Olfactory system second messengers

Some sensory neurons of the VNO express two gene superfamilies, termed Vlr and V2r, that encode over 240 proteins of the seven-transmembrane type (Matsunami and Buck, 1997). These G-protein-linked putative pheromone receptors are distantly related to the main olfactory system s receptors. Receptors of the VNO are linked to different G-proteins, and their extracellular N-terminal domains are longer than those of the receptors in the main olfactory system. (Vi receptors are linked to Gi-proteins and V2 receptors to Go-proteins). The intracellular excitation mechanism in VNO sensory neurons also differs from that in the main olfactory systems instead of linking to adenylyl cyclase, the VNO receptors activate the phosphoinositol second messenger system. This has been demonstrated in several mammalian species. In hamsters, aphrodisin increases inositol 1,4,5-trisphosphate (IP3) levels in VNO membranes. Boar seminal fluid and urine stimulate increases of IP3 in the VNO of the female pig. (However, in the pig, the VNO is not necessarily essential for responses to pheromones [Dorries etal., 1997]). 

In all systems studied thus far, olfactory-stimulated signal transduction is mediated by G-protein-coupled receptor (GPCR) pathways involving the synthesis of second messengers such as cyclic AMP (cAMP) and/or inositol 1,4,5-triphosphate (IP3) (Boekhoff et al., 1994 Reed, 1992). Moreover, several proteins that are involved in fundamental aspects of olfactory signal transduction have... 

The olfactory receptors belong to the 7-transmembrane-G-protein coupled family of receptors. A schematic structure is shown in Figure 10.6. They are coupled to G-proteins in the same way as is rhodopsin and generate the nerve signal through a similar second messenger system to that used by rhodopsin (see Chapter 8). They differ from rhodopsin in that they lack the additional tether back into the cell... 

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