PGO waves in rats in the non-paradoxical sleep states

Experiments were conducted in rats in order to characterize further the occurrence of PGO waves recorded from the region of the locus coeruleus, which were elicited by auditory stimuli. Repeated stimulus presentations at regular interstimulus intervals resulted in a consistent, state-dependent pattern in the occurrence and amplitudes of elicited PGO waves. The probability of eliciting a PGO wave was lowest during paradoxical sleep and highest immediately afterwards. This was followed by a gradual decline in responding during the ensuing period of slow-wave sleep. These results were obtained regardless of at what point in the rats' sleep cycles presentations of the auditory stimuli began. In other experiments, a comparison of elicited PGO waves with the acoustic startle reflex revealed major differences between these 2 responses. PGO waves could be elicited with very low intensity stimuli that were below the threshold for producing a behavioral response, while much louder stimuli were required to elicit a startle. The startle reflex habituated in a linear fashion and did not display state-related changes. It was concluded that elicited PGO waves are an electrophysiological sign of sensory responsiveness, and that their spontaneous appearance during and preceding paradoxical sleep reflects endogenously generated reticular activation.