Menthol acts as a positive allosteric modulator on nematode levamisole sensitive nicotinic acetylcholine receptors

Abstract

The ongoing and widespread emergence of resistance to the existing anti-nematodal pharmacopeia has made it imperative to develop new anthelminthic agents. Historically, plants have been important sources of therapeutic compounds and offer an alternative to synthetic drugs. Monoterpenoids are phytochemicals that have been shown to produce acute toxic effects in insects and nematodes. Previous studies have shown nicotinic acetylcholine receptors (nAChRs) to be possible targets for naturally occurring plant metabolites such as carvacrol and carveol. In this study we examined the effects of monoterpenoid compounds on a levamisole sensitive nAChR from Oesophagostomum dentatum and a nicotine sensitive nAChR from Ascaris suum. We expressed the receptors in Xenopus laevis oocytes and used two-electrode voltage-clamp to characterize the effect of various compounds on these cys-loop receptors. At 100 μM the majority of these compounds acted as antagonists. Interestingly, further experiments revealed that both 0.1 μM and 10 μM menthol potentiated acetylcholine and levamisole responses in the levamisole sensitive receptor but not the nicotine sensitive receptor. We also investigated the effects of 0.1 μM menthol on the contractility of A. suum somatic muscle strips. Menthol produced significant potentiation of peak contractions at each concentration of acetylcholine. The positive allosteric modulatory effects of menthol in both in vivo and in vitro experiments suggests menthol as a promising candidate for combination therapy with cholinergic anthelmintics.

Action of Carvacrol on <em>Parascaris</em> sp. and Antagonistic Effect on Nicotinic Acetylcholine Receptors

“…S1). As previously described [27], 100 µM carvacrol led to a significant decrease in the ACh maximum response (73.6 ± 1.7 %, p < 0.05, n=6). Increasing the carvacrol concentration to 300 µM further drastically reduced the effect of ACh (19.6 ± 1.5 %, p < 0.05, n=5).…”

“…and from the closely related species A. suum were able to form homomeric functional N-AChRs when expressed in Xenopus oocytes [35,36]. Recently, carvacrol proved to be a noncompetitive inhibitor of the Asu-N-AChR [27]. In order to investigate the mode of action of carvacrol in Parascaris sp., we applied carvacrol on oocytes expressing the Parascaris sp.…”

“…However, the antiparasitic mode of action of monoterpenoid compounds is poorly understood and their potential on horse parasites has not been investigated so far. Previous studies evidenced that the anthelmintic effect of carvacrol might be mediated through different ligand-gated ion channel subtypes including tyramine, acetylcholine and GABA receptors of nematodes [18,[25][26][27][28] as well as acetylcholinesterase [14].…”

EAT-18 is an essential auxiliary protein interacting with the non-alpha nAChR subunit EAT-2 to form a functional receptor

“…Two-electrode voltage-clamp in Xenopus oocytes. Oocyte injections and two-electrode electrophysiology recordings were performed as previously described [48].…”

Anthelmintic activity of Eucalyptus citriodora essential oil and its major component, citronellal, on sheep gastrointestinal nematodes

“…This monoterpenoid was recently shown to be a positive allosteric modulator of Oesophagostomum dentatum levamisole-sensitive nicotinic acetylcholine receptor. Thus, its use is suggested in combination with cholinomimetic drugs to increase anthelmintic effectiveness (CHOUDHARY et al, 2019).…”