The role of the posterior hypothalamus in controlling the paradoxical phase of sleep

Chronic experiments were performed on seven cats to study the effects of high-frequency electrical stimulation of the posterior hypothalamic area on the characteristics of paradoxical sleep; the excitability of this structure at different stages of paradoxical sleep was determined. These studies showed that at the stage showing ECoG desynchronization and phasic events (stage 1), the response threshold for behavioral arousal resulting from stimulation of posterior hypothalamus was 20-30% higher than at the stage characterized by α-like activity in the ECoG and the absence of phasic phenomena (stage 2). Transient stimulation of the posterior hypothalamus at stage 1, at a level which was subthreshold for arousal from this state, led to a transition to stage 2 or a reduction in phasic events in paradoxical sleep without altering the qualitative characteristics of phase 1. Stimulation of the posterior hypothalamus at a level subthreshold for arousal from stage 2 and applied continuously during paradoxical sleep led to a reduction in the duration of this stage by 25-50% and to an increase in the proportion of stage 2 in the structure of paradoxical sleep. These results provide evidence that the posterior hypothalamus is involved in the inhibitory control of the 'executive' mechanisms of paradoxical sleep responsible for the ECoG desynchronization and the phasic manifestations of this state. It is suggested that the functional activity of the posterior hypothalamus at stage 1 also increases at stage 2, thus evidently fulfilling a 'guard' function.