Odorant receptor expression as a function of neuronal maturity in the adult rodent olfactory system

Odorant receptors (ORs) are expressed in a spatially restricted manner in the mammalian olfactory epithelium (OE), and this patterning probably contributes to innervation specificity within the olfactory bulb (OB). Furthermore, glomerular targeting appears to be contingent on receptor choice. Central to the mechanism by which ORs influence axonal specificity is the timing of OR expression during the life cycle of the olfactory sensory neurons (OSNs). Data indicate that OSNs express ORs in the absence of the OB but do not address whether OR expression is an early event in OSN differentiation. Accordingly, we evaluated whether ORs are expressed in mature [olfactory marker protein (OMP(+))] and/or immature [growth-associated protein of 43 kDa m.w. (GAP-43(+))] OSNs by assessing the expression of the P2 OR subtype via immunostaining for beta-gal and concurrent OMP or GAP-43 expression in P2-IRES-tauLacZ mice. Nearly 90% of P2(+) OSNs expressed OMP, whereas approximately 10% expressed GAP-43. One month after unilateral bulb ablation, the number of P2(+) OSNs decreased on the lesioned side; however, the percent of P2(+)/GAP-43(+) OSNs dramatically increased. We also determined that onset of P2 OR expression is slightly delayed when evaluated in the context of neuronal differentiation. Additionally, we defined the expression of OR(+) OSNs in the OE of rats via in situ hybridization with a panel of eight ORs followed by OMP immunostaining. All eight ORs were found in neurons situated throughout the height of the OE, including those OSNs deep to OMP staining, thus demonstrating definitively that ORs are expressed prior to the maturational state defined by OMP expression.