Effects of cocaine on sensory inhibition in rats: preliminary data.

The purpose of this study was to determine the effect of cocaine on inhibitory sensory processing mechanisms in the brain. To accomplish this aim, recording electrodes were surgically placed into the vertex region of 12 rats. After recovery from surgery, rats were injected once daily for 5 days with either cocaine (20 mg/kg, IP) or saline. Immediately and 23 hr after each injection, the rats were tested for sensory gating mechanisms. They were presented with a series of two clicking sounds, a conditioning and testing click, delivered 0.5 sec apart, and the amplitude of the N40 responses to each of these clicks was recorded. The ratios of the amplitude of the N40 response to the testing click over that of the conditioning click (T/C ratio) were calculated for each animal for each testing period. The T/C ratios of the control (Saline-injected) animals were less than one, indicating that the conditioning stimulus was able to activate inhibitory neural pathways, producing a decrease in the response to the testing stimulus. The T/C ratios of the cocaine-treated animals were significantly greater than those of controls when the tests were conducted either immediately after injection or 23 hr later. These observations suggest that cocaine can impair mechanisms involved in the gating of responses to auditory stimuli. The higher T/C ratio found at 23 hr after cocaine injection suggests that an impairment in the gating mechanism may be produced by an arousal response that is associated with the environment in which the animals had been injected with cocaine.