In the diencephalons of the adult zebrafish brain, all catecholamine-containing neurons are dopaminergic. The organization and projection pattern of these neurons are studied using tyrosine hydroxylase immunocytochemistry. By their locations, 3 neuronal complexes and 17 cell groups are identified on the bases of their morphology, staining intensity, and projection pattern: 1) the preoptic complex (5 groups); 2) the posterior tuberal complex (4 groups); and 3) the hypothalamic complex (5 groups). In addition, three other groups can be distinguished: one group in the ventral thalamus; one in the pretectal area, and one found in the postoptic commissure and above the pituitary stalk in a few brains. Two dopaminergic pathways are defined: 1) the preoptico-hypophyseal tract runs in close association with the lateral forebrain bundle along the base of the brain between the preoptic area and the pituitary stalk, and neurons of the preoptic complex are major contributors to this pathway; additional fibers come from the large periventricular organ-associated neurons of the posterior tuberal; 2) the endohypothalamic tract links neurons of the hypothalamic complex and consists mainly of processes from hypothalamic neurons. Axons from neurons of the suprachiasmatic, periventricular organ-associated, and posterior tuberal nuclei also join this pathways after entering the hypothalamus. Several groups of neurons contact the cerebrospinal fluid. These appear to be primarily local neurons because none have processes that join the two major pathways. The preoptic area, dorsal thalamus, tuberal and hypothalamic areas, optic tectum, and pituitary are the major targets of diencephalic dopaminergic neurons. The dorsal telencephalon does not receive input from these cells. The large periventricular organ-accompanying neurons have descending projections beyond the diencephalon and isthmus. Some cells of this group terminate in the crista cerebellaris. A few axons also exit the medulla via a branch of the octavolateralis nerve.

The organization of the brainstem trigeminal complex (BTC) of the mouse is described, with emphasis on the normal organization of the vibrissal representations. Thionin staining for Nissal substance was employed to reveal the cytoarchitecture. Cytochrome oxidase histochemistry was used to reveal the chemoarchitecture. Golgi impregnation methods, in combination with thionin staining, were used to examine the neuronal dendritic morphology within a defined cytoarchitectonic context. An in vitro horseradish peroxidase labelling method was used to study the distribution and morphology of primary trigeminal afferent terminals within the BTC. The BTC consists of four distinct subnuclei: principalis (nVp), oralis (nVo), interpolaris (nVi), and caudalis (nVc). The present study shows that these sub-nuclei can be distinguished from each other on the basis of several anatomical criteria, including the distribution and density of neuronal size classes, histochemical staining intensity, morphology and orientation of neuronal dendrites, and size and texture of primary afferent terminal arbors. Anatomical manifestation of vibrissal representations within the BTC can be described in nVp, nVi, and nVc, but not in nVo. Within the three subnuclei where they are found, anatomical vibrissal representations are composed to architectural subunits that form an overall pattern homeomorphic to the pattern of vibrissae on the face of the animal. Each sub-unit forms a cylindrical tube running in a rostrocaudal orientation within the BTC. These sub-units will be called barrelettes. Cytologically, each barrelette consists of cell-dense "sides," surrounding a practically cell-free "hollow." Individual sub-units are separated by narrow, cell-free "septa." Histochemically, each subunit is manifested as a discrete patch of positive-staining reaction products. Differential interference contrast optics shows that these patches correspond precisely to the barrelette hollows. Evidence is presented to show that the barrelettes are the functional units for the processing of vibrissal sensory information. Terminal arborizations of individual primary afferents seem to be confined to the hollow of single barrelettes. The majority of neurons that form the sides of a barrelette have bitufted dendritic arbors, which project predominantly into the barrelette hollow, although a minority of neurons, particularly in nVi and nVc, also extend part of their dendritic arbors into adjacent barrelette hollows. The barrelette hollows are thus the principal neuropil region in which primary afferents and their target neurons interact. Contacts are made mainly between en passant varicosities and terminal boutons on primary afferent collaterals and dendritic spines and shafts of second order neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

The locus coeruleus is a widely projecting isthmal noradrenergic nucleus. In the zebrafish, it consists of between three and ten neurons, most of which have multiple, bilaterally projecting axons. Immunohodological studies show that the locus coeruleus provides most, if not all, of the noradrenergic innervation of the brain rostral to the isthmus. The pathways and targets in the zebrafish are similar to ascending coeruleal projections of other vertebrates. Axons ascend through two main pathways: the longitudinal catecholamine bundle and the periventricular catecholamine pathway. The former is a dense meshwork of varicosity-bearing axons which ascends along the lateral longitudinal fasciculus into the mesencephalon. In the posterior tuberal area, this bundle dives ventrally and assumes a lateral position. In the diencephalon, it takes up a position ventral to the medial forebrain bundle, and follows this bundle into the telencephalon, where it joins the medial olfactory tract to enter the olfactory bulb. The periventricular catecholamine pathway is a diffuse pathway consisting of thick, smooth axons. It is associated with the medial longitudinal fasciculus. Rostral to the nucleus of the medial longitudinal fasciculus, this pathway joins the longitudinal catecholamine bundle around the medial forebrain bundle. The periventricular pathway gives rise to coarse terminal arbors with large but sparse varicosities, whereas the longitudinal catecholamine bundle gives rise to terminal plexuses with fine and dense fibers and varicosities. Among the more densely innervated regions are the raphé nucleus, the interpeduncular nucleus, the torus semicircularis, parts of the hypothalamus, and the suprachiasmatic and preoptic areas. The torus longitudinalis, optic tectum, cerebellum, habenular complex, the dorsomedial zone of area dorsalis telencephali, and the olfactory bulb are moderately innervated. The nucleus glomerulosus, the torus lateralis and lateral subnuclei of the nucleus diffusus, and the anterior tuberal nucleus are devoid of noradrenergic innervation.

Nitric oxide has recently been implicated as a neurotransmitter, and may modulate synaptic transmission, cerebral blood flow, and neurotoxicity. NADPH diaphorase histochemistry has been shown to be a reliable marker for nitric oxide synthase, the enzyme that synthesizes nitric oxide, in the nervous system. Because monoaminergic neurons frequently contain co-transmitters, we examined whether these cells also exhibit NADPH diaphorase activity. Frozen sections from postnatal and adult rat brains were stained for NADPH diaphorase activity and either serotonin-like immunoreactivity or tyrosine hydroxylase-like immunoreactivity. Numerous neurons in the mesopontine serotoninergic cell groups (including the caudal linear, dorsal, median, supralemniscal, and pontine raphe nuclei) contained both serotonin-like immunoreactivity and NADPH diaphorase activity. Within the dorsal raphe nucleus, approximately 70% of the serotoninergic neurons in the medial subnuclei displayed NADPH diaphorase activity, while less than 10% of the serotoninergic neurons in the lateral subnuclei were doubly labeled. Retrograde labeling with fluorescent microspheres indicated that many raphe-cortical neurons contained NADPH diaphorase activity. No NADPH diaphorase activity was detected in serotoninergic neurons in the medullary nuclei (including the raphe magnus, raphe pallidum, and raphe obscurus). Only a small proportion of tyrosine hydroxylase-like immunoreactive neurons in the periaqueductal gray, rostral linear nucleus, and rostrodorsal ventral tegmental area contained NADPH diaphorase activity. Tyrosine hydroxylase-like immunoreactive neurons in the substantia nigra, locus coeruleus, hypothalamus, olfactory bulb, and dorsal raphe nucleus did not contain detectable NADPH diaphorase activity. The observation that many mesopontine (but not medullary) serotoninergic neurons contain NADPH diaphorase activity suggests that these neurons may release both serotonin and nitric oxide.

The locus coeruleus is a noradrenergic nucleus located in the isthmal tegmentum. In mammals, it contains several thousand neurons that have diverse projection patterns and contain various neuropeptides. In fishes, this nucleus contains few neurons. This study attempts to define and quantify morphological types of locus coeruleus neurons, and search for neurochemical subpopulations in the zebrafish. In this fish, the locus coeruleus contains between 3 and 10 neurons, and most nuclei contain between 5 and 8 cells. Nuclei in more inbred lines of fish have a narrower range of neurons. The difference in neuron number between the two sides of the same brain is small, but only 24% of the brains have identical numbers on both sides. These observations suggest that there is a two-step control of neuron number: the genetic constitution of the fish determines the approximate number of cells, while epigenetic factors determine the final number. Based on dendritic orientation, three types of cells are identified: (1) V type, neurons with only ventrally projecting dendrites; (2) L type, neurons with only laterally projecting dendrites; and (3) VL type, neurons with both ventrally and laterally projecting dendrites. Over 65% of the neurons are of the V type; some nuclei have V type cells only. There is a correlation between the total number of neurons and the ratio of each cell type. In nuclei with five cells or fewer, over 80% of the neurons are V type; higher percentages of the other two types are seen in nuclei with 6 or more neurons. The dendritic morphology and orientation suggest that various types of neurons may receive different inputs. Cholinesterases are not detectable in locus coeruleus neurons. Immunocytochemical staining for a number of neuropeptides also fails to demonstrate detectable levels. Neuropeptide Y is present in some cells abutting the locus coeruleus, but these are probably not catecholamine-containing neurons. Some neurons contain choline-acetyltransferase. These observations suggest that locus coeruleus neurons of the zebrafish may be morphologically and neurochemically heterogeneous.