptor-dependent+Ca2++increases+in+hippocampal+astrocytes+does+not+affect+baseline+CA1+pyramidal+neuron+synaptic+activity.">Loss of IP3 receptor-dependent Ca2+ increases in hippocampal astrocytes does not affect baseline CA1 pyramidal neuron synaptic activity.</a> J. Neurosci. 28, 4967-4973 (2008).</li><li>Nett, W. J., Oloff, S. H., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hippocampal+astrocytes+in+situ+exhibit+calcium+oscillations+that+occur+independent+of+neuronal+activity.">Hippocampal astrocytes in situ exhibit calcium oscillations that occur independent of neuronal activity.</a> J. Neurophysiol. 87, 528-537 (2002).</li><li>Garaschuk, O., Milos, R. I., Konnerth, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Targeted+bulk-loading+of+fluorescent+indicators+for+two-photon+brain+imaging+in+vivo.">Targeted bulk-loading of fluorescent indicators for two-photon brain imaging in vivo.</a> Nat Protoc. 1, 380-386 (2006).</li><li>Nimmerjahn, A., Kirchhoff, F., Kerr, J. N., Helmchen, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sulforhodamine+101+as+a+specific+marker+of+astroglia+in+the+neocortex+in+vivo.">Sulforhodamine 101 as a specific marker of astroglia in the neocortex in vivo.</a> Nat. Methods. 1, 31-37 (2004).</li><li>Prezeau, L., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Changes+in+the+carboxyl-terminal+domain+of+metabotropic+glutamate+receptor+1+by+alternative+splicing+generate+receptors+with+differing+agonist-independent+activity.">Changes in the carboxyl-terminal domain of metabotropic glutamate receptor 1 by alternative splicing generate receptors with differing agonist-independent activity.</a> Mol. Pharmacol. 49, 422-429 (1996).</li><li>Shao, Y., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Quantitative+relationship+between+alpha+1-adrenergic+receptor+density+and+the+receptor-mediated+calcium+response+in+individual+astroglial+cells.">Quantitative relationship between alpha 1-adrenergic receptor density and the receptor-mediated calcium response in individual astroglial cells.</a> Mol. Pharmacol. 44, 247-254 (1993).</li><li>Wang, S. S., Thompson, S. H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+changes+in+functional+muscarinic+acetylcholine+receptor+density+in+single+neuroblastoma+cells+using+calcium+release+kinetics.">Measurement of changes in functional muscarinic acetylcholine receptor density in single neuroblastoma cells using calcium release kinetics.</a> Cell Calcium. 15, 483-496 (1994).</li><li>Ostasov, P., Krusek, J., Durchankova, D., Svoboda, P., Novotny, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Ca2++responses+to+thyrotropin-releasing+hormone+and+angiotensin+II:+the+role+of+plasma+membrane+integrity+and+effect+of+G11alpha+protein+overexpression+on+homologous+and+heterologous+desensitization.">Ca2+ responses to thyrotropin-releasing hormone and angiotensin II: the role of plasma membrane integrity and effect of G11alpha protein overexpression on homologous and heterologous desensitization.</a> Cell Biochem. Funct. 26, 264-274 (2008).</li><li>Shelton, M. K., McCarthy, K. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Hippocampal+astrocytes+exhibit+Ca2+-elevating+muscarinic+cholinergic+and+histaminergic+receptors+in+situ.">Hippocampal astrocytes exhibit Ca2+-elevating muscarinic cholinergic and histaminergic receptors in situ.</a> J. Neurochem. 74, 555-563 (2000).</li><li>Hermans, E., Challiss, R. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Structural+signalling+and+regulatory+properties+of+the+group+I+metabotropic+glutamate+receptors:+prototypic+family+C+G-protein-coupled+receptors.">Structural signalling and regulatory properties of the group I metabotropic glutamate receptors: prototypic family C G-protein-coupled receptors.</a> Biochem. J. 359, 465-484 (2001).</li><li>Zur Nieden, R., Deitmer, J. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+Role+of+Metabotropic+Glutamate+Receptors+for+the+Generation+of+Calcium+Oscillations+in+Rat+Hippocampal+Astrocytes+In+Situ.">The Role of Metabotropic Glutamate Receptors for the Generation of Calcium Oscillations in Rat Hippocampal Astrocytes In Situ.</a> Cortex. 16 (5), 676-687 (2005).</li><li>de Ligt, R. A., Kourounakis, A. P., AP, I. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inverse+agonism+at+G+protein-coupled+receptors:+(patho)physiological+relevance+and+implications+for+drug+discovery.">Inverse agonism at G protein-coupled receptors: (patho)physiological relevance and implications for drug discovery.</a> Br. J. Pharmacol. 130, 1-12 (2000).</li><li>Kang, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sulforhodamine+101+induces+long-term+potentiation+of+intrinsic+excitability+and+synaptic+efficacy+in+hippocampal+CA1+pyramidal+neurons.">Sulforhodamine 101 induces long-term potentiation of intrinsic excitability and synaptic efficacy in hippocampal CA1 pyramidal neurons.</a> Neuroscience. 169, 1601-1609 (2010).</li><li>Tong, X., Shigetomi, E., Looger, L. L., Khakh, B. S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Genetically+encoded+calcium+indicators+and+astrocyte+calcium+microdomains.">Genetically encoded calcium indicators and astrocyte calcium microdomains.</a> Neuroscientist. 19, 274-291 (2013).</li><li>Akerboom, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Optimization+of+a+GCaMP+Calcium+Indicator+for+Neural+Activity+Imaging.">Optimization of a GCaMP Calcium Indicator for Neural Activity Imaging.</a> J. Neurosci. 32, 13819-13840 (2012).</li><li>Shigetomi, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+calcium+microdomains+within+entire+astrocyte+territories+and+endfeet+with+GCaMPs+expressed+using+adeno-associated+viruses.">Imaging calcium microdomains within entire astrocyte territories and endfeet with GCaMPs expressed using adeno-associated viruses.</a> J. Gen. Physiol. 141, 633-647 (2013).</li><li>Ortinski, P. I., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+induction+of+astrocytic+gliosis+generates+deficits+in+neuronal+inhibition.">Selective induction of astrocytic gliosis generates deficits in neuronal inhibition.</a> Nat. Neurosci. 13, 584-591 (2010).</li><li>Fiacco, T. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Selective+stimulation+of+astrocyte+calcium+in+situ+does+not+affect+neuronal+excitatory+synaptic+activity.">Selective stimulation of astrocyte calcium in situ does not affect neuronal excitatory synaptic activity.</a> Neuron. 54, 611-626 (2007).</li><li>Echegoyen, J., Neu, A., Graber, K. D., Soltesz, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Homeostatic+plasticity+studied+using+in+vivo+hippocampal+activity-blockade:+synaptic+scaling,+intrinsic+plasticity+and+age-dependence.">Homeostatic plasticity studied using in vivo hippocampal activity-blockade: synaptic scaling, intrinsic plasticity and age-dependence.</a> PLoS One. 2, (2007).</li><li>Aguado, F., Espinosa-Parrilla, J. F., Carmona, M. A., Soriano, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Neuronal+activity+regulates+correlated+network+properties+of+spontaneous+calcium+transients+in+astrocytes+in+situ.">Neuronal activity regulates correlated network properties of spontaneous calcium transients in astrocytes in situ.</a> J. Neurosci. 22, 9430-9444 (2002).</li><li>Takata, N., Hirase, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cortical+layer+1+and+layer+2/3+astrocytes+exhibit+distinct+calcium+dynamics+in+vivo.">Cortical layer 1 and layer 2/3 astrocytes exhibit distinct calcium dynamics in vivo.</a> PLoS ONE. 3, e2525 (2008).</li><li>Salahpour, A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=BRET+biosensors+to+study+GPCR+biology,+pharmacology,+and+signal+transduction.">BRET biosensors to study GPCR biology, pharmacology, and signal transduction.</a> Front. Endocrinol. 3 (105), (2012).</li><li>Di Castro, M. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Local+Ca2++detection+and+modulation+of+synaptic+release+by+astrocytes.">Local Ca2+ detection and modulation of synaptic release by astrocytes.</a> Nat. Neurosci. 14, 1276-1284 (2011).</li><li>Sun, W., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glutamate-dependent+neuroglial+calcium+signaling+differs+between+young+and+adult.">Glutamate-dependent neuroglial calcium signaling differs between young and adult.</a> Science. 339, 197-200 (2013).</li></ol>

The authors wish to disclose that the tetrodotoxin used in these studies was purchased from Abcam. Abcam had no involvement in the hypotheses, design, or collection of data. All communication regarding sponsorship of the work by Abcam occurred after the peer review process was complete.

The described scaling models represent practical methods for researching long-term plasticity of astrocytic group I mGluRs. Imaging spontaneous and evoked Ca2+ events provides a sensitive assay for measuring changes in astrocytic Gq GPCR activity, as firm evidence has been established that astrocyte Ca2+ elevations occur following release from IP3R-sensitive stores downstream of Gq GPCR activation10,12,17,18. The percentage of astrocytes in the population responding to group I mGluR agonist and the pattern of such Ca2+ responses report changes in group I mGluRs by astrocytes.
The specific technique used to load astrocytes with Ca2+ indicator is an important consideration in the design of experiments to look for changes in astrocytic Gq GPCR activity. Bolus-loading or bulk-loading multiple astrocytes, or patch-clamp loading of individual astrocytes can be used to image Ca2+ transients in astrocytes. Each approach offers certain advantages and disadvantages. Directly filling astrocytes with Ca2+ indicator via patch clamp allows unequivocal identification of the cell as an astrocyte without need for a secondary marker such as SR-101. Patch-clamp delivery of indicator also enables recording of Ca2+ activity from small astrocytic compartments including the bushy fine processes, potentially deeper in the slice where cells are healthier and with more intact interactions with synapses (depending on the laser power available). However, patch-clamp loading suffers from low throughput as data are collected one cell at a time. Bulk-loading, by contrast, allows a large number of astrocytes to be loaded with Ca2+ indicator and imaged simultaneously. However, only astrocytes near the surface (&#60;20 &#181;m) of the slice are loaded, with associated concerns about cell health and intact synapses.
The backpressure bolus-loading protocol presented here offers a middle ground, with relatively high throughput and the ability to monitor Ca2+ activity deeper within the slice (40-75 &#181;m). A significant increase in the percentage of spontaneously active astrocytes using the bolus-loading technique is observed compared to bulk-loading, suggesting that the connections among neuronal synapses and the astrocytic processes are more complete15. With good loading, one can often monitor Ca2+ activity in main processes of astrocytes (data not shown) or potentially even smaller compartments using 2-photon microscopy. However, care would need to be exercised in assigning the smaller processes to a particular astrocyte, as the boundaries blend into the nonspecific background staining. An additional concern with the use of bulk-loading or bolus-loading procedures is the need for a secondary marker for astrocyte identification. While it has been known for many years that astrocytes preferentially take up AM ester Ca2+ indicators, the secondary marker SR-101 is often used to verify the loaded cells as astrocytes. SR-101 may in itself alter the intrinsic excitability of neurons29. Use of SR-101 corroborates the need to perform all astrocyte Ca2+ measurements in TTX to limit possible SR-101 effects on neuronal excitability. Assuming that both control and experimental groups include SR-101, the marker in itself should not account for the effects observed in astrocyte Ca2+ signaling following long-term manipulation of neuronal action potentials. SR-101 may be more of a concern in high K+ experiments, however, as it may reduce the difference between 2.5 mM K+ vs. 5.0 mM K+ if the basal firing rate is not altered proportionally.
A very promising approach to deliver Ca2+ indicator to astrocytes has emerged recently which offers an attractive alternative to the more traditional approaches using Ca2+ dyes. Significant advances have been made over the past few years with genetically encoded calcium indicators (GECIs) targeted to astrocytes30-32. GECIs can be delivered to astrocytes by in vivo microinjection of adeno-associated viral vectors into a brain region of interest such as the hippocampus. Expression of GECIs is achieved after approximately two weeks following viral infection32. There are numerous advantages presented by the use of GECIs in astrocytes. First, the vectors are targeted to astrocytes using an astrocyte-specific promoter, so the labeled cells are astrocytes32. Second, the signal-to-noise now seems comparable to what can be obtained using patch-clamp delivery of dye, but without the invasiveness of having had a patch pipette on the cell32. Third, the indicators can be delivered and expressed in adult tissue, which is problematic using bulk-loading delivery methods. Furthermore, the expression is mosaic, offering the ability to differentiate among multiple astrocytes. Thus, several astrocytes can potentially be imaged simultaneously, while also recording in the soma and fine branchlets at the same time. Therefore, potentially one single technique could be used in place of three separate techniques (bulk-loading, bolus-loading, and patch-clamp loading) to record scaling activity of astrocytic Gq GPCRs, greatly increasing efficiency.
One potential drawback of using viral-mediated delivery of Ca2+ indicators to astrocytes is the possible effect on slice health32. The adeno-associated viral vectors used to deliver the GECIs have been shown previously to cause reactive gliosis of astrocytes33. Preparation of brain slices in general likely initiates early stages of pathology including release of inflammatory molecules10. Therefore, combined with the long incubation times required to induce scaling of astrocytic receptors, use of GECIs delivered using viral vectors would need to receive additional consideration in the context of slice health in these types of experiments.
When employing this protocol, it is important to keep in mind that the application time for agonist to produce a response will vary as a function of receptor availability. For a given concentration of agonist, the application time will have to be longer if receptors have scaled down, and shorter if receptors have scaled up, for the drug to reach an adequate concentration in the tissue to activate receptors sufficiently to produce a Ca2+ response. Therefore, drug application times, and potentially their concentrations, may have to be adjusted depending upon the intended direction of the scaling. For example, the agonist concentration may need to be lowered in the case of TTX to avoid saturating responses, and increased after incubating slices in high K+ to even see a response. Specifically, the DHPG concentration was shifted from 5-15 &#181;M after TTX treatment to 30-50 &#181;M after 5.0 mM K+ treatment in order to study Ca2+ response patterns, as 5-15 &#181;M was often too low to produce reliable responses in astrocytes after scaling down of group I mGluRs.
Recording of astrocyte Ca2+ activity provides no direct evidence of receptor insertion or internalization to or from the plasma membrane. However, based on the remarkable similarity of the data with data from previous studies in vitro which examined the direct relationship between Gq GPCR expression levels and spontaneous and evoked Ca2+ transients21-24, the most logical interpretation of the changes in Ca2+ signaling is that the astrocyte surface receptor expression levels have changed. A complementary approach may be an important consideration if one wants to provide additional evidence about the locus of the effect on Ca2+ activity. A strategy we used was to examine the effect of TTX incubation on hippocampal slices from astrocytic MrgA1R mice. These transgenic mice express a foreign Gq GPCR (the MrgA1R) only in astrocytes. Because this receptor is not native to the brain, there is no endogenous neurotransmitter present to change its activity levels. Previous work suggested that this receptor engages the same intracellular signaling molecules as endogenous group I mGluRs in the same astrocytes34. After long-term incubation of slices from MrgA1R mice in TTX, no differences in agonist-evoked MrgA1R responses compared to littermate control incubated slices would provide evidence that the effect on astrocyte Ca2+ activity is due to changes localized to the surface receptor, especially if group I mGluR responses are still significantly enhanced in the same astrocytes. An alternative, though perhaps more involved strategy would be to isolate astrocytes from the slices for Western blot analysis, as long as a membrane fraction could be analyzed for changes in surface receptor expression levels. Fluorescence Activated Cell Sorting (FACS) or flow cytometry may be helpful here.
The possible applications of this technique to the study of neurons, astrocytes and astrocyte-neuronal interactions are many. In our experiments, only DHPG-evoked group I mGluR astrocytic Ca2+ responses were studied, in isolated acute hippocampal slices from juvenile mice. This preparation not only has the intact afferents (Schaffer collaterals), but also the neurons that give rise to them (CA3 pyramidal cells), making it possible to manipulate the firing rates of these glutamatergic neurons onto the postsynaptic cells (CA1 pyramidal cells) and the astrocytes in stratum radiatum whose processes associate with these synapses. The acute hippocampal slice may not be the best preparation for manipulating firing rates of other types of neurons, however, as many afferents are severed from the neurons that give rise to them. Nevertheless, it may be possible in certain slice preparations to observe plasticity of other astrocytic Gq GPCR subtypes. For example, slices could be prepared with basal forebrain cholinergic neurons and their projections to hippocampus intact. Incubation of these slices in TTX or elevated K+ would affect basal firing rates of cholinergic neurons, leading to scaling of mAchRs in astrocytes of stratum oriens, which receive a significant portion of the cholinergic input1. An alternative yet untested approach to study astrocytic receptor scaling within a specific area of the brain, with all of the connections intact while scaling occurs, could be to use an in vivo model where a sustained release of TTX is achieved by implantation of a plastic polymer Elvax 40W above the region of interest35. This approach has been used previously in a study of neuronal scaling but should also be applicable to astrocytic scaling. Finally, with the proper readout, future studies could examine other GPCR families, including changes in Gs or Gi GPCRs. One might predict astrocytic GABAB Gi GPCRs to be affected following inhibition of firing in locally projecting GABA interneurons within any slice preparation. Development of new indicators targeting other signaling molecules, such as a real-time indicator of the second messenger cAMP, would open up a whole new area of research on neuron-to-astrocyte receptor communication.
Bidirectional scaling of astrocytic mGluRs by manipulation of basal neuron firing rates provides a measure of the sensitivity of astrocytes to AP-mediated release of neurotransmitter. Astrocytes can apparently sense spontaneous APs and glutamate release at Schaffer collateral-CA1 pyramidal cell synapses even when extracellular K+ is within a physiological range. While acute application of TTX does not reduce the frequency of spontaneous astrocyte Ca2+ activity18,36,37, the Ca2+ activity among the astrocytes in the population becomes decorrelated36, providing evidence that astrocytic receptors are AP detectors. This suggests that astrocytes sense spontaneous neuronal APs with no affect on their overall Ca2+ activity. It is widely accepted that intracellular concentrations of IP3 need to reach a threshold level to stimulate IP3Rs sufficiently to lead to a detectable Ca2+ elevation. Could spontaneous neuronal APs activate astrocytic GPCRs without producing measurable Ca2+ elevations? Future studies could utilize Fluorescence Resonance Energy Transfer (FRET) or a similar technique (such as BRET) to examine the relationship between G protein coupling to the receptor (a measure of receptor activation) and Ca2+ release from internal stores. BRET has been used extensively in vitro to detect G protein-to-GPCR coupling38, although it may be some time before this technology becomes available for use in intact tissue preparations. It is possible that astrocytic Gq GPCRs are being activated much more frequently than can be recorded using the currently available Ca2+ imaging tools. In addition to sensing action potentials, astrocytic Gq GPCRs may also be able to detect miniature quantal release of neurotransmitter as reported in a recent study39. The bidirectional scaling method described here may be used in future studies to provide a measure of the extent to which astrocytic Gq GPCRs detect quantal vesicular release of neurotransmitter, by including bafilomycin A1 in the incubation protocol.
Thus far, the scaling protocols have only been used in hippocampal slices from juvenile mice (p12-p18). Therefore, it is currently unknown if astrocytic receptor scaling could also be induced in tissue obtained from adult mice. A compelling recent study suggests that group I mGluR expression in astrocytes diminishes considerably after the first week of age and continues to decline until adulthood, with very low levels of receptor expression in adult astrocytes40. It would therefore be interesting to determine if astrocytic mGluRs scale up following long-term inhibition of neuronal firing in adult mouse hippocampal slices to levels approaching those seen in astrocytes from juvenile mice. This finding would suggest that astrocytic receptor expression is not static at a given age but can change rapidly depending upon levels of neuronal activity. In contrast to reduced expression of group I mGluRs in adult mice, evidence is emerging that adrenergic receptors, including &#945;1A, &#945;2A, and &#946;1 subtypes, are predominantly expressed by astrocytes in the adult brain3,4. The &#945;1A adrenergic Gq GPCR may be an attractive target for future studies of neuron-to-astrocyte&#160; communication, including whether these receptors are sensitive to changes in adrenergic neuron firing rates.&#160;

Astrocytes respond within seconds to stimulation of neurons or neuronal axons with increases in cytoplasmic Ca2+ resulting almost exclusively from activation of astrocytic Gq GPCRs. For example, muscarinic acetylcholine receptors1, cannabinoid receptors2, &#945;1A adrenergic receptors3,4, and group I mGluRs (see below) are all astrocytic Gq GPCR subtypes that acutely respond to neuronal activity. Activation of astrocytic group I mGluRs has been demonstrated most extensively, following stimulation of neuronal glutamatergic afferents in situ (such as acute hippocampal slices)5-7, as well as in adult mouse cortex in vivo following sensory stimulation8. The outcome of activation of astrocytic Gq GPCR signaling on the biology and physiology of astrocytes, neurons, or astrocyte-neuron interactions has been a matter of debate9-12. It will be some time before the function of neuron-to-astrocyte receptor signaling is fully appreciated.&#160;
While it is clear that neurons can activate astrocytic receptors using experimental protocols, there are aspects of neuron-to-astrocyte receptor communication that remain poorly understood. First, the actual amount of neuronal activity required to activate astrocytic Gq GPCRs is not well-defined, and second, the ability of astrocytic receptors to exhibit use-dependent plasticity has received little attention. To begin to address these questions, we recently developed a protocol to induce bidirectional scaling of astrocytic group I mGluRs in acute juvenile hippocampal slices in response to long-term changes in neuronal action potential (AP)-dependent synaptic activity. Similar to what has been discovered for bidirectional homeostatic plasticity of neuronal ionotropic glutamate receptors13,14, astrocytic group I mGluRs scale up following blockade of neuronal action potentials and scale down when neuronal action potential frequency is increased15. These compensatory changes in astrocytic receptors can be measured by recording spontaneous and evoked astrocyte Ca2+ transients and comparing the properties of these events to those from astrocytes in control conditions. In this manuscript, we describe the complete methodology for use of this protocol, including preparation of acute hippocampal slices, incubation conditions to induce astrocyte receptor scaling, astrocyte Ca2+ indicator dye loading, Ca2+ imaging techniques using confocal microscopy, and expected effects on astrocyte Gq GPCR activity. Predictable effects on astrocyte Ca2+ signaling properties - which match those recorded previously in cultured cells transfected with different expression levels of Gq GPCRs - provide a &#8220;roadmap&#8221; that can be used in future studies to assay for changes in astrocytic GPCR expression. The ramifications and potential applications for use of this technique will contribute to our understanding of astrocyte-neuronal interactions in the healthy and diseased brain.

<table border="0" cellpadding="0" cellspacing="0" width="587">
	<colgroup>
		<col />
		<col />
		<col />
		<col />
	</colgroup>
	<tbody>
		<tr height="32">
			<td height="32" style="height:32px;width:163px;">Chamber Supplies</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="66">
			<td height="66" style="height:66px;width:163px;">Brinkmann pipette storage container</td>
			<td style="width:135px;">Fisher Scientific</td>
			<td style="width:131px;">03-491</td>
			<td style="width:159px;">Use the drawer portion as the incubation chamber</td>
		</tr>
		<tr height="28">
			<td height="28" style="height:28px;width:163px;">Electrical drill</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:163px;">Flexible tubing</td>
			<td style="width:135px;">Tygon</td>
			<td style="width:131px;">R-3603</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:163px;">Silicone seam sealant&#160;</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;">Also called aquarium seam sealer</td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:163px;">Gas tank</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;">95% oxygen, 5% carbon dioxide</td>
		</tr>
		<tr height="46">
			<td height="46" style="height:46px;width:163px;">Natural beveled pipette tip</td>
			<td style="width:135px;">USA Scientific</td>
			<td style="width:131px;">1111-1000</td>
			<td style="width:159px;">Cut-to-fit to connect oxygenate lines</td>
		</tr>
		<tr height="46">
			<td height="46" style="height:46px;width:163px;">One-to-six lines manifold</td>
			<td style="width:135px;">Warner Instruments</td>
			<td style="width:131px;">64-0210 &#160;(MP-6)</td>
			<td style="width:159px;">For the microloader-manifold apparatus</td>
		</tr>
		<tr height="59">
			<td height="65" rowspan="2" style="height:65px;width:163px;">Eppendorf microloader</td>
			<td rowspan="2" style="width:135px;">Eppendorf</td>
			<td rowspan="2" style="width:131px;">5242 956.003</td>
			<td rowspan="2" style="width:159px;">For the microloader-manifold apparatus, cut-to-fit</td>
		</tr>
		<tr height="6">
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:163px;">Floating Bubble Rack</td>
			<td style="width:135px;">Bel Art Scienceware</td>
			<td style="width:131px;">F18875-0400</td>
			<td style="width:159px;">For slice holder</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:163px;">600 micron Sefar Nitex nylon mesh</td>
			<td style="width:135px;">ELKO filtering Co.</td>
			<td style="width:131px;">&#160;06-600/51</td>
			<td style="width:159px;">For slice holder</td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">Krazy Glue</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;">For slice holder</td>
		</tr>
		<tr height="32">
			<td height="32" style="height:32px;width:163px;">Reagent</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:163px;">Isoflurane</td>
			<td style="width:135px;">Baxter</td>
			<td style="width:131px;">1001936060</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">NaCl</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">S271-3</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:163px;">KCl</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">P333-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;width:163px;">CaCl2</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">C79-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="26">
			<td height="26" style="height:26px;width:163px;">MgCl2</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">M33-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="23">
			<td height="23" style="height:23px;width:163px;">NaH2PO4</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">S369-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;width:163px;">NaHCO3</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">S233-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">Glucose</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">Fisher</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="63">
			<td height="63" style="height:63px;width:163px;">(&#177;)-6-Hydroxy-2,5,7,8- tetramethylchromane-2-carboxylic acid (Trolox)</td>
			<td style="width:135px;">Acros Organics</td>
			<td style="width:131px;">53188-07-1</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">Ascorbic acid</td>
			<td style="width:135px;">Acros Organics</td>
			<td style="width:131px;">401471000</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Tetrodotoxin citrate (TTX)</td>
			<td style="width:135px;">Ascent Scientific</td>
			<td style="width:131px;">Asc-055</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Sulforhodamine 101 (SR-101)</td>
			<td style="width:135px;">Sigma-Aldrich</td>
			<td style="width:131px;">284912</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Dimethyl Sulfoxide (DMSO)&#160;</td>
			<td style="width:135px;">Fisher Scientific</td>
			<td style="width:131px;">D128-500</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Pluronic Acid F-127</td>
			<td style="width:135px;">Invitrogen, Molecular Probes</td>
			<td style="width:131px;">P6867</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:163px;">Oregon Green 488 BAPTA-1 AM *cell permeant (special packaging)</td>
			<td style="width:135px;">Invitrogen, Molecular Probes</td>
			<td style="width:131px;">O6807</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">(RS)-3,5-DHPG</td>
			<td style="width:135px;">Ascent</td>
			<td style="width:131px;">Asc-020</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">Histamine</td>
			<td style="width:135px;">Sigma-Aldrich&#160;</td>
			<td style="width:131px;">H7125</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="46">
			<td height="46" style="height:46px;width:163px;">Carbamoylcholine chloride (Carbachol)</td>
			<td style="width:135px;">Sigma-Aldrich</td>
			<td style="width:131px;">C4382</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="44">
			<td height="44" style="height:44px;width:163px;">Adenosine 5&#8217;-ATP disodium (Na-ATP)</td>
			<td style="width:135px;">Sigma-Aldrich</td>
			<td style="width:131px;">A7699</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="32">
			<td height="32" style="height:32px;width:163px;">Dissection Tools</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="25">
			<td height="25" style="height:25px;width:163px;">Single-edge razor blade&#160;</td>
			<td style="width:135px;">GEM</td>
			<td style="width:131px;">62-0161</td>
			<td style="width:159px;">For bisection&#160;</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Double edge razor blade</td>
			<td style="width:135px;">TED PELLA, INC.</td>
			<td style="width:131px;">121-6</td>
			<td style="width:159px;">For cutting slices</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Mayo Scissors, supercut</td>
			<td style="width:135px;">WPI</td>
			<td style="width:131px;">14010-15</td>
			<td style="width:159px;">For decapitation</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:163px;">Fine iris scissors, straight</td>
			<td style="width:135px;">Fine Science Tools</td>
			<td style="width:131px;">14094-11</td>
			<td style="width:159px;">For cutting the skull</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:163px;">Iris forceps, curved</td>
			<td style="width:135px;">WPI</td>
			<td style="width:131px;">15915</td>
			<td style="width:159px;">To remove the skin and skull</td>
		</tr>
		<tr height="44">
			<td height="44" style="height:44px;width:163px;">Small spatula</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;">To remove/transfer the brain</td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:163px;">Dumostar Dumont #5 Biologie Tip forceps</td>
			<td style="width:135px;">Fine Science Tools</td>
			<td style="width:131px;">11295-10</td>
			<td style="width:159px;">For hippocampus dissection</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Glass Pasteur pipette</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">13-678-20B</td>
			<td style="width:159px;">For transferring brain slices</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Pasteur pipette rubber bulb</td>
			<td style="width:135px;">Fisher</td>
			<td style="width:131px;">03-448-22</td>
			<td style="width:159px;">For transferring brain slices</td>
		</tr>
		<tr height="39">
			<td height="39" style="height:39px;width:163px;">Polystyrene 100-mm tissue culture dishes</td>
			<td style="width:135px;">Corning</td>
			<td style="width:131px;">25020</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="32">
			<td height="32" style="height:32px;width:163px;">Equipment</td>
			<td style="width:135px;"></td>
			<td style="width:131px;"></td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;width:163px;">Vibratome</td>
			<td style="width:135px;">Leica</td>
			<td style="width:131px;">VT 1200S</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="20">
			<td height="23" rowspan="2" style="height:23px;width:163px;">Water bath</td>
			<td rowspan="2" style="width:135px;">Fisher</td>
			<td rowspan="2" style="width:131px;">ISOTEMP 210</td>
			<td rowspan="2" style="width:159px;">For warm incubation</td>
		</tr>
		<tr height="3">
		</tr>
		<tr height="43">
			<td height="43" style="height:43px;width:163px;">Micropipette puller</td>
			<td style="width:135px;">Narishige</td>
			<td style="width:131px;">PC-10</td>
			<td style="width:159px;">For bolus-loading pipette</td>
		</tr>
		<tr height="42">
			<td height="42" style="height:42px;width:163px;">Confocal microscope</td>
			<td style="width:135px;">Olympus</td>
			<td style="width:131px;">Olympus Fluoview 1000</td>
			<td style="width:159px;"></td>
		</tr>
		<tr height="85">
			<td height="85" style="height:85px;width:163px;">Low Profile Open Diamond Bath Imaging Chamber with PM-1 platform</td>
			<td style="width:135px;">Warner Instruments</td>
			<td style="width:131px;">RC-26GLP</td>
			<td style="width:159px;">diamond bath with low profile</td>
		</tr>
		<tr height="45">
			<td height="45" style="height:45px;width:163px;">Borosilicate glass pipette</td>
			<td style="width:135px;">World Precision Instruments</td>
			<td style="width:131px;">TW150F-4</td>
			<td style="width:159px;">For bolus-loading pipette</td>
		</tr>
		<tr height="22">
			<td height="25" rowspan="2" style="height:25px;width:163px;">Micromanipulator&#160;</td>
			<td rowspan="2" style="width:135px;">Sutter instrument</td>
			<td rowspan="2" style="width:131px;">ROE-200</td>
			<td rowspan="2" style="width:159px;">For bolus-loading pipette</td>
		</tr>
		<tr height="3">
		</tr>
		<tr height="64">
			<td height="64" style="height:64px;width:163px;">Spin-X centrifuge tube filter with 0.22 &#181;m cellulose acetate&#160;</td>
			<td style="width:135px;">Costar</td>
			<td style="width:131px;">8161</td>
			<td style="width:159px;"></td>
		</tr>
	</tbody>
</table>

inducing plasticity of astrocytic receptors by manipulation of neuronal firing rates

Close to two decades of research has established that astrocytes in situ and in vivo express numerous G protein-coupled receptors (GPCRs) that can be stimulated by neuronally-released transmitter. However, the ability of astrocytic receptors to exhibit plasticity in response to changes in neuronal activity has received little attention. Here we describe a model system that can be used to globally scale up or down astrocytic group I metabotropic glutamate receptors (mGluRs) in acute brain slices. Included are methods on how to prepare parasagittal hippocampal slices, construct chambers suitable for long-term slice incubation, bidirectionally manipulate neuronal action potential frequency, load astrocytes and astrocyte processes with fluorescent Ca2+ indicator, and measure changes in astrocytic Gq GPCR activity by recording spontaneous and evoked astrocyte Ca2+ events using confocal microscopy. In essence, a &#8220;calcium roadmap&#8221; is provided for how to measure plasticity of astrocytic Gq GPCRs. Applications of the technique for study of astrocytes are discussed. Having an understanding of how astrocytic receptor signaling is affected by changes in neuronal activity has important implications for both normal synaptic function as well as processes underlying neurological disorders and neurodegenerative disease.

Representative results in Figure 3 show the effect of incubation of acute mouse hippocampal slices in TTX for 4-6 hr on s.r. astrocyte Ca2+ activity. Data include both spontaneous Ca2+ transients and DHPG-evoked group I mGluR Ca2+ responses, from slices incubated in control ACSF vs. ACSF plus TTX. Other than basic characteristic morphological features, stellate process assembly and small soma size (~10 &#181;m), astrocytes are identified in the s.r. by overlay of the Ca2+ indicator OGB-1 AM with the selective astrocyte marker SR-10119,20 (Figure 3A). The numbered boxes over astrocyte cell bodies correspond to the numbered fluorescence over time traces shown in Figure 3B. The group 1 mGluR agonist (RS)-3.5-DHPG is applied to determine the specific scaling effects on group 1 mGluRs in astrocytes. To discriminate between&#160;physiology&#160;in the specificity of the scaling effect to group 1 mGluRs versus other Gq GPCRs, a cocktail of agonists is applied at the end of each experiment. Here we used 10 &#181;M each of the Gq GPCR agonists histamine, carbamylcholine chloride (carbachol) and adenosine 5&#8217;-ATP disodium (Na-ATP). The agonist cocktail also serves as a positive control to identify viable, responsive astrocytes in cases where cells do not respond to DHPG, presumably because those particular astrocytes do not express sufficient amounts of the receptor to elicit a response to DHPG.
We used different concentrations of DHPG, including 5 &#181;M and 15 &#181;M (Figure 3B) as well as 30 &#181;M and 50 &#181;M (Figure 4A) to assist in revealing changes in astrocytic group I mGluRs. The relationship between cellular Ca2+ responses and Gq GPCR expression levels has been examined previously in vitro21-24. First, the threshold to respond to a particular agonist concentration depends on the density of receptors expressed by each cell. In a population of cells, more cells respond with a Ca2+ elevation to a given concentration of agonist when the cells are transfected with higher densities of receptors. After incubating slices in TTX, the percentage of astrocytes in the population responding to a fixed concentration of agonist increases (Figures 3B and 3C). We have found that 5 &#181;M and 15 &#181;M DHPG reveal obvious differences in the percentage of responsive astrocytes between the control and TTX treated cells, whereas 30 and 50 &#181;M DHPG are required to compare group I mGluR responses in 5.0 mM K+ treated vs. control cells (Figure 4A).
The relationship between astrocytic Ca2+ response patterns and agonist concentration has also been examined in situ. Increasing the agonist concentration shifts the pattern of the Ca2+ response in astrocytes from single peak Ca2+ elevations to multipeak and plateau Ca2+ elevations25-27. Based on these previous findings, we predicted that the response pattern to a single concentration of agonist will shift if there are changes in the level of receptor expression. Thus, depending on which of the two scaling methods is being utilized (inhibiting neuronal firing or increasing it), the concentration of agonist necessary to produce a particular response pattern will increase or decrease. For example, astrocytes incubated in TTX shift their DHPG-evoked Ca2+ response pattern to more plateau-type Ca2+ elevations and respond to lower agonist concentrations compared to control astrocytes (Figure 3C). Taking into account the earlier studies, these observations suggest that the group I mGluR receptor expression levels in astrocytes has increased. &#160;
Rise time and onset (latency) of the Ca2+ elevations have also been shown to directly correlate to changes in Gq GPCR expression levels in cultured cells22-24. Higher receptor expression levels result in shorter response latencies and faster rise times while reduction in receptor density produces the opposite effect. For astrocytes incubated in TTX, Ca2+ transients evoked by application of DHPG have significantly faster rise times compared to astrocytes incubated in control ACSF (Figure 3D). As previously mentioned, amplitudes of agonist-evoked Ca2+ responses remain unchanged regardless of the agonist concentration or the scaling model15 (Figure 3D).
In addition to the changes observed by directly activating group I mGluRs with DHPG, spontaneous astrocyte Ca2+ activity is also significantly affected by this manipulation. We observed a 2.26 fold increase in the percentage of spontaneously active astrocytes incubated in TTX versus control. This is an increase from only 12.9% of control astrocytes exhibiting spontaneous activity in the soma to 42.1% in the TTX incubated astrocytes (Figure 3E). Because it is known that GPCRs exhibit &#8220;intrinsic&#8221; or constitutive activity in the absence of agonist21,26,28, and that the level of this intrinsic activity increases with increasing receptor expression levels, these data suggest that the density of astrocytic Gq GPCRs increases following long-term reduction in neuronal action potential firing. Similar to agonist-evoked responses, the rise times of the spontaneous Ca2+ elevations are also increased (Figure 3E).
Representative data using the second protocol, incubation in elevated extracellular potassium (5.0 mM), is depicted in Figure 4. An increase in extracellular K+ from 2.5-5.0 mM results in a significant increase in basal CA3 neuron action potential frequency15. Higher concentrations of DHPG (30 &#181;M and 50 &#181;M) are necessary in order to evoke group I mGluR Ca2+ responses from the astrocytes incubated in high potassium (Figures 4A and&#160;4B). This is consistent with a reduced level of group I mGluR responsiveness in astrocytes following a long-term increase in neuronal action potentials. In addition, the evoked response pattern to a fixed concentration of DHPG shifts from plateau-like responses to weaker single-peak responses (Figure 4B). Examining the percentage of spontaneously active astrocytes in the two potassium conditions reveals that fewer astrocytes incubated in high K+ are spontaneously active compared to the control condition (Figure 4C). This effect is in the opposite direction of the TTX condition in which the percentage of astrocytes exhibiting spontaneous Ca2+ elevations is increased. Last, both evoked and spontaneous Ca2+ elevations have slower rise times in astrocytes incubated in high K+ versus the control condition (Figures&#160;4C and 4D). Overall these data suggest that astrocytic Gq GPCR expression levels scale bidirectionally depending upon the level of neuronal action potential activity over an extended time period.
<img alt="Figure 1" fo:content-width="5in" src="/files/ftp_upload/51458/51458fig1highres.jpg" width="500" /> 
Figure 1.&#160;Slice incubation chamber fabrication and set up. (A) The drawer portion of a Brinkmann pipette storage container along with its air-tight lid is used to construct the slice incubation chamber. (B) Drill a hole in the side of the container approximately 1&#188; in from the top and &#188; in from the side. Fit in a piece of flexible tubing. (C) Connect the microloader-manifold apparatus to the inside end of the flexible tubing. Note cut-to-fit natural 200 &#181;l pipette tip (white arrow). (D) Six 20 &#181;l Eppendorf microloaders are cut-to-fit to a one-to-six line manifold to create a microloader-manifold apparatus. (E) Drill two small holes on the lid. (F) A Floating Bubble Rack for making the slice holder. (G) The bottom &#8220;legs&#8221; of the floating bubble rack are removed so that a piece of Nylon mesh material can be glued to the bottom. (H) Fill the incubation chamber with sufficient amount of ACSF so that the slice holder floats. Adjust the tubing length so the tips of the microloaders can rest at one corner of the chamber floor. When placing the incubation chamber in the water bath, the water level in the bath should be at the same level as the ACSF in the chamber. <a href="https://www.jove.com/files/ftp_upload/51458/51458fig1highres.jpg" target="_blank">Click here to view larger image.</a>
<img alt="Figure 2" fo:content-width="5in" src="/files/ftp_upload/51458/51458fig2highres.jpg" width="500" /> 
Figure 2.&#160;Estimation of slice health. (A) A healthy-looking hippocampal slice using differential interference contrast (DIC) optics. Healthy slices have a smooth, velvety appearance and a high percentage of healthy CA1 pyramidal neurons. Note the apical dendrites projecting into stratum radiatum. Patch-clamp of neurons that look like those shown here reveal a low resting membrane potential (-61 to -62 mV) in standard 2.5 mM K+ ACSF with few spontaneous action potentials. Membrane potential and firing rates will vary as a function of extracellular K+ (Xie et al.15). Arrows point to putative astrocytes. Abbreviations: s.r., stratum radiatum; s.py., stratum pyramidale. Scale bar, 10 &#181;m. (B) Unhealthy slices will have a high percentage of dead CA1 pyramidal neurons, which have the appearance of fried eggs (white arrows point to nuclei of dead neurons - the &#8220;yolk&#8221; of the fried egg). <a href="https://www.jove.com/files/ftp_upload/51458/51458fig2highres.jpg" target="_blank">Click here to view larger image.</a>
<img alt="Figure 3" fo:content-width="4.5in" src="/files/ftp_upload/51458/51458fig3highres.jpg" width="500" /> 
Figure 3.&#160;Recording amplified Gq GPCR activity and evoked group I mGluR Ca2+ responses after long-term inhibition of neuronal APs by incubation in TTX. (A) Representative images of cells in the recording field incubated in control conditions (upper panels) or in TTX (lower panels) that have taken up Oregon Green BAPTA-1 AM Ca2+ indicator dye (left panels) and SR-101 (middle panels). Scale bar is 10 &#181;m. Overlay of both signals (&#8220;merge&#8221;) indicates that astrocytes load with Ca2+ indicator. Boxes are drawn over individual astrocyte soma to record fluorescence intensity over time in the green channel to monitor Ca2+ activity in astrocytes. (B) Sample traces from the recording boxes in A) of Ca2+ activity in the astrocytes. Astrocytes incubated in TTX show increased spontaneous activity and more robust evoked group I mGluR Ca2+ responses as evidenced by changes in the pattern of response. Examples of single peak (circle), multipeak (rectangle), and plateau (dotted rectangle) Ca2+ transients are shown. (C) Changes in response patterns are especially evident using different concentrations of the group I mGluR agonist DHPG. More multipeak and plateau responses are evident after incubation in TTX compared to control at a given agonist concentration. (D) Rise times of DHPG-evoked Ca2+ responses are faster in astrocytes incubated in TTX compared to control (upper panel), while amplitudes do not change (lower panel), indicative of &#8220;all-or-none&#8221; response amplitudes once the threshold to respond has been reached. (E) Rise times of spontaneous astrocyte Ca2+ transients are also faster in TTX incubated vs. control incubated astrocytes (upper panel), while the percentage of astrocytes in the population exhibiting spontaneous Gq GPCR Ca2+ activity increases (lower panel). <a href="https://www.jove.com/files/ftp_upload/51458/51458fig3highres.jpg" target="_blank">Click here to view larger image.</a>
<img alt="Figure 4" fo:content-width="6in" src="/files/ftp_upload/51458/51458fig4highres.jpg" width="500" /> 
Figure 4.&#160;Recording diminished astrocytic Gq GPCR activity and evoked group I mGluR Ca2+ responses following long-term increase in neuronal APs by incubation in elevated extracellular potassium. (A) Representative traces of astrocyte Ca2+ recordings from slices incubated in 5.0 mM K+ ACSF to depolarize neurons and increase their basal firing rate compared to control ACSF (2.5 mM K+ ACSF). Astrocytes incubated with 5.0 mM K+ ACSF exhibit fewer spontaneous somatic Ca2+ transients and weaker DHPG evoked responses compared to astrocytes incubated in control ACSF. (B) A comparison of patterns of evoked responses to multiple concentrations of DHPG reveals weaker response types after long-term increase in neuronal APs. (C) A reduction in the percentage of astrocytes in the population exhibiting spontaneous Ca2+ activity is observed after long-term incubation in elevated K+ compared to control ACSF (upper panel), while rise times of the spontaneous activity become slower (lower panel). (D) Rise times of evoked astrocyte Ca2+ responses to different concentrations of DHPG become slower following 5.0 mM K+ treatment&#160; compared to astrocytes incubated in control ACSF. <a href="https://www.jove.com/files/ftp_upload/51458/51458fig4highres.jpg" target="_blank">Click here to view larger image.</a>

Watch this Scientific Journal Video about Inducing Plasticity of Astrocytic Receptors by Manipulation of Neuronal Firing Rates at JoVE.com

Inducing Plasticity of Astrocytic Receptors by Manipulation of Neuronal Firing Rates

Here we describe an adaptation of protocols used to induce homeostatic plasticity in neurons for the study of plasticity of astrocytic G protein-coupled receptors. Recently used to examine changes in astrocytic group I mGluRs in juvenile mice, the method can be applied to measure scaling of various astrocytic GPCRs, in tissue from adult mice in situ and in vivo, and to gain a better appreciation of the sensitivity of astrocytic receptors to changes in neuronal activity.

Close to two decades of research has established that astrocytes in situ and in vivo express numerous G protein-coupled receptors (GPCRs) that can be ...

inducing-plasticity-astrocytic-receptors-manipulation-neuronal-firing

Research

JoVE Journal

Neuroscience

14.1K Views.  University of California Riverside. Here we describe an adaptation of protocols used to induce homeostatic plasticity in neurons for the study of plasticity of astrocytic G protein-coupled receptors. Recently used to examine changes in astrocytic group I mGluRs in juvenile mice, the method can be applied to measure scaling of various astrocytic GPCRs, in tissue from adult mice in situ and in vivo, and to gain a better appreciation of the sensitivity of astrocytic receptors to changes in ne...

Video: Inducing Plasticity of Astrocytic Receptors by Manipulation of Neuronal Firing Rates

Preparation of Oligomeric &beta;-amyloid1-42 and Induction of Synaptic Plasticity Impairment on Hippocampal Slices

Impairment of synaptic connections is likely to underlie the subtle amnesic changes occurring at the early stages of Alzheimer s Disease (AD). &beta;-amyloid (A&beta;), a peptide produced in high amounts in AD, is known to reduce Long-Term Potentiation (LTP), a cellular correlate of learning and memory. Indeed, LTP impairment caused by A&beta; is a useful experimental paradigm for studying synaptic dysfunctions in AD models and for screening drugs capable of mitigating or reverting such synaptic impairments. Studies have shown that A&beta; produces the LTP disruption preferentially via its oligomeric form. Here we provide a detailed protocol for impairing LTP by perfusion of oligomerized synthetic A&beta;1-42 peptide onto acute hippocampal slices. In this video, we outline a step-by-step procedure for the preparation of oligomeric A&beta;1-42. Then, we follow an individual experiment in which LTP is reduced in hippocampal slices exposed to oligomerized A&beta;1-42 compared to slices in a control experiment where no A&beta;1-42 exposure had occurred.

Preparation of Oligomeric &amp;#946;-amyloid1-42 and Induction of Synaptic Plasticity Impairment on Hippocampal Slices

One feature of Alzheimer's Disease is the elevation of A&beta;1-42 peptide. Here we provide a protocol for preparing synthetic A&beta;1-42 oligomers, which impairs hippocampal Long-Term Potentiation, a cellular correlate of memory. This procedure is useful for investigating mechanisms of A&beta;-induced pathology and drug screening.

Impairment of synaptic connections is likely to underlie the subtle amnesic changes occurring at the early stages of Alzheimer s Disease (AD). ...

Mapping Inhibitory Neuronal Circuits by Laser Scanning Photostimulation

Inhibitory neurons are crucial to cortical function. They comprise about 20% of the entire cortical neuronal population and can be further subdivided into diverse subtypes based on their immunochemical, morphological, and physiological properties1-4. Although previous research has revealed much about intrinsic properties of individual types of inhibitory neurons, knowledge about their local circuit connections is still relatively limited3,5,6. Given that each individual neuron's function is shaped by its excitatory and inhibitory synaptic input within cortical circuits, we have been using laser scanning photostimulation (LSPS) to map local circuit connections to specific inhibitory cell types. Compared to conventional electrical stimulation or glutamate puff stimulation, LSPS has unique advantages allowing for extensive mapping and quantitative analysis of local functional inputs to individually recorded neurons3,7-9. Laser photostimulation via glutamate uncaging selectively activates neurons perisomatically, without activating axons of passage or distal dendrites, which ensures a sub-laminar mapping resolution. The sensitivity and efficiency of LSPS for mapping inputs from many stimulation sites over a large region are well suited for cortical circuit analysis.
Here we introduce the technique of LSPS combined with whole-cell patch clamping for local inhibitory circuit mapping. Targeted recordings of specific inhibitory cell types are facilitated by use of transgenic mice expressing green fluorescent proteins (GFP) in limited inhibitory neuron populations in the cortex3,10, which enables consistent sampling of the targeted cell types and unambiguous identification of the cell types recorded. As for LSPS mapping, we outline the system instrumentation, describe the experimental procedure and data acquisition, and present examples of circuit mapping in mouse primary somatosensory cortex. As illustrated in our experiments, caged glutamate is activated in a spatially restricted region of the brain slice by UV laser photolysis; simultaneous voltage-clamp recordings allow detection of photostimulation-evoked synaptic responses. Maps of either excitatory or inhibitory synaptic input to the targeted neuron are generated by scanning the laser beam to stimulate hundreds of potential presynaptic sites. Thus, LSPS enables the construction of detailed maps of synaptic inputs impinging onto specific types of inhibitory neurons through repeated experiments. Taken together, the photostimulation-based technique offers neuroscientists a powerful tool for determining the functional organization of local cortical circuits.

This paper introduces an approach of combining laser scanning photostimulation with whole cell recordings in transgenic mice expressing GFP in limited inhibitory neuron populations. The technique allows for extensive mapping and quantitative analysis of local synaptic circuits of specific inhibitory cortical neurons.

Inhibitory neurons are crucial to cortical function. They comprise about 20% of the entire cortical neuronal population and can be further subdivided into ...

Lateral Diffusion and Exocytosis of Membrane Proteins in Cultured Neurons Assessed using Fluorescence Recovery and Fluorescence-loss Photobleaching

Membrane proteins such as receptors and ion channels undergo active trafficking in neurons, which are highly polarised and morphologically complex. This directed trafficking is of fundamental importance to deliver, maintain or remove synaptic proteins.
Super-ecliptic pHluorin (SEP) is a pH-sensitive derivative of eGFP that has been extensively used for live cell imaging of plasma membrane proteins1-2. At low pH, protonation of SEP decreases photon absorption and eliminates fluorescence emission. As most intracellular trafficking events occur in compartments with low pH, where SEP fluorescence is eclipsed, the fluorescence signal from SEP-tagged proteins is predominantly from the plasma membrane where the SEP is exposed to a neutral pH extracellular environment. When illuminated at high intensity SEP, like every fluorescent dye, is irreversibly photodamaged (photobleached)3-5. Importantly, because low pH quenches photon absorption, only surface expressed SEP can be photobleached whereas intracellular SEP is unaffected by the high intensity illumination6-10. 
FRAP (fluorescence recovery after photobleaching) of SEP-tagged proteins is a convenient and powerful technique for assessing protein dynamics at the plasma membrane. When fluorescently tagged proteins are photobleached in a region of interest (ROI) the recovery in fluorescence occurs due to the movement of unbleached SEP-tagged proteins into the bleached region. This can occur via lateral diffusion and/or from exocytosis of non-photobleached receptors supplied either by de novo synthesis or recycling (see Fig. 1). The fraction of immobile and mobile protein can be determined and the mobility and kinetics of the diffusible fraction can be interrogated under basal and stimulated conditions such as agonist application or neuronal activation stimuli such as NMDA or KCl application8,10. 
We describe photobleaching techniques designed to selectively visualize the recovery of fluorescence attributable to exocytosis. Briefly, an ROI is photobleached once as with standard FRAP protocols, followed, after a brief recovery, by repetitive bleaching of the flanking regions. This 'FRAP-FLIP' protocol, developed in our lab, has been used to characterize AMPA receptor trafficking at dendritic spines10, and is applicable to a wide range of trafficking studies to evaluate the intracellular trafficking and exocytosis.

This report describes the use of live cell imaging and photobleach techniques to determine the surface expression, transport pathways and trafficking kinetics of exogenously expressed, pH-sensitive GFP-tagged proteins at the plasma membrane of neurons.

Membrane proteins such as receptors and ion channels undergo active trafficking in neurons, which are highly polarised and morphologically complex. This ...

Simultaneous Electroencephalography, Real-time Measurement of Lactate Concentration and Optogenetic Manipulation of Neuronal Activity in the Rodent Cerebral Cortex

Although the brain represents less than 5% of the body by mass, it utilizes approximately one quarter of the glucose used by the body at rest1. The function of non rapid eye movement sleep (NREMS), the largest portion of sleep by time, is uncertain. However, one salient feature of NREMS is a significant reduction in the rate of cerebral glucose utilization relative to wakefulness2-4. This and other findings have led to the widely held belief that sleep serves a function related to cerebral metabolism. Yet, the mechanisms underlying the reduction in cerebral glucose metabolism during NREMS remain to be elucidated.
	One phenomenon associated with NREMS that might impact cerebral metabolic rate is the occurrence of slow waves, oscillations at frequencies less than 4 Hz, in the electroencephalogram5,6. These slow waves detected at the level of the skull or cerebral cortical surface reflect the oscillations of underlying neurons between a depolarized/up state and a hyperpolarized/down state7. During the down state, cells do not undergo action potentials for intervals of up to several hundred milliseconds. Restoration of ionic concentration gradients subsequent to action potentials represents a significant metabolic load on the cell8; absence of action potentials during down states associated with NREMS may contribute to reduced metabolism relative to wake.
	Two technical challenges had to be addressed in order for this hypothetical relationship to be tested. First, it was necessary to measure cerebral glycolytic metabolism with a temporal resolution reflective of the dynamics of the cerebral EEG (that is, over seconds rather than minutes). To do so, we measured the concentration of lactate, the product of aerobic glycolysis, and therefore a readout of the rate of glucose metabolism in the brains of mice. Lactate was measured using a lactate oxidase based real time sensor embedded in the frontal cortex. The sensing mechanism consists of a platinum-iridium electrode surrounded by a layer of lactate oxidase molecules. Metabolism of lactate by lactate oxidase produces hydrogen peroxide, which produces a current in the platinum-iridium electrode. So a ramping up of cerebral glycolysis provides an increase in the concentration of substrate for lactate oxidase, which then is reflected in increased current at the sensing electrode. It was additionally necessary to measure these variables while manipulating the excitability of the cerebral cortex, in order to isolate this variable from other facets of NREMS.
	We devised an experimental system for simultaneous measurement of neuronal activity via the elecetroencephalogram, measurement of glycolytic flux via a lactate biosensor, and manipulation of cerebral cortical neuronal activity via optogenetic activation of pyramidal neurons. We have utilized this system to document the relationship between sleep-related electroencephalographic waveforms and the moment-to-moment dynamics of lactate concentration in the cerebral cortex. The protocol may be useful for any individual interested in studying, in freely behaving rodents, the relationship between neuronal activity measured at the electroencephalographic level and cellular energetics within the brain.

A procedure is described for manipulating the activity of cerebral cortical pyramidal neurons optogenetically while the electroencephalogram, electromyogram, and cerebral lactate concentration are monitored. Experimental recordings are performed on cable-tethered mice while they undergo spontaneous sleep/wake cycles. Optogenetic equipment is assembled in our laboratory; recording equipment is commercially available.

Although the brain represents less than 5% of the body  by mass, it utilizes approximately one quarter of the glucose used by the body  at rest1. The ...

Fast Micro-iontophoresis of Glutamate and GABA: A Useful Tool to Investigate Synaptic Integration

One of the fundamental interests in neuroscience is to understand the integration of excitatory and inhibitory inputs along the very complex structure of the dendritic tree, which eventually leads to neuronal output of action potentials at the axon. The influence of diverse spatial and temporal parameters of specific synaptic input on neuronal output is currently under investigation, e.g. the distance-dependent attenuation of dendritic inputs, the location-dependent interaction of spatially segregated inputs, the influence of GABAergig inhibition on excitatory integration, linear and non-linear integration modes, and many more.
With fast micro-iontophoresis of glutamate and GABA it is possible to precisely investigate the spatial and temporal integration of glutamatergic excitation and GABAergic inhibition. Critical technical requirements are either a triggered fluorescent lamp, light-emitting diode (LED), or a two-photon scanning microscope to visualize dendritic branches without introducing significant photo-damage of the tissue. Furthermore, it is very important to have a micro-iontophoresis amplifier that allows for fast capacitance compensation of high resistance pipettes. Another crucial point is that no transmitter is involuntarily released by the pipette during the experiment.
Once established, this technique will give reliable and reproducible signals with a high neurotransmitter and location specificity. Compared to glutamate and GABA uncaging, fast iontophoresis allows using both transmitters at the same time but at very distant locations without limitation to the field of view. There are also advantages compared to focal electrical stimulation of axons: with micro-iontophoresis the location of the input site is definitely known and it is sure that only the neurotransmitter of interest is released. However it has to be considered that with micro-iontophoresis only the postsynapse is activated and presynaptic aspects of neurotransmitter release are not resolved. In this article we demonstrate how to set up micro-iontophoresis in brain slice experiments.

In this article we introduce fast micro-iontophoresis of neurotransmitters as a technique to investigate integration of postsynaptic signals with high spatial and temporal precision.

One of the fundamental interests in neuroscience is to understand the integration of excitatory and inhibitory inputs along the very complex structure of ...

Deriving the Time Course of Glutamate Clearance with a Deconvolution Analysis of Astrocytic Transporter Currents

The highest density of glutamate transporters in the brain is found in astrocytes. Glutamate transporters couple the movement of glutamate across the membrane with the co-transport of 3 Na+ and 1 H+ and the counter-transport of 1 K+. The stoichiometric current generated by the transport process can be monitored with whole-cell patch-clamp recordings from astrocytes. The time course of the recorded current is shaped by the time course of the glutamate concentration profile to which astrocytes are exposed, the kinetics of glutamate transporters, and the passive electrotonic properties of astrocytic membranes. Here we describe the experimental and analytical methods that can be used to record glutamate transporter currents in astrocytes and isolate the time course of glutamate clearance from all other factors that shape the waveform of astrocytic transporter currents. The methods described here can be used to estimate the lifetime of flash-uncaged and synaptically-released glutamate at astrocytic membranes in any region of the central nervous system during health and disease.

We describe an analytical method to estimate the lifetime of glutamate at astrocytic membranes from electrophysiological recordings of glutamate transporter currents in astrocytes.

The highest density of glutamate transporters in the brain is found in astrocytes. Glutamate transporters couple the movement of glutamate across the ...

Genetic Manipulation of Cerebellar Granule Neurons In Vitro and In Vivo to Study Neuronal Morphology and Migration

Developmental events in the brain including neuronal morphogenesis and migration are highly orchestrated processes. In vitro and in vivo analyses allow for an in-depth characterization to identify pathways involved in these events. Cerebellar granule neurons (CGNs) that are derived from the developing cerebellum are an ideal model system that allows for morphological analyses. Here, we describe a method of how to genetically manipulate CGNs and how to study axono- and dendritogenesis of individual neurons. With this method the effects of RNA interference, overexpression or small molecules can be compared to control neurons. In addition, the rodent cerebellar cortex is an easily accessible in vivo system owing to its predominant postnatal development. We also present an in vivo electroporation technique to genetically manipulate the developing cerebella and describe subsequent cerebellar analyses to assess neuronal morphology and migration.

Genetic Manipulation of Cerebellar Granule Neurons In Vitro and In Vivo to Study Neuronal Morphology and Migration

Neuronal morphogenesis and migration are crucial events underlying proper brain development. Here, we describe methods to genetically manipulate cultured cerebellar granule neurons and the developing cerebellum for the assessment of morphology and migratory characteristics of neurons.

Developmental events in the brain including neuronal morphogenesis and migration are highly orchestrated processes. In vitro and in vivo analyses allow ...

An In Vitro Adult Mouse Muscle-nerve Preparation for Studying the Firing Properties of Muscle Afferents

Muscle sensory neurons innervating muscle spindles and Golgi tendon organs encode length and force changes essential to proprioception. Additional afferent fibers monitor other characteristics of the muscle environment, including metabolite buildup, temperature, and nociceptive stimuli. Overall, abnormal activation of sensory neurons can lead to movement disorders or chronic pain syndromes. We describe the isolation of the extensor digitorum longus (EDL) muscle and nerve for in vitro study of stretch-evoked afferent responses in the adult mouse. Sensory activity is recorded from the nerve with a suction electrode and individual afferents can be analyzed using spike sorting software. In vitro preparations allow for well controlled studies on sensory afferents without the potential confounds of anesthesia or altered muscle perfusion. Here we describe a protocol to identify and test the response of muscle spindle afferents to stretch. Importantly, this preparation also supports the study of other subtypes of muscle afferents, response properties following drug application and the incorporation of powerful genetic approaches and disease models in mice.

An In Vitro Adult Mouse Muscle-nerve Preparation for Studying the Firing Properties of Muscle Afferents

Muscle sensory neurons are involved in proprioceptor signaling and also report on metabolic state and injury related events. We describe an adult mouse in vitro muscle-nerve preparation for studies on stretch-activated muscle afferents.

Muscle sensory neurons innervating muscle spindles and Golgi tendon organs encode length and force changes essential to proprioception. Additional ...

Pentylenetetrazole-Induced Kindling Mouse Model

Pentylenetetrazole (PTZ) is a GABA-A receptor antagonist. An intraperitoneal injection of PTZ into an animal induces an acute, severe seizure at a high dose, whereas sequential injections of a subconvulsive dose have been used for the development of chemical kindling, an epilepsy model. A single low-dose injection of PTZ induces a mild seizure without convulsion. However, repetitive low-dose injections of PTZ decrease the threshold to evoke a convulsive seizure. Finally, continuous low-dose administration of PTZ induces a severe tonic-clonic seizure. This method is simple and widely applicable to investigate the pathophysiology of epilepsy, which is defined as a chronic disease that involves repetitive seizures. This chemical kindling protocol causes repetitive seizures in animals. With this method, vulnerability to PTZ-mediated seizures or the degree of aggravation of epileptic seizures was estimated. These advantages have led to the use of this method for screening anti-epileptic drugs and epilepsy-related genes. In addition, this method has been used to investigate neuronal damage after epileptic seizures because the histological changes observed in the brains of epileptic patients also appear in the brains of chemical-kindled animals. Thus, this protocol is useful for conveniently producing animal models of epilepsy.

This protocol describes a method of chemical kindling with pentylenetetrazole and provides a mouse model of epilepsy. This protocol can also be used to investigate vulnerability to seizure induction and pathogenesis after epileptic seizures in mice.

Pentylenetetrazole (PTZ) is a GABA-A receptor antagonist. An intraperitoneal injection of PTZ into an animal induces an acute, severe seizure at a high ...

Non-invasive Strategies for Chronic Manipulation of DREADD-controlled Neuronal Activity

Chemogenetic strategies have emerged as reliable tools for remote control of neuronal activity. Among these, designer receptors exclusively activated by designer drugs (DREADDs) have become the most popular chemogenetic approach used in modern neuroscience. Most studies deliver the ligand clozapine-N-oxide (CNO) using a single intraperitoneal injection, which is suitable for the acute activation/inhibition of the targeted neuronal population. There are, however, only a few examples of strategies for chronic modulation of DREADD-controlled neurons, the majority of which rely on the use of delivery systems that require surgical intervention. Here, we expand on two non-invasive strategies for delivering the ligand CNO to chronically manipulate neural population in mice. CNO was administered either by using repetitive (daily) eye-drops, or chronically through the animal&#39;s drinking water. These non-invasive paradigms result in robust activation of the designer receptors that persisted throughout the CNO treatments. The methods described here offer alternatives for the chronic DREADD-mediated control of neuronal activity and may be useful for experiments designed to evaluate behavior in freely moving animals, focusing on less-invasive CNO delivery methods.

Here we describe two non-invasive methods to chronically control neuronal activity using chemogenetics in mice. Eye-drops were used to deliver clozapine-N-oxide (CNO) daily. We also describe two methods for prolonged administration of CNO in drinking water. These strategies for chronic neuronal control require minimal intervention reducing animals&#8217; stress.

Chemogenetic strategies have emerged as reliable tools for remote control of neuronal activity. Among these, designer receptors exclusively activated by ...