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 (FaNaCs) show both structural and functional similarities to the mammalian acid-sensing ion channels (ASICs). Both channel types are related to the epithelial 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 FaNaCs. We chose to examine this possibility at a site of FaNaC expression in the 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 ASICs (ASIC3, 22%). In situ hybridization was used to map the LsFaNaC expression pattern in the brain and to identify the right pedal giant1 (RPeD1) neuron as a site where the properties of the endogenous channel could be studied. In RPeD1 neurons isolated in culture, we demonstrated the presence of an FMRFamide-gated sodium current with features expected for a FaNaC: amiloride sensitivity, sodium selectivity, specificity for FMRFamide and Phe-Leu-Arg-Phe-amide (FLRFamide), and no dependency on G-protein coupling. The sodium current also exhibited rapid desensitization in response to repeated FMRFamide 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 by the FaNaC. Acidification also prevented the desensitization of the FMRFamide-induced inward current. The acid sensitivity of LsFaNaC is consistent with the hypothesis that FaNaCs share a common ancestry with the ASICs.
Perry
S J
SJ
Sussex Centre for Neuroscience, University of Sussex, Falmer, Brighton, BN1 9QG, United Kingdom.
Straub
V A
VA
Schofield
M G
MG
Burke
J F
JF
Benjamin
P R
PR
D000143:Acids / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism; D002021:Buffers; D003001:Cloning, Molecular; D019835:FMRFamide / Q000378:metabolism* / Q000494:pharmacology; D006863:Hydrogen-Ion Concentration / Q000187:drug effects; D017403:In Situ Hybridization; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology*; D008195:Lymnaea; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008969:Molecular Sequence Data; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009928:Organ Specificity; D018408:Patch-Clamp Techniques; D017386:Sequence Homology, Amino Acid; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000235:genetics* / Q000378:metabolism*
D000143:Acids; D002021:Buffers; C097095:FaNaCh protein, Helix aspersa; D015222:Sodium Channels; D019835:FMRFamide; D000584:Amiloride; D012964:Sodium