The neurotransmitter dopamine is known to regulate locomotion and egg-laying in C. elegans. In vertebrate systems dopamine is involved in more complex roles such reward and cognition, disruptions in which can lead to altered behavior and disease. Mutations in a D1-like dopamine receptor,
dar-1 (
ev748), were recently characterized;
dar-1::GFP was shown to be expressed in the mechanoreceptor neurons ALM and PLM. Since
dar-1 was expressed in the touch cells we reasoned that the mutant might have altered response to mechanical stimuli. Although
dar-1 mutant animals appeared normal in an assay for light touch sensitivity (Chalfie et al., 1985), they showed altered response to a non-localized mechanical stimulus, or tap, administered to the culture plate. Wildtype animals respond to tap by switching from reversals, mediated largely by ALM or, less frequently, by accelerating forward, a process mediated largely by PLM. Repeated stimulation of wild-type animals results in a decrease in the frequency of tap-induced reversals as well as a reduction in the distance traveled during a reversal, a process known as tap habituation (Rose and Rankin, 2001). We subjected the
dar-1 mutant and wild-type animals to a series of 30 taps administered 10 seconds apart, and measured frequency of reversals and distance traveled during reversals. We observed that
dar-1 and wild-type animals both initially exhibited high rates of reversals in response to tap stimuli. However, the
dar-1 mutants showed a much more rapid decline in reversal rate in response to habituation training than the wild-type strain. These alterations in tap habituation were rescued by a wild-type
dar-1 transgene expressed under its own promoter, indicating that the phenotype was indeed a consequence of the
dar-1 loss-of-function mutation. A habituation phenotype similar to
dar-1 was also observed in
cat-2 (
e1112), a tyrosine hydroxylase required for dopamine synthesis. Additionally
dar-1 was rescued by a touch cell specific, pmec7::
dar-1 transgene, indicating a cell autonomous role for
dar-1 in the touch cells. Optical imaging studies of neuronal activity should help resolve how dopamine is modulating habituation of the touch circuit.