State-dependent GABAergic inhibition of sciatic nerve-evoked responses of dorsal spinocerebellar tract neurons.

ABSTRACT

Peripheral nerve-evoked potentials recorded in the cerebellum 35 yr ago inferred that sensory transmission via the dorsal spinocerebellar tract (DSCT) is reduced occasionally and only during eye movements of active sleep compared with wakefulness or quiet sleep. A reduction or withdrawal of primary afferent input and/or ongoing inhibition of individual lumbar DSCT neurons may underlie this occurrence. This study distinguished between these possibilities by examining whether peripheral nerve-evoked responses recorded from individual DSCT neurons are suppressed specifically during active sleep, and if so, whether GABA mediates this phenomenon. Synaptic responses to threshold stimuli applied to the sciatic nerve were characterized by a single spike response at short latency and/or a longer latency burst of action potentials. During the state of quiet wakefulness, response magnitude did not differ from that observed during quiet sleep. During active sleep, short and long latency responses were suppressed by 26 and 14%, respectively, and returned to pre-active sleep levels following awakening from active sleep. Sciatic nerve-evoked early and late responses were further analyzed in a paired fashion around computer-tagged eye movement events that hallmark the state of active sleep. Response magnitude was suppressed by 14.4 and 11.5%, respectively, during eye movement events of active sleep. The GABA(A) antagonist bicuculline, applied juxtacellularly by microiontophoresis, abolished response suppression during non-eye movement periods and eye movement events of active sleep. In conclusion, synaptic transmission via DSCT neurons is inhibited by GABA tonically during non-eye movement periods and phasically during eye movement events of active sleep.