Inositol trisphosphate , second messenger systems

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]). 

FIGURE 12-19 Hormone-activated phospholipase C and IP3. Two intracellular second messengers are produced in the hormone-sensitive phosphatidylinositol system inositol 1,4,5-trisphosphate (IP3) and diacylglycerol. Both contribute to the activation of protein kinase C. By raising cytosolic [Ca2+], IP3 also activates other Ca2+-dependent enzymes thus Ca2+ also acts as a second messenger. 

There are two major types of vasopressin receptors, VI and V2. The VI receptor occurs in vascular smooth muscle and is coupled via to activation of the phosphoinositide cascade-signaling system and generation of the second messenger inositol trisphosphate (IP3) and diacylglycerol. V2 receptors are found in kidney and are coupled via to activation of adenylate cyclase and production of the second messenger cyclic AMP. 

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