Local alpha-bungarotoxin-sensitive nicotinic receptors in the nucleus accumbens modulate nicotine-stimulated dopamine secretion in vivo

Abstract

Nicotinic cholinergic receptors in the ventral tegmental area are required for the accumbal dopamine response to systemic nicotine. In contrast, the role of nicotinic receptors located within the nucleus accumbens itself has not been clarified for systemically administered nicotine. In the present study, in vivo microdialysis of accumbal dopamine secretion and receptor antagonist blockade in both the ventral striatal nucleus accumbens and the midbrain ventral tegmental area were used to evaluate this question. The nicotinic receptor antagonists methyllycaconitine or mecamylamine were delivered through the accumbal dialysis probe, followed by 0.09mg/kg nicotine (i.v.). The alpha7 subunit antagonist methyllycaconitine inhibited 71% of the dopamine response (P<0.01), whereas mecamylamine was completely ineffective. In addition, the classical alpha7 subunit antagonist alpha-bungarotoxin infused into the nucleus accumbens adjacent to the microdialysis probe, significantly reduced dopamine release by 0.065 or 0.09mg/kg nicotine (i.v.; P<0. 05). Combined, these data indicate the involvement of alpha7 subunit-containing nicotinic receptors in the nucleus accumbens. In contrast, local infusion of mecamylamine into the ventral tegmental area effectively blocked nicotine-induced accumbal dopamine release. Simultaneous infusions of methyllycaconitine into the accumbens and mecamylamine into the ventral tegmental area induced greater blockade of nicotine-stimulated dopamine secretion than methyllycaconitine or mecamylamine alone. In conclusion, the present study demonstrates that different types of nicotinic cholinergic receptors, located in the ventral striatal nucleus accumbens (alpha-bungarotoxin sensitive and mecamylamine insensitive) and the midbrain ventral tegmental area (mecamylamine sensitive), may be required for the full effects of nicotine on the mesostriatal dopaminergic pathway. While activation of nicotinic cholinergic receptors in the ventral tegmentum is required for the accumbal dopamine response to systemic nicotine, accumbal nicotinic receptors themselves act as modulators of this response. This fine tuning of the dopamine reward pathway through alpha7 nicotinic cholinergic receptors in the nucleus accumbens may amplify the secretion of dopamine, allowing a subthreshold brain concentration of nicotine to become an effective stimulus for dopamine secretion.