Nuclear and Cytoplasmic Triiodothyronine-Binding Sites in Primary Sensory Neurons and Schwann Cells: Radioautographic Study During Development

The effects of the thyroid hormones on target cells are mediated through nuclear T₃ receptors. In the peripheral nervous system, nuclear T₃ receptors were previously detected with the monoclonal antibody 283 rnAb in all the primary sensory neurons throughout neuronal life and in peripheral glia at the perinatal period only (Eur. J. Neurosci. 5, 379, 7993). To determine whether these nuclear T₃ receptors correspond to functional ones able to bind T₃, cryostat sections and in vitro cell cultures of dorsal root ganglion (DRG) or sciatic nerve were incubated with 0.1 nM [1251]-labeled T₃, either alone to visualize the total T₃-binding sites or added with a lo3 fold excess of unlabeled T₃ to estimate the part due to the non-specific T₃-binding. After glutaraldehyde fixation, radioautography showed that the specific T₃-binding sites were largely prevalent. The T₃-binding capacity of peripheral glia in DRG and sciatic nerve was restricted to the perinatal period in vivo and to Schwann cells cultured in vitro. In all the primary sensory neurons, specific T₃-binding sites were disclosed in foetal as well as adult rats. The detection of the T₃-binding sites in the nucleus indicated that the nuclear T₃ receptors are functional. Moreover the concomitant presence of both T₃-binding sites and T₃ receptors CI isoforms in the perikaryon of DRG neurons infers that: 1) [1251]-labeied T₃ can be retained on the T₃-binding ‘E’ domain of nascent CI, isoform molecules newly-synthesized on the perikaryal ribosomes; 2) the CI isoforms translocated to the nucleus are modified by posttranslational changes and finally recognized by 2B3 rnAb as nuclear T₃ receptor. In conclusion, the radioautographic visualization of the T₃-binding sites in peripheral neurons and glia confirms that the nuclear T₃ receptors are functional and contributes to clarify t h e discordant intracellular localization provided by the immunocytochemical detection of nuclear T₃ receptors and T₃ receptor CI isoforms.