The C. elegans G protein subunit G-alpha-0 is encoded by the gene G-alpha-0. It has been implicated in mediating the effect of 5-HT on locomotion, defecation and egg laying (S!galat et al., 1995). 5-HT also stimulates pharyngeal muscle but
goa-1 mutations do not alter this response suggesting that 5-HT signaling in this muscle does not involve G-alpha-0. However, G-alpha-0 is expressed in the pharyngeal muscle and
goa-1 mutations, such as
n1134, cause defective pharyngeal pumping. This indicates that G-alpha-0 is involved in the regulation of pumping, but by a neurotransmitter other than 5-HT. Here we present electrophysiological data indicating that octopaminergic modulation of pharyngeal pumping differs between wild-type and
goa-1 mutants. The anterior region of C. elegans was sectioned from the body and placed in a perfusion chamber on an inverted microscope stage in modified Dent!s saline (composition in mM: NaCl 144, MgCl2 10, CaCl2 1, KCl 6, HEPES 5, pH 7.4). Intracellular recordings were made from pharyngeal muscle (60-100 MOhm microelectrodes; 3M KAcetate), connected to an Axoclamp 2A amplifier. Drugs were applied by perfusion . Firstly we made a quantitative comparison of the increase in action potential frequency elicited by 5-HT in the pharynxes of wild-type and
n1134 C. elegans, to confirm the results of the behavioural studies that suggested that the 5-HT response was not mediated by Go. The 5-HT concentration-response curves were not different: EC50 in wild type was 124 nM (95% confidence limits 28-558 nM; n=6) and for
n1134 it was 90 nM (95% confidence limits, 21-383 nM; n=6). We then went on to look at the wild-type response to octopamine. The preparation was perfused with 500 nM 5-HT to maintain pumping. Octopamine (threshold 500 nM) decreased action potential frequency (n=10). In some preparations this inhibition was followed by a transient excitation. For
n1134, octopamine was significantly less effective at reducing action potential frequency compared to wild type (n=6). We are currently investigating responses elicited in a second
goa-1 mutant,
n363 and in a
goa-1(xs) mutant. Reference: S!galat, L., D. A. Elkes and J. M. Kaplan. 1995. Modulation of serotonin-controlled behaviors by Go in Caenorhabditis elegans. Science 267: 1648-1651. Acknowledgments: Many thanks to Laurent S!galat for providing the
n1134 C. elegans. Chris Franks is funded by the BBSRC.