Differing Effects of Comparable Serum Testosterone Concentration on Gonadotropin Secretion in Pre-and Postpuberal Orchidectomized Rats1

Abstract

At the time of orchidectomy adult and immature rats received a subcutaneous implant consisting of a polydimethylsiloxane capsule containing crystalline testosterone. Implant size (and testosterone dosage) were adjusted on the basis of body weight and measurements of testosterone concentrations indicated that comparable serum testosterone concentrations were achieved in both age groups. The testosterone treatment proved to be adequate to prevent the postcastration increase in serum LH and FSH concentrations in the immature rats but was totally ineffective in this regard in the adult animals. The data support the concept that the hypothalamo-hypophyseal system of the immature rat is more sensitive to inhibition by testosterone (or its metabolites) than that of the adult and suggest that age related variations in the serum testosterone concentrations produced by such treatment do not account for this differing sensitivity.