The availability of synthetic thyroid hormone receptor agonists provides a valuable tool to analyze whether specific receptor isoforms mediate specific physiological responses to thyroid hormone. GC-1 is a thyroid hormone analog displaying selectivity for thyroid hormone receptor β. We have ana lyzed the effect of GC-1 on expression of thyroid hormone target genes in the cerebrum and cerebellum. Congenitally hypothyroid rats were treated with single daily doses of either T3 or GC-1. Both compounds similarly induced Purkinje cell protein-2 (PCP-2) in the cerebellum. Expression of RC3 and Rhes in the caudate, and hairless, neurotrophin-3, Reelin, and Rev-ErbAα in the cerebellum, was analyzed by in situ hybridization on postnatal d 16. Hypothyroidism strongly decreased expression of RC3 and Rhes in the caudate, and hairless, RevErbAα, and neurotrophin-3 in the cerebellum, and increased Reelin. T3 treatment normalized the expression of all genes. However, GC-1 effectively normalized expression of Rhes and Reelin only. The lack of a GC-1 effect on most cerebellar genes can be explained by the known distribution of thyroid hormone receptor α and β isoforms. However, in the caudate, RC3 and Rhes are expressed in the same cells, and therefore, they may represent specific gene responses linked to specific thyroid hormone receptor isoforms.
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