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The effects of a potent gonadotrophin-releasing hormone (GnRH) antagonist, (N-Ac-D-p-Cl-Phe^1,2,D-Trp³,D-Arg⁶,D-Ala¹⁰)-GnRH (Org 30276), on pituitary and testicular function of adult macaque monkeys were investigated. After a study to find the correct dose in castrated monkeys, five intact adult male animals were treated with daily s.c. injections of 5 mg antagonist for 9 weeks. The treatment resulted in an immediate decline in serum LH and testosterone in three out of five animals. The two hormones remained suppressed during the 9-week treatment period. Testosterone and LH responses to a bolus injection of GnRH (50 µg i.v.) were blunted or abolished during the antagonist treatment. Testicular volumes decreased markedly and ejaculates obtained at the end of treatment were azoospermic or contained only few dead sperm. Histological examination of the testes showed complete disruption of seminiferous epithelium in these animals. A decrease of body weight was observed in the treated animals. When the treatment was ceased, all inhibitory effects of GnRH antagonists were reversible. In the other two animals no consistent suppression of pituitary or testicular function could be observed during this period, nor was a doubling of the treatment dose for a further 8 weeks capable of fully suppressing endocrine and seminal parameters in these monkeys.

It is concluded that GnRH antagonist treatment is capable of rapidly decreasing serum LH and testosterone and disrupting spermatogenesis in this primate species. Suppression effected by antagonist treatment is more rapid than that caused by GnRH agonists. The individual responses to the tested doses, however, vary markedly.

J. Endocr. (1985) 104, 345–354