GnRH agonists and antagonists stimulate recovery of fertility in irradiated LBNF1 rats.

The goal of this study was to determine whether both gonadotropin-releasing hormone (GnRH) agonists and antagonists could enhance fertility in rats given sterilizing doses of irradiation, to quantify the levels of fertility, and to measure their relative effectiveness in stimulating recovery of spermatogenesis. Irradiated rats were treated with either the GnRH agonist Lupron or the GnRH antagonist Cetrorelix, which have different mechanisms of action. The antagonist suppressed luteinizing hormone (LH), reducing intratesticular testosterone from 75 ng/g-testis to about 5 ng/g-testis, whereas the agonist reduced intratesticular testosterone only moderately to about 20 ng/g-testis, presumably by direct action on the Leydig cell since LH was elevated. These differences were reflected in Leydig cell morphology. When hormone treatment was started immediately after 3.7-Gy irradiation, fertility was normal at week 20 in the agonist-treated rats and was near normal in antagonist-treated rats, whereas irradiated-only rats were sterile. At week 22 in the GnRH antagonist-treated rats, testicular weights and sperm counts were maintained at greater than 80% of control values; in GnRH agonist-treated rats, they were slightly but significantly lower than in GnRH antagonist-treated rats, and in irradiated-only rats, they were very low. When the treatment was initiated 10 weeks after 5-Gy irradiation, after spermatogenesis had ceased, fertility was restored at week 30 to subnormal levels in 83% of GnRH agonist- and 50% of GnRH antagonist-treated rats. Testis weights and sperm counts were restored to about 50% and 20% of control levels, respectively. The percentages of tubules with differentiated germ cells were higher in all groups of antagonist-treated rats than in those of agonist-treated rats. Thus, both GnRH agonists and antagonists produced dramatic recovery of spermatogenesis and fertility in irradiated rats, although there were differences in mechanism and perhaps also in effectiveness.

Gonadotropin-releasing hormone analogs stimulate and testosterone inhibits the recovery of spermatogenesis in irradiated rats.

We investigated the effects of GnRH analogs, different doses of testosterone (T), an androgen receptor antagonist (flutamide), and combinations of these on the recovery of spermatogenesis after irradiation. Treatment with a GnRH agonist (Lupron) for 10 weeks after irradiation reduced the intratesticular T concentration (ITT) to 4% of that in irradiated rats and serum FSH to undetectable levels without altering serum LH levels. Injection of a GnRH antagonist (Cetrorelix) at 3 weeks after irradiation suppressed LH, FSH, and ITT to <7%, 32%, and 10%, respectively, of levels in irradiated-only rats within 2 weeks; suppression was maintained for approximately 3 to 4 weeks. The percentage of tubules with differentiated germ cells (repopulation index, RI) was <0.6% at weeks 10 to 20 after irradiation. Spermatogenic recovery was induced by both the GnRH agonist (RI = 58% at week 10; 91% at week 20) and antagonist (RI = 70% at week 13). There was a dose-dependent suppression of testicular germ cell repopulation when T was combined with GnRH analogs. The ability of T to abolish the spermatogenic stimulatory effect of the GnRH antagonist was evident by the similar RI obtained for irradiated rats given antagonist + T or T alone. This suppression of GnRH-induced recovery of spermatogenesis by T could be reversed by flutamide. The RI best correlated with the degree of ITT suppression. In ITT-suppressed rats, the RI also showed an inverse correlation with serum T levels. Thus, T and/or its androgenic metabolites either directly or indirectly inhibit spermatogenic recovery after irradiation through an androgen receptor-mediated process. In addition, there was a close negative correlation between RI and FSH levels, and hence, a spermatogenic inhibitory role for FSH in the irradiated rats cannot be ruled out.

Enhancement of A spermatogonial proliferation and differentiation in irradiated rats by gonadotropin-releasing hormone antagonist administration.

The initial changes in the numbers, proliferation, and differentiation of A spermatogonia in irradiated rats after the administration of a GnRH antagonist, which is known to induce differentiation in this system, were investigated. LBNF1 rats were given 6 Gy of gamma-irradiation; some were treated with the GnRH antagonist Cetrorelix beginning 15 weeks after irradiation. Although the spermatogonia in the irradiated rats without hormone treatment continue to proliferate (labeling and mitotic indexes of 24% and 18%, respectively), they underwent apoptosis (apoptotic indexes of 21% by the terminal transferase-mediated end labeling assay and 9% by nuclear morphology), resulting in a constant number of A spermatogonia. Whole mount analysis of clones ofA spermatogonia revealed that larger clones were more likely to undergo apoptosis than mitosis. Hormone administration decreased the intratesticular testosterone concentration to 6% of the level in irradiated rats within 1 week. Concomitantly, there was a decrease in spermatogonial apoptotic indexes to 43% of levels in irradiated-only rats, leading to an increases in their numbers by 150%, their diameters by 11%, and their labeling indexes by 31%. The sizes of the mitotic clones gradually increased, and clones of more than eight cells appeared at week 3 of hormone treatment. A spermatogonial differentiation began at week 4, and by week 6.6, differentiation occurred in 30% of the tubules. Thus, suppression of intratesticular testosterone by the GnRH antagonist may be responsible for the immediate changes in spermatogonial numbers and kinetics, but several additional steps are required before differentiation begins, which did not occur until week 4.