The olfactory bulbs tonically inhibit serum gonadotropin and prolactin levels in male hamsters on long or short photoperiod.

Abstract Bilateral removal of the olfactory bulbs (BX) inhibits the testicular regression associated with maintenance of golden hamsters on short photoperiod (10L:14D). The present study was done to examine the reproductive endocrine changes following BX of hamsters, to test whether BX increases gonadotropin secretion by enhancing the rate of metabolism of peripheral testosterone, and to determine whether BX inhibits the response to photoperiod by blocking chemosensory signals from conspecifics. BX resulted in a marked increase in tonic serum gonadotropin levels in pre-pubertal (23 days old) and adult hamsters maintained on long photoperiod (14L:10D). Maintenance on 10L14D inhibited gonadotropin secretion in BX hamsters, but this only reduced the previously elevated levels to those of the sham group on stimulatory photoperiod, and the testes therefore remained large. BX hamsters on 10L:14D had a higher post-castration increase in serum luteinizing hormone than sham-operated hamsters. Following testosterone replacement (20 mm Silastic capsules), BX animals had lower serum testosterone and higher serum follicle- stimulating hormone levels than the sham group. BX hamsters had a shorter mean half-time for disappearance of testosterone from serum following removal of the capsule, but some animals in the sham group also metabolized testosterone rapidly. Isolation in cages receiving air filtered to remove pheromonal type molecules did not affect the rate or degree of testicular regression in response to short days. We conclude that the olfactory bulbs tonically inhibit gonadotropin release in golden hamsters on long or short photoperiod. The olfactory bulbs may facilitate the negative feedback of testosterone and may inhibit testosterone metabolism, but there were also steroid-independent effects. The influence of the olfactory bulbs on the hamsters' response to short days is apparently not related to chemosensory information from other hamsters.