Feeding effects of growth hormone-releasing factor in rats are photoperiod sensitive

By examining the influence of photoperiod, this study extended previous findings showing that centrally administered growth hormone-releasing factor (GRF) increases feeding. Rats received intracerebroventricular injections of GRF (0, 0.4, 4.0, and 40.0 pmol) during either the light or dark phase of the photocycle. GRF produced a dose-dependent increase in 90-min food intake during the light photoperiod but a dose-dependent suppression during the dark photo-period. GRF did not significantly alter 24-hr feeding or locomotor activity, and a biologically inactive analog of GRF had no effect on feeding. Possible mechanisms underlying GRF's photoperiod dependency are discussed, including a suggestion that endogenous GRF may play a role in the regulation of circadian feeding.