Astrocytic Plasticity and Inhibitory Transmission in Orexin Neurons Following Acute Sleep Deprivation

Abstract

Sleep and wakefulness are regulated by numerous neuronal populations throughout the brain, including the wake-promoting orexin (ORX) and sleep-promoting melanin-concentrating hormone (MCH) neurons in the lateral hypothalamus. Increasing evidence indicates the roles of astrocytes in regulating various behaviors, and astrocytes have been shown to modulate excitatory synaptic transmission to ORX and MCH neurons through glutamate transporter 1 (GLT1) depending on sleep history. However, it is unknown whether astrocytes regulate inhibitory synaptic transmission to ORX and MCH neurons. The first study used immunoconfocal microscopy to investigate whether appositions of astrocytic GABA transporters GAT1 and GAT3 with ORX and MCH neurons are altered following 6 h of sleep deprivation (SD), which increases sleep need, compared with rested (Rest) control rats. After SD, GAT1 but not GAT3 appositions on ORX neurons decreased while appositions of either GAT with MCH neurons remained unchanged, suggesting astrocytic responses to SD at inhibitory synapses to ORX neurons. The decreases in appositions of the astrocytic transporters after SD could be due to structural repositioning of perisynaptic astrocytes. We investigated this possibility by using serial section electron microscopy. After 6 h of SD, both astrocytic contact with synapses and astrocytic coverage per synapse decreased at synapses to ORX neurons, indicating retraction of astrocyte processes at these synapses. Synaptic densities remained unchanged between Rest and SD. In addition, the data under control (Rest) condition revealed the following: the vast majority of synapses with ORX neurons were contacted by astrocytes; the density of synapses was greater on dendrites than the soma; and synaptic clefts were larger at synapses on the soma than on dendrites. Finally, to investigate whether the GAT1 apposition reduction on ORX neurons is associated with functional changes in inhibitory transmission, patch-clamp recordings were conducted in acute hypothalamic slices. Surprisingly, there were no differences in either spontaneous or evoked inhibitory postsynaptic currents between the Rest and SD conditions. This is likely due to compensatory action of GAT3, as only simultaneous inhibition of both GAT1 and GAT3 but not either one alone resulted in reduction in evoked inhibitory postsynaptic current. Collectively, these findings suggest that astrocytes predominantly modulate excitatory rather than inhibitory synapses in wake-promoting ORX neurons in the lateral hypothalamus through retraction of perisynaptic astrocyte processes. We propose that astrocytic process remodeling at synapses to ORX neurons acts as a cellular mechanism for homeostatic regulation of sleep.