Detection of extended distribution of beta2-thyroid hormone receptor messenger ribonucleic acid (RNA) in adult rat brain using complementary RNA in situ hybridization histochemistry.

In situ hybridization histochemistry using a complementary RNA probe, directed at the entire length of the thyroid hormone receptor beta2 (TRbeta2)-specific region (-56 to 495 bp), was used to evaluate expression of TRbeta2 messenger RNA (mRNA) in coronal sections of adult rat brain. An extended distribution and intense expression of TRbeta2 mRNA was found in several regions of rat brain, including regions where TRbeta2 mRNA had not been previously identified using PCR or in situ hybridization histochemistry. These areas included hippocampus (dentate gyrus and C1, -2, and -3), cerebral cortex (predominantly layer 3), arcuate nucleus, median eminence, medial geniculate nucleus, tegmental bundle, medial and lateral lemniscus, Purkinje layer of the cerebellum, and several brain stem nuclei. In conclusion, we have developed a highly sensitive and specific method to demonstrate TRbeta2 mRNA expression in adult rat brain. The present findings are in agreement with immunoreactive TRbeta2 studies of rat brain and argue against the presence of an unidentified T3-binding protein to explain the previous discordant results of TRbeta2 mRNA and protein studies. In addition, the specificity of distribution of TR beta2 mRNA to certain brain nuclei, particularly those involved in hearing, implies a specific functional role of this receptor subtype and provides a physiological basis to understand the effects of hypothyroidism on brain development.