Study of the Direct Action of Luteinizing Hormone-Releasing Hormone Agonists at the Testicular Level in Intact Rats Treated with an Antiluteinizing Hormone Serum*
Is it well known that LHRH agonists can inhibit testicular functions by gonadal desensitization secondary to endogenous LH release and that a direct action at the gonadal level has also been demonstrated. Since an excess of anti-LH serum can be used, as an alternative to hypophysectomy, to neutralize the influence of endogenous LH release, the relative importance of the testis and pituitary gland in the inhibitory effect of a LHRH agonist, [D-Ser-(TBU)6, des-Gly-NH210]LHRH ethylamide (Buserelin), was studied on gonadal gonadotropin receptors in intact adult male rats treated with equine anti-LH or normal horse serum (NHS). A single administration of increasing doses (1–100 ng) of Buserelin leads to a progressive inhibition of testicular LH and PRL receptor levels by 70% and 40%, respectively, in animals injected with NHS. Treatment with the anti-LH serum completely prevents this inhibitory effect of a single dose of the LHRH agonist. After two successive injections of Buserelin in NHS-treated animals, testicular LH receptors are reduced by 70% and 80% with the 100- and 500-ng doses, respectively, while testicular PRL receptors are inhibited by 40– 60%. In animals treated with the anti-LH serum, the inhibition of testicular LH receptors is reduced by only 18% (100 ng Buserelin) and 55% (500 ng Buserelin), while the inhibitory effect on PRL receptors is abolished. The present data show that endogenous LH release induced by a single injection of a LHRH agonist plays an essential role in the loss of testicular LH receptors measured 2 days later. Moreover, upon repeated injection of the LHRH agonist, neutralization of endogenous LH release by an anti-LH serum markedly reduces the inhibitory effect of the LHRH agonist on LH receptors, while it completely prevents the effect on PRL receptors, suggesting that the inhibitory effect of the LHRH agonist in the male rat is predominantly due to endogenous LH release rather than to a direct action of the peptide at the testicular level.