Diurnal variations of the fattening response to prolactin in the golden topminnow, Fundulus chrysotus

Fattening was induced in sexually mature golden topminnows, Fundulus chrysotus, by daily injections of prolactin or luteinizing hormone. However, the time of treatment was of fundamental importance. Whereas injectins given during the middle of a 16‐hour photoperiod stimulated an increase in body weight and fat stores, injections given early in the light period caused a loss in body weight and lipid reserves. These results suggest that the phasing of diurnal rhythms of prolactin and/or luteinizing hormone release from the pituitary may be important in regulating the annual cycle of fat stores. Prolactin, alone, did not induce substantial fattening, if any at all, in hypophysectomized topminnows. Accordingly, the necessity for other pituitary factors at the midphotoperiod interval is apparent. In addition, the deoreases in the lipid and dry nonlipid components of hypophysectomized F. chrysotus by prolactin injections, early in the photoperiod or at midday, suggest there is at least one extra‐hypophyseal factor which exhibits a diurnal periodicity affecting the responses to prolactin.