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Transient expression of 3,5,3′‐triiodothyronine nuclear receptors in rat oligodendrocytes: In vivo and in vitro immunocytochemical studies

It is generally accepted that the action of thyroid hormones is mediated through specific nuclear receptors. Recent studies have demonstrated the homology of the thyroid receptor with the cellular product of the oncogen v‐erbA. So far, two genes have been identified and classified as α and β subtypes. In this study, the expression of nuclear triiodothyronine (T3) receptors (NT3RS) was examined in secondary cultures containing 85–90% oligodendrocytes (OL) prepared from newborn rat brain primary cultures enriched in OL. These cultures, which are able to produce myelin membranes, were examined by double immunolabelling with a monoclonal antibody (2B3) raised against purified rat liver NT3Rs and with antibodies against two maturation markers of OL: an early marker, galactocerebroside (GC), and myelin basic protein (MBP), which is expressed later than GC. 2B3 recognized three nuclear proteins with the same molecular weights as β, α1, and α2 sybtypes with different capacities for binding T3. In 5‐day ‐old OL secondary cultures (25 days, total time in culture), 2B3‐NT3R immunoreactivity was located in 77% of morphologically immature OL (GC)+ cells, whereas only 44% of morphologically mature OL were immunoreactive. Only 35% of the MBP+ cells co‐expressed NT3Rs. In the corpus callosum of devloping rat brain, at all ages studied from 7–60 days postnatal, the total absence of NT3Rs in dark OL (morphologically mature), confirmed by ultrastructural immunocytochemistry, indicates an even more dramatic decrease during maturation. Furthermore, the percentage of medium OL (less mature) stained by 2B3 is reduced by approximately half in 60‐ compared to 20‐day‐old rat brain. It is of interest to note that the in vitro observation with maturation markers mirrors the in vivo decrease of NT3R expression during development. It is intersting that NT3Rs are absent in vivo before the critical period of active myelination. These data indicate the presence of a nuclear T3 binding protein in the nuclei of OL at the time of myelination both in vitro and in vivo. The transient expression of these NT3Rs during active myelination argues in favour of a direct effect of thyroid hormones on OL. © 1994 Wiley‐Liss, Inc.