GROWTH HORMONE-RELEASING HORMONE MEDIATES FEEDING-SPECIFIC FEEDBACK TO THE SUPRACHIASMATIC CIRCADIAN CLOCK

Growth hormone-releasing hormone (GHRH) is known to stimulate food int ake in a circadian phase-dependent manner in rats. The suprachiasmatic nucleus (SCN) is an important site of action for this effect. In ligh t of the central role played by the SCN in the control of circadian rh ythms, together with the phase-dependent nature of GHRH-induced feedin g, we sought to determine the possible involvement of SCN GHRH activit y in the regulation of circadian rhythmicity. Two studies were conduct ed using hamsters as subjects. Study one replicated the daytime feedin g-stimulatory effects of GHRH in hamsters, thereby validating its appe titive effects in this species. Study two showed that, in free-running hamsters, intra-SCN microinjections of GHRH produced phase advances w hen injected during the subjective day while having little effect duri ng the subjective night. The GHRH phase-response curve was found to re semble that observed for nonphotic influences on the clock. It is sugg ested that GHRH input to the SCN is a neural representation of a nonph otic influence (perhaps feeding specific) on the clock.