Name: ex vivo optogenetic interrogation of long-range synaptic transmission and plasticity from medial prefrontal cortex to lateral entorhinal cortex
Uploaded: 2022-02-25T13:00:00
Description: Watch this Scientific Journal Video about Ex Vivo Optogenetic Interrogation of Long-Range Synaptic Transmission and Plasticity from Medial Prefrontal Cortex to Lateral Entorhinal Cortex at JoVE.com

This work is supported by Wellcome grant 206401/Z/17/Z. We would like to thank Zafar Bashir for his expert mentorship and Dr. Clair Booth for technical assistance and comments on the manuscript.

All animal procedures were conducted in accordance with the United Kingdom Animals Scientific Procedures Act (1986) and associated guidelines as well as local institutional guidelines.
1. Stereotaxic viral injection
NOTE: The current protocol requires anatomical, but not post-synaptic cell type, specificity.
<ol>
	<li>Choose the appropriate animal. Male wild-type Lister hooded rats were used in this protocol (300-350 g, approximately 3 months old)...

<ol>
	<li>Martin, S., Grimwood, P., Morris, R. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Synaptic+plasticity+and+memory:+an+evaluation+of+the+hypothesis.">Synaptic plasticity and memory: an evaluation of the hypothesis.</a> Annual Review of Neuroscience. 23, 649-711 (2000).</li><li>Poorthuis, R. 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The protocol presented here describes a method to explore highly specific long-range synaptic projections using a combination of stereotaxic surgery to deliver AAVs encoding optogenetic constructs, and electrophysiology in acute brain slices (Figure 1). Together these techniques offer tools to characterize the physiology and plasticity of brain circuitry with high precision in long-range and anatomically diffuse pathways that were previously inaccessible using traditional, non-specific, elec...

Understanding the physiology of synapses and how they undergo plastic changes is fundamental for understanding how brain networks function in the healthy brain1, and how they malfunction in brain disorders. The use of acute ex vivo brain slices allows for the recording of the electrical activity of synapses from single neurons with a high signal-to-noise ratio using whole-cell patch-clamp recordings. Control of membrane potential and straightforward pharmacological manipulation allows isolation of receptor subtypes. These recordings can be made with exquisite specificity to identify the post-synaptic neuron, including laminar and sub-regional position2, cellular morphology3, presence of molecular markers4, its afferent projections5, or even if it was recently active6.
Achieving specificity of pre-synaptic inputs is, however, somewhat more challenging. The conventional method has used stimulation electrodes to excite the axons which run in a particular lamina. An example of this is in the hippocampus where local stimulation in the stratum radiatum activates synapses that project from the CA3 to the CA1 subfield7. In this instance, presynaptic specificity is achieved as CA3 input represents the sole excitatory input located within stratum radiatum which projects to CA1 pyramidal cells8. This high degree of input specificity achievable with conventional electrical presynaptic activation of CA3-CA1 axons is, however, an exception which is reflected in the intense study that this synapse has been subject to. In other brain regions, axons from multiple afferent pathways co-exist in the same lamina, for example, in layer 1 of neocortex9, thus rendering input-specific presynaptic stimulation impossible with conventional stimulating electrodes. This is problematic as different synaptic inputs may have divergent physiological properties; therefore, their co-stimulation may lead to mischaracterization of synaptic physiology.
The advent of optogenetics, the genetic encoding of photosensitive membrane proteins (opsins) such as channelrhodopsin-2 (ChR2), has allowed a vast expansion of possibilities for studying isolated synaptic projections between brain regions10,11. Here we describe a generalizable and low-cost solution to studying long-range synaptic physiology and plasticity. The optogenetic constructs are delivered in a highly specific manner using viral vectors allowing for extremely precise control of the pre-synaptic region of interest. Efferent projections will express the light-activated channel allowing for activation of these fibers in a target region. Thus, long-range, anatomically diffuse pathways that cannot be independently activated by traditional, non-specific, electrical stimulation can be studied.
We describe, as an example pathway, transduction of medial prefrontal cortex (mPFC) with adeno-associated viruses (AAVs) encoding excitatory cation-channel opsins. We then describe the preparation of acute slices from lateral entorhinal cortex (LEC), patch-clamp recordings from layer 5 LEC pyramidal neurons, and light-evoked activation of glutamatergic mPFC-LEC projections (Figure 1). We also describe the histological assessment of the injection site to confirm the location of the pre-synaptic region of interest and identification of post-synaptic cell morphology.

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			<td>0.2 mL tube</td>
			<td>Fisher Scientific Ltd</td>
			<td>12134102</td>
			<td></td>
		</tr>
		<tr>
			<td>10 &#181;L pipette</td>
			<td>Gilson</td>
			<td>FD10001</td>
			<td></td>
		</tr>
		<tr>
			<td>24 well plate</td>
			<td>SARSTEDT</td>
			<td>83.3922</td>
			<td></td>
		</tr>
		<tr>
			<td>3 way luer valve</td>
			<td>Cole-Parmer</td>
			<td>WZ-30600-02</td>
			<td></td>
		</tr>
		<tr>
			<td>3,3&#8242;-Diaminobenzidine (DAB) substrate</td>
			<td>Vector Laboratories</td>
			<td>SK-4105</td>
			<td></td>
		</tr>
		<tr>
			<td>40x objective</td>
			<td>Olympus</td>
			<td>LUMPLFLN40XW</td>
			<td></td>
		</tr>
		<tr>
			<td>4-aminopyridine</td>
			<td>Hello Bio</td>
			<td>HB1073</td>
			<td></td>
		</tr>
		<tr>
			<td>4x objective</td>
			<td>Olympus</td>
			<td>PLN4X/0.1</td>
			<td></td>
		</tr>
		<tr>
			<td>AAV9-CaMKiia-hChR2(E123T/T159C)-mCherry</td>
			<td>Addgene</td>
			<td>35512</td>
			<td>Viral titre: 3.3x1013 GC/ml</td>
		</tr>
		<tr>
			<td>Achromatic lens</td>
			<td>Edmund Optics</td>
			<td>49363</td>
			<td>Focusses visual spectrum and near-IR</td>
		</tr>
		<tr>
			<td>Benchtop microcentrifuge</td>
			<td>Benchmark Scientific</td>
			<td>C1005*</td>
			<td></td>
		</tr>
		<tr>
			<td>Biocytin</td>
			<td>Sigma-Aldrich</td>
			<td>B4261</td>
			<td></td>
		</tr>
		<tr>
			<td>Borosillicate glass capillary</td>
			<td>Warner Instruments</td>
			<td>G150F-6</td>
			<td></td>
		</tr>
		<tr>
			<td>Burr</td>
			<td>Fine science tools</td>
			<td>19008-07</td>
			<td></td>
		</tr>
		<tr>
			<td>CaCl2</td>
			<td>Sigma-Aldrich</td>
			<td>C5670</td>
			<td></td>
		</tr>
		<tr>
			<td>Camera - Qimaging Retiga Electro</td>
			<td>Photometrics</td>
			<td>01-ELECTRO-M-14-C</td>
			<td></td>
		</tr>
		<tr>
			<td>Carbachol</td>
			<td>Tocris</td>
			<td>2810</td>
			<td></td>
		</tr>
		<tr>
			<td>Chlorhexidine surgical scrub</td>
			<td>Vetasept</td>
			<td>XHG008</td>
			<td></td>
		</tr>
		<tr>
			<td>Clippers</td>
			<td>Andis</td>
			<td>22445</td>
			<td>AGC Super 2-Speed Detachable Blade Clipper</td>
		</tr>
		<tr>
			<td>Collimation condenser lens</td>
			<td>ThorLabs</td>
			<td>ACL2520-A</td>
			<td></td>
		</tr>
		<tr>
			<td>Coverslips</td>
			<td>Fisher Scientific Ltd</td>
			<td>10011913</td>
			<td></td>
		</tr>
		<tr>
			<td>Cryostat</td>
			<td>Leica</td>
			<td>CM3050 S</td>
			<td></td>
		</tr>
		<tr>
			<td>CsMeSO4</td>
			<td>Sigma-Aldrich</td>
			<td>C1426</td>
			<td></td>
		</tr>
		<tr>
			<td>Cyanoacrylate glue</td>
			<td>Rapid Electronics Ltd</td>
			<td>84-4557</td>
			<td></td>
		</tr>
		<tr>
			<td>Data acquisition device</td>
			<td>National Instruments</td>
			<td>USB-6341 BNC</td>
			<td></td>
		</tr>
		<tr>
			<td>D-glucose</td>
			<td>Sigma-Aldrich</td>
			<td>G8270</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichroic mirror 500 nm long-pass</td>
			<td>Edmund Optics</td>
			<td>69899</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichroic mirror 600 nm long-pass</td>
			<td>Edmund Optics</td>
			<td>69901</td>
			<td></td>
		</tr>
		<tr>
			<td>Dichroic mirror cube</td>
			<td>ThorLabs</td>
			<td>CM1-DCH/M</td>
			<td></td>
		</tr>
		<tr>
			<td>EGTA</td>
			<td>Millpore</td>
			<td>324626</td>
			<td></td>
		</tr>
		<tr>
			<td>Electrode holder with side port</td>
			<td>HEKA</td>
			<td>895150</td>
			<td></td>
		</tr>
		<tr>
			<td>Emission filter</td>
			<td>Chroma</td>
			<td>59022m</td>
			<td></td>
		</tr>
		<tr>
			<td>Excitation filter</td>
			<td>Chroma</td>
			<td>ET570/20x</td>
			<td></td>
		</tr>
		<tr>
			<td>Eye gel</td>
			<td>Dechra</td>
			<td>Lubrithal</td>
			<td></td>
		</tr>
		<tr>
			<td>Fine paint brush</td>
			<td>Scientific Laboratory Supplies</td>
			<td>BRU2052</td>
			<td></td>
		</tr>
		<tr>
			<td>Guillotine</td>
			<td>World Precision Instruments</td>
			<td>DCAP</td>
			<td></td>
		</tr>
		<tr>
			<td>HEPES</td>
			<td>Sigma-Aldrich</td>
			<td>H3375</td>
			<td></td>
		</tr>
		<tr>
			<td>Hydrogen peroxide solution</td>
			<td>Sigma-Aldrich</td>
			<td>H1009</td>
			<td>30% (w/w)</td>
		</tr>
		<tr>
			<td>Isoflurane</td>
			<td>Henry Schein</td>
			<td>988-3245</td>
			<td></td>
		</tr>
		<tr>
			<td>Isopentane</td>
			<td>Sigma-Aldrich</td>
			<td>M32631</td>
			<td></td>
		</tr>
		<tr>
			<td>KCl</td>
			<td>Sigma-Aldrich</td>
			<td>P3911</td>
			<td></td>
		</tr>
		<tr>
			<td>k-gluconate</td>
			<td>Sigma-Aldrich</td>
			<td>G4500</td>
			<td></td>
		</tr>
		<tr>
			<td>Kinematic fluorescence filter cube</td>
			<td>ThorLabs</td>
			<td>DFM1T1</td>
			<td></td>
		</tr>
		<tr>
			<td>LED driver</td>
			<td>ThorLabs</td>
			<td>LEDD1B</td>
			<td></td>
		</tr>
		<tr>
			<td>Lidocaine ointment</td>
			<td>Teva</td>
			<td>80007150</td>
			<td></td>
		</tr>
		<tr>
			<td>MgATP</td>
			<td>Sigma-Aldrich</td>
			<td>A9187</td>
			<td></td>
		</tr>
		<tr>
			<td>MgCl</td>
			<td>Sigma-Aldrich</td>
			<td>M2670</td>
			<td></td>
		</tr>
		<tr>
			<td>MgSO4</td>
			<td>Sigma-Aldrich</td>
			<td>M7506</td>
			<td></td>
		</tr>
		<tr>
			<td>Micro drill</td>
			<td>Harvard Apparatus</td>
			<td>75-1887</td>
			<td></td>
		</tr>
		<tr>
			<td>Microelectrode puller</td>
			<td>Sutter instruments</td>
			<td>P-87</td>
			<td></td>
		</tr>
		<tr>
			<td>Microinjection syringe</td>
			<td>Hamilton</td>
			<td>7634-01/00</td>
			<td></td>
		</tr>
		<tr>
			<td>Microinjection syringe needle</td>
			<td>Hamilton</td>
			<td>7803-05</td>
			<td>Custom specification: gauge 33, length 15mm, point style 4 - 12&#176;</td>
		</tr>
		<tr>
			<td>Microinjection syringe pump</td>
			<td>World Precision Instruments</td>
			<td>UMP3T-1</td>
			<td></td>
		</tr>
		<tr>
			<td>Mounted blue LED</td>
			<td>ThorLabs</td>
			<td>M470L5</td>
			<td></td>
		</tr>
		<tr>
			<td>Mounted green LED</td>
			<td>ThorLabs</td>
			<td>M565L3</td>
			<td></td>
		</tr>
		<tr>
			<td>Na2HPO4.7H2O</td>
			<td>Sigma-Aldrich</td>
			<td>S9390</td>
			<td></td>
		</tr>
		<tr>
			<td>NaCl</td>
			<td>Sigma-Aldrich</td>
			<td>S9888</td>
			<td></td>
		</tr>
		<tr>
			<td>NaGTP</td>
			<td>Sigma-Aldrich</td>
			<td>G8877</td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4</td>
			<td>Sigma-Aldrich</td>
			<td>S0751</td>
			<td></td>
		</tr>
		<tr>
			<td>NaH2PO4.H2O</td>
			<td>Sigma-Aldrich</td>
			<td>S9638</td>
			<td></td>
		</tr>
		<tr>
			<td>NaHCO3</td>
			<td>Sigma-Aldrich</td>
			<td>S5761</td>
			<td></td>
		</tr>
		<tr>
			<td>NIR LED</td>
			<td>OSRAM</td>
			<td>SFH4550</td>
			<td>Used for refracted IR imaging of slice, differential interference contrast (DIC) optics is another commonly used method</td>
		</tr>
		<tr>
			<td>OCT medium</td>
			<td>VWR International</td>
			<td>RAYLLAMB/OCT</td>
			<td>Optimal cutting temperature medium</td>
		</tr>
		<tr>
			<td>Paraformaldehyde</td>
			<td>Sigma-Aldrich</td>
			<td>158127</td>
			<td></td>
		</tr>
		<tr>
			<td>Paraformaldehyde</td>
			<td>Sigma-Aldrich</td>
			<td>P6148</td>
			<td></td>
		</tr>
		<tr>
			<td>Patch clamp amplifier</td>
			<td>Molecular Devices</td>
			<td>700A</td>
			<td></td>
		</tr>
		<tr>
			<td>Peristaltic pump</td>
			<td>World Precision Instruments</td>
			<td>Ministar</td>
			<td></td>
		</tr>
		<tr>
			<td>Poly-L-lysine coated microscope slides</td>
			<td>Fisher Scientific Ltd</td>
			<td>23-769-310</td>
			<td></td>
		</tr>
		<tr>
			<td>Recording chamber</td>
			<td>Warner Instruments</td>
			<td>RC-26G</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel blade</td>
			<td>Swann Morton</td>
			<td>#24</td>
			<td></td>
		</tr>
		<tr>
			<td>Slice anchor</td>
			<td>Warner Instruments</td>
			<td>SHD-26-GH/15</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereotaxic frame</td>
			<td>Kopf</td>
			<td>Model 902</td>
			<td></td>
		</tr>
		<tr>
			<td>Stereotaxic holder for micro drill</td>
			<td>Harvard Apparatus</td>
			<td>75-1874</td>
			<td></td>
		</tr>
		<tr>
			<td>Sucrose</td>
			<td>Sigma-Aldrich</td>
			<td>S0389</td>
			<td></td>
		</tr>
		<tr>
			<td>Surgical Microscope</td>
			<td>Carl Zeiss</td>
			<td>OPMI 1 FR pro</td>
			<td></td>
		</tr>
		<tr>
			<td>Suture</td>
			<td>Ethicon</td>
			<td>W577H</td>
			<td></td>
		</tr>
		<tr>
			<td>Syringe filter for intracellular recording solution</td>
			<td>Thermo Scientific Nalgene</td>
			<td>171-0020</td>
			<td></td>
		</tr>
		<tr>
			<td>Tetrodotoxin citrate</td>
			<td>Hello Bio</td>
			<td>HB1035</td>
			<td></td>
		</tr>
		<tr>
			<td>Transfer pipettes</td>
			<td>Fisher Scientific Ltd</td>
			<td>10458842</td>
			<td></td>
		</tr>
		<tr>
			<td>Triton X-100</td>
			<td>Sigma-Aldrich</td>
			<td>X100</td>
			<td></td>
		</tr>
		<tr>
			<td>Upright fluorescence microscope</td>
			<td>Leica</td>
			<td>DM6 B</td>
			<td></td>
		</tr>
		<tr>
			<td>VECTASHIELD Antifade Mounting Medium with DAPI</td>
			<td>Vector Laboratories</td>
			<td>H-1200-10</td>
			<td></td>
		</tr>
		<tr>
			<td>VECTASTAIN ABC-HRP kit</td>
			<td>Vector Laboratories</td>
			<td>PK-4000</td>
			<td></td>
		</tr>
		<tr>
			<td>Vibratome</td>
			<td>Campden Instruments</td>
			<td>7000smz-2</td>
			<td></td>
		</tr>
		<tr>
			<td>WinLTP</td>
			<td>https://www.winltp.com/</td>
			<td>Version 2.32</td>
			<td>Data acquisition software</td>
		</tr>
		<tr>
			<td>Solution</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>aCSF</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>sucrose cutting solution</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>PFA</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
		<tr>
			<td>Intracellular?</td>
			<td></td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

ex vivo optogenetic interrogation of long-range synaptic transmission and plasticity from medial prefrontal cortex to lateral entorhinal cortex

Studying the physiological properties of specific synapses in the brain, and how they undergo plastic changes, is a key challenge in modern neuroscience. Traditional in vitro electrophysiological techniques use electrical stimulation to evoke synaptic transmission. A major drawback of this method is its nonspecific nature; all axons in the region of the stimulating electrode will be activated, making it difficult to attribute an effect to a particular afferent connection. This issue can be overcome by replacing electrical stimulation with optogenetic-based stimulation. We describe a method for combining optogenetics with in vitro patch-clamp recordings. This is a powerful tool for the study of both basal synaptic transmission and synaptic plasticity of precise anatomically defined synaptic connections and is applicable to almost any pathway in the brain. Here, we describe the preparation and handling of a viral vector encoding channelrhodopsin protein for surgical injection into a pre-synaptic region of interest (medial prefrontal cortex) in the rodent brain and making of acute slices of downstream target regions (lateral entorhinal cortex). A detailed procedure for combining patch-clamp recordings with synaptic activation by light stimulation to study short- and long-term synaptic plasticity is also presented. We discuss examples of experiments that achieve pathway- and cell-specificity by combining optogenetics and Cre-dependent cell labeling. Finally, histological confirmation of the pre-synaptic region of interest is described along with biocytin labeling of the post-synaptic cell, to allow further identification of the precise location and cell type.

In this protocol, we describe how to study long-range synaptic physiology and plasticity using viral delivery of optogenetic constructs. The protocol can be very easily adapted to studying almost any long-range connection in the brain. As an example, we describe the injection of AAVs encoding an opsin into rat mPFC, the preparation of acute slices from LEC, patch-clamp recordings from layer 5 LEC pyramidal neurons, and light-evoked activation of mPFC terminals in LEC (Figure 1).
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Watch this Scientific Journal Video about Ex Vivo Optogenetic Interrogation of Long-Range Synaptic Transmission and Plasticity from Medial Prefrontal Cortex to Lateral Entorhinal Cortex at JoVE.com

Ex Vivo Optogenetic Interrogation of Long-Range Synaptic Transmission and Plasticity from Medial Prefrontal Cortex to Lateral Entorhinal Cortex

Here we present a protocol describing viral transduction of discrete brain regions with optogenetic constructs to permit synapse-specific electrophysiological characterization in acute rodent brain slices.

Ex Vivo Optogenetic Interrogation of Long-Range Synaptic Transmission and Plasticity from Medial Prefrontal Cortex to Lateral Entorhinal Cortex

Studying the physiological properties of specific synapses in the brain, and how they undergo plastic changes, is a key challenge in modern neuroscience. ...

Virally delivered optogenetic constructs permit detailed electrophysiological characterization of the physiology and plasticity of specific synapses in acute brain slices. The primary advantages of activating synapses optogenetically is the ability to study long-range pathways, and the selective stimulation of axons which are not anatomically separated. Demonstrating the procedure will be Dr.Lisa Kinnavane, a research associate from our laboratory.To begin, load a Hamilton syringe into a microinjection syringe pump attached to a movable arm mounted to a stereotaxic frame. Then place a five-microliter aliquot of the virus in a microcentrifuge. Spin the tube for a few seconds and pipette two microliters of the viral preparation into the tube's lid.To fill the syringe with the viral preparation, view the needle tip with a surgical microscope. Then manually place the bolus of the virus at the tip of the needle, and withdraw the syringe plunger using the pump controls. Set the pump injection volume to 300 nanoliters and flow rate to 100 nanoliters per minute.Run the pump and confirm proper flow by observing the droplet of the virus at the needle tip. Absorb the virus on a cotton bud, and clean the needle with 70%ethanol. Next, using the adjuster screws on the stereotaxic frame, navigate the needle tip to the bregma, and take note of the stereotaxic measurements observed on the three vernier scales on the frame.Add, or subtract these distances from bregma coordinates. Make a burr hole at the skull surface using a micro-drill mounted to the stereotaxic arm. Insert the needle into the brain at the predetermined dorsoventral coordinate, and infuse a predetermined volume of the viral preparation.After infusion, leave the needle in situ for 10 minutes to allow for the diffusion of the bolus. Then remove the needle, and run the pump to ensure that the needle is not blocked. Two weeks after the viral injection, euthanize the rat, rapidly dissect the entire brain, and transfer it into the sucrose cutting solution.Using a metal teaspoon, pick up the brain, discard the excess solution, and place it onto a filter paper. Using a scalpel, remove the cerebellum, and cut the cerebrum in the coronal plane approximately halfway along its length. The posterior half is the LEC tissue block.Return the tissue block and the remaining brain to the sucrose cutting solution. Next, place a drop of cyanoacrylate glue onto a vibratome stage, and spread it into a thin layer. Then using a teaspoon, pick the LEC tissue block, and transfer it onto the glue patch, such that the anterior coronal cut has adhered.Install the stage into the vibratome tissue chamber, and quickly pour sufficient sucrose cutting solution to submerge the tissue. After orienting the ventral surface of the tissue block toward the blade, cut 350-micrometer thick slices from ventral to dorsal using a slow blade advancement speed. Typically, seven slices can be obtained per hemisphere.Transfer the slices to the slice collection chamber. Then place the collection chamber in a 34 degree Celsius water bath for one hour before returning to room temperature. Place the remainder of the brain in paraformaldehyde for 48 hours.For target cell identification, place the slice into the recording chamber, and immobilize it using a slice anchor. Under low magnification, using infrared illumination, navigate to the LEC layer five. Then change to the high magnification water immersion objective, and identify the pyramidal neurons.Mark the position of the cell on the monitor with tape. Next, to form a whole cell patch clamp, fabricate a borosilicate glass micro-pipette, and fill it with intracellular recording solution. Place the filled micro-pipette in the electrode holder and apply positive pressure by blowing hard into a mouthpiece connected to the electrode holder side port.Raise the microscope objective such that a meniscus forms and insert the electrode into the meniscus until it can be seen on the microscope. Open the Seal Test window, and determine whether the pipette resistance is three to five megaohms. Then touch the identified cell with a pipette tip, resulting in an indentation in the cell membrane.Next, apply negative pressure by suction at the mouthpiece, increasing pipette resistance. Continue applying negative pressure gradually until the cell membrane ruptures, resulting in whole cell capacitance transients. To enter the current clamp configuration, use a mounted LED directed into the microscope light path with filter cubes and appropriate optics, and apply light pulses to the slice via the 40X objective.Deliver trains of multiple light pulses at five hertz, 10 hertz, and 20 hertz to investigate the presynaptic release properties. To allow biocytin to fill the neuron, wait for at least 15 minutes after entering the whole cell configuration. In voltage clamp, monitor the membrane capacitance and input resistance.Then slowly withdraw the pipette along the approach angle away from the cell's soma, observing the slow disappearance of capacitance transients and membrane current, indicating the resealing of the cell membrane and formation of an outside-out patch at the pipette tip. Place the slice into paraformaldehyde in a 24-well plate and incubate overnight. Using OCT medium, attach a tissue block to the cryostat specimen disc.To freeze the tissue, place isopentane in an appropriate container and submerge the specimen disc, ensuring the tissue is above the level of the isopentane. Then lower the container of isopentane into liquid nitrogen, and allow the tissue to freeze. Once the tissue is completely frozen, leave the tissue block in the cryostat chamber at minus 20 degrees Celsius for 30 minutes to allow the temperature of the block to equilibrate.After equilibration, cut 40-micrometer thick sections in the cryostat, and use a fine paintbrush to guide the sections off the blade. Adhere these frozen sections to a room temperature poly-L-lysine-coated glass microscope slide by touching the slide to the sections. After adding 150 microliters of mounting medium to each slide, apply the cover slip, and remove any air bubbles by gently pressing on the cover slip.Cover the slides to protect against photobleaching, and let them air dry for 12 hours. Then using a fluorescence microscope, examine the location of the viral injection site. For biocytin staining, incubate the slices in 3%hydrogen peroxide in PBS for 30 minutes to block any endogenous peroxidase activity.Then wash the brain slices with PBS until no further oxygen bubbles are visible. Next, incubate the slices for three hours in 1%avidin-biotinylated HRP complex solution in PBS containing 0.1%Triton X-100. After six PBS washes, incubate each slice in DAB solution until the biocytin staining of neuronal structures becomes visible.Stop the reaction by transferring the slices in cold PBS. Then mount the slices onto the glass microscope slides using a brush. After removing excess PBS, add mounting medium.Cover the slices using cover slips, and let them air dry, as demonstrated earlier. A healthy pyramidal cell was located and patched. If post-synaptic cell identification is required, a cell expressing the fluorescent marker should be localized using widefield optics.Single light pulses of two milliseconds resulted in simple waveform optogenetic excitatory post-synaptic potentials. Short-term plasticity of the synapse is examined by applying five, 10, and 20 hertz trains of light stimulation. While investigating long-term plasticity, adding cholinergic agonist carbachol to the circulating aCSF for 10 minutes caused a long-term depression that was still evident 40 minutes after removal of the ligand.The length of the injection site was examined histologically. The fluorescent reporter mCherry was localized to the deeper layers of the prelimbic and infralimbic cortex. Further, staining a biocytin-filled cell confirmed its location and morphology.The critical steps of this protocol are accurate transduction of the afferent brain region, which can be verified post hoc, and rapid dissection of the brain when preparing acute slices. This procedure is easily combined with fluorescent labeling of an individual cell type, such as a particular interneuron sub-class. To do this, we'd use a transgenic animal that expresses a recombinase in that particular cell type.Virally delivered optogenetic constructs have allowed rapid advancements in the fields of systems and circuit neuroscience.

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Research

JoVE Journal

Neuroscience

Correlating Behavioral Responses to fMRI Signals from Human Prefrontal Cortex: Examining Cognitive Processes Using Task Analysis

The aim of this methods paper is to describe how to implement a neuroimaging technique to examine complementary brain processes engaged by two similar tasks. Participants' behavior during task performance in an fMRI scanner can then be correlated to the brain activity using the blood-oxygen-level-dependent signal. We measure behavior to be able to sort correct trials, where the subject performed the task correctly and then be able to examine the brain signals related to correct performance. Conversely, if subjects do not perform the task correctly, and these trials are included in the same analysis with the correct trials we would introduce trials that were not only for correct performance. Thus, in many cases these errors can be used themselves to then correlate brain activity to them. We describe two complementary tasks that are used in our lab to examine the brain during suppression of an automatic responses: the stroop1 and anti-saccade tasks. The emotional stroop paradigm instructs participants to either report the superimposed emotional 'word' across the affective faces or the facial 'expressions' of the face stimuli1,2. When the word and the facial expression refer to different emotions, a conflict between what must be said and what is automatically read occurs. The participant has to resolve the conflict between two simultaneously competing processes of word reading and facial expression. Our urge to read out a word leads to strong 'stimulus-response (SR)' associations; hence inhibiting these strong SR's is difficult and participants are prone to making errors. Overcoming this conflict and directing attention away from the face or the word requires the subject to inhibit bottom up processes which typically directs attention to the more salient stimulus. Similarly, in the anti-saccade task3,4,5,6, where an instruction cue is used to direct only attention to a peripheral stimulus location but then the eye movement is made to the mirror opposite position. Yet again we measure behavior by recording the eye movements of participants which allows for the sorting of the behavioral responses into correct and error trials7 which then can be correlated to brain activity. Neuroimaging now allows researchers to measure different behaviors of correct and error trials that are indicative of different cognitive processes and pinpoint the different neural networks involved.

The goal of our research is to correlate behavior to brain activity. Accurate behavioral measures and imaging techniques allow us to elucidate brain-behavior relationships.

The aim of this methods paper is to describe how to implement a neuroimaging technique to examine complementary brain processes engaged by two similar ...

Methods for Study of Neuronal Morphogenesis: Ex vivo RNAi Electroporation in Embryonic Murine Cerebral Cortex

The cerebral cortex directs higher cognitive functions. This six layered structure is generated in an inside-first, outside-last manner, in which the first born neurons remain closer to the ventricle while the last born neurons migrate past the first born neurons towards the surface of the brain1. In addition to neuronal migration2, a key process for normal cortical function is the regulation of neuronal morphogenesis3. While neuronal morphogenesis can be studied in vitro in primary cultures, there is much to be learned from how these processes are regulated in tissue environments.
We describe techniques to analyze neuronal migration and/or morphogenesis in organotypic slices of the cerebral cortex4,6. A pSilencer modified vector is used which contains both a U6 promoter that drives the double stranded hairpin RNA and a separate expression cassette that encodes GFP protein driven by a CMV promoter7-9. Our approach allows for the rapid assessment of defects in neurite outgrowth upon specific knockdown of candidate genes and has been successfully used in a screen for regulators of neurite outgrowth8. Because only a subset of cells will express the RNAi constructs, the organotypic slices allow for a mosaic analysis of the potential phenotypes. Moreover, because this analysis is done in a near approximation of the in vivo environment, it provides a low cost and rapid alternative to the generation of transgenic or knockout animals for genes of unknown cortical function. Finally, in comparison with in vivo electroporation technology, the success of ex vivo electroporation experiments is not dependant upon proficient surgery skill development and can be performed with a shorter training time and skill.

Methods for Study of Neuronal Morphogenesis: Ex vivo RNAi Electroporation in Embryonic Murine Cerebral Cortex

To conduct a rapid assessment of the function of genes in the development of cerebral cortex, we describe methods involving the ex vivo electroporation of plasmids co-expressing inhibitory RNA (RNAi) and GFP in murine embryonic cortex. This protocol is amenable to the study of various aspects of neurodevelopment such as neurogenesis, neuronal migration and neuronal morphogenesis including dendrite and axon outgrowth.

The cerebral cortex directs higher cognitive functions. This six layered structure is generated in an inside-first, outside-last manner, in which the ...

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

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

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

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

Preparation of Parasagittal Slices for the Investigation of Dorsal-ventral Organization of the Rodent Medial Entorhinal Cortex

Computation in the brain relies on neurons responding appropriately to their synaptic inputs. Neurons differ in their complement and distribution of membrane ion channels that determine how they respond to synaptic inputs. However, the relationship between these cellular properties and neuronal function in behaving animals is not well understood. One approach to this problem is to investigate topographically organized neural circuits in which the position of individual neurons maps onto information they encode or computations they carry out1. Experiments using this approach suggest principles for tuning of synaptic responses underlying information encoding in sensory and cognitive circuits2,3.
The topographical organization of spatial representations along the dorsal-ventral axis of the medial entorhinal cortex (MEC) provides an opportunity to establish relationships between cellular mechanisms and computations important for spatial cognition. Neurons in layer II of the rodent MEC encode location using grid-like firing fields4-6. For neurons found at dorsal positions in the MEC the distance between the individual firing fields that form a grid is on the order of 30 cm, whereas for neurons at progressively more ventral positions this distance increases to greater than 1 m. Several studies have revealed cellular properties of neurons in layer II of the MEC that, like the spacing between grid firing fields, also differ according to their dorsal-ventral position, suggesting that these cellular properties are important for spatial computation2,7-10.
Here we describe procedures for preparation and electrophysiological recording from brain slices that maintain the dorsal-ventral extent of the MEC enabling investigation of the topographical organization of biophysical and anatomical properties of MEC neurons. The dorsal-ventral position of identified neurons relative to anatomical landmarks is difficult to establish accurately with protocols that use horizontal slices of MEC7,8,11,12, as it is difficult to establish reference points for the exact dorsal-ventral location of the slice. The procedures we describe enable accurate and consistent measurement of location of recorded cells along the dorsal-ventral axis of the MEC as well as visualization of molecular gradients2,10. The procedures have been developed for use with adult mice (&gt; 28 days) and have been successfully employed with mice up to 1.5 years old. With adjustments they could be used with younger mice or other rodent species. A standardized system of preparation and measurement will aid systematic investigation of the cellular and microcircuit properties of this area.

We describe procedures for preparation and electrophysiological recording from brain slices that maintain the dorsal-ventral axis of the medial entorhinal cortex (MEC). Because neural encoding of location follows a dorsal-ventral organization within the MEC, these procedures facilitate investigation of cellular mechanisms important for navigation and memory.

Computation in the brain relies on neurons responding appropriately to their synaptic inputs. Neurons differ in their complement and distribution of ...

Simultaneous Two-photon In Vivo Imaging of Synaptic Inputs and Postsynaptic Targets in the Mouse Retrosplenial Cortex

This video shows the craniotomy procedure that allows chronic imaging of neurons in the mouse retrosplenial cortex (RSC) using in vivo two-photon microscopy in Thy1-GFP transgenic mouse line. This approach creates a possibility to investigate the correlation of behavioural manipulations with changes in neuronal morphology in vivo.
The cranial window implantation procedure was considered to be limited only to the easily accessible cortex regions such as the barrel field. Our approach allows visualization of neurons in the highly vascularized RSC. RSC is an important element of the brain circuit responsible for spatial memory, previously deemed to be problematic for in vivo two-photon imaging.
The cranial window implantation over the RSC is combined with an injection of mCherry-expressing recombinant adeno-associated virus (rAAVmCherry) into the dorsal hippocampus. The expressed mCherry spreads out to axonal projections from the hippocampus to RSC, enabling the visualization of changes in both presynaptic axonal boutons and postsynaptic dendritic spines in the cortex. 
This technique allows long-term monitoring of experience-dependent structural plasticity in RSC.

Simultaneous Two-photon In Vivo Imaging of Synaptic Inputs and Postsynaptic Targets in the Mouse Retrosplenial Cortex

This video shows the craniotomy procedure that allows chronic imaging of neurons in mouse retrosplenial cortex using in vivo two photon microscopy in Thy1-GFP transgenic line. This approach is combined with injection of mCherry-expressing adeno-associated virus into dorsal hippocampus. These techniques allow long-term monitoring of experience-dependent structural plasticity in RSC.

This video shows the craniotomy procedure that allows chronic imaging of neurons in the mouse retrosplenial cortex (RSC) using in vivo two-photon ...

Pentylenetetrazole-Induced Kindling Mouse Model

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

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

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

Combined Transcranial Magnetic Stimulation and Electroencephalography of the Dorsolateral Prefrontal Cortex

Transcranial magnetic stimulation (TMS) is a non-invasive method that produces neural excitation in the cortex by means of brief, time-varying magnetic field pulses. The initiation of cortical activation or its modulation depends on the background activation of the neurons of the cortical region activated, the characteristics of the coil, its position and its orientation with respect to the head. TMS combined with simultaneous electrocephalography (EEG) and neuronavigation (nTMS-EEG) allows for the assessment of cortico-cortical excitability and connectivity in almost all cortical areas in a reproducible manner. This advance makes nTMS-EEG a powerful tool that can accurately assess brain dynamics and neurophysiology in test-retest paradigms that are required for clinical trials. Limitations of this method include artifacts that cover the initial brain reactivity to stimulation. Thus, the process of removing artifacts may also extract valuable information. Moreover, the optimal parameters for dorsolateral prefrontal (DLPFC) stimulation are not fully known and current protocols utilize variations from the motor cortex (M1) stimulation paradigms. However, evolving nTMS-EEG designs hope to address these issues. The protocol presented here introduces some standard practices for assessing neurophysiological functioning from stimulation to the DLPFC that can be applied in patients with treatment resistant psychiatric disorders that receive treatment such as transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), magnetic seizure therapy (MST) or electroconvulsive therapy (ECT).

The protocol presented here is for TMS-EEG studies utilizing intracortical excitability test-retest design paradigms. The intent of the protocol is to produce reliable and reproducible cortical excitability measures for assessing neurophysiological functioning related to therapeutic interventions in the treatment of neuropsychiatric diseases such as major depression.

Transcranial magnetic stimulation (TMS) is a non-invasive method that produces neural excitation in the cortex by means of brief, time-varying magnetic ...

In Vivo Intracerebral Stereotaxic Injections for Optogenetic Stimulation of Long-Range Inputs in Mouse Brain Slices

Knowledge of cell-type specific synaptic connectivity is a crucial prerequisite for understanding brain-wide neuronal circuits. The functional investigation of long-range connections requires targeted recordings of single neurons combined with the specific stimulation of identified distant inputs. This is often difficult to achieve with conventional and electrical stimulation techniques, because axons from converging upstream brain areas may intermingle in the target region. The stereotaxic targeting of a specific brain region for virus-mediated expression of light-sensitive ion channels allows selective stimulation of axons originating from that region with light. Intracerebral stereotaxic injections can be used in well-delimited structures, such as the anterior thalamic nuclei, in addition to other subcortical or cortical areas throughout the brain.
Described here is a set of techniques for precise stereotaxic injection of viral vectors expressing channelrhodopsin in the mouse brain, followed by photostimulation of axon terminals in the brain slice preparation. These protocols are simple and widely applicable. In combination with whole-cell patch clamp recording from a postsynaptically connected neuron, photostimulation of axons allows the detection of functional synaptic connections, pharmacological characterization, and evaluation of their strength. In addition, biocytin filling of the recorded neuron can be used for post-hoc morphological identification of the postsynaptic neuron.

This protocol describes a set of methods to identify the cell-type specific functional connectivity of long-range inputs from distant brain regions using optogenetic stimulations in ex vivo brain slices.

Knowledge of cell-type specific synaptic connectivity is a crucial prerequisite for understanding brain-wide neuronal circuits. The functional ...

Rapid Golgi Stain for Dendritic Spine Visualization in Hippocampus and Prefrontal Cortex

Golgi impregnation, using the Golgi staining kit with minor adaptations, is used to impregnate dendritic spines in the rat hippocampus and medial prefrontal cortex. This technique is a marked improvement over previous methods of Golgi impregnation because the premixed chemicals are safer to use, neurons are consistently well impregnated, there is far less background debris, and for a given region, there are extremely small deviations in spine density between experiments. Moreover, brains can be accumulated after a certain point and kept frozen until further processing. Using this method any brain region of interest can be studied. Once stained and cover slipped, dendritic spine density is determined by counting the number of spines for a length of dendrite and expressed as spine density per 10 &#181;m dendrite.

The protocol describes a modification of the rapid Golgi method, which can be adapted to any part of the nervous system, for staining neurons in the hippocampus and medial prefrontal cortex of the rat.

Golgi impregnation, using the Golgi staining kit with minor adaptations, is used to impregnate dendritic spines in the rat hippocampus and medial ...

Organotypic Cultures of Adult Human Cortex as an Ex vivo Model for Human Stem Cell Transplantation and Validation

Neurodegenerative disorders are common and heterogeneous in terms of their symptoms and cellular affectation, making their study complicated due to the lack of proper animal models that fully mimic human diseases and the poor availability of post-mortem human brain tissue. Adult human nervous tissue culture offers the possibility to study different aspects of neurological disorders. Molecular, cellular, and biochemical mechanisms could be easily addressed in this system, as well as testing and validating drugs or different treatments, such as cell-based therapies. This method combines long-term organotypic cultures of the adult human cortex, obtained from epileptic patients undergoing resective surgery, and ex vivo intracortical transplantation of induced pluripotent stem cell-derived cortical progenitors. This method will allow the study of cell survival, neuronal differentiation, the formation of synaptic inputs and outputs, and the electrophysiological properties of human-derived cells after transplantation into intact adult human cortical tissue. This approach is an important step prior to the development of a 3D human disease modeling platform that will bring basic research closer to the clinical translation of stem cell-based therapies for patients with different neurological disorders and allow the development of new tools for reconstructing damaged neural circuits.

Organotypic Cultures of Adult Human Cortex as an Ex vivo Model for Human Stem Cell Transplantation and Validation

This protocol describes long-term organotypic cultures of adult human cortex combined with ex vivo intracortical transplantation of induced pluripotent stem cell-derived cortical progenitors, which present a novel methodology to further test stem cell-based therapies for human neurodegenerative disorders.

Neurodegenerative disorders are common and heterogeneous in terms of their symptoms and cellular affectation, making their study complicated due to the ...