The voltage-dependent, slow inward current induced by the neuropeptide FMRFamide in Aplysia neuron R14

The effects of the peptide FMRFamide (Phe-Met-Arg-Phe-NH2) on the soma of neuron R14 in the abdominal ganglion of Aplysia californica and A. brasiliana were characterized. Pressure-ejected FMRFamide caused 3 types of responses, (1) a fast outward current (duration, less than 30 sec), (2) a fast inward current (duration, less than 20 sec), and (3) a slow inward current (peak at 0.5-1 min; duration, 2-3 min). The slow inward current, the chief object of this study, arises from a voltage-dependent conductance increase. The FMRFamide-elicited slow inward current is largest between -40 mV and -20 mV, the region of a negative slope resistance in the normal current-voltage relationship for R14. The slow FMRFamide-induced inward current is largely carried by Na+. This current is independent of external [K+] but depends inversely on external [Ca2+] and [Cl-]. The concentrations of the latter ions may influence the voltage dependence of the response. The slow inward current has many properties in common with inward currents induced in other molluscan neurons by applications of neuropeptides or intracellular injections of cyclic nucleotides.