The vertebrate reticular formation, containing over 30 nuclei in mammals, is a core brainstem area with a long evolutionary history. However, not all reticular nuclei are equally old. Nuclei that are widespread among the vertebrate classes are probably ones that evolved early. We describe raphe nuclei in the reticular formation of three cartilaginous fishes that diverged from a common ancestor over 350 million years ago. These fishes are Hydrolagus colliei, a holocephalan, Squalus acanthias, a small-brained shark, and Heterodontus francisci, a large-brained shark. Nuclear identification was based on immunohistochemical localization of serotonin and leu-enkephalin, on brainstem location, and on cytoarchitectonics. Raphe nuclei are clustered in inferior and superior cell groups, but within these groups individual nuclei can be identified: raphe pallidus, raphe obscurus, and raphe magnus in the inferior group and raphe pontis, raphe dorsalis, raphe centralis superior, and raphe linearis in the superior group. Hydrolagus lacked a dorsal raphe nucleus, but the nucleus was present in the sharks. The majority of immunoreactive cells are found in the superior group, especially in raphe centralis superior, but immunoreactive cells are present from spinal cord to caudal mesencephalon. The distribution and cytoarchitectonics of serotoninergic and enkephalinergic cells are similar to each other, but raphe nuclei contain fewer enkephalinergic than serotoninergic cells. The cytoarchitectonics of immunoreactive raphe cells in cartilaginous fishes are remarkably similar to those described for raphe nuclei in mammals; however, the lack of a raphe dorsalis in Hydrolagus indicates that either it evolved later than the other raphe nuclei or it was lost in holocephalan fishes.
S-nitrosocysteine is a potent vasodilatory agent capable of overcoming the hypoxic vasoconstrictive response of the lung. Our results suggest it may prove useful as a pulmonary vasodilatory agent at low doses. Higher dose infusions reduce mean systemic pressure and lead to compensatory reductions in renal and mesenteric blood flow without a decrease in cardiac output.
We examined the distribution of immunoreactivity to serotonin (5-HT), leu-enkephalin (LENK), tyrosine-hydroxylase (TH), and substance P (SP) within the primary visceral sensory region of cartilaginous fish. Two genera of sharks, Squalus and Heterodontus, a skate, Raja, a ray, Myliobatis, and a holocephalian, Hydrolagus, were used. Cranial nerves, VII, IX, and X enter the visceral sensory complex from the lateral aspect and divide it into lobes. Based on sagittally cut sections, there are four lobes in Hydrolagus and five in Squalus, corresponding to the number of gill arches. The neurochemicals are differentially distributed within each lobe. LENK+ and 5-HT+ fibers are located in all regions within the visceral sensory complex. SP+ fibers are extremely dense in a dorsolateral subdivision and do not extend as far ventrally as 5-HT+ and LENK+ fibers. The lobes lack 5-HT+ cells, but contain a few LENK+ and SP+ cells. Many TH+ cells are distributed in dorsomedial portions of the complex, but there are few TH+ fibers. Thus, the visceral sensory area of cartilaginous fish contains several divisions that can be distinguished by their neurochemical content.
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