Activation Of A Peptidergic Synapse Locally Modulates Postsynaptic Calcium Influx

We examined the synaptic connection between Phe-Met-Arg-Phe-NH₂ (FMRFamide)-immunoreactive neurone VD4 and its target neurone Pl, both found in the central nervous system of the pond snail Helisoma trivolvis. The major FMRFamide-like peak in neurone VD4 appears to be FMRFamide itself, based on its high performance liquid chromatography (HPLC) elution time and immunoreactivity before and after oxidation, but small peaks are also present at the elution times of Phe-Leu-Arg-Phe-NH₂ (FLRFamide) and Gly-Asp-Pro-Phe-Leu-Arg-Phe-NH₂ (GDPFLRFamide).

The modulatory actions of the neuropeptides found in neurone VD4 were tested on the postsynaptic target cell Pl. Bath application of both the tetrapeptides FMRFamide and FLRFamide at a concentration of 10⁻⁵ mol l⁻¹ reduced the macroscopic voltage-sensitive calcium current of neurone Pl in culture; FMRFamide by 45% and FLRFamide by 51%. Bath application of the heptapeptide GDPFLRFamide (10⁻⁵ mol l⁻¹) reduced the calcium current by only 8%.

We reconstructed the synaptic connection between VD4 and Pl in culture. Action-potential-evoked calcium transients in neurites growing from Pl cells in culture were monitored using Fura-2. Addition of FMRFamide, FLRFamide or GDPFLRFamide reduced the magnitude of the calcium transient in Pl. Stimulation of VD4 mimicked the effects of peptide application and caused localized reductions in the action-potential-evoked calcium transients in Pl at the points of contact between the neurites of neurones VD4 and Pl. These results suggest that neurone VD4 modulates the calcium influx of neurone Pl through the release of endogenous FMRFamide-related peptides and that this modulatory action is restricted to sites of synaptic interaction.