ORCHIECTOMY SELECTIVELY INCREASES FOLLICLE-STIMULATING-HORMONE SECRETION IN GONADOTROPIN-RELEASING-HORMONE ANTAGONIST-TREATED MALE-RATS

The pituitary component of the feedback mechanisms exerted by testicul ar factors on gonadotropin secretion was analyzed in adult male rats t reated with a potent gonadotropin-releasing hormone (GnRH) antagonist. In order to discriminate between androgens and testicular peptides, g roups of males were orchidectomized (to eliminate androgens and non-an drogenic testicular factors) or injected with ethylene dimethane sulfo nate (EDS), a selective toxin for Leydig cells (to eliminate selective ly androgens) and treated for 15 days with vehicle or the GnRH antagon ist e-D-Trp-Ser-Tyr-D-Arg-Leu-Arg-Pro-D-Ala-NH2CH3COOH (Org.30276, 5 m g/kg/72 hours). Serum concentrations of luteinizing hormone (LH) and f ollicle-stimulating hormone (FSH) were measured 7 and 14 days after th e beginning of treatment. We found that: in males treated with GnRH an tagonist, orchidectomy or EDS treatment did not induce any increase in LH secretion; and orchidectomy, but not EDS treatment, increased FSH secretion in GnRH-treated males. The present results show that negativ e feedback of testicular factors on LH secretion is mediated completel y through changes in GnRH actions. In contrast, a part of the inhibito ry action of the testis on FSH secretion is exerted directly at the pi tuitary level. It can be hypothesized that non-leydig cell testicular factor(s) inputs at different levels of the hypothalamic-pituitary axi s in controlling LH and FSH secretion.