Evidence to suggest that cytosolic acetylcholine in rat hippocampal nerve terminals is not directly transferred into synaptic vesicles for release

Rat hippocampal minces were loaded with [acetyl 1-C-14]acetylcholine ((IC)-C-14]ACh) in the presence of the ''poorly penetrating'' acetylcholinesterase (EC 3.1.1.7; AChE) inhibitor echothiophate and the effect of high K+ depolarization determined on the subcellular storage and release of [C-14]ACh and its metabolites. Results indicated that high K+ did not augment the release of [C-14]ACh. Rather, it increased the release of [C-14]acetate while simultaneously reducing the level of [C-14]ACh in the cytosolic (S-3) fraction. When the identical experiment was performed with paraoxon, a ''penetrating'' AChE inhibitor, high K+ still did not increase the release of [C-14]ACh. However, paraoxon prevented the K+-induced loss of [C-14]ACh from the cytosolic fraction as well as the K+-induced gain of [C-14]acetate in the release medium. When minces were loaded with [C-14]ACh in the presence of echothiophate and subsequently subjected to high K+ depolarization in the absence or presence of vesamicol (AH5183; (-)-trans-2-[4-phenylpiperidino] cyclohexanol), a drug which blocks the refilling of synaptic vesicles with ACh, the amount of endogenous ACh released was reduced approximately 50%. Conversely, the amount of [C-14]ACh released was not reduced at all. These results suggest that cytosolic ACh is not directly transported into synaptic vesicles for release when hippocampal nerve terminals are depolarized. Rather, its hydrolysis is accelerated in response to depolarization. A working hypothesis explaining the importance of the depolarization-induced breakdown of cytosolic ACh to central ACh metabolism is presented.