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Using the whole-cell voltage-clamp technique, the effects of the neuropeptide Phe-Met-Arg-Phe-NH 2 (FMRFa) on two types of dihydropyridine-sensitive, high-voltage-activated calcium currents were investigated in isolated neuroendocrine caudo-dorsal cells (CDCs), which control egg-laying in the mollusc Lymnaea stagnalis. These currents are: (1) a transient current ('~inact = -10-25 ms) with an activation threshold of-40 mV and maximal amplitude at + 10 mV and (2) a sustained current ('Cinac t = -100-300 ms) with a threshold of -10 mV and a peak at +30 mV.FMRFa caused a partial block of the calcium current that was rapid, reversible and dose-dependent (ED50 = 4.3 nM). The FMRFa-sensitive and insensitive currents differed in voltage-dependence of activation and inactivation, steadystate inactivation characteristics and time course of recovery from inactivation, all indicating that FMRFa selectively suppressed the sustained calcium current.Internal perfusion of CDCs with GTP-?-S or GDP-[3-S depressed the FMRFa response, suggesting the involvement of G-proteins. Experiments aimed at elucidation of the signal transduction pathway between the FMRFa receptor and the calcium channel revealed no involvement of second messengers and protein kinases. The FMRFa-induced inhibition of the sustained calcium current probably results from a direct interaction between a G-protein, activated by the FMRFa receptor, and the calcium channel.The selective inhibition of this calcium current is likely to decrease the influx of calcium during the action potential, which will reduce the release of autoexcitatory CDC-peptides and contribute to a suppression of excitability.

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