Pattern of Nitrergic Neuronal System Organization in the Brain of Two Holostean Fishes (Actinopterygii: Ginglymodi)

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The study of the nitrergic system, formed by the networks of neurons containing the enzyme nitric oxide synthase (NOS), has been extremely useful in unraveling neuroanatomical features of the organization of the central nervous system of vertebrates. Thus, data are available for representatives of most vertebrate classes and, in particular, several studies have detailed the organization of this system in teleosts. In contrast, no information is available regarding this neurotransmission system in the brains of holosteans, an early diverged and poorly understood group of actinopterygian fishes, currently considered a sister group of teleosts that contains only 8 species. The present study provides the first detailed information on the distribution of nitrergic cell bodies and fibers in 2 holostean species of the genus Lepisosteus, the spotted gar L. oculatus and the Florida gar L. platyrhincus. NOS immunohistochemistry and the NADPH diaphorase (NADPH-d) histochemical reaction were used, and both techniques yielded identical results, with the exception of the primary olfactory and terminal nerve fibers, which only labeled for NADPH-d exclusively in L. oculatus. Double immunohistochemistry was conducted for the simultaneous detection of NOS with tyrosine hydroxylase, choline acetyltransferase, calbindin, calretinin, and serotonin to accurately establish the localization of the nitrergic neurons and fibers in the brain of holosteans, the neuroanatomy of which has been mostly neglected, and to assess possible interactions between these neuroactive substances. Distinct groups of nitrergic cells were located in subpallial areas, the basal hypothalamus, posterior tubercle, optic tectum and mesencephalic tegmentum, reticular formation, solitary tract nucleus, spinal cord, and amacrine cells in the retina. In addition, low numbers of nitrergic cells were observed in the pallium, suprachiasmatic nucleus, prethalamic and thalamic areas, torus lateralis and torus semicircularis, cerebellar and laterodorsal tegmental nuclei, and the ventral octavolateral area. Comparison of these results with those from other classes of vertebrates, and including a segmental analysis to correlate cell populations, reveals that the pattern of the nitrergic system in holosteans is very close to that in ancestral actinopterygian fishes and highlights conserved and derived traits.