Recently, the zebrafish has been extensively used for studying the development of the central nervous system (CNS). However, the zebrafish CNS has been poorly analyzed in the adult. The cholinergic/cholinoceptive system of the zebrafish CNS was analyzed by using choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry in the brain, retina, and spinal cord. AChE labeling was more abundant and more widely distributed than ChAT immunoreactivity. In the telencephalon, ChAT-immunoreactive (ChAT-ir) cells were absent, whereas AChE-positive neurons were observed in both the olfactory bulb and the telencephalic hemispheres. The diencephalon was the region with the lowest density of AChE-positive cells, mainly located in the pretectum, whereas ChAT-ir cells were exclusively located in the preoptic region. ChAT-ir cells were restricted to the periventricular stratum of the optic tectum, but AChE-positive neurons were observed throughout the whole extension of the lamination except in the marginal stratum. Although ChAT immunoreactivity was restricted to the rostral tegmental, oculomotor, and trochlear nuclei within the mesencephalic tegmentum, a widespread distribution of AChE reactivity was observed in this region. The isthmic region showed abundant AChE-positive and ChAT-ir cells in the isthmic, secondary gustatory and superior reticular nucleus and in the nucleus lateralis valvulae. ChAT immunoreactivity was absent in the cerebellum, although AChE staining was observed in Purkinje and granule cells. The medulla oblongata showed a widespread distribution of AChE-positive cells in all main subdivisions, including the octavolateral area, reticular formation, and motor nuclei of the cranial nerves. ChAT-ir elements in this area were restricted to the descending octaval nucleus, the octaval efferent nucleus and the motor nuclei of the cranial nerves. Additionally, spinal cord motoneurons appeared positive to both markers. Substantial differences in the ChAT and AChE distribution between zebrafish and other fish species were observed, which could be important because zebrafish is widely used as a genetic or developmental animal model.

The distribution and morphological characterization of nicotinamide adenine dinucleotide phosphate-diaphorase (ND)-positive cells and fibers in the tench central nervous system was mapped by using a direct histochemical method. This enzyme was observed in specific cell populations throughout all main divisions of the tench brain.In the telencephalon, we found strongly labeled olfactory fibers, as well as positive cells and fibers in the area ventralis of the telencephalic lobes. Positive staining was observed in the following diencephalic nuclei: nucleus preopticus magnocellularis pars magnocellularis, nucleus recessus lateralis, nucleus recessus posterioris, nucleus posterior tuberis, and nucleus diffusus torus lateralis, as well as small cells with a diffuse distribution surrounding the diencephalic ventricle. In the mesencephalon, heavily stained ND-positive neurons were observed in the nucleus fasciculi longitudinalis medialis, nucleus nervi oculomotorius, and nucleus nervi trochlearis. In the hindbrain the most evident staining was observed as large neurons located in the nuclei of the cranial nerves, scattered positive cells located between the negative fibers of the cranial nerves, and in the nucleus fasciculi solitari. Finally, in the spinal cord, ND-positive cells and fibers were mainly located in the ventral horn.This distribution of ND labeling in the brain of the tench is significantly different from previous data on ND activity in the brain of terrestrial vertebrates and does not correlate with the presence and distribution patterns of several neurotransmitters and neuroactive substances in the teleost brain. o 199.5 Wiley-Liss, Inc.

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