Patch-clamp experiments on the C2 neurone of Helix aspersa have shown that the neuropeptide Phe-Met-Arg-Phe-NH2 (FMRFamide) directly gates a Na+ channel. The channel is amiloride-sensitive. Activation of this channel is responsible for the fast excitatory action of the peptide. Using primers based on amiloride-sensitive epithelial Na+ channels, a complete cDNA sequence (FaNaCh) was cloned and sequenced from a Helix library. The sequence is predicted to have just two membrane-spanning regions and a large extracellular loop. When expressed in Xenopus laevis oocytes, the channel responded to FMRFamide. Taken together, these data provide the first evidence for a peptide-gated ion channel. Comparison of the properties of the expressed FaNaCh with the native neuronal channel show small differences in the sensitivities to some drugs and in channel conductance. It is not yet clear whether the native channel is a homo-oligomer or comprises other subunits. The peptide FKRFamide is an effective antagonist of FMRFamide on the expressed and neuronal channels. Nucleotide sequences encoding similar channel proteins occur in neurones of species as dissimilar as man and Caenorhabditis elegans. Some channels are thought to be associated with mechano-sensation, at least one is a proton-gated channel and others may also be ligand-gated channels.

This content is only available via PDF.