Ith the receptor in its closed stateEffect of menthol on other ligand gated channelsThe observed inhibitory action of menthol appeared to become dependent on the duration permitted for interaction involving the menthol as well as the nAChR at the same time because the conformational state with the receptor protein itself. Allowing menthol to interact with nAChR before channel opening resulted in a rise of its inhibitory activity around the nAChR by ;80 (from 37 to 66 ). Conversely, when the interaction between menthol and nAChR occurred following channel opening, the efficacy of its inhibitory activity was reduced to six . Rising the menthol concentration from 100 to 200 lM didn’t result in a additional enhance of current inhibition. The tiny degree of inhibition observed with all the nAChR inside the open conformation is unlikely as a result of lowered interaction time in between the menthol along with the receptor, as saturation of the present inhibition is reached inside 60 on the total menthol application time (200 ms, see Figure 1B). These findings suggest that interaction between menthol and nAChR is facilitated if the channel protein is in the closed state conformation. Transition from the nAChR to its open conformation obscures the menthol interaction website, which consequently results within a lower efficacy of menthol around the protein complicated.Menthol inhibits the nAChR by allosteric modulationBesides its 5945-86-8 site modulator impact on opioid receptors (Galeotti et al. 2002), menthol has recently been shown to be a particular modulator of ionotropic inhibitory receptors. For example, (+) menthol acts as a optimistic modulator of recombinant GABAA and glycine receptors expressed in Xenopus oocytes (Hall et al. 2004). In these situations, the allosteric-binding web-site for menthol is also a binding site for other pharmacologically active substances such as the anesthetic propofol (Watt et al. 2008). For that reason, it would be of interest to analyze if, for example, propofol, which has some structural similarities with menthol, exerts effects on the nAChR and if it may bind to a common website.Menthol and nicotine interactionThe most recent findings by Willis et al. (2011) showed that menthol acts as a broad-spectrum counterirritant since it reduced respiratory irritation response of various respiratory irritants identified in tobacco smoke. Their data recommend a role of TRPM8 pathways by way of which activation of TRPM8 by menthol leads to inhibition on the respiratory irritation response. The mechanism underlying this action is currently unknown. Our data extend the findings by Willis et al. (2011) and show that menthol can act as counterirritant directly in the receptor of a major irritant contained in tobacco smoke, nicotine (Lee et al. 2007).AcknowledgementsOur benefits indicate that the effect of menthol will not rely on a competitive antagonism. This can be recommended by the finding that the EC50 values in the dose esponse curve for nicotine and nicotine plus menthol, respectively, are certainly not substantially unique. On the other hand, the dose esponse curve is shifted downward reflecting the reduction of the present amplitude more than the whole concentration variety. It might be ruled out that menthol acts as competitive antagonist around the nAChR. Within this case, 1 would anticipate a slowing of activation kinetics of whole-cell currents, which was not observed in our experiments (see Figure 2A). For noncompetitive inhibition, a single can distinguish at the very least two different mechanisms. Menthol could act as pore blocker and sterically interfere wi.
