Regulation of transmitter release by presynaptic receptors at a cholinergic neuro-neuronal synapse

The modulation of evoked transmitter release by presynaptic receptors was studied at an identified cholinergic synapse in the buccal ganglion of Aplysia. Two auto-receptors affecting acetylcholine release in opposite ways were identified. Additionally acetylcholine (ACh) release was found to be facilitated by the peptide FLRFamide and inhibited by histamine. Ca2+ channels appeared as the final effectors controlled by these non-cholinergic presynaptic receptors whereas the activation of cholinergic presynaptic receptors did not affect the Ca2+ influx. The intracellular pathway activated by FLRFamide receptors was investigated in detail. The facilitation of transmitter release induced by this peptide was prevented by bath application of H-7, a protein kinase C inhibitor. Moreover, a diacylglycerol analog mimicked the action of FLRFamide. These results suggest that activation of protein kinase C leading to the phosphorylation of Ca2+ channels could be the mechanism through which presynaptic FLRFamide receptors increase evoked quantal release of acetylcholine at this synapse.