Multiple actions of a molluscan cardioexcitatory neuropeptide and related peptides on identified Helix neurones.

Abstract

The effects of the molluscan neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) and related peptides were tested on H. aspersa neurons. Ionophoretic application of FMRFamide depolarized and excited some neurons, but hyperpolarized and inhibited others. In some neurons the sign of the response was dependent on the membrane potential. Two responses resulted from an increase in membrane conductance, a depolarizing response mediated mainly by an increase in Na+ permeability, and a hyperpolarizing response mediated by an increase in K+ permeability. In the C1 neuron a voltage-dependent response was observed, which only occurred when the neuron was depolarized from its resting level. This response was recorded as an inward current during voltage clamp and resulted from a decrease in K current(s), possibly Ca-activated K current. More than one response may occur in a single neuron. In the C1 neuron, the K-mediated hyperpolarization occurred as well as the voltage-dependent response, while the depolarization seen in the F2 neuron was a combination of an increase in Na conductance and an increase in K conductance.