This is the continuation of a project to investigate how mammalian circadian rhythms are generated, and synchronized with the 24 hr light-dark (LD) cycle. The role played by the putative neurochemical messengers found within the suprachiasmatic nucleus (SCN) (i.e., vasopressin, somatostatin, vasoactive intestinal peptide and gamma aminobutyric acid) in the generation of circadian rhythms will be investigated by determining how the microinjection of these substances (and antagonists and antisera) influence the circadian control of hamster wheelrunning activity. Whether a second circadian clock exists within the hamster circadian system will be determined by destroying the SCN and examining whether the circadian rhythm of core body temperature persists, and whether it can be synchronized by the LD cycle. The functional, neurochemical and anatomical bases of the synchronization of circadian rhythms to the LD cycle will be investigated in both a nocturnal (hamster) and diurnal (ground squirrel) mammal by: 1) defining the importance of each specific component of the LD cycle (i.e., light, the transition between light and dark; darkness, the transition between dark and light) in the synchronization of circadian rhythms by determining how each of these LD cycle components phase shift circadian rhythms, 2) defining whether the putative neurochemical messengers that appear to be contained in the two major visual afferents of the SCN (i.e., LANT-6 in the retinohypothalamic tract and neuropeptide Y in the lateral geniculate - SCN projection) can mimic the phase shifts produced by specific components of the LD cycle when microinjected into the SCN region, and 3) determine whether different visual projections to the SCN communicte different types of visual information about the LD cycle to the SCN by determining how the different components of the LD cycle phase shift circadian rhythms following lesions that destroy the lateral geniculate nucleus. By understanding how circadian rhythms are generated and synchronized with the environment, it will be possible to provide effective treatments for the increasing number of health problems linked to disorders of the circadian timing system.
