A diurnal rhythm of pituitary prolactin activity with diurnal effects of mammalian and teleostean prolactin on total body lipid deposition and liver lipid metabolism in teleost fishes

Previous investigations have shown that mammalian prolactin has a pronounced effect on lipid deposition in teleost fishes (e.g. Meier et al., 1971; de Vlaming & Sage, 1972). Prolactin can be either lipogenic or lipolytic depending on when during the day it is injected. Studies were initiated to determine whether the diurnal effects of prolactin on lipid metabolism in Fundulus similis vary with season or photoperiod acclimation. Data presented here indicate that the diurnal rhythm of response to prolactin persists in the pre-spawning and post-spawning seasons if F. similis is maintained on a long photoperiod. When animals are acclimated to a short photoperiod, treatment with mammalian prolactin either early or late in the photoperiod causes lipid depletion. Homogenates of the portion of the pituitary containing only prolactin cells cause lipid depletion in F. similis when injected early during a long photoperiod, but stimulate lipogenesis if injected late in the photoperiod; these results imply that fish prolactin may be involved in lipid metabolism. Furthermore, prolactin activity in the pituitary of F. similis varies diurnally. In vitro studies of lipid metabolism in Notemigonous crysoleucas and Ictalurus natalis suggest that the liver may be one of the target organs for prolactin.