The Olfactory Bulbs Enhance Reproductive Hormone Secretion in Male Rats on Long or Short Photoperiod

Abstract

Unlike seasonally breeding species such as the Syrian hamster, Sprague-Dawley laboratory rats do not normally respond to short photoperiod (6L:18D) with reproductive regression. However, removal of the olfactory bulbs (BX) unmasks a photoperiodic response in pre-pubertal rats so that blinding or short photoperiod results in an inhibition of reproductive hormone secretion and/or a delay in pubertal development. This is apparently mediated by pineal melatonin which inhibits gonadotropin and/or prolactin secretion, but the mechanism by which BX facilitates the response to photoperiod is not clear.Experiment I was performed to determine serum levels of reproductive hormones at frequent intervals following BX and/or maintenance on short days. Twenty-three-day old male rats were BX or underwent sham BX (SH). They were thereafter maintained on a 14L:lOD (long photoperiod) or 6L:18D photoperiod for the duration of the study. At 6 weeks following surgery, BX rats on either photoperiod had smaller testes than the SH groups. At week 8, the BX group on 6L:18D had smaller testes than the other three groups. There were transient reductions in serum luteinizing hormone and follicle-stimulating hormone in the BX rats on short photoperiod, but there were prolonged effects of BX decreasing prolactin levels in rats on long or short photoperiod. In SH rats, testosterone was elevated for weeks 6 through 10 of the study, and BX blocked this increase.Experiment II was performed to determine whether BX alters testosterone feedback on gonadotropin secretion. Twenty-three-day old male rats were BX or underwent SH operation and were then returned to a room on 14L:lOD. Six to 8 weeks later, all animals were castrated and placed on 6L:18D or returned to 14L:lOD. Eight weeks following castration, the rats were implanted with Silastic capsules containing 0, 10, 20 or 40 mm testosterone. The post-castration increase in serum luteinizing hormone and folliclestimulating hormone was lower in the BX khan SH rats. In long photoperiod, serum luteinizing hormone and follicle-stimulating hormone were often lower in the BX rats receiving no testosterone replacement or lower doses of testosterone than in the SH group receiving similar capsules. Maintenance on short photoperiod increased the responsiveness to testosterone so that even the rats receiving low doses of testosterone had very low luteinizing hormone and follicle-stimulating hormone levels whether they were SH or BX.In summary, BX rats on long or short photoperiod had lower serum prolactin and testosterone levels than the comparable SH group and BX inhibited the post-castration increase in gonadotropin secretion. The data therefore suggest that the olfactory bulbs tonically enhance reproductive hormone secretion (especially around the time of pubertal development).Golden hamsters undergo marked seasonal variations in reproductive competence (1-3). The inhibition of gonadotropin secretion associated with winter-like photoperiods is induced by pineal melatonin, which acts in large part b...

Social Cues Attenuate Photoresponsiveness of the Male Reproductive System in Siberian Hamsters (Phodopus sungorus)

“…Other factors have been shown to influence gonadal response to short photoperiods. For example, in the Syrian hamster, olfactory bulbectomy eliminates responses to short photoperiods by directly stimulating gonadotropin-releasing hormone-secreting neurons (Pieper et al, 1990;Pieper and Lobocki, 1993). Melatonin secretion is not affected by this procedure (Pieper et al, 1988).…”

“…In the seasonally breeding Syrian hamster (Mesocricetus auratus), olfactory bulbectomy inhibits gonadal regression in response to short photoperiods (Pieper et al, 1984;Clancy et al, 1986). Olfactory bulbectomy acts on the reproductive axis by inducing an increased secretion of gonadotropins (Pieper et al, 1990). This stimulation of the reproductive axis overrides, but does not prevent, the change in melatonin release in response to short photoperiods (Pieper et al, 1987(Pieper et al, , 1988.…”

Food and Neonatal Androgen Interact with Photoperiod to Inhibit Reproductive Maturation in Fischer 344 Rats1

“…However, a reproductive response to short days (SD) and/or melatonin treatment can be induced in adult male Sprague-Dawley, Wistar, and some other strains of rats by several procedures: food deprivation, neonatal androgen treatment, chronic exposure to exogenous testosterone (T), or olfactory bulbectomy ( [7][8][9][10][11], but see [12]). …”

Olfactory Bulb Removal Does Not Prevent Gonadotropin or Prolactin Responses to Changing Photoperiod in the Ewe

“…Despite such degeneration, neither photoperiod-induced changes, nor basal gonadotropin levels were affected. These results suggest that in the ewe, unlike in the rat [36] and hamster [37], the olfactory bulbs do not tonically influence gonad otropin or prolactin secretion.…”