Differential expression of odorant receptor mRNA in rat tissues

Odorant receptors are a multigene family encoding seven-transmembrane-domain receptors and belong to the superfamily of G-protein coupled receptors. Approximately 1000 different odorant receptors are thought to exist within the rat olfactory epithelium, distributed in specific zones of the olfactory sheet. Some odorant-like receptors are found in the accessory system, the vomeronasal organ and few receptors are described in testis and sperms. The question arouse if odorant receptors were also expressed in other non-neuronal tissues. We did a systematic analysis of the expression of odorant receptor mRNAs in tissues of the rat (olfactory mucosa, olfactory bulb, cerebral cortex, eye, adrenal gland, kidney, heart, skeletal muscle, intestine, lung, liver, testis) by using RT-397 398 487 PCR and the appropriate primers. The primers were designed to recognize specifically one type of odorant receptors (OR-F2, F3, F5, F6, F12, I3, I7, I8, I9, I14, I15; OR-nomenclature according to Buck \& Axel, 1991). All mRNAs could be demonstrated in the olfactory epithelium and olfactory bulb, additionally some odorant receptors were expressed in several non-olfactory tissues with a differential expression pattern among olfactory receptors and across tissues. Several mRNAs were found in liver, partially the same and others in kidney, testis, brain and the other non-olfactory tissues. None of the investigated olfactory receptor mRNAs was found in skeletal muscle. Each odorant receptor is capable of interacting with one or a small number of ligands to detect odor signals. The expression of odorant receptors in non-olfactory tissues raises the question of their function in those tissues. Odorant receptors are seven-transmembrane-domain proteins, a structural characteristic that is also seen in other ''chemo'' receptors such as neurotransmitter and hormonal receptors. Dependent on the linkage to the signal cascade of G-proteins or other second messengers within the cell, the odorant receptors in non-olfactory tissues might act as such chemosensitive proteins probably detecting metabolic substances