Differential testicular responses to short daylength in deer mice are reflected by regional and morphological differences in the GnRH neuronal system.

Male deer mice (Peromyscus maniculatus) respond to short photoperiod with a range of spermatogenic responses. About one-third of all short day exposed mice exhibit a complete cessation of spermatogenesis (reproductive responsive), while about an equal number remain reproductively competent (reproductive nonresponsive). These differential spermatogenic responses are mirrored by endocrine measures; reproductive responsive males exhibit reduced circulating levels of testosterone and luteinizing hormone, while reproductive nonresponsive males exhibit long day levels of both hormones. A variety of evidence indicates that despite individual differences in reproductive responses, all short day exposed mice detect and respond to the change in photoperiod at the level of the hypothalamus, irrespective of gonadal response. We investigated whether deer mice exhibiting disparate gonadal responses to short days differed in aspects of the GnRH neuronal system, as revealed by immunocytochemistry. Reproductively mature males were maintained on either long (16L:8D) or short (8L:16D) photoperiod for 8 weeks. Thereafter, regional and morphological differences in GnRH-containing neurons were compared among long day (n = 8) and short day reproductive responsive (n = 8) and nonresponsive (n = 8) deer mice. Results demonstrate both an effect of photoperiod and reproductive state on the number, location and morphology of immunoreactive-GnRH neurons. Short days caused immunoreactive GnRH cell number to increase in both short day phenotypes relative to long day mice. Significant differences among treatment groups in the numbers of cells per region were confined to the lateral hypothalamus and lateral preoptic area. Both short day phenotypes exhibited an increase in cell number in the lateral preoptic area, compared to long day mice.(ABSTRACT TRUNCATED AT 250 WORDS)