Distribution of the mRNAs encoding the thyrotropin-releasing hormone (TRH) precursor and three TRH receptors in the brain and pituitary of Xenopus laevis: effect of background color adaptation on TRH and TRH receptor gene expression.

In amphibians, thyrotropin-releasing hormone (TRH) is a potent stimulator of alpha-melanotropin (alpha-MSH) secretion, so TRH plays a major role in the neuroendocrine regulation of skin-color adaptation. We have recently cloned a third type of TRH receptor in Xenopus laevis (xTRHR3) that has not yet been characterized in any other vertebrate species. In the present study, we have examined the distribution of the mRNAs encoding proTRH and the three receptor subtypes (xTRHR1, xTRHR2, and xTRHR3) in the frog CNS and pituitary, and we have investigated the effect of background color adaptation on the expression of these mRNAs. A good correlation was generally observed between the expression patterns of proTRH and xTRHR mRNAs. xTRHRs, including the novel receptor subtype xTRHR3, were widely expressed in the telencephalon and diencephalon, where two or even three xTRHR mRNAs were often simultaneously observed within the same brain structures. In the pituitary, xTRHR2 was expressed selectively in the distal lobe, and xTRHR3 was found exclusively in the intermediate lobe. Adaptation of frog skin to background illumination had no effect on the expression of proTRH and xTRHRs in the brain. In contrast, adaptation of the animals to a white background provoked an 18-fold increase in xTRHR3 mRNA concentration in the intermediate lobe of the pituitary. These data demonstrate that, in amphibians, the effect of TRH on alpha-MSH secretion is mediated through the novel receptor subtype xTRHR3.