Effects of physiological and abnormally elevated prolactin levels on the pituitary-testicular axis.

Prolactin can influence testicular function both directly, and indirectly via altering release of gonadotropins from the pituitary. Although numerous effects of prolactin on male reproductive functions have been described, only a few were demonstrated in more than one species. These include effects on male accessory reproductive glands, on testicular luteinizing hormone receptors, on the release of gonadotropins and on sexual behavior. In rodents, prolactin appears to play a physiological role in the pituitary regulation of testicular function. This is especially pronounced in the golden hamster. In this seasonally-breeding species, alteration of prolactin release is one of the mechanisms mediating the effects of photoperiod on the testis. In the hamster, prolactin is required for the maintenance of luteinizing hormone and prolactin receptors in the testis and treatment with prolactin can completely reverse testicular atrophy induced by exposure to short photoperiod. In both men and rats, excessive release of prolactin (hyperprolactinemia) leads to suppression of sexual behavior and gonadotropin release. These effects appear to be due to the action of prolactin on the central nervous system. Most, if not all, of the effects of prolactin exhibit striking variability among species. Moreover, prolactin can exert differential effects on the same target tissue in the same species, depending primarily on the dose.