Effects of Arg–Phe-amide peptides on identified motor neurons in the hydromedusa Polyorchis penicillatus

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Abstract
The effects were examined of several peptides (RFamide, FMRFamide, LPLRFamide, and pEGRFamide), all with the carboxy terminus Arg–Phe-amide, on the activity of partially isolated swimming motor neuron networks of the hydrozoan jellyfish Polyorchis penicillatus. Intracellular microelectrode recordings revealed the following responses to RFamide peptide solutions: an initial transitory hyperpolarization, which was often not present, followed by a long-duration (up to 3 min) excitation. Excitation was usually in the form of a spike train which increased rapidly in frequency and then decreased more slowly. Such spike trains were usually associated with an underlying depolarization and (or) a reduction in the spike after-hyperpolarization. The initial hyperpolarization was seen frequently with RFamide (supplied by Bachem), FMRFamide, and FLRFamide. This latter response could be partially accounted for by the acidity of the solutions. The order of effectiveness in eliciting excitation was: pEGRFamide (Anthopleura peptide), RFamide, FMRFamide, LPLRFamide, and FLRFamide. The effect of concentration on this response was examined for pEGRFamide. The responses described are interpreted in terms of possible conductance changes.