Regulation of synaptic acetylcholine concentrations by acetylcholine transport in rat striatal cholinergic transmission

摘要

Abstract In addition to hydrolysis by acetylcholine esterase ( AC hE), acetylcholine ( AC h) is also directly taken up into brain tissues. In this study, we examined whether the uptake of AC h is involved in the regulation of synaptic AC h concentrations. Superfusion experiments with rat striatal segments pre‐incubated with [ 3 H]choline were performed using an ultra‐mini superfusion vessel, which was developed to minimize superfusate retention within the vessel. Hemicholinium‐3 ( HC ‐3) at concentrations less than 1?μM, selectively inhibited the uptake of [ 3 H]choline by the high affinity‐choline transporter 1 and had no effect on basal and electrically evoked [ 3 H]efflux in superfusion experiments. In contrast, HC ‐3 at higher concentrations, as well as tetraethylammonium (10?μM), which inhibited the uptake of both [ 3 H]choline and [ 3 H] AC h, increased basal [ 3 H]overflow and potentiated electrically evoked [ 3 H]efflux. These effects of HC ‐3 and tetraethylammonium were also observed under conditions where tissue AC hE was irreversibly inactivated by diisopropylfluorophosphate. Specifically, the potentiation of evoked [ 3 H]efflux was significantly higher in AC hE‐inactivated preparations and was attenuated by atropine. On the other hand, striatal segments pre‐incubated with [ 3 H] AC h failed to increase [ 3 H]overflow in response to electrical stimulation. These results show that synaptic AC h concentrations are significantly regulated by the postsynaptic uptake of AC h, as well as by AC hE hydrolysis and modulation of AC h release mediated through presynaptic muscarinic AC h receptors. In addition, these data suggest that the recycling of AC h‐derived choline may be minor in cholinergic terminals. This study reveals a new mechanism of cholinergic transmission in the central nervous system.