Diurnal variations of the fattening response to prolactin in the White-throated Sparrow, Zonotrichia albicollis

Abstract

Increases in body weight and fat deposits were induced in the photorefactory White-throated Sparrow, Zonotrichia albicollis, by injections of prolactin. The increases in lipid reserves were comparable to photoperiodically induced fattening and to that observed during the migratory seasons. However, the time of injection was of fundamental importance. Daily injections of prolactin at 5 and 10 hours after the beginning of a 16-hour photoperiod were stimulatory to fattening; injections made at 0 and 5 hours after the beginning of the photoperiod suppressed body weight and lipid reserves. These results suggest that the phasing of diurnal rhythms of prolactin release from the pituitary by the photoperiod may be important in regulating the annual cycle of fat stores.