Recording Large-scale Neuronal Ensembles with Silicon Probes in the Anesthetized Rat

The overall goal of this procedure is to describe an acute, large scale neuronal recording from the left and right fore limb somatosensory cortex, simultaneously in an anesthetized rat using a silicon probe. This is accomplished by first anesthetizing the animal with urethane, and later by placing the rat in the stereotaxic frame to make a two to three centimeter incision over the skull. The second step is to place the ground and reference screws.

Then two craniotomies are made above the somatosensory courtesies and the dura mater is removed. The final step is to carefully introduce the silicone probes into the brain and lower them slowly to the desired area. This procedure allows the recording of local field potentials and spiking activity from tens of neurons.

The main advantage of this technique is that silicone probes allows to record from large number of recording sites. Moreover, multiple recording sites can be arranged over distance of millimeters, allowing to record simultaneously across multiple cortical columns or across multiple layers in one cortical column. Importantly, geometrically precise distribution of recording sites allows to investigate special relations between neuronal populations.

This method can help answer key questions in the neuroscience field, such as how neuronal population activity is involved in memory plasticity, stimulus coding, decision making, as well as how these neural interactions are affected by different drugs. To begin this procedure, anesthetize the rat with iso fluorine at a concentration of three to 4%and maintain at 2%to prevent the animal from getting stressed by injection. Then inject 7.5 milliliters per kilogram of 20%diluted urethane solution intraperitoneal, and divide this injection into four applications each separated by approximately 30 minute intervals.

Next, clean the silicone probes with distilled water under the microscope. Paint the probes with a fluorescent marker for marking the positions of the probes in the histological analysis. This procedure should always be done under a microscope to make sure the probes are not being damaged or bent.

After that, shave the fur along the incision site. Then apply ointment to both eyes. Administer five milligrams per milliliter of Dexamethasone subcutaneously to reduce brain inflammation.

At this point, fix the animal in the stereotaxic frame. Next, prepare the surgical site antiseptically using chlorhexidine soap and ethanol before making the incision administer lidocaine and epinephrine subcutaneously at the incision site for local anesthesia, and to reduce the potential risk of bleeding. Then make a two centimeter incision along the scalp midline, and remove the skin.

To expose the skull, clear the dorsal scalp from the fascia by blunt dissection and control bleeding using bone wax. Next, drill two holes in the occipital bone of the skull. Attach the small screws to the holes for ground and reference electrodes.

The reference screw must be in contact with the dura. After that, make two one to two millimeter diameter craniotomies in the coordinates corresponding to the fore limb. Somatosensory cortices.

Use compressed air to remove the bone dust as needed during craniotomy to prevent the skull warming up from drilling. Apply drops of saline solution. Then build a small barrier of acrylic resin surrounding the two craniotomies in order to keep PBS on top of the exposed dura, and to prevent it from drying.

Apply PBS constantly during the whole experiment. Next, use two 30 and a half gauge needles with bent tips to remove the dura mater of both craniotomies. At this point, place the tips of the silicon probes just in contact with the pia.

Then slowly introduce the silicon probes, making sure that their shanks can be inserted into the brain without resistance or bending. Carefully lower the probes until they reach the desired recording areas. If needed, add drops of PBS on the top of the craniotomies to prevent the tissue from drying.

Shown here is a representative electrophysiological recording of the local field potential for 500 milliseconds from one tero. An example of the spike wave form is in the insert shown. Here are examples of the two dimensional views of unit clusters obtained from one tetro.

Each cluster represents the spikes from a single neuron. The averaged spike wave forms from the representative units at each of the four channels from a single tetro are shown here. The waveform energy of the recorded spikes over time is plotted in this graph.

Note that the relatively constant energy value for each unit indicates stable recording over the displayed period. After watching this video, you should be able to perform silicon probe recordings in multiple brain areas from more than a hundred neural simultaneously. This is achieved by completing the following steps and izing the animal, placing the reference and ground screws, drilling the craniotomies, removing dura, and finally inserting the pros in the desired recording area.