Neuronal expression of an FMRFamide-gated Na+ channel and its modulation by acid pH

The molluscan Phe-Met-Arg-Phe-amide (FMRFamide)-gated sodium channels (FaNa Cs) show both structural and functional similarities to the mammalian acid- sensing ion channels (ASICs). Both channel types are related to the epithel ial sodium channels and, although the neuropeptide FMRFamide directly gates the FaNaCs, it also modulates the proton-gating properties of ASICs. It is not yet known whether protons can alter the gating properties of the FaNaC s. We chose to examine this possibility at a site of FaNaC expression in th e nervous system of the mollusk Lymnaea stagnalis. We cloned a putative L. stagnalis FaNaC (LsFaNaC) that exhibited a high degree of sequence identity to the Helix aspersa FaNaC (HaFaNaC, 60%), and a weaker homology to the AS ICs (ASIC3, 22%). In situ hybridization was used to map the LsFaNaC express ion pattern in the brain and to identify the right pedal giant1 (RPeD1) neu ron as a site where the properties of the endogenous channel could be studi ed. In RPeD1 neurons isolated in culture, we demonstrated the presence of a n FMRFamide-gated sodium current with features expected for a FaNaC: amilor ide sensitivity, sodium selectivity, specificity for FMRFamide and Phe-LeuA rg-Phe-amide (FLRFamide), and no dependency on G-protein coupling. The sodi um current also exhibited rapid desensitization in response to repeated FMR Famide applications. Lowering of the pH of the bathing solution reduced the amplitude of the FMRFamide-gated inward current, while also activating an additional sustained weak inward current that was apparently not mediated b y the FaNaC. Acidification also prevented the desensitization of the FMRFam ide-induced inward current. The acid sensitivity of LsFaNaC is consistent w ith the hypothesis that FaNaCs share a common ancestry with the ASICs.