Differential inhibition of nicotine- and acetylcholine-evoked currents through α4β2 neuronal nicotinic receptors by tobacco cembranoids in Xenopus oocytes

Abstract

In tobacco, there are two types of compounds that interact with neuronal nicotinic acetylcholine receptors (nnAChRs) in the brain. The first is the addictive component of tobacco and an agonist of these receptors, nicotine. The second are cyclic diterpenoids called cembranoids that non-competitively inhibit many types of nnAChRs. Nictotinic receptors composed of α4β2 subunits are the predominant type of nicotinic receptors in the brain. These α4β2 receptors are up-regulated upon chronic exposure to nicotine and have been implicated in nicotine addiction. The present study was designed to determine whether the inhibitory effects of two cembranoids from tobacco [(1 S, 2 E, 4 R, 6 R, 7 E, 11 E)-2,7,11-cembratriene-4,6-diol (4 R) and its diastereoisomer (1 S, 2 E, 4 S, 6 R, 7 E, 11 E)-2,7,11-cembratriene-4,6-diol (4 S)] were comparable on acetylcholine (ACh) and nicotine-evoked currents through α4β2 nnAChRs. α4β2 nnAChRs from rat brain were expressed in Xenopus oocytes and studied using the two-electrode voltage-clamp technique. The dose–response curves for acetylcholine and nicotine were hyperbolic and bell-shaped, respectively. Although there was no difference in the potency between cembranoids 4 R and 4 S, both of these cembranoids more potently inhibited nicotine-induced currents than acetylcholine-induced currents. Furthermore, both cembranoids were more potent inhibitors of this receptor when they were preincubated for 1 min prior to application of agonist. The finding that cembranoids preferentially inhibit nicotine-induced currents over those elicited by the natural neurotransmitter acetylcholine may have important implications when developing strategies to prevent nicotine addiction and tobacco use.