Diurnal variations of metabolic responses to prolactin in lower vertebrates

Abstract

Several responses to prolactin are shown to vary diurnally. On 16-hour photoperiods, midday injections induce fattening in the golden topminnow, Fundulus chrysotus, the leopard frog, Rana pipiens, the green anole, Anolis carolinensis, and the white-throated sparrow, Zonotrichia albicollis. Such injections, carried out over a period of 1 week, increase the total body lipid to levels found during the annual cycle when lipid deposits are greatest. Midday injections also promote a catabolic effect in the fish and the bird, and induce nocturnal restlessness in the white-throated sparrow, a nocturnal migrant. On the other hand, early injections (2 hours after the beginning of the photoperiod) cause a loss in fat stores in the fish and the bird and fail to produce nocturnal restlessness in the bird. Early injections, however, promote growth responses (increase in dry nonlipid materials) in all four vertebrates. The fattening and catabolic responses to prolactin injections at midday are still present in unfed fish. The diurnal variation in responses to prolactin offer an explanation for the conflicting and diverse reports regarding the function of prolactin and emphasize the importance of time in understanding the physiology of prolactin.