Effects of exogenous triiodothyronine on fast axonal transport during tadpole metamorphosis

Bullfrog tadpoles at metamorphic stages V, X and XVIII were immersed in 25 nM triiodothyronine (T3) to assess whether the 4-5 fold increase in fast axonal transport (FAxT) previously observed during this span of spontaneous metamorphosis (1) could be mimicked by precocious application of thyroid hormone. The trend initially observed was for T3 to stimulated [35S]methionine incorporation into lumbar DRG and inhibit incorporation in tail DRG. Both effects, however, appeared to be exerted primarily on satellite cells rather than neurons since most of the T3-induced changes in DRG were of a similar magnitude to those in the respective nerve trunks. Findings consistent with this observation resulted from use of the retrogradely transported lectin, ricin120, to determine the proportion of DRG incorporation occurring in neurons. When incorporation of [35S]methionine in lumbar DRG neurons was examined, T3 had no stimulatory effect at any of the metamorphic stages examined. When FAxT was assessed as [35S]protein accumulating proximal to a nerve trunk ligature, and expressed as a percentage of newly-synthesized protein in lumbar DRG neurons, no stimulatory effect of T3 was detected. The question remains whether the changes in FAxT in peripheral neurons observed during spontaneous metamorphosis may be induced by circulating hormones other than T3 or are secondary to changes in the target tissues.