Drosophila olfactory system

The powerful analysis that is possible with the detailed description of the Drosophila olfactory system may well be hampered by a lack of knowledge in two areas of research that need more attention the chemistry and behavioral ecology of the adequate stimuli for this system. 

Couto A., Alenius, M. and Dickson, B. J. (2005). Molecular, anatomical, and functional organization of the Drosophila olfactory system. Curr. Biol., 15,1535-1547. 

Couto A, Alenius M, Dickson BJ (2005) Molecular, anatomical, and functional organization of the Drosophila olfactory system. Curr Biol 15 1535-1547 Crittenden JR, Skoulakis EM, Han KA, Kalderon D, Davis RL (1998) Tripartite mushroom body architecture revealed by antigenic markers. Learn Mem 5 38-51 Dahanukar A, Foster K, van der Goes van Naters WM, Carlson JR (2001) A Gr receptor is required for response to the sugar trehalose in taste neurons of Drosophila. Nat Neurosci 4 1182-1186 Dahanukar A, Lei Y-T, Kwon JY, Carlson JR (2007) Two Gr genes underlie sugar reception in Drosophila. Neuron 56 503-516... 

In this model, OBPs participate in the selective transport of pheromone and other semiochemicals to their olfactory receptors. The selectivity of the system is likely to be achieved by layers of filters [ 16], i.e., by the participation of compartmentalized OBPs and olfactory receptors. It seems that OBPs transport only a subset of compounds that reach the pore tubules. Some of these compounds may not bind to the receptors compartmentalized in the particular sensilla. The odorant receptors, on the other hand, are activated by a subset of compounds, as indicated by studies in Drosophila, showing that a single OR is activated by multiple compounds [66]. If some potential receptor ligand reaches the pore tubules but are not transported by OBPs, receptor firing is prevented because the receptors are protected by the sensillar lymph. In other words, even if neither OBPs nor odorant receptors (ORs) are extremely specific, the detectors (olfactory system) can show remarkable selectivity if they function in a two-step filter. 

At first glance, labeled-line coding of olfactory signals may seem in contrast to the ensemble or across-fiber code (Shepherd, 1985) where complex mixtures of odorants or even individual odorant components are perceived as patterns of activity across an ensemble of neurons and AL glomeruli. However, recent experiments examining odor coding of individual ORNs in Drosophila and mammalian olfactory systems demonstrate that individual ORNs are capable of a wide spectrum of responses. In the fly, a particular odor can excite one neuron while inhibiting another, and a particular neuron can be excited by one odor and... 

McKenna M. P., Hekmat-Scafe D. S., Gaines P. and Carlson J. R. (1994) Putative Drosophila pheromone-binding proteins expressed in a subregion of the olfactory system. J. Biol. Chem. 269, 16340-16347. 

Recently, a putative olfactory receptor from Drosophila, Or43a (Clyne et al., 1999 Vosshall et al., 1999), has been expressed in Xenopus laevis oocytes (Wetzel et al., 2001). The receptor expressed in a heterologous cell system was activated by four odorants, i.e. cyclohexanone, cyclohexanol, benzaldehyde, and benzyl alcohol (Wetzel et al., 2001). These experiments not only provided direct evidence for the function of the Or gene, but also demonstrated that the olfactory receptor can be stimulated without an odorant-binding protein. It was demonstrated earlier that PBP was not necessary to obtain pheromone-dependent responses in cultured olfactory receptor neurons of Manduca sexta (Stengl et al., 1992). The possibility that OBPs have been produced in vitro and were present in cultured ORNs could not be excluded. The same argument can not be raised for the heterologous expression of the Drosophila olfactory receptor. While the evidence that Xenopus oocytes responded to odorants in the absence of OBPs does not support the OBP-odorant complex model, it also demonstrated that OBPs are essential for the kinetics of the olfactory system (see below). 

Stocker, 1994). While mammals possess millions of OSNs that relay information to thousands of olfactory bulb glomeruli, adult Drosophila have a mere 1300 OSNs connected to 43 antennal lobe glomeruli. This vastly simplified olfactory system, that nevertheless retains many of the anatomical features found in the mammalian olfactory system, make the fly an excellent model system in which to study the sense of smell. 

This chapter will discuss the isolation of Drosophila odorant receptor (DOR) genes, how these genes have expanded our understanding of the development and functional anatomy of the olfactory system, how the odor response profiles of OSNs respond to odorants, and the mechanisms by which odor-specific activity is relayed to the brain. 

Figure 23.5 Overview of pathways for olfactory processing in the Drosophila brain. Schematic outline of major wiring principles in the olfactory system (not to scale).

Ayyub C., Rodrigues V., Hasan G. and Siddiqi O. (2000) Genetic analysis of olfC demonstrates a role for the position-specific integrals in the olfactory system of Drosophila melanogaster. Mol. Gen. Genet. 263, 498-504. 

Gerber, Stocker, Tanimura and Thum use Drosophila to elucidate the generation of behavior from olfactory and gustatory sensation. The functional anatomy of Drosophila olfactory receptor neurons is described both for mature flies and larvae, which emerge as simpler model system with fewer olfactory receptors and with attraction and repulsion as easily testable, behavioral outcomes. 

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