Tissue distribution and subcellular localization of Na+ channel mRNA in the nervous system of the squid, Loligo opalescens.

Recent cloning of a putative Na+ channel alpha subunit cDNA, GFLN1, from the squid stellate ganglion has allowed us to study the expression of this ion channel at a cellular level. In situ hybridizations with a probe derived from and specific to 3' untranslated and coding sequence of GFLN1 were used to determine its tissue distribution as well as its subcellular localization. In sections of the stellate ganglion, the probe labeled all of the cells in the giant fiber lobe (GFL) and most cells in the cellular layer of the main ganglion. In these non-GFL portions of the stellate ganglion, labeling was particularly intense in the ventral large cells and weak or absent in the dorsal small cells. In the optic lobe, only a select group of cells, the second-order visual giant neurons, were intensely labeled. These results are consistent with electrophysiological data that show GFL-like Na+ currents in rare large cells dissociated from the optic lobe and in most but not all cells from the non-GFL part of the stellate ganglion. In sections of the subesophageal mass of the central nervous system, strong labeling for GFLN1 mRNA occurred in the fin lobe, posterior chromatophore lobe, central and latero-ventral palliovisceral lobes, and posterior pedal lobe. In all cases, labeling was detected only in the cellular layer of these tissues and never in nerves or neuropil. In situ hybridization with dissociated GFL neurons maintained in primary culture verified that Na+ channel mRNA is confined to the cell body. These results indicate that GFLN1 is expressed predominately in large cells with large or long axons, and that this mRNA is restricted to the cell bodies of these neurons.