Mechanism for increase in expression of cerebral diazepam binding inhibitor mRNA by nicotine: involvement of L-type voltage-dependent calcium channels

Abstract

We investigated the mechanisms underlying the increase in diazepam binding inhibitor (DBI) and its mRNA expression induced by nicotine (0.1 μM) exposure for 24 h using mouse cerebral cortical neurons in primary culture. Nicotine-induced (0.1 μM) increases in DBI mRNA expression were abolished by hexamethonium, a nicotinic acetylcholine (nACh) receptor antagonist. Agents that stabilize the neuronal membrane, including tetrodotoxin (TTX), procainamide (a Na + channel inhibitor), and local anesthetics (dibucaine and lidocaine), dose-dependently inhibited the increased expression of DBI mRNA by nicotine. The nicotine-induced increase in DBI mRNA expression was inhibited by L-type voltage-dependent Ca 2+ channel (VDCC) inhibitors such as verapamil, calmodulin antagonist (W-7), and Ca 2+/calmodulin-dependent protein kinase II (CAM II kinase) inhibitor (KN-62), whereas P/Q- and N-type VDCC inhibitors showed no effects. In addition, nicotine exposure for 24 h induced [ 3H]nicotine binding to the particulate fractions of the neurons with an increased B max value and no changes in K d. Under these conditions, the 30 mM KCl- and nicotine-induced 45Ca 2+ influx into the nicotine-treated neurons was significantly higher than those into non-treated neurons. These results suggest that the nicotine-stimulated increase in DBI mRNA expression is mediated by CAM II kinase activation resulting from the increase in intracellular Ca 2+ through L-type VDCCs subsequent to the neuronal membrane depolarization associated with nACh receptor activation.