Abstract
Sleep-wake behavior is regulated by a circadian rhythm, homeostatically and by additional mechanisms that determine the timing of slow-wave sleep and rapid eye movement sleep (REMS) episodes. The posterior hypothalamus coordinates the neural and humoral signals with the rest-activity cycle. It contains wake-active neurons, and is a site where stimulation of inhibitory GABAA receptors promotes sleep, whereas their antagonism enhances wakefulness. We explored whether GABAergic mechanisms present in the posterior hypothalamus contribute to the homeostatic and other aspects of sleep-wake regulation. Using micropunches of tissue extracted from either the perifornical (PF) or dorsomedial (DM) regions of the posterior hypothalamus of rats, we determined that mRNA levels for selected subunits of GABAA receptors (β1, β3 and ε) were higher at the end of the active period or following sleep deprivation, when the need for sleep is high, than after several hours of sleep, when sleep need is partially fulfilled. Such a pattern was present in the PF region only, and was consistent with changes in β1 subunit and GABA synthesizing enzyme (GAD) protein levels. In contrast, in the DM region, the levels of GABAA receptor subunit mRNAs and proteins (α1, α2, β1) and GAD varied with circadian time, but were not responsive to sleep deprivation. Separate experiments with sleep-wake monitoring and local perfusion of the PF region with the GABAA receptor antagonist bicuculline revealed that the antagonist had a weaker sleep-reducing effect when sleep need was enhanced by sleep deprivation and that the increased amount of REMS characteristic of the late sleep period was dependent on endogenous GABAergic inhibition. These results support the concept that a varying magnitude of GABAergic inhibition exerted within the PF region contributes to the homeostatic regulation of sleep and shapes its temporal pattern, whereas GABAergic mechanisms in the DM region contribute to circadian regulation.
Highlights
Sleep-wake behavior is regulated by the circadian rhythm and homeostatic drive, with the strength of the latter increasing with the duration of prior time spent awake [1,2,3]
We found that the strength of GABAergic inhibition in the PF region increases when the drive for sleep is increased, that changes in GABAA receptors (GABAARs) expression likely contribute to this process, and that sleep drive-related changes in the PF inhibition mediated by GABAARs differentially affect slow-wave sleep (SWS), rapid eye movement sleep (REMS) and motor activity during wakefulness
GABAA Receptor Subunit and GABA Synthesizing Enzyme, glutamic acid decarboxylase (GAD), mRNA Levels Vary in the Posterior Hypothalamus with Sleep Need and Circadian Time
Summary
Sleep-wake behavior is regulated by the circadian rhythm and homeostatic drive, with the strength of the latter increasing with the duration of prior time spent awake [1,2,3]. Each of the two major sleep states, slow-wave sleep (SWS) and rapid eye movement sleep (REMS), has features of a homeostatically regulated process, and a temporal structure, such that the relative amounts of SWS and REMS gradually shift in favor of the latter with the duration of the sleep period [25,26]. To orchestrate this complex regulation, multiple mechanisms must act both locally and systemically, and their contributions must be coordinated by a neural network to achieve a sleep-wake behavior pattern that optimally meets the diverse needs of the entire organism [27]
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