Neuronal Intermediate Filament Expression During Neurite Outgrowth from Explanted Goldfish Retina: Effect of Retinoic Acid

Regulation of the goldfish neuronal intermediate filament proteins ON 1 and ON 2 was investigated in a retinal explant system. The synthesis of these proteins in explanted retina decreased with increasing time in culture, despite continuing neurite outgrowth. Thus, ON 1 /ON 2 neurofilament expression is regulated independently from neurite outgrowth. During regeneration of the goldfish optic nerve in vivo, the expression of these proteins increased during the later phase of the process, when growing axons make contact with the optic tectum. The declining synthesis of ON 1 and ON 2 during neurite outgrowth in culture suggests that factors extrinsic to the retina are necessary to support synthesis of these proteins. Treating retinal explants with retinoic acid stimulated the synthesis of the ON 1 /ON 2 proteins in a dose‐dependent manner. This stimulation was effective during a period of declining synthesis of the ON 1 /ON 2 proteins, restoring their synthesis towards initial levels of expression. These results show that retinoic acid serves as a modulator of neurofilament expression in this in vitro model of nerve regeneration.