Mammals rely heavily on olfaction to interact adequately with each other and with their environment. The mammalian olfactory system recognizes diverse chemical stimuli conveying information about such things as food quality, the genetic identity or sexual status of potential mates, and even stress’:’. A paper by Riviere et al in Nature May 2009 ) describes the identification of a chemosensory neuron in the rodent nose that responds to stimuli associated with cell damage, disease and inflammation, These results should help us to understand how animals identify pathogens or assess the health status of potential partners.
Until recently it was believed that the olfactory system of most mammals was of 2 types : a main olfactory system that detects environmental odours, for instance those emitted by food or predators, and an accessory (vomeronasal) olfactory system that detects pheromones – intraspecies chemical signals that elicit a stereotyped behavioural or hormonal change. It is now clear that the sense of smell is much more complex. Indeed, the main and accessory olfactory systems each respond to both general odours and pheromones.
Each olfactory division contains several types of sensory cell identified by the receptors and other proteins they express, the connections they make in the olfactory part of the brain, and the chemical stimuli to which they respond. This diversity of sensory cells in the nose has given rise to the concept of olfactory subsystems, each dedicated to a particular chemosensory role.
In addition animals use olfaction to assess whether other organisms may be dangerous, or even to judge the health status of potential partners. Mice use olfactory cues to avoid potential mates that are infected with parasites’, whereas nematode worms develop aversions to odours given off by harmful bacteria, thereby avoiding toxic food, However, although such olfactory-based aversion behaviours have been documented, no olfactory subsystem that is dedicated to the assessment of health status or disease has been identified in mammals.
They make use of seven transmembrane G-protein-coupled receptors to identify odorants and pheromones. These receptors are present on dendrites of olfactory sensory neurons found in the main olfactory or vomeronasal sensory epithelia, and are involved in the odorant, trace amineassociated receptor and vomeronasal type 1 receptor superfamilies. The newly described formyl peptide receptor-related genes and vomeronasal sensory neurons, are found in multiple mammalian species. They are similar to the four known olfactory receptor gene classes, these genes encode seven-transmembrane proteins, and are characterized by monogenic transcription and a punctate expression pattern in the sensory neuroepithelium.

Munger Noses within noses Nature vol 459, 521-2

Riviere et al 2009 Formyl peptide receptor – like proteins are a novel family of vomeronasal chemosensors Nature vol 459, 574-577

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