THE EFFECTS OF PHOTOPERIOD ON OLFACTORY C-FOS EXPRESSION IN PRAIRIE VOLES, MICROTUS-OCHROGASTER

Reproduction stops among the majority of prairie voles (Microtus ochro gaster) during the winter. Short day lengths suppress male reproductiv e function dramatically in the laboratory, but photoperiod exerts only subtle effects on female reproductive function. Thus, the regulation of seasonal breeding in this species remains partially unspecified. In contrast to commonly studied rodents, female prairie voles do not und ergo spontaneous estrous cycles; rather, they are induced into estrus by exposure to chemosignals expressed in conspecific male urine. In th e present study, the hypothesis was tested that seasonal breeding amon g female prairie voles in the field reflects photoperiod-mediated chan ges in the responsiveness of the chemosensory system to male urine. Re sponsiveness was assessed by localizing the product of the c-fos immed iate early gene with an immunocytochemical procedure. Female prairie v oles were maintained in either long (LD 16:8) or short (LD 8:16) photo periods from birth until adulthood, and exposed to either male urine o r skim milk. Immunocytochemistry for fos protein revealed an increased number of immunoreactive cells within the accessory olfactory system of female prairie voles, including the accessory olfactory bulbs, gran ule cell layer, as well as the medial and cortical divisions of the am ygdala 1 h after exposure to a single drop of urine as compared to ind ividuals exposed to skim milk. The number of immunoreactive fos cells induced in females by conspecific male urine was also affected by phot operiod; short day females displayed fewer immunoreactive fos neurons in the accessory olfactory system as compared to long-day animals. Tak en together, these results indicate that similar mechanisms underlie t he responses of different rodent species to the chemosignals of conspe cifics and that the pattern of fos expression observed in the present study has functional significance for the regulation of reproduction i n prairie voles.