Sodium channels in astrocytes of rat optic nerve in situ: immuno-electron microscopic studies

Abstract

Immuno-electron microscopic localization of sodium channels within astrocyte somata and processes of adult rat optic nerve was demonstrated with polyclonal antibody 7493. In immunoblots of crude glycoproteins from adult rat optic nerve, antisera 7493, which is directed against purified rat brain sodium channels, recognizes a 260 kDa protein. Antisera 7493 intensely immunostains axon membrane at nodes of Ranvier. Associated perinodal astrocyte processes are also stained with antisera 7493. In addition, astrocyte cell bodies and major processes exhibit immunoreactivity with antibody 7493. Immunostaining with antisera 7493 is heterogeneously distributed within astrocyte cytoplasm and also appears to be associated with some regions of astrocyte plasmalemma. Glial filaments are not immunostained with 7493 antisera. Astrocyte processes forming the glial limitans and surrounding blood vessels display reduced immunoreactivity to 7493 compared to longitudinally oriented or perinodal astrocyte processes. However, some focal regions of the glial limitans exhibit robust 7493 immunostaining. Oligodendrocytes do not display 7493 antisera immunoreactivity. Optic nerve sections incubated with preimmune sera or with 7493 antisera that had been previously adsorbed with purified sodium channel protein, exhibited no immunoreactivity. These results demonstrate localization of sodium channels within astrocytes in situ of rat optic nerve and extend previous electrophysiological and pharmacological findings of sodium channels in cultured astrocytes. Possible functional roles of sodium channels within astrocytes are discussed.