C. elegans muscle Cys-loop receptors as novel targets of terpenoids

Abstract

In Caenorhabditis elegans, the appropriate balance of ACh and GABA signaling is required for coordinated movement. The muscle levamisole-sensitive acetylcholine receptor (L-AChR) and the GABA A receptor (UNC-49) are involved in C. elegans muscle contraction and are targets of anthelmintic drugs. We have identified three terpenoids, thymol, carvacrol and eugenol, that paralyze C. elegans by acting at these receptors. By using a primary culture of C. elegans L1 muscle cells we explore their actions at the single-channel and macroscopic current levels. Single-channel activity of L-AChR elicited by ACh can be readily detected from muscle cells. All compounds (< 0.1 mM) produce a dramatic reduction in the frequency of opening events without affecting channel properties. Channel activity elicited by ACh remains constant during the recording but disappears within the first minutes in the presence of the compounds, indicating increased desensitization or slow open-channel block. For evaluating their actions at both ACh and GABA receptors, we recorded macroscopic currents from cultured L1 muscle cells. Thymol is not capable of eliciting responses but it significantly inhibits ACh and GABA-responses. Behavioral assays show that thymol potentiates the paralyzing actions of established anthelmintic agents, indicating that it can be used in combination, which is as a strategy to reduce acquisition of resistance. By acting at the NMJ these terpenoids emerge as novel anthelmintic agents.