Hormonal interrelationships between hypothalamus, pituitary and testis of rams and bulls

This mini-review aims to summarize some of our recent findings relating to testicular function and feedback control of the hypothalamic-pituitary axis by testicular steroids in rams and bulls. Testosterone secretion in intact males is not tonic, but is characterized by episodic pulses. This pattern of secretion is dictated by inputs of the central nervous system via secretions of the hypothalamus (luteinizing hormone-releasing hormone; LHRH) and anterior pituitary (luteinizing hormone; LH). A temporal relationship exists between concentrations of LH and testosterone in serum and evidence is presented that strongly suggests that their episodic secretion is dependent on discrete episodes of LHRH discharge from the hypothalamus. Based on data from experiments with rams and bulls, I suggest that acutely castrated males (but not chronic castrates) remain susceptible to the negative feedback effects of testosterone, i.e., LH concentrations remain suppressed in serum of animals given testosterone replacement therapy immediately following castration. Estradiol-17 beta, on the other hand, abolishes pulsatile LH release and suppresses mean LH concentrations in both acute and chronic castrates. Therefore, testosterone feedback on LH secretion may, in part, involve extragonadal conversion to estradiol-17 beta to block pulsatile LHRH release. The potent inhibitory effects of estradiol on LH secretion provide an experimental probe for future investigations relating to mechanisms controlling male reproduction.