Diversity and expression of odorant receptors in Drosophila

Olfactory perception translates abstract chemical features of odorants into meaningful neural information to elicit appropriate behavioral responses (Shepherd, 1994; Buck, 1996). Specialized bipolar olfactory sensory neurons (OSNs) are responsible for the initial events in odor recognition. These have ciliated dendrites exposed to the environment, and a single axon that extends into the brain and forms synapses with second order projection neurons (PNs) (Shepherd, 1994; Buck, 1996). In arthropods and mammals, the first olfactory synapse is organized into glomeruli, spherical structures in which afferent olfactory neuron axons synapse with projection neuron dendrites (Hildebrand and Shepherd, 1997). Molecular recognition of thousands of diverse odorants is mediated by a large family of odorant receptor (OR) genes, each encoding a different seven transmembrane domain G protein-coupled receptor (GPCR). OR genes were first identified in the rat by Buck and Axel (1991), using an innovative approach which assumed that the ORs would be members of the GPCR super family, encoded by an extremely large gene family expressed only in olfactory tissues. These assumptions proved to be correct and led to the identification of several hundred rat OR genes, selectively expressed in OSNs. Using degenerate oligonucleotide primers that would anneal with conserved regions in the transmembrane domains of the GPCR super family, Buck and Axel used the polymerase chain reaction (PCR) to identify OR genes in olfactory epithelium mRNA.