PHEROMONES ELICIT EQUIVALENT LEVELS OF FOS-IMMUNOREACTIVITY IN PREPUBERTAL AND ADULT MALE SYRIAN-HAMSTERS

Male reproductive behavior in the Syrian hamster is dependent on both pheromones from the female and the presence of gonadal steroid hormone s. The pheromones are contained within female hamster vaginal secretio ns (FHVS) and stimulate anogenital investigation and mounting by the m ale. Administration of testosterone to castrated male hamsters facilit ates anogenital investigation, mounts, and intromissions in adults, bu t elicits only anogenital investigation in prepubertal males. One hypo thesis for why the full complement of reproductive behaviors is not ac tivated by testosterone in prepubertal males is that the neural proces sing of pheromonal cues encountered during anogenital investigation is different in juveniles and adults. In the present experiment, we inve stigated the influence of sexual maturity on Fos expression in respons e to FHVS in the male Syrian hamster. We predicted a greater increase in Fos-immunoreactivity after exposure to FHVS within the neural circu it mediating male reproductive behaviors in adult compared to prepuber tal males. Intact adult and prepubertal males were exposed to either a clean cotton swab or a swab containing FHVS. We found that, compared to animals exposed to a clean cotton swab, both prepubertal and adult males exposed to FHVS have a greater amount of Fos-immunoreactivity wi thin several brain nuclei comprising the neural circuit mediating male reproductive behavior. Furthermore, this Fos response was equivalent in the two age groups. These results suggest that the inability of the prepubertal male hamster to perform the full repertoire of male repro ductive behaviors is not due to a lack of a neuronal activation in res ponse to the pheromonal cues present in FHVS. (C) 1998 Academic Press.