An immunohistochemical study of the telencephalon of the senegal bichir (Polypterus senegalus)

Transgenic tools targeting the basal ganglia reveal both evolutionary conservation and specialization of neural circuits in zebrafish

The cortico-basal ganglia circuit mediates decision making. Here, we generated transgenic tools for adult zebrafish targeting specific subpopulations of the components of this circuit and utilized them to identify evolutionary homologs of the mammalian direct- and indirect-pathway striatal neurons, which respectively project to the homologs of the internal and external segment of the globus pallidus (dorsal entopeduncular nucleus [dEN] and lateral nucleus of the ventral telencephalic area [Vl]) as in mammals. Unlike in mammals, the Vl mainly projects to the dEN directly, not by way of the subthalamic nucleus. Further single-cell RNA sequencing analysis reveals two pallidal output pathways: a major shortcut pathway directly connecting the dEN with the pallium and the evolutionarily conserved closed loop by way of the thalamus. Our resources and circuit map provide the common basis for the functional study of the basal ganglia in a small and optically tractable zebrafish brain for the comprehensive mechanistic understanding of the cortico-basal ganglia circuit.

Telencephalic eversion in embryos and early larvae of four teleost species

The telencephalon of ray-finned fishes undergoes eversion, which is very different to the evagination that occurs in most other vertebrates. Ventricle morphogenesis is key to build an everted telencephalon. Thus, here we use the apical marker zona occludens 1 to understand ventricle morphology, extension of the tela choroidea and the eversion process during early telencephalon development of four teleost species: giant danio (Devario aequipinnatus), blind cavefish (Astyanax mexicanus), medaka (Oryzias latipes), and paradise fish (Macroposus opercularis). In addition, by using immunohistochemistry against tubulin and calcium-binding proteins, we analyze the general morphology of the telencephalon, showing changes in the location and extension of the olfactory bulb and other telencephalic regions from 2 to 5 days of development. We also analyze the impact of abnormal eye and telencephalon morphogenesis on eversion, showing that cyclops mutants do undergo eversion despite very dramatic abnormal eye morphology. We discuss how the formation of the telencephalic ventricle in teleost fish, with its characteristic shape, is a crucial event during eversion.

Serotonin-immunoreactivity in the brain of the cichlid fish Oreochromis mossambicus

Serotonin (5-HT) is an evolutionarily conserved monoaminergic neurotransmitter found in the central nervous system and peripheral nervous system across invertebrates and vertebrates. Although the distribution of 5-HT-immunoreactive (5-HT-ir) neurons is investigated in various fish species, the organization of these neurons in cichlid fishes is poorly understood. These fish are known for their adaptability to diverse environments, food habits, and complex mating and breeding behaviors, including parental care. In this paper, we describe the organization of 5-HT-ir neurons in the brain of the cichlid fish Oreochromis mossambicus. Aggregations of 5-HT-ir neurons were spotted in the granule cell layer of the olfactory bulb and near the ventricular border in the preoptic area and magnocellular subdivisions of the nucleus preopticus. Although the presence of 5-HT-ir cells and fibers in the hypothalamic and thalamic regions, cerebellum, and raphe nuclei was comparable to that of other teleosts, the current study reveals the occurrence of 5-HT-ir cells and fibers for the first time in some areas, such as the nucleus posterior tuberis, nucleus oculomotorius, and nucleus paracommissuralis in the tilapia. While the presence of 5-HT-ir cells and fibers in gustatory centers suggests a role for serotonin in the processing of gustatory signals, distinctive pattern of 5-HT immunoreactivity was seen in the telencephalon, pretectal areas, mesencephalic, and rhombencephalic regions, suggesting a cichlid fish specific organization of the serotonergic system. In conclusion, the 5-HT system in the tilapia brain may serve several neuroendocrine and neuromodulatory roles, including regulation of reproduction and sensorimotor processes.

A Critical Review of Zebrafish Neurological Disease Models-2. Application: Functional and Neuroanatomical Phenotyping Strategies and Chemical Screens

Extensive phylogenetic conservation of molecular pathways and neuroanatomical structures, associated with efficient methods for genetic modification, have been exploited increasingly to generate zebrafish models of human disease. A range of powerful approaches can be deployed to analyze these models with the ultimate goal of elucidating pathogenic mechanisms and accelerating efforts to find effective treatments. Unbiased neurobehavioral assays can provide readouts that parallel clinical abnormalities found in patients, although some of the most useful assays quantify responses that are not routinely evaluated clinically, and differences between zebrafish and human brains preclude expression of the full range of neurobehavioral abnormalities seen in disease. Imaging approaches that use fluorescent reporters and standardized brain atlases coupled with quantitative measurements of brain structure offer an unbiased means to link experimental manipulations to changes in neural architecture. Together, quantitative structural and functional analyses allow dissection of the cellular and physiological basis underlying neurological phenotypes. These approaches can be used as outputs in chemical modifier screens, which provide a major opportunity to exploit zebrafish models to identify small molecule modulators of pathophysiology that may be informative for understanding disease mechanisms and possible therapeutic approaches.

Role of Habenula in Social and Reproductive Behaviors in Fish: Comparison With Mammals

Social behaviors such as mating, parenting, fighting, and avoiding are essential functions as a communication tool in social animals, and are critical for the survival of individuals and species. Social behaviors are controlled by a complex circuitry that comprises several key social brain regions, which is called the social behavior network (SBN). The SBN further integrates social information with external and internal factors to select appropriate behavioral responses to social circumstances, called social decision-making. The social decision-making network (SDMN) and SBN are structurally, neurochemically and functionally conserved in vertebrates. The social decision-making process is also closely influenced by emotional assessment. The habenula has recently been recognized as a crucial center for emotion-associated adaptation behaviors. Here we review the potential role of the habenula in social function with a special emphasis on fish studies. Further, based on evolutional, molecular, morphological, and behavioral perspectives, we discuss the crucial role of the habenula in the vertebrate SDMN.

Organization of the Catecholaminergic System in the Short-Lived Fish Nothobranchius furzeri

The catecholaminergic system has received much attention based on its regulatory role in a wide range of brain functions and its relevance in aging and neurodegenerative diseases. In the present study, we analyzed the neuroanatomical distribution of catecholaminergic neurons based on tyrosine hydroxylase (TH) immunoreactivity in the brain of adult Nothobranchius furzeri. In the telencephalon, numerous TH+ neurons were observed in the olfactory bulbs and the ventral telencephalic area, arranged as strips extending through the rostrocaudal axis. We found the largest TH+ groups in the diencephalon at the preoptic region level, the ventral thalamus, the pretectal region, the posterior tuberculum, and the caudal hypothalamus. In the dorsal mesencephalic tegmentum, we identified a particular catecholaminergic group. The rostral rhombencephalon housed TH+ cells in the locus coeruleus and the medulla oblongata, distributing in a region dorsal to the inferior reticular formation, the vagal lobe, and the area postrema. Finally, scattered TH+ neurons were present in the ventral spinal cord and the retina. From a comparative perspective, the overall organization of catecholaminergic neurons is consistent with the general pattern reported for other teleosts. However, N. furzeri shows some particular features, including the presence of catecholaminergic cells in the midbrain. This work provides a detailed neuroanatomical map of the catecholaminergic system of N. furzeri, a powerful aging model, also contributing to the phylogenetic understanding of one of the most ancient neurochemical systems.

Serotonin (5-hydroxytryptamine)-immunoreactive neurons in the brain of the viviparous fish Gambusia affinis

The monoaminergic neurotransmitter serotonin (5-HT) acts as a neuromodulator and is associated with a wide range of functions in fish. In this investigation, 5-HT immunoreactivity was studied in the central nervous system (CNS) of the viviparous mosquitofish Gambusia affinis. 5-HT-immunoreactive (5-HT-ir) cells/fibres were observed throughout the subdivisions of ventral and dorsal telencephalon including the olfactory bulb. Several intensely stained 5-HT-ir cells and/or fibres were detected in different areas of the hypothalamus as well as the proximal pars distalis of the pituitary gland. 5-HT-ir cells were restricted to the dorsal and ventral part of the pretectal diencephalic cluster, but only fibres were detected in the anterior, ventromedial and posterior subdivisions of the thalamic nucleus and in the preglomerular complex. In the mesencephalon, 5-HT-ir perikarya, and fibres were seen in the optic tectum, midbrain tegmentum and torus semicircularis. A cluster of prominently labelled 5-HT-ir neurons was observed in the superior raphe nucleus, whereas numerous 5-HT-ir fibres were distributed throughout the rhombencephalic divisions. In addition, a bundle of rostrocaudally running 5-HT-ir fibres was noticed in the spinal cord. This is the first detailed neuroanatomical study in a viviparous teleost, reporting a widespread distribution of 5-HT-ir somata and fibres in the CNS. The results of this study provide new insights into the evolutionarily well conserved nature of the monoaminergic system in the CNS of vertebrates and suggest a role for 5-HT in regulation of several physiological, behavioural and neuroendocrine functions in viviparous teleosts.

Analysis of pallial/cortical interneurons in key vertebrate models of Testudines, Anurans and Polypteriform fishes

The organization of the pallial derivatives across vertebrates follows a comparable elementary arrangement, although not all of them possess a layered cortical structure as sophisticated as the cerebral cortex of mammals. However, its expansion along evolution has only been possible by the development and coevolution of the cellular networks formed by excitatory neurons and inhibitory interneurons. Thus, the comparative analysis of interneuron types in vertebrate models of key evolutionary significance will provide important information, due to the extraordinary anatomical sophistication of their interneuron systems with simpler behavioral implications. Particularly in mammals, the main consensus for classifying interneuron types is based on non-overlapping markers, which do not form a single population, but consist of several distinct classes of inhibitory cells showing co-expression of other markers. In our study, we analyzed immunohistochemically the expression of the main markers like somatostatin (SOM), parvalbumin (PV), calretinin (CR), calbindin (CB), neuropeptide Y (NPY) and/or nitric oxide synthase (NOS) at the pallial regions of three different models of Osteichthyes. First, we selected two tetrapods, one amniote from the genus Pseudemys belonging to the order Testudine, at the base of the amniote diversification and with a three-layered simple cortex, and the Anuran Xenopus laevis, an anamniote tetrapod with a non-layered evaginated pallium, and finally the order Polypteriform, a small fish group at the base of the actinopterygian diversification with an everted telencephalon. SOM was the most conserved interneuron type in terms of its distribution and co-expression with other markers such as CR, in contrast to PV, which showed a different pattern between the models analyzed. In addition, the SOM expression supports a homological relationship between the medial pallial derivatives in all the models. CR and CB expressions in the tetrapods were observed, particularly, CR expressing cells were detected in the medial and the dorsal pallial derivatives, in contrast to CB, which appeared only in discrete scattered populations. However, the pallium of Polypteriforms fishes was almost devoid of CR cells, in contrast to the important number of CB cells observed in all the pallial regions. The NPY immunoreactivity was detected in all the pallial domains of all the models, as well as cells coexpressing CR. Finally, the pallial nitrergic expression was also conserved, which allows to postulate the homological relationships between the ventropallial and the amygdaloid derivatives. In summary, even in basal pallial models the neurochemically characterized interneurons indicate that their first appearance took place before the common ancestor of amniotes. Thus, our results suggest a shared pattern of interneuron types in the pallium of all Osteichthyes.

Serotonin systems in three socially communicating teleost species, the grunting toadfish (Allenbatrachus grunniens), a South American marine catfish (Ariopsis seemanni), and the upside-down catfish (Synodontis nigriventris)

We investigated immunohistochemically the distribution of serotonergic cell populations in three teleost species (one toadfish, Allenbatrachus grunniens, and two catfishes, Synodontis nigriventris and Ariopsis seemanni). All three species exhibited large populations of 5-HT positive neurons in the paraventricular organ (PVO) and the dorsal (Hd) and caudal (Hc) periventricular hypothalamic zones, plus a smaller one in the periventricular pretectum, a few cells in the pineal stalk, and - only in catfishes - in the preoptic region. Furthermore, the rhombencephalic superior and inferior raphe always contained ample serotonergic cells. In each species, a neuronal mass extended into the hypothalamic lateral recess. Only in the toadfish, did this intraventricular structure contain serotonergic cells and arise from Hd, whereas in the catfishes it emerged from medially and represents the dorsal tuberal nucleus seen in other catfishes as well. Serotonergic cells in PVO, Hd and Hc were liquor-contacting. Those of the PVO extended into the midline area of the periventricular posterior tubercular nucleus in both catfishes. Dopaminergic, liquor-contacting neurons were additionally investigated using an antibody against tyrosine hydroxylase (TH) in S. nigriventris showing that TH was never co-localized with serotonin. Because TH antibodies are known to reveal mostly or only the TH1 enzyme, we hypothesize that th1-expressing dopamine cells (unlike th2-expressing ones) do not co-localize with serotonin. Since the three investigated species engage in social communication using swim bladder associated musculature, we investigated the serotonergic innervation of the hindbrain vocal or electromotor nuclei initiating the social signal. We found in all three species serotonergic fibers seemingly originating from close-by serotonergic neurons of inferior raphe or anterior spinal cord. Minor differences appear to be rather species-specific than dependent on the type of social communication.

Evolution of the Chordate Telencephalon

The dramatic evolutionary expansion of the neocortex, together with a proliferation of specialized cortical areas, is believed to underlie the emergence of human cognitive abilities. In a broader phylogenetic context, however, neocortex evolution in mammals, including humans, is remarkably conservative, characterized largely by size variations on a shared six-layered neuronal architecture. By contrast, the telencephalon in non-mammalian vertebrates, including reptiles, amphibians, bony and cartilaginous fishes, and cyclostomes, features a great variety of very different tissue structures. Our understanding of the evolutionary relationships of these telencephalic structures, especially those of basally branching vertebrates and invertebrate chordates, remains fragmentary and is impeded by conceptual obstacles. To make sense of highly divergent anatomies requires a hierarchical view of biological organization, one that permits the recognition of homologies at multiple levels beyond neuroanatomical structure. Here we review the origin and diversification of the telencephalon with a focus on key evolutionary innovations shaping the neocortex at multiple levels of organization.

Atlas of the telencephalon based on cytoarchitecture, neurochemical markers, and gene expressions in Rhinogobius flumineus [Mizuno, 1960]

Gobiida is a basal subseries of percomorphs in teleost fishes, holding a useful position for comparisons with other orders of Percomorpha as well as other cohort of teleosts. Here, we describe a telencephalic atlas of a Gobiida species Rhinogobius flumineus (Mizuno, Memoirs of the College of Science, University of Kyoto, Series B: Biology, 1960; 27, 3), based on cytoarchitectural observations, combined with analyses of the distribution patterns of neurochemical markers and transcription factors. The telencephalon of R. flumineus shows a number of features distinct from those of other teleosts. Among others, the followings were of special note. (a) The lateral part of dorsal telencephalon (Dl), which is known as a visual center in other teleosts, is composed of as many as seven regions, some of which are conspicuous, circumscribed by cell plates. These subdivisions of the Dl can be differentiated clearly by differential soma size and color with Nissl-staining, and distribution patterns of neural markers. (b) Cell populations continuous with the ventral region of dorsal part of ventral telencephalon (vVd) exhibit extensive dimension. Especially, portion 1 of the central part of ventral telencephalon appears to represent a cell population laterally translocated from the vVd, forming a large cluster of small cells that penetrate deep into the central part of dorsal telencephalon. (c) The magnocellular subdivision of dorsal part of dorsal telencephalon (Ddmg) contains not only large cells but also vglut2a-positive clusters of small cells that cover a wide range of the caudal Ddmg. Such clusters of small cells have not been observed in the Ddmg of other teleosts.

Distribution and female reproductive state differences in orexigenic and anorexigenic neurons in the brain of the mouth brooding African cichlid fish, Astatotilapia burtoni

Integration of reproduction and metabolism is necessary for species survival. While the neural circuits controlling energy homeostasis are well-characterized, the signals controlling the relay of nutritional information to the reproductive axis are less understood. The cichlid fish Astatotilapia burtoni is ideal for studying the neural regulation of feeding and reproduction because females cycle between a feeding gravid state and a period of forced starvation while they brood developing young inside their mouths. To test the hypothesis that candidate neuropeptide-containing neurons known to be involved in feeding and energy homeostasis in mammals show conserved distribution patterns, we performed immunohistochemistry and in situ hybridization to localize appetite-stimulating (neuropeptide Y, NPY; agouti-related protein, AGRP) and appetite-inhibiting (cocaine and amphetamine-regulated transcript, CART; pro-opiomelanocortin, pomc1a) neurons in the brain. NPY, AGRP, CART, and pomc1a somata showed distribution patterns similar to other teleosts, which included localization to the lateral tuberal nucleus (NLT), the putative homolog of the mammalian arcuate nucleus. Gravid females also had larger NPY and AGRP neurons in the NLT compared to brooding females, but brooding females had larger pomc1a neurons compared to gravid females. Hypothalamic agrp mRNA levels were also higher in gravid compared to brooding females. Thus, larger appetite-stimulating neurons (NPY, AGRP) likely promote feeding while females are gravid, while larger pomc1a neurons may act as a signal to inhibit food intake during mouth brooding. Collectively, our data suggest a potential role for NPY, AGRP, POMC, and CART in regulating energetic status in A. burtoni females during varying metabolic and reproductive demands.

Inhibiting effects of rhynchophylline on methamphetamine-dependent zebrafish are related with the expression of tyrosine hydroxylase (TH)

In this study, to study the effect of rhynchophylline on TH in midbrain of methamphetamine-induced conditioned place preference (CPP) adult zebrafish, place preference adult zebrafish models were established by methamphetamine (40μg/g) and the expression of TH was observed by immunohistochemistry technique and Western blot. Ketamine (150μg/g), high dose of rhynchophylline (100μg/g) group can significantly reduce the place preference; immunohistochemistry results showed that the number of TH-positive neurons in midbrain was increased in the methamphetamine model group, whereas less TH-positive neurons were found in the ketamine group and high dosage rhynchophylline group. Western blot results showed that the expression of TH protein was significantly increased in the model group, whereas less expression was found in the ketamine group, high dosage rhynchophylline group. Our data pointed out that TH plays an important role in the formation of methamphetamine-induced place preference in adult zebrafish. Rhynchophylline reversed the expression of TH in the midbrain demonstrates the potential effect of mediates methamphetamine induced rewarding effect.

Inhibiting effects of rhynchophylline on zebrafish methamphetamine dependence are associated with amelioration of neurotransmitters content and down-regulation of TH and NR2B expression

Others and we have reported that rhynchophylline reverses amphetamine-induced conditioned place preference (CPP) effect which may be partly mediated by amelioration of central neurotransmitters and N-methyl-d-aspartate receptor 2B (NR2B) levels in the rat brains. The current study investigated the inhibiting effects of rhynchophylline on methamphetamine-induced (METH-induced) CPP in adult zebrafish and METH-induced locomotor activity in tyrosine hydroxylase-green fluorescent protein (TH-GFP) transgenic zebrafish larvae and attempted to confirm the hypothesis that these effects were mediated via regulation of neurotransmitters and dopaminergic and glutamatergic systems. After baseline preference test (on days 1-3), zebrafish were injected intraperitoneally METH (on days 4, 6 and 8) or the same volume of fish physiological saline (on days 5 and 7) and were immediately conditioned. Rhynchophylline was administered at 12h after injection of METH. On day 9, zebrafish were tested for METH-induced CPP. Results revealed that rhynchophylline (100mg/kg) significantly inhibited the acquisition of METH-induced CPP, reduced the content of dopamine and glutamate and down-regulated the expression of TH and NR2B in the CPP zebrafish brains. Furthermore, the influence of rhynchophylline on METH-induced locomotor activity was also observed in TH-GFP transgenic zebrafish larvae. Results showed that rhynchophylline (50mg/L) treatment led to a significant reduction on the locomotor activity and TH expression in TH-GFP transgenic zebrafish larvae. Taken together, these data indicate that the inhibition of the formation of METH dependence by rhynchophylline in zebrafish is associated with amelioration of the neurotransmitters dopamine and glutamate content and down-regulation of TH and NR2B expression.

Organization of the nitrergic neuronal system in the primitive bony fishes Polypterus senegalus and Erpetoichthys calabaricus (Actinopterygii: Cladistia)

Cladistians are a group of basal actinopterygian fishes that constitute a good model for studying primitive brain features, most likely present in the ancestral bony fishes. The analysis of the nitrergic neurons (with the enzyme nitric oxide synthase; NOS) has helped in understanding important aspects of brain organization in all vertebrates studied. We investigated the nitrergic system of two cladistian species by means of specific antibodies against NOS and NADPH-diaphorase (NADPH-d) histochemistry, which, with the exception of the primary olfactory and terminal nerve fibers, labeled only for NADPH-d, yielded identical results. Double immunohistochemistry was conducted for simultaneous detection of NOS with tyrosine hydroxylase, choline acetyltransferase, calbindin, calretinin, and serotonin, to establish accurately the localization of the nitrergic neurons and fibers and to assess possible interactions between these neuroactive substances. The pattern of distribution in both species showed only subtle differences in the density of labeled cells. Distinct groups of NOS-immunoreactive cells were observed in pallial and subpallial areas, paraventricular region, tuberal and retromammillary hypothalamic areas, posterior tubercle, prethalamic and thalamic areas, optic tectum, torus semicircularis, mesencephalic tegmentum, interpeduncular nucleus, superior and middle reticular nuclei, magnocellular vestibular nucleus, solitary tract nucleus, nucleus medianus magnocellularis, the spinal cord and amacrine cells in the retina. Large neurons in cranial nerve sensory ganglia were also labeled. The comparison of these results with those from other vertebrates, using a neuromeric analysis, reveals a conserved pattern of organization of the nitrergic system from this primitive fish group to amniotes, including mammals.

The effect of observers on behavior and the brain during aggressive encounters

What effect does an audience have on an animal's behavior and where is this influence registered in the brain? To answer these questions, we analyzed male cichlid fish fighting in the presence of audiences of various compositions and measured expression of immediate early genes in the brain as a proxy for neural activity. We hypothesized their behavior would change depending on who was watching them. We measured behavioral responses from both the "watchers" and the "watched" during aggressive encounters and found that males fighting in the presence of an audience were more aggressive than males fighting without an audience. Depending on the nature of the audience, immediate early gene expression in key brain nuclei was differentially influenced. Both when an audience of larger males watched fighting males, and when they were watching larger males fighting, nuclei in the brain considered homologous with mammalian nuclei known to be associated with anxiety showed increased activity. When males were in the presence of any audience or when males saw any other males fighting, nuclei in the brain known to be involved in reproduction and aggression were differentially activated relative to control animals. In all cases, there was a close relationship between patterns of brain gene expression between fighters and observers. This suggests that the network of brain regions known as the social behavior network, common across vertebrates, are activated not only in association with the expression of social behavior but also by the reception of social information.

Organization of the orexin/hypocretin system in the brain of two basal actinopterygian fishes, the cladistians Polypterus senegalus and Erpetoichthys calabaricus

Cladistians are primitive actinopterygian fishes mostly neglected in neuroanatomical studies. In the present study, the detailed neuroanatomical distribution of orexin (hypocretin)-like immunoreactive (OX-ir) cell bodies and fibers was analyzed in the brain of two species representative of the two extant genera of cladistians. Antibodies against mammalian orexin-A and orexin-B peptides were used. Simultaneous detection of orexins with neuropeptide Y (NPY), tyrosine hydroxylase (TH), and serotonin (5-HT) was used to establish accurately the topography of the orexin system and to evaluate the possible interactions with NPY and monoaminergic systems. A largely common pattern of OX-ir distribution in the two cladistian species was observed. Most OX-ir cells were located in the suprachiasmatic nucleus and tuberal hypothalamus, whereas scarce cells were observed in the posterior tubercle. In addition, a population of OX-ir cells was found in the preoptic area only in Polypterus and some cells also contained TH. The observed widespread distribution of OX-ir fibers was especially abundant in the retrobulbar area, subpallial areas, preoptic area, suprachiasmatic nucleus, tuberal hypothalamic area, prethalamus, thalamus, pretectum, optic tectum, and tegmentum. Low innervation was found in relation to monoaminergic cell groups, whereas a high NPY innervation was observed in all OX-ir cell groups. These relationships would represent the anatomical substrate for the functional interdependence between these systems. The organization of the orexin system in cladistians revealed a pattern largely consistent with those reported for all studied groups of vertebrates, suggesting that the primitive organization of this peptidergic system occurred in the common ancestor of gnathostome vertebrates.

Evolutionary and developmental contributions for understanding the organization of the basal ganglia

Herein we take advantage of the evolutionary developmental biology approach in order to improve our understanding of both the functional organization and the evolution of the basal ganglia, with a particular focus on the globus pallidus. Therefore, we review data on the expression of developmental regulatory genes (that play key roles in patterning, regional specification and/or morphogenesis), gene function and fate mapping available in different vertebrate species, which are useful to (a) understand the embryonic origin and basic features of each neuron subtype of the basal ganglia (including neurotransmitter/neuropeptide expression and connectivity patterns); (b) identify the same (homologous) subpopulations in different species and the degree of variation or conservation throughout phylogeny, and (c) identify possible mechanisms that may explain the evolution of the basal ganglia. These data show that the globus pallidus of rodents contains two major subpopulations of GABAergic projection neurons: (1) neurons containing parvalbumin and neurotensin-related hexapetide (LANT6), with descending projections to the subthalamus and substantia nigra, which originate from progenitors expressing Nkx2.1, primarily located in the pallidal embryonic domain (medial ganglionic eminence), and (2) neurons containing preproenkephalin (and possibly calbindin), with ascending projections to the striatum, which appear to originate from progenitors expressing Islet1 in the striatal embryonic domain (lateral ganglionic eminence). Based on data on Nkx2.1, Islet1, LANT6 and proenkephalin, it appears that both cell types are also present in the globus pallidus/dorsal pallidum of chicken, frog and lungfish. In chicken, the globus pallidus also contains neurons expressing substance P (SP), perhaps originating in the striatal embryonic domain. In ray-finned and cartilaginous fishes, the pallidum contains at least the Nkx2.1 lineage cell population (likely representing the neurons containing LANT6). Based on the presence of neurons containing enkephalin or SP, it is possible that the pallidum of these animals also includes the Islet1 lineage cell subpopulation, and both neuron subtypes were likely present in the pallidum of the first jawed vertebrates. In contrast, lampreys (jawless fishes) appear to lack the pallidal embryonic domain and the Nkx2.1 lineage cell population that mainly characterize the pallidum in jawed vertebrates. In the absence of data in other jawless fishes, the ancestral condition in vertebrates remains to be elucidated. Perhaps, a major event in telencephalic evolution was the novel expression of Nkx2.1 in the subpallium, which has been related to Hedgehog expression and changes in the regulatory region of Nkx2.1.

Immunohistochemical distribution of calretinin and calbindin (D-28k) in the brain of the cladistian Polypterus senegalus

Polypteriform fishes are believed to be basal to other living ray-finned bony fishes, and they may be useful for providing information of the neural organization that existed in the brain of the earliest ray-finned fishes. The calcium-binding proteins calretinin (CR) and calbindin-D28k (CB) have been widely used to characterize neuronal populations in vertebrate brains. Here, the distribution of the immunoreactivity against CR and CB was investigated in the olfactory organ and brain of Polypterus senegalus and compared to the distribution of these molecules in other ray-finned fishes. In general, CB-immunoreactive (ir) neurons were less abundant than CR-ir cells. CR immunohistochemistry revealed segregation of CR-ir olfactory receptor neurons in the olfactory mucosa and their bulbar projections. Our results confirmed important differences between pallial regions in terms of CR immunoreactivity of cell populations and afferent fibers. In the habenula, these calcium-binding proteins revealed right-left asymmetry of habenular subpopulations and segregation of their interpeduncular projections. CR immunohistochemistry distinguished among some thalamic, pretectal, and posterior tubercle-derived populations. Abundant CR-ir populations were observed in the midbrain, including the tectum. CR immunoreactivity was also useful for characterizing a putative secondary gustatory/visceral nucleus in the isthmus, and for distinguishing territories in the primary viscerosensory column and octavolateral region. Comparison of the data obtained within a segmental neuromeric context indicates that some CB-ir and CR-ir populations in polypteriform fishes are shared with other ray-finned fishes, but other positive structures appear to have evolved following the separation between polypterids and other ray-finned fishes.

Orexin and neuropeptide Y: tissue specific expression and immunoreactivity in the hypothalamus and preoptic area of the cichlid fish Cichlasoma dimerus

Neuropeptide Y (NPY) and orexin are neuropeptides involved in the regulation of feeding in vertebrates. In this study we determined the NPY and orexin mRNA tissue expression and their immunoreactivity distribution in both preoptic area and hypothalamus, regions involved in the regulation of feeding behavior. Both peptides presented a wide expression in all tissues examined. The NPY-immunoreactive (ir) cells were localized in the ventral nucleus posterioris periventricularis (NPPv) and numerous ir-NPY fibers were found in the nucleus lateralis tuberis (NLT), the nucleus recess lateralis (NRL) and the neurohypophysis. Ir-orexin cells were observed in the NPPv, dorsal NLT, ventral NLT, lateral NLT (NLTl) and the lateral NRL. Ir-orexin fibers were widespread distributed along all the hypothalamus, especially in the NLTl. Additionally, we observed the presence of ir-orexin immunostaining in adenohypophyseal cells, especially in somatotroph cells and the presence of a few ir-orexin-A fibers in the neurohypophysis. In conclusion, both peptides have an ubiquitous mRNA tissue expression and are similarly distributed in the hypothalamus and preoptic area of Cichlasoma dimerus. The presence of ir-orexin in adenohypohyseal cells and the presence of ir-orexin and NPY fibers in the neurohypophysis suggest that both peptides may play an important neuroendocrine role in anterior pituitary.

Expression of the cannabinoid CB1 receptor in the gymnotiform fish brain and its implications for the organization of the teleost pallium

Cannabinoid CB1 receptors (CB1R) are widely distributed in the brains of many vertebrates, but whether their functions are conserved is unknown. The weakly electric fish, Apteronotus leptorhynchus (Apt), has been well studied for its brain structure, behavior, sensory processing, and learning and memory. It therefore offers an attractive model for comparative studies of CB1R functions. We sequenced partial AptCB1R mRNAs and performed in situ hybridization to localize its expression. Partial AptCB1R protein sequence was highly conserved to zebrafish (90.7%) and mouse (81.9%) orthologs. AptCB1R mRNA was highly expressed in the telencephalon. Subpallial neurons (dorsal, central, intermediate regions and part of the ventral region, Vd/Vc/Vi, and Vv) expressed high levels of AptCB1R transcript. The central region of dorsocentral telencephalon (DC(core) ) strongly expressed CB1R mRNA; cells in DC(core) project to midbrain regions involved in electrosensory/visual function. The lateral and rostral regions of DC surrounding DC(core) (DC(shell) ) lack AptCB1R mRNA. The rostral division of the dorsomedial telencephalon (DM1) highly expresses AptCB1R mRNA. In dorsolateral division (DL) AptCB1R mRNA was expressed in a gradient that declined in a rostrocaudal manner. In diencephalon, AptCB1R RNA probe weakly stained the central-posterior (CP) and prepacemaker (PPn) nuclei. In mesencephalon, AptCB1R mRNA is expressed in deep layers of the dorsal (electrosensory) torus semicircularis (TSd). In hindbrain, AptCB1R RNA probe weakly labeled inhibitory interneurons in the electrosensory lateral line lobe (ELL). Unlike mammals, only few cerebellar granule cells expressed AptCB1R transcripts and these were located in the center of eminentia granularis pars posterior (EGp), a cerebellar region involved in feedback to ELL.

Organization of afferents to the striatopallidal systems in the fire-bellied toad Bombina orientalis

The cerebral hemispheres of amphibians display paired dorsal and ventral striatum (commonly referred to as striatum proper and nucleus accumbens, respectively). Each striatal region is proposed to be closely associated with a pallidal structure located caudal to it to form a striatopallidal system. In the present study, afferents to the dorsal and ventral striatopallidal systems of the fire-bellied toad (Bombina orientalis) were investigated using the neuronal tracer biocytin. A quantitative analysis of the topographical distribution of afferent neurons from the thalamus and posterior tubercle/ventral tegmentum was emphasised. The main results show that inputs to the two striatopallidal systems originate from distinct dorsal thalamic nuclei, with dorsal and ventral striatopallidal afferent neurons favouring strongly the lateral/central and anterior thalamic nuclei, respectively. However, afferent neuron distribution in the dorsal thalamus does not differ in the rostrocaudal axis of the brain. Afferent neurons from the posterior tubercle and ventral tegmentum, on the other hand, are organised topographically along the rostrocaudal axis. About 85 % of afferent neurons to the dorsal striatopallidal system are located rostrally in the posterior tubercle, while 75 % of afferent neurons to the ventral striatopallidal system are found more caudally in the ventral tegmentum. This difference is statistically significant and confirms the presence of distinct mesostriatal pathways in an amphibian. These findings demonstrate that an amphibian brain displays striatopallidal systems integrating parallel streams of sensory information potentially under the influence of distinct ascending mesostriatal pathways.

What's in a name? Considerations of homologies and nomenclature for vertebrate social behavior networks

Behavioral neuroendocrinology is an integrative discipline that spans a wide range of taxa and neural systems, and thus the appropriate designation of homology (sameness) across taxa is critical for clear communication and extrapolation of findings from one taxon to another. In the present review we address issues of homology that relate to neural circuits of social behavior and associated systems that mediate reward and aversion. We first address a variety of issues related to the so-called "social behavior network" (SBN), including homologies that are only partial (e.g., whereas the preoptic area of fish and amphibians contains the major vasopressin-oxytocin cell groups, these populations lie in the hypothalamus of other vertebrates). We also discuss recent evidence that clarifies anterior hypothalamus and periaqueductal gray homologies in birds. Finally, we discuss an expanded network model, the "social decision-making network" (SDM) which includes the mesolimbic dopamine system and other structures that provide an interface between the mesolimbic system and the SBN. This expanded model is strongly supported in mammals, based on a wide variety of evidence. However, it is not yet clear how readily the SDM can be applied as a pan-vertebrate model, given insufficient data on numerous proposed homologies and a lack of social behavior data for SDM components (beyond the SBN nodes) for amphibians, reptiles or fish. Functions of SDM components are also poorly known for birds. Nonetheless, we contend that the SDM model provides a very sound and important framework for the testing of many hypotheses in nonmammalian vertebrates.

Neuroanatomical organization of the cholinergic system in the central nervous system of a basal actinopterygian fish, the senegal bichir Polypterus senegalus

Polypterid bony fishes are believed to be basal to other living ray-finned fishes, and their brain organization is therefore critical in providing information as to primitive neural characters that existed in the earliest ray-finned fishes. The cholinergic system has been characterized in more advanced ray-finned fishes, but not in polypterids. In order to establish which cholinergic neural centers characterized the earliest ray-finned fishes, the distribution of choline acetyltransferase (ChAT) is described in Polypterus and compared with the distribution of this molecule in other ray-finned fishes. Cell groups immunoreactive for ChAT were observed in the hypothalamus, the habenula, the optic tectum, the isthmus, the cranial motor nuclei, and the spinal motor column. Cholinergic fibers were observed in both the telencephalic pallium and the subpallium, in the thalamus and pretectum, in the optic tectum and torus semicircularis, in the mesencephalic tegmentum, in the cerebellar crest, in the solitary nucleus, and in the dorsal column nuclei. Comparison of the data within a segmental neuromeric context indicates that the cholinergic system in polypterid fishes is generally similar to that in other ray-finned fishes, but cholinergic-positive neurons in the pallium and subpallium, and in the thalamus and cerebellum, of teleosts appear to have evolved following the separation of polypterids and other ray-finned fishes.

Assessments of the effects of nicotine and ketamine using tyrosine hydroxylase-green fluorescent protein transgenic zebrafish as biosensors

Transgenic zebrafish are a common vertebrate model system for the study of addictive behavior. In the present study, plasmid constructs containing green fluorescent protein (GFP) and the promoter of tyrosine hydroxylase (TH), a key synthetic enzyme for catecholamines, were produced. The TH-GFP constructs were microinjected into zebrafish embryonic cells. Three days post-fertilization, GFP began expressing in distinct catecholaminergic areas. The TH-GFP transgenic zebrafish were employed as live biosensors to test the effects of the commonly abused drugs nicotine and ketamine. First, locomotion assays were used to study the general excitatory effects of the drugs. Maximal locomotor activity was obtained after treatment with a high concentration of nicotine (10 μM), but with a much lower concentration of ketamine (0.1 μM). Second, TH protein levels in zebrafish brains were assessed by Western blot. TH protein levels were significantly increased, with maximal protein levels found after treatment with the same drug concentrations that gave maximal locomotor activity. Importantly, analysis of GFP in the zebrafish catecholaminergic areas revealed the same expression patterns as was obtained by Western blot. The present results indicate that increased locomotor activity can be correlated to TH protein expression, as indicated by Western blot and expression of TH-GFP. We have shown that TH-GFP expression is a reliable method to show the effects of drugs on TH expression that may be employed as a novel high-throughput live biosensor for screening drugs of abuse.

Evolution of gnathostome prodynorphin and proenkephalin: characterization of a shark proenkephalin and prodynorphin cDNAs

Analyses of prodynorphin and proenkephalin cDNAs cloned from the central nervous system of the shark, Heterodontus portusjacksoni, provided additional evidence that these two opioid precursor-coding genes were most likely directly derived from a common ancestral gene. The two cDNAs could be aligned by inserting only seven gaps. The prodynorphin cDNA encodes five opioid sequences which could be aligned to opioid positions B through F in the proenkephalin cDNA. The sequence identity within the opioid positions was 59% at the amino acid level. Shark α-neo-endorphin, dynorphin A, and dynorphin B have amino acid motifs in common with shark met-enkephalin-8, and shark proenkephalin opioid positions E and F, respectively, which have not been observed in other gnathostome prodynorphin and proenkephalin precursor sequences. Shark prodynorphin encodes both kappa (α-neo-endorphin, dynorphin A, and dynorphin B) and delta (met-enkephalin and leu-enkephalin) opioid sequences. Mixed function prodynorphin precursors (encoding both enkephalins and dynorphins) are also found in representatives of the teleost fishes, lungfishes, and amphibians. It appears that only mammals evolved a prodynorphin precursor that exclusively encodes kappa opioid agonists (dynorphins).

New insights into the neuroanatomical distribution and phylogeny of opioids and POMC-derived peptides in fish

This review re-evaluates the use of immunological probes to map enkephalinergic, dynorphinergic, and endorphinergic circuits in the CNS of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes in light of the characterization of proenkephalin, prodynorphin, and POMC sequences from representatives of these groups of fish over the past 20 years. The use of α-MSH specific antisera is a reliable method for detecting POMC immunopositive cell bodies and fibers. Since α-MSH and β-endorphin are co-localized in the same neurons, these studies also reveal the distribution of endorphinergic networks. Met-enkephalin specific antisera can be used to detect enkephalinergic circuits in the CNS of gnathostomes because of the ubiquitous presence of this pentapeptide in the proenkephalin sequences of gnathostomes. However, the use of leu-enkephalin specific antisera to detect enkephalinergic networks is more problematic. While this immunological probe is appropriate for analyzing enkephalinergic networks in mammals and perhaps teleosts, for the lungfishes and cartilaginous fishes this probe is more likely able to detect dynorphinergic circuits. In this regard, there is a need to re-examine dynorphinergic networks in non-mammalian gnathostomes by using species specific antisera directed against dynorphin end-products.

Lungfish aestivating activities are locked in distinct encephalic γ-aminobutyric acid type A receptor α subunits

Ammonia in dipnoans plays a crucial role on neuronal homeostasis, especially for those brain areas that maintain torpor and awakening states in equilibrium. In the present study, specific α subunits of the major neuroreceptor inhibitory complex (GABA(A) R), which predominated during some phases of aestivation of the lungfish Protopterus annectens, turned out to be key adaptive factors of this species. From the isolation, for the first time, of the encoding sequence for GABA(A) R α₁, α₄ , and α₅ subunits in Protopterus annectens, qPCR and in situ hybridization levels of α₄ transcript in thalamic (P < 0.001) and mesencephalic (P < 0.01) areas proved to be significantly higher during long aestivating maintenance states. Very evident α₅ mRNA levels were detected in diencephalon during short inductive aestivating states, whereas an α₄ /α₁ turnover characterized the arousal state. Contextually, the recovery of physiological activities appeared to be tightly related to an evident up-regulation of α₁ transcripts in telencephalic and cerebellar sites. Surprisingly, TUNEL and amino cupric silver methods corroborated apoptotic and neurodegenerative cellular events, respectively, above all in telencephalon and cerebellum of lungfish exposed to long maintenance aestivating conditions. Overall, these results tend to underlie a novel GABAergic-related ON/OFF molecular switch operating during aestivation of the lungfish, which might have a bearing on sleeping disorders.

Effects of stress and motivation on performing a spatial task

Learning is ubiquitous in the animal kingdom but has been studied extensively in only a handful of species. Moreover, learning studied under laboratory conditions is typically unrelated to the animal's natural environment or life history. Here, we designed a task relevant to the natural behavior of male African cichlid fish (Astatotilapia burtoni), to determine if they could be trained on a spatial task to gain access to females and shelter. We measured both how successfully animals completed this task over time and whether and how immediate early gene and hormone expression profiles were related to success. While training fish in a maze, we measured time to task completion, circulating levels of three key hormones (cortisol, 11-ketotestosterone, and testosterone) and mRNA abundance of seven target genes including three immediate early genes (that served proxies for brain activity) in nine brain regions. Data from our subjects fell naturally into three phenotypes: fish that could be trained (learners), fish that could not be trained (non-learners) and fish that never attempted the task (non-attempters). Learners and non-learners had lower levels of circulating cortisol compared to fish that never attempted the task. Learners had the highest immediate early gene mRNA levels in the homologue of the hippocampus (dorsolateral telencephalon; Dl), lower cortisol (stress) levels and were more motivated to accomplish the task as measured by behavioral observations. Fish that never attempted the task showed the lowest activity within the Dl, high stress levels and little to no apparent motivation. Data from non-learners fell between these two extremes in behavior, stress, and motivation.

Female genomic response to mate information

Females should be choosier than males about prospective mates because of the high costs of inappropriate mating decisions. Both theoretical and empirical studies have identified factors likely to influence female mate choices. However, male-male social interactions also can affect mating decisions, because information about a potential mate can trigger changes in female reproductive physiology. We asked how social information about a preferred male influenced neural activity in females, using immediate early gene (IEG) expression as a proxy for brain activity. A gravid female cichlid fish (Astatotilapia burtoni) chose between two socially equivalent males and then saw fights between these two males in which her preferred male either won or lost. We measured IEG expression levels in several brain nuclei including those in the vertebrate social behavior network (SBN), a collection of brain nuclei known to be important in social behavior. When the female saw her preferred male win a fight, SBN nuclei associated with reproduction were activated, but when she saw her preferred male lose a fight, the lateral septum, a nucleus associated with anxiety, was activated instead. Thus social information alone, independent of actual social interactions, activates specific brain regions that differ significantly depending on what the female sees. In female brains, reproductive centers are activated when she chooses a winner, and anxiety-like response centers are activated when she chooses a loser. These experiments assessing the role of mate-choice information on the brain using a paradigm of successive presentations of mate information suggest ways to understand the consequences of social information on animals using IEG expression.

What do fish make of mirror images?

Fish act aggressively towards their mirror image suggesting that they consider it another individual, whereas in some mammals behavioural response to mirrors may be an evidence of self-recognition. Since fish cannot self-recognize, we asked whether they could distinguish between fighting a mirror image and fighting a real fish. We compared molecular, physiological and behavioural responses in each condition and found large differences in brain gene expression levels. Although neither levels of aggressive behaviour nor circulating androgens differed between these conditions, males fighting a mirror image had higher immediate early gene (IEG) expression in brain areas homologous to the amygdala and hippocampus than controls. Since amygdalar responses are associated with fear and fear conditioning in other species, higher levels of brain activation when fighting a mirror suggest fish experience fear in response to fights with a mirror image. Clearly, the fish recognize something unusual about the mirror image and the differential brain response may reflect a cognitive distinction.

Transgenic zebrafish models of neurodegenerative diseases

Since the introduction of the zebrafish as a model for the study of vertebrate developmental biology, an extensive array of techniques for its experimental manipulation and analysis has been developed. Recently it has become apparent that these powerful methodologies might be deployed in order to elucidate the pathogenesis of human neurodegenerative diseases and to identify candidate therapeutic approaches. In this article, we consider evidence that the zebrafish central nervous system provides an appropriate setting in which to model human neurological disease and we review techniques and resources available for generating transgenic models. We then examine recent publications showing that appropriate phenotypes can be provoked in the zebrafish through transgenic manipulations analogous to genetic abnormalities known to cause human tauopathies, polyglutamine diseases or motor neuron degenerations. These studies show proof of concept that findings in zebrafish models can be applicable to the pathogenic mechanisms underlying human diseases. Consequently, the prospects for providing novel insights into neurodegenerative diseases by exploiting transgenic zebrafish models and discovery-driven approaches seem favorable.

Increased DJ-1 expression under oxidative stress and in Alzheimer's disease brains

Mutations in the DJ-1 gene have been linked to autosomal recessive familial Parkinson's disease. To understand the function of DJ-1, we determined the DJ-1 expression in both zebrafish and post mortem human brains. We found that DJ-1 was expressed early during zebrafish development and throughout adulthood. Knock down (KD) of DJ-1 by injection of morpholino did not cause dramatic morphologic alterations during development, and no loss of dopaminergic neurons was observed in embryos lacking DJ-1. However, DJ-1 KD embryos were more susceptible to programmed cell death. While a slight reduction in staining for islet-1 positive neurons was observed in both DJ-1 KD and H2O2 treated embryos, the number of apoptotic cells was significantly increased in both KD and H2O2 treated embryos. Interestingly, DJ-1 expression was increased in brains of zebrafish under conditions of oxidative stress, indicating that DJ-1 is a part of stress-responsive machinery. Since oxidative stress is one of the major contributors to the development of Alzheimer's disease (AD), we also examined DJ-1 expression in AD brains. Using DJ-1 specific antibodies, we failed to detect a robust staining of DJ-1 in brain tissues from control subjects. However, DJ-1 immunoreactivity was detected in hippocampal pyramidal neurons and astrocytes of AD brains. Therefore, our results strongly suggest that DJ-1 expression is not necessary during zebrafish development but can be induced in zebrafish exposed to oxidative stress and is present in human AD brains.

The goldfish (Carassius auratus) as a model for neuroendocrine signaling

Goldfish (Carassius auratus) are excellent model organisms for the neuroendocrine signaling and the regulation of reproduction in vertebrates. Goldfish also serve as useful model organisms in numerous other fields. In contrast to mammals, teleost fish do not have a median eminence; the anterior pituitary is innervated by numerous neuronal cell types and thus, pituitary hormone release is directly regulated. Here we briefly describe the neuroendocrine control of luteinizing hormone. Stimulation by gonadotropin-releasing hormone and a multitude of classical neurotransmitters and neuropeptides is opposed by the potent inhibitory actions of dopamine. The stimulatory actions of gamma-aminobutyric acid and serotonin are also discussed. We will focus on the development of a cDNA microarray composed of carp and goldfish sequences which has allowed us to examine neurotransmitter-regulated gene expression in the neuroendocrine brain and to investigate potential genomic interactions between these key neurotransmitter systems. We observed that isotocin (fish homologue of oxytocin) and activins are regulated by multiple neurotransmitters, which is discussed in light of their roles in reproduction in other species. We have also found that many novel and uncharacterized goldfish expressed sequence tags in the brain are also regulated by neurotransmitters. Their sites of production and whether they play a role in neuroendocrine signaling and control of reproduction remain to be determined. The transcriptomic tools developed to study reproduction could also be used to advance our understanding of neuroendocrine-immune interactions and the relationship between growth and food intake in fish.