Early events of secondary degeneration after partial optic nerve transection: an immunohistochemical study

Secondary degeneration in the central nervous system involves indirect damage to neurons and glia away from the initial injury. Partial transection of the dorsal optic nerve (ON) results in precise spatial separation of the primary trauma from delayed degenerative events in ventrally placed axons and parent somata. Here we conduct an immunohistochemical survey of secondary cellular changes in and around axons and their parent retinal ganglion cell (RGC) somata during the first 3 days after a restricted, dorsal ON transection. This is before the secondary loss of RGCs and axons projecting through the uninjured, ventral portion of the ON. Within 5 min, manganese superoxide dismutase (MnSOD; a marker of oxidative stress) co-localizes within the astrocytic network across the entire profile of the ON. Secondary astrocyte hypertrophy of immunofluorescent labeling was evident from 3 h, with sustained increases in myelin basic protein immunoreactivity across the nerve by 24 h. Increases in NG-2-positive oligodendrocyte precursor cells, ED-1-positive activated microglia/macrophages, and Iba1-positive reactive resident microglia/macrophage numbers were only seen in ON vulnerable to secondary degeneration by 3 days. Changes within RGC somata exclusively vulnerable to secondary degeneration were detected at 24 h, as evidenced by increases in MnSOD immunoreactivity, followed by increases in c-jun immunoreactivity at 3 days. Treatment with the voltage-gated calcium channel blocker lomerizine did not alter any measured outcome. We conclude that oxidative stress spreading via the astrocytic network and from injured axons to parent RGC somata is an early event during secondary degeneration, and containment is likely to be required in order to prevent further damage to the nerve.