Examining the Effects of (α4)3(β2)2 Nicotinic Acetylcholine Receptor-Selective Positive Allosteric Modulator on Acute Thermal Nociception in Rats

Advances in small molecule selective ligands for heteromeric nicotinic acetylcholine receptors

The study of nicotinic acetylcholine receptors (nAChRs) has significantly progressed in the last decade, due to a) the improved techniques available for structural studies; b) the identification of ligands interacting at orthosteric and allosteric recognition sites on the nAChR proteins, able to tune channel conformational states; c) the better functional characterization of receptor subtypes/subunits and their therapeutic potential; d) the availability of novel pharmacological agents able to activate or block nicotinic-mediated cholinergic responses with subtype or stoichiometry selectivity. The copious literature on nAChRs is related to the pharmacological profile of new, promising subtype selective derivatives as well as the encouraging preclinical and early clinical evaluation of known ligands. However, recently approved therapeutic derivatives are still missing, and examples of ligands discontinued in advanced CNS clinical trials include drug candidates acting at both neuronal homomeric and heteromeric receptors. In this review, we have selected heteromeric nAChRs as the target and comment on literature reports of the past five years dealing with the discovery of new small molecule ligands or the advanced pharmacological/preclinical investigation of more promising compounds. The results obtained with bifunctional nicotinic ligands and a light-activated ligand as well as the applications of promising radiopharmaceuticals for heteromeric subtypes are also discussed.

Potentiation of (α4)2(β2)3, but not (α4)3(β2)2, nicotinic acetylcholine receptors reduces nicotine self-administration and withdrawal symptoms

The low sensitivity (α4)3(β2)2 (LS) and high sensitivity (α4)2(β2)3 (HS) nAChR isoforms may contribute to a variety of brain functions, pathophysiological processes, and pharmacological effects associated with nicotine use. In this study, we examined the contributions of the LS and HS α4β2 nAChR isoforms in nicotine self-administration, withdrawal symptoms, antinociceptive and hypothermic effects. We utilized two nAChR positive allosteric modulators (PAMs): desformylflustrabromine (dFBr), a PAM of both the LS and HS α4β2 nAChRs, and CMPI, a PAM selective for the LS nAChR. We found that dFBr, but not CMPI, decreased intravenous nicotine self-administration in male mice in a dose-dependent manner. Unlike dFBr, which fully reverses somatic and affective symptoms of nicotine withdrawal, CMPI at doses up to 15 mg/kg in male mice only partially reduced nicotine withdrawal-induced somatic signs, anxiety-like behavior and sucrose preference, but had no effects on nicotine withdrawal-induced hyperalgesia. These results indicate that potentiation of HS α4β2 nAChRs is necessary to modulate nicotine's reinforcing properties that underlie nicotine intake and to reverse nicotine withdrawal symptoms that influence nicotine abstinence. In contrast, both dFBr and CMPI enhanced nicotine's hypothermic effect and reduced nicotine's antinociceptive effects in male mice. Therefore, these results indicate a more prevalent role of HS α4β2 nAChR isoforms in mediating various behavioral effects associated with nicotine, whereas the LS α4β2 nAChR isoform has a limited role in mediating body temperature and nociceptive responses. These findings will facilitate the development of more selective, efficacious, and safe nAChR-based therapeutics for nicotine addiction treatment.