Ryanodine-, IP3- and NAADP-dependent calcium stores control acetylcholine release

Injections of inositol trisphosphate (IP3) or nicotinamide adenine dinucleo tide phosphate (NAADP) into the presynaptic neurone of an identified cholin ergic synapse in the buccal ganglion of Aplysia californica increased the a mplitude of the inhibitory postsynaptic current evoked by a presynaptic act ion potential. This suggests that Ca2+ release from various Ca2+ stores can modulate acetylcholine (ACh) release. Specific blockade of the calcium-ind uced calcium release (CICR) mechanism with ryanodine, or of IP3-induced cal cium release with heparin, abolished the effects Of IP3, but not the effect s of NAADP, suggesting the presence of an intracellular Ca2+ pool independe nt of those containing ryanodine receptors (RyR) or IP3 receptors. To reinf orce electrophysiological observations, intracellular [Ca2+], changes were measured using the fluorescent dye rhod-2. Injections of cyclic ADP-ribose (an activator of RyR), IP3 or NAADP into the presynaptic neurone induced tr ansient increases in the free intracellular Ca2+ concentration. RyR- and IP 3-induced increases were prevented by application of respective selective a ntagonists but not NAADP-induced increases. Our results show that RyR-depen dent, IP3-dependent, and NAADP-dependent Ca2+ stores are present in the sam e presynaptic terminal but are differently involved in the regulation of th e presynaptic Ca2+ concentration that triggers transmitter release.