Direct inhibition of testicular function by gonadotropin-releasing hormone: mediation by specific gonadotropin-releasing hormone receptors in interstitial cells.

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Abstract
Agonist analogs of gonadotropin-releasing hormone (gonadoliberin GnRH) exert antigonadal effects in male and female animals. In hypophysectomized male rats treated with follicle-stimulating hormone [follitropin] administration of a potent GnRH agonist [[D-Ser(+Bo)6]des-Gly10-GnRH-N-ethylamide] caused depletion of luteinizing hormone [lutropin] and prolactin receptors and marked suppression of serum testosterone levels. The possibility that such direct effects of GnRH agonists on testicular function could be expressed through specific receptors located in the interstitial cells of the testis was supported by the selective concentration of a 125I-labeled GnRH agonist by the testis in vivo. Specific receptors for the releasing hormone were demonstrated in testis particles and dispersed interstitial cells by direct binding analysis with the 125I-labeled GnRH agonist. The binding affinity (Ka = 5 .times. 109 M-1) and peptide specificity of the testicular GnRH binding sites were similar to those of anterior pituitary and ovarian GnRH receptors. The presence of GnRH receptors in the testis indicates that these sites mediate the direct inhibitory actions of GnRH agonists upon testicular endocrine function.