Evidence to suggest that extracellular acetate is accumulated by rat hippocampal cholinergic nerve terminals for acetylcholine formation and release

It is well established that extracellular choline is transported into central cholinergic nerve terminals by `high' and `low' affinity processes to form the neurotransmitter acetylcholine (ACh). The intent of the present investigation was to ascertain whether extracellular acetate might also be transported into central cholinergic nerve terminals to form ACh. To test this possibility, rat hippocampal tissue was incubated with varying concentrations of extracellular [1-¹⁴C]acetate (0.1–100 μM) and the uptake of [1-¹⁴C]acetate and the amount of [¹⁴C]ACh formed by the tissue determined. The results indicated that the uptake of extracellular [1-¹⁴C]acetate was temperature-dependent and saturable having an apparent Michaelis constant (K_m) of 22 μM. The formation of [¹⁴C]ACh in the tissue as a function of extracellular [1-¹⁴C]acetate appeared to occur by both `high' and `low' affinity processes with apparent K_m values of 0.5 and 19.6 μM, respectively. In other experiments, three inhibitors (lithium, allicin and sodium) of acetyl CoA synthetase (EC 6.2.1.1 acetate: CoA ligase), the enzyme which converts acetate to acetyl CoA when ATP and CoA are present, inhibited [1-¹⁴C]acetate uptake and the amount of [¹⁴C]ACh formed from that [1-¹⁴C]acetate. Additionally, vesamicol, an inhibitor of ACh transport into synaptic vesicles, blocked the filling of a synaptic vesicle-enriched fraction of hippocampal tissue with newly synthesized [¹⁴C]ACh formed from extracellular [1-¹⁴C]acetate. High K⁺ depolarization of hippocampal tissue loaded with extracellular [1-¹⁴C]acetate not only increased the synthesis but also the release of [¹⁴C]ACh. These results suggest that extracellular acetate is recycled by rat hippocampal cholinergic nerve terminals for the formation and release of ACh. They also suggest that the enzyme acetyl CoA synthetase mediates extracellular acetate uptake into hippocampal cholinergic nerve terminals by metabolizing it to acetyl CoA and thereby creating a diffusion gradient for it to follow. © 1997 Elsevier Science B.V. All rights reserved.