Up-regulation of Nicotinic Receptors by Nicotine Varies with Receptor Subtype

Heather Walsh,Ryan Mastro,J.C. Hoda,Daniel Bertrand,Anitha P. Govind,William N. Green,Yolanda Vallejo

doi:10.1074/jbc.m703432200

Abstract

Recent evidence suggests that in addition to alpha4beta2 and alpha3-containing nicotinic receptors, alpha6-containing receptors are present in midbrain dopaminergic neurons and involved in the nicotine reward pathway. Using heterologous expression, we found that alpha6beta2, like alpha3beta2 and alpha4beta2 receptors, formed high affinity epibatidine binding complexes that are pentameric, trafficked to the cell surface, and produced acetylcholine-evoked currents. Chronic nicotine exposure up-regulated alpha6beta2 receptors with differences in up-regulation time course and concentration dependence compared with alpha4beta2 receptors, the predominant high affinity nicotine binding site in brain. The alpha6beta2 receptor up-regulation required higher nicotine concentrations than for alpha4beta2 but lower than for alpha3beta2 receptors. The alpha6beta2 up-regulation occurred 10-fold faster than for alpha4beta2 and slightly faster than for alpha3beta2. Our data suggest that nicotinic receptor up-regulation is subtype-specific such that alpha6-containing receptors up-regulate in response to transient, high nicotine exposures, whereas sustained, low nicotine exposures up-regulate alpha4beta2 receptors.

Highlights

After chronic nicotine exposure, high-affinity agonist binding to nicotinic receptors is increased in murine [9, 11, 62] and human brains [12]
We find that the time course and concentration dependence of ␣6␤2 and ␣3␤2 receptor up-regulation is similar but ϳ10-fold faster than ␣4␤2 receptor up-regulation, requires much higher nicotine concentrations to up-regulate, and occurs without the 2–3-h delay observed with ␣4␤2 receptor up-regulation
These differences suggest that up-regulation of these nicotinic receptor subtypes could occur during different phases of nicotine intake that are observed during smoking

Summary

Introduction

High-affinity agonist binding to nicotinic receptors is increased in murine [9, 11, 62] and human brains [12]. We first assayed expression of ␣6␤2 or ␣3␤2 receptors by binding the agonist, 125I-labeled epibatidine (Fig. 1, A and B), to stably transfected ␣6FLAG␤2HA cells and transiently transfected ␣3FLAG␤2HA cells. 125I-Labeled epibatidine binding and subunit assembly of ␣6␤2 and ␣3␤2 receptors expressed at 30 or 37 °C.

Results

Conclusion