Interactions of Photoperiod and Ectopic Pituitary Grafts on Hypothalamic and Pituitary Function in Male Hamsters

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Abstract
Exposure of adult male hamsters to short days (<12.5 light/day) leads to suppression of gonadal function which is secondary to reductions in gonadotropin and prolactin (PRL) secretion. PRL secretion is decreased in short days despite a reduction of dopaminergic (DA) input from the hypothalamus, suggesting that the pituitary may become more sensitive to the inhibitory effects of DA. Although hypothalamic DA metabolism is altered by short-day exposure, it is not known whether the DA system can respond to PRL feedback or whether these changes in DA or PRL levels are responsible for the observed changes in gonadotropin secretion. To address these questions, the effects of PRL-secreting ectopic pituitary grafts on hypothalamic catecholamine metabolism and the effects of experimental manipulations of catecholamine metabolism on PRL and gonadotropin secretion were evaluated in adult male hamsters exposed to a 14 h light: 10 h dark (l4L:lOD) or a 5L:19D photoperiod. Short-photoperiod exposure led to expected reductions in testes weight, plasma PRL levels, and in vitro PRL secretion, but circulating levels of luteinizing hormone or follicle-stimulating hormone were not affected. Norepinephrine and DA turnover in the median eminence and in the medial basal hypothalamus was also reduced in the 5L:19D as compared to the l4L:lOD animals. Pituitary grafts elevated PRL levels and hypothalamic DA turnover in animals from either photoperiod, but in vitro PRL secretion was reduced only from the pituitaries of 14L:10D hamsters. Short-photoperiod exposure increased the ability of DA to suppress PRL secretion, and this effect could be reversed by the presence of an ectopic pituitary graft. The manipulation of photoperiod and plasma PRL levels also affected circulating gonadotropin levels, possibly by altering catecholamine metabolism in the medial basal hypothalamus and in the medial preoptic area. From these data, we conclude that short-photoperiod exposure causes complex changes in hypothalamic catecholamine metabolism and responsiveness that may partially explain photoperiod-mediated changes in hormonal feedback control systems.