Molecular and Neural Mechanisms of Pheromone Reception in the Rat Vomeronasal System and Changes in the Pheromonal Reception by the Maturation and Sexual Experiences

Excerpt

Pheromones, which provide specific information concerning the identity, gender, and endocrine status in a variety of mammals have been found in saliva, skin gland secretions, and urine (Halpern and Martinez-Marcos 2003; Powers and Winans 1975; Wysocki and Meredith 1987). The modulation of gonadal function by the smell of urine has been well established in rodent VNOs (Halpern and Martinez-Marcos 2003; Keverne et al. 1986; Wysocki and Meredith 1987). In rats, pheromones in urine excreted from males and females 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 estrous cycle of females from 5 to 4 days (Chateau et al. 1976), and estrous synchrony among females living together (McClintock 1978). In addition, exposure to male pheromones elicits the release of luteinizing hormone (Rajendren et al. 1990) and estrous synchrony among females that are living together (McClintock 1978).

The vomeronasal system is a second olfactory system organized in parallel with the main olfactory system. The VNO, which is the peripheral chemoreceptor organ of the vomeronasal system, forms a bilateral tubular structure in the ventral part of the nasal cavity of rats. The interaction of a pheromone with receptors of VNSs initiates a sequential molecular event leading to action potential initiation. In the present chapter, we first addressed the mechanism of pheromone reception by the VNO in rats.

Chemical signals excreted from animals affect the sexual behavior of conspecific male and female animals. Information regarding the females’ endocrine state is transmitted to males by means of urinary pheromones. Sexually experienced male rats prefer estrous to diestrous urine odor (Lydell and Doty 1972; Pfaff and Pfaffmann 1969). The VSNs project information to the mitral cells (MTCs) in the AOB located on the dorsocaudal surface of the main olfactory bulb (MOB) (Rajendren et al. 1990). Pheromonal information transmitted via the MTC is modified by GABA-immunoreactive interneurons at the periglomerular cell (PGC) layer and by granule cells (GCs) (Keverne et al. 1986; Halpern and Martinez-Marcos 2003; Wysocki and Meredith 1987).

Comparison of c-Fos expression, which is correlated with the neural activity of sexually experienced and inexperienced males after exposure to estrous or diestrous urine shows changes in cellular responses to urinary pheromones as related to sexual experience (Honda et al. 2008). In the lateral and rostral sectors of the PGC, many more Fos-immunoreactive (Fos-ir) cells were observed in the sexually experienced rats than in the inexperienced rats. In the latter part of this chapter, I will describe possible mechanisms concerning changes in neural function with sexual experiences and aging.