Menthol Stereoisomers Exhibit Different Effects on alpha4beta2 nAChR Upregulation and Dopamine Neuron Spontaneous Firing.
Menthol Stereoisomers Exhibit Different Effects on alpha4beta2 nAChR Upregulation and Dopamine Neuron Spontaneous Firing.
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DOI：
10.1523/eneuro.0465-18.2018
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发表时间：
2018-11-01
期刊：
影响因子：
3.4
通讯作者：
Lester, Henry A
中科院分区：
文献类型：
--
作者：
Henderson, Brandon J;Grant, Stephen;Lester, Henry A
薄荷醇导致吸烟者戒烟率低，部分原因是薄荷醇增强了尼古丁的奖励和强化。加香烟草产品含有不同比例的（-）-薄荷醇和（+）-薄荷醇。我们研究了这两种薄荷醇立体异构体上调α 4 β 2 nAChR的能力，并使用与吸烟无关的长期低剂量（≤500 nm）暴露来改变多巴胺神经元放电频率。我们发现（-）-薄荷醇上调α 4 β 2 nAChR，而（+）-薄荷醇不上调。我们还发现，（-）-薄荷醇降低多巴胺神经元基线放电和多巴胺神经元兴奋性，而（+）-薄荷醇表现出无影响。然后，我们使用非洲爪蟾卵母细胞表达系统检查了两种立体异构体在较高浓度（>10 μ M）下抑制α 4 β 2 nAChR功能的能力。为了探测薄荷醇的潜在结合位点，我们进行了洪水模拟和定点诱变。我们发现薄荷醇可能结合到TM 2（跨膜M2）螺旋上的9位。我们发现薄荷醇的抑制作用依赖于9位残基侧链的端到端距离。此外，我们已经发现，在抑制野生型α 4 β 2 nAChR和一系列L9突变nAChR方面，（-）-薄荷醇仅比（+）-薄荷醇略微（25%）更有效。这些数据表明，薄荷醇表现出立体选择性nAChRs的效果和立体化学效果是更大的长期，亚微摩尔暴露在小鼠比急性，更高水平的曝光。我们假设，这两个薄荷醇立体异构体，只有（-）-薄荷醇发挥作用，通过多巴胺神经元上的nAChR增强尼古丁的奖励。
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 alpha4beta2 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 alpha4beta2 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 alpha4beta2 nAChR function at higher concentrations (>10 m) 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 alpha4beta2 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.