Source: Dissertation Abstracts International, Volume: 78-11(E), Section: B.

Adviser: John R. Carlson.

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Nitrogenous compounds play an important role in the insect world. They include amino acids, which are building blocks and breakdown products of proteins. Proteins are composed of one or more polypeptide chains, each of which is a string of amino acids linked by peptide bonds. Another nitrogenous compound is ammonia, a compound released by humans that is believed to elicit a host-seeking response in insects such as Anopheles gambiae, a vector for plasmodia that cause malaria. Amino acids and ammonia are two components of proteins that are found ubiquitously in nature; however, little is known about the taste responses they elicit from Drosophila melanogaster or the cellular mechanisms underlying these responses.

The fly has four taste organs: the labellum, pharynx, legs, and wings. The principal taste organ on the head, the labellum, contains 31 chemosensory hairs composed of 3 morphological classes of sensilla arranged in a stereotyped pattern. In order to characterize the cellular basis of amino acid response in the Drosophila labellum, we used single-unit electrophysiological to measure systematically the response of all 31 labellar sensilla to 19 amino acids. We also used behavioral experiments to measure the responses of the fly to amino acids. We found that S sensilla yielded the strongest responses, and that S sensilla were heterogeneous in their response profiles. Different amino acids elicited different responses from the various S sensilla. The strongest responses of the S sensilla were to tryptophan. We found no responses to tryptophan in S sensilla in which bitter-sensing neurons were ablated, suggesting that these neurons may be required in S sensilla for tryptophan response. Tryptophan, despite its relatively strong physiological response, yielded little if any behavioral response in a CAFE assay. Phenylalanine, which elicited a relatively strong behavioral response, also elicits a physiological response from S sensilla, but with a different profile than tryptophan. The differences in profiles between phenylalanine and tryptophan support the possibility that different amino acids may be differentially encoded by the taste neuron repertoire.

In a separate set of experiments, we investigated ammonia's role in both the gustatory and olfactory system. In the taste system, I found that ammonia is an aversive taste cue using a CAFE behavioral assay. Through electrophysiology experiments, we found that nearly all sensilla of the Drosophila labellum respond to neutral solutions of ammonia. Ammonia is toxic at high levels to many organisms, and we found that it has a negative valence in a second behavioral paradigm operating over a shorter period of time. Physiological and behavioral responses to ammonia depend at least in part on Gr66a + bitter-sensing taste neurons, which activate a circuit that deters feeding. The Amt transporter, a critical component of olfactory responses to ammonia, is widely expressed in taste neurons but is not required for taste responses. This work establishes ammonia as an ecologically important taste cue in Drosophila, and shows that it can activate circuits that promote opposite behavioral outcomes via different sensory systems.

In the olfactory system, I found that an ammonium transporter gene that is essential for ammonia responses in a class of coeloconic olfactory receptor neurons (ORNs) is strongly expressed in the antennae, with weak expression in the leg, using RT-PCR. Further analysis from our group found that the transporter is not expressed in ORNs, but rather in neighboring auxiliary cells. We found an unexpected non-cell autonomous role for a component that is essential to the olfactory response to ammonia. The defective response observed in a Drosophila mutant of this gene was rescued by its Anopheles ortholog, and orthologs are found in virtually all insect species examined, suggesting that its role is conserved.

Taken together, these works characterize and provide cellular insight into the detection of amino acids and ammonia. We hope that understanding this chemosensory signaling will provide fundamental information useful in combating insect vectors.