| Summary: | This is a study of the control of luteinining hormone (LH) secretion in the marmoset monkey, Callithrix jacchus. The way in which the hypothalamo-hypophysial-gonadal system operates to regulate LH secretion in primates is reviewed. A double antibody hetorologous radioimmunoassay for measuring LH in marmoset plasma was developed and validated. Intra-muscular administration of synthetic luteinising hormone-releasing hormone (LH-RH) induces a marked increase in plasma LH concentrations in the marmoset, suggesting that the releasing hormone has an important physiological role in controlling LH secretion in this species. The effects of steroid hormones on pituitary response to exogenous LH-RH were examined. Pituitary responsiveness to LH-RH was enhanced in long term, but, not in short term, gonadeotomised animals. Whereas oestradiol-174 implants inhibited pituitary response to LH-RH in long term castrates, implants of progesterone augmented the response. A direct action of gonadal steroids on the pituitary gland is therefore suggested. The way in which steroid hormones influence the hypothalamo-hypophysial system was further studied by examining their ability to depressor increase circulating LH concentrations. Gonadectomy resulted in an increase in plasma LH levels indicating that LH secretion is normally suppressed by the action of gonadal steroids. In the "open-loop" situation the elevated LH levels are the result of an episodic secretion of the hormone by the pituitary gland. Closure of this feedback loop with oestradiol-17f caused a chronic suppression of LH secretion, suggesting that this steroid is an important component of the negative feedback mechanism regulating tonic LH secretion. The effects of progesterone, testosterone and dihydrotestosterone on LH secretion in gonadectomised marmosets were also tested. Whereas these hormones prevented the post-castration rise in IX concentrations, they were apparently ineffective in suppressing LH levels in long term gonadectomised animals. A decrease in the sensitivity of the hypothalamic-pituitary system to negative feedback as the interval from castration increases is suggested. Positive feedback control of LH secretion was also examined. A single injection of oestradiol benzoate had a biphasic effect on LH secretion, with an initial negative feedback effect characteristically preceding the positive response. Oestrogen induced LH release was observed in castrated and intact males as well as in castrated females, suggesting that the positive feedback response to oestrogen in the marmoset is not a sexually dimorphic characteristic. The ability of progesterone, testosterone, and dibydrotestosterone to induce, or modify oestrogen induced, positive feedback was also assessed in gonadectomised animals. The effects of inhibition of LH-RH were also examined. Active immunisation against LH-RH induced a breakdown of the hypothalamo-hypophysial-gonadal system, resulting in inhibition of gonadal and pituitary function. LH-RH induced LH release was suppressed by the use of competitive antagonist analogues of LH-RH. The potential application of these procedures in fertility control was investigated.
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