Influence of continuous gonadotropin-releasing hormone (GnRH) agonist treatment on luteinizing hormone and testosterone secretion, the response to GnRH, and the testicular response to human chorionic gonadotropin in male rhesus monkeys.

Five adult male rhesus monkeys were continuously infused for 56 days with 25 micrograms/day of GnRH agonist (Wy-40972; Ag) using an implanted osmotic pump. Bioassayable serum levels of LH were elevated 8-fold on the second day of Ag treatment and then declined precipitously to below pretreatment levels by day 15. Serum levels of testosterone (T) changed similarly during Ag treatment, except that the fall in serum LH levels preceded the decline in serum levels of T by at least 2 days. Ag administration also eliminated the diurnal variation in serum LH and T. GnRH administration (50 micrograms) induced a 13- to 20-fold rise in serum LH and a 3- to 7-fold increase in serum T in control monkeys. After 4 weeks of Ag administration , none of the animals responded to GnRH. Both control and experimental monkeys had a rise in serum T in response to hCG after 7 weeks of Ag treatment. Basal levels of LH and T returned to normal by 12 days posttreatment, and the serum LH and T responses to GnRH were normal 19 days posttreatment. These results indicate that 1) continuous administration of Ag is an effective method of inducing antiferility effects in male rhesus monkeys; 2) pituitary desensitization is a major factor involved in Ag-induced gonadal dysfunction in this species; and 3) the method of administration may be the critical factor in determining the effectiveness of GnRH agonists.