Inhibition of the Synthesis of Acetylcholine in Rat Brain Slices by (−)‐Hydroxycitrate and Citrate

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Abstract
Slices of rat caudate nucleus were incubated in a solution of 123 mM‐NaCl, 5 mM‐KCl, 1.2 mM‐MgCl2, 1.2 mM‐NaH2PO4, 25 mM‐NaHCO3, 0.2 mM‐choline chloride, 0.058 mM‐paraoxon, 1 mM‐EGTA, and oxidizable substrates. (−)‐Hydroxycitrate, a specific inhibitor of ATP‐citrate lyase (EC 4.1.3.8), used at a concentration of 2.5 mM, inhibited the synthesis of acetylcholine (ACh) from [1,5‐14C]citrate by 82–86%, but that from [U‐14C]glucose by only 33%, from [2‐14C]pyruvate by 24% and from [1‐14C‐acetyl]carnitine by 8%; the production of 14CO2 from these substrates was not substantially changed. The synthesis of ACh from glucose and pyruvate was in hibited also by citrate; 2.5 mM‐ and 5 mM‐citrate diminished it by 43% and 66%, respectively; the production of from [U‐14C]glucose and from [1‐14C]pyruvate was not affected. The mechanism of the inhibitory effect of citrate on the synthesis of ACh is not clear; the possibility is discussed that citrate alters the intracellular milieu in cholinergic neurons by chelating the intracellular Ca2+ and decreases the supply of mitochondrial acetyl‐CoA to the cytosol. The results with (−)‐hydroxycitrate indicate that the cleavage of citrate by ATP‐citrate lyase is not responsible for the supply of more than about one‐third of the acetyl‐CoA which is used for the synthesis of ACh when glucose or pyruvate are the main oxidizable substrates. This proportion may be even smaller, since (−)‐hydroxycitrate possibly affects the synthesis of ACh from glucose and pyruvate by a mechanism (unknown) similar to that of citrate, rather than by the inhibition of ATP‐citrate lyase.