Biochemical studies of olfaction: role of cilia in odorant recognition.

Chemoreception in vertebrates is beginning to be understood. Numerous anatomical, behavioral, and physiological studies are now available. Current research efforts are examining the molecular basis of chemoreception. Rainbow trout (Salmo gairdneri) have a functional olfactory system and are a suitable vertebrate model for studying odorant interactions with receptors. Using a biochemical approach, initial events of olfactory recognition were examined; the aim was to determine the location and specificity of odor receptors. Cilia occupy the distal region of the receptor neuron on the trout olfactory epithelium, and their membranes are the postulated locus of odorant receptor sites. A cilia preparation was isolated from the olfactory rosette. The preparation was characterized by quantifying biochemical markers for cilia, along with electron microscopy, all of which substantiated enrichment of cilia. Functional activity was assessed by quantifying binding of several radioactively labeled odorant amino acids. The odorants bound to the cilia in a manner similar to the sedimentable preparation previously isolated from t h e olfactory rosette of the same animal, thus verifying the presence of odor receptors in the cilia preparation. Evidence also confirmed a site TSA which binds L-threonine, L-serine, and L-alanine and a site L which binds L-lysine (and L-arginine). Binding of L-serine and D-alanine showed evidence for a single affinity site while the others showed two affinity sites. Separation of membrane fractions from the cilia preparation revealed that binding activity is associated with a very low density membrane fraction B.