Nitric oxide stimulates Ca²⁺-independent synaptic vesicle release

A new fluorescence method using the dye FM1-43 was used to examine exocytotic release from hippocampal synaptosomes. Nitric oxide caused a marked transient stimulation of vesicle release. Several structurally unrelated nitric oxide donors, sodium nitroprusside, S-nitroso-N-acetylpenicillamine, 3-morpholino-sydnonimine, and acidified sodium nitrite, were effective. Release stimulated by nitric oxide and KCI were comparable in time course, using both the fluorescence assay and [³H]l-glutamate to monitor neurotransmitter release. Activation of guanylyl cyclase was not responsible for nitric oxide-stimulated release. Unlike release stimulated by KCl or A23187, nitric oxide-stimulated release was found to be independent of a rise in intrasynaptosomal Ca²⁺. Indo-1/AM measurements indicated that nitric oxide actually decreased intracellular Ca²⁺, and the Ca²⁺ channel blocker Cd²⁺ did not affect nitric oxide-stimulated vesicle release. Nitric oxide does, however, appear to act on the Ca²⁺-sensitive pool of vesicles. Nitric oxide may be the first physiological mediator that induces vesicle exocytosis without raising Ca²⁺ and may provide an interesting new tool for the study of molecules involved in vesicle exocytosis.