Effects of different secretagogues and intracellular messengers on the muscarinic modulation of [3H]acetylcholine release.

We have investigated whether muscarinic receptors modulate the release of [3H]ACh elicited by secretagogues that act by different mechanisms in rat cerebral cortical synaptosomes. Oxotremorine (10 microM) reduced the calcium-dependent [3H]ACh release induced by mild K+-depolarization (10 and 15 mM K+), but not that by higher K+ concentrations. The ACh-release induced by A23187 (0.2-5 micrograms/ml), liposomes laden with 113 mM CaCl2, or 4-aminopyridine (1-10 mM) was not modulated by oxotremorine. Ouabain (100 microM)-induced release of [3H]ACh was reduced by oxotremorine in normal but not calcium-free KR, indicating that extracellular calcium-uptake but not Na+, K+-ATPase activity may be necessary for release-modulation. With respect to possible second messenger systems, dibutyrylcyclic AMP (0.1-2 mM), dibutyrylcyclic GMP (0.1-2 mM), forskolin (100 microM), and phorbol ester (0.3-3 micrograms/ml) were without effect on release or release-modulation. These results are consistent with an involvement of K+-channels and voltage-sensitive calcium-channels in the muscarinic release-inhibition process. They argue against an involvement of Na+, K+-ATPase, adenylate cyclase, guanylate cyclase, and phosphatidylinositol turnover in the release-modulation process.