Bilateral orchidectomy and concomitant testosterone replacement in the juvenile male rhesus monkey (Macaca mulatta) receiving an invariant intravenous gonadotropin-releasing hormone (GnRH) infusion results, as in the hypothalamus lesioned GnRH-driven adult male, in a selective hypersecretion of follicle-stimulating hormone.

The purpose of this experiment was to determine whether the testes of the juvenile male rhesus monkey, receiving an invariant intermittent iv infusion of GnRH, produce a specific FSH secretion-inhibiting hormone that exerts its action directly at the level of the pituitary gland. To this end, five male rhesus monkeys between 13-18 months of age were treated with a chronic intermittent iv infusion of GnRH (0.1 microgram/min for 3 min every 3 h) for 10 weeks to elicit an adult-like pattern in the episodic activity of the pituitary-Leydig cell axis. Animals were then bilaterally orchidectomized, and on the day of castration testosterone replacement with testosterone-containing Silastic capsules that maintained circulating levels of the steroid at approximately 6 ng/ml was initiated. Sequential blood samples were collected before castration and at 4-5, 11-13, and 18-19 days thereafter. Removal of the testes resulted in a marked and selective hypersecretion of FSH, a response very similar to that observed previously in hypothalamus-lesioned GnRH-treated adult males. This finding indicates that it will be possible to substitute the juvenile male for the much larger hypothalamus-lesioned adult in future studies requiring a hypophysiotropic clamp preparation. Such a modification of this experimental model will facilitate an examination of the effects on FSH secretion of passive immunization with inhibin antisera and of administering pure inhibin peptides.