Monitoring the Brain Cortex with a Multi-Electrode Array in Response to Seizure-Inducing Agents

研究方案

All procedures involving animal samples have been reviewed and approved by the appropriate animal ethical review committee.

1. Preparing Experimental Solution and Equipment for MEA (Multielectrode Array) Recording

1. Prepare artificial cerebral spinal fluid (aCSF; 124 mM NaCl, 4.4 mM KCl, 1 mM NaH₂PO₃, 2 mM MgSO₄, 2 mM CaCl₂, 25 mM NaHCO₃, and 10 mM glucose, bubbled with 95% O₂ and 5% CO₂).
2. Use two types of MEA probes: 6 x 10 planar MEA and 8 x 8 MEA. The former probe covers the region that comprises the cortex, striatum, and thalamus. The latter probe covers only the cortical region.
3. Use a 60-channel amplifier with a band-pass filter set between 0.1 Hz and 3 kHz at 1,200 amplification. Acquire data at a 10 kHz sampling rate.
4. Place two AgCl-coated silver wires inside the MEA chamber for DCS( Direct current stimulation). Then, use the AgCl-coated silver wires to produce electric fields generated by an isolated stimulator.
5. Place a tungsten electrode (diameter, 127 µm; length, 7.62 cm; 8° AC tapered tip; resistance, 5 MΩ) for thalamic stimulation, and place the reference electrode in the MEA chamber. Deliver the tungsten electrode's currents using an isolated stimulator that is controlled by a pulse generator (Figure 1).

2. Brain Slice Preparation

1. Use male C57BL/6J mice, 4-8 weeks old. House the animals in an air-conditioned room (21-23 °C; 50% humidity; 12 hr/12 hr light /dark cycle, lights on at 8:00 AM) with free access to food and water.
2. Take a 250 ml aliquot of the aCSF prepared in Step 1.1 and place it in an ice-filled beaker. At the same time, supply continuous gas composed of 95% O₂ and 5% CO₂.
3. Surgery
   1. Anesthetize the animal with 4% isoflurane in a glass box for approximately 3 min. Once the animal reaches a surgical depth of anesthesia (indicated by the lack of a response to toe pinch), place it on a shallow tray that is filled with crushed ice and remove the head using scissors.
   2. Expose the skull and trim off the remaining muscle. Next, using rongeurs, peel away the dorsal surface of the skull from the brain. Trim away the sides of the skull using rongeurs. Sterilize all of the surgical instruments with a 75% ethanol solution.
   3. Using a spatula, cut the olfactory bulbs and nerve connections along the ventral surface of the brain and remove the brain. After decapitation, and quickly transfer the brain to a beaker filled with ice-cold oxygenated aCSF.
4. Preparation of Medial Thalamus (MT)-ACC (Anterior cingulate cortex) Brain Slice
   Note: Prepare slices that contain the pathway from the MT to ACC
   1. Hand-cut the brain block with two sagittal cuts 2.0 mm lateral to the midline in each hemisphere to display the subcortical anatomy. Then, make two angled cuts. Make the first cross-cut parallel to the visible fiber tract in the striatum.
   2. Make the second cross-cut from the connection between the cerebellum and visual cortex to the midpoint between the anterior commissure and optic tract that are ventral and parallel to the thalamocingulate pathway.
   3. Attach the brain block to an angular plate (~120°) with cyanoacrylate adhesive and make a cut just above the turning point of the pathway. Unfold the plate, flatten it, and glue it onto the chamber stage of a vibratome.
   4. Make medial thalamus-ACC brain slices (500 µm thick) and then immerse them in ice-cold oxygenated aCSF.Transfer slices to the recording chamber and keep at 32 °C under continuous perfusion (12 ml/min) with oxygenated aCSF for 1 hr.

3. Preparation of Perfusion Chamber for Multielectrode Array Recording

1. Preparation of Perfusion Chamber
   1. Place an MEA probe on a multi-channel system, and use two separate polyethylene tubes to connect the probe to a peristaltic pump. Use one tube to guide the aCSF into the MEA chamber and the other tube to guide the aCSF out of the chamber. Finally, continuously perfuse the preparation with warm (29-30 °C) oxygenated aCSF (8 ml/min).
2. Transfer brain slice to MEA. Hold down the brain slice on the MEA using a wet cotton swab. Carefully move the brain slice to ensure the ACC is oriented above the electrodes.
3. Use slice anchor kits and hold-downs to press the brain slice. This step ensures a good electrical connection between the slice and electrodes.

4. Generation of Electric Fields by DCS

Note: The definition of the electric field orientation was based on the direction of the axodendritic axis in the ACC. The orientations of dendrite and soma compartments were confirmed using Golgi staining12.

1. Place the AgCl electrode (defined as the anode) proximal to the ACC and place the other electrode (defined as the cathode) distal to the ACC. Record the field strength that is generated by the two field orientations (parallel and perpendicular to the ACC axodendritic fibers) by the MEA, and deliver the currents of the electric fields using a stimulator.
2. Fix the distance of the AgCl electrodes (about 1.5-2 cm), and adjust the stimulator's current strength to make the DCS between 0.5 and 2 mA.

5. Electrically-induced Cortical Synaptic Responses

Note: Induce synaptic responses in the ACC by electrical stimulation in the MT, in which a programmable electrical stimulus generator produces rectangular biphasic current pulses.

1. Repeat Section 3 above.
2. Place a tungsten electrode in the MT, and deliver pulses from the stimulator to the thalamic region of the slices via bipolar tungsten electrodes.
3. Use various current intensities to determine the threshold that elicits an ACC response. Here, use an intensity of ±150 µA and duration of 200 µsec, which elicited an 80% maximal response in the ACC in most slices.
4. Move the tungsten electrode along the thalamocingulate pathway (from MT to corpus callosum) in the MT-ACC slice to obtain the optimal response profiles.
5. Make 10-20 sweeps of ACC responses, and use the software to automatically average all of the ACC evoked by MT stimulation. The result iss the synaptic responses in ACC induced from MT stimulation by MT-ACC pathway.

6. Electrically-induced Seizure-like Activity

Note: Seizure-like activity was induced by the application of 4-aminopyridine (4-AP; 250 µM) and bicuculline (5 µM). Previous time-control studies showed that maximal and stable responses appeared 2-3 hr after drug application14.

1. Repeat Section 5 above.
2. Add drugs to the perfusion solution. Use 4-AP (4-Aminopyridine)(250 µM) and bicuculline (5 µM). Mix the drugs uniformly and continue perfusion for 2-3 hr.
3. To facilitate seizure-like activity, maintain the perfusion pump at a relatively fast perfusion rate (8 ml/min), which can also help prevent the build-up of a pH gradient.
4. Place a tungsten electrode in the MT, and deliver electrical stimulation (150 µA, 200 µsec duration) to obtain ACC response profiles.
5. Make 10-20 sweeps and average the responses.
6. Replace the perfusion solution with fresh aCSF to wash out the drugs. Repeat Step 6.5.

结果

Figure 1: Preparation of the Thalamocingulate Slice and MEA Recording System Setup. (A) MT-ACC slice procedure (a) Remove the brain and (b) transfer to cool oxygenated aCSF. (c) Make two parasagittal cuts from the midline. (d) Side view of the medial part of the brain block. (e) Make two angled ventral cuts of the brain block. (f) Glue the brain block onto an angled plastic plate. (g) Make a dors...

材料

Name	Company	Catalog Number	Comments
Isoflurane	Halocarbon Products Corporation	NDC 12164-002-25	4%
aCSF (total:1 L):
D(+)-Glucose	MERCK	1.08337.1000	10 mM
Sodium hydrogen carbonate	MERCK	1.06329.0500	25 mM
Sodium chloride	MERCK	1.06404.1000	124 mM
(+)-Sodium L-ascorbate, >=98%	SIGMA	A4034-100G	0.15 g/2 c.c
Magnesium sulfate, anhydrous, ReagentPlus	SIGMA	M7506-500G	2 mM
Calcium chloride dihydrate	MERCK	1.02382.1000	2 mM
Sodium dihydrogen phosphate monohydrate	MERCK	1.06346.1000	1 mM
Potassium chloride	May & Baker LTD Dagenham England	MS 7616	4.4 mM
(+)-Bicuculline	TOCRIS	130	5 µM in aCSF
4-Aminopyridine	TOCRIS	940	250 µM in aCSF
Vibratome	Vibratome	Series 1000	Block slicing into 500 µm thick slices
Multielectrode array (MEA) probes: 6 x 10 planar MEA	Multi Channel Systems	60MEA500/30iR-Ti-pr MEAS 6x10	electrode diameter, 30 µm; electrode spacing, 500 µm; impedance, 50 kΩ at 200 Hz
Multielectrode array (MEA) probes: 8 x 8 MEA	Ayanda Biosystems	60MEA200/10iR-Ti-pr MEAS 8x8	pyramidal-shaped electrode; diameter, 40 µm; tip height, 50 µm; electrode spacing, 200 µm; impedance, 1,000 kΩ at 200 Hz
A 60-channel amplifier was used with a band-pass filter set between 0.1 Hz and 3 KHz at 1,200X amplification	Multi-Channel Systems	MEA-1060-BC
MC Rack software at a 10 KHz sampling rate	Multi-Channel Systems		Software for data collect and recordings
control of a pulse generator	Multi-Channel Systems	STG 1002
slice anchor kits and hold-downs	Warner Instruments	SHD-26H/10; WI64-0250
Peristaltic Pump-minipuls3	Gilsom	MINIPULS3	perfusion rate : 8 ml/min
Isolated stimulator	A-M Systems	Model 2100	intensity of ±350 μA , duration of 200 μsec
Tungsten electrode	A-M Systems	575300	placed in thalamus