Temporal synergism of corticosterone and prolactin controlling fat storage in the white-throated sparrow, Zonotrichia albicollis

Abstract

Increases in body fat were induced by daily injections of corticosterone. However, the time of injection is of critical importance. Injections given 6 hr after the onset of a daily photoperiod of 6 hr caused fat increases, whereas injections at 12 or 18 hr after “dawn” were ineffective. In addition to fat increases, injections at 6 hr after “dawn” also induced nocturnal locomotor activity in the white-throated sparrow, a nocturnal migrant. Nocturnal activity was absent in the untreated birds as well as in those receiving corticosterone at 12 or 18 hr after “dawn.” Apparently, the daily variations in responses to corticosterone depend on the circadian release of endogenous pituitary prolactin. In birds kept on continuous light, it was discovered that a temporal synergism of injected corticosterone and injected prolactin can regulate the level of fat stores. Daily injections of prolactin at 4 or 12 hr after corticosterone produced high levels of obesity in lean photorefractory birds after 5 days. Injections at 8 or 20 hr after corticosterone depressed fat stores, and injections at 0 or 16 hr after the adrenal steroid produced intermediate levels of fat. It is concluded that seasonal levels of fat stores are the cumulative results of daily changes that are regulated by a temporal synergism of corticosterone and prolactin.