The hypocretins (orexins) are a recently discovered neuropeptide family thought to play a role in the regulation of sleep, feeding, and neuroendocrine secretion. Neurons that manufacture hypocretins are located exclusively in and around the perifornical nucleus and lateral hypothalamus. Most behavioral studies to date involving hypocretins have focused on their effects on sleep and arousal or feeding. This proposal aims to examine a possible role for the hypocretins in the regulation of circadian rhythms. There is very dense hypocretin innervation of several brain regions known to be involved in the regulation of circadian rhythms, including the midbrain raphe nuclei and the intergeniculate leaflet of the thalamus. In addition, hypocretins are thought to be released directly into the ventricular system, raising the possiblilty that hypocretins could act in brain regions that don't display hypocretin-immunoreactive fibers. Since hypocretins appear to stimulate arousal /waking, and induced arousal and sleep deprivation are capable of shifting the phase of the circadian clock, this proposal will investigate hypocretin as a potential mediator of the effects of arousal state on circadian activity rhythms. Microinjections of hypocretin-1 and hypocretin-2 will be given into several brain nuclei or the lateral ventricle during the middle of the subjective day. This is the time of the circadian day when behavioral stimuli have maximal effects on circadian clock phase. These brain regions will include the suprachiasmatic nucleus, the intergeniculate leaflet, the dorsal raphe, the median raphe, and several other regions with dense hypocretin innervation. This will tell us whether hypocretin is capable of shifting the circadian clock. In addition, the neural pathways between the hypocretin-containing cells and the suprachiasmatic nucleus will be determined using a combination of anterograde and retrograde tracers. Overall, this proposal will begin the process of determining the mechanisms by which hypocretin may be involved in the regulation of circadian rhythms, and lead to greater understanding of interactions between sleep and circadian rhythmicity.
