Thyrotropin-releasing hormone (TRH): immunohistochemical distribution in tadpole and frog brain.

The anatomical localization of immunoreactive TRH (IR-TRH) was demonstrated by the peroxidase-antiperoxidase technique in the brain and pituitary gland of larval and adult Rana catesbeiana. In the adult frog main sites of IR-TRH are perikarya and neuronal fibers in the preoptic and infundibular nuclei of the hypothalamus and in the amygdala and diagonal band of Broca of the telencephalon. In addition, TRH-positive neuronal fibers and endings were found in the septum, pallium, and brain stem as well as in the preoptico-hypophyseal tract, the external zone of the median eminence (which matures during late larval stages), and the pars nervosa; fibers were less extensive in the pars intermedia, and were absent from the pars distalis. In early larval stages, the magnocellular nucleus of the posterior preoptic area is the main site of immunoreactive perikarya. During late stages the extensive adult pattern of distribution of IR-TRH becomes established. The study represents the first immunohistochemical demonstration of IR-TRH in larval anurans, and serves as a basis for clarification of the neuroendocrine regulation of metamorphosis.