Augmentation of sensitivity to urinary pheromone and excreting of urinary pheromone by sexual experiences.

Pheromonal signals provide specific information concerning the identity, gender, endocrine, and social status of different members of the population in a variety of mammals (Halpern, 1987; Wysocki and Meredith, 1987). Pheromones in urine excreted from male and female rats induce various changes in gonadal functions such as reflex ovulation in the absence of coitus and mounting (Johns et al., 1978), a reduction in the oestrous cycle of female rats from 5 to 4 days (Chateau et al., 1976) and oestrous synchrony among female rats living together (McClintock, 1978). The vomeronasal organ is the peripheral chemoreceptor organ of the vomeronasal system. Regulation of gonadal functions by urinary pheromones has been well established in the rodent vomeronasal organ. Vomeronasal sensory neurons project information to the accessory olfactory bulb (AOB) located on the dorso-caudal surface of the main olfactory bulb. The induction of Fos has been widely used as an assay for studying the excitability of populations of neurons within many different regions of the brain. Immunohistological methods have been used to visualize Fos as a means of identifying neurons that are activated by stimulation. The urinary pheromone-induced increases in Fos-immunoreactivities were eliminated by the removal of the vomeronasal organ in the AOB of rats, indicating that pheromonal information is transmitted to neurons at the AOB (Inamura et al., 1999a).