Preoptic area of hypothalamus (POA)
This important anterior thalamic area is the main site of integration of homeostatic and circadian drives. It includes the VLPO and VMPO, which are close to the SCN, as well as osmoreceptors which secrete arginine vasopressin (AVP).
Ventrolateral preoptic nuclei (VLPO)
These are situated inferiorly to the SCN and lateral to the third ventricle, adjacent to the ventromedial preoptic nuclei (VMPO). Their neurones secrete both GABA and galanine which are inhibitory neurotransmitters. They inhibit all the aminergic brainstem arousal nuclei, including the locus coeruleus, raphe nuclei, mesolimbic system and also the tuberomammillary nuclei.
The dorsal part of the VLPO promotes NREM sleep, and its medial extension, which projects particularly to the LDT/PPT, induces REM sleep. The VLPO activity is not related to circadian rhythms, but increases with sleep deprivation. The VLPO is active in sleep and inactive during wakefulness. Its widespread inhibitory connections give it the potential to cause a synchronized reactivation of the sleep promoting centres, but it is inhibited by the aminergic arousal systems, particularly the tuberomammillary nuclei, as well as the cholinergic basal forebrain.
Ventromedial preoptic nuclei (VMPO)
These are particularly involved with temperature control, but also modify sleep–wake function.
Median preoptic nucleus (MPN)
This is located in the hypothalamus, dorsal to the third ventricle, and is GABA-ergic. It receives input from the SCN and projects to the cholinergic neurones in the basal forebrain and to the perifornical nuclei. It is active during sleep, particularly stages 3 and 4 NREM sleep.