Effects of phencyclidine (PCP) on the visual evoked potentials in the rhesus monkey

Effects of acute and chronic administration of phencyclidine (PCP) on both neocortical and subcortical visual potentials (VEPs) and on spontaneous EEGs were studied in the rhesus monkeys with permanently implanted brain electrodes. VEPs were evoked by brief single photo-stimulator flashes (0.8 pps. 10 microsec duration). Injection of PCP (0.5 to 4.0 mg/kg doses, IV) in monkeys produced a significant inhibition on the peak-amplitude of major VEP components predominantly in the occipital lobe and hippocampus. The PCP-induced VEP inhibition persisted in the presence of occipital and hippocampal theta-activities. Nystagmus persisted throughout the 6 to 8 hours course of PCP-induced behavior. A biphasic pattern of inhibitory and excitatory effects on EEGs and behavior was also observed during the 6 to 8 hours observation period. Chronic administration of PCP (2.0 and 4.0 mg/kg dose IV daily) produced a significant decrease in its inhibitory effects on VEPs, suggesting tolerance development to the inhibitory effect on VEPs. The results suggest that the hippocampus has important implications in the modulation of PCP effects on CNS activities related to the visual function of the rhesus monkey.