Editors' ChoiceIon Channels

Escape by Trace Amine

Science Signaling  02 Jun 2009:
Vol. 2, Issue 73, pp. ec180
DOI: 10.1126/scisignal.273ec180

The trace amine tyramine is found in the nervous systems of mammals and nematodes at lower concentrations than other biogenic amines (such as dopamine or serotonin). In Caenorhabditis elegans, tyramine is released by RIM motor neurons, which control locomotion and head movements thought to be involved in escape responses. Finding that exogenous tyramine inhibited head oscillations and induced backward locomotory body movements in wild-type animals, Pirri et al. (see also Branicky and Schafer) sought mutants resistant to the effects of exogenous tyramine. Their genetic screen identified lgc-55, which encoded a protein resembling a cysteine-loop ligand-gated ion channel. Electrophysiological analysis revealed that LGC-55 was a chloride channel activated by tyramine. A fluorescent expression reporter indicated that LGC-55 was present in neck muscles and neurons postsynaptic to the RIM motor neurons. When gently touched on the anterior portion of the body, wild-type C. elegans suppress exploratory head oscillations and reverse the direction of their locomotion. In response to anterior touch, lgc-55 loss-of-function mutants did not suppress head oscillations and did not move backward as far as wild-type animals moved (although they initiated this backward movement normally). Expression of an lgc-55 minigene in neck muscles rescued the defect in suppression of head oscillations, whereas expression of the minigene in AVB neurons restored reversal behavior. Because depolarization of the AVB neuron is required for forward movement, the authors proposed that tyramine inhibits forward locomotion by activating LGC-55 and hyperpolarizing the AVB neuron. Accordingly, exogenous tyramine increased reverse locomotion in lgc-55 mutants expressing the lgc-55 minigene only in AVB neurons. Thus, tyramine mediates fast inhibitory synaptic transmission involved in escape responses in C. elegans.

J. K. Pirri, A. D. McPherson, J. L. Donnelly, M. M. Francis, M. J. Alkema, A tyramine-gated chloride channel coordinates distinct motor programs of a Caenorhabditis elegans escape response. Neuron 62, 526–538 (2009). [PubMed]

R. Branicky, W. R. Schafer, Tyramine: A new receptor and a new role at the synapse. Neuron 62, 458–460 (2009). [PubMed]