Menthol Binding and Inhibition of α7-Nicotinic Acetylcholine Receptors

Abstract

Menthol is a common compound in pharmaceutical and commercial products and a popular additive to cigarettes. The molecular targets of menthol remain poorly defined. In this study we show an effect of menthol on the α7 subunit of the nicotinic acetylcholine (nACh) receptor function. Using a two-electrode voltage-clamp technique, menthol was found to reversibly inhibit α7-nACh receptors heterologously expressed in Xenopus oocytes. Inhibition by menthol was not dependent on the membrane potential and did not involve endogenous Ca2+-dependent Cl− channels, since menthol inhibition remained unchanged by intracellular injection of the Ca2+ chelator BAPTA and perfusion with Ca2+-free bathing solution containing Ba2+. Furthermore, increasing ACh concentrations did not reverse menthol inhibition and the specific binding of [125I] α-bungarotoxin was not attenuated by menthol. Studies of α7- nACh receptors endogenously expressed in neural cells demonstrate that menthol attenuates α7 mediated Ca2+ transients in the cell body and neurite. In conclusion, our results suggest that menthol inhibits α7-nACh receptors in a noncompetitive manner.

Menthol Enhances the Desensitization of Human α3β4 Nicotinic Acetylcholine Receptors

“…Their data also suggested that menthol acted as a negative allosteric modulator. Ashoor et al (2013) found that menthol attenuated function of a7 nAChRs expressed in Xenopus oocytes but did not compete for a7 receptor binding sites, again suggesting a negative allosteric effect.…”

“…These modulators include metal ions (Vernino et al, 1992;Zwart et al, 1995;Hsiao et al, 2001), steroid hormones (Valera et al, 1992;Ke and Lukas, 1996;Paradiso et al, 2000;Curtis et al, 2002), and small synthetic ligands (Bertrand and Gopalakrishnan, 2007;Moaddel et al, 2007;Henderson et al, 2010). In fact, menthol has recently been reported to allosterically inhibit currents mediated by a4b2 (Hans et al, 2012) and a7 (Ashoor et al, 2013) nAChRs. However, to our knowledge menthol is the first example of a drug demonstrated to act allosterically to augment desensitization of a neuronal nAChR without activating or even binding to the receptor's orthosteric site.…”

“…Interestingly, two recent studies revealed that menthol inhibits ACh and nicotine-stimulated currents in heterologously expressed a4b2 (Hans et al, 2012) and a7 (Ashoor et al, 2013) nAChRs, subtypes predominantly expressed in the brain, and produces a slow, time-dependent inhibition of ACh-and nicotine-evoked currents in cells from the trigeminal ganglia (Hans et al, 2012). In each case, menthol appeared to act allosterically.…”

Allosteric modulation of nicotinic acetylcholine receptors

“…These include psychoactive drugs such as mecamylamine and phencyclidine [105][106][107][108][109], philanthotoxin [110], general anaesthetics [111], antidepressants [112][113][114], monoterpines such as camphor, menthol and propofol [115][116][117][118][119], aminoglycoside antibiotics [120] antipsychotics [121], the fluorophore crystal violet [122]; zinc [99,123] and some detergents [124]. Several endogenous ligands have also been reported to have negative allosteric effects on nAChRs.…”

“…Despite relatively small changes in chemical structure, the compounds examined displayed five distinct pharmacological effects on α7 nAChRs. These included effects typical of type I PAMs, type II PAMs, NAMs, SAMs and allosteric agonists [33] and it has been proposed that all of these pharmacological effects can arise from ligands binding to a broadly similar or overlapping site located within the previously identified intra-subunit transmembrane cavity A transmembrane binding site in nAChRs has also been proposed for monoterpine compounds such as menthol and propofol [116,117] and is consistent with evidence supporting a transmembrane binding site for monoterpines on other pentameric LGICs [143][144][145][146][147]. Affinity labelling studies with a photoreactive analogue of propofol identified three sites at which it bound within the transmembrane domain of muscle-type nAChRs, but concluded that the functionally relevant site for the inhibitory action of propofol was an intra-subunit site [117], similar to that described earlier for α7-selective allosteric modulators such as NS-1738, PNU-120596 and 4BP-TQS [30,139].…”