Immunohistochemical localization of thyrotropin-releasing hormone in the brain of reptiles

To gain insight into the evolution of the thyrotropin-releasing hormone (TRH) system in the brain of vertebrates we have conducted a comparative analysis of the distribution of TRH immunoreactive cell bodies and fibers in two reptiles, the turtle Pseudemys scripta elegans and the lizard Gekko gecko. Double labeling for TRH and tyrosine hydroxylase, the main catecholamine marker, was made to help the correct localization of the labeled structures and to evaluate the possible interaction of these two systems. Cells containing TRH were found in the hypothalamic paraventricular and periventricular nuclei and their projections to the median eminence were corroborated. In addition, with some species variation, extrahypothalamic cells were present in the medial amygdala, mesencephalic tegmentum, descending vestibular nucleus and in the retina. Fiber labeling was observed in all main brain subdivisions but was more abundant in regions such as the striatum, septum, amygdaloid complex, dorsal thalamus and tegmento-reticular zones. Actual colocalization of TRH and catecholamines in the same neurons was not observed but the codistribution of cells and fibers labeled for TRH and tyrosine hydroxylase strongly suggests that they can interact in diverse regions, not only in the hypothalamo–hypophysial system. The comparison of the distribution of TRH immunoreactive neurons and fibers found in reptiles with those reported for other vertebrates reveals a strong resemblance but also notable variations, not only across vertebrate classes but also within the same class.