Gonadotrope Function in Ovariectomized Ewes Actively Immunized against Gonadotropin-Releasing Hormone (Gn RH)1

The gonadotrope cells of the ovine anterior pituitary were insulated from hypothalamic inputs by imposing an immunologic barrier generated by active immunization of ovariectomized ewes against gonadotropin-releasing hormone (GnRH) conjugated to keyhole limpet hemocyanin (KLH) through a p-aminophenylacetic acid bridge. All GnRH-KLH animals immunized developed titers of anti-GnRH that exceeded 1:5000. The antisera were specific for GnRH and cross-reacted with GnRH agonists modified in position 10 to an extent that was < 0.01%. Ewes actively immunized against GnRH-KLH displayed levels of basal and GnRH agonist-induced gonadotropin secretion that were markedly lower (p<0.05) than comparable parameters in ewes actively immunized against KLH. In contrast, basal and thyrotropin-releasing hormone (TRH)-induced prolactin (PRL) secretion were not compromised by active immunization. Immunization against the GnRH-KLH conjugate, but not KLH alone, prevented expression of the positive feedback response to exogenous estradiol (E2). Pituitary stores of immunoactive luteinizing hormone (LH) and follicle-stimulating hormone (FSH) were significantly (p<0.001) reduced in ewes immunized against GnRH-KLH but stores of PRL were not affected by such immunization. Further, the biopotency of the residual LH stores in tissue of animals from the anti-GnRH group was significantly (p<0.05) lower than LH biopotency in anti-KLH animals. Serum levels of LH in anti-GnRH ewes were restored by circhoral administration of a GnRH agonist that did not cross-react with the antisera generated. Pulsatile delivery of GnRH agonist in anti-GnRH ewes significantly (p<0.05) elevated serum LH within 48 h and reestablished LH levels comparable to anti-KLH ewes within 6 days of treatment. These data indicate that active immunization against GnRH-KLH conjugate establishes a selective immunologic barrier between the anterior pituitary and hypothalamus. The functional integrity of the gonadotrope cells is markedly reduced without compromising mammotrope function. The antigonadotrope effect of active immunization against GnRH-KLH can be reversed by pulsatile administration of GnRH agonists that do not cross-react with the antisera produced.