27. Odorant Sensing

Chemosensory perception is one of the most important systems for appraisal of the environment. In contrast to physical sensory modalities, whose stimuli nature is constant (light, sound), the ever-changing odorous environment requires that the chemosensory system can cope with varying situations. This is accomplished by a large number of odorant receptors, most of them with a broad recognition spectrum due to a plasticity of the binding cavity. This apparent fuzziness is mandatory for the combinatorial mode of odorant recognition that allows reception of an almost infinite number of odorants with an enormous discriminatory power. Odorant receptors are expressed in olfactory sensory neurons (OSNs) following the one-neuron one-receptor rule; only one from more than a thousand receptor genes is expressed, in fact either from the maternal or the paternal allele. Thus, the receptor type determines the molecular receptive range of a particular chemosensory neuron. The choice and continuous expression of a particular receptor gene is supposed to result from a hierarchy of regulatory processes, involving cis-regulatory deoxyribonucleic acid (DNA) elements, epigenetics and a negative feedback mechanism mediated by the receptor protein itself. The chemoelectrical transduction process in OSNs is mediated by an intracellular reaction cascade leading to the generation of action potentials that are conveyed to the brain. An individual OSN extends its axon directly into the olfactory bulb where it targets onto a distinct spherical neuropil – the olfactory glomerulus – that connects sensory input with output neurons and local modulatory interneurons. The connectivity is remarkably precise such that only axons from neurons with the same receptor type innervate a specific glomerulus. The molecular determinants that control the complex process of axonal pathfinding, segregation, and targeting that lead to a receptor-specific wiring of the olfactory system have been only partially unraveled.