We thank Hans Reiner Polder, Martin Fuhrmann and Walker Jackson for carefully reading the manuscript. The authors received funding that was provided by the ministry of research MIWF of the state Northrhine-Westfalia (S.R.), the BMBF-Projektr&auml;ger DLR US-German collaboration in computational neuroscience (CRCNS; S.R.), Centers of Excellence in Neurodegenerative Diseases (COEN; S.R.), and the University of Bonn intramural funding program (BONFOR; S.R.).

1. System Requirements
<ol>
	<li>Microscope system: Good visualization of the dendrite is crucial. If available, use a two-photon laser scanning microscope system. In our experiments we used a TRIM Scope II, LaVision Biotec, Bielefeld, Germany or an Ultima IV system, Prairie Technologies, Middleton, Wisconsin equipped with a Ti:Sapphire ultrafast-pulsed laser (Chameleon Ultra II, Coherent) and a high NA objective (60X, 0.9 NA, Olympus) to visualize the dendrites which we had filled with a fluorescent dye via the patch ...

<ol>
	<li>Megias, M., Emri, Z., Freund, T. F., Gulyas, A. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Total+number+and+distribution+of+inhibitory+and+excitatory+synapses+on+hippocampal+CA1+pyramidal+cells.">Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells.</a> Neuroscience. 102, 527-540 (2001).</li><li>Gasparini, S., Migliore, M., Magee, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=On+the+initiation+and+propagation+of+dendritic+spikes+in+CA1+pyramidal+neurons.">On the initiation and propagation of dendritic spikes in CA1 pyramidal neurons.</a> J. Neurosci. 24, 11046-11056 (2004).</li><li>Losonczy, A., Magee, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Integrative+properties+of+radial+oblique+dendrites+in+hippocampal+CA1+pyramidal+neurons.">Integrative properties of radial oblique dendrites in hippocampal CA1 pyramidal neurons.</a> Neuron. 50, 291-307 (2006).</li><li>Remy, S., Csicsvari, J., Beck, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Activity-dependent+control+of+neuronal+output+by+local+and+global+dendritic+spike+attenuation.">Activity-dependent control of neuronal output by local and global dendritic spike attenuation.</a> Neuron. 61, 906-916 (2009).</li><li>Stuart, G., Schiller, J., Sakmann, B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Action+potential+initiation+and+propagation+in+rat+neocortical+pyramidal+neurons.">Action potential initiation and propagation in rat neocortical pyramidal neurons.</a> J. Physiol. 505 (Pt. 3), 617-632 (1997).</li><li>Magee, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Dendritic+integration+of+excitatory+synaptic+input.">Dendritic integration of excitatory synaptic input.</a> Nat. Rev. Neurosci. 1, 181-190 (2000).</li><li>Amaral, D. G., Witter, M. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+three-dimensional+organization+of+the+hippocampal+formation:+a+review+of+anatomical+data.">The three-dimensional organization of the hippocampal formation: a review of anatomical data.</a> Neuroscience. 31, 571-591 (1989).</li><li>Miles, R., Toth, K., Gulyas, A. I., Hajos, N., Freund, T. F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Differences+between+somatic+and+dendritic+inhibition+in+the+hippocampus.">Differences between somatic and dendritic inhibition in the hippocampus.</a> Neuron. 16, 815-823 (1996).</li><li>Muller, C., Beck, H., Coulter, D., Remy, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Inhibitory+control+of+linear+and+supralinear+dendritic+excitation+in+CA1+pyramidal+neurons.">Inhibitory control of linear and supralinear dendritic excitation in CA1 pyramidal neurons.</a> Neuron. 75, 851-864 (2012).</li><li>Murnick, J. G., Dube, G., Krupa, B., Liu, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=High-resolution+iontophoresis+for+single-synapse+stimulation.">High-resolution iontophoresis for single-synapse stimulation.</a> J. Neurosci. Methods. 116, 65-75 (2002).</li><li>Liu, G., Choi, S., Tsien, R. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Variability+of+neurotransmitter+concentration+and+nonsaturation+of+postsynaptic+AMPA+receptors+at+synapses+in+hippocampal+cultures+and+slices.">Variability of neurotransmitter concentration and nonsaturation of postsynaptic AMPA receptors at synapses in hippocampal cultures and slices.</a> Neuron. 22, 395-409 (1999).</li><li>Renger, J. J., Egles, C., Liu, G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+developmental+switch+in+neurotransmitter+flux+enhances+synaptic+efficacy+by+affecting+AMPA+receptor+activation.">A developmental switch in neurotransmitter flux enhances synaptic efficacy by affecting AMPA receptor activation.</a> Neuron. 29, (2001).</li><li>Heine, M., 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=Surface+mobility+of+postsynaptic+AMPARs+tunes+synaptic+transmission.">Surface mobility of postsynaptic AMPARs tunes synaptic transmission.</a> Science. 320, 201-205 (2008).</li><li>Somogyi, P., Klausberger, T. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Defined+types+of+cortical+interneurone+structure+space+and+spike+timing+in+the+hippocampus.">Defined types of cortical interneurone structure space and spike timing in the hippocampus.</a> J. Physiol. 562, 9-26 (2005).</li><li>Pugh, J. R., Jahr, C. E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Axonal+GABAA+receptors+increase+cerebellar+granule+cell+excitability+and+synaptic+activity.">Axonal GABAA receptors increase cerebellar granule cell excitability and synaptic activity.</a> J. Neurosci. 31, 565-574 (2011).</li><li>Mozrzymas, J. W., Zarnowska, E. D., Pytel, M., Mercik, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Modulation+of+GABA(A)+receptors+by+hydrogen+ions+reveals+synaptic+GABA+transient+and+a+crucial+role+of+the+desensitization+process.">Modulation of GABA(A) receptors by hydrogen ions reveals synaptic GABA transient and a crucial role of the desensitization process.</a> J. Neurosci. 23, 7981-7992 (2003).</li><li>Pasternack, M., Smirnov, S., Kaila, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Proton+modulation+of+functionally+distinct+GABAA+receptors+in+acutely+isolated+pyramidal+neurons+of+rat+hippocampus.">Proton modulation of functionally distinct GABAA receptors in acutely isolated pyramidal neurons of rat hippocampus.</a> Neuropharmacology. 35, 1279-1288 (1996).</li><li>. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Single-Channel Recording. , (2009).</li><li>Davie, J. T., 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=Dendritic+patch-clamp+recording.">Dendritic patch-clamp recording.</a> Nat. Protoc. 1, 1235-1247 (2006).</li><li>Major, G., Polsky, A., Denk, W., Schiller, J., Tank, D. W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spatiotemporally+graded+NMDA+spike/plateau+potentials+in+basal+dendrites+of+neocortical+pyramidal+neurons.">Spatiotemporally graded NMDA spike/plateau potentials in basal dendrites of neocortical pyramidal neurons.</a> J. Neurophysiol. 99, 2584-2601 (2008).</li><li>Rose, G. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Combining+pharmacology+and+whole-cell+patch+recording+from+CNS+neurons,+in+vivo.">Combining pharmacology and whole-cell patch recording from CNS neurons, in vivo.</a> J. Neurosci Methods. , (2012).</li><li>Behrends, J. C., Lambert, J. C., Jensen, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Repetitive+activation+of+postsynaptic+GABA(A+)receptors+by+rapid,+focal+agonist+application+onto+intact+rat+striatal+neurones+in+vitro.">Repetitive activation of postsynaptic GABA(A )receptors by rapid, focal agonist application onto intact rat striatal neurones in vitro.</a> Pflugers Arch. 443, 707-712 (2002).</li><li>Hahnel, C., Kettenmann, H., Grantyn, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Practical Electrophysiological methods. , (1992).</li><li>Wetzel, C. H., 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=Specificity+and+sensitivity+of+a+human+olfactory+receptor+functionally+expressed+in+human+embryonic+kidney+293+cells+and+Xenopus+Laevis+oocytes.">Specificity and sensitivity of a human olfactory receptor functionally expressed in human embryonic kidney 293 cells and Xenopus Laevis oocytes.</a> J. Neurosci. 19, 7426-7433 (1999).</li><li>Cash, S., Yuste, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Linear+summation+of+excitatory+inputs+by+CA1+pyramidal+neurons.">Linear summation of excitatory inputs by CA1 pyramidal neurons.</a> Neuron. 22, 383-394 (1999).</li><li>Eccles, J. C., Jaeger, J. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+relationship+between+the+mode+of+operation+and+the+dimensions+of+the+junctional+regions+at+synapses+and+motor+end-organs.">The relationship between the mode of operation and the dimensions of the junctional regions at synapses and motor end-organs.</a> Proc. R. Soc. Lond. B. Biol. Sci. 148, 38-56 (1958).</li><li>Kwon, H. B., Sabatini, B. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Glutamate+induces+de+novo+growth+of+functional+spines+in+developing+cortex.">Glutamate induces de novo growth of functional spines in developing cortex.</a> Nature. 474, 100-104 (2011).</li><li>Fino, 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=RuBi-Glutamate:+Two-Photon+and+Visible-Light+Photoactivation+of+Neurons+and+Dendritic+spines.">RuBi-Glutamate: Two-Photon and Visible-Light Photoactivation of Neurons and Dendritic spines.</a> Front Neural Circuits. 3, 2 (2009).</li></ol>

Here we explain how to apply fast micro-iontophoresis of neurotransmitters to investigate synaptic integration on dendrites. This technique has been successfully used to investigate glutamatergic and GABAergic synaptic transmission in different brain regions in vitro and in vivo9,20-22. Micro-iontophoresis has been used for more than 60 years, but in early years it was mostly used to either locally apply neurotransmitters and drugs on slow or intermediate timescales23 or for microi...

Neurons in the central nervous system receive a variety of synaptic inputs on their thin and ramified dendritic processes1. There, the majority of the excitatory dendritic inputs are mediated by glutamatergic synapses. These synapses can be activated in a spatially distributed way, resulting in postsynaptic linear integration of excitatory postsynaptic potentials (EPSPs). If the synapses are activated simultaneously and in spatial proximity on the dendrite, these excitatory inputs can be integrated supra-linearly and generate dendritic spikes2-5.
Furthermore, the integration of excitatory inputs depends on the location of the input on the dendritic tree. Signals that arrive at the distal tuft region are much more attenuated than proximal inputs due to cable filtering6. In the hippocampus, distant inputs to the apical tuft dendrites are generated by a different brain region than those on proximal dendrites7. An exciting question is therefore, how synaptic input is processed by different dendritic compartments and if dendritic integration regulates the influence of these layered inputs on neuronal firing in different ways.
Not only the functional properties, morphological features of the dendrite, the location and clustering of the inputs are affecting the dendritic integration of excitatory inputs, also the additional inhibitory inputs from GABAergic terminals crucially determine the efficacy of the glutamatergic synapses8,9. These different aspects of synaptic integration can be ideally investigated using neurotransmitter micro-iontophoresis, which allows spatially defined application of different neurotransmitters to dendritic domains. We demonstrate here how to successfully establish micro-iontophoresis of glutamate and GABA to investigate signal integration in neurons.
For this application, fine-tipped high resistance pipettes filled with concentrated neurotransmitter solutions are used. These pipettes are positioned close to the outer membrane of the cell, where the neurotransmitter receptors are located. A good visualization of the dendritic branches is required. This is best achieved using fluorescent dyes, which are introduced via the patch pipette. Then a very short (&lt;1 msec) current pulse (in the range of 10 - 100 nA) is used to eject the charged neurotransmitter molecules. With these short pulses and effective capacitance compensation, postsynaptic potentials or currents can be evoked with high temporal and spatial precision, which means the location of the excitatory input is precisely known. Glutamate micro-iontophoresis can activate synapses in a defined radius, which is smaller than 6 &mu;m as shown here (Figure 19), but it is also possible to reach single synapse resolution10-12.
Heine, M., et al showed that the spatial resolution of fast micro-iontophoresis can even be adjusted to spot sizes below 0.5 &mu;m, which is smaller than spot sizes regularly achieved with two-photon uncaging of glutamate13. With fast micro-iontophoresis it is easily possible to use two or more iontophoretic pipettes and place them at different, even distant spots on the dendritic tree. In this way, integration of excitatory events, including those from different pathways, can be investigated. It is also possible to use a glutamate and a GABA filled iontophoretic pipette at the same time. In this way the effect of GABAergic inhibition at different locations relative to the excitatory input (on-path, off-path inhibition) can be studied. Also, the impact of inhibition by interneurons targeting specific neuronal domains, like distal dendrites, soma or axons14, can be investigated using GABA iontophoresis. In cultured neurons, fast micro-iontophoresis offers the opportunity to investigate single synapse distribution and the elementary aspects of synaptic communication in neurons in much more detail10,11.
In this article we demonstrate in detail how to establish glutamate and GABA iontophoresis for the use in acute brain slices, which allows investigating synaptic integration of excitatory and inhibitory inputs in dependence of input location, input strengths, and timing, alone or in interplay. We will point out the advantages and limitations of this technique and how to troubleshoot successfully.

<table border="1">
	<tbody>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>Material</td>
		</tr>
		<tr>
			<td>Two-photon laser scanning microscope (TRIM Scope II), and Ultima IV, Prairie Technologies, Middleton, Wisconsin)</td>
			<td>LaVision Biotec, Bielefeld, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Two-photon laser scanning microscope Ultima IV</td>
			<td>Prairie Technologies, Middleton, Wisconsin, USA</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Ti:Sapphire ultrafast-pulsed laser</td>
			<td>Chameleon Ultra II, Coherent</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>60X Objective, NA 0.9</td>
			<td>Olympus</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Zeiss Axioskop 2 FS upright microscope</td>
			<td>TILLPhotonics, Gr&auml;felfing, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Monochromator</td>
			<td>TILLPhotonics, Gr&auml;felfing, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Micro-iontophoresis system MVCS-02</td>
			<td>NPI Electronics, Tamm, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Sutter puller P-97</td>
			<td>Sutter Instrument Company, Novato, CA</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glass filaments (150 GB F 8P)</td>
			<td>Science Products, Hofheim, Germany</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>&nbsp;</td>
			<td>Reagent</td>
		</tr>
		<tr>
			<td>Alexa Fluor 488 hydrazide</td>
			<td>Molecular Probes life technologies</td>
			<td>A-10436</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Alexa Fluor 594</td>
			<td>Molecular Probes life technologies</td>
			<td>A-10438</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Sigma Aldrich</td>
			<td>S7653</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma Aldrich</td>
			<td>P9333</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td>Sigma Aldrich</td>
			<td>S8282</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>Sigma Aldrich</td>
			<td>S6297</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma Aldrich</td>
			<td>S7903</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Sigma Aldrich</td>
			<td>C5080</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>MgCl2</td>
			<td>Sigma Aldrich</td>
			<td>M2670</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glucose</td>
			<td>Sigma Aldrich</td>
			<td>G7528</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>K-Gluconate</td>
			<td>Sigma Aldrich</td>
			<td>G4500</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>HEPES-acid</td>
			<td>Sigma Aldrich</td>
			<td>H4034</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Phosphocreatin</td>
			<td>Sigma Aldrich</td>
			<td>P7936</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Sigma Aldrich</td>
			<td>E3889</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>Glutamic acid</td>
			<td>Sigma Aldrich</td>
			<td>G8415</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>GABA</td>
			<td>Sigma Aldrich</td>
			<td>A5835</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>NaOH</td>
			<td>Merck</td>
			<td>1.09137.1000</td>
			<td>&nbsp;</td>
		</tr>
		<tr>
			<td>HCl</td>
			<td>Merck</td>
			<td>1.09108.1000</td>
			<td>&nbsp;</td>
		</tr>
	</tbody>
</table>

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.

A simple approach to determine the spatial spread of iontophoresis is to retract the iontophoretic pipette stepwise from the dendrite, while keeping the ejected glutamate constant. We found that the spatial extent of a micro-iontophoretic stimulation had a diameter of approximately 12 &mu;m (Figure 1 showing radius). How deep in the tissue the iontophoresis can be used depends on the rigidity of the pipette. However, the iontophoretic pipettes needed for experiments in slices (Figure 2),...

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

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 ...

fast-micro-iontophoresis-glutamate-gaba-useful-tool-to-investigate

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Neuroscience

19.3K vues.  University of Bonn. Dans cet article, nous introduisons rapide micro-iontophorèse de neurotransmetteurs comme une technique pour étudier l'intégration des signaux post-synaptiques avec une grande précision spatiale et temporelle.

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

Cut-loading: A Useful Tool for Examining the Extent of Gap Junction Tracer Coupling Between Retinal Neurons

In addition to chemical synaptic transmission, neurons that are connected by gap junctions can also communicate rapidly via electrical synaptic transmission. Increasing evidence indicates that gap junctions not only permit electrical current flow and synchronous activity between interconnected or coupled cells, but that the strength or effectiveness of electrical communication between coupled cells can be modulated to a great extent1,2. In addition, the large internal diameter (~1.2 nm) of many gap junction channels permits not only electric current flow, but also the diffusion of intracellular signaling molecules and small metabolites between interconnected cells, so that gap junctions may also mediate metabolic and chemical communication. The strength of gap junctional communication between neurons and its modulation by neurotransmitters and other factors can be studied by simultaneously electrically recording from coupled cells and by determining the extent of diffusion of tracer molecules, which are gap junction permeable, but not membrane permeable, following iontophoretic injection into single cells. However, these procedures can be extremely difficult to perform on neurons with small somata in intact neural tissue.
Numerous studies on electrical synapses and the modulation of electrical communication have been conducted in the vertebrate retina, since each of the five retinal neuron types is electrically connected by gap junctions3,4. Increasing evidence has shown that the circadian (24-hour) clock in the retina and changes in light stimulation regulate gap junction coupling3-8. For example, recent work has demonstrated that the retinal circadian clock decreases gap junction coupling between rod and cone photoreceptor cells during the day by increasing dopamine D2 receptor activation, and dramatically increases rod-cone coupling at night by reducing D2 receptor activation7,8. However, not only are these studies extremely difficult to perform on neurons with small somata in intact neural retinal tissue, but it can be difficult to adequately control the illumination conditions during the electrophysiological study of single retinal neurons to avoid light-induced changes in gap junction conductance.
Here, we present a straightforward method of determining the extent of gap junction tracer coupling between retinal neurons under different illumination conditions and at different times of the day and night. This cut-loading technique is a modification of scrape loading9-12, which is based on dye loading and diffusion through open gap junction channels. Scrape loading works well in cultured cells, but not in thick slices such as intact retinas. The cut-loading technique has been used to study photoreceptor coupling in intact fish and mammalian retinas7, 8,13, and can be used to study coupling between other retinal neurons, as described here.

An easy and convenient method to determine the extent of gap junction tracer coupling between retinal neurons is described. This technique enables one to investigate the function of the electrical synapses between neurons in the intact retina under different illumination conditions and at different times of the day and night.

In addition to chemical synaptic transmission, neurons that are connected by gap junctions can also communicate rapidly via electrical synaptic ...

MPI CyberMotion Simulator: Implementation of a Novel Motion Simulator to Investigate Multisensory Path Integration in Three Dimensions

Path integration is a process in which self-motion is integrated over time to obtain an estimate of one's current position relative to a starting point 1. Humans can do path integration based exclusively on visual 2-3, auditory 4, or inertial cues 5. However, with multiple cues present, inertial cues - particularly kinaesthetic - seem to dominate 6-7. In the absence of vision, humans tend to overestimate short distances (&lt;5 m) and turning angles (&lt;30&deg;), but underestimate longer ones 5. Movement through physical space therefore does not seem to be accurately represented by the brain.
Extensive work has been done on evaluating path integration in the horizontal plane, but little is known about vertical movement (see 3 for virtual movement from vision alone). One reason for this is that traditional motion simulators have a small range of motion restricted mainly to the horizontal plane. Here we take advantage of a motion simulator 8-9 with a large range of motion to assess whether path integration is similar between horizontal and vertical planes. The relative contributions of inertial and visual cues for path navigation were also assessed.
16 observers sat upright in a seat mounted to the flange of a modified KUKA anthropomorphic robot arm. Sensory information was manipulated by providing visual (optic flow, limited lifetime star field), vestibular-kinaesthetic (passive self motion with eyes closed), or visual and vestibular-kinaesthetic motion cues. Movement trajectories in the horizontal, sagittal and frontal planes consisted of two segment lengths (1st: 0.4 m, 2nd: 1 m; &plusmn;0.24 m/s2 peak acceleration). The angle of the two segments was either 45&deg; or 90&deg;. Observers pointed back to their origin by moving an arrow that was superimposed on an avatar presented on the screen.
Observers were more likely to underestimate angle size for movement in the horizontal plane compared to the vertical planes. In the frontal plane observers were more likely to overestimate angle size while there was no such bias in the sagittal plane. Finally, observers responded slower when answering based on vestibular-kinaesthetic information alone. Human path integration based on vestibular-kinaesthetic information alone thus takes longer than when visual information is present. That pointing is consistent with underestimating and overestimating the angle one has moved through in the horizontal and vertical planes respectively, suggests that the neural representation of self-motion through space is non-symmetrical which may relate to the fact that humans experience movement mostly within the horizontal plane.

An efficient way to gain insight into how humans navigate themselves in three dimensions is described. The method takes advantage of a motion simulator capable of moving observers in ways unattainable by traditional simulators. Results confirm that movement in the horizontal plane is underestimated, while vertical movement is overestimated.

Path integration is a process in which self-motion is integrated over time to obtain an estimate of one's current position relative to a starting point 1. ...

Automated Quantification of Synaptic Fluorescence in C. elegans

Synapse strength refers to the amplitude of postsynaptic responses to presynaptic neurotransmitter release events, and has a major impact on overall neural circuit function. Synapse strength critically depends on the abundance of neurotransmitter receptors clustered at synaptic sites on the postsynaptic membrane. Receptor levels are established developmentally, and can be altered by receptor trafficking between surface-localized, subsynaptic, and intracellular pools, representing important mechanisms of synaptic plasticity and neuromodulation. Rigorous methods to quantify synaptically-localized neurotransmitter receptor abundance are essential to study synaptic development and plasticity. Fluorescence microscopy is an optimal approach because it preserves spatial information, distinguishing synaptic from non-synaptic pools, and discriminating among receptor populations localized to different types of synapses. The genetic model organism Caenorhabditis elegans is particularly well suited for these studies due to the small size and relative simplicity of its nervous system, its transparency, and the availability of powerful genetic techniques, allowing examination of native synapses in intact animals.
Here we present a method for quantifying fluorescently-labeled synaptic neurotransmitter receptors in C. elegans. Its key feature is the automated identification and analysis of individual synapses in three dimensions in multi-plane confocal microscope output files, tabulating position, volume, fluorescence intensity, and total fluorescence for each synapse. This approach has two principal advantages over manual analysis of z-plane projections of confocal data. First, because every plane of the confocal data set is included, no data are lost through z-plane projection, typically based on pixel intensity averages or maxima. Second, identification of synapses is automated, but can be inspected by the experimenter as the data analysis proceeds, allowing fast and accurate extraction of data from large numbers of synapses. Hundreds to thousands of synapses per sample can easily be obtained, producing large data sets to maximize statistical power. Considerations for preparing C. elegans for analysis, and performing confocal imaging to minimize variability between animals within treatment groups are also discussed. Although developed to analyze C. elegans postsynaptic receptors, this method is generally useful for any type of synaptically-localized protein, or indeed, any fluorescence signal that is localized to discrete clusters, puncta, or organelles.
The procedure is performed in three steps: 1) preparation of samples, 2) confocal imaging, and 3) image analysis. Steps 1 and 2 are specific to C. elegans, while step 3 is generally applicable to any punctate fluorescence signal in confocal micrographs.

Automated Quantification of Synaptic Fluorescence in C. elegans

The abundance of neurotransmitter receptors clustered at synapses strongly influences synaptic strength. This method quantifies fluorescently-labeled neurotransmitter receptors in three dimensions with single-synapse resolution in C. elegans, allowing hundreds of synapses to be rapidly characterized within a single sample without distortions introduced by z-plane projection.

Synapse strength refers to the amplitude of postsynaptic responses to presynaptic neurotransmitter release events, and has a major impact on overall ...

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 ...

Using an &#945;-Bungarotoxin Binding Site Tag to Study GABA A Receptor Membrane Localization and Trafficking

It is increasingly evident that neurotransmitter receptors, including ionotropic GABA A receptors (GABAAR), exhibit highly dynamic trafficking and cell surface mobility1-7. To study receptor cell surface localization and endocytosis, the technique described here combines the use of fluorescent &#945;-bungarotoxin with cells expressing constructs containing an &#945;-bungarotoxin (Bgt) binding site (BBS). The BBS (WRYYESSLEPYPD) is based on the &#945; subunit of the muscle nicotinic acetylcholine receptor, which binds Bgt with high affinity8,9. Incorporation of the BBS site allows surface localization and measurements of receptor insertion or removal with application of exogenous fluorescent Bgt, as previously described in the tracking of GABAA and metabotropic GABAB receptors2,10. In addition to the BBS site, we inserted a pH-sensitive GFP (pHGFP11) between amino acids 4 and 5 of the mature GABAAR subunit by standard molecular biology and PCR cloning strategies (see Figure 1)12. The BBS is 3&#39; of the pH-sensitive GFP reporter, separated by a 13-amino acid alanine/proline linker. For trafficking studies described in this publication that are based on fixed samples, the pHGFP serves as a reporter of total tagged GABAAR subunit protein levels, allowing normalization of the Bgt labeled receptor population to total receptor population. This minimizes cell to cell Bgt staining signal variability resulting from higher or lower baseline expression of the tagged GABAAR subunits. Furthermore the pHGFP tag enables easy identification of construct expressing cells for live or fixed imaging experiments.

Using an &amp;#945;-Bungarotoxin Binding Site Tag to Study GABA A Receptor Membrane Localization and Trafficking

Here we demonstrate the use of fluorescent Alexa dye coupled to &#945;-bungarotoxin to measure GABA A receptor surface localization and endocytosis in hippocampal neurons. Through the use of constructs bearing a short extracellular tag that binds &#945;-bungarotoxin, analysis of plasma membrane protein endocytic trafficking can be achieved.

It is increasingly evident that neurotransmitter receptors, including ionotropic GABA A receptors (GABAAR), exhibit highly dynamic trafficking and cell ...

Induction of an Isoelectric Brain State to Investigate the Impact of Endogenous Synaptic Activity on Neuronal Excitability In Vivo

The way neurons process information depends both on their intrinsic membrane properties and on the dynamics of the afferent synaptic network. In particular, endogenously-generated network activity, which strongly varies as a function of the state of vigilance, significantly modulates neuronal computation. To investigate how different spontaneous cerebral dynamics impact single neurons&#39; integrative properties, we developed a new experimental strategy in the rat consisting in suppressing in vivo all cerebral activity by means of a systemic injection of a high dose of sodium pentobarbital. Cortical activities, continuously monitored by combined electrocorticogram (ECoG) and intracellular recordings are progressively slowed down, leading to a steady isoelectric profile. This extreme brain state, putting the rat into a deep comatose, was carefully monitored by measuring the physiological constants of the animal throughout the experiments. Intracellular recordings allowed us to characterize and compare the integrative properties of the same neuron embedded into physiologically relevant cortical dynamics, such as those encountered in the sleep-wake cycle, and when the brain was fully silent.

Induction of an Isoelectric Brain State to Investigate the Impact of Endogenous Synaptic Activity on Neuronal Excitability In Vivo

This procedure performs long-lasting in vivo intracellular recordings from single neurons during physiologically relevant cerebral states and after complete abolition of ongoing electrical activities, resulting in an isoelectric brain state. The physiological constants of the animal are carefully monitored during the transition to the artificial comatose condition.

The way neurons process information depends both on their intrinsic membrane properties and on the dynamics of the afferent synaptic network. In ...

Using Enzyme-based Biosensors to Measure Tonic and Phasic Glutamate in Alzheimer's Mouse Models

Neurotransmitter disruption is often a key component of diseases of the central nervous system (CNS), playing a role in the pathology underlying Alzheimer's disease, Parkinson's disease, depression, and anxiety. Traditionally, microdialysis has been the most common (lauded) technique to examine neurotransmitter changes that occur in these disorders. But because microdialysis has the ability to measure slow 1-20 minute changes across large areas of tissue, it has the disadvantage of invasiveness, potentially destroying intrinsic connections within the brain and a slow sampling capability. A relatively newer technique, the microelectrode array (MEA), has numerous advantages for measuring specific neurotransmitter changes within discrete brain regions as they occur, making for a spatially and temporally precise approach. In addition, using MEAs is minimally invasive, allowing for measurement of neurotransmitter alterations in vivo. In our laboratory, we have been specifically interested in changes in the neurotransmitter, glutamate, related to Alzheimer's disease pathology. As such, the method described here has been used to assess potential hippocampal disruptions in glutamate in a transgenic mouse model of Alzheimer's disease. Briefly, the method used involves coating a multi-site microelectrode with an enzyme very selective for the neurotransmitter of interest and using self-referencing sites to subtract out background noise and interferents. After plating and calibration, the MEA can be constructed with a micropipette and lowered into the brain region of interest using a stereotaxic device. Here, the method described involves anesthetizing rTg(TauP301L)4510 mice and using a stereotaxic device to precisely target sub-regions (DG, CA1, and CA3) of the hippocampus.

Using Enzyme-based Biosensors to Measure Tonic and Phasic Glutamate in Alzheimer&#039;s Mouse Models

Here, we describe the setup, software navigation, and data analysis for a spatially and temporally precise method of measuring tonic and phasic extracellular glutamate changes in vivo using enzyme-linked microelectrode arrays (MEA).

Neurotransmitter disruption is often a key component of diseases of the central nervous system (CNS), playing a role in the pathology underlying ...

Culturing In Vivo-like Murine Astrocytes Using the Fast, Simple, and Inexpensive AWESAM Protocol

The AWESAM (a low-cost easy stellate astrocyte method) protocol entails a fast, simple, and inexpensive way to generate large quantities of in vivo-like mouse and rat astrocyte monocultures: Brain cells can be isolated from different brain regions, and after a week of cell culture, non-astrocytic cells are shaken off by placing the culture dishes on a shaker for 6 h in the incubator. The remaining astrocytes are then passaged into new plates with an astrocyte-specific medium (termed NB+H). NB+H contains low concentrations of heparin-binding EGF-like growth factor (HBEGF), which is used in place of serum in medium. After growing in NB+H, AWESAM astrocytes have a stellate morphology and feature fine processes. Moreover, these astrocytes have more in vivo-like gene expression than astrocytes generated by previously published methods. Ca2+ imaging, vesicle dynamics, and other events close to the membrane can thus be studied in the fine astrocytic processes in vitro, e.g., using live cell confocal or TIRF microscopy. Notably, AWESAM astrocytes also exhibit spontaneous Ca2+ signaling similar to astrocytes in vivo.

Culturing In Vivo-like Murine Astrocytes Using the Fast, Simple, and Inexpensive AWESAM Protocol

The AWESAM protocol described here is optimal for culturing murine astrocytes in isolation from other brain cells in a fast, simple, and inexpensive manner. AWESAM astrocytes exhibit spontaneous Ca2+ signaling, morphology, and gene expression profiles similar to astrocytes in vivo.

The AWESAM (a low-cost easy stellate astrocyte method) protocol entails a fast, simple, and inexpensive way to generate large quantities of in vivo-like ...

Combining Optogenetics with Artificial microRNAs to Characterize the Effects of Gene Knockdown on Presynaptic Function within Intact Neuronal Circuits

The purpose of this protocol is to characterize the effect of gene knockdown on presynaptic function within intact neuronal circuits. We describe a workflow on how to combine artificial microRNA (miR)-mediated RNA interference with optogenetics to achieve selective stimulation of manipulated presynaptic boutons in acute brain slices. The experimental approach involves the use of a single viral construct and a single neuron-specific promoter to drive the expression of both an optogenetic probe and artificial miR(s) against presynaptic gene(s). When stereotactically injected in the brain region of interest, the expressed construct makes it possible to stimulate with light exclusively the neurons with reduced expression of the gene(s) under investigation. This strategy does not require the development and maintenance of genetically modified mouse lines and can in principle be applied to other organisms and to any neuronal gene of choice. We have recently applied it to investigate how the knockdown of alternative splice isoforms of presynaptic P/Q-type voltage-gated calcium channels (VGCCs) regulates short-term synaptic plasticity at CA3 to CA1 excitatory synapses in acute hippocampal slices. A similar approach could also be used to manipulate and probe the neuronal circuitry in vivo.

This protocol provides a workflow on how to combine artificial microRNA-mediated RNA interference with optogenetics to stimulate specifically presynaptic boutons with reduced expression of selective gene(s) within intact neuronal circuits.

The purpose of this protocol is to characterize the effect of gene knockdown on presynaptic function within intact neuronal circuits. We describe a ...

A High-throughput Calcium-flux Assay to Study NMDA-receptors with Sensitivity to Glycine/D-serine and Glutamate

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are classified as ionotropic glutamate receptors and have critical roles in learning and memory. NMDAR malfunction, expressed as either over- or under-activity caused by mutations, altered expression, trafficking, or localization, can contribute to numerous diseases, especially in the central nervous system. Therefore, understanding the receptor&#39;s biology as well as facilitating the discovery of compounds and small molecules is crucial in ongoing efforts to combat neurological diseases. Current approaches to studying the receptor have limitations including low throughput, high cost, and the inability to study its functional abilities due to the necessary presence of channel blockers to prevent NMDAR-mediated excitotoxicity. Additionally, the existing assay systems are sensitive to stimulation by glutamate only and lack sensitivity to stimulation by glycine, the other co-ligand of the NMDAR. Here, we present the first plate-based assay with high-throughput power to study an NMDA receptor with sensitivity to both co-ligands, glutamate and D-serine/glycine. This approach allows the study of different NMDAR subunit compositions and allows functional studies of the receptor in glycine- and/or glutamate-sensitive modes. Additionally, the method does not require the presence of inhibitors during measurements. The effects of positive and negative allosteric modulators can be detected with this assay and the known pharmacology of NMDAR has been replicated in our system. This technique overcomes the limitations of existing methods and is cost-effective. We believe that this novel technique will accelerate the discovery of therapies for NMDAR-mediated pathologies.

The goal of this protocol is to facilitate the study of NMDA-receptors (NMDAR) at a larger scale and allow the examination of modulatory effects of small molecules and their therapeutic applications.

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are classified as ionotropic glutamate receptors and have critical roles in learning and memory. NMDAR ...