A procedure for recording electroretinogram and visual evoked potential in freely moving cats

A procedure for recording the electroretinogram (ERG) and the visual evoked potential (VEP) in freely moving cats was developed and used to test the visual toxicity of nalidixic acid. The electrodes for recording the ERG and VEP were implanted chronically in the sclera of the eye and on the surface of the visual cortex, respectively. Both the ERG and VEP were simultaneously recorded in freely moving cats using a slip ring that allows the cat to move freely without twisting the cables. After a 15-min dark-adaptation period, 20 responses to repetitive photic stimulation with an intensity of 2 joules and a 10-sec interstimulus interval were averaged by means of a minicomputer. The waveform of the ERG consisted of a negative wave (a-wave) followed by a positive wave (b-wave) with 2 to 3 oscillatory potentials on the rising slope. The VEP consisted of initial positive wave (P1 wave) followed by a negative wave (N1 wave) and late positive and negative deflections. The amplitude of the components of the ERG and VEP varied considerably from animal to animal, whereas within the same animal they were very stable. Using this technique, the effects of nalidixic acid, a retinotoxic compound, on the ERG and VEP were studied. Nalidixic acid produced a marked, but transient, decrease in the amplitude of the ERG b-wave after a single intravenous injection at a dose of 10 mg/kg. After repeated doses at 40 mg/kg/day for 2 weeks, the reduction in the amplitude of the b-wave was stronger, but remained transient. Upon ophthalmologic and histopathologic examination of the retina, no abnormalities indicating visual toxicity were observed. These results indicate that this procedure for recording the ERG and VEP is useful for evaluating the visual toxicity of drugs in freely moving cats.