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 M) 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 g/ml), liposomes laden with 113 mM CaCl₂, or 4-aminopyridine (1–10 mM) was not modulated by oxotremorine. Ouabain (100 M)-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 M), and phorbol ester (0.3–3 g/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.