This study was supported by the following grants (to D.A.T.): NIA RO1 AG074999, NIA R21AG051103, VA I21RX002223, and VA I21 BX003023.

All the animal procedures were approved by the Institutional Animal Care and Use Committee at Duke University or the equivalent local authority regulating research involving animals. See the Table of Materials for details about all the materials, instruments, and equipment used in this protocol.
1. Instrument preparation
<ol>
	<li>Make sure that all the required items and surgical instruments are in place (Figure 2): scalp cleaning solution (alcohol pads), tape, forceps, scissors, and a drill for placing the small (0.5 mm) burr holes.</li>
	<li>Prepare the extracranial surface electrodes for skull application, and make sure any surgical glue has been cleaned from them if they have been used previously.</li>
	<li>Verify the impedance of these tACS electrodes directly before applying them to the skull. For this, use the built-in measurement function of the tACS stimulator with both electrodes placed in a saline bath. 
	NOTE: The preferred impedance is &#60;5 K&#8486; per electrode pair to allow sufficient current to be passed across the skull. The stimulator device checks the impedance prior to delivering constant-current pulses and gives the value directly.</li>
</ol>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/65195/65195fig02.jpg" /> 
Figure 2: A photograph of the required instrumentation, including dissecting instruments and scissors, for preparing the extracranial stimulation. 1. Micro dissecting scissors, 11.5 cm; 2. Forceps, 11.5 cm, slight curve, serrated; 3. Dumont #7 forceps, curved; 4. Dumont #5 forceps; 5. Micro curette, 13 cm; 6. Q-tips; 7. Surgical tape; 8. Alcohol pads. <a href="https://www.jove.com/files/ftp_upload/65195/65195fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
2. Preparation of the animal for surgery
NOTE: For these experiments, we used 14 C57BL/6&#160;control mice between 12 weeks and 33 weeks of age, of which five were male and nine were female.
<ol>
	<li>Anesthetize the animal in an induction chamber with isoflurane in 30% O2 at ~1.5 L/min, with ~4% initially to induce and ~1.25%-1.5% to maintain at a level of anesthesia with spontaneous breathing and sufficient to eliminate the tail pinch response.</li>
	<li>Transfer the animal to the stereotaxic frame after induction, and then secure the head into the nose cone and ear bars for the subsequent electrode application and burr hole procedure (Figure 1 and Figure 3).</li>
	<li>Connect the nose cone of the stereotaxic frame to the vaporizer via an inlet and to an outlet to remove any isoflurane residual through a scavenger system (e.g., charcoal or a vacuum). Make sure there are no air leaks from the nose cone, both to maintain the anesthesia level with the isofluorane and to prevent accidental leakage into the room air (Figure 3).</li>
	<li>Check the position of the mouse in the stereotaxic frame, including the position of the nose cone, to allow spontaneous respiration without intubation, as well as appropriate anesthesia recovery and scavenging to protect the research personnel (Figure 3).</li>
	<li>Place the probes for measuring the pulse, pulse oxygen saturation (pulse OX), blood pressure, and temperature on the animal; ensure that the minimum pulse oxygenation is 90% and the pulse is &#62;450/min (the alarm lower limit is shown as 380 pulses/min). Record these parameters during the procedure at regular intervals or continuously, depending on the recording system (Figure 3).</li>
	<li>Prior to starting the procedure, check the level of sedation of the animal using (for example) a toe pinch to check the reflexes. If there is no reflex, then the level of sedation is optimal, as long as the animal maintains spontaneous respiration and adequate pulse oxygenation. If there is a reflex, increase the delivery of isoflurane to deepen the level of the anesthesia, and then recheck the reflex. Continuously observe and monitor the respiratory frequency of the animal, and adjust the isoflurane delivery accordingly.</li>
	<li>Shave the scalp hair or remove the hair with depilatory cream (clean the residual cream with alcohol pad passages).</li>
	<li>Apply eye ointment, and then aseptically clean the scalp with three passages of iodine and alcohol prior to excision using scissors.</li>
</ol>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/65195/65195fig03.jpg" /> 
Figure 3: An image of the animal in the stereotactic frame, with the skull exposed and only the tACS stimulator electrodes in place (prior to the burr hole placement). Note the blood pressure device around the tail and the pulse oximeter on the paw, with the reading on the left. There are scavenging tubes for the isoflurane around the nose cone. Abbreviation: tACS = transcranial alternating current stimulation. <a href="https://www.jove.com/files/ftp_upload/65195/65195fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
3. Surgical procedure: Applying the stimulating electrodes and making the burr holes
<ol>
	<li>For a terminal study, remove the scalp using surgical scissors, and expose the skull ~3 mm from the lambdoid suture caudally and ~3 mm frontal to the bregma to expose part of the posterior frontal suture. Excise the scalp parietally to expose the initial part of the temporal muscle on both sides (Figure 3).</li>
	<li>Remove any residual subcutaneous connective tissues so that the skull is clean and dry for the application of the stimulating electrodes.</li>
	<li>Apply conductive gel or paste to the side of the electrodes that will be in contact with the skull, and secure the electrodes with surgical superglue around the edge at intermittent spots. 
	NOTE: Do not allow the conducting gel to interfere with the surgical superglue in order to allow a better bond to the skull surface. The outer surface of the electrodes can also be insulated (from the scalp if this is closed during a survival surgery) using surgical superglue.</li>
	<li>Use either commercial flat electrodes, or create in-house electrodes using insulated wire with a diameter of 100 &#181;m (soldered to the plate) and a 1 mm x 3 mm flexible, insulated (on one surface) copper plate cut according to the size of the skull.</li>
	<li>Apply lidocaine paste on the temporal muscle and scalp on both sides without disturbing the electrodes to reduce the muscle and peripheral nerve activation.</li>
	<li>Once the extracranial stimulating electrodes are in place 4 mm laterally to each side of the skull (between the bregma and lambda), drill two 0.5 mm burr holes for the glass electrodes 2 mm on each side of the midline, 4 mm apart from each other, orthogonal to the sagittal suture (Figure 1). Fill these burr holes with sterile mineral oil to prevent current ingress into the skull from the extracranial electrodes.</li>
	<li>If desired for a particular experiment to induce spreading depression (i.e., potassium-induced spreading depression [K+-SD]), add, on the right side of the skull, a third 0.5 mm burr hole ~1.5 mm rostral to the coronal suture and ~1 mm lateral to the posterior frontal suture. Fill this burr hole with saline for later 1 M KCl application to induce K+-SD.</li>
	<li>Test the impedance of the extracranial stimulating electrodes both prior to the burr hole placement (and compared to the same electrodes placed in a saline bath) and after the burr hole placement to verify that the burr holes do not interfere with the current flow into the brain (i.e., ensure that the resistance is unchanged). 
	NOTE: The impedance measurement is provided directly by the stimulating device. Generally, we have found the overall system impedance (i.e., from the extracranial electrodes across the skull/brain pathway, typically ~3 K&#8486;) to be relatively constant regardless of the burr holes and glass microelectrodes, indicating that there is minimal current leakage directly into the brain through the burr holes.</li>
	<li>Place the chronic transcranial stimulation electrodes for chronic stimulation in a similar fashion. In this case, insulate the outer surface of the electrodes, close the scalp, and either tunnel the insulated wires out through the scalp or route them into a fixed head stage mounted to the skull.</li>
</ol>
4. Physiological procedure
<ol>
	<li>Commence with the physiological aspects of the experiment, once the animal is fully prepared for the non-survival, physiological experiment. Maintain the anesthesia level sufficient for both spontaneous respiration and adequate pulse Ox, respiratory, and pulse levels.</li>
	<li>Measure the CBF resulting from the extracranial stimulation by one of the following two methods.
	<ol>
		<li>Place the mouse under a laser speckle imaging device with or without intracranial recording electrodes to measure the intracranial electrical field during stimulation episodes (Figure 3).</li>
		<li>Transfer the animal to a physiological preparation for the placement of bilateral laser Doppler probes and intracranial electrodes to measure the intracranial electrical field during stimulation episodes (Figure 1).</li>
	</ol>
	</li>
</ol>
5. Placement of bilateral laser Doppler and glass electrodes
<ol>
	<li>Transfer the animal to a microscope stage for the application of bilateral laser Doppler probes. Place the probes on the top of the skull surface between the bilateral burr holes and the coronal suture (Figure 1).</li>
	<li>Fill pulled glass microelectrodes (~0.1 &#181;M, 2-6 M&#8486; impedance) with 0.2 M NaCl, and place them using a micromanipulator into the two burr holes placed laterally to the sagittal suture3,14&#160;(Figure 1). 
	NOTE: These burr holes are between the two symmetrical extracranial stimulation electrodes (Figure 1).</li>
	<li>Once inserted into the brain, ensure these glass microelectrodes are ~1 mm within the cerebral cortex. Perform depth profiles at various symmetrical depths. Refill the burr holes with sterile mineral oil to insulate this pathway for current flow.</li>
</ol>
6. Stimulation procedure and measurement of the intensity of the transcranial alternating current stimulation (tACS) or transcranial direct current stimulation (tDCS)
<ol>
	<li>Record continuous data from the dual laser Doppler probes on the skull and the two intracranial microelectrode outputs (recorded using a DC amplifier with headstages) using a digitizing system and software with at least four channels (at a sampling rate of 1 KHz). Once all the values have been recorded over a sufficiently stable baseline duration (i.e., &#62;10 min), test the extracranial stimulation. 
	NOTE: Figure 4 shows an example of the four channels with the two intracranial recording electrodes in the upper channels and the CBF response in the lower channels.</li>
	<li>Apply brief periods of on/off stimulation at various amplitudes (i.e., 20-30 s, 0.5-2.0 mA, in the tolerable range) to obtain a clear baseline before and after stimulation (Figure 4). Apply the stimulation between the two skull tACS electrodes on either side (Figure 1) using a commercial, human-compatible stimulating device that delivers a constant current.</li>
	<li>Closely observe the mouse for muscle twitches or other responses to the tACS, such as a change in the pulse or respiration, to create an upper limit of tolerability (generally ~2 mA).</li>
	<li>Continue to monitor the impedance across the electrodes with stimulation epochs to ensure this is constant.</li>
	<li>Add a small amount (2-3 &#181;L) of 1 M KCl to the anterior burr hole14 to induce spontaneous K+-SD events. These generate a large CBF response and interactions between the K+-SD-induced CBF response and the CBF response. Estimate the tACS CBF response, applying the tACS stimulation both before and after the occurrence of the SD.</li>
	<li>At the end of the experiment, perform euthanasia through an overdose of isoflurane (5%) and then decapitate once the respirations and heartbeats have ceased.</li>
</ol>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/65195/65195fig04.jpg" /> 
Figure 4: Data showing four channels of raw data in response to low-intensity tACS. The data are arranged with the upper two rows as the intracranial, direct DC electrical recordings (labeled as Input 1 [IN0] and input 2 [IN1]) and the lower two rows as the bilateral laser Doppler recordings of the cerebral blood flow. Note that the responses are asymmetrical between the right (upper) and left (lower) electrical and cerebral blood flow traces. (A) A small response (16% increase in blood flow) in response to a 1.2 mV/mm 20 s stimulus (0.75 mA). (B) A larger response (21% increase in blood flow) in response to a 1.4 mV/mm stimulus (1.0 mA). Abbreviation: tACS = transcranial alternating current stimulation. <a href="https://www.jove.com/files/ftp_upload/65195/65195fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
7. Calculation of the electrical field
<ol>
	<li>Measure the difference in the output from the two intracranial electrodes using the difference in the half-wave (one cycle) of the two sine waves recorded (the upper two traces in Figure 4). Divide this difference (mV) by the distance between the two electrodes (mm, here ~4 mm but directly measured in each case) to arrive at the field strength (mV/mm)3,6.</li>
</ol>

Mapping Cerebral Blood Flow and Electrical Fields: Electrode Placement in Mice

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The authors have no conflicts of interest to declare.

This protocol focuses on the in vivo, anesthetized measurement of the CBF response as a biomarker to estimate the brain response to tES14. Longer-term biomarkers of the tES response include histological treatment effects, such as the prevention of or changes in amyloid plaque formation (i.e., with gamma stimulation at 40 Hz in several AD models)16,17,18,19, but ...

Transcranial electrical stimulation (tES; with sine wave stimulation, tACS) is a common, external, non-invasive approach to brain neuromodulation1,2. Previously, we hypothesized that at certain doses, tES (and particularly tACS) may increase the cerebral blood flow (CBF) in the underlying brain regions3. Further, a dose-response relationship may exist between either the external current applied or the intracranial electrical field and the resulting CBF responses. However, most clinical stimulation protocols have focused on a maximal comfortable skin level of stimulation (i.e., ~ 2 mA) for scheduled periods of time (i.e., 30-45 min) as a treatment protocol4,5. In rodents, it is possible to use invasive, extracranial brain electrodes applied directly to the skull to investigate the electrical fields in the brain induced by tES6. Hence, the goal of this approach is to determine the effects of the intensity of tACS at relevant frequencies on CBF changes in terms of the dose-response relationship. This dose-response curve is based on a short-term physiological biomarker-direct measurements of the CBF-in relation to the electrical field imposed on the brain3. We have previously shown that, at larger amplitudes, typically beyond the range of electrical fields within the brain induced by tACS clinically, there is a near-linear correlation between the induced electrical field and the CBF in the cortex3. However, smaller-field stimulation (i.e., 1-5 mV/mm intensity) may be more relevant and feasible for use in humans; hence, we have modified our techniques to detect smaller CBF changes.
This paper describes a protocol to analyze the effects of lower-field strength tES alternating sine currents (tACS) on CBF (i.e., 0.5-2.0 mA current, 1-5 mV/mm electrical field), which can be tolerated by awake rodents5. This protocol involves the use of novel laser speckle imaging during tACS, as well as dual intracranial glass electrodes, to determine both the spread of active tACS within the brain (as monitored by the CBF) and the intracranial electrical field intensity, which is shown both as a diagram and an actual experimental photograph (Figure 1). There are many possible physiological effects of tES within the brain, including direct neuronal modulation, neural plasticity, and astrocyte activation7,8. Though CBF has been measured with tDCS9,10, these measurements were slow, indirect, and insufficient for assessing the dose-response function in the brain. Therefore, by using appropriate short-term biomarkers (i.e., CBF, electrical fields) and rapid on/off sequences of tACS, we can now estimate the dose-response function more accurately. Further, we can apply different techniques to measure the CBF, including both focal laser Doppler probes (LD) and laser speckle imaging (LSI) with defined regions of interest.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/65195/65195fig01.jpg" /> 
Figure 1: Transcranial stimulation diagram and photographic example. (A) Diagram of the transcranial stimulation setup. The diagram shows a mouse skull with coronal and sagittal sutures. The transcranial electrodes are placed laterally and symmetrically on the skull and are mounted with surgical glue and conductive paste between the electrodes and the skull. These electrodes are connected to a human-compatible, constant-current stimulation device, which can specify the frequency, amplitude, and duration of stimulation. For the assessment of intracranial electrical fields, bilateral glass electrodes (~2 M&#8486;) are placed in the cerebral cortex (i.e., within 1 mm of the inner aspect of the skull through small burr holes), and these are sealed with mineral oil and have AgCl grounds in the neck muscle (shown as larger wires in the center buried into the subcutaneous neck tissue). These glass electrodes are connected to a DC amplifier, and their outputs are recorded through a digitizer with at least four channels. Bilateral laser Doppler probes are also placed on the skull for recordings. The entire skull is also imaged with either a laser speckle imaging device or a high-resolution (at least 1,024 x 1,024 pixels, 12-14 bit pixel depth) cooled camera for intrinsic optical signal detection. The hemoglobin isosbestic frequency is typically chosen (i.e., 562 nm) for illumination for blood flow imaging. (B) A close-up image of an actual experiment, showing the bilateral laser Doppler probes (to the left), the (bilateral) intracranial glass recording microelectrodes placed through the burr holes, and with the tACS stimulating electrodes laterally. Abbreviation: tACS = transcranial alternating current stimulation. <a href="https://www.jove.com/files/ftp_upload/65195/65195fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
As a way of assessing the mechanisms, we can also interrogate interactions with other physiological processes that also alter the CBF, such as K+-induced spreading depolarization11. Further, rather than scheduled sessions at regular times, it is also possible to develop a closed-loop system based on additional biomarkers for a variety of diseases, as has been proposed for epilepsy treatment12 (i.e., clinical Neuropace devices). For example, closed-loop brain stimulation for Parkinson&#39;s disease is commonly based on the intrinsic, abnormal local field potentials (LFPs) intrinsic to this disease in the absence of sufficient dopamine (typically &#946;-band LFPs)13.

<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td>Alcohol pads</td>
			<td>HenryShein</td>
			<td>112-6131</td>
			<td></td>
		</tr>
		<tr>
			<td>Baby mineral oil</td>
			<td>Johnson &#38; Johnson</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>BD 1 mL syringe</td>
			<td>Becton Dikinson</td>
			<td>REF 305699</td>
			<td></td>
		</tr>
		<tr>
			<td>C3 Flat Surface Electrodes</td>
			<td>Neuronexus</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>C57BI mice</td>
			<td></td>
			<td></td>
			<td>from NIH colonies&#160;</td>
		</tr>
		<tr>
			<td>Copper skull electrods</td>
			<td>In house preparation</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Digidata 1440, Clampex</td>
			<td>Axon Instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont #5 forceps</td>
			<td>FST</td>
			<td>#5</td>
			<td></td>
		</tr>
		<tr>
			<td>Dumont #7 forceps curved</td>
			<td>Dumont</td>
			<td>RS-5047</td>
			<td></td>
		</tr>
		<tr>
			<td>Eye ointment</td>
			<td>Major</td>
			<td>LubiFresh P.M. NDC-0904-6488-38</td>
			<td></td>
		</tr>
		<tr>
			<td>Flaming/Brown micropipette puller</td>
			<td>Sutter instrument Co.</td>
			<td>Model P-87</td>
			<td></td>
		</tr>
		<tr>
			<td>Forceps 11.5 cm slight curve&#160; serrated</td>
			<td>Roboz</td>
			<td>RS-8254</td>
			<td></td>
		</tr>
		<tr>
			<td>Intramedic needle 23 G</td>
			<td>Becton Dikinson</td>
			<td>REF 427565</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl 1 M</td>
			<td>In house preparation</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Laser Doppler Probes</td>
			<td>Moor Instruments</td>
			<td>0.46 mm laser doppler probes</td>
			<td></td>
		</tr>
		<tr>
			<td>Laser Speckle Imaging Device</td>
			<td>RWD</td>
			<td>RFLSI-ZW</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro curette 13 cm</td>
			<td>FST</td>
			<td>10080-05</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro Dissecting Scissors, 11.5 cm</td>
			<td>Roboz</td>
			<td>RS-5914</td>
			<td></td>
		</tr>
		<tr>
			<td>Mouse anesthesia fixation</td>
			<td>Stoelting</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Neuroconn-DS</td>
			<td>Neurocare</td>
			<td>DC-Stimulator Plus</td>
			<td></td>
		</tr>
		<tr>
			<td>PhysioSuite Monitoring</td>
			<td>Kent Scientific</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Q-tips</td>
			<td>Fisherbrand</td>
			<td>22363167</td>
			<td></td>
		</tr>
		<tr>
			<td>Saline 0.9% NaCl solution</td>
			<td>Baxter</td>
			<td>281322</td>
			<td></td>
		</tr>
		<tr>
			<td>Sensicam QE</td>
			<td>PCO Instruments</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Software</td>
			<td>Axon Instruments Clampex</td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Surgical glue</td>
			<td>Covetrus</td>
			<td>31477</td>
			<td></td>
		</tr>
		<tr>
			<td>Surgical tape</td>
			<td>3M Transpore</td>
			<td>T9784</td>
			<td></td>
		</tr>
	</tbody>
</table>

placement of extracranial stimulating electrodes and measurement of cerebral blood flow and intracranial electrical fields in anesthetized mice

The detection of cerebral blood flow (CBF) responses to various forms of neuronal activation is critical for understanding dynamic brain function and variations in the substrate supply to the brain. This paper describes a protocol for measuring CBF responses to transcranial alternating current stimulation (tACS). Dose-response curves are estimated both from the CBF change occurring with tACS (mA) and from the intracranial electric field (mV/mm). We estimate the intracranial electrical field based on the different amplitudes measured by glass microelectrodes within each side of the brain. In this paper, we describe the experimental setup, which involves using either bilateral laser Doppler (LD) probes or laser speckle imaging (LSI) to measure the CBF; as a result, this setup requires anesthesia for the electrode placement and stability. We present a correlation between the CBF response and the current as a function of age, showing a significantly larger response at higher currents (1.5 mA and 2.0 mA) in young control animals (12-14 weeks) compared to older animals (28-32 weeks) (p &lt; 0.005 difference). We also demonstrate a significant CBF response at electrical field strengths &lt;5 mV/mm, which is an important consideration for eventual human studies. These CBF responses are also strongly influenced by the use of anesthesia compared to awake animals, the respiration control (i.e., intubated vs. spontaneous breathing), systemic factors (i.e., CO2), and local conduction within the blood vessels, which is mediated by pericytes and endothelial cells. Likewise, more detailed imaging/recording techniques may limit the field size from the entire brain to only a small region. We describe the use of extracranial electrodes for applying tACS stimulation, including both homemade and commercial electrode designs for rodents, the concurrent measurement of the CBF and intracranial electrical field using bilateral glass DC recording electrodes, and the imaging approaches. We are currently applying these techniques to implement a closed-loop format for augmenting the CBF in animal models of Alzheimer's disease and stroke.

Representative results are shown in Figure 4, Figure 5, and Figure 6. Figure 4 shows an example of the four channels with the two intracranial recording electrodes on the upper channels and the CBF responses on the lower channels. The tACS is symmetrical across the skull, but generally, the intracranial field response is slightly asymmetrical for applied AC currents, with one side showing a larger respo...

Watch this Scientific Journal Video about Stimulation-Based Approach to Improve Cerebral Blood Flow in Alzheimer's Model at JoVE.com

Author Spotlight: Stimulation-Based Approach to Improve Cerebral Blood Flow in Alzheimer's Model 

We describe a protocol for assessing dose-response curves for extracranial stimulation in terms of brain electrical field measurements and a relevant biomarker-cerebral blood flow. Since this protocol involves invasive electrode placement into the brain, general anesthesia is needed, with spontaneous breathing preferred rather than controlled respirations.

Placement of Extracranial Stimulating Electrodes and Measurement of Cerebral Blood Flow and Intracranial Electrical Fields in Anesthetized Mice

The detection of cerebral blood flow (CBF) responses to various forms of neuronal activation is critical for understanding dynamic brain function and ...

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1.0K Views.  Duke University Medical Center. We describe a protocol for assessing dose-response curves for extracranial stimulation in terms of brain electrical field measurements and a relevant biomarker-cerebral blood flow. Since this protocol involves invasive electrode placement into the brain, general anesthesia is needed, with spontaneous breathing preferred rather than controlled respirations.

Video: Author Spotlight: Stimulation-Based Approach to Improve Cerebral Blood Flow in Alzheimer's Model 

Cerebral Blood Oxygenation Measurement Based on Oxygen-dependent Quenching of Phosphorescence

Monitoring of the spatiotemporal characteristics of cerebral blood and tissue oxygenation is crucial for better understanding of the 
neuro-metabolic-vascular relationship. Development of new pO2 measurement modalities with simultaneous monitoring of pO2 in larger fields of view with higher 
spatial and/or temporal resolution will enable greater insight into the functioning of the normal brain and will also have significant impact on diagnosis 
and treatment of neurovascular diseases such as stroke, Alzheimer's disease, and head injury.
Optical imaging modalities have shown a great potential to provide high spatiotemporal resolution and quantitative imaging of pO2 based on hemoglobin absorption in visible and near infrared range of optical spectrum. However, multispectral measurement of cerebral blood oxygenation relies on photon migration through the highly scattering brain tissue. Estimation and modeling of tissue optical parameters, which may undergo dynamic changes during the experiment, is typically required for accurate estimation of blood oxygenation. On the other hand, estimation of the partial pressure of oxygen (pO2) based on oxygen-dependent quenching of phosphorescence should not be significantly affected by the changes in the optical parameters of the tissue and provides an absolute measure of pO2. Experimental systems that utilize oxygen-sensitive dyes have been demonstrated in in vivo studies of the perfused tissue as well as for monitoring the oxygen content in tissue cultures, showing that phosphorescence quenching is a potent technology capable of accurate oxygen imaging in the physiological pO2 range. 
Here we demonstrate with two different imaging modalities how to perform measurement of pO2 in cortical vasculature based on phosphorescence lifetime imaging. In first demonstration we present wide field of view imaging of pO2 at the cortical surface of a rat. This imaging modality has relatively simple experimental setup based on a CCD camera and a pulsed green laser. An example of monitoring the cortical spreading depression based on phosphorescence lifetime of Oxyphor R3 dye was presented. In second demonstration we present a high resolution two-photon pO2 imaging in cortical micro vasculature of a mouse. The experimental setup includes a custom built 2-photon microscope with femtosecond laser, electro-optic modulator, and photon-counting photo multiplier tube. We present an example of imaging the pO2 heterogeneity in the cortical microvasculature including capillaries, using a novel PtP-C343 dye with enhanced 2-photon excitation cross section.
Click here to view the related article <a href="http://www.jove.com/Details.php?ID=1731">Synthesis and Calibration of Phosphorescent Nanoprobes for Oxygen Imaging in Biological Systems.</a>

We present an experimental procedure for measuring the partial pressure of oxygen (pO2) in cerebral vasculature based on oxygen-dependent quenching of phosphorescence. Animal preparation and imaging procedures were outlined for both large field of view CCD-based imaging of pO2 in rats and 2-photon excitation based imaging of pO2 in mice.

Monitoring of the spatiotemporal characteristics of cerebral blood and tissue oxygenation is crucial for better understanding of the 
...

Intracranial Injection of Adeno-associated Viral Vectors

Intracranial injection of viral vectors engineered to express a fluorescent protein is a versatile labeling technique for visualization of specific subsets of cells in different brain regions both in vivo and in brain sections. Unlike the injection of fluorescent dyes, viral labeling offers targeting of individual cell types and is less expensive and time consuming than establishing transgenic mouse lines. In this technique, an adeno-associated viral (AAV) vector is injected intracranially using stereotaxic coordinates, a micropipette and an automated pump for precise delivery of AAV to the desired area with minimal damage to the surrounding tissue. Injection parameters can be tailored to individual experiments by adjusting the animal age at injection, injection location, volume of injection, rate of injection, AAV serotype and the promoter driving gene expression. Depending on the conditions chosen, virally-induced transgene expression can allow visualization of groups of cells, individual cells or fine cellular processes, down to the level of dendritic spines. The experiment shown here depicts an injection of double-stranded AAV expressing green fluorescent protein for the labeling of neurons and glia in the mouse primary visual cortex.

Here we present the intracranial injection of AAV vectors for fluorescent labeling of neurons and glia in the visual cortex.

Intracranial injection of viral vectors engineered to express a fluorescent protein is a versatile labeling technique for visualization of specific ...

Intracranial Orthotopic Allografting of Medulloblastoma Cells in Immunocompromised Mice

Medulloblastoma is the most common pediatric tumor of the nervous system. A large body of animal studies has focused on cerebellar granule neuron precursors (CGNPs) as the cell-of-origin for medulloblastoma1-4. However, the diverse clinical presentations of medulloblastoma subtypes in human patients (nodular, desmoplastic, classical and large cell/anaplastic), and the fact that medulloblastoma is found in a subset of human patients with no ectopic expression of CGNP marker5, suggest that the cellular and molecular origins of medulloblastoma are more complex and far from being completely deciphered. Therefore, it is essential to determine whether there is an alternative medulloblastoma tumor cell-of-origin based on which cell-type specific therapeutic modality can be developed. To this end, intracranial orthotopic allografting of genetically marked tumor cell types followed by subsequent analyses of secondary tumor development in recipients will allow determination of the cellular origin of tumor-initiating cells. Here we describe the experimental protocol for intracranial orthotopic allografting of medulloblastoma cells derived from primary tumor tissue, and this procedure can also be used for transplanting cells from established cell lines.

This protocol describes the isolation and dissociation of mouse medulloblastoma tissue, and subsequent allografting of the tumor cells into immunocompromised recipient mice in order to initiate secondary medulloblastoma.

Medulloblastoma is the most common pediatric tumor of the nervous system. A large body of animal studies has focused on cerebellar granule neuron ...

Comprehensive Profiling of Dopamine Regulation in Substantia Nigra and Ventral Tegmental Area

Dopamine is a vigorously studied neurotransmitter in the CNS. Indeed, its involvement in locomotor activity and reward-related behaviour has fostered five decades of inquiry into the molecular deficiencies associated with dopamine regulation. The majority of these inquiries of dopamine regulation in the brain focus upon the molecular basis for its regulation in the terminal field regions of the nigrostriatal and mesoaccumbens pathways; striatum and nucleus accumbens. Furthermore, such studies have concentrated on analysis of dopamine tissue content with normalization to only wet tissue weight. Investigation of the proteins that regulate dopamine, such as tyrosine hydroxylase (TH) protein, TH phosphorylation, dopamine transporter (DAT), and vesicular monoamine transporter 2 (VMAT2) protein often do not include analysis of dopamine tissue content in the same sample. The ability to analyze both dopamine tissue content and its regulating proteins (including post-translational modifications) not only gives inherent power to interpreting the relationship of dopamine with the protein level and function of TH, DAT, or VMAT2, but also extends sample economy. This translates into less cost, and yet produces insights into the molecular regulation of dopamine in virtually any paradigm of the investigators' choice.
We focus the analyses in the midbrain. Although the SN and VTA are typically neglected in most studies of dopamine regulation, these nuclei are easily dissected with practice. A comprehensive readout of dopamine tissue content and TH, DAT, or VMAT2 can be conducted. There is burgeoning literature on the impact of dopamine function in the SN and VTA on behavior, and the impingements of exogenous substances or disease processes therein 1-5. Furthermore, compounds such as growth factors have a profound effect on dopamine and dopamine-regulating proteins, to a comparatively greater extent in the SN or VTA 6-8. Therefore, this methodology is presented for reference to laboratories that want to extend their inquiries on how specific treatments modulate behaviour and dopamine regulation. Here, a multi-step method is presented for the analyses of dopamine tissue content, the protein levels of TH, DAT, or VMAT2, and TH phosphorylation from the substantia nigra and VTA from rodent midbrain. The analysis of TH phosphorylation can yield significant insights into not only how TH activity is regulated, but also the signaling cascades affected in the somatodendritic nuclei in a given paradigm.
We will illustrate the dissection technique to segregate these two nuclei and the sample processing of dissected tissue that produces a profile revealing molecular mechanisms of dopamine regulation in vivo, specific for each nuclei (Figure 1).

Dopamine is distinctly regulated in the midbrain nuclei, which contain the cell bodies and dendrites of the dopamine neurons. Here we describe a dissection and sample-handling approach to maximize results, and thus conclusions and insights, on dopamine regulation in the midbrain nuclei of the substantia nigra (SN) and ventral tegmental area (VTA) in rodents.

Dopamine is a vigorously studied neurotransmitter in the CNS. Indeed, its involvement in locomotor activity and reward-related behaviour has fostered five ...

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 ...

Dual Electrophysiological Recordings of Synaptically-evoked Astroglial and Neuronal Responses in Acute Hippocampal Slices

Astrocytes form together with neurons tripartite synapses, where they integrate and modulate neuronal activity. Indeed, astrocytes sense neuronal inputs through activation of their ion channels and neurotransmitter receptors, and process information in part through activity-dependent release of gliotransmitters. Furthermore, astrocytes constitute the main uptake system for glutamate, contribute to potassium spatial buffering, as well as to GABA clearance. These cells therefore constantly monitor synaptic activity, and are thereby sensitive indicators for alterations in synaptically-released glutamate, GABA and extracellular potassium levels. Additionally, alterations in astroglial uptake activity or buffering capacity can have severe effects on neuronal functions, and might be overlooked when characterizing physiopathological situations or knockout mice. Dual recording of neuronal and astroglial activities is therefore an important method to study alterations in synaptic strength associated to concomitant changes in astroglial uptake and buffering capacities. Here we describe how to prepare hippocampal slices, how to identify stratum radiatum astrocytes, and how to record simultaneously neuronal and astroglial electrophysiological responses. Furthermore, we describe how to isolate pharmacologically the synaptically-evoked astroglial currents.

The preparation of acute brain slices from isolated hippocampi, as well as the simultaneous electrophysiological recordings of astrocytes and neurons in stratum radiatum during stimulation of schaffer collaterals is described. The pharmacological isolation of astroglial potassium and glutamate transporter currents is demonstrated.

Astrocytes form together with neurons tripartite synapses, where they integrate and modulate neuronal activity. Indeed, astrocytes sense neuronal inputs ...

Video Imaging and Spatiotemporal Maps to Analyze Gastrointestinal Motility in Mice

The enteric nervous system (ENS) plays an important role in regulating gastrointestinal (GI) motility and can function independently of the central nervous system. Changes in ENS function are a major cause of GI symptoms and disease and may contribute to GI symptoms reported in neuropsychiatric disorders including autism. It is well established that isolated colon segments generate spontaneous, rhythmic contractions known as Colonic Migrating Motor Complexes (CMMCs). A procedure to analyze the enteric neural regulation of CMMCs in ex vivo preparations of mouse colon is described. The colon is dissected from the animal and flushed to remove fecal content prior to being cannulated in an organ bath. Data is acquired via a video camera positioned above the organ bath and converted to high-resolution spatiotemporal maps via an in-house software package. Using this technique, baseline contractile patterns and pharmacological effects on ENS function in colon segments can be compared over 3-4 hr. In addition, propagation length and speed of CMMCs can be recorded as well as changes in gut diameter and contraction frequency. This technique is useful for characterizing gastrointestinal motility patterns in transgenic mouse models (and in other species including rat and guinea pig). In this way, pharmacologically induced changes in CMMCs are recorded in wild type mice and in the Neuroligin-3R451C mouse model of autism. Furthermore, this technique can be applied to other regions of the GI tract including the duodenum, jejunum and ileum and at different developmental ages in mice.

This article describes a video imaging technique and high-resolution spatiotemporal mapping to identify changes in the neural regulation of colonic motility in adult mice. Subtle effects on gastrointestinal (GI) function can be detected using this approach in isolated tissue preparations to advance our understanding of GI disease.

The enteric nervous system (ENS) plays an important role in regulating gastrointestinal (GI) motility and can function independently of the central ...

Simultaneous Recording of Electroretinography and Visual Evoked Potentials in Anesthetized Rats

The electroretinogram (ERG) and visual evoked potential (VEP) are commonly used to assess the integrity of the visual pathway. The ERG measures the electrical responses of the retina to light stimulation, while the VEP measures the corresponding functional integrity of the visual pathways from the retina to the primary visual cortex following the same light event. The ERG waveform can be broken down into components that reflect responses from different retinal neuronal and glial cell classes. The early components of the VEP waveform represent the integrity of the optic nerve and higher cortical centers. These recordings can be conducted in isolation or together, depending on the application. The methodology described in this paper allows simultaneous assessment of retinal and cortical visual evoked electrophysiology from both eyes and both hemispheres. This is a useful way to more comprehensively assess retinal function and the upstream effects that changes in retinal function can have on visual evoked cortical function.

This protocol describes simultaneous measurement of electroretinogram and visual evoked potentials in anesthetized rats.

The electroretinogram (ERG) and visual evoked potential (VEP) are commonly used to assess the integrity of the visual pathway. The ERG measures the ...

An Improved Method for Collection of Cerebrospinal Fluid from Anesthetized Mice

The cerebrospinal fluid (CSF) is a valuable body fluid for analysis in neuroscience research. It is one of the fluids in closest contact with the central nervous system and thus, can be used to analyze the diseased state of the brain or spinal cord without directly accessing these tissues. However, in mice it is difficult to obtain from the cisterna magna due to its closeness to blood vessels, which often contaminate samples. The area for CSF collection in mice is also difficult to dissect to and often only small samples are obtained (maximum of 5-7 &#181;L or less). This protocol describes in detail a technique that improves on current methods of collection to minimize contamination from blood and allow for the abundant collection of CSF (on average 10-15 &#181;L can be collected). This technique can be used with other dissection methods for tissue collection from mice, as it does not impact any tissues during CSF extraction. Thus, the brain and spinal cord are not affected with this technique and remain intact. With greater CSF sample collection and purity, more analyses can be used with this fluid to further aid neuroscience research and better understand diseases affecting the brain and spinal cord.

This protocol describes an improved technique for the abundant collection of cerebrospinal fluid (CSF) with no contamination from blood. With greater sample collection and purity, more analyses can be performed using CSF to further our understanding of diseases that affect the brain and spinal cord.

The cerebrospinal fluid (CSF) is a valuable body fluid for analysis in neuroscience research. It is one of the fluids in closest contact with the central ...

Intracranial Pharmacotherapy and Pain Assays in Rodents

Pain is a salient sensory experience with affective and cognitive dimensions. However, central mechanisms for pain remain poorly understood, hindering the development of effective therapeutics. Intracranial pharmacology presents an important tool for understanding the molecular and cellular mechanisms of pain in the brain, as well as for novel treatments. Here we present a protocol that integrates intracranial pharmacology with pain behavior testing. Specifically, we show how to infuse analgesic drugs into a select brain region, which may be responsible for pain modulation. Furthermore, to determine the effect of the candidate drug in the central nerve system, pain assays are performed after intracranial treatment. Our results demonstrate that intracranial administration of analgesic drugs in a targeted region can provide relief of pain in rodents. Thus, our protocol successfully demonstrates that intracranial pharmacology, combined with pain behavior testing, can be a powerful tool for the study of pain mechanisms in the brain.

Here we present a protocol to perform intracranial pharmacological experiments followed by pain behavior assays in rodents. This protocol allows researchers to deliver molecular and cellular targets in the brain, for pharmacologic agents in the treatment of pain.

Pain is a salient sensory experience with affective and cognitive dimensions. However, central mechanisms for pain remain poorly understood, hindering the ...