The ASH sensory neuron in C. elegans is polymodal, responding to both noxious (high osmolarity/volatile repellents) and mechanical (nose touch) stimuli to initiate backward locomotion. Examination of one ASH-mediated locomotory behavior, avoidance to octanol, has revealed that modulation of the ASH neural circuit is complex and involves multiple monoamines and distinct amine receptors (Wragg et al., 2007; Harris et al., 2009). In the present study, we have identified the monoamine receptors involved in another 5-HT stimulated ASH-mediated aversive response, nose touch. Like responses to octanol, multiple 5-HT receptors also have been identified in the food/5-HT sensitization of nose touch. For example, the expression of
ser-5 in the ASHs appears to be essential for 5-HT dependent increases in aversive responses to nose touch. In contrast, both TA and OA inhibit nose touch and two monoamine receptors (F14D12.6 and SER-3) appear to be involved in octopaminergic inhibition. These receptors are currently being localized in the ASH locomotory circuit. Using the calcium indicator cameleon, we analyzed ASH Ca2+ responses to nose touch in
ser-5 null animals. ASH Ca2+ transients were independent of exogenous 5-HT and more robust in
ser-5 null animals than in wild-type or
ser-4;
mod-1;
ser-7 ser-1 quadruple null animals that presumably only express
ser-5. Since we predict that SER-5 stimulates neurotransmitter release at the ASH synapse, the increased Ca2+ signaling in the ASH soma was surprising. We are currently exploring whether SER-5 signaling has other effects on the ASH or whether SER-5 also modulates the release of other ligands, perhaps peptides, that inhibit ASH Ca2+ dynamics.