Thyroid hormone receptor beta is widely expressed in the brain and pituitary of the Japanese eel, Anguilla japonica.

In a previous study, we cloned the thyroid hormone receptor betaB (TRbetaB) from a teleostean fish, the Japanese conger eel (Conger myriaster). The gene encoding this receptor is expressed in the brain and pituitary. In this study, we cloned TRbetaB from the brain of the Japanese eel (Anguilla japonica) to extend our studies into thyroid hormone function in fish. RT-PCR analysis demonstrated that the TRbetaB transcripts were abundant in both the brain and pituitary. The TRbetaB cDNA encoded a 379 amino acid protein with much higher homology to the conger eel TRbetaB than to other fish TRs, supporting the existence of a TRbetaB isoform, at least in anguilliforms. In a transiently transfected Japanese eel cell line, Hepa-E1, TRbetaB and its splice variants in the ligand-binding domain (TRbetaBL) showed thyroid hormone (TH)-dependent activation of transcription from the TH-responsive promoter. In situ hybridization studies revealed the presence of TRbetaB transcripts in the pars distalis of the pituitary.