Single-unit In vivo Recordings from the Optic Chiasm of Rat

Summary

Retinal ganglion cells transmit visual information from the eye to the brain with sequences of action potentials. Here, we demonstrate how to record the action potentials of single ganglion cells in vivo from anesthetized rats.

Abstract

Information about the visual world is transmitted to the brain in sequences of action potentials in retinal ganglion cell axons that make up the optic nerve. In vivo recordings of ganglion cell spike trains in several animal models have revealed much of what is known about how the early visual system processes and encodes visual information. However, such recordings have been rare in one of the most common animal models, the rat, possibly owing to difficulty in detecting spikes fired by small diameter axons. The many retinal disease models involving rats motivate a need for characterizing the functional properties of ganglion cells without disturbing the eye, as with intraocular or in vitro recordings. Here, we demonstrate a method for recording ganglion cell spike trains from the optic chiasm of the anesthetized rat. We first show how to fabricate tungsten-in-glass electrodes that can pick up electrical activity from single ganglion cell axons in rat. The electrodes outperform all commercial ones that we have tried. We then illustrate our custom-designed stereotaxic system for in vivo visual neurophysiology experiments and our procedures for animal preparation and reliable and stable electrode placement in the optic chiasm.

Protocol

Part I: Fabrication of Tungsten-in-Glass Electrodes

1. A graphite ring is placed in a beaker filled with a concentrated solution of sodium nitrate (0.71g/mL) and potassium hydroxide (0.34g/mL) dissolved in water¹, and the beaker is placed under the sewing arm of a modified sewing machine.
2. Tungsten wire (0.5mm diameter, 6cm length) is taped to the sewing arm so that it is partially immersed (1-2cm) in solution.
3. 2V are applied across the tungsten wire by attaching a positive lead to the sewing arm and a negative lead to the graphite ring.
4. The sewing machine is turned on and the tungsten wire is dipped repetit....

Discussion

Optic fiber recordings are an attractive approach for addressing experimental questions about retinal information encoding and transmission that require an intact eye. Moreover, the signaling properties of both eyes can be studied in virtually the same physiological state if the electrode is positioned in the optic chiasm or tract where the activity of crossed and uncrossed optic nerve fibers can be recorded with a single electrode penetration. Optic fiber recordings are common in cat but not in other popular animals mod.......