Biological activities of essential oils on Caenorhabditis elegans: from molecular target to anthelmintic therapeutic strategies

Abstract

Plants, herbal preparations and essential oils are used for health and medical functions since ancient times. Essential oils (EOs) have been extensively applied for human and veterinary health; their active agents have been isolated and incorporated into many current pharmaceutical preparations. Our study involves the use of Caenorhabditis elegans as a model of parasitic nematodes due to its simplicity and easy maintenance in the laboratory. We start from the hypothesis that EOs of aromatic plants used for aromatherapy or as food additives have anthelmintic action. We developed behavioral and molecular assays in wild-type worms, and analyzed mutant strains lacking receptors involved in locomotion to identify the pharmacological targets mediating the anthelmintic activities. We also explored the combination of current anthelmintics together with the addition of EOs purified compounds as a strategy to reduce resistance. In paralysis assays on agar plates, all tested EOs inhibited with different potencies C. elegans locomotion as well as egg hatching. The major compounds present in the tested EOs, transcinnamaldehyde (TC), geraniol, citronellol and linalool, were active at C. elegans. The combination of TC with the commercial anthelmintics levamisole and monepantel showed synergistic paralysis effects, while its combination with piperazine or ivermectin produced antagonic effects. Mutant worms lacking the levamisole-sensitive nicotinic receptor (L-AChR), GABA and glutamate receptors were partially resistant to these compounds. By single-channel recordings from C. elegans muscle cultured cells we deciphered how L-AChRs are modulated by TC. The analysis revealed that TC acts as an allosteric inhibitor of L-AChRs with key roles in nematode locomotion. Overall, we identified essential oils and their components as novel anthelmintic drugs and revealed their main pharmacological targets. Our results propose EOs as sources of natural compounds with promising polypharmacological profiles for anthelmintic therapeutics, and provide data on the efficacies of combinations that emerge as strategies to reduce drug resistance in nematodes