acr-16 encodes an essential subunit of the levamisole-resistant nicotinic receptor at the Caenorhabditis elegans neuromuscular junction

The Caenorhabditis elegans neuromuscular junction (NMJ) contains three pharmacologically distinct ionotropic receptors: gamma-aminobutyric acid receptors, levamisole-sensitive nicotinic receptors, and levamisole-insensitive nicotinic receptors. The subunit compositions of the gamma-aminobutyric acid- and levamisole-sensitive receptors have been elucidated, but the levamisole-insensitive acetylcholine receptor is uncharacterized. To determine which of the similar to 40 putative nicotinic receptor subunit genes in the C. elegans genome encodes the levamisole-resistant receptor, we utilized MAPCeL, a microarray profiling strategy. Of seven nicotinic receptor subunit transcripts found to be enriched in muscle, five encode the levamisole receptor subunits, leaving two candidates for the levamisole-insensitive receptor: acr-8 and acr-16. Electrophysiological analysis of the acr-16 deletion mutant showed that the levamisole-insensitive muscle acetylcholine current was eliminated, whereas deletion of acr-8 had no effect. These data suggest that ACR-16, like its closest vertebrate homolog, the nicotinic receptor alpha 7-subunit, may form homomeric receptors in vivo. Genetic ablation of both the levamisole-sensitive receptor and acr-16 abolished all cholinergic synaptic currents at the NMJ and severely impaired C. elegans locomotion. Therefore, ACR-16-containing receptors account for all non-levamisole-sensitive nicotinic synaptic signaling at the C. elegans NMJ. The determination of subunit composition for all three C. elegans body wall muscle ionotropic receptors provides a critical foundation for future research at this tractable model synapse.