Inhibition of Human α4β2 Neuronal Nicotinic Acetylcholine Receptors by Volatile Aromatic Anesthetics Depends on Drug Hydrophobicity

Volatile aromatic compounds such as benzene are general anesthetics that produce amnesia, hypnosis, and immobility in response to noxious stimuli when inhaled.1 Although these compounds are not used clinically, they are frequently found in commercial items such as solvents and household cleaning products and are abused as inhalant drugs. They are also used as pharmacological tools for assessing the importance of structural properties in defining anesthetic potency on relevant molecular targets. In a previous study, Raines et al.2 demonstrated that the inhibitory potencies of volatile aromatic anesthetics for human NR1/NR2B N-methyl-D-aspartate (NMDA) receptors correlate strongly with the abilities of the drugs to form cation-π binding interactions. Anesthetics with high π-electron density (such as benzene) were nearly 100-fold more potent than highly fluorinated compounds with low π-electron density (such as hexafluorobenzene), suggesting that aromatic anesthetics ind to an NMDA receptor site that contains a positive charge, leading to subsequent inhibition of receptor function. Recent studies have shown that volatile aromatic anesthetics also produce γ-aminobutyric acid type A (GABAA) receptor enhancement3 and voltage-gated sodium channel inhibition,4 which may also contribute to the behavioral effects of these drugs. Neuronal nicotinic acetylcholine (nACh) receptors are cation-selective ligand-gated ion channels that are widely expressed in the brain. Postsynaptic neuronal nACh receptors mediate excitatory synaptic transmission, whereas presynaptic receptors modulate the release of several neurotransmitters including GABA, dopamine, and glutamate.5–7 Neuronal nACh receptors are thought to mediate a variety of important brain functions such as learning and memory.8 –10 One of the most common neuronal nACh receptor subtypes found in the brain is a heteromer composed of α4 and β2 subunits,11 and α4β2 nACh receptors have been shown to be highly sensitive to a wide range of inhaled and IV general anesthetics.12–17 Some inhalation anesthetics such as cyclopropane and butane, which do not appreciably enhance the function of GABAA receptors at clinically relevant concentrations, are potent inhibitors of α4β2 nACh receptors.18 Although nACh receptors do not mediate anesthetic-induced immobility,19,20 nonimmobilizing halogenated alkanes such as F6 have been shown to inhibit α4β2 nACh receptors at concentrations that cause memory impairment in rats,18,21 suggesting that neuronal nACh receptors may play a role in mediating anesthetic-induced amnesia. Because neuronal nACh receptors are inhibited by many inhaled and IV general anesthetic drugs, we hypothesized that they are also reversibly inhibited by volatile aromatic anesthetics. We used electrophysiological techniques to assess the inhibitory potencies of 8 aromatic compounds for human α4β2 nACh receptors and used a similar approach from the previous study with NMDA receptors2 to establish the structural correlates of α4β2 nACh receptor inhibition.