Distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of urodele amphibians.

To improve knowledge of the peptidergic systems in the brain of amphibians we have conducted a comparative analysis of the distribution of TRH immunoreactive cell bodies and fibers in three species of urodeles. Fiber labeling was observed in all main brain subdivisions suggesting different control functions for TRH in extrahypothalamic systems. However, as in other vertebrates, TRH neurons were abundant in the hypothalamic nuclei that presumably project to the median eminence and the neural lobe of the hypophysis. Considerable interspecies differences were noted mainly related to innervation of the olfactory and visual centers (thalamus and mesencephalic tectum) and the precise localization of immunoreactive cell bodies, which was assessed by double labeling with tyrosine hydroxylase. The comparison of the distribution of TRH immunoreactive neurons and fibers found in urodeles with those reported for other vertebrates, in particular with anamniotes, reveals a strong resemblance but also notable variations not only across vertebrate classes but also within the same class. In this respect, the virtual lack in urodeles of TRH innervation of the intermediate lobe of the hypophysis clearly contrasts with the innervation found in anurans. Therefore, the important role of skin color adaptation proposed for TRH in anurans on the basis of the direct innervation of the intermediate lobe is not applicable for urodeles.