Evidence for photoperiodic control of prolactin release in rams

Abstract

Prolactin is known to increase the effect of testosterone on male accessory sex glands, and to have a synergistic action with LH on spermatogenesis and on testicular androgen secretion. It has been shown also that the male pituitary releases prolactin into the blood in the rat and the human. Little is known, however, about factors which regulate prolactin secretion in this sex. The influence of androgens and light on prolactin secretion have been examined by determining plasma levels of prolactin in normal and castrated rams under various photoperiod lengths. The main feature is the parallelism between variations of light photoperiod and the plasma prolactin levels, both in intact rams (correlation coefficient, r=+0.888, P< 0.001) and castrated animals (r=+0.887, P< 0.001). In both groups, maximal prolactin values were found when animals were exposed to light for 16 hr; conversely, the lowest values corresponded to the shortest light photoperiod. Plasma prolactin levels did not differ in androgen treated and control animals. The absence of variations in prolactin concentration following either castration or testosterone propionate injections suggests that there is no antagonism between the release of prolactin and of gonadotrophins. In particular, androgenic treatment does not influence prolactin secretion in a range where it markedly decreases LH release. Consequently, it appears from this study that there is little interference by testicular androgens on prolactin release.