Development of tyrosine hydroxylase-, dopamine- and dopamine β-hydroxylase-immunoreactive neurons in a teleost, the three-spined stickleback

Dopaminergic modulation of olfactory-evoked motor output in sea lampreys (Petromyzon marinus L.)

Detection of chemical cues is important to guide locomotion in association with feeding and sexual behavior. Two neural pathways responsible for odor-evoked locomotion have been characterized in the sea lamprey (Petromyzon marinus L.), a basal vertebrate. There is a medial pathway originating in the medial olfactory bulb (OB) and a lateral pathway originating from the rest of the OB. These olfactomotor pathways are present throughout the life cycle of lampreys, but olfactory-driven behaviors differ according to the developmental stage. Among possible mechanisms, dopaminergic (DA) modulation in the OB might explain the behavioral changes. Here, we examined DA modulation of olfactory transmission in lampreys. Immunofluorescence against DA revealed immunoreactivity in the OB that was denser in the medial part (medOB), where processes were observed close to primary olfactory afferents and projection neurons. Dopaminergic neurons labeled by tracer injections in the medOB were located in the OB, the posterior tuberculum, and the dorsal hypothalamic nucleus, suggesting the presence of both intrinsic and extrinsic DA innervation. Electrical stimulation of the olfactory nerve in an in vitro whole-brain preparation elicited synaptic responses in reticulospinal cells that were modulated by DA. Local injection of DA agonists in the medOB decreased the reticulospinal cell responses whereas the D2 receptor antagonist raclopride increased the response amplitude. These observations suggest that DA in the medOB could modulate odor-evoked locomotion. Altogether, these results show the presence of a DA innervation within the medOB that may play a role in modulating olfactory inputs to the motor command system of lampreys.

Comparative Analysis of the Organization of the Catecholaminergic Systems in the Brain of Holostean Fishes (Actinopterygii/Neopterygii)

Living holosteans, comprising 8 species of bowfins and gars, form a small monophyletic group of actinopterygian fishes, which are currently considered as the sister group to the enormously numerous teleosts and have largely been neglected in neuroanatomical studies. We have studied the catecholaminergic (CAergic) systems by means of antibodies against tyrosine hydroxylase (TH) and dopamine (DA) in the brain of representative species of the 3 genera included in the 2 orders of holostean fishes: Amia calva (Amiiformes) and Lepisosteus platyrhincus, Lepisosteus oculatus, and Atractosteus spatula (Lepisosteiformes). Different groups of TH/DA-immunoreactive (ir) cells were observed in the olfactory bulb, subpallium, and preoptic area of the telencephalon. Hypothalamic groups were labeled in the suprachiasmatic nucleus, tuberal (only in A. calva), retrotuberal, and retromamillary areas; specifically, the paraventricular organ showed only DA immunoreactivity. In the diencephalon, TH/DA-ir groups were detected in the prethalamus, posterior tubercle, and pretectum. In the caudal hindbrain, the solitary tract nucleus and area postrema presented TH/DA-ir cell groups, and also the spinal cord and the retina. Only in A. calva, particular CAergic cell groups were observed in the habenula, the mesencephalic tegmentum, and in the locus coeruleus. Following a neuromeric analysis, the comparison of these results with those obtained in other classes of fishes and tetrapods shows many common traits of CAergic systems shared by most vertebrates and in addition highlights unique features of actinopterygian fishes.

Changes in behavior and brain immediate early gene expression in male threespined sticklebacks as they become fathers

Motherhood is a period of intense behavioral and brain activity. However, we know less about the neural and molecular mechanisms associated with the demands of fatherhood. Here, we report the results of two experiments designed to track changes in behavior and brain activation associated with fatherhood in male threespined stickleback fish (Gasterosteus aculeatus), a species in which fathers are the sole providers of parental care. In experiment 1, we tested whether males' behavioral reactions to different social stimuli depends on parental status, i.e. whether they were providing parental care. Parental males visited their nest more in response to social stimuli compared to nonparental males. Rates of courtship behavior were high in non-parental males but low in parental males. In experiment 2, we used a quantitative in situ hybridization method to compare the expression of an immediate early gene (Egr-1) across the breeding cycle - from establishing a territory to caring for offspring. Egr-1 expression peaked when the activities associated with fatherhood were greatest (when they were providing care to fry), and then returned to baseline levels once offspring were independent. The medial dorsal telencephalon (basolateral amygdala), lateral part of dorsal telencephalon (hippocampus) and anterior tuberal nucleus (ventral medial hypothalamus) exhibited high levels of Egr-1 expression during the breeding cycle. These results help to define the neural circuitry associated with fatherhood in fishes, and are consistent with the hypothesis that fatherhood - like motherhood - is a period of intense behavioral and neural activity.

Connectivity and ultrastructure of dopaminergic innervation of the inner ear and auditory efferent system of a vocal fish

Dopamine (DA) is a conserved modulator of vertebrate neural circuitry, yet our knowledge of its role in peripheral auditory processing is limited to mammals. The present study combines immunohistochemistry, neural tract tracing, and electron microscopy to investigate the origin and synaptic characteristics of DA fibers innervating the inner ear and the hindbrain auditory efferent nucleus in the plainfin midshipman, a vocal fish that relies upon the detection of mate calls for reproductive success. We identify a DA cell group in the diencephalon as a common source for innervation of both the hindbrain auditory efferent nucleus and saccule, the main hearing endorgan of the inner ear. We show that DA terminals in the saccule contain vesicles but transmitter release appears paracrine in nature, due to the apparent lack of synaptic contacts. In contrast, in the hindbrain, DA terminals form traditional synaptic contacts with auditory efferent neuronal cell bodies and dendrites, as well as unlabeled axon terminals, which, in turn, form inhibitory-like synapses on auditory efferent somata. Our results suggest a distinct functional role for brain-derived DA in the direct and indirect modulation of the peripheral auditory system of a vocal nonmammalian vertebrate.

Deep-brain photoreception links luminance detection to motor output in Xenopus frog tadpoles

Nonvisual photoreceptors are widely distributed in the retina and brain, but their roles in animal behavior remain poorly understood. Here we document a previously unidentified form of deep-brain photoreception in Xenopus laevis frog tadpoles. The isolated nervous system retains sensitivity to light even when devoid of input from classical eye and pineal photoreceptors. These preparations produce regular bouts of rhythmic swimming activity in ambient light but fall silent in the dark. This sensitivity is tuned to short-wavelength UV light; illumination at 400 nm initiates motor activity over a broad range of intensities, whereas longer wavelengths do not cause a response. The photosensitive tissue is located in a small region of caudal diencephalon-this region is necessary to retain responses to illumination, whereas its focal illumination is sufficient to drive them. We present evidence for photoreception via the light-sensitive proteins opsin (OPN)5 and/or cryptochrome 1, because populations of OPN5-positive and cryptochrome-positive cells reside within the caudal diencephalon. This discovery represents a hitherto undescribed vertebrate pathway that links luminance detection to motor output. The pathway provides a simple mechanism for light avoidance and/or may reinforce classical circadian systems.

Casting a wider fish net on animal models in neuropsychiatric research

Neuropsychiatric disorders, such as schizophrenia, are associated with abnormal brain development. In this review, we discuss how studying dimensional components of these disorders, or endophenotypes, in a wider range of animal models will deepen our understanding of how interactions between biological and environmental factors alter the trajectory of neurodevelopment leading to aberrant behavior. In particular, we discuss some of the advantages of incorporating studies of brain and behavior using a range of teleost fish species into current neuropsychiatric research. From the perspective of comparative neurobiology, teleosts share a fundamental pattern of neurodevelopment and functional brain organization with other vertebrates, including humans. These shared features provide a basis for experimentally probing the mechanisms of disease-associated brain abnormalities. Moreover, incorporating information about how behaviors have been shaped by evolution will allow us to better understand the relevance of behavioral variation to determine their physiological underpinnings. We believe that exploiting the conservation in brain development across vertebrate species, and the rich diversity of fish behavior in lab and natural populations will lead to significant new insights and a holistic understanding of the neurobiological systems implicated in neuropsychiatric disorders.

Dopaminergic and noradrenergic circuit development in zebrafish

Dopaminergic and noradrenergic neurons constitute some of the major far projecting systems in the vertebrate brain and spinal cord that modulate the activity of circuits controlling a broad range of behaviors. Degeneration or dysfunction of dopaminergic neurons has also been linked to a number of neurological and psychiatric disorders, including Parkinson's disease.Zebrafish (Danio rerio) have emerged over the past two decades into a major genetic vertebrate model system,and thus contributed to a better understanding of developmental mechanisms controlling dopaminergic neuron specification and axonogenesis. In this review, we want to focus on conserved and dynamic aspects of the different catecholaminergic systems, which may help to evaluate the zebrafish as a model for dopaminergic and noradrenergic cellular specification and circuit function as well as biomedical aspects of catecholamine systems.

Evolution of a vertebrate social decision-making network

Animals evaluate and respond to their social environment with adaptive decisions. Revealing the neural mechanisms of such decisions is a major goal in biology. We analyzed expression profiles for 10 neurochemical genes across 12 brain regions important for decision-making in 88 species representing five vertebrate lineages. We found that behaviorally relevant brain regions are remarkably conserved over 450 million years of evolution. We also find evidence that different brain regions have experienced different selection pressures, because spatial distribution of neuroendocrine ligands are more flexible than their receptors across vertebrates. Our analysis suggests that the diversity of social behavior in vertebrates can be explained, in part, by variations on a theme of conserved neural and gene expression networks.

Expression of the paralogous tyrosine hydroxylase encoding genes th1 and th2 reveals the full complement of dopaminergic and noradrenergic neurons in zebrafish larval and juvenile brain

The development of dopaminergic and noradrenergic neurons has received much attention based on their modulatory effect on many behavioral circuits and their involvement in neurodegenerative diseases. The zebrafish (Danio rerio) has emerged as a new model organism with which to study development and function of catecholaminergic systems. Tyrosine hydroxylase is the entry enzyme into catecholamine biosynthesis and is frequently used as a marker for catecholaminergic neurons. A genome duplication at the base of teleost evolution resulted in two paralogous zebrafish tyrosine hydroxylase-encoding genes, th1 and th2, the expression of which has been described previously only for th1. Here we investigate the expression of th2 in the brain of embryonic and juvenile zebrafish. We optimized whole-mount in situ hybridization protocols to detect gene expression in the anatomical three-dimensional context of whole juvenile brains. To confirm whether th2-expressing cells may indeed use dopamine as a neurotransmitter, we also included expression of dopamine beta hydroxylase, dopa decarboxylase, and dopamine transporter in our analysis. Our data provide the first complete account of catecholaminergic neurons in the zebrafish embryonic and juvenile brain. We identified four major th2-expressing neuronal groups that likely use dopamine as transmitter in the zebrafish diencephalon, including neurons of the posterior preoptic nucleus, the paraventricular organ, and the nuclei of the lateral and posterior recesses in the caudal hypothalamus. th2 expression in the latter two groups resolves a previously reported discrepancy, in which strong dopamine but little tyrosine hydroxylase immunoreactivity had been detected in the caudal hypothalamus. Our data also confirm that there are no mesencephalic DA neurons in zebrafish.

Development of the serotonergic system in the central nervous system of the sea lamprey

Lampreys belong to the most primitive extant group of vertebrates, the Agnathans, which is considered the sister group of jawed vertebrates. Accordingly, characterization of neuronal groups and their development appears useful for understanding early evolution of the nervous system in vertebrates. Here, the development of the serotonergic system in the central nervous system of the sea lamprey, Petromyzon marinus, was investigated by immunohistochemical analysis of specimens ranging from embryos to adults. The different serotonin-immunoreactive (5-HT-ir) neuronal populations that are found in adults were observed between the embryonic and metamorphic stages. The earliest serotonergic neurons were observed in the basal plate of the isthmus region of late embryos. In prolarvae, progressive appearance of new serotonergic cell groups was observed: firstly in the spinal cord, then in the pineal organ, tuberal region, zona limitans intrathalamica, rostral isthmus, and the caudal part of the rhombencephalon. In early larvae a new group of serotonergic cells was observed in the mammillary region, whereas in the pretectal region and the parapineal organ the first serotonergic cells were seen in the middle and late larval stages, respectively. The first serotonergic fibres appeared in early prolarvae, with fibres that ascend and descend from the isthmic cell group, and the number of immunoreactive fibres increased progressively until the adult stage. The results reveal strong resemblances between lampreys and other vertebrates in the spatio-temporal pattern of development of brainstem populations. This study also reveals a shared pattern of early ascending and descending serotonergic pathways in lampreys and jawed vertebrates.

Immunohistochemical Organization of the Forebrain in the White Sturgeon, Acipenser transmontanus

The distribution of substance P (SP), leucine-enkephalin (LENK), serotonin (5HT), dopamine (DA), and tyrosine hydroxylase (TH) was examined in the forebrain of the white sturgeon in order to evaluate several anatomical hypotheses based on cytoarchitectonics, and to gain a better understanding of the evolution of the forebrain in ray-finned fishes. The subpallium of the telencephalon has the highest concentration of the neuropeptides SP and LENK, allowing the pallial-subpallial border to be easily distinguished. The distribution of dopamine is similar to that of serotonin in the subpallium, fibers positive for these transmitters are particularly dense in the dorsal and ventral divisions of the subpallium. In addition, a small population of DA- and 5HT-positive cell bodies--which appear to be unique to sturgeons--was identified at the level of the anterior commissure. The internal granular layer of the olfactory bulbs had large numbers of TH-positive cell bodies and fibers, as did the rostral subpallium. The occurrence of cell bodies positive for LENK in the dorsal nucleus of the rostral subpallium supports the hypothesis that this nucleus is homologous to the striatum in other vertebrates. This is further reinforced by the apparent origin of an ascending dopaminergic pathway from cells in the posterior tubercle that are likely homologous to the ventral tegmental area/substantia nigra in land vertebrates. Finally, the differential distribution of SP and TH in the pallium supports the hypothesis that the pallium, or area dorsalis, can be divided medially into a rostral division (Dm), a caudal division (Dp) that is the main pallial target of secondary olfactory projections, and a narrow lateral division (Dd+Dl) immediately adjacent to the attachment of the tela choroidea along the entire rostrocaudal length of the telencephalic hemisphere.

Studies of threespine stickleback developmental evolution: progress and promise

A promising route for understanding the origin and diversification of organismal form is through studies at the intersection of evolution and development (evo-devo). While much has been learned over the last two decades concerning macroevolutionary patterns of developmental change, a fundamental gap in the evo-devo synthesis is the integration of mathematical population and quantitative genetics with studies of how genetic variation in natural populations affects developmental processes. This micro-evo-devo synthesis requires model organisms with which to ask empirical questions. Threespine stickleback fish (Gasterosteus aculeatus), long a model for studying behavior, ecology and evolution, is emerging as a prominent model micro-evo-devo system. Research on stickleback over the last decade has begun to address the genetic basis of morphological variation and sex determination, and much of this work has important implications for understanding the genetics of speciation. In this paper we review recent threespine stickleback micro-evo-devo results, and outline the resources that have been developed to make this synthesis possible. The prospects for stickleback research to speed the micro-(and macro-) evo-devo syntheses are great, and this workhorse model system is well situated to continue contributing to our understanding of the generation of diversity in organismal form for many more decades.

Tyrosine hydroxylase immunoreactivity in the developing visual pathway of the zebrafish

We analyzed the distribution of tyrosine hydroxylase immunoreactivity in the central nervous zones involved in the processing of visual information during zebrafish ontogeny, employing a segmental approach. In the retina, we observed immunolabeled cells in the inner nuclear layer after hatching. From the juvenile stages onwards, some of these cells presented two immunolabeled processes towards the inner and outer plexiform layers of the retina, which are identified as interplexiform cells. In the adult zebrafish retina, we have identified two cellular types displaying immunoreactivity for tyrosine hydroxylase: interplexiform and amacrine cells. In the optic tectum, derived from the mesencephalon, no immunolabeled neurons were observed in any of the stages analyzed. The periventricular gray zone and the superficial white zone display immunostained neuropile from the end of fry life onwards. At the 30-day postfertilization, the tyrosine hydroxylase immunoreactive neuropile in the optic tectum presents two bands located within the retinorecipient strata and deeper strata, respectively. All diencephalic regions, which receive direct retinal inputs, show immunolabeled cells in the preoptic area, in the pretectum, and in the ventral thalamus from embryonic stages onwards. During the fry development, the immunolabeled neurons can be observed in the periventricular pretectum from 15-days postfertilization and in both the ventrolateral thalamic nucleus and suprachiasmatic nucleus from 30-days postfertilization. The transient expression of tyrosine hydroxylase is observed in fibers of the optic tract during fry and juvenile development. The existence of immunolabeled neuropile in the zebrafish retinorecipient strata could be related to the turnover of retinotectal projections.

Temporal and spatial organization of tyrosine hydroxylase-immunoreactive cell groups in the embryonic brain of an elasmobranch, the lesser-spotted dogfish Scyliorhinus canicula

We have studied the development of catecholaminergic (CA) neuronal groups in the brain of the dogfish Scyliorhinus canicula using immunohistochemistry to tyrosine hydroxylase (TH). The earliest TH-immunoreactive (THir) cells were detected in the primordia of the posterior tubercle and suprachiasmatic nuclei (PTN and SCN, respectively) of stage 26 embryos. At stage 28, THir cells were also seen extending between the SCN and the PTN at ventral thalamic levels. At stage 30, some THir cerebrospinal fluid-contacting neurons and migrated THir cells were found in the walls of the posterior recess, and a few weakly THir cells also appeared at the isthmus level (locus coeruleus) and in the caudal rhombencephalic tegmentum. At stage 31, further THir cell groups appeared in the synencephalon and midbrain (ventral tegmental area/substantia nigra, VTA/SN), and the rhombencephalon (viscerosensory and visceromotor columns). At stage 32, the first THir cells appeared in the pallium, the olfactory bulb and the preoptic area. THir cells are seen in the retina from stage 33. The developmental sequence of THir cell groups in dogfish appears to be rather similar to that described for teleosts, apart from the appearance of the VTA/SN and pallial cells, which lack in teleosts.

Development of the dopamine-immunoreactive system in the central nervous system of the sea lamprey

The development of dopamine-immunoreactive (DAir) populations in the central nervous system of the sea lamprey, a modern representative of the earliest vertebrates, was studied to achieve further understanding of dopaminergic systems in vertebrates. The first DAir cell groups appeared in the spinal cord, the posterior tubercle nucleus and the dorsal hypothalamic nucleus of prolarval stages. In larvae, new DAir cell groups were observed in the caudal preoptic nucleus, the postoptic commissure nucleus, the postinfundibular commissure nucleus and the caudal rhombencephalon. All these DAir cell groups observed in larvae were also DA-positive in adults, which showed only one new DAir cell group found in the ventral hypothalamic nucleus. Occasional DAir cells were observed in the telencephalon, the ventral thalamus and/or the isthmus of large larvae and adults. From medium-sized larvae to adults, the regions most richly innervated by DAir fibers were the neurohypophysis, the striatum, the pretectum and the midbrain tegmentum. Striking differences are observed between lampreys and other vertebrates in regard to the relative time of appearance of DAir cells, which is probably related with the complex life cycle of the sea lamprey.

Development of catecholaminergic systems in the spinal cord of the dogfish Scyliorhinus canicula (Elasmobranchs)

The development of catecholamine-synthesizing cells and fibers in the spinal cord of dogfish (Scyliorhinus canicula L.) was studied by means of immunohistochemistry using antibodies against tyrosine hydroxylase (TH). The only TH-immunoreactive (TH-ir) cells already present in the spinal cord of stage 26 embryos were of cerebrospinal fluid-contacting (CSF-c) type. These cells were the first catecholaminergic neurons of the dogfish CNS. The number of these TH-ir cells increased very considerably in later embryos and adult dogfish. In later embryos (stage 33; prehatching), faintly TH-ir non-CSF-contacting neurons were observed in the ventral horn throughout most of the spinal cord. In adult dogfish, some non-CSF-contacting TH-ir cells were observed ventral or lateral to the central canal. In the rostral spinal cord, the catecholaminergic neurons observed in dorsal regions were continuous with caudal rhombencephalic populations. Numerous TH-ir fibers were observed in the spinal cord of later embryos and in adults, both intrinsic and descending from the brain, innervating many regions of the cord including the dorsal and ventral horns. In addition, some TH-ir fibers innervated the marginal nucleus of the spinal cord. The early appearance of catecholaminergic cells and fibers in the embryonic spinal cord of the dogfish, and the large number of these elements observed in adults, suggests an important role for catecholamines through development and adulthood in sensory and motor functions.

Development of the catecholaminergic system in the early zebrafish brain: an immunohistochemical study

Tyrosine hydroxylase-containing cells (TH cells) were investigated immunohistochemically in early and late postembryonic zebrafish brain sections (at 2 and 5 days postfertilization [dpf]) yielding an improved neuroanatomical resolution of spatiotemporal developmental dynamics of the catecholaminergic system. Additionally, double-immunolabel preparations for visualizing TH cells and cells containing the proliferating cell nuclear antigen (PCNA cells) were carried out allowing for a prosomeric interpretation of early forebrain TH cell clusters. Many TH cell populations recently described in the adult zebrafish brain could be identified in the present study by location and cell type already in the 5 dpf (e.g. eight of 12 adult diencephalic TH cell populations) and 2 dpf (e.g. five of 12 adult TH cell populations) zebrafish brain. Early and adult diencephalic TH cells are restricted to the pretectum (P1) and ventral thalamus (P3) in the alar plate, and to various TH groups in the basal plate posterior tuberculum (P3), as well as to various populations in the hypothalamus (secondary prosencephalon). The alar plate ventral thalamic and most anterodorsal posterior tubercular TH cell populations range among the earliest detectable ones. There was no indication of migration of TH cells from the midbrain-hindbrain boundary or anterior neural ridge into the diencephalon.

Distribution of tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) immunoreactivity in the central nervous system of two chondrostean fishes (Acipenser baeri and Huso huso)

To obtain a better understanding of the evolution of the brain catecholaminergic systems of fishes, we have examined the distribution of catecholamine-synthesizing enzymes in two species of sturgeon (Acipenser baeri and Huso huso) using antibodies against tyrosine hydroxylase (TH) and dopamine-beta -hydroxylase (DBH; only analyzed in Acipenser). Both sturgeons showed TH-immunoreactive (THir) neurons widely distributed in most regions of the brain, the highest number of THir cells being located in the forebrain (olfactory bulb, preoptic area, and posterior tuberculum). THir cells were also seen in other forebrain areas (retrobulbar area, dorsal and ventral telencephalic areas, hypothalamus, ventral thalamus, pretectal area) and in the brainstem (locus coeruleus, viscerosensory area, caudal reticular formation, and area postrema). Immunoreactive fibers and varicosities showed a wide distribution, being particularly abundant in the diencephalon and mesencephalon. DBH-immunoreactive (DBHir) cells were observed in the anterior tuberal nucleus, where these cells were TH-negative, and in the locus coeruleus and the caudal rhombencephalon (vagal reticular formation), where the DBHir cells were also THir. DBHir fibers were scarce in the telencephalon and very abundant in the diencephalon, mesencephalon, and rhombencephalon. The comparative analysis of the catecholaminergic systems of chondrosteans and those observed in other groups of fishes and tetrapods indicate a similar organization of many nuclei, as well as characteristics that are probably primitive, such as the presence of a large number of forebrain catecholaminergic groups.

Development of tyrosine hydroxylase-immunoreactive systems in the brain of the larval lamprey Lampetra fluviatilis

The development of the catecholaminergic system of the brain of the lamprey (Lampetra fluviatilis) was studied with immunocytochemistry in a series of larvae of different sizes by using two different antibodies directed against tyrosine hydroxylase (TH), the rate-limiting enzyme of catecholamine synthesis. In group 1 larvae (length: 29-54 mm, ages: 8 months to 1.5 years), the only TH-immunoreactive somata observed were located in the caudal wall of the recessus praeopticus (RP) and in the nucleus tuberculi posterioris (NTP). In group 2 larvae (length: 55-80 mm, ages: 1.5-2.5 years), the somata of immunolabeled cells of the NTP give rise to fibers, most of which are ascending and terminate in the corpus striatum. Additional immunoreactive cells are observed in the nucleus praeopticus (NP), which has differentiated, and in the spinal cord. In group 3 larvae (length: 81-110 mm, ages: 2.5-4 years), the spatial distribution of TH-immunoreactive elements (somata, fibers, and terminals) bears many resemblances to that seen in the adult. Immunolabeled cells may be observed in the olfactory bulb, in the nucleus commissurae postopticae (NCP), and in the nucleus dorsalis hypothalami (NDH). Nevertheless, some groups of TH-immunoreactive cells found in the adult are not observed in group 3 larvae; these may appear during the metamorphic phase. By comparative analysis, we show that, in spite of several differences, the spatiotemporal sequence of appearance of TH-immunoreactive cell bodies and fibers in the lamprey presents many similarities to that described in gnathostomes.

Descending supraspinal pathways in amphibians: III. Development of descending projections to the spinal cord in Xenopus laevis with emphasis on the catecholaminergic inputs

In developmental stages of the clawed toad, Xenopus laevis, we describe the ontogeny of descending supraspinal connections, catecholaminergic projections in particular, by means of retrograde tracing techniques with dextran amines. Already at embryonic stages (stage 40), spinal projections from the reticular formation, raphe nuclei, Mauthner neurons, vestibular nuclei, the locus coeruleus, the interstitial nucleus of the medial longitudinal fasciculus, the posterior tubercle, and the periventricular nucleus of the zona incerta are well developed. At the beginning of the premetamorphic period (stage 46), spinal projections arise from the suprachiasmatic nucleus, the torus semicircularis, the pretectal region, and the ventral telencephalon. After stage 48, tectospinal and cerebellospinal projections develop, with spinal projections from the preoptic area following at stage 51. Rubrospinal projections are present at stage 50. During the prometamorphic period, spinal projections arise in the nucleus of the solitary tract, the lateral line nucleus, and the mesencephalic trigeminal nucleus. With in vitro double-labeling methods, based on retrograde tracing of dextran amines in combination with tyrosine hydroxylase (TH) immunohistochemistry, we show that at stage 40/41, catecholaminergic (CA) neurons in the posterior tubercle are the first to project to the spinal cord. Subsequently, at stage 43, new projections arise in the periventricular nucleus of the zona incerta and the locus coeruleus. The last CA projection to the spinal cord originates from neurons in the nucleus of the solitary tract at the beginning of prometamorphosis (stage 53). Our data show a temporal, rostrocaudal sequence in the development of the CA cell groups projecting to the spinal cord. Moreover, the early appearance of CA fibers, preterminals and terminal-like structures in dorsal, intermediate, and ventral zones of the embryonic spinal cord, suggests an important role for catecholamines during development in nociception, autonomic functions, and motor control at the spinal level.

Comparative anatomy of the histaminergic and other aminergic systems in zebrafish (Danio rerio)

The histaminergic system and its relationships to the other aminergic transmitter systems in the brain of the zebrafish were studied by using confocal microscopy and immunohistochemistry on brain whole-mounts and sections. All monoaminergic systems displayed extensive, widespread fiber systems that innervated all major brain areas, often in a complementary manner. The ventrocaudal hypothalamus contained all monoamine neurons except noradrenaline cells. Histamine (HA), tyrosine hydroxylase (TH), and serotonin (5-HT) -containing neurons were all found around the posterior recess (PR) of the caudal hypothalamus. TH- and 5-HT-containing neurons were found in the periventricular cell layer of PR, whereas the HA-containing neurons were in the surrounding cell layer as a distinct boundary. Histaminergic neurons, which send widespread ascending and descending fibers, were all confined to the ventrocaudal hypothalamus. Histaminergic neurons were medium in size (approximately 12 microm) with varicose ascending and descending ipsilateral and contralateral fiber projections. Histamine was stored in vesicles in two types of neurons and fibers. A close relationship between HA fibers and serotonergic raphe neurons and noradrenergic locus coeruleus neurons was evident. Putative synaptic contacts were occasionally detected between HA and TH or 5-HT neurons. These results indicate that reciprocal contacts between monoaminergic systems are abundant and complex. The results also provide evidence of homologies to mammalian systems and allow identification of several previously uncharacterized systems in zebrafish mutants.

Dopamine transporter expression distinguishes dopaminergic neurons from other catecholaminergic neurons in the developing zebrafish embryo

To characterize the formation of the dopaminergic system in the developing zebrafish CNS, we cloned cDNAs encoding tyrosine hydroxylase (th), an enzyme in dopamine synthesis, and the dopamine transporter (dat), a membrane transport protein which terminates dopamine action by re-uptake. Dopaminergic neurons are first detected between 18 and 19 h post-fertilization in a cluster of cells in the ventral diencephalon. Subsequently, th and dat detection identifies dopaminergic neurons in the olfactory bulb, the pretectum, the retina and the locus coeruleus. Neurons expressing th but not dat are adrenergic or noradrenergic, and are found in the locus coeruleus, the medulla, the likely analog of the carotid body, and precursors of the enteric and sympathetic nervous system.

The teleostean (zebrafish) dopaminergic system ascending to the subpallium (striatum) is located in the basal diencephalon (posterior tuberculum)

Tyrosine hydroxylase immunohistochemistry is used to demonstrate catecholaminergic neuronal populations in the fore- and midbrain of adult zebrafish (Danio rerio). While no catecholaminergic neurons are found in the midbrain, various immunoreactive populations were found in the diencephalon (hypothalamus, posterior tuberculum, ventral thalamus, pretectum) and telencephalon (preoptic region, subpallium, olfactory bulb). The posterior tubercular catecholaminergic cells include three cytological types (small round, large pear-shaped, and bipolar liquor-contacting cells). Furthermore, the retrograde neuronal tracers DiI or biocytin were applied to demonstrate ascending projections to the basal telencephalon (incl. the striatum). A double-label approach was used - together with tyrosine hydroxylase immunohistochemistry - in order to visualize neurons positive for tyrosine hydroxylase and a retrograde tracer. Double-labeled cells were identified in two locations in the posterior tuberculum (i.e, small round neurons in the periventricular nucleus of the posterior tuberculum and large pear-shaped cells adjacent to it). They are interpreted as the teleostean dopaminergic system ascending to the striatum, since previous work [16] established that no noradrenergic neurons exist in the forebrain of the adult zebrafish.

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.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

The zebrafish young mutation acts non-cell-autonomously to uncouple differentiation from specification for all retinal cells

Embryos from mutagenized zebrafish were screened for disruptions in retinal lamination to identify factors involved in vertebrate retinal cell specification and differentiation. Two alleles of a recessive mutation, young, were isolated in which final differentiation and normal lamination of retinal cells were blocked. Early aspects of retinogenesis including the specification of cells along the inner optic cup as retinal tissue, polarity of the retinal neuroepithelium, and confinement of cell divisions to the apical pigmented epithelial boarder were normal in young mutants. BrdU incorporation experiments showed that the initiation and pattern of cell cycle withdrawal across the retina was comparable to wild-type siblings; however, this process took longer in the mutant. Analysis of early markers for cell type differentiation revealed that each of the major classes of retinal neurons, as well as non-neural Muller glial cells, are specified in young embryos. However, the retinal cells fail to elaborate morphological specializations, and analysis of late cell-type-specific markers suggests that the retinal cells were inhibited from fully differentiating. Other regions of the nervous system showed no obvious defects in young mutants. Mosaic analysis demonstrated that the young mutation acts non-cell-autonomously within the retina, as final morphological and molecular differentiation was rescued when genetically mutant cells were transplanted into wild-type hosts. Conversely, differentiation was prevented in wild-type cells when placed in young mutant retinas. Mosaic experiments also suggest that young functions at or near the cell surface and is not freely diffusible. We conclude that the young mutation disrupts the post-specification development of all retinal neurons and glia cells.

Comparative immunocytochemical study of FMRFamide neuronal system in the brain of Danio rerio and Acipenser ruthenus during development

The distribution of FMRFamide-like immunoreactive (ir) neurons and fibers was investigated in the central nervous system of developing zebrafish and juvenile sturgeon (sterlet). Adult zebrafish was also studied. In zebrafish embryos FMRFamide-ir elements first appeared 30 h post-fertilization (PF). Ir somata were located in the olfactory placode and in the ventral diencephalon. FMRFamide-ir fibers originating from diencephalic neurons were found in the ventral telencephalon and in ventral portions of the brainstem. At 48 h PF, the ir perikarya in the olfactory placode displayed increased immunoreactivity and stained fibers emerged from the somata. At 60 h PF, bilaterally, clusters of FMRFamide-ir neurons were found along the rostro-caudal axis of the brain, from the olfactory placode to rostral regions of the ventro-lateral telencephalon. At 60 h PF, numerous ir fibers appeared in the dorsal telencephalon, optic lobes, optic nerves, and retina. Except for ir fibers in the hypophysis at the age of 72 h PF, and a few ir cells in the nucleus olfacto-retinalis (NOR) at the age of 2 months PF, no major re-organization was noted in subsequent ontogenetic stages. The number of stained NOR neurons increased markedly in sexually mature zebrafish. In adult zebrafish, other ir neurons were located in the dorsal zones of the periventricular hypothalamus and in components of the nervus terminalis. We are inclined to believe that neurons expressing FMRFamide originate in the olfactory placode and in the ventricular ependyma in the hypothalamus. On the same grounds, a dual origin of FMRFamide-ir neurons is inferred in the sturgeon, an ancestral bony fish: prior to the observation of ir cells in the nasal area and in the telencephalon stained neurons were noted in circumventricular hypothalamic regions.

Development of immunoreactivity to neuropeptide Y in the brain of brown trout (Salmo trutta fario)

The development of neuropeptide Y-immunoreactive (NPY-ir) neurons in the brain of the brown trout, Salmo trutta fario, was studied by using the streptavidin-biotin immunohistochemical method. Almost all NPY-ir neurons found in the brain of adults already appeared in embryonic stages. The earliest NPY-ir neurons were observed in the laminar nucleus, the locus coeruleus, and the vagal region of 9-mm-long embryos. In the lateral area of the ventral telencephalon, habenula, hypothalamus, optic tectum, and saccus vasculosus, NPY-ir cells appeared shortly after (embryos 12-14 mm in length). The finding of NPY-ir cells in the saccus vasculosus and the vagal region expand the NPY-ir structures known in teleosts. Among the regions of the trout brain most richly innervated by NPY-ir fibers are the hypothalamus, the isthmus, and the complex of the nucleus of the solitary tract/area postrema, suggesting a correlation of NPY with visceral functions. Two patterns of development of NPY-ir populations were observed: Some populations showed a lifetime increase in cell number, whereas, in other populations, cell number was established early in development or even diminished in adulthood. These developmental patterns were compared with those found in other studies of teleosts and with those found in other vertebrates. J. Comp. Neurol. 414:13-32, 1999.

Development of NADPH-diaphorase activity in the central nervous system of the cichlid fish, Tilapia mariae

The distribution of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity was studied in the cichlid fish Tilapia mariae during ontogenesis by the histochemical reaction of NADPH-diaphorase that indicates, in aldehyde-fixed tissue, the presence of nitric oxide synthase, which is the enzyme responsible for nitric oxide production. The first appearance of NADPH-diaphorase-positive neurons has a striking bilateral symmetry and occurs 20 h after fertilization (stage 8) in the olfactory placodes and in the neural tube where two clusters of positive neurons were seen in the diencephalon and in the rhombomere r4 of the hindbrain. Two days after fertilization (stage 10), the clusters of positive neurons showed labeled axons. The two longitudinal fiber bundles that arose from the diencephalic positive neurons ran caudally in the tract of the postoptic commissure. At stage 12 (3.5 days after fertilization), new populations of NADPH-diaphorase-positive neurons appeared in the telencephalon, in some diencephalic nuclei, and in the hypothalamus. Several trigeminal motor neurons showed strong NADPH-diaphorase activity, whereas the optic tectum and cerebellum were completely free of enzymatic activity. In the hindbrain, clusters of positive neurons were seen in the octavolateral region and in the region defined by the exit of the vagus nerve. In the cervical spinal cord, some ventral putative motor neurons were labeled. At stage 14 (5.5 days after fertilization), several periventricular neurons of the optic tectum and some neurons of the cerebellar lamina were labeled. Dorsal neurons, including a few large superficial neurons were also labeled in the cervical spinal cord. NADPH-diaphorase activity was seen in the neuropil area of the telencephalon, the target of olfactory inputs, and in the sensory dorso-lateral area of the spinal cord.

Mutations in the zebrafish unmask shared regulatory pathways controlling the development of catecholaminergic neurons

The mechanism by which pluripotent progenitors give rise to distinct classes of mature neurons in vertebrates is not well understood. To address this issue we undertook a genetic screen for mutations which affect the commitment and differentiation of catecholaminergic (CA) [dopaminergic (DA), noradrenergic (NA), and adrenergic] neurons in the zebrafish, Danio rerio. The identified mutations constitute five complementation groups. motionless and foggy affect the number and differentiation state of hypothalamic DA, telencephalic DA, retinal DA, locus coeruleus (LC) NA, and sympathetic NA neurons. The too few mutation leads to a specific reduction in the number of hypothalamic DA neurons. no soul lacks arch-associated NA cells and has defects in pharyngeal arches, and soulless lacks both arch-associated and LC cell groups. Our analyses suggest that the genes defined by these mutations regulate different steps in the differentiation of multipotent CA progenitors. They further reveal an underlying universal mechanism for the control of CA cell fates, which involve combinatorial usage of regulatory genes.

Development of catecholaminergic neurons in the pond snail, Lymnaea stagnalis: II. Postembryonic development of central and peripheral cells

Catecholamines have long been thought to play important roles in different mollusc neural functions. The present study used glyoxylate- and aldehyde-induced histofluorescence to identify central and peripheral catecholaminergic neurons in the snail Lymnaea stagnalis. The majority of these cells were also found to react to antibodies raised against tyrosine hydroxylase. A minority of the catecholaminergic neurons, however, exhibited no such immunoreactivity. The number of central catecholaminergic neurons nearly doubled (from about 45 to about 80 cells) during the first 2-3 days of postembryonic development. Thereafter, catecholaminergic neurons again doubled in number and generally grew by about 100-200% in soma diameter as the snails grew by 1,000% in overall linear measurements. In contrast to the relatively meager addition of central catecholaminergic neurons, several thousand catecholaminergic somata were added to different peripheral tissues during postembryonic development. These small, centrally projecting neurons were particularly concentrated in the lips, esophagus, anterior margin of the foot, and different regions of the male and female reproductive tracts. Chromatographic analyses indicated that dopamine was the major catecholamine present in the central ganglia, foot, and esophagus, although detectable levels of norepinephrine (approximately 20% of dopamine levels) were also found in the ganglia. The total content but not the concentration of dopamine increased within the tissue samples during postembryonic development. The companion study (Voronezhskaya et al. [1999] J. Comp. Neurol. 404:285-296) and the present study furnish a complete description of central and peripheral catecholaminergic neurons from their first appearance in early embryonic development to adulthood.

Actual problems of the cerebrospinal fluid-contacting neurons

Cerebrospinal fluid (CSF)-contacting neurons form a part of the circumventricular organs of the central nervous system. Represented by different cytologic types and located in different regions, they constitute a CSF-contacting neuronal system, the most central periventricular ring of neurons in the brain organized concentrically according to our concept. Because the central nervous system of deuterostomian echinoderm starfishes and the prochordate lancelet is composed mainly of CSF-contacting-like neurons, we hypothesize that this cell type represents ancient cells, or protoneurons, in the vertebrate brain. Neurons may contact the ventricular CSF via their dendrites, axons, or perikarya. Most of the CSF-contacting nerve cells send their dendritic processes into the ventricular cavity, where they form ciliated terminals. These ciliated endings resemble those of known sensory cells. By means of axons, the CSF-contacting neurons also may contact the external CSF space, where the axons form terminals of neurohormonal type similar to those known in the neurohemal areas. The most simple CSF-contacting neurons of vertebrates are present in the terminal filum, spinal cord, and oblongate medulla. The dendritic pole of these medullospinal CSF-contacting neurons terminates with an enlargement bearing many stereocilia in the central canal. These cells are also supplied with a 9 x 2 + 2 kinocilium that may contact Reissner's fiber, the condensed secretory material of the subcommissural organ. The Reissner's fiber floating freely in the CSF leaves the central canal at the caudal open end of the terminal filum in lower vertebrates, and open communication is thus established between internal CSF and the surrounding tissue spaces. Resembling mechanoreceptors cytologically, the spinal CSF-contacting neurons send their axons to the outer surface of the spinal cord to form neurosecretory-type terminals. They also send collaterals to local neurons and to higher spinal segments. In the hypothalamic part of the diencephalon, neurons of two circumventricular organs, the paraventricular organ and the vascular sac, of the magnocellular neurosecretory nuclei and several parvocellular nuclei, form CSF-contacting dendritic terminals. A CSF-contacting neuronal area also was found in the telencephalon. The CSF-contacting dendrites of all these areas bear solitary 9 x 2 + 0 cilia and resemble chemoreceptors and developing photoreceptors cytologically. In electrophysiological experiments, the neurons of the paraventricular organ are highly sensitive to the composition of the ventricular CSF. The axons of the CSF-contacting neurons of the paraventricular organ and hypothalamic nuclei terminate in hypothalamic synaptic zones, and those of magno- and parvocellular neurosecretory nuclei also form neurohormonal terminals in the median eminence and neurohypophysis. The axons of the CSF-contacting neurons of the vascular sac run in the nervus and tractus sacci vasculosi to the nucleus (ganglion) sacci vasculosi. Some hypothalamic CSF-contacting neurons contain immunoreactive opsin and are candidates to represent the "deep encephalic photoreceptors." In the newt, cells derived from the subependymal layer develop photoreceptor outer segments protruding to the lumen of the infundibular lobe under experimental conditions. Retinal and pineal photoreceptors and some of their secondary neurons possess common cytologic features with CSF-contacting neurons. They contact the retinal photoreceptor space and pineal recess, respectively, both cavities being derived from the third ventricle. In addition to ciliated dendritic terminals, there are intraventricular axons and neuronal perikarya contacting the CSF. Part of the CSF-contacting axons are serotoninergic; their perikarya are situated in the raphe nuclei. Intraventricular axons innervate the CSF-contacting dendrites, intraventricular nerve cells, and/or the ventricular surface of the ependyma. (ABSTRACT TRUNCATED)

Melatonin accelerates metamorphosis in Xenopus laevis

Delayed metamorphosis associated with large body size has been observed in Woodhousei fowleri tadpoles reared in continuous dark (DD). To evaluate the mechanism by which DD delayed metamorphosis, light-cycle exposure was controlled and thyroxine (T4), melatonin, or drugs that alter prolactin (Prl) concentrations were given to Xenopus laevis tadpoles. It was hypothesized that exogenous melatonin would delay metamorphosis and increase body size, and that elevation of Prl concentrations would have effects similar to melatonin exposure. Xenopus laevis tadpoles were randomized to three light conditions [light/dark (LD, 12 h/12 h), DD, and continuous light (LL)] and subgroups in each light condition were treated with T4, melatonin, bromocriptine (Bro), haloperidol (Hal), or no drug. Each subgroup included 12 tadpoles. Drugs were administered in the water either continuously or daily from 07.00 to 19.00 h (Intermittent). Measurements of total length, leg length, and stage of metamorphosis were obtained at regular intervals. DD resulted in delayed metamorphosis, while LL did not. T4 accelerated metamorphosis as expected, countering the delaying effects of DD. In contrast to the hypothesis, melatonin accelerated metamorphosis and impaired body size compared to controls. Intermittent Hal also accelerated metamorphosis, while Bro delayed it. In DD, both T4 and melatonin led to increased tadpole size in contrast to their counterparts in LD or LL. Delayed metamorphosis in DD is not caused by increased melatonin production. Melatonin and Hal (as given in this study) accelerate metamorphosis. Melatonin acceleration of metamorphosis may occur through alteration of the concentration of prolactin.

Tyrosine hydroxylase-negative, dopaminergic neurons are targets for transmitter-depleting action of haloperidol in the snail brain

1. The effects of long term administration of micromolar concentrations of the D2 antagonist haloperidol upon monoaminergic neurons in the snail Lymnaea stagnalis was investigated. 2. Treatment by bath application with 0.5-2.0 micromolar haloperidol, caused a significant, continuous depletion of dopamine levels in the nervous system as revealed by high performance liquid chromatography. 3. A transient depletion of serotonin was also observed, but DOPA and norepinephrine levels were unaffected. Similar depletion of dopamine was observed after the land snail, Achatina fulica, was injected with haloperidol on each of 4 consecutive days. 4. The depletion of dopamine as revealed with glyoxylate-induced fluorescence in Lymnaea appears to be restricted to a subpopulation of catecholaminergic neurons which are immuno-negative for tyrosine hydroxylase, the synthetic enzyme responsible for the conversion of tyrosine to DOPA. 5. The results thus demonstrate a depleting action of low micromolar doses of chronic haloperidol on specific subsets of dopaminergic neurons and a novel preparation for studying catecholaminergic mechanisms operating across the animal kingdom.

Ontogeny of GABA-immunoreactive neurons in the central nervous system in a teleost, gasterosteus aculeatus L

The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is known to exert various neurotrophic actions in the developing nervous system, but little is known about its distribution in the central nervous system during early development. We have studied the development of GABA-immunoreactive (GABAir) neurons during embryogenesis of a teleost fish, the three-spined stickleback. As early as 51 h postfertilization (PF; hatching occurs 144-168 h PF, and the first monoaminergic neurons appear around 72 h PF) GABAir neurons appear in the ventral prosencephalon caudal to the optic recess, in the ventral mesencephalon, and in the spinal cord. Then, there is a gradual addition of GABAir cell groups in the rostral prosencephalon and ventral rhombencephalon (66 h PF), dorsal and caudal hypothalamus and pretectum (72 h PF), ventral hypothalamus (78 h PF), preoptic region, thalamus, and in the mesencephalon and rhombencephalon (96 h PF). GABAir axons appear in the spinal cord already at 51 h PF, and then gradually appear in the various tracts of the early axonal scaffold of pathfinding fibers, so that by 96 h PF the entire axonal scaffold contains GABAir fibers. It appears likely that GABAergic axons contribute a major population to the formation of the axonal scaffold. Moreover, in the prosencephalon GABAir neurons are arranged in clusters that may reflect a neuromeric organization with six prosencephalic neuromeres.

Development of catecholamine systems in the central nervous system of the newt Pleurodeles waltlii as revealed by tyrosine hydroxylase immunohistochemistry

The aim of the present study was to extend our knowledge of the development of catecholamine (CA) systems in the class of amphibians to the order of urodeles. In contrast to previous studies of urodeles, the present study with antisera against tyrosine hydroxylase (TH) and dopamine revealed that CA systems are already present at early embryonic stages of the newt, Pleurodeles waltlii. Although the development from fertilized egg to juvenile in the urodele Pleurodeles lasts twice as long as that in the anuran, Xenopus laevis, and shows less dramatic changes in external morphology, the spatiotemporal sequence of appearance of TH-immunoreactive cell groups is rather similar. An early appearance of TH-immunoreactive cell bodies occurs in the olfactory bulb, the posterior tubercle, the accompanying cell group of the hypothalamic periventricular organ, the suprachiasmatic nucleus, the locus coeruleus, an area immediately ventral to the central canal of the spinal cord, and in the retina. Somewhat later, immunoreactive cells are detected in the posterior thalamic nucleus and in the rostral portion of the midbrain tegmentum, whereas the preoptic cell group is the last one to become TH immunoreactive. The presence of CA systems at early embryonic stages of both anurans and urodeles suggests that these systems are already of functional significance early in development. The maturation of CA neuronal structures in the olfactory and retinal circuitries, which takes place during development earlier in amphibians than in mammals, supports that notion.

Hypophysiotrophic systems in the brain of the Atlantic salmon. Neuronal innervation of the pituitary and the origin of pituitary dopamine and nonapeptides identified by means of combined carbocyanine tract tracing and immunocytochemistry

The neuroanatomical organization of neurons projecting to the pituitary and the origin of pituitary dopamine and nonapeptides were investigated in the brain of the Atlantic salmon (Salmo salar). Carbocyanine tract tracing in combination with tyrosine hydroxylase, arginine vasotocin and isotocin immunocytochemistry for double labelling revealed a previously unknown organization of hypophysiotrophic cell groups and their extrahypothalamic projections, and provide the first direct identification in a teleost fish of the origin of the dopaminergic and nonapeptidergic innervation of the pituitary. The present data include identification of (1) hypophysiotrophic neurons in the ventral telencephalon and in the periventricular preoptic nucleus, (2) large (magnocellular) vasotocinergic hypophysiotrophic neurons in the most rostral extension of the preoptic area, (3) a distinct neuronal group located in a supraoptic/suprachiasmatic position in the anterior periventricular nucleus, that seems to be the major source of dopaminergic innervation of the pituitary, (4) the nonapeptidergic hypophysiotrophic neurons in the preoptic nucleus, (5) hypophysiotrophic neurons in the ventral and posterior hypothalamus of which some are of liquor-contacting type, (6) projections from hypophysiotrophic and non-hypophysiotrophic neurons in the preoptic nucleus to extrahypothalamic areas such as thalamic and periventricular pretectal nuclei, and (7) subdivisions within the preoptic nucleus that exhibit different combinations of hypophysiotrophic and extrahypothalamic efferent connections. Together with previous studies of retinohypothalamic projections and neurochemical organization of hypothalamic/preoptic areas, the present data suggest that the preoptic nucleus and the anterior periventricular nucleus in teleosts possess functional subdivisions with features that resemble those of the paraventricular, supraoptic and suprachiasmatic nuclei of other vertebrates. In the Atlantic salmon, specific dopaminergic and nonapeptidergic neuronal subdivisions are proposed to play a role for photoperiod control of endocrine activity.

Development of catecholamine systems in the brain of the lizard Gallotia galloti

For a better insight into general and derived traits of developmental aspects of catecholaminergic (CA) systems in amniotes, we have studied the development of these systems in the brain of a lizard, Gallotia galloti, with tyrosine hydroxylase (TH)- and dopamine (DA) immunohistochemical techniques. Two main groups of TH-immunoreactive (THi) perikarya appear very early in development: one group in the midbrain which gives rise to the future ventral tegmental area, substantia nigra and retrorubral cell groups, and another group in the tuberomammillary hypothalamus. Somewhat later in development, TH/DA-immunoreactive cells are observed in the thalamus, rostrodorsal hypothalamus and spinal cord, and, with another delay, in the suprachiasmatic nucleus, the periventricular organ, and the pretectal posterodorsal nucleus. CA cell groups that appear rather late in development include the cells in the olfactory bulb, the locus coeruleus and the caudal brainstem. As expected, the development of immunoreactive fibers stays behind that of the cell bodies, but reaches the adult-like pattern just prior to hatching. The present study revealed considerable variation in the relation between the state of cytodifferentiation and first expression of TH/DA immunoreactivity between CA cell groups. Catecholamine cells in the midbrain and tuberomammillary hypothalamus are still migrating, immature (absence of dendrites) and express only TH immunoreactivity at the time of first detection. Cells which appear at later developmental stages lie already further away from the ventricle, possess two or more dendritic processes, and generally express both TH- and DA immunoreactivity.

Ontogeny of catecholamine systems in the central nervous system of anuran amphibians: an immunohistochemical study with antibodies against tyrosine hydroxylase and dopamine

To get more insight into developmental aspects of catecholamine systems in vertebrates, in particular anuran amphibians, these systems were studied immunohistochemically in embryos and larvae of Xenopus laevis and Rana ridibunda. Antisera against tyrosine hydroxylase (TH) and dopamine (DA) revealed that catecholamine systems are already present at early embryonic stages. The first dopamine group to be detected was found ventral to the central canal of the spinal cord of Xenopus, soon followed by DA cell groups in the posterior tubercle, the hypothalamic periventricular organ, the accompanying cell group of the periventricular organ, and the suprachiasmatic nucleus. Although weakly TH-immunoreactive cells were found in the olfactory bulb at about the same embryonic stages, DA immunoreactivity was not detected until premetamorphic stage 49. Dopamine cell groups in the caudal brainstem, midbrain, and pretectum appeared at late premetamorphic and prometamorphic stages, whereas the preoptic group was first observed at the metamorphic climax stage. Rana showed an almost similar timetable of development of catecholamine cell groups, except for the caudal brainstem group which was already present at the end of the embryonic period. When compared with previous studies by means of formaldehyde-induced fluorescence technique, it becomes clear that TH/DA immunohistochemistry enables an earlier detection of catecholamine cell groups and fiber systems in anuran amphibians. The present study also revealed that the DA-immunoreactive cells of the hypothalamic periventricular organ never stained with the TH antiserum during development, thus supporting their putatively DA accumulating nature. Another notable result is the site of origin and rather late appearance of the midbrain dopaminergic cell group. It is suggested that the latter cell group only partly corresponds to the ventral tegmental area and substantia nigra of amniotes.

Developmental changes in the brain-stem serotonergic nuclei of teleost fish and neural plasticity

1. During early ontogeny, the serotonergic neurons in the brain stem of the three-spined stickleback shows a temporal and spatial developmental pattern that closely resembles that of amniotes. 2. However, in the adult fish, only the midline nuclei of the rostral group (dorsal and median raphe nuclei) and the dorsal lateral tegmental nucleus are consistently serotonin-immunoreactive (5-HTir), whereas the groups of the upper and lower rhombencephalon (raphe pontis, raphe magnus, and raphe pallidus/obscurus nuclei) are variable and, when present, contain relatively small numbers of 5-HTir neurons. 3. Using specific antisera against tryptophan 5-hydroxylase and aromatic L-amino acid decarboxylase, we have shown that the lateral B9 group and the groups of the upper and lower rhombencephalon are consistently present in adult sticklebacks. The results are discussed in relation to other known instances of neurotransmitter plasticity or transient neurotransmitter expression in teleost fish. 4. While there are several instances of transient expression of neurotransmitter markers by discrete neuronal populations, there is so far no evidence of changes from one neurotransmitter phenotype to another in the brain of teleost fish. However, there are indications of plasticity of expression of catecholamines and indoleamines, and their respective synthesizing enzymes, as reflected in age-dependent changes and variation between individuals of different physiological status. 5. As the brain grows continuously in teleost fish, and new neurons are added from proliferative regions, synaptic connections may be expected to undergo remodeling in all brain regions throughout life. Thus, the teleostean brain may be considered a suitable model for experimental studies of different aspects of neural plasticity.

Catecholaminergic systems in the zebrafish. I. Number, morphology, and histochemical characteristics of neurons in the locus coeruleus

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.

Immunolocalization of aromatic L-amino acid decarboxylase in goldfish (Carassius auratus) brain

The distribution of monoamines (catecholamines and serotonin) in fishes has been previously studied by immunohistochemistry of both the monoamines themselves and their biosynthetic enzymes. But the distribution of neurons containing aromatic L-amino acid decarboxylase, an enzyme involved in the biosynthesis of both catecholamines and serotonin, has up to now not been investigated. In order to improve knowledge about the localization of aromatic L-amino acid decarboxylase, neurons containing this enzyme were mapped immunohistochemically in the goldfish brain. Furthermore, neurons bearing aromatic L-amino acid decarboxylase immunoreactivity have been compared with those containing tyrosine hydroxylase and serotonin immunoreactivities. Our results show that distribution of aromatic L-amino acid decarboxylase immunoreactivity generally coincides with that of tyrosine hydroxylase and serotonin. Nevertheless, the presence of nine D cell groups (containing aromatic L-amino acid decarboxylase but lacking both catecholamines and serotonin) and six groups of neurons which are aromatic L-amino acid decarboxylase-immunonegative but contain tyrosine hydroxylase, and might produce L-DOPA, have been revealed. The occurrence of both D cell groups and presumptive L-DOPA neurons in goldfish brain is discussed in relation to similar findings in fish and mammalian brain.