DIFFERENTIAL EXPRESSION OF C-ERBA MESSENGER-RNAS IN THE DEVELOPING CEREBELLUM AND CEREBRAL-CORTEX OF THE RAT

Expression of c-erbA genes, which encode thyroid hormone receptors, has been shown to be tissue and age specific. The differential regulation of thyroid hormone receptor mRNAs in rat brain was examined in the present study using Northern hybridization techniques. Ontogenic profiles of two known mRNAs for rat thyroid hormone receptors (rTR), rTRalpha and rTRbeta, were determined in the rat cerebellum and cerebral cortex. Three bands of approximately 6, 5 and 2.6 kb in size were detected at 1, 6, 12 and 21 days of age (preweanling period) and in the adult, in both brain regions using a rTRalpha-common DNA probe. The 2.6-kb band was expressed at much higher levels than either the 5- or 6-kb band at all ages tested. The 6- and 5-kb bands were expressed at similar levels throughout the preweanling period in the cerebellum, but only at 1 and 6 days of age in the cerebral cortex. The 6-kb transcript was expressed at a much lower level than the 5-kb mRNA by postnatal day 12 in the cerebral cortex. A similar change in the relative level of 6- to 5-kb mRNA expression was noted in cerebellar mRNA isolated from adult animals. The overall expression of rTRalpha mRNA was less in the adult cerebellum and cerebral cortex compared to that in the preweanling period. The rTRbeta transcripts were present in very low levels in the cerebral cortex and were undetectable in the cerebellum. These results are consistent with the fact that the developing brain is more sensitive to thyroid hormone than the mature rat brain. Therefore, developmental processes in the brain may be sensitive to effects of thyroid hormone mediated through this receptor.