Blockade of singular follicle-stimulating hormone secretion and testicular development in photostimulated Djungarian hamsters (Phodopus sungorus) by a gonadotropin-releasing hormone antagonist

Photostimulated male Djungarian hamsters following placement in a long- day photoperiod exhibit a characteristic rise in serum FSH levels that occurs in the absence of a simultaneous rise in LH levels. It is not known whether this singular FSH secretion is dependent upon a differential responsiveness of the gonadotrophs to the pattern of pulsatile GnRH release or is instead driven by a GnRH-independent mechanism. We have assessed the GnRH dependence of this singular FSH secretion by testing the ability of a potent GnRH antagonist (GnRHa: WY-45760) to block FSH and testicular responses to photostimulation. Photoinhibited hamsters were transferred from a short-day (6L:18D) to a long-day photoperiod (16L:8D). Hamsters received two daily injections of a GNRH antagonist or vehicle (VEH). After 0 (short day), 3, 5, 10, 30, or 40 days the hamsters were killed; plasma was assayed for FSH, LH, and testosterone (T), and testes weights were recorded. Testes were sectioned and analyzed for tubular development. In VEH-treated animals, testicular weights increased after photostimulation, reaching mean values of 514 mg by 30 days. Treatment with GnRHa resulted in a significant (p < 0.01) attenuation of testicular growth after 30 days of photostimulation (mean testes weight = 110.1 mg). In VEH-treated hamsters there was a rapid increase in FSH levels after photostimulation that became significant by 5 days and peaked at 10 days. In the GnRHa-treated group, however, these FSH increments were completely blocked at 5 days and significantly reduced at 10 days compared to the values in the corresponding VEH-treated groups. Serum LH and T levels remained relatively low following photostimulation in both groups. These data demonstrate that, in Djungarian hamsters, the singular rise in FSH secretion observed after the transfer from short to long days is primarily dependent upon endogenous GnRH stimulation. This GnRH-dependent FSH rise is, in turn, necessary for normal testicular development following photostimulation. Thus alterations in FSH-releasing factor activity or gonadal hormone feedback need not be invoked to explain the differential effects of photostimulation on LH and FSH release during the early stages of photic-induced changes in neuroendocrine-gonadal activity. We hypothesize that photostimulation of hamsters induces a change in the pattern of endogenous GnRH neurosecretion, which in turn conveys FSH-specific signals for the secretion of the gonadotropin.