Vertebrate Olfactory Reception

Through his sense of smell man has long encountered the phenomenon of molecular recognition. As early as first century BC, the Roman poet and philosopher Lucretius wrote (based on Democritus' atomic theory), Such substances as agreeably titilate the senses are composed of smooth round atoms; those that seem bitter and harsh are more tightly compacted of hooked particles . . . (Lucretius 55 BC). The last century has brought the realization that similar recognition of atomic or molecular configurations constitutes the basis of all life processes. Probably because olfaction has been a subject of study and speculation long before the advent of modern biochemistry, numerous olfactory theories have been proposed (Davies 1971, Amoore 1982). However, in the eyes of a contemporary molecular biologist, a special theory accounting for the initial steps in olfaction may seem superfluous, as the sensory cells in the nose are not unique in their ability to receive chemical signals selectively. It would be most parsi­ monious to assume that odorant recognition involves membrane protein receptors and transduction components analogous to those which mediate the specific responses to hormones, growth factors, neurotransmitters, and antigens. This notion, which corresponds to the combined Stereo­ chemical/Functional-group theories of olfaction (Amoore 1982, Beets 1971), is validated by many of the studies reviewed here. Olfactory recognition is mediated by a large ensemble' of sensory cells, each conveying a fraction of the information that signifies the nature of the odorant and its concentration. The sensory neurons receive chemical signals at their dendritic membrane and respond by firing action potentials down their axons. The olfactory response is therefore analogous to neurotransmitter reception, and the sensory organ is a simple but