Spontaneous and elicited PGO spikes in rats.

High-amplitude waves, similar in distribution and configuration to ponto-geniculo-occipital (PGO) spikes recorded in cats, were recorded from the area of the locus coeruleus of chronically implanted, unanesthetized albino rats. The mean frequency of spiking during paradoxical sleep (PS) was 19 per min and the amplitudes of the spikes ranged from 0.07 to 0.28 mV. Based on these findings and those of a previous report, it was concluded that these spikes are homologous with PGO spikes recorded in cats. It was also found that spikes identical to the spontaneously occurring spikes of PS could be produced during wakefulness and in all states of sleep with external auditory stimulation. These elicited spikes were not accompanied by eye movements. Initial stimulus presentations resulted in EEG desynchronization, orienting movements and PGO spikes. With continued stimulus presentations, EEG and other signs of alerting would habituate followed by a gradual habituation of the spikes. The latency to the appearance of an evoked spike ranged from 20 to 24 msec. These findings are in agreement with recent studies in cats showing that PGO spikes occur in response to external, auditory stimuli in wakefulness and all stages of sleep in addition to their spontaneous occurrence just prior to and during PS.