Predominant Suppression of FSHβ-immunoreactivity after Long-Term Treatment of Intact and Castrate Adult Male Rats with the GnRH Agonist Deslorelin

GnRH agonists are used to treat gonadal steroid-dependent disorders in humans and contracept animals. These agonists are thought to work by desensitizing gonadotropes to GnRH, thereby suppressing FSH and LH secretion. It is not known whether changes occur in the cellular composition of the pituitary gland following chronic GnRH agonist exposure. Adult male Sprague-Dawley rats were treated with a sham, deslorelin, or deslorelin plus testosterone implant for 41.0±0.6 days. In a second experiment, rats were castrated and treated with deslorelin and/or testosterone. Pituitary sections were labeled immunocytochemically for FSHβ and LHβ, or αGSU. Deslorelin suppressed testis weight by two thirds and reduced plasma FSH and LH in intact rats. Deslorelin decreased the percentage of gonadotropes but the effect was specific to the FSHβ-ir cells. Testosterone did not reverse the deslorelin-induced reduction in the overall gonadotrope population. However, in the presence of testosterone, the proportion of gonadotropes that was FSHβ-ir increased in the remaining gonadotropes. There was no effect of treatment on the total LHβ-ir cell population although the loss of FSHβ in bi-hormonal cells increased the proportion of mono-hormonal LHβ-ir gonadotropes. The castration-induced plasma LH and FSH increases were suppressed by deslorelin, testosterone or both. Castration increased both LH-ir and FSH-ir without increasing the overall gonadotrope population; thus increasing the proportion of bi-hormonal cells. Deslorelin suppressed these increases. Testosterone increased FSH-ir in deslorelin-treated castrate rats. Deslorelin did not affect αGSU immunoreactivity, suggesting that the gonadotrope population per se is not eliminated by deslorelin but the ability of gonadotropes to synthesize FSHβ is compromised. We hypothesize that the FSH dominant suppression may be central to the long-term contraceptive efficacy of deslorelin in the male.