Neurons expressing thyrotropin-releasing hormone-like messenger ribonucleic acid are widely distributed in Xenopus laevis brain.
Neurons containing messenger ribonucleic acid encoding thyrotropin-releasing hormone (TRH) in the brain of Xenopus laevis were visualized using in situ hybridization histochemistry. TRH-like mRNA-containing cells were detected in each coronal section tested, from the rostral tip of the telencephalon to the brainstem. In the telencephalon, labeled cells were observed in the olfactory nucleus, nucleus accumbens, striatum, diagonal band, and amygdala. Many intensely labeled cells were found in the hypothalamus, especially the preoptic nucleus and infundibular nuclei (dorsal and ventral). TRH-like mRNA-containing cells were also observed in the thalamus, optic tectum, and brain stem. These results suggest that the TRH neuronal system of X. laevis is organized in a manner similar to that of mammals, in that it is widely distributed throughout forebrain. However, Northern blot analysis of RNA extracted from Xenopus brain indicated that additional mRNAs possessing a TRH-like sequence may be present, suggesting that the regulation of TRH gene expression in Xenopus is different from that of the rat.