ENHANCEMENT OF GLYCINE RELEASE FROM HUMAN BRAIN CORTEX SYNAPTOSOMES BY ACETYLCHOLINE ACTING AT M(4)-MUSCARINIC RECEPTORS

Synaptosomes prepared from fresh specimens of human cerebral cortex we re labeled with [H-3]glycine ([H-3]Gly) and distributed in parallel su perfusion chambers. Exposure to 15 mM KCl evoked a tritium overflow wh ich was largely prevented by 10 mM Mg++, suggesting a consistent compo nent of Ca++-dependent [H-3]Gly release. Acetylcholine (ACh; 1-100 muM ), added during K+-depolarization, increased the release of tritium in a concentration-dependent manner (maximal effect, 60%; EC50 = 7 muM). Oxotremorine (1-100 muM) mimicked ACh. The effect of 10 muM ACh was i nsensitive to the nicotinic antagonist mecamylamine (100 muM), but it was blocked by the muscarinic antagonist atropine (0.1 muM). Three mus carinic receptor antagonists, pirenzepine, AF-DX 116 -hydro-6H-pyrido- [2-3-b][1,4]benzodiazepine-6-one} and himbacine, endowed with relative selectivity for various muscarinic receptor subtypes, prevented with differential affinities the effect of 10 muM ACh. Himbacine was the mo st potent antagonist of ACh, its pA2 (8.34) being 20- or 50-fold highe r than that of pirenzepine (7.27) or AF-DX 116 (6.65). It is concluded that: 1) ACh can increase the release of Gly in human cerebral cortex ; 2) the interaction occurs through muscarinic receptors which resembl e most the M4 subtype; and 3) considering that Gly is required to acti vate the N-methyl-D-aspartate glutamate receptor, the ACh-evoked Gly r elease may represent a linkage between cholinergic and glutamatergic t ransmission, two systems strongly implicated in cognitive processes.