Regulation of the distribution and function of [(125)I]epibatidine binding sites by chronic nicotine in mouse embryonic neuronal cultures.

Chronic nicotine produces up-regulation of α4ß2* nicotinic acetylcholine receptors (nAChRs) (* denotes that an additional subunit may be part of the receptor). However, the extent of up-regulation to persistent ligand exposure varies across brain regions. The aim of this work was to study the cellular distribution and function of nAChRs after chronic nicotine treatment in primary cultures of mouse brain neurons. Initially, high-affinity [(125)I]epibatidine binding to cell membrane homogenates from primary neuronal cultures obtained from diencephalon and hippocampus of C57BL/6J mouse embryos (embryonic days 16-18) was measured. An increase in α4ß2*-nAChR binding sites was observed in hippocampus, but not in diencephalon, after 24 h of treatment with 1 µM nicotine. However, a nicotine dose-dependent up-regulation of approximately 3.5- and 0.4-fold in hippocampus and diencephalon, respectively, was found after 96 h of nicotine treatment. A significant fraction of total [(125)I]epibatidine binding sites in both hippocampus (45%) and diencephalon (65%) was located on the cell surface. Chronic nicotine (96 h) up-regulated both intracellular and surface binding in both brain regions without changing the proportion of those binding sites compared with control neurons. The increase in surface binding was not accompanied by an increase in nicotine-stimulated Ca(2+) influx, suggesting persistent desensitization or inactivation of receptors at the plasma membrane occurred. Given the differences observed between hippocampus and diencephalon neurons exposed to nicotine, multiple mechanisms may play a role in the regulation of nAChR expression and function.