Distribution of serotonin in the brain stem and spinal cord of the lizard Varanus exanthematicus: an immunohistochemical study

The distribution of serotonin-containing nerve cell bodies, fibers and terminals in the lizard Varanus exanthematicus was studied with the indirect immunofluorescence technique, using antibodies to serotonin. Most of the serotonin-containing cell bodies were found in the midline, in both of the raphe nuclei, i.e. the nuclei raphes superior and inferior. A considerable number of more laterally shifted serotonergic neurons was found particularly at three levels of the brain stem, viz. in the caudal mesencephalic tegmentum, at the isthmic level, and over a long distance in the medulla oblongata. These laterally situated serotonin-positive neurons were partly found within the confines of the substantia nigra, the nucleus reticularis superior and the lateral part of the nucleus reticularis medius and ventrolateral part of the nucleus reticularis inferior, respectively. No serotonergic cell bodies were found in the spinal cord. In the brain stem a dense serotonergic innervation was observed in all of the motor nuclei of the cranial nerves, in two layers of the tectum mesencephali, in the nucleus interpeduncularis pars ventralis, the nucleus profundus mesencephali pars rostralis, the periventricular grey, the nucleus parabrachialis, the vestibular nuclear complex, the nucleus descendens nervi trigemini, the nucleus raphes inferior, and parts of the nucleus tractus solitarii. Descending serotonergic pathways could be traced into the spinal cord via the dorsolateral, ventral and ventromedial funiculi, and were found to innervate mainly three parts of the spinal grey throughout the spinal cord, i.e. the dorsal part of the dorsal horn, the motoneuron area in the ventral horn, and the intermediate zone just lateral to the central canal. The results obtained in the present study suggest a close resemblance of the organization of the serotonergic system in reptiles and mammals, especially as to the serotonergic innervation of the spinal cord.