Distribution of choline acetyltransferase (ChAT) immunoreactivity in the central nervous system of a chondrostean, the siberian sturgeon (Acipenser baeri)

All studies to date of cholinergic systems of bony fishes have been done in teleosts. To gain further insight into the evolution of the cholinergic systems of bony fishes, we have studied the brain of a chondrostean fish, the Siberian sturgeon (Acipenser baeri, Brandt), by using an antibody against choline acetyltransferase (ChAT). This study showed the presence of ChAT-immunoreactive (ChAT-ir) neurons in the preoptic region (parvocellular and magnocellular preoptic nuclei and suprachiasmatic nucleus), the periventricular and tuberal hypothalamus, the saccus vasculosus, the dorsal thalamus, and the habenula. The mesencephalic tegmentum contained ChAT-ir cells in the torus semicircularis and torus lateralis. The isthmus contained several cholinergic populations: the nucleus isthmi, the lateral nucleus of the valvula, the secondary visceral nucleus, and the dorsal tegmental nucleus. The motor neurons of the cranial nerves and the spinal motor column were strongly immunoreactive. The medial (sensory) trigeminal nucleus also contained a ChAT-ir neuronal population. The distribution of ChAT-ir neurons in the sturgeon brain showed some notable differences with that observed in teleosts, such as the absence of cholinergic cells in the telencephalon and the optic tectum. Several brain regions were richly innervated by ChAT-ir fibers, particularly the telencephalon, optic tectum, thalamus, posterior tubercle, and interpeduncular nucleus. The hypothalamo-hypophyseal tract, the tract of the saccus vasculosus, the fasciculus retroflexus, and an isthmo-mesencephalo-thalamic tract were the most conspicuous cholinergic bundles. Comparative analysis of these results suggests that teleosts have conserved most traits of the cholinergic system of the sturgeon, having acquired new cholinergic populations during evolution.     

Distribution of choline acetyltransferase (ChAT) immunoreactivity in the brain of the adult trout and tract-tracing observations on the connections of the nuclei of the isthmus

The distribution of cholinergic neurons and fibers was studied in the brain and rostral spinal cord of the brown trout and rainbow trout by using an antiserum against the enzyme choline acetyltransferase (ChAT). Cholinergic neurons were observed in the ventral telencephalon, preoptic region, habenula, thalamus, hypothalamus, magnocellular superficial pretectal nucleus, optic tectum, isthmus, cranial nerve motor nuclei, and spinal cord. In addition, new cholinergic groups were detected in the vascular organ of the lamina terminalis, the parvocellular and magnocellular parts of the preoptic nucleus, the anterior tuberal nucleus, and a mesencephalic tegmental nucleus. The presence of ChAT in the magnocellular neurosecretory system of trout suggests that acetylcholine is involved in control of hormone release by neurosecretory terminals. In order to characterize the several cholinergic nuclei observed in the isthmus of trout, their projections were studied by application of 1,1;-dioctadecyl-3,3,3;, 3;-tetramethylindocarbocyanine perchlorate (DiI) to selected structures of the brain. The secondary gustatory nucleus projected mainly to the lateral hypothalamic lobes, whereas the nucleus isthmi projected to the optic tectum and parvocellular superficial pretectal nucleus, as previously described in other teleost groups. In addition, other isthmic cholinergic nuclei of trout may be homologs of the mesopontine system of mammals. We conclude that the cholinergic systems of teleosts show many primitive features that have been preserved during evolution, together with characteristics exclusive to the group.     

Catecholaminergic systems in the zebrafish. IV. Organization and projection pattern of dopaminergic neurons in the diencephalon

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.     

Distribution of galanin-like immunoreactivity in the brain of the Siberian sturgeon (Acipenser baeri)

Galanin is a 29-amino acid peptide widely distributed in the central nervous system of vertebrates. The organization of galaninergic systems is well known in teleosts, the most advanced actinopterygians, but no data are available on primitive bony fish. To extend the evolutionary analysis of galaninergic systems we studied the distribution of galanin-like immunoreactive (GAL-ir) cells and fibers in the sturgeon brain, since chondrosteans are among the most primitive extant actinopterygians. Double-immunolabeling experiments were performed to compare the distribution of galanin with that of neurophysin, tyrosine hydroxylase, and serotonin. Numerous GAL-ir cells of cerebrospinal fluid-contacting (CSF-C) type were found in the ventral telencephalon, preoptic area, and in the tuberal and caudal hypothalamus. The distribution of GAL-ir elements in the sturgeon brain shows many similarities to that observed in other vertebrates, but also important differences, such as the abundance of GAL-ir CSF-C cells, which appear to be a primitive characteristic. GAL-ir neurons observed in the sturgeon telencephalic hemispheres perhaps represent the basic organization of common ancestors of bony fishes and tetrapods. In the preoptic-hypophyseal system, GAL-ir cells appeared to be related not only with neurophysin-expressing neurons (in the tuberal hypothalamus) but also with serotoninergic and catecholamines-synthesizing neurons (in preoptic and tuberal nuclei). Numerous GAL-ir fibers were observed in the median eminence and neural lobe of the hypophysis, indicating that galanin may play a role in the modulation of hypophyseal secretion. GAL-ir neurons were absent from the sturgeon brainstem, suggesting that their presence in other vertebrates could represent an evolutionary recent acquisition.     

Fiber connections of the lateral valvular nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

Fiber connections of the lateral valvular nucleus were investigated in a percomorph teleost, the tilapia (Oreochromis niloticus), by tract-tracing methods. Following tracer injections into the lateral valvular nucleus, neurons were labeled in the ipsilateral dorsal part of dorsal telencephalic area, corpus glomerulosum pars anterior, dorsomedial thalamic nucleus, central nucleus of the inferior lobe, mammillary body, semicircular torus, valvular and cerebellar corpus, in the bilateral rostral regions of the central part of dorsal telencephalic area, dorsal region of the medial part of dorsal telencephalic area, habenula, anterior tuberal nucleus, posterior tuberal nucleus, and spinal cord, and in the contralateral lateral funicular nucleus. Labeled fibers and terminals were found in the ipsilateral cerebellar corpus and bilateral valvula of the cerebellum. Tracers were injected into portions of the telencephalon, pretectum, inferior lobe, and cerebellum to confirm reciprocally connections with the lateral valvular nucleus and to determine afferent terminal morphology in the lateral valvular nucleus. Telencephalic fibers terminated mainly in a dorsolateral portion of the lateral valvular nucleus. Terminals from the corpus glomerulosum pars anterior, central nucleus of the inferior lobe, and mammillary body showed more diffuse distributions and were not confined to particular portions of the lateral valvular nucleus. Labeled terminals in the lateral valvular nucleus were cup-shaped or of beaded morphology. These results indicate that the lateral valvular nucleus receives projections from various sources including the telencephalon, pretectum, and inferior lobe to relay information to the valvular and cerebellar corpus. In addition, the corpus glomerulosum pars anterior in tilapia is considered to be homologous to the magnocellular part of superficial pretectal nucleus in cyprinids.     

Comparative distribution of mammalian GnRH (gonadotrophin-releasing hormone) and chicken GnRH-II in the brain of the immature siberian sturgeon (Acipenser baeri)

The brain of the sturgeon has recently been shown to contain at least two forms of GnRH (gonadotropin-releasing hormone), mammalian GnRH (mGnRH) and chicken GnRH-II (cGnRH-II). In this study, we compared the distribution of immunoreactive (ir) mGnRH and cGnRH-II in the brain of immature Siberian sturgeons (Acipenser baeri). The overall distribution of mGnRH was very similar to the distribution of sGnRH in teleosts such as salmonids or cyprinids. mGnRH-ir perikarya were observed in the olfactory nerves and bulbs, the telencephalon, the preoptic region, and the mediobasal hypothalamus. All these cell bodies are located along a continuum of ir-fibers that could be traced from the olfactory nerve to the nerve to the hypothalamopituitary interface. No ir-fibers were observed in the anterior lobe of the pituitary, but a few were seen to enter the neurointermediate lobe. mGnRH-ir fibers were detected in many parts of the brain, particularly in the forebrain. mGnRH-ir cerebrospinal fluid-containing cells were observed in the telencephalon, the preoptic region, and the mediobasal hypothalamus. In contrast, cGnRH-II was present mainly in the posterior brain, although a few ir axons were seen in the above-mentioned territories. In particular, cGnRH-II-ir cell bodies, negative for mGnRH, were consistently observed in the nucleus of the medial longitudinal fasciculus of the midbrain tegmentum. The cGnRH-II innervation in the optic tectum, cerebellum, vagal lobe, and medulla oblongata was more abundant than the mGnRH innervation in the same areas. This study provides evidence that the organization of the GnRH systems in a primitive bony fish is, highly similar to that reported in teleosts and further documents the differential distribution of two forms of GnRH in the brain of vertebrates. © 1993 Wiley-Liss,Inc.     

Distribution of thyrotropin-releasing hormone (TRH) immunoreactivity in the brain of the zebrafish (Danio rerio)

The distribution of thyrotropin-releasing hormone (TRH) in the brain of the adult zebrafish was studied with immunohistochemical techniques. In the telencephalon, abundant TRH-immunoreactive (TRHir) neurons were observed in the central, ventral, and supra- and postcommissural regions of the ventral telencephalic area. In the diencephalon, TRHir neurons were observed in the anterior parvocellular preoptic nucleus, the suprachiasmatic nucleus, the lateral hypothalamic nucleus, the rostral parts of the anterior tuberal nucleus and torus lateralis, and the posterior tuberal nucleus. Some TRHir neurons were also observed in the central posterior thalamic nucleus and in the habenula. The mesencephalon contained TRHir cells in the rostrodorsal tegmentum, the Edinger-Westphal nucleus, the torus semicircularis, and the nucleus of the lateral lemniscus. Further TRHir neurons were observed in the interpeduncular nucleus. In the rhombencephalon, TRHir cells were observed in the nucleus isthmi and the locus coeruleus, rostrally, and in the vagal lobe and vagal motor nucleus, caudally. In the forebrain, TRHir fibers were abundant in several regions, including the medial and caudodorsal parts of the dorsal telencephalic area, the ventral and commissural parts of the ventral telencephalic area, the preoptic area, the posterior tubercle, the anterior tuberal nucleus, and the posterior hypothalamic lobe. The dorsal thalamus exhibited moderate TRHir innervation. In the mesencephalon, the optic tectum received a rich TRHir innervation between the periventricular gray zone and the stratum griseum centrale. A conspicuous TRHir longitudinal tract traversed the tegmentum and extended to the rhombencephalon. The medial and lateral mesencephalic reticular areas and the interpeduncular nucleus were richly innervated by TRHir fibers. In the rhombencephalon, the secondary gustatory nucleus received abundant TRHir fibers. TRHir fibers moderately innervated the ventrolateral and ventromedial reticular area and richly innervated the vagal lobe and Cajal's commissural nucleus. Some TRHir fibers coursed in the lateral funiculus of the spinal cord. Some TRHir amacrine cells were observed in the retina. The wide distribution of TRHir neurons and fibers observed in the zebrafish brain suggests that TRH plays different roles. These results in the adult zebrafish reveal a number of differences with respect to the TRHir systems reported in other adult teleosts but were similar to those found during late developmental stages of trout (Díaz et al., 2001).     

Development of galanin-like immunoreactivity in the brain of the brown trout (Salmo trutta fario), with some observations on sexual dimorphism

The development of galanin-like immunoreactive (GAL-ir) cells and fibers was investigated in the brain of brown trout embryos, alevins, juveniles, and adults (some spontaneously releasing their gametes). The earliest GAL-ir neurons appeared in the preoptic region and the primordial hypothalamic lobe of 12-mm embryos. After hatching, new GAL-ir neurons appeared in the lateral, anterior, and posterior tuberal nuclei, and in late alevins, GAL-ir neurons appeared in the area postrema. In juveniles, further GAL-ir populations appeared in the nucleus subglomerulosus and magnocellular preoptic nucleus. The GAL-ir neuronal groups present in juveniles were also observed in sexually mature adults, although the area postrema of males lacked immunoreactive neurons. Moreover, spawning males exhibited GAL-ir somata in the olfactory bulb and habenula, which were never observed in adult females or in developing stages. In adults, numerous GAL-ir fibers were observed in the ventral telencephalon, preoptic area, hypothalamus, neurohypophysis, mesencephalic tegmentum, ventral rhombencephalon, and area postrema. Moderate to low GAL-ir innervation was seen in the olfactory bulbs, dorsomedial telencephalon, epithalamus, medial thalamus, optic tectum, cerebellum, and rhombencephalic alar plate. There were large differences among regions in the GAL-ir innervation establishment time. In embryos, GAL-ir fibers appeared in the preoptic area and hypothalamus, indicating early expression of galanin in hypophysiotrophic centers. The presence of galanin immunoreactivity in the olfactory, reproductive, visual, and sensory-motor centers of the brain suggest that galanin is involved in many other brain functions. Furthermore, the distribution of GAL-ir elements observed throughout trout development indicates that galaninergic system maturation continues until sexual maturity.     

Corticotropin-releasing hormone (CRH)-containing neurons in the immature rat hippocampal formation: Light and electron microscopic features and colocalization with glutamate decarboxylase and parvalbumin

Corticotropin-releasing hormone (CRH) excites hippocampal neurons and induces death of selected CA3 pyramidal cells in immature rats. These actions of CRH require activation of specific receptors that are abundant in CA3 during early postnatal development. Given the dramatic effects of CRH on hippocampal neurons and the absence of CRH-containing afferents to this region, we hypothesized that a significant population of CRHergic neurons exists in developing rat hippocampus. This study defined and characterized hippocampal CRH-containing cells by using immunocytochemistry, ultrastructural examination, and colocalization with gamma-aminobutyric acid (GABA)-synthesizing enzyme and calcium-binding proteins. Numerous, large CRH-immunoreactive (ir) neurons were demonstrated in CA3 strata pyramidale and oriens, fewer were observed in the corresponding layers of CA1, and smaller CRH-ir cells were found in stratum lacunosum-moleculare of Ammon's horn. In the dentate gyrus, CRH-ir somata resided in the granule cell layer and hilus. Ultrastructurally, CRH-ir neurons had aspiny dendrites and were postsynaptic to both asymmetric and symmetric synapses. CRH-ir axon terminals formed axosomatic and axodendritic symmetric synapses with pyramidal and granule cells. Other CRH-ir terminals synapsed on axon initial segments of principal neurons. Most CRH-ir neurons were coimmunolabeled for glutamate decarboxylase (GAD)-65 and GAD-67 and the majority also contained parvalbumin, but none were labeled for calbindin. These results confirm the identity of hippocampal CRH-ir cells as GABAergic interneurons. Further, a subpopulation of neurons immunoreactive for both CRH and parvalbumin and located within and adjacent to the principal cell layers consists of basket and chandelier cells. Thus, axon terminals of CRH-ir interneurons are strategically positioned to influence the excitability of the principal hippocampal neurons via release of both CRH and GABA. Hippocampus 1998;8: 231–243. © 1998 Wiley-Liss, Inc.     

Distribution of corticotropin-releasing hormone in the developing zebrafish brain

Corticotropin-releasing hormone (CRH) plays a central role in the physiological regulation of the hypothalamus-pituitary-adrenal/interrenal axis mediating endocrine, behavioral, autonomic, and immune responses to stress. Despite the wealth of knowledge about the physiological roles of CRH, the genetic mechanisms by which CRH neurons arise during development are poorly understood. As a first step toward analyzing the molecular and genetic pathways involved in CRH lineage specification, we describe the developmental distribution of CRH neurons in the embryonic zebrafish, a model organism for functional genomics and developmental biology. We searched available zebrafish expressed sequence tag (EST) databases for CRH-like sequences and identified one EST that contained the complete zebrafish CRH open reading frame (ORF). The CRH precursor sequence contained a signal peptide, the CRH peptide, and a cryptic peptide with a conserved sequence motif. RT-PCR analysis showed crh expression in a wide range of adult tissues as well as during embryonic and larval stages. By whole-mount in situ hybridization histochemistry, discrete crh-expressing cell clusters were found in different parts of the embryonic zebrafish brain, including telencephalon, preoptic region, hypothalamus, posterior tuberculum, thalamus, epiphysis, midbrain tegmentum, and rostral hindbrain and in the neural retina. The localization of crh mRNA within the preoptic region is consistent with the central role of CRH in the teleost stress response through activation of the hypothalamic-pituitary-interrenal axis. The widespread distribution of CRH-synthesizing cells outside the preoptic region suggests additional functions of CRH in the embryonic zebrafish brain.     

Distribution and acute stressor-induced activation of corticotrophin-releasing hormone neurones in the central nervous system of Xenopus laevis

In mammals, corticotrophin-releasing hormone (CRH) and related peptides are known to play essential roles in the regulation of neuroendocrine, autonomic and behavioural responses to physical and emotional stress. In nonmammalian species, CRH-like peptides are hypothesized to play similar neuroendocrine and neurocrine roles. However, there is relatively little detailed information on the distribution of CRH neurones in the central nervous system (CNS) of nonmammalian vertebrates, and there are currently no comparative data on stress-induced changes in CRH neuronal physiology. We used a specific, affinity-purified antibody raised against synthetic Xenopus laevis CRH to map the distribution of CRH in the CNS of juvenile South African clawed frogs. We then analysed stress-induced changes in CRH immunoreactivity (CRH-ir) throughout the CNS. We found that CRH-positive cell bodies and fibres are widely distributed throughout the brain and rostral spinal cord of juvenile X. laevis. Strong CRH-immunoreactivity (ir) was found in cell bodies and fibres in the anterior preoptic area (POA, an area homologous to the mammalian paraventricular nucleus) and the external zone of the median eminence. Specific CRH-ir cell bodies and fibres were also identified in the septum, pallium and striatum in the telencephalon; the amygdala, bed nucleus of the stria terminalis and various hypothalamic and thalamic nuclei in the diencephalon; the tectum, torus semicircularis and tegmental nuclei of the mesencephalon; the cerebellum and locus coeruleus in the rhombencephalon; and the ventral horn of the rostral spinal cord. To determine if exposure to an acute physical stressor alters CRH neuronal physiology, we exposed juvenile frogs to shaking/handling and conducted morphometric analysis. Plasma corticosterone was significantly elevated by 30 min after exposure to the stressor and continued to increase up to 6 h. Morphometric analysis of CRH-ir after 4 h of stress showed a significant increase in CRH-ir in parvocellular neurones of the anterior preoptic area, the medial amygdala and the bed nucleus of the stria terminalis, but not in other brain regions. The stress-induced increase in CRH-ir in the POA was associated with increased Fos-like immunoreactivity (Fos-LI), and confocal microscopy showed that CRH-ir colocalized with Fos-LI in a subset of Fos-LI-positive neurones. Our results support the view that the basic pattern of CNS CRH expression arose early in vertebrate evolution and lend further support to earlier studies suggesting that amphibians may be a transitional species for descending CRH-ergic pathways. Furthermore, CRH neurones in the frog brain exhibit changes in response to a physical stressor that parallel those seen in mammals, and thus are likely to play an active role in mediating neuroendocrine, behavioural and autonomic stress responses.     

Experimental study of the connections of the telencephalon in the rainbow trout (Oncorhynchus mykiss). II: Dorsal area and preoptic region

In this study and the accompanying article (Folgueira et al., 2004a), the fluorescent carbocyanine dye 1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) was used in fixed tissue to comprehensively analyze the connections of the different regions of the telencephalic lobes and the preoptic region of the rainbow trout. Here, we analyze the connections of the dorsal area (D; pallium) of the telencephalon, and the preoptic region, as well as the telencephalic connections of several structures in the diencephalon and brainstem of juvenile trout. The dorsal plus dorsolateral pallial zone of D (Dd+Dl-d) receives afferents from contralateral Dd+Dl-d, the ventral area of the telencephalon, preoptic nucleus, suprachiasmatic nucleus, medial thalamus, preglomerular complex, anterior and lateral tuberal nuclei, posterior tuberal nucleus, posterior hypothalamic lobe, superior raphe nucleus, and the rhombencephalic central gray and reticular formation, and projects to the central zone of D (Dc), medial thalamus, and some caudomedial hypothalamic regions. The medial zone of D (Dm) maintains reciprocal connections with the preglomerular complex and also receives afferents from the preoptic nucleus, suprachiasmatic nucleus, anterior tuberal nucleus, preglomerular tertiary gustatory nucleus, posterior tubercle, superior raphe nucleus, locus coeruleus, and the rhombencephalic central gray, and reticular formation. Dc receives fibers mainly from Dd+Dl-d, preoptic nucleus, preglomerular complex, and torus semicircularis and projects to several extratelencephalic centers, including the paracommissural nucleus, optic tectum, torus semicircularis, thalamus, preglomerular complex, posterior tubercle nuclei, and inferior hypothalamic lobes. The posterior zone of D (Dp) is mainly connected with the olfactory bulbs, the ventral and supracommissural nuclei of the ventral area (subpallium), the preoptic nucleus, and the preglomerular complex and projects to wide hypothalamic and posterior tubercular regions. The preoptic nucleus projects to the olfactory bulb, to most regions of the telencephalic lobes, and to several diencephalic and brainstem structures. These results reveal complex and specialized connectional patterns in the rainbow trout dorsal telencephalon and preoptic region. Most of these connections have not been described previously in salmonids. These connections indicate that the salmonid telencephalon is involved in multisensorial processing and modulation of brain activity.     

Ascending general visceral sensory pathways from the brainstem to the forebrain in a cichlid fish,Oreochromis (Tilapia) niloticus

Fiber connections of the general visceral sensory centers in the brainstem were studied with tract‐tracing methods in a percomorph teleost, tilapia Oreochromis niloticus. General visceral afferents of the vagal nerve from abdominal viscera terminated bilaterally in the commissural nucleus of Cajal (NCC) and area postrema (AP). The NCC and AP projected bilaterally to the secondary general visceral nucleus (SVN), four diencephalic nuclei (the preglomerular general visceral nucleus [pVN], nucleus of the lateral recess, posterior thalamic nucleus, and lateral tuberal area), preoptic area, and ventral telencephalon (supracommissural, dorsal, and ventral parts) in addition to the glossopharyngeal and vagal lobes and medullary reticular formation. Injections to the SVN resulted in labeled terminals in the forebrain structures that receive fibers from the primary centers and additionally in the diffuse nucleus of the inferior lobe, lateral torus, and inferior subdivision of lateral torus. The present study suggests that the ascending general visceral projections arising from the brainstem centers in teleosts are quite similar to those in mammals and birds. Descending pathways were also notable. In addition to descending projections from the SVN and medullary structures to the primary centers, long descending pathways to the SVN, NCC, and AP were found to originate from the pVN, nucleus of the lateral recess, posterior thalamic nucleus, and preoptic area. The SVN was found to receive fibers from the ventral telencephalon as well. Therefore, the present study indicates that most of the general visceral structures in the forebrain are reciprocally connected with the brainstem centers. J. Comp. Neurol. 518:3570–3603, 2010. © 2010 Wiley‐Liss, Inc.     

Immunohistochemical localization of three different prepro-GnRHs in the brain and pituitary of the European sea bass (Dicentrarchus labrax) using antibodies to the corresponding GnRH-associated peptides

The distribution of the cells expressing three prepro-gonadotrophin-releasing hormones (GnRH), corresponding to salmon GnRH (sGnRH), seabream GnRH (sbGnRH), and chicken GnRH-II (cGnRH-II) forms, was studied in the brain and pituitary of the sea bass (Dicentrarchus labrax) by using immunohistochemistry. To circumvent the cross-reactivity problems of antibodies raised to GnRH decapeptides, we used specific antibodies generated against the different sea bass GnRH-associated peptides (GAP): salmon GAP (sGAP), seabream GAP (sbGAP), and chicken-II GAP (cIIGAP). The salmon GAP immunostaining was mostly detected in terminal nerve neurons but also in ventral telencephalic and preoptic perikarya. Salmon GAP-immunoreactive (ir) fibers were observed mainly in the forebrain, although sGAP-ir projections were also evident in the optic tectum, mesencephalic tegmentum, and ventral rhombencephalon. The pituitary only receives a few sGAP-ir fibers. The seabream GAP-ir cells were mainly detected in the preoptic area. Nevertheless, sbGAP-ir neurons were also found in olfactory bulbs, ventral telencephalon, and ventrolateral hypothalamus. The sbGAP-ir fibers were only observed in the ventral forebrain, innervating strongly the pituitary gland. Finally, chicken-II GAP immunoreactivity was only detected in large synencephalic cells, which are the origin of a profuse innervation reaching the telencephalon, preoptic area, hypothalamus, thalamus, pretectum, posterior tuberculum, mesencephalic tectum and tegmentum, cerebellum, and rhombencephalon. However, no cIIGAP-ir fibers were detected in the hypophysis. These results corroborate the overlapping of sGAP- and sbGAP-expressing cells in the forebrain of the sea bass, and provide, for the first time, unambiguous information on the distribution of projections of the three different GnRH forms expressed in the brain of a single species.     

Localization of immunoreactive tyrosine hydroxylase in the goldfish brain

This report describes the distribution of tyrosine hydroxylase immunoreactive (TH-ir) structures in the brain of the goldfish (Carassius auratus). The localization of TH-ir cell groups revealed by immunocytochemical techniques is largely in accordance with catecholamine distribution previously reported in teleosts by using monoamine fluorescence; however, in the telencephalon and diencephalon, several new cell groups are elucidated. In the telencephalon, TH-ir cell bodies are observed in the olfactory bulb, area ventralis telencephali, and the central zone of the area dorsalis telencephali. TH-ir fibers and terminals are moderately dense throughout the telencephalon except for a sparse innervation of the area dorsalis, pars medialis. Immunostained cells are present in the suprachiasmatic nucleus and magnocellular and parvicellular components of the preoptic nucleus. Immunoreactive fibers from preoptic cells can be traced caudally in two main tracts to the infundibulum. Dense immunoreactivity around cells in the pituitary provides anatomical support for catecholamine involvement in the neuroendocrine axis probably via preopticohypophysial connections. At middiencephalic levels, immunoreactive cells are present in the ventral thalamus, nucleus pretectalis periventricularis, pars ventralis, and paraventricular organ pars anterioris. In the caudal diencephalon, TH-ir cells are seen within the posterior tuberal nuclei and dorsal to posterior recess. No immunostained cells are observed in the midbrain. In the hindbrain, tyrosine hydroxylase containing cells comprise three groups similar to that described using Falck-Hillarp histofluorescence (Parent et al., '78), i.e., isthmal, central medullary, and medullospinal groups. Tyrosine hydroxylase immunoreactivity is interpreted as evidence for the presence of catecholamines and not only provides an anatomical basis for the functional significance of catecholamines in teleosts, but may be useful in elucidating homologous structures in tetrapod vertebrates, although certain sites of immunoreactivity may prove to be unique to teleosts.     

Visual receptive thalamopetal neurons in the optic tectum of teleosts (holocentridae)

Tectal neurons previously known to receive retinofugal input were herein shown to project to the nucleus prethalamicus. Following HRP injections into the nucleus prethalamicus, pyriform neurons in the stratum periventiculare and stratum album centrale, and fusiform neurons in the stratum griseum centrale, were retrogradely labeled. Because the labeled types of neurons have been characterized as the main visual receptive neurons of the optic tectum, and because the nucleus prethalamicus of teleosts projects to the telencephalon, this nucleus can now be considered homologous to the nucleus rotondus of reptiles and birds and to the nucleus lateralis postterior-pulvinar complex of mammals, that is, it provides a relay for retinotectal visual input to the telencephalon. Orthogradely labeled terminals as well as retrogradely labeled neurons were also found in the dorsal area of the telencephalon. The tecto-prethalamotelencephalic projections are only ipsilateral.     

Connections of the ventral telencephalon (subpallium) in the zebrafish (Danio rerio)

Connections of the medial precommissural subpallial ventral telencephalon, i.e., dorsal (Vd, interpreted as part of striatum) and ventral (Vv, interpreted as part of septum) nuclei of area ventralis telencephali, were studied in the zebrafish (Danio rerio) using two tracer substances (DiI or biocytin). The following major afferent nuclei to Vd/Vv were identified: medial and posterior pallial zones of dorsal telencephalic area, and the subpallial supracommissural and postcommissural nuclei of the ventral telencephalic area, the olfactory bulb, dorsal entopeduncular, anterior and posterior parvocellular preoptic and suprachiasmatic nuclei, anterior, dorsal and central posterior dorsal thalamic, as well as rostrolateral nuclei, periventricular nucleus of the posterior tuberculum, posterior tuberal nucleus, various tuberal hypothalamic nuclei, dorsal tegmental nucleus, superior reticular nucleus, locus coeruleus, and superior raphe nucleus. Efferent projections of the ventral telencephalon terminate in the supracommissural nucleus of area ventralis telencephali, the posterior zone of area dorsalis telencephali, habenula, periventricular pretectum, paracommissural nucleus, posterior dorsal thalamus, preoptic region, midline posterior tuberculum (especially the area dorsal to the posterior tuberal nucleus), tuberal (midline) hypothalamus and interpeduncular nucleus. Strong reciprocal interconnections likely exist between septum and preoptic region/midline hypothalamus and between striatum and dorsal thalamus (dopaminergic) posterior tuberculum. Regarding ascending activating/modulatory systems, the pallium shares with the subpallium inputs from the (noradrenergic) locus coeruleus, and the (serotoninergic) superior raphe, while the subpallium additionally receives such inputs from the (dopaminergic) posterior tuberculum, the (putative cholinergic) superior reticular nucleus, and the (putative histaminergic) caudal hypothamalic zone.     

Gonadotropin-releasing hormone (GnRH) immunoreactive system in the brain of the dwarf gourami (Colisa lalia) as revealed by light microscopic immunocytochemistry using a monoclonal antibody to common amino acid sequence of GnRH

The present paper aims to give a morphological basis for the study of the terminal nerve system and its relation to the whole gonadotropin-releasing hormone (GnRH) immunoreactive (ir) neuronal system. We examined the GnRH-ir neuronal system of a tropical fish, the dwarf gourami, by using a recently developed monoclonal antibody against GnRH (LRH13) which recognizes the amino acid sequence common to all known variants of GnRH (Park and Wakabayashi, Endocrinol. Jpn. 33:257-272, '86). The ganglion cells of the terminal nerve (TN-ggl cells) in the transitional area between the olfactory bulb and the telencephalon reacted strongly with the LRH13. A distinct bundle of axons emanating from the TN-ggl cells ran caudally through the ventral telencephalon and the preoptic area. Some of these axons entered the optic nerve and innervated the retina. The remaining axons continued caudally to enter the hypothalamus and the midbrain. A second group of GnRH-ir cell bodies was found in the preoptic area. A distinct bundle of GnRH-ir fibers originating from these cell bodies innervated the pituitary. This pathway is equivalent to the preoptico-infundibular pathway of other vertebrates, and the GnRH in this pathway is presumed to function as hypophysiotrophic hormone to facilitate the release of gonadotropins from the pituitary. The distribution of GnRH-ir fibers in the brain was extensive. Most fibers apparently originated from the TN-ggl cells and covered various brain regions from the olfactory bulb to the spinal cord. They were especially abundant in the olfactory bulb, ventral telencephalon, preoptic area, optic tectum, and some hypothalamic areas. Thus, GnRH might function as a neuromodulator and/or neurotransmitter in these areas. The abundant GnRH-ir fibers in the ventral telencephalon and the preoptic area might affect some aspects of sexual behavior, since these areas have been suggested to be involved in the control of sexual behavior in teleosts.     

Projections of the sensory trigeminal nucleus in a percomorph teleost, tilapia (Oreochromis niloticus)

The sensory trigeminal nucleus of teleosts is the rostralmost nucleus among the trigeminal sensory nuclear group in the rhombencephalon. The sensory trigeminal nucleus is known to receive the somatosensory afferents of the ophthalmic, maxillar, and mandibular nerves. However, the central connections of the sensory trigeminal nucleus remain unclear. Efferents of the sensory trigeminal nucleus were examined by means of tract-tracing methods, in a percomorph teleost, tilapia. After tracer injections to the sensory trigeminal nucleus, labeled terminals were seen bilaterally in the ventromedial thalamic nucleus, periventricular pretectal nucleus, medial part of preglomerular nucleus, stratum album centrale of the optic tectum, ventrolateral nucleus of the semicircular torus, lateral valvular nucleus, prethalamic nucleus, tegmentoterminal nucleus, and superior and inferior reticular formation, with preference for the contralateral side. Labeled terminals were also found bilaterally in the oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, medial funicular nucleus, and contralateral sensory trigeminal nucleus and inferior olive. Labeled terminals in the oculomotor nucleus and trochlear nucleus showed similar densities on both sides of the brain. However, labelings in the trigeminal motor nucleus, facial motor nucleus, facial lobe, descending trigeminal nucleus, and medial funicular nucleus showed a clear ipsilateral dominance. Reciprocal tracer injection experiments to the ventromedial thalamic nucleus, optic tectum, and semicircular torus resulted in labeled cell bodies in the sensory trigeminal nucleus, with a few also in the descending trigeminal nucleus.     

Distribution of serotonin in the brain of the mormyrid teleost Gnathonemus petersii

The distribution of serotonin-immunoreactive neurons and fibers was studied in the highly developed brain of the weakly electric fish Gnathonemus petersii with the aid of specific antibodies against serotonin. Serotoninergic cell bodies occur in three regions: the raphe region of the brainstem, the hypothalamus, and the transition zone between the dorsal thalamus and the pretectum. Serotoninergic raphe neurons are clustered in three groups: nucleus raphes superior, intermedius, and inferior. The latter has not been described in other teleosts and thus might be the source of the serotoninergic innervation of specific mormyrid electrosensory brain regions. Most hypothalamic serotoninergic neurons have cerebrospinal-fluid (CSF)-contacting processes and thus belong to the paraventricular organ (PVO), which in Gnathonemus is located around a number of small infundibular recesses. The distribution of serotonin in the PVO precisely matches the distribution of dopamine, as described previously. Serotoninergic cells in the thalamopretectal transition zone also have been described in other teleosts, but not in other vertebrate groups, and thus seem to represent a teleostean specialization. Serotoninergic fiber density is especially high in the medial forebrain bundle and surrounding preoptic and hypothalamic regions as well as in several telencephalic and preoptic subependymal plexus. Serotoninergic fibers appear to be almost completely absent in the large and differentiated corpus and valvula cerebelli. Comparison with the literature on teleostean serotoninergic innervation patterns reveals several mormyrid specializations, including the absence of serotonin in large parts of the mormyrid telencephalic lobes, a differentiated innervation pattern of distinct electrosensory and mechanosensory subnuclei of the torus semicircularis, a refined serotoninergic lamination pattern in the midbrain tectum, and a prominent innervation of the electrosensory lateral line lobe, the associated caudal cerebellar lobe, and the electromotor medullary relay nucleus. A distinct innervation of several types of (pre)motor neurons, such as the Mauthner cells and facial motor neurons, has not been reported previously for other teleosts. Consequently, the distribution of serotoninergic fibers as well as neurons in the mormyrid brain is substantially adapted to the high degree of differentiation of its electrosensory and telencephalic brain regions, but serotoninergic innervation is not involved in the circuitry of the most impressive part of the mormyrid brain; i.e., its large corpus and valvula cerebelli.