PRESYNAPTIC RECEPTORS FOR FMRFAMIDE, HISTAMINE AND BUCCALIN REGULATE ACETYLCHOLINE-RELEASE AT A NEURO-NEURONAL SYNAPSE OF APLYSIA BY MODULATING N-TYPE CA2+ CHANNELS

At an identified neuro-neuronal synapse of the buccal ganglion of Aplysia, quantal release of acetylcholine (ACh) is increased by FMRFamide and decreased by histamine or buccalin. Activation of presynaptic receptors for these neuromodulators modifies a presynaptic Ca2+ current which is nifedipine-resistant and omega-conotoxin-sensitive. The voltage-sensitivity of these N-type Ca2+ channels is increased by FMRFamide and decreased by histamine through the intermediate of G proteins. Buccalin does not implicate G proteins and reduces the Ca2+ current without affecting the voltage-sensitivity of N-type Ca2+ channels. The possibility of relating the shifts in voltage-dependence of the Ca2+ current induced by FMRFamide and histamine to the phosphorylation state of the N-type Ca2+ channels is discussed. A scheme for the complex regulation of ACh release by presynaptic auto- and heteroreceptors is proposed.