POSITIVE AND NEGATIVE EFFECTS OF TACRINE (TETRAHYDROAMINOACRIDINE) AND METHOXYTACRINE ON THE METABOLISM OF ACETYLCHOLINE IN BRAIN CORTICAL PRISMS INCUBATED UNDER RESTING CONDITIONS

The effects of tacrine (1,2,3,4-tetrahydro-9-aminoacridine) and 7-methoxytacrine on the metabolism of acetylcholine were investigated in experiments on prisms of rat cerebral cortex incubated in vitro in low-potassium (3 mmol/1 K+) media; cholinesterases were inactivated by paraoxon to avoid any action of tacrine and methoxytacrine via their inhibition. Under "resting" conditions, tacrine and methoxytacrine increased the synthesis of unlabeled acetylcholine in the prisms; at the same time, they inhibited the uptake of [C-14]choline from the medium and the synthesis of [C-14]acetylcholine. The concentration of free choline was not increased by tacrine or methoxytacrine in either the tissue or the medium. The contradiction between the increased synthesis of unlabeled and the diminished synthesis of labeled acetylcholine indicates that the utilization of intracellular choline (which is presumably mobilized from intracellular choline esters) for the synthesis of acetylcholine is increased by tacrine and methoxytacrine. This conclusion is supported by the observation that the inhibition of acetylcholine synthesis during incubation with hemicholinium-3 (an inhibitor of choline transport into cholinergic nerve terminals) was overcome when tacrine was present simultaneously with hemicholinium-3. When the prisms were preincubated with [C-14]choline and incubated with tacrine or methoxytacrine only after this, the amount of [C-14]acetylcholine recovered in the tissue plus the medium was higher at the end of incubation with tacrine or methoxytacrine than without them, again suggesting that the drugs were able to increase the utilization of intracellular [C-14]choline or its esters for acetylcholine synthesis. Most experiments were performed with tacrine and methoxytacrine at concentrations of 100-mu-mol/L, but a positive action of tacrine on the synthesis of acetylcholine was apparent at a concentration as low as 10-mu-mol/L. The mechanism by which tacrine and methoxytacrine increase the utilization of intracellular choline or its esters for the synthesis of acetylcholine has not been identified.