In addition, we were able to integrate the functional and expression data and predict a function for one Gr ( Figure 9). While our data support the hypothesis that Gr59c encodes a bitter receptor for BER, DEN, and LOB, Gr59c is not sufficient for responses to these compounds in sugar neurons. It is also apparently not necessary, in the sense that physiological responses to these tastants
were observed in S-a sensilla that do not express the Gr59c driver. These observations suggest that there is another receptor for BER, DEN, and LOB that may recognize a different moiety of these tastants, providing multiple means of detecting some of the most behaviorally aversive bitter tastants in the panel. We note that 38 of the Gr-GAL4 drivers, slightly more than half, showed expression in the labellum. The other selleck chemicals llc Selleckchem Ku 0059436 Grs are probably expressed in other chemosensory neurons of the adult and larva ( Dunipace et al., 2001, Jones et al., 2007, Kwon et al., 2007,
Scott et al., 2001 and Thorne and Amrein, 2008) (unpublished data, A.D., J.Y.K., L.A.W., F. Ling, and J.R.C.). Of the 38 labellar Gr-GAL4 drivers, 33 are expressed in bitter neurons, and only a few in sugar neurons. It seems probable that a high fraction of Grs are devoted to bitter perception because of the number and structural complexity of bitter compounds ( Schoonhoven et al., 2005 and Schwab, 2003). Sugars are simpler and more similar in structure. In order to detect the wide diversity of noxious bitter substances that an animal may encounter, a larger and more versatile repertoire of receptors is likely needed. We note that
in mice and rats, 36 bitter receptors have been identified ( Wu et al., 2005), but few sugar receptors ( Montmayeur et al., 2001 and Nelson et al., 2001). Among the Grs mapped to bitter neurons, five map to all bitter neurons: Gr32a, Gr33a, Gr39a.a, Gr66a, and Gr89a. Some or all of these “core bitter Grs” may function as coreceptors, perhaps forming multimers with other Grs. These core Grs might play a role analogous to Or83b, an Or that is MTMR9 broadly expressed in olfactory receptor neurons and that functions in the transport of other Ors and as a channel, rather than conferring odor specificity per se (Benton et al., 2006, Sato et al., 2008 and Wicher et al., 2008). If so, the core Grs may be useful in deorphanizing other Grs in heterologous expression systems. We note that in mammals, T1R3 functions as a common coreceptor with either T1R1 or T1R2 to mediate gustatory responses to amino acids or sugars, respectively (Zhao et al., 2003). We note finally that the receptor-to-neuron map defines intriguing developmental problems.