In mammals, olfactory stimuli are detected by sensory neurons at two distinct sites: the olfactory epithelium (OE) of the nasal cavity and the neuroepithelium of the vomeronasal organ (VNO). While the OE can detect volatile chemicals released from numerous sources, the VNO appears to be specialized to detect pheromones that are emitted by other animals and that convey information of behavioral or physiological importance. The Communication among mammals via chemical messengers known as pheromones may play important roles in a variety of social interactions, especially those associated with reproduction (1-4). Pheromones present in urine or vaginal secretions have been implicated in such effects as the initiation of copulatory behavior in male hamsters and alterations in reproductive state in mice, including acceleration or suppression of puberty and block of pregnancy. The accessory olfactory system, which originates in the sensory neuroepithelium of the vomeronasal organ (VNO), is thought to be specialized to detect pheromones (1-4).In rodents, the VNO is a cartilage-encased tubular structure, which is located at the base of the nasal septum and opens into the nasal cavity via a single duct. Molecules dissolved in nasal mucus are pumped into the VNO lumen by changes in VNO blood volume (5). Like the olfactory epithelium (OE) of the nasal cavity, which detects volatile odorants as well as some pheromones, the VNO neuroepithelium contains sensory neurons that project axons to the olfactory bulb of the brain. However, the pathways followed by sensory information that enters the olfactory system through the VNO and OE, the "accessory" and "main" olfactory pathways, respectively, are distinct and remain separate at all levels of the nervous system (6). OE-derived signals ultimately reach multiple brain regions, including the frontal cortex, which is thought to mediate the conscious perception of odors. In contrast, VNO-derivedThe publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. signals are targeted to hypothalamic structures implicated in reproductive physiology and behavior (7,8).The neuroepithelium of the VNO is derived embryologically as an evagination from the developing OE and resembles the OE morphologically (9). VNO and OE neurons are both highly unusual among neurons in that they are short-lived and are replaced from a local stem cell population throughout life (10, 11). In addition, both express olfactory marker protein (OMP) (12) and both are bipolar cells that extend fine processes (cilia or microvilli) into the external environment and project axons to the olfactory bulb of the brain (1, 6, 11). These similarities have suggested that neurons in the OE and VNO might use the same mechanisms, perhaps even the same molecules, to transduce sensory stimuli.Very little is known about sensory transduction in the VNO. However, extensive studies of s...