Presynaptic mechanisms regulating Ca2+ concentration triggering acetylcholine release at an identified neuro-neuronal...

We have used an identified cholinergic neuro-neuronal synapse in the buccal ganglion of Aplysia to determine which types of Ca2+ channels are involved in triggering transmitter release. omega-Conotoxin as well as funnel web spider toxin partially reduced acetylcholine release indicating that both N- and P-type Ca2+ channels are involved. Nifedipine-sensitive L-type Ca2+ channels are also present but they are not directly implicated in acetylcholine release. We have identified presynaptic receptors to two peptides. FMRFamide and buccalin and to the neurotransmitter histamine. FMRFamide facilitates acetylcholine release by increasing the presynaptic Ca2+ influx whereas buccalin and histamine have an opposite effect. These neuromodulators control only the influx of Ca2+ through N-type Ca2+ channels since their action on transmitter release can be prevented by omega-conotoxin but not by funnel web spider toxin. FMRFamide and histamine, respectively, increased and decreased Ca2+ influx by shifting in opposite ways the voltage sensitivity to activation of the channels. Buccalin reduced Ca2+ influx by decreasing the number of available channels. 2,5-Diterbutyl 1,4-benzohydroquinone, a blocker of the reticulum Ca2+ pump, increased evoked transmitter release by increasing the intracellular concentration of Ca2+ without affecting the presynaptic Ca2+ influx. It is suggested that a reticulum-like Ca2+ buffer, in close proximity to N- and P-type Ca2+ channels, controls the intracellular concentrations of Ca2+ actually triggering acetylcholine release.