The relationship between dopaminergic neuronal structures and dopaminoceptive structures in the amphibian brain and spinal cord are assessed by means of single and double immunohistochemical techniques with antibodies directed against DARPP-32 (a phosphoprotein related to the dopamine D(1)-receptor) and tyrosine hydroxylase (TH) applied to the brain of the anurans Rana perezi and Xenopus laevis. The DARPP-32 antibody yielded a well-differentiated pattern of staining in the brain of these anurans. In general, areas that are densely innervated by TH-immunoreactive fibers such as the nucleus accumbens, striatum, amygdaloid complex, thalamus, optic tectum, torus semicircularis and spinal cord display a remarkable immunoreactivity for DARPP-32 in cell bodies and neuropil. Distinct cellular DARPP-32 immunoreactivity was also found in the septum, preoptic area, suprachiasmatic nucleus, tuberal hypothalamic region, habenula, retina, midbrain tegmentum, rhombencephalic reticular formation and solitary tract nucleus. Hodological data supported that striatal projection neurons were DARPP-32 immunoreactive. Double immunohistofluorescence staining revealed that catecholaminergic cells generally do not stain for DARPP-32, except for some cells in the ventral mesencephalic tegmentum of Xenopus and cells in the nucleus of the solitary tract of Rana. Several interspecies differences were noted for the DARPP-32 distribution in the brain of the two anurans, namely in the habenula, the thalamus and prethalamus, the cerebellum and octavolateral area and the structures with DARPP-32/TH colocalization. However, in general, the distribution of DARPP-32 in the brain of the anuran amphibians resembles in many aspects the pattern observed in amniotes, especially in 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.