Removal of the Olfactory Bulbs Modifies the Gonadal Responses to Photoperiod in the Lesser Mouse Lemur (Microcebus Murinus)

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Abstract
The sexual activity of the lesser mouse lemur, unlike that of most living primates, is clearly dependent on photoperiod. Males responded to long daylengths (> 12 h light/day) by rapid testicular development and an increase in testosterone levels reaching 65.8 +/- 1.5 ng/ml (n = 12) after 3 wk of inductive photoperiod. Levels were maintained unchanged until 14 wk of exposure to long photoperiod, when spontaneous gonadal regression was observed. Under short daylengths, intact males exhibited regressed gonadal function with low plasma concentrations of testosterone: 5.4 +/- 1 ng/ml. To investigate the effect of olfactory bulb deprivation on gonadal responses to photoperiod, 12 males were bulbectomized (BX) during exposure to either long (n = 8, BX1) or short (n = 4, BX2) daylengths. In both groups of BX males, responses to long or short daylengths were maintained. However, the timing and the amplitude of gonadal responses to photoperiod were modified; BX males underwent a significant delay in testis development and also had reduced testosterone levels (53.3 +/- 2.3 ng/ml and 37.3 +/- 0.9 ng/ml for BX1 and BX2 groups, respectively). Moreover, BX males did not exhibit spontaneous gonadal regression after 14 wk of exposure to long daylengths, indicating that the photorefractory state did not appear, at least under our experimental conditions. In this primate, olfactory bulbs exert an important effect on the neuroendocrinological control of reproductive responsiveness to photoperiod.

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