CHANGES IN ULTRASTRUCTURE AND VOLTAGE-DEPENDENT CURRENTS AT THE GLIA-LIMITANS OF THE FROG OPTIC-NERVE FOLLOWING RETINAL ABLATION

The surface of the frog optic nerve consists of astrocytic processes separated by narrow extracellular clefts underlying a pial sheath of loose connective tissue. Macroscopic voltage dependent currents can be recorded from this surface using the loose patch-clamp technique. In this study the changes in ultrastructure and voltage dependent Na currents have been studied for up to 1 year following removal of the retina. During the first 1-4 weeks, many of the myelinated and unmyelinated axons of the retinal ganglion cells degenerate, and the debris is phagocytosed by macrophages and glial cells. However, some morphologically intact axons remain even 12 weeks after surgery. Finally, after 16 weeks all the axons have disappeared, leaving a nerve consisting only of glial cells, some of which contain phagosomes. At 40-52 weeks after enucleation, the nerve persists, at 20-40% of the normal diameter, consisting mostly of normal looking astrocytes. The amplitude of the voltage dependent Na currents recorded from nerves during the first 1-4 weeks after enucleation, with the pial sheath intact, decreases by about 50%. After 8 weeks, the Na current recorded from the surface is about 30% of control. At 16-52 weeks after removal of the retina, when there are no intact axons, the Na current is reduced by 90%. If, however, the pial sheath is stripped away, the Na currents recorded from the glial surface are 40-50% of control during this same 16- to 52-week period, suggesting that in the all-glia nerve, the currents are shunted by the relatively thicker pial sheath. In contrast to their normal TTX sensitivity, the voltage dependent Na currents recorded from these all glial nerves are insensitive to TTX (5 muM) but disappear when the external Na is replaced by TMA. The results suggest that in situ glial voltage dependent membrane properties depend on interactions between the glial cells and the neighboring axons.