Inhibition of axoplasmic transport in the developing visual system of the rat-II, Quantitative analysis of alterations in transport of tritiated proline or fucose.

Abstract Developmental alterations in the amount of tritiated proline and fucose incorporated by retinal neurons and transported within axons of the optic nerve to the dorsal lateral geniculate nucleus and superior colliculus were measured at 1, 5, 10 and 15 days postnatal using quantitative autoradiography and liquid scintillation analysis. The amount of axon transport inhibition induced by intraocular injections of colchicine (10 −4 M−5 × 10 −3 M) and xylocaine (10 −3 M−10 −1 M) was also determined by this methodology. Grain counts of retinal autoradiograms obtained from animals of all ages employed in this study demonstrated that [ 3 H]proline is rapidly incorporated by all retinal neurons but becomes increasingly concentrated within the inner nuclear, ganglion cell and optic fiber layers between 2 h and 2 days after injection. [ 3 H]fucose is preferentially taken up and concentrated within the plexiform and sensory element layers. Intraocular colchicine administered 24 h prior to isotope injection exhibited no significant effect on incorporation but depressed the amount of activity in the layer of optic fibers. Comparison of the effects of colchicine and xylocaine on axon transport of [ 3 H]proline injected at 3 days of age revealed dose-dependent suppression of transport occurring up to six hr after isotope injection; however, with longer survival periods the effects of xylocaine were no longer significant whereas colchicine maintained suppression of axon transport to 20% of normal for periods of up to 10 days. Additionally, the rate and quantity of [ 3 H]proline transported to the dorsal lateral geniculate nucleus and tectum of 1–15-day old animals was found to be inversely proportional to the age of the animal. Maximally-effective concentrations of colchicine, determined for each age level examined by the previous study to be highest compatible with the viability of the retino-fugal projection, 34 also reduced transport to from 20–40% of normal, being most effective in animals of 1–10 days of age. Autoradiographs of the optic disc demonstrated a colchicine-induced suppression of the amount of label in the exiting optic fibers and similar preparations of the tectum revealed an elimination of activity in the stratum griseum superficialis and stratum opticum which are normally heavily labeled following intraocular [ 3 H]proline or fucose. Following [ 3 H]fucose injection at 10 days of age, isotope appears in the contralateral visual cortex between 3 and 10 days later, indicating a substantial amount of transneuronal transfer of label which appeared to occur in concert with periods of maximal synaptogenesis. Injection of colchicine reduced the amount of label in the cortex to background levels. Concomitantly, autoradiographs of the dorsal lateral geniculate nucleus revealed cytoplasmic labeling of geniculate neurons in control animals but this was eliminated following colchicine injection. It was concluded from this analysis that xylocaine is not suitable for long-term studies of the inhibition of axon transport, whereas application of colchicine results in a relatively long-term suppression of transport without significant disruption of protein synthesis in retinal neurons.