Phenotypic differences in the GnRH neuronal system of deer mice Peromyscus maniculatus under a natural short photoperiod.

The neural mechanisms by which short photoperiod induces gonadal regression among seasonally breeding mammals are not well understood. One hypothesis suggests that the proximate cause of seasonal gonadal regression is a photoperiod-induced modification in GnRH secretion. This hypothesis is indirectly supported by our recent findings using immunocytochemistry which identified specific photoperiod-induced adjustments in the number and morphology of GnRH containing neurones between reproductively competent and reproductively regressed laboratory housed male deer mice. Herein, we report that the GnRH neuronal system is similarly affected in reproductively responsive and nonresponsive wild male deer mice Peromyscus maniculatus exposed to a natural short photoperiod. The distribution of immunoreactive (IR)-GnRH neurones was nearly identical in field caught animals and those housed under artificial photoperiod in the laboratory. Compared with reproductively nonresponsive males, reproductively responsive mice from the field population possessed a greater total number of IR-GnRH neurones, a greater number of IR-GnRH neurones within the lateral hypothalamus, and a greater proportion of bipolar IR-GnRH neurones. Each of these distributional and morphological characters was consistent with our findings in laboratory housed male deer mice exposed to an artificial short photoperiod. Taken together, these data underscore the validity of using an artificial photoperiod to evaluate seasonal adjustments in reproductive function in the laboratory.