Biol. Pharm. Bull. 28(9) 1770—1772 (2005)

pheromones in many vertebrates. Pheromones have been found in saliva, skin gland secretions, and urine. For example, pheromones in the urine excreted from male and female rats induce various changes in gonadal function and endocrine state, including reflex ovulation in the absence of coitus and mounting, and a reduction in the oestrous cycle of female rats from 5 to 4 d, and oestrous synchrony among female rats housed together. The vomeronasal organ exists in many vertebrates for the purpose of receiving pheromones, which in turn affect sexual and social behavior. In a previous study, we demonstrated that the sensory neurons in the vomeronasal organ (VNO) of female rats, which respond to male Wistar urine, are localized in the apical layer of the epithelium, where one type of Gi2a is selectively expressed. Exposure of the VNO of the female Wistar rat to male Wistar urine induces c-Fos expression, which is correlated with cellular activity, primarily in the rostral region of the accessory olfactory bulb (AOB), suggesting that pheromonal information received by the sensory neurons in the VNO is transmitted to neurons in the AOB. Exposure of the VNO of female rats to protease-sensitive urinary pheromones excreted from male rats induced the expression of c-Fos-immunoreactive cells in the AOB. In the present study, in order to characterize the effects of protease-sensitive pheromones present in male urine on the endocrine state of female rats, the plasma progesterone concentration in female rats was measured after the female rats had been exposed to male urinary pheromones with and without protease treatment.