Suppression of plasma luteinizing hormone by prolactin in the male rat.

Although a direct effect of PRL on gonadotropins has been previously suggested, it has not been convincingly demonstrated. The secretion of LH and FSH was studied in response to the stimuli of castration and LH releasing hormone administration in adult male rats made hyperprolactinemic with ectopic pituitary glands. Although plasma LH and FSH levels were similar in non-castrate hyperprolactinemic rats vs. controls, LH concentrations 24 h postcastration were less in hyperprolactinemic animals as compared to controls (P less than 0.001). The level of LH achieved was inversely correlated with the PRL concentration generated (r = -0.71; P less than 0.01). LH suppression was evident in hyperprolactinemic rats at 1 and 3 days postcastration but was no longer observable at 7 days postcastration. After LH releasing hormone administration to non-castrate rats the rise in plasma LH was significantly less in the hyperprolactinemic animals (P less than 0.05). These experiments support the hypothesis that PRL directly inhibits LH secretion, presumably at the pituitary level.