Nicotine's remarkable propensity for inducing addiction starts with its ability to bind to brain acetylcholine (ACh) receptors with high affinity. If it activated the ACh receptors in muscle, nearly identical to those in the brain, with similar efficiency, smoking would cause severe muscle contractions. That this doesn't happen has been a long-running pharmacological puzzle, now solved in an in-depth study of the chemistry of nicotine's interaction with the two receptor types. Binding to the α4 and β2 receptor subunits that underlie nicotine addiction involves both hydrogen bond formation and strong cation–π interaction between the positive charge of nicotine and a specific, conserved tryptophan residue. Muscle-type receptors also contain this tryptophan, but the cation–π interaction does not exist and the hydrogen bond is weaker. This appears to be due to differences in the overall shape of the binding pocket, associated with a single point mutation near the key tryptophan residue. As wells as solving a molecular mystery, these results provide guidance for the development of new analogues of nicotine for possible therapeutic use in neurological conditions and smoking cessation. This paper investigates why nicotine selectively activates neuronal and not muscular acetylcholine receptors, finding that a strong cation–π interaction, and also a hydrogen bond, form between nicotine and a specific tryptophan residue in receptors composed of α4 and β2 subunits — the subunit combination thought to underlie nicotine addiction. Muscle-type receptors also contain this tryptophan residue, but the cation–π interaction does not exist and the hydrogen bond is weaker, apparently due to the overall shape of the binding pocket. Nicotine addiction begins with high-affinity binding of nicotine to acetylcholine (ACh) receptors in the brain. The end result is over 4,000,000 smoking-related deaths annually worldwide and the largest source of preventable mortality in developed countries. Stress reduction, pleasure, improved cognition and other central nervous system effects are strongly associated with smoking. However, if nicotine activated ACh receptors found in muscle as potently as it does brain ACh receptors, smoking would cause intolerable and perhaps fatal muscle contractions. Despite extensive pharmacological, functional and structural studies of ACh receptors, the basis for the differential action of nicotine on brain compared with muscle ACh receptors has not been determined. Here we show that at the α4β2 brain receptors thought to underlie nicotine addiction, the high affinity for nicotine is the result of a strong cation–π interaction to a specific aromatic amino acid of the receptor, TrpB. In contrast, the low affinity for nicotine at the muscle-type ACh receptor is largely due to the fact that this key interaction is absent, even though the immediate binding site residues, including the key amino acid TrpB, are identical in the brain and muscle receptors. At the same time a hydrogen bond from nicotine to the backbone carbonyl of TrpB is enhanced in the neuronal receptor relative to the muscle type. A point mutation near TrpB that differentiates α4β2 and muscle-type receptors seems to influence the shape of the binding site, allowing nicotine to interact more strongly with TrpB in the neuronal receptor. ACh receptors are established therapeutic targets for Alzheimer's disease, schizophrenia, Parkinson's disease, smoking cessation, pain, attention-deficit hyperactivity disorder, epilepsy, autism and depression1. Along with solving a chemical mystery in nicotine addiction, our results provide guidance for efforts to develop drugs that target specific types of nicotinic receptors.

Nicotine binding to brain receptors requires a strong cation–π interaction