Nicotine-evoked [3H]5-hydroxytryptamine release from rat striatal synaptosomes

The aim of this study was to characterize the pharmacology of presynaptic nicotinic cholinoceptors (nAChRs) that modulate release of 5-hydroxytryptamine (5-HT) from superfused rat brain synaptosomes preloaded with [3H]5-HT. Nicotine increased 5-HT release from striatal synaptosomes (maximally by 15-30%) but not from cerebral cortex or hippocampal synaptosomes. Release of striatal 5-HT was increased in a concentration-dependent manner by nicotine, epibatidine, cytisine, and ACh (with added esterase inhibitor and muscarinic antagonist). Respective EC50 values were: 0.5, 0.003, 0.1 and 0.7 microM. The maximal effect of each agonist was virtually completely blocked by a high concentration of the insurmountable nicotinic antagonist mecamylamine; at a higher concentration of epibatidine (3 microM), a mecamylamine-insensitive effect was revealed. Nicotine, ACh and epibatidine appeared equally efficacious, whereas cytisine was of lower efficacy (60-70% of ACh). Release evoked by a half-maximal concentration of nicotine was inhibited by the nicotinic antagonists dihydro-beta-erythroidine (IC50 0.04 microM) and methyllycaconitine (IC50 0.06 microM). Nicotine-evoked 5-HT release was not reduced by tetrodotoxin given in a concentration that blocked veratridine-evoked release. These findings provide functional evidence for a direct action of nicotine on 5-HT neurons in the brain. The presynaptic nAChRs that modulate striatal 5-HT release appear to possess a novel pharmacological profile.