TY - JOUR T1 - Menthol Stereoisomers Exhibit Different Effects on α4β2 nAChR Upregulation and Dopamine Neuron Spontaneous Firing JF - eneuro JO - eNeuro DO - 10.1523/ENEURO.0465-18.2018 VL - 5 IS - 6 SP - ENEURO.0465-18.2018 AU - Brandon J. Henderson AU - Stephen Grant AU - Betty K. Wong AU - Rezvan Shahoei AU - Stephanie M. Huard AU - Shyam S. M. Saladi AU - Emad Tajkhorshid AU - Dennis A. Dougherty AU - Henry A. Lester Y1 - 2018/11/01 UR - http://www.eneuro.org/content/5/6/ENEURO.0465-18.2018.abstract N2 - Menthol contributes to poor cessation rates among smokers, in part because menthol enhances nicotine reward and reinforcement. Mentholated tobacco products contain (−)-menthol and (+)-menthol, in varying proportions. We examined these two menthol stereoisomers for their ability to upregulate α4β2 nAChRs and to alter dopamine neuron firing frequency using long-term, low-dose (≤500 nm) exposure that is pharmacologically relevant to smoking. We found that (−)-menthol upregulates α4β2 nAChRs while (+)-menthol does not. We also found that (−)-menthol decreases dopamine neuron baseline firing and dopamine neuron excitability, while (+)-menthol exhibits no effect. We then examined both stereoisomers for their ability to inhibit α4β2 nAChR function at higher concentrations (>10 µm) using the Xenopus oocyte expression system. To probe for the potential binding site of menthol, we conducted flooding simulations and site-directed mutagenesis. We found that menthol likely binds to the 9´ position on the TM2 (transmembrane M2) helix. We found that menthol inhibition is dependent on the end-to-end distance of the side chain at the 9´ residue. Additionally, we have found that (−)-menthol is only modestly (∼25%) more potent than (+)-menthol at inhibiting wild-type α4β2 nAChRs and a series of L9´ mutant nAChRs. These data reveal that menthol exhibits a stereoselective effect on nAChRs and that the stereochemical effect is much greater for long-term, submicromolar exposure in mice than for acute, higher-level exposure. We hypothesize that of the two menthol stereoisomers, only (−)-menthol plays a role in enhancing nicotine reward through nAChRs on dopamine neurons. ER -