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Distribution of tyrosine hydroxylase- and serotonin-immunoreactive cells in the central nervous system of the thornback guitarfish, Platyrhinoidis triseriata.

Brainstem reticular nuclei of amniotes (mammals, birds and reptiles) may share a common phylogenetic origin as demonstrated by their many shared features (hodology, cytoarchitectonics, presence of neurochemicals). By studying characteristics of these nuclei in outgroups of amniotes, we hope to obtain clues about the phylogeny of the reticular formation. In this paper we report the distribution of immunoreactivity to tyrosine hydroxylase (TH) and serotonin (5-HT) in the brain of an elasmobranch, the thornback guitarfish, Platyrhinoidis triseriata. Our working hypothesis is that if morphologically and immunohistochemically similar cell groups are present, they are homologous to cell groups in amniotes. Thus we have used mammalian terminology. The dorsal and lateral pallium of the telencephalon and many diencephalic nuclei contained TH+ cells. In the mesencephalon, TH+ cell groups were located in raphe linearis, the ventral tegmentum and substantia nigra. The rhombencephalon contained TH+ cells in a putative locus coeruleus (A6), and a subcoeruleus group. Probable A5, A2/C2 and A1/C1 groups were also located. A few 5-HT+ cells were located in the telencephalon and many were found in the diencephalon. In the mesencephalon, 5-HT+ cells were located in the nucleus reticularis pedunculopontinus pars dissipatus (B9). Metencephalic cells were found in reticularis pontis oralis lateralis and medialis, the reticulotegmental nucleus, nucleus centralis superior (B8), reticularis magnocellularis and reticularis pontis caudalis. In the myelencephalon, 5-HT+ cells were contained in raphe pallidus, reticularis paragigantocellularis lateralis and reticularis ventralis pars alpha. The cell shapes, locations, and neurochemical content of Platyrhinoidis reticular groups were very similar to those of amniotes. This elasmobranch has most of the 5-HT+ and TH+ cell groups found in mammals with the major exception that no 5-HT+ cells were in a nucleus which might correspond to raphe dorsalis.