The detection of bitter and sweet compounds by individual taste receptors in Insects

The gustatory system is essential for determining the palpability of potential food sources and avoiding the ingestion of toxic compounds. The genetic model insect, Drosophila melanogaster, utilizes a family of 68 Gustatory receptor (Gr) genes to detect bitter and sweet substances. In a previous study, we designed a unique system to functionally characterize individual taste receptors. Using the ab1C olfactory neuron, we individually expressed sweet receptors from a highly conserved clade and recorded neural activity using a panel of nine sweet stimuli. We discovered that each Gr is directly involved in the detection of sweet compounds. Additionally, each receptor detects unique but overlapping subsets of sugars. After successfully characterizing Grs in the sweet clade, we expanded our analysis to include Gr43a, an internal fructose receptor, and its Anopheles gambiae ortholog, AgGr25. Both receptors function in the Drosophila ab1C neuron and are directly involved in the detection of similar panels of sweet compounds, including fructose. Based on recent studies, we know that sweet neurons can be inhibited by bitter alkaloids in the absence of bitter Grs. One possible mechanism is that sweet Gr proteins are directly inhibited by bitter compounds. By using mixtures of sweet and bitter compounds, we tested this hypothesis with individual sweet receptors expressed in the ab1C neuron. We discovered that bitter compounds could directly inhibit sweet receptors, and in fact, most sweet receptors exhibited unique bitter inhibition profiles. One exception, Gr43a is immune to such inhibition. In summary, our results reveal that sweet receptors have complex interactions with both sweet and bitter compounds.