Effects of gonadotropin-releasing hormone pulse frequency modulation on the reproductive axis of photoinhibited male Siberian hamsters

In Siberian hamsters, photostimulation evokes differential release of the gonadotropins, with FSH rising rapidly and LH levels rising much later. We have tested the hypothesis that differential release of gonadotropins in this species can be mediated by changes in the frequency of pulsatile GnRH stimulation. Photoinhibited Siberian hamsters received GnRH pulses at frequencies of 1 pulse every 45 (fast), 90 (medium), or 180 min (slow). Animals were killed at 0, 3, 5, 10, 20, and 30 days after treatment. There was a clear GnRH pulse frequency effect on LH release, with fast pulses > medium pulses > slow pulses > short-day (SD) controls. In addition, 10 days of fast-frequency GnRH pulses produced LH levels significantly greater than LH levels in animals exposed to 10 days of medium or slow GnRH pulse frequencies. Pulsatile GnRH produced the following serum FSH relationships: medium pulses > fast pulses > SD. The FSH response to slow GnRH frequency fell between the two faster frequencies. The effect of GnRH pulse frequency on paired testes weight was as follows: fast pulses = medium pulses > slow pulses > SD controls. The differing GnRH pulse frequencies produced the following testosterone relationships; fast pulses > medium pulses = slow pulses = SD controls. These results agree with studies showing that slower GnRH pulse frequencies facilitate FSH release, while faster GnRH pulse frequencies favor LH release. Our observations are also consistent with the idea that the singular release of FSH after transfer of hamsters to a long- day photoperiod is mediated by alterations in the frequency of endogenous pulsatile GnRH release.