Histamine and FLRFamide regulate acetylcholine release at an identified synapse in Aplysia in opposite ways.

Abstract

1. The effects of histamine and FLRFamide (Phe-Leu-Arg-Phe-NH2) on acetylcholine (ACh) release were studied in the buccal ganglion of Aplysia californica on an identified synapse (buccal ganglion inhibitory synapse, BGIS) involved in a small neuronal circuit controlling the feeding behaviour. The inhibitory postsynaptic current (IPSC) evoked by a presynaptic spike in the voltage-clamped postsynaptic neurone was decreased by histamine and increased by FLRFamide. 2. Histamine and FLRFamide modified the amplitude of the presynaptic spike. To test if these drugs acted directly on presynaptic calcium influx, we evoked transmitter relase by 3 s depolarizations of the presynaptic neurone (to + 10 mV) under voltage clamp to avoid modifications of presynaptic membrane polarization induced by changes in presynaptic voltage-dependent K+ and/or Na+ conductances. 3. Statistical analysis of this evoked long-duration (3 s) induced postsynaptic current (LDIPSC) allowed us to calculate the amplitude and the decay time of the miniature postsynaptic current and consequently the number of quanta released by the presynaptic terminal. 4. The amplitude of the LDIPSC decreased during the 3 s presynaptic depolarization. This was not due to a lack of available transmitter, since LDIPSC amplitude could be maintained constant by a ''clamp of the release of ACh'' which adequately depolarized the presynaptic neurone, but rather to changes in the calcium influx into the presynaptic neurone. 5. FLRFamide increased more the initial portions of the LDIPSC than the final portions. This effect of FLRFamide was only reduced and delayed by atropine or curare, antagonists of muscarinic-like and nicotinic-like autoreceptors previously demonstrated to be present at the same terminal. Activation of the nicotine-like receptors, which also increased transmitter release, induced a modification of the shape of the LDIPSC which was completely different from that due to FLRFamide. 6. Histamine decreased the amplitude of the LDIPSC. This effect was more pronounced at the beginning of the response. The effects of histamine were insensitive to curare and atropine, but were completely blocked by cimetidine, a specific histamine receptor antagonist. 7. The modifications of the shape and of the amplitude of the LDIPSC by FLRFamide and histamine suggested that these molecules alter presynaptic influx of calcium. This was confirmed by the analysis of calcium current recorded from the presynaptic neurone: the calcium inward current in the presynaptic neurone was increased by FLRFamide and reduced by histamine, whereas the activation of autoreceptors had no measurable effect on calcium current. 8. We conclude that FLRFamide and histamine, besides other possible modifications like that of the presynaptic voltage-dependent Na+ and/or K+ conductances, regulate the synaptic efficacy at the BGIS by acting on two specific presynaptic heteroreceptors modulating calcium current in the presynaptic neurone in opposite ways.