Pharmacological studies of thyrotropin-releasing hormone (TRH) receptors from Xenopus laevis: is xTRHR3 a TRH receptor?

Previously, three receptors, xTRHR1, xTRHR2, and xTRHR3, were cloned from brain tissue of Xenopus laevis with primers designed using sequences from the mammalian TRH receptor (TRHR) subtype 1. We expressed the Xenopus receptors in HEK 293EM cells and studied their binding and signaling properties using a series of TRH analogs substituted at the first, second, and third positions. We observed that the three Xenopus receptors exhibited binding and signaling properties that were distinct. Although xTRHR1 was most similar to mouse TRHR1 (mTRHR1), it exhibited binding affinities that were different from mTRHR1. In contrast to mTRHR2, xTRHR2 exhibited lower affinities and potencies for all TRH analogs than mTRHR1. The xTRHR3 displayed very low affinities and potencies for TRH and TRH analogs and showed little discrimination for TRH analogs; it is likely, therefore, that another peptide is the cognate ligand for xTRHR3. Our findings show differences between TRHR1 and TRHR2 from Xenopus and mammals and suggest that xTRHR3 is a receptor for a ligand other than TRH.