Amino acids as taste stimuli for tsetse flies

This paper reports the responses of taste cells on the legs of the blood-feeding tsetse fly Glossina fuscipes fuscipes Newstead 1910 (Diptera: Glossinidae) to twenty protein amino acids and to their mixture as it is present in human sweat. It is investigated whether the mixture is sensed differently than the amino acids singly. The taste cells are most sensitive to phenylalanine (K approximately equal to 1 micromolar) and tyrosine; and they respond in a lesser degree to methionine, valine, isoleucine, cysteine, tryptophan, histidine, alanine, and threonine. The amino acids serine, proline, asparagine, arginine, glutamine, lysine, aspartic acid, glutamic acid, and glycine give little or no response even at 10 mM. As the succession of effectiveness of the amino acids appears to be the same for all cells, it is deduced that the flies are unable to discriminate the amino acids by comparing responses across sensory cells. A temporal coding of quality does not seem feasible either. Thus, the properties of the taste cells limit the sense to assessing the intensity of an amino acid stimulus and not its identity. Although several parameters in the response adaptation curves are concentration-dependent, it is suggested that the flies judge intensity of a stimulus only from the first 50 or so milliseconds. Although other studies and these indicate that a multiplicity of binding sites may be responsible for the reception of amino acids, the response to the mixture can be predicted from a no-interaction model, whereby each ligand's access to the binding sites is proportional to its mole fraction. It is argued that this may be the case for more of the naturally occurring mixtures which comprise structurally similar ligands. The responses to the mixture and to phenylalanine alone are equally susceptible to inhibition by sodium chloride. It is suggested that, although discrimination of hosts probably requires another sense, the sense of taste is an excellent tool to detect a host underfoot during the local search for a feeding site.