Negative feedback regulation of the secretion and actions of GnRH in male ruminants.

The roles of testicular hormones in the negative feedback regulation of the secretion and actions of GnRH in male domestic ruminants are reviewed, concentrating mainly on research conducted with rams. Testicular steroids have major feedback actions directly at the hypothalamus to inhibit the secretion of GnRH, although it is apparent that, under certain circumstances, the steroids also have actions directly at the pituitary gland. Further research is necessary to delineate these actions and to determine the contribution of testosterone and its primary metabolites, dihydrotestosterone and oestradiol, to negative feedback on the hypothalamo-pituitary unit. Since GnRH neurones do not possess receptors for steroids, testicular steroids must evoke other neuronal pathways to influence GnRH producing neurones. While the opioids may be important in this regard, it is necessary to determine which other neuronal pathways may also be involved to understand fully the mechanism of action of testicular steroids. It appears that the feedback regulation of the secretion of LH can be accounted for by testicular steroids, whereas the secretion of FSH is influenced by inhibin and steroids, and possibly the recently isolated proteins follistatin and activin. The actions of inhibin to suppress the secretion of FSH occur at the pituitary gland and not on the synthesis or secretion of GnRH. There is a complex interaction between testosterone and inhibin in the control of FSH secretion that results in synergistic effects during the non-breeding season but not during the breeding season. Activin has been shown to have FSH-stimulating properties and follistatin has been shown to have FSH-inhibiting properties, but it is unknown if these proteins play a physiological role in the feedback regulation of FSH in male domestic ruminants.