The Molecular Architecture of Odor and Pheromone Sensing in Mammals

exposure of a female to a male that is genetically different from the inseminating male (the Bruce Effect). In the Boston, Massachusetts 02115 Bruce Effect, it appears that a pheromone effect can be coupled with the detection of " individuality cues " that result from genetic variation within a species, including Olfaction is an ancient sense. Its precursors can be differences at major histocompatibility loci. found in the most primitive single-celled organisms, reflecting the need of every organism to sense its chemical milieu. In mammals, the olfactory system can detect and Odor and Pheromone Circuits distinguish a vast number of volatile chemicals with a In mammals, odorants are detected in the olfactory epi-large variety of structures (Beets, 1970; Shepherd, 1988). thelium (OE) that lines the nasal cavity (Shepherd, 1988) It seems to have evolved to sense almost any volatile (Figure 1). Signals generated in olfactory sensory neu-chemical it might encounter. However, the olfactory sys-rons in the OE in response to odorants are relayed tem is also responsible for the sensing of pheromones, through the main olfactory bulb (MOB) to the olfactory chemicals released by animals that act on conspecifics cortex (OC) and then to other brain areas. Via these to regulate populations of animals and their social inter-pathways, odor signals ultimately reach higher cortical actions (Wilson, 1963; Shepherd, 1988). Pheromones areas involved in the conscious perception of odors, as elicit programmed neuroendocrine changes and innate well as limbic areas, such as the amygdala and hypothal-behaviors, suggesting a need for a very precise recogni-amus, that are involved in emotional and motivational tion process. responses. How does the olfactory system meet these dual re-Most mammals have a second olfactory sense organ, quirements for general odor sensing and the generation called the vomeronasal organ (VNO) (Figure 1). The VNO of stereotyped responses to specific pheromones? is a tubular structure in the nasal septum that is con-Studies in both insects and mammals suggest that it nected to the nasal cavity by a small duct. Removal of does it by segregating odor and pheromone detection the VNO, or severing its connection to the brain, inter-in different sensory neurons and in different neural path-feres with pheromone effects but not with general odor, these observations have sug-functional studies of these pathways have provided in-gested that the VNO may be specialized to detect phero-sight into many aspects of odor and pheromone sensing In recent years, the there, signals …