Changes in fast axonal transport in sensory neurons during tadpole metamorphosis

Nitric oxide synthase expression and cell changes in dorsal root ganglia and spinal dorsal horn of developing and adultRana esculenta indicate a role of nitric oxide in limb metamorphosis

Metamorphosis of amphibians requires reconfiguration of sensory and locomotor neural networks. In view of such plastic changes and implications of nitric oxide (NO) in neural developmental shaping, we examined via histochemistry and immunohistochemistry its synthetic enzyme nitric oxide synthase (NOS) in dorsal root ganglia (DRGs) and dorsal horn of the developing and adult frog Rana esculenta. In limb DRGs, NOS positivity was first and selectively detected just before limb bud appearance, increased during metamorphosis, and was then down-regulated. In adulthood, NOS was expressed in some DRG neurons at all segmental levels. Similar features were detected in the dorsal horn neuropil. In limb DRGs, cell counts in Nissl-stained sections revealed a twofold increase of differentiated neurons during metamorphosis and an additional twofold increase in adulthood. Perikaryal sizes in limb DRGs did not vary during metamorphosis but increased and were more heterogeneous in the adult frog, probably reflecting adaptation to body size. NOS and cell changes during metamorphosis were much less marked in DRGs at other levels. Carbocyanine tracing documented selective labeling of NOS-expressing hindlimb DRG neurons from the spinal nerve at the time of initiation of hindlimb movements. The findings show that, in limb DRG neurons, NOS parallels cell differentiation and limb development during metamorphosis. The data also provide evidence of NOS expression in DRG cells innervating the hindlimbs when sensorimotor circuits become functionally mature. This study indicates a key role of NO production in the maturation of sensory functions that subserves in amphibians the transition from swimming to tetrapod locomotion.

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.