The mechanism of tetrahydroaminoacridine-evoked release of endogenous 5-hydroxytryptamine and dopamine from rat brain tissue prisms.

1 Tetrahydroaminoacridine (THA) is an acetylcholinesterase (AChE) inhibitor which may have a greater therapeutic effect in Alzheimer-type dementia (ATD) than other cholinergic agents. This suggests possible non-cholinergic properties. We have therefore studied the effects of THA on the release of endogenous 5-hydroxytryptamine (5-HT) from rat cortical prisms and dopamine from striatal prisms. 2 In the presence of K+ (1 mM), THA stimulated release of both 5-HT and dopamine. THA (100 microM)-evoked monoamine release was comparable, but not additive with the release produced by K+ (35 mM). The effect was not maximal at 1 mM THA. THA-evoked release of 5-HT was independent of the presence of Ca2+ in the external medium. 3 Drugs acting on the cholinergic system, nicotine, mecamylamine, atropine, oxotremorine, physostigmine and neostigmine (all 10 microM) had no effect on 5-HT and dopamine-release. 4-Aminopyridine (4-AP), a potent acetylcholine-releasing agent, had no effect on 5-HT release and was approximately 100 fold less active than THA on dopamine release. 4 Both THA and reserpine enhanced the release of 5-HT in the presence of the monoamine oxidase inhibitor, pargyline. Reserpine- but not THA-evoked release was abolished in the absence of pargyline. Reserpine (5 mg kg-1, i.p.) markedly depleted brain monoamine concentrations 3 h after injection, while THA (15 mg kg-1, i.p.) had no effect. 5 Chloroamphetamine and fenfluramine both released 5-HT in a Ca2(+)-independent manner and with a similar potency to THA, while (+)-amphetamine released dopamine with a similar potency to THA. The effects of the amphetamines were not maximal at 1 mM. However, unlike THA, chloroamphetamine-evoked release of 5-HT was additive with release evoked by K+ (35 mM). 6 Clomipramine (IC50 = 0.036 microM) and THA (IC50 = 19.9 microM) all inhibited the uptake of [3H]-5-HT into a P2 membrane preparation. However, none of these compounds inhibited [3H]-5-HT uptake into tissue prisms during the release experiments in which the reuptake inhibitor fluoxetine (5 microM) was present. 7 We conclude that THA does not release endogenous 5-HT through a cholinergic, reserpine- or amphetamine-like mechanism or through inhibition of reuptake. The possibility exists that the release may occur via blockade of 4-AP-insensitive K+ channels.