Induction of FOS immunoreactivity in central accessory olfactory structures of the female rat following exposure to conspecific males

Abstract

Reproductive events in the female rat can be influenced by exposure to the odors of conspecific males. Much evidence indicates that these pheromonal effects are mediated by the accessory olfactory system (AOS); however, individual cells within the AOS that are stimulated following exposure to male odors have not yet been visualized. The present experiment was designed to determine the effect of exposure to conspecific males and male odors on signal transduction in central AOS neurons as measured by immunohistochemical detection of the induction of the fos-like protein. AOS structures examined included the accessory olfactory bulb (AOB), medial amygdala (mAMYG), and bed nucleus of the stria terminalis (BNST). Due to its importance in the control of reproductive activities and its direct link to the AOS, the ventromedial nucleus of the hypothalamus (VMH) was also examined. Adult, ovariectomized rats were injected with estradiol benzoate (EB) and 48 h later were placed in cages containing bedding material soiled by conspecific males or placed in cages containing clean bedding material. After exposure durations ranging from 10 to 180 min, the animals were sacrificed and the brains were immunohistochemically processed for detection of fos-like immunoreactivity. Another group of ovariectomized, EB-injected females was repeatedly paired with conspeciflc males for 15 min followed by 15 min of rest. Repeated matings were conducted over a 60-, 120-, or 180-min period while control animals were repeatedly exposed to clean bedding material. Quantitative analysis of the number of fos-immunopositive cells in the AOB revealed that continuous exposure to male-soiled bedding or repeated mating resulted in significant induction of foslike immunoreactivity compared to controls. Both treatments produced similar numbers of fos-like immunoreactive cells in the mitral and granule cell layers of the AOB. Fos induction was apparent after 60 min of treatment but was more prominent at 120 and 180 min. In the mAMYG, BLAST, and VMH, differences between the two treatments were noted. Exposure to male-soiled bedding for 60 min produced scattered staining in the mAMYG, BLAST, and VMH, whereas 60 min of repetitive mating resulted in a more dense distribution of fos-like immunoreactive cells in these areas. Strikingly distinct patterns of fos-like immunoreactive cells were observed in the mAMYG, BLAST, and VMH following 120 or 180 min of repetitive mating. These patterns were not present in animals exposed to male odors. The findings indicate that exposure of female rats to reproductively relevant stimuli resulted in induction of fos-like immunoreactivity within the AOS and that both olfactory and nonolfactory cues probably contributed to this effect.