The composition of the GABA receptor at the Caenorhabditis elegans neuromuscular junction

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Abstract
The unc‐49 gene of the nematode Caenorhabditis elegans encodes three γ‐aminobutyric acid type A (GABAA) receptor subunits. Two of these, UNC‐49B and UNC‐49C, are expressed at high abundance and co‐localize at the neuromuscular junction. The UNC‐49B subunit is sufficient to form a GABAA receptor in vitro and in vivo. Furthermore, all loss‐of‐function unc‐49 alleles lack functional UNC‐49B. No mutations specifically inactivate UNC‐49C. Thus, UNC‐49C appears to be dispensable for receptor function; however, UNC‐49C has been conserved among different nematode species, suggesting it plays a necessary role. To ascertain whether UNC‐49C is part of the GABAA receptor in vivo, we performed patch‐clamp electrophysiology on C. elegans muscle cells. Sensitivity to GABA, and to the antagonists picrotoxin and pregnenolone sulfate, matched the UNC‐49B/C heteromer rather than the UNC‐49B homomer, for both exogenous and synaptically‐released GABA. The synaptic localization of UNC‐49C requires the presence of UNC‐49B, indicative of a physical association between the two subunits in vivo. Thus, the in vivo receptor is an UNC‐49B/C heteromer. UNC‐49C plays a negative modulatory role. Using the rapid ligand‐exchange technique in vitro, we determined that UNC‐49C causes accelerated receptor desensitization. Previously, UNC‐49C was shown to reduce single‐channel conductance in UNC‐49B/C heteromers. Thus, the function of UNC‐49B is to provide GABA responsiveness and localization to synapses, while the function of UNC‐49C is to negatively modulate receptor function and precisely shape inhibitory postsynaptic currents. British Journal of Pharmacology (2005) 144, 502–509. doi:10.1038/sj.bjp.0706052