ABSTRACT
Growth hormone-releasing hormone (GHRH) is known to stimulate food intake in a circadian phase-dependent manner in rats. The suprachiasmatic nucleus (SCN) is an important site of action for this effect. In light of the central role played by the SCN in the control of circadian rhythms, together with the phase-dependent nature of GHRH-induced feeding, we sought to determine the possible involvement of SCN GHRH activity in the regulation of circadian rhythmicity. Two studies were conducted using hamsters as subjects. Study one replicated the daytime feeding-stimulatory effects of GHRH in hamsters, thereby validating its appetitive effects in this species. Study two showed that, in free-running hamsters, intra-SCN microinjections of GHRH produced phase advances when injected during the subjective day while having little effect during the subjective night. The GHRH phase-response curve was found to resemble that observed for nonphotic influences on the clock. It is suggested that GHRH input to the SCN is a neural representation of a nonphotic influence (perhaps feeding specific) on the clock.
