University of Stuttgart-Hohenheim, Institute of Physiology, 70593 Stuttgart, Germany.
Chemo-electrical transduction in vertebrate olfaction is mediated via g protein-coupled intracellular reaction cascades. studies analyzing a large variety of different odorous compounds indicate that in rat olfactory cilia, stimulation with distinct odors cause a rapid and transient elevation in either adenosine 3',5'-cyclic monophosphate (cAMP) or inositol-trisphosphate (IP3) concentration. recent studies with selective inhibitors for effector enzymes indicate that the two second messenger pathways involved in olfactory transduction show a functional antagonism, thus allowing the olfactory sensory receptor cell not only to work as a transducer but also to serve as an integrating unit, which might be important for coding olfactory information of complex odor blends. In addition to the main olfactory system, which is responsible for the conventional sense of smell, most terrestrial vertebrates possess an anatomically and functionally distinct sensory organ, the vomeronasal organ, which appears to play a key role in the detection of pheromones. Recent studies indicate that stimulation of female rat microvillar preparation with male rat urine-derived compounds induce a rapid and transient increase in the concentration of IP3, resembling second messenger signalling in the main olfactory system. Using different fractions of male rat urine, it was observed that lipophilic volatile odorants activate phospholipase C (PLC) by a Gi-protein-subtype, whereas Go-controlled PLC-stimulation was only observed upon application of alpha2u-globulin, a major urinary protein of the lipocalin superfamiliy. Since each G protein-subtype is stereotypically coexpressed with one of the two structurally different candidate pheromone receptors (V1R and V2R, respectively), the results provide first experimental evidence that V1Rs coexpressed with Gi may be activated by lipophilic, probably volatile odorants, whereas V2Rs coexpressed with Go seem to be spezialized to interact with pheromonal components of proteinaceous nature.