Presence of growth inhibitors in fish optic nerve myelin: Postinjury changes

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Abstract
This study shows that the fish optic nerve, which is able to regenerate after injury, contains myelin‐associated growth inhibitors similar to the growth inhibitors present in mammalian central nervous system (CNS) myelin. The ability of nerves to regenerate was previously correlated with the ability of sections from these nerves to support neuronal attachment to and axonal growth in vitro. Thus neuroblastoma cells or embryonic neurons became attached to and grew axons on sections of rat sciatic nerve or fish optic nerve, which are spontaneously regenerating systems, but not on sections of rat optic nerve, a nongenerating system. Failure of the latter to support axonal growth has been attributed, at least in part, to growth inhibitors. Recently it was shown that adult neurons, which differ in their growth requirement from embryonic neurons, are unable to extend neurites on sections of normal sciatic nerve but are able to extend neurites on sections of sciatic nerve that was injured prior to its excision. We found a similar in the fish optic nerve, i.e. that the nerve is normally not permissive to growth of adult retinal axons but becomes growth permissive after injury. The nonpermissiveness of the normal fish optic nerve was found to correlate with the presence of myelin‐associated growth‐inhibitory molecules. This inhibitory activity of fish myelin was neutralized by IN‐1 antibodies, known to neutralize rat myelin growth inhibitors apparently similar or even identical to those of rat, but possibly present in lower amounts than in the rat. Results are discussed with respect to the possibility that fish optic nerve, like the rat sciatic regeneration of adult neurons. Such changes might include elimination or neutralization of growth inhibitors.