Pharmacological evidence of specific acetylcholine transport in rat cerebral cortex and other brain regions

Abstract

Functional acetylcholine receptors (AChRs) were recently demonstrated to exist not only in the plasma membrane but also intracellularly in brain tissues. In order to activate intracellular AChRs, endogenous hydrophilic ACh must cross the plasma membrane. Here, we examined the pharmacological characteristics of this process, including whether it is mediated by active ACh uptake. When ACh esterase (AChE) was suppressed by diisopropylfluorophosphate, [3H]ACh was effectively taken up into segments of rat cerebral cortex and other brain regions, in contrast to peripheral tissues such as liver and kidney. The uptake of [3H]ACh in rat cerebral cortex was temperature-dependent, and the uptake capacity was comparable to that of [3H]choline. However, [3H]ACh uptake was inhibited by lower concentrations of ACh, carbachol, tetraethylammonium (TEA), compared with uptake of [3H]choline. Uptake of [3H]ACh was also inhibited by several organic cations, including choline, hemicholinium-3 (HC-3), quinidine, decynium 22, clonidine, diphenhydramine, but was little affected by some amino acids and biogenic amines, corticosterone, spermine, atropine, and tetrodotoxin. Unlike diisopropylfluorophosphate, several ACh esterase inhibitors, including drugs for Alzheimer's disease, such as donepezil, galantamine, and rivastigmine, also suppressed the uptake of [3H]ACh, but not [3H]choline. These results indicate that in the brain, ACh is specifically taken up through a unique transport system with different pharmacological properties from known organic cation transporters (OCTs), and suggest that this mechanism may be involved in intracellular cholinergic transmission in the brain. (Figure presented.).