Immunohistochemical localization of serotonin in the brain stem of Rana pipiens (amphibia)

The indirect antibody peroxidase-anti-peroxidase technique was used to immunocytochemically localize serotonin (5HT) neurons in the frog brain stem. The 5HT neurons were found as expected in the nucleus isthmi, and also the different nuclei. They were located in the region extending from posterior of the cranial nerve VIII level (caudally) to anterior of nucleus interpeduncularis level (rostrally), both close to the midline and also in the lateral area. The midline raphe reticular nuclei with 5HT neurons were the raphe obscurus, raphe pallidus, raphe magnus, raphe pontis, raphe dorsalis, raphe centralis superior and raphe linearis. The laterally-located reticular nuclei with 5HT neurons were the reticularis magnocellularis, reticularis gigantocellularis, reticularis paragigantocellularis, reticularis pontis oralis pars medialis and pars lateralis, reticularis pedunculopontis, reticularis cuneiformis and reticularis sub-cuneiformis. Previously it has been suggested that the 5HT neurons of non-mammalian veterbrates are confined to only the midline raphe nuclei. However, a lateralization of the 5HT system from the midline raphe region was found in Rana pipiens. The frog reticular groups with 5HT neurons were similar to those found in the mammalian reticular system. Thus, the organization of 5HT system in the frog suggests that the organization is phylogenetically old