Bioscientifica Proceedings

In males, including the ram, testosterone, acting via its primary metabolites oestradiol and dihydrotestosterone (DHT), suppresses circulating LH concentrations. This effect is due primarily, although not totally, to decreased frequency of gonadotrophin-releasing hormone (GnRH) pulses. The arcuate–ventromedial region (ARC–VMR) of the mediobasal hypothalamus and possibly the medial preoptic area (mPOA) are sites at which oestradiol acts to suppress GnRH, but the site of DHT action is not known. Given that native GnRH neurones appear to contain few or no oestrogen or androgen receptors, the effects of testosterone metabolites probably are exerted by modulating activity of inhibitory interneurone systems such as (ß-endorphin, dopamine, and γ-aminobutyric acid (GABA). Although (ß-endorphin clearly inhibits GnRH secretion, the observation that testosterone treatment during a long-day photoperiod reduced proopiomelanocortin (POMC) mRNA in the arcuate nucleus while coincidentally suppressing GnRH release indicates that ß-endorphin does not mediate the inhibitory effect of testosterone on GnRH. Activation of GABA_A receptors in either the mPOA or ARC–VMR suppressed LH, whereas activation of GABA_B receptors in the ARC–VMR increased LH pulse amplitude. Therefore, it is suggested that GABA acts in both regions to regulate LH. Whereas testosterone affects GABA metabolism in the rat hypothalamus, its effect in the ram hypothalamus is yet to be determined. Testosterone treatment activated dopaminergic cells in the retrochiasmatic A15 area in the same animals in which it suppressed POMC mRNA in the arcuate nucleus. This dopaminergic system may partially mediate the negative feedback effect of testosterone in the ram analogous to its role in partially mediating the negative effect of oestrogen in the ewe. Future studies must concentrate on determining how these and other putative inhibitory neuronal systems interact and how they in turn are regulated by environmental factors such as photoperiod.

© 1999 Journals of Reproduction and Fertility Ltd