GONADOTROPIN AND PROLACTIN SECRETION IN CASTRATED MALE SHEEP FOLLOWING SUBCUTANEOUS OR INTRACRANIAL TREATMENT WITH TESTICULAR HORMONES

Interactions between testosterone, estradiol, and inhibin in the contr ol of gonadotrophin secretion in males are poorly understood. Castrate d rams were treated with steroid-free bovine follicular fluid (bFF), t estosterone, or estradiol and for 7 d (2 x 2 x 2 factorial design). Gi ven independently, none of the exogenous hormones affected follicle-st imulating hormone (FSH) concentrations, but the combination of one or both steroids with bFF reduced FSH secretion. Testosterone and estradi ol reduced luteinizing hormone (LH) pulse frequency (there was no syne rgism), and bFF had no effect. Plasma prolactin concentrations were no t affected by any treatment. To locate the central sites of steroid ac tion, castrated rams were bilaterally implanted in the preoptic area ( POA), ventromedial nucleus (VMH), or arcuate nucleus (ARC). These impl ants did not affect FSH or prolactin concentrations, or LH pulse ampli tude. The frequency of the LH pulses was not affected by testosterone in any site. Estradiol located in the ARC, but not the POA or VMH, dec reased LH pulse frequency. In summary, FSH secretion is controlled by synergistic interactions between inhibin and estradiol or testosterone , whereas GnRH/LH pulse frequency is controlled by testicular steroids . Estradiol acts partly, at least, in the ARC, but the central site of action, testosterone remains unknown.