Our finding of functional heterogeneity in labellar sensilla is c

Our finding of functional heterogeneity in labellar sensilla is consistent with the finding that two taste sensilla on the prothoracic leg responded to this website BER but not quinine, whereas another sensillum responded to quinine but not BER (Meunier et al., 2003). A recent study found that N,N-diethyl-m-toluamide (DEET) elicited different responses from several labellar sensilla tested (Lee et al., 2010). Functionally distinct bitter neurons have also been described in taste organs of caterpillars, and in the case

of the Manduca larva, ARI and salicin activate spike trains that differ in dynamics ( Glendinning et al., 2002 and Glendinning et al., 2006). The functional differences among neurons in the Drosophila labellum suggested underlying molecular differences. In particular, we wondered whether the four classes of bitter taste neurons defined by physiological

analysis could be distinguished by molecular analysis. We constructed a receptor-to-neuron map of the entire IOX1 solubility dmso Gr repertoire and found that four classes of bitter taste neurons emerged on the basis of receptor expression, classes that coincided closely with the four functional classes. Moreover, the neuronal classes that were more broadly tuned expressed more receptors. While the physiological and molecular analyses support each other well, there are limitations to each analysis that raise interesting considerations. Our functional analysis is based on a limited number of taste stimuli. We selected bitter tastants that were structurally diverse, but bitter compounds vary enormously in structure and only a small fraction during of them can be sampled. It is possible that by testing more tastants, by testing them over a greater concentration range, or by analyzing temporal dynamics in greater detail that even more diversity

would become apparent among the bitter-sensing neurons. There are also limitations to our receptor-to-neuron map. First, the map considers exclusively the 68 Grs. There are at least two additional receptors that can mediate bitter taste. DmXR, a G protein-coupled receptor, is expressed in bitter neurons of the labellum and is required for behavioral avoidance of L-canavanine, a naturally occurring insecticide (Mitri et al., 2009); the TRPA1 cation channel, also expressed in a subset of bitter neurons in the labellum, is required for behavioral and electrophysiological responses to ARI (Kim et al., 2010). Second, Gr-GAL4 drivers may not provide a fully accurate representation of Gr gene expression in every case. Genetic analysis has shown that Gr64a is required for the physiological responses of labellar sensilla to some sugars and is therefore expected to be expressed in labellar sugar neurons ( Dahanukar et al., 2007). Our Gr64a-GAL4 driver, however, is not expressed in these neurons, suggesting the lack of a regulatory element.

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