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 l ater. We have tested the hypothesis that differential release of gonad otropins in this species can be mediated by changes in the frequency o f pulsatile GnRH stimulation. Photoinhibited Siberian hamsters receive d 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 (S D) controls. In addition, 10 days of fast-frequency GnRH pulses produc ed LH levels significantly greater than LH levels in animals exposed t o in days of medium or slow GnRH pulse frequencies. Pulsatile GnRH pro duced the following serum FSH relationships: medium pulses > fast puls es > SD. The FSH response to slow GnRH frequency fell between the two faster frequencies. The effect of GnRH pulse frequency an paired teste s weight was as follows: fast pulses = medium pulses > slow pulses > S D controls. The differing GnRH pulse frequencies produced the followin g testosterone relationships; fast pulses > medium pulses = slow pulse s = 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 wi th the idea that the singular release of FSH after transfer of hamster s to a long-day photoperiod is mediated by alterations in the frequenc y of endogenous pulsatile GnRH release.