Effect of short photoperiods on the in vitro GnRH release by hypothalamic explants in intact and castrated male Syrian hamsters: relation to testicular regression and recrudescence.

Prolonged exposure of adult Syrian hamsters to short days decreases LH and FSH circulating levels within 2-4 weeks, then induces testicular regression. After 18 weeks of short days, the testis size and gonadotropin levels increase spontaneously. This study investigated whether these phases of photosensitivity and photorefractoriness corresponded to variations of in vitro GnRH release. Male hamsters were either kept under long days (LD 16:8) or transferred to short days (SD 6:18) and sacrificed from 2-26 weeks after transfer. To separate the effects of testis feedback from a possible direct photoperiodic drive on the hypothalamus, males were bilaterally castrated, kept under LD or transferred to SD, and sacrificed from 2-14 weeks after transfer. Hypothalamic explants were incubated in a saline buffer for three periods of 15 min and exposed to KCl (60 mM) for 15 min. The return to basal values was followed for six periods of 15 min, then the explants were stimulated with copper complexed equimolarly with histidine (Cu/His, 200 microM) and prostaglandin E2 (PGE2, 10 microM). At the end of the incubation period, the concentration of GnRH remaining in the explants was measured. In intact males, GnRH release in vitro increased significantly between 2 and 4 weeks after transfer to short days; it returned to values similar to LD ones between 6 and 12 weeks, during the phase of testis involution. At the beginning of photorefractoriness (SD 14-18), it increased transiently and returned to values similar to LD ones from SD 20, during the testis spontaneous recrudescence. After castration, the in vitro GnRH release decreased significantly under LD and SD. The transfer of castrated hamsters to SD resulted in transient increases of GnRH release (SD 4, 8 and 14), and in a progressive loss of the explant's ability to release GnRH in vitro. These results showed a photoperiodic regulation of in vitro GnRH release and a testis feedback effect on this release. They demonstrated an inverse relationship between the readily releasable pool of GnRH and the circulating levels of gonadotrophins at the beginning of photosensitive and photorefractory phases and after castration.