Avian Olfactory Receptors: Differentiation of Olfactory Neurons under Normal and Experimental Conditions

Abstract

AbstractThe bird olfactory system has a simple structure and affords an attractive developmental model for the study of olfactory morphogenesis and differentiation. We have cloned and characterized several chick olfactory receptor (COR) genes belonging to the superfamily of seven-transmembrane domain proteins.In situhybridization analysis of their spatiotemporal patterns of expression during development reveals several important characteristics.CORexpression starts early in placodal cells (Embryonic Day 5, E5). Changes in their expression pattern then correlate with the onset of synaptogenesis (E8). The adult pattern, achieved before hatching, shows that cells expressing a particularCORare not regionalized within the epithelium. By double-labelin situhybridization, we clearly demonstrate that a single cell does not coexpress differentCORgenes (or subsets ofCORs) at any stage of development. Following bulbar deafferentation,CORexpression ceases more rapidly than expected from previous axotomy experiments. Concomitantly, a reactivation of theCash-1gene, which is involved in early neuronal specification, could be an early sign of olfactory neuronal regeneration. Modulation ofCORandCash-1expression points to a simultaneous process of neuronal degeneration and regeneration in the olfactory epithelium after axotomy.CORexpression is restricted to the olfactory epithelium except during early stages (before synaptogenesis). At that time, cells distributed along the olfactory nerve, from the placode to the anterior telencephalon, also expressCORs. This cell population is different from the luteinizing hormone releasing hormone neurons migrating from the placode. Our results show that the olfactory neurons or neuroblasts choose to express oneCORbefore establishing functional connections with the bulb. Later on, bulboepithelial connections seem important not only for olfactory neuron survival but also for stimulation ofCORexpression. In addition, beyond their implication in functional odor detection, CORs could be involved, at early stages, in processes of olfactory morphogenesis, including the establishment of a bulbar chemotopy