Sugar receptor specificity in the fleshfly, Sarcophaga bullata.

The sugar receptor of insects is a useful preparation for studies of receptor specificity compared to the more complex sensors of vertebrates. Unlike the vertebrates, insects appear to respond behaviorally only to carbohydrates. Von Frisch v and Dethier a using the honey bee, Apis mellifera, and the blowfly, Phormia regina, respectively, were able to make inferences concerning a common structure for behaviorally attractive and therefore presumably sweet-tasting molecules. They concluded that (1) The size of the sapid molecule is important. Molecules smaller than pentoses are uniformly ineffective; monoand disaccharides are generally effective whereas only a few trisaccharides are effective. (2) Acyclic polyols such as D-glucitol are ineffective; in this instance, there appears to be a requirement for a ring structure. (3) The ring oxygen is not required; at least one hexahydroxycyclohexane isomer is active. (4) Sugars with a-glycosidic linkages are more effective than the corresponding fl-anomers. Using the blowfly, Phormia regina, Evans 5 initiated correlative studies on chemical structure and the gustatory response of insects. The principle conclusion was that sugars interacting with a putative 'glucose site' must be in the pyranose form and that the configuration of the C-3 and C-4 hydroxyl groups determines the responsiveness. Additional interest in this model was generated by the demonstration of electrophysiological responses from insect labellar hairs generally correlated with the positive behavioral response and therefore, by implication, identified as a sweet receptor spike 11. Subsequent electrophysiologica119, ~0 and behavioral 4 studies have indicated that in some flies there is only one variety of sweet taste receptor cell, but that it may have multiple receptor sites. Previous studies with the inositol receptor of the silkworm, Bombyx mori ~5, suggested to us that the cyclitols might be relatively simple and suitable model compounds for study of the stereospecificity of the sugar receptor in the fly. In the present investigation we have in addition measured electrophysiological responses to a number of previously examined as well as many previous-