Effect of tacrine on in vivo release of dopamine and its metabolites in the striatum of freely moving rats.

The effects of tacrine (THA) on extracellular concentrations of dopamine (DA), 3,4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid were investigated in the striatum of freely moving rats, using a microdialysis technique in which tacrine was administered locally via the microdialysis membrane. THA in concentrations of 10(-8) to 10(-5) M, significantly elevated the levels of the DA metabolites 3,4-dihydroxyphenylacetic acid and homovanillic acid, whereas a significantly increased content of extracellular DA was observed at higher concentrations of THA (10(-3) to 10(-2) M). Local administration of the muscarinic antagonist atropine (10(-6) M) or the nicotinic antagonist mecamylamine (10(-5) M) both prevented the effects of THA on DA and its metabolites. In vitro receptor binding studies showed that THA displaced the binding of muscarinic antagonists [3H]pirenzepine (IC50, 2.1 +/- 0.4 microM) and [3H]AFDX 384 (IC50, 3.4 +/- 0.2 microM) equally in striatal tissue, suggesting that THA binds with equal affinity to M1 and M2 muscarinic receptor subtypes. THA showed a 20-fold lower affinity to high-affinity nicotinic receptors compared with muscarinic receptors when determined by [3H]cytisine competition curves. The study indicates that THA enhances monoamine neurotransmission in the rat striatum, probably via an interaction with both muscarinic and nicotinic heteroreceptors.