Synaptic and Intrinsic Plasticity in Lateral Hypothalamic Sleep/Wake Regulatory Neurons Following Acute Sleep Deprivation: Implications for Sleep Homeostasis
Sleep/wake behaviour is regulated by a network of sleep- and wake-promoting neurons that exhibit reciprocal activity patterns. This circuit is under the influence of the circadian clock, which maintains stable diurnal sleep/wake rhythms, and homeostatic control, which regulates sleep based on prior time spent awake. Astrocytes can modulate synaptic plasticity through high-affinity transporters like glutamate transporter 1 (GLT1). It was previously unknown whether GLT1 plays a role in the coordinated activation and silencing of sleep/wake neurons during challenges to sleep homeostasis. With quantitative immunoconfocal analysis, I examined the spatial relationship between GLT1 and sleep/wake neurons from two brain regions involved in the homeostatic regulation of sleep. These include orexin (ORX) and melanin concentrating hormone (MCH) neurons in the lateral hypothalamus (LH), and cholinergic and parvalbumin-expressing neurons in the basal forebrain (BF) of the rat. Following acute sleep deprivation (SD; 6 h) there was no change in GLT1 juxtaposition with BF neurons compared to a time-matched undisturbed group (Rest). In the LH, SD decreased GLT1 juxtaposition with wake-promoting ORX neurons and increased GLT1 juxtaposition with sleep-promoting MCH neurons compared to Rest. These changes were reversed by a short subsequent period of sleep opportunity (3 h). Using whole-cell patch-clamp electrophysiology, I demonstrated that SD-induced changes in GLT1 juxtaposition modulated excitatory synaptic transmission to LH neurons. Following SD, but not Rest, there was tonic presynaptic inhibition of excitatory transmission to ORX neurons through group III metabotropic glutamate receptors. This may represent a homeostatic mechanism promoting transition to sleep. In contrast, no presynaptic change was detected in MCH neurons, yet a large, slow kainate receptor-mediated EPSC induced by train-stimulation was significantly reduced following SD. This synaptic plasticity in the LH is consistent with decreased glutamate clearance at times when GLT1 juxtaposition is reduced. Finally, SD increased the intrinsic excitability of both ORX and MCH neurons. ORX neurons exhibited a reduced afterhyperpolarizing potential and spike frequency adaptation, while MCH neurons exhibited reduced A-current, following SD. These synaptic and intrinsic changes may have novel and important implications for homeostatic and non-homeostatic sleep regulation.