Cellular Mechanisms of Olfactory Signal Transduction

Abstract

The sense of smell has the exquisite capacity to recognize and discriminate among an immense variety of volatile chemical compounds that are present in the environment. In mammals, olfactory cues are detected by sensory neurons belonging to three different sensory modalities harboured in the nasal cavity. Olfactory and vomeronasal primary sensory neurons (OSNs and VSNs, respectively) detect odorants and pheromones, whereas free nerve endings of the V cranial nerve (N. trigeminus) constitute the common chemical sense and convey pain, touch, temperature and chemosensory information. Our interest is to understand the molecular and cellular mechanisms of chemosensation in the diverse subsystems. A first common step in signal transduction is the specific binding of odorant molecules to receptor proteins located in specialized membrane protrusions: the cilia, microvilli or free nerve endings innervating the nasal mucosa. Olfactory and pheromone receptor proteins of OSNs and VSNs, respectively, are identified and functionally characterized to some extent. The intracellular mechanisms that transduce the initial chemical information into an electrical signal in OSNs involve the adenylate cyclase–cyclic nucleotide-gated channel pathway in addition to a recently identified phosphoinositides pathway. In contrast, a phospholipase C–diacylglycerol– transient receptor potential (TRP) channel pathway seems to play the major role in VSNs. However, the molecular mechanisms used by trigeminal neurons to detect odorant molecules are largely unknown. The beginning characterization of thermosensitive TRP channels among other receptors and ion channels expressed in trigeminal ganglion cells revealed their role in detection of chemical compounds and chemosensory signalling in these cells. The detection and interpretation of chemical cues in an animal’s