Molecular approaches to pheromone detection in mammals

The mammalian vomeronasal system mediates the detection of pheromones-chemical stimuli that are emitted by one individual and detected by another to elicit genetically preprogrammed physiological and behavioral responses. Although the responses elicited by pheromones have been well characterized, the nature of the molecules that function as mammalian pheromones is largely unknown. It is also not understood how those compounds are detected and decoded by the vomeronasal system to generate appropriate responses.; Two diverse families of G protein-coupled receptors are expressed by two distinct and non-overlapping subpopulations of vomeronasal neurons. The purpose of such a numerous and diverse repertoire of chemosensory receptors, and the way in which individual receptors contribute to the overall pheromone response, is poorly understood. Characterization of vomeronasal receptors should aid in the identification of compounds that may participate in the pheromone response, and also enable us to gain insights into basic principles of pheromone sensory coding.; Two complementary molecular approaches were utilized to investigate the molecular logic of pheromone detection in the vomeronasal organ. We first attempted to functionally express vomeronasal receptors in a heterologous expression system as a way to identify the compounds that function as the mammalian pheromones, and to investigate how both the individual receptor genes and the highly homologous subfamilies of receptor genes contribute to the detection and processing of pheromonal signals. Unfortunately, detection of pheromone-mediated responses using this system was unsuccessful; we concluded that, as is the case for some other chemosensory receptors, vomeronasal receptor function required additional molecular component(s) missing from our heterologous system.; To overcome this obstacle, we took an in vivo approach to study vomeronasal receptor activation. Individual vomeronasal receptor neurons activated by putative pheromonal compounds were identified and isolated. Single cell cDNA analysis allowed identification of the receptor genes expressed by these neurons, and a transgenic strategy was used to genetically label VNO neurons expressing that receptor. Preliminary data suggests that VNO neurons labeled in the transgenic animals respond to the same compound that activated the original neuron, supporting the hypothesis that we have developed a method to associate individual ligands with specific vomeronasal receptors. Moreover, since this strategy also labels the regions of the brain innervated by neurons expressing the transgene, this approach will allow us to investigate how activation of individual vomeronasal receptors generates the patterns of neuronal activity that lead to behavioral responses.