A vulnerable period of colchicine toxicity during goldfish optic nerve regeneration

The effects of intraocular (i.o.) administration of the alkaloid colchicine on visual recovery following axotomy of the goldfish optic nerve were investigated. Under the experimental conditions used, control goldfish recovered vision, measured behaviorally, within 5-7 weeks of retro-orbital optic nerve crush. Fish injected i. o. with 0.1 microg of colchicine within 3 days of optic nerve crush (post-crush; PC) recovered vision after some delay relative to control fish, while injection with colchicine between 7 and 14 days PC produced a much more profound inhibition of recovery of vision, in most cases a complete block for the duration of the study (98 days). Further evidence for a delayed susceptibility of the regenerating optic nerve to colchicine following crush was reflected in a suppression of neurite outgrowth normally seen in explanted retinal tissue taken from PC goldfish. In addition, retrograde transport of the fluorescent dye 4-(4-didecylaminostyryl)-N-methylpyridinium iodide from the optic tectum to the retina as a measure of axonal continuity revealed substantially less labeling following i.o. administration of colchicine 1 week PC when compared to retinas from fish receiving colchicine at the time of optic nerve crush. Histological sections of the retina showed no evidence of residual retinal damage resulting from the colchicine injections or from interactions of axotomy and the drug administration. These results indicate a period of increased vulnerability of the regenerating visual system to the toxic effects of i.o. administered colchicine, beginning 3-5 days PC, and remaining until regenerating optic nerve fibers have begun to reach the tectum. While colchicine has many known effects on nerve function, it is proposed that the delayed susceptibility to disruption of regeneration observed in these experiments is largely, if not entirely, attributable to a colchicine-induced accumulation of tubulin heterodimers, which are known to block microtubule assembly and to participate in a feedback inhibition of tubulin synthesis. Thus, it is during the maximal induction of tubulin synthesis and of microtubule formation which normally occurs several days following axotomy that colchicine has its greatest effect. The results suggest that colchicine may be especially neurotoxic during neural development and regeneration.