Do teleost fishes possess a homolog of mammalian isocortex?

Analysis of visual discrimination in Japanese eel (Anguilla japonica)

Japanese eels were trained to discriminate between a checkerboard panel and a plain gray panel in a circular pool with three pipes. One of the pipes was open, whereas the others were closed. The correct choice of discriminative stimulus was reinforced by entering the pipe. When the panels were displayed vertically (on the wall), the eels successfully learned discrimination, but they were unable to acquire the task when the panels were presented horizontally (on the floor). Enucleation of the retina impaired discrimination, whereas ablation of the olfactory plates did not. In the second experiment, the eels underwent three tests after discriminative training with vertical stimuli displayed. When plain black or white panels were presented instead of a checkerboard panel, the eels could not discriminate. Thus, the discriminative stimulus must have both black and white components. The eels exhibited a generalization gradient along the fines of the checkerboard. Finally, the pallium was damaged by coagulation, and the eels did not maintain the discrimination after the lesions. The behavioral deficits were classified into successful relearning and no relearning. Damage to the dorso-lateral (DL) or dorso-central (DC) pallium was associated with severe impairment (no relearning), although it was not possible to isolate the particular brain area or combination of brain areas which was required. The DL damage probably causes memory deficits, but the deficits caused by the DC damage might be motor or motivational deficits.

The evolutionary origins of the Global Neuronal Workspace in vertebrates

The Global Neuronal Workspace theory of consciousness offers an explicit functional architecture that relates consciousness to cognitive abilities such as perception, attention, memory, and evaluation. We show that the functional architecture of the Global Neuronal Workspace, which is based mainly on human studies, corresponds to the cognitive-affective architecture proposed by the Unlimited Associative Learning theory that describes minimal consciousness. However, we suggest that when applied to basal vertebrates, both models require important modifications to accommodate what has been learned about the evolution of the vertebrate brain. Most importantly, comparative studies suggest that in basal vertebrates, the Global Neuronal Workspace is instantiated by the event memory system found in the hippocampal homolog. This proposal has testable predictions and implications for understanding hippocampal and cortical functions, the evolutionary relations between memory and consciousness, and the evolution of unified perception.

Boundary vector cells in the goldfish central telencephalon encode spatial information

Navigation is one of the most fundamental cognitive skills for the survival of fish, the largest vertebrate class, and almost all other animal classes. Space encoding in single neurons is a critical component of the neural basis of navigation. To study this fundamental cognitive component in fish, we recorded the activity of neurons in the central area of the goldfish telencephalon while the fish were freely navigating in a quasi-2D water tank embedded in a 3D environment. We found spatially modulated neurons with firing patterns that gradually decreased with the distance of the fish from a boundary in each cell's preferred direction, resembling the boundary vector cells found in the mammalian subiculum. Many of these cells exhibited beta rhythm oscillations. This type of spatial representation in fish brains is unique among space-encoding cells in vertebrates and provides insights into spatial cognition in this lineage.

The Cytoarchitecture of the Tectal-Related Pallium of Squirrelfish, Holocentrus sp

The squirrelfish, which live in visually complex coral reefs, have very large eyes and a special dual-system “day and night vision” retina. They also have atypical expansions of brain areas involved in processing visual information. The midbrain tectum sends information via diencephalic relay to two enlarged dorsal telencephalic regions. The latter include a superficial dorsal/lateral “cortex-like area” of small to medium-sized cells [area dorsalis telencephali, pars lateralis-dorsal region (dorsal segment); Dld1] which projects to an underlying dorsocentral region of relatively large cells (the area dorsalis telencephali, pars centralis-dorsal region; Dcd) which in turn reconnects with the tectum. Additionally, the cerebellum is also involved in this pathway. The hypertrophied pallial regions, termed the tectal-related pallium (TRP), most likely exert major influences on a variety of visually-related sensorimotor systems. This research aimed at better establishing the cellular structures and possible connections within the TRP. Nissl and rapid Golgi staining, biotinylated dextran amine tracing and cell-filling, and electron microscopy were used in this study. For gross observation of the pallial areas and plotting of the study sites, a mini-atlas of transverse and horizontal sections was constructed. This research better documented the known cellular elements of the TRP and defined two novel cell types. Species differences in the TRP may be related to possible differences in behavior and ecology.

The neuroecology of the water-to-land transition and the evolution of the vertebrate brain

The water-to-land transition in vertebrate evolution offers an unusual opportunity to consider computational affordances of a new ecology for the brain. All sensory modalities are changed, particularly a greatly enlarged visual sensorium owing to air versus water as a medium, and expanded by mobile eyes and neck. The multiplication of limbs, as evolved to exploit aspects of life on land, is a comparable computational challenge. As the total mass of living organisms on land is a hundredfold larger than the mass underwater, computational improvements promise great rewards. In water, the midbrain tectum coordinates approach/avoid decisions, contextualized by water flow and by the animal's body state and learning. On land, the relative motions of sensory surfaces and effectors must be resolved, adding on computational architectures from the dorsal pallium, such as the parietal cortex. For the large-brained and long-living denizens of land, making the right decision when the wrong one means death may be the basis of planning, which allows animals to learn from hypothetical experience before enactment. Integration of value-weighted, memorized panoramas in basal ganglia/frontal cortex circuitry, with allocentric cognitive maps of the hippocampus and its associated cortices becomes a cognitive habit-to-plan transition as substantial as the change in ecology. This article is part of the theme issue 'Systems neuroscience through the lens of evolutionary theory'.

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.

Identifying Amygdala-Like Territories in Scyliorhinus canicula (Chondrichthyan): Evidence for a Pallial Amygdala

To identify the putative amygdalar complex in cartilaginous fishes, our first step was to obtain evidence that supports the existence of a pallial amygdala in the catshark Scyliorhinus canicula, at present the prevailing chondrichthyan model in comparative neurobiology and developmental biology. To this end, we analyzed the organization of the lateral walls of the telencephalic hemispheres of adults, juveniles, and early prehatching embryos by immunohistochemistry against tyrosine hydroxylase (TH), somatostatin (SOM), Pax6, serotonin (5HT), substance P (SP), and Met-enkephalin (MetEnk), calbindin-28k (CB), and calretinin (CR), and by in situ hybridization against regulatory genes such as Tbr1, Lhx9, Emx1, and Dlx2. Our data were integrated with those available from the literature related to the secondary olfactory projections in this shark species. We have characterized two possible amygdalar territories. One, which may represent a ventropallial component, was identified by its chemical signature (moderate density of Pax6-ir cells, scarce TH-ir and SOM-ir cells, and absence of CR-ir and CB-ir cells) and gene expressions (Tbr1 and Lhx9 expressions in an Emx1 negative domain, as the ventral pallium of amniotes). It is perhaps comparable to the lateral amygdala of amphibians and the pallial amygdala of teleosts. The second was a territory related to the pallial-subpallial boundary with abundant Pax6-ir and CR-ir cells, and 5HT-ir, SP-ir, and MetEnk-ir fibers capping dorsally the area superficialis basalis. This olfactory-related region at the neighborhood of the pallial-subpallial boundary may represent a subpallial amygdala subdivision that possibly contains migrated cells of ventropallial origin.

Impairments in spatial learning by telencephalic lesions in Japanese eels (Anguilla japonica)

This study aimed to use Japanese eels (Anguilla japonica) as subjects to examine the effects of telencephalic lesions on spatial learning. Ten Japanese eels were trained on a Morris-type spatial learning task. Four pipes were placed in a pool; however, the eels could hide in only one of these pipes. The learning task ensured that the eels learned about the position of the open pipe. Subsequently, their telencephalons were damaged. The lesioned eels could not maintain their learning and demonstrated deficits in re-learning as some of them were unable to relearn the task. An analysis of the lesion sizes revealed that while damage to the dorsolateral pallium correlates with maintenance of learning, damage to the dorsomedial pallium correlates with re-learning.

The Lamprey Forebrain - Evolutionary Implications

The forebrain plays a critical role in a broad range of neural processes encompassing sensory integration and initiation/selection of behaviour. The forebrain functions through an interaction between different cortical areas, the thalamus, the basal ganglia with the dopamine system, and the habenulae. The ambition here is to compare the mammalian forebrain with that of the lamprey representing the oldest now living group of vertebrates, by a review of earlier studies. We show that the lamprey dorsal pallium has a motor, a somatosensory, and a visual area with retinotopic representation. The lamprey pallium was previously thought to be largely olfactory. There is also a detailed similarity between the lamprey and mammals with regard to other forebrain structures like the basal ganglia in which the general organisation, connectivity, transmitters and their receptors, neuropeptides, and expression of ion channels are virtually identical. These initially unexpected results allow for the possibility that many aspects of the basic design of the vertebrate forebrain had evolved before the lamprey diverged from the evolutionary line leading to mammals. Based on a detailed comparison between the mammalian forebrain and that of the lamprey and with due consideration of data from other vertebrate groups, we propose a compelling account of a pan-vertebrate schema for basic forebrain structures, suggesting a common ancestry of over half a billion years of vertebrate evolution.

Neural substrates involved in the cognitive information processing in teleost fish

Over the last few decades, it has been shown that fish, comprising the largest group of vertebrates and in many respects one of the least well studied, possess many cognitive abilities comparable to those of birds and mammals. Despite a plethora of behavioural studies assessing cognition abilities and an abundance of neuroanatomical studies, only few studies have aimed to or in fact identified the neural substrates involved in the processing of cognitive information. In this review, an overview of the currently available studies addressing the joint research topics of cognitive behaviour and neuroscience in teleosts (and elasmobranchs wherever possible) is provided, primarily focusing on two fundamentally different but complementary approaches, i.e. ablation studies and Immediate Early Gene (IEG) analyses. More recently, the latter technique has become one of the most promising methods to visualize neuronal populations activated in specific brain areas, both during a variety of cognitive as well as non-cognition-related tasks. While IEG studies may be more elegant and potentially easier to conduct, only lesion studies can help researchers find out what information animals can learn or recall prior to and following ablation of a particular brain area.

Neural Activation Patterns Associated with Maternal Mouthbrooding and Energetic State in an African Cichlid Fish

Parental care is widespread in the animal kingdom, but for many species, provisioning energetic resources must be balanced with trade-offs between self-promoting and offspring-promoting behaviors. However, little is known about the neural mechanisms underlying these motivational decisions. Mouthbrooding is an extreme form of parental care most common in fishes that provides an ideal opportunity to examine which brain regions are involved in parenting and energetics. The African cichlid fish Astatotilapia burtoni is a maternal mouthbrooder in which females hold developing young inside their mouths for 2 weeks. This brood care makes feeding impossible, so females undergo obligatory starvation. We used immunohistochemistry for the neural activation marker pS6 to examine which brain regions were involved in processing salient information in mouthbrooding, starved, and fed females. We identified brain regions more associated with maternal brood care (TPp, Dc-4/-5), and others reflective of energetic state (Dl-v, NLTi). Most nuclei examined, however, were involved in both maternal care and energetic status. Placement of each of the 16 examined nuclei into these functional categories was supported by node by node comparisons, co-activity networks, hierarchical clustering, and discriminant function analysis. These results reveal which brain regions are involved in parental care and food intake in a species where provisioning is skewed towards the offspring when parental feeding is not possible. This study provides support for both distinct and shared circuitry involved in regulation of maternal care, food intake, and energy balance, and helps put the extreme parental case of mouthbrooding into a comparative and evolutionary context.

Representation of edges, head direction, and swimming kinematics in the brain of freely-navigating fish

Like most animals, the survival of fish depends on navigation in space. This capacity has been documented in behavioral studies that have revealed navigation strategies. However, little is known about how freely swimming fish represent space and locomotion in the brain to enable successful navigation. Using a wireless neural recording system, we measured the activity of single neurons in the goldfish lateral pallium, a brain region known to be involved in spatial memory and navigation, while the fish swam freely in a two-dimensional water tank. We found that cells in the lateral pallium of the goldfish encode the edges of the environment, the fish head direction, the fish swimming speed, and the fish swimming velocity-vector. This study sheds light on how information related to navigation is represented in the brain of fish and addresses the fundamental question of the neural basis of navigation in this group of vertebrates.

The Zebrafish Amygdaloid Complex - Functional Ground Plan, Molecular Delineation, and Everted Topology

In mammals and other tetrapods, a multinuclear forebrain structure, called the amygdala, forms the neuroregulatory core essential for emotion, cognition, and social behavior. Currently, higher circuits of affective behavior in anamniote non-tetrapod vertebrates (“fishes”) are poorly understood, preventing a comprehensive understanding of amygdala evolution. Through molecular characterization and evolutionary-developmental considerations, we delineated the complex amygdala ground plan of zebrafish, whose everted telencephalon has made comparisons to the evaginated forebrains of tetrapods challenging. In this radical paradigm, thirteen telencephalic territories constitute the zebrafish amygdaloid complex and each territory is distinguished by conserved molecular properties and structure-functional relationships with other amygdaloid structures. Central to our paradigm, the study identifies the teleostean amygdaloid nucleus of the lateral olfactory tract (nLOT), an olfactory integrative structure that links dopaminergic telencephalic groups to the amygdala alongside redefining the putative zebrafish olfactory pallium (“Dp”). Molecular characteristics such as the distribution of substance P and the calcium-binding proteins parvalbumin (PV) and calretinin (CR) indicate, that the zebrafish extended centromedial (autonomic and reproductive) amygdala is predominantly located in the GABAergic and isl1-negative territory. Like in tetrapods, medial amygdaloid (MeA) nuclei are defined by the presence of substance P immunoreactive fibers and calretinin-positive neurons, whereas central amygdaloid (CeA) nuclei lack these characteristics. A detailed comparison of lhx5-driven and vGLut2a-driven GFP in transgenic reporter lines revealed ancestral topological relationships between the thalamic eminence (EmT), the medial amygdala (MeA), the nLOT, and the integrative olfactory pallium. Thus, the study explains how the zebrafish amygdala and the complexly everted telencephalon topologically relate to the corresponding structures in mammals indicating that an elaborate amygdala ground plan evolved early in vertebrates, in a common ancestor of teleosts and tetrapods.

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.

Expression of RNA-binding protein Rbfox1l demarcates a restricted population of dorsal telencephalic neurons within the adult zebrafish brain

Rbfox RNA-binding proteins are expressed in the adult mammalian brain and are required for proper brain development and function. Studies in mice and humans have implicated Rbfox1/RBFOX1 in autism, neuronal excitation and epilepsy, and Rbfox2/RBFOX2 in cerebellar development. The zebrafish has emerged as a prominent model system for brain study, possessing neuroanatomical conservation with mammals and an extensive capacity for adult neurogenesis and plasticity. In this study, we characterize Rbfox1l and Rbfox2 expression in the adult zebrafish brain. While Rbfox2 is expressed broadly, Rbfox1l is expressed in restricted populations of neurons in the dorsal telencephalon and cerebellum. In the dorsal telencephalon, Rbfox1l is expressed in a specific population of neurons spanning Dm and Dc regions. In the cerebellum, Rbfox1l and Rbfox2 are expressed in the Purkinje cell layer, reminiscent of Rbfox1 and Rbfox2 expression in the mammalian cerebellum. Our findings motivate future studies of Rbfox function in the zebrafish brain.

BDNF, Brain, and Regeneration: Insights from Zebrafish

Zebrafish (Danio rerio) is a teleost fish widely accepted as a model organism for neuroscientific studies. The adults show common basic vertebrate brain structures, together with similar key neuroanatomical and neurochemical pathways of relevance to human diseases. However, the brain of adult zebrafish possesses, differently from mammals, intense neurogenic activity, which can be correlated with high regenerative properties. Brain derived neurotrophic factor (BDNF), a member of the neurotrophin family, has multiple roles in the brain, due also to the existence of several biologically active isoforms, that interact with different types of receptors. BDNF is well conserved in the vertebrate evolution, with the primary amino acid sequences of zebrafish and human BDNF being 91% identical. Here, we review the available literature regarding BDNF in the vertebrate brain and the potential involvement of BDNF in telencephalic regeneration after injury, with particular emphasis to the zebrafish. Finally, we highlight the potential of the zebrafish brain as a valuable model to add new insights on future BDNF studies.

Life-Long Neurogenic Activity of Individual Neural Stem Cells and Continuous Growth Establish an Outside-In Architecture in the Teleost Pallium

Spatiotemporal variations of neurogenesis are thought to account for the evolution of brain shape. In the dorsal telencephalon (pallium) of vertebrates, it remains unresolved which ancestral neurogenesis mode prefigures the highly divergent cytoarchitectures that are seen in extant species. To gain insight into this question, we developed genetic tools to generate here the first 4-dimensional (3D + birthdating time) map of pallium construction in the adult teleost zebrafish. Using a Tet-On-based genetic birthdating strategy, we identify a “sequential stacking” construction mode where neurons derived from the zebrafish pallial germinal zone arrange in outside-in, age-related layers from a central core generated during embryogenesis. We obtained no evidence for overt radial or tangential neuronal migrations. Cre-lox-mediated tracing, which included following Brainbow clones, further demonstrates that this process is sustained by the persistent neurogenic activity of individual pallial neural stem cells (NSCs) from embryo to adult. Together, these data demonstrate that the spatiotemporal control of NSC activity is an important driver of the macroarchitecture of the zebrafish adult pallium. This simple mode of pallium construction shares distinct traits with pallial genesis in mammals and non-mammalian amniotes such as birds or reptiles, suggesting that it may exemplify the basal layout from which vertebrate pallial architectures were elaborated.

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Neurons of the teleost pallium are arranged in concentric age-dependent layers

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Neurons of the central pallial domain, Dc, are born during embryogenesis

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Most pallial neurons are generated from ventricular her4-positive radial glia

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The majority of individual pallial radial glia are neurogenic throughout life

The Cytoarchitecture of the Telencephalon of Betta Splendens Regan 1910 (Perciformes: Anabantoidei) with a Stereological Approach to the Supracommissural and Postcommissural Nuclei

Teleostean fish brains are useful models to study cellular and functional specializations along the phylogenesis. The Betta splendens Regan 1910 (Siamese fighting fish; Perciformes:Anabantoidei) is known for its aggressive display, courtship behavior, nest building, and offspring care. Here, we present novel and detailed data about the cytoarchitecture of the olfactory bulb and the telencephalic hemispheres of this fish. The hematoxylin-eosin and Nissl techniques served to identify brain nuclei (n = 19 males and n = 21 females) and for the stereological evaluation of the numerical density of cells and the proportion of neurons and glial cells in the ventral telencephalon supracommissural (Vs) and postcommissural (Vp) nuclei of adult males and females. These nuclei are putative homologs of the sexually dimorphic medial amygdala in mammals. The olfactory bulb of Betta splendens consists of 5 concentrically arranged layers plus ganglion cells of the terminal nerves. The dorsal telencephalon consists of 16 different cell groups. The ventral telencephalon has 8 nuclei, plus the lateral septal organ and the nuclei of the preoptic area forming an anatomical continuum. The rostrocaudal extent of the Vs and Vp is not different between sexes. In both nuclei, the proportion of neurons to glial cells is approximately 2:1 and the density of neurons and glial cells is not different between sexes. These morphological findings can subserve future research on the brain function of the Betta splendens and the search for neural sex differences in other central areas of this same species, in other teleost species, or yet in other related vertebrate group. Anat Rec, 00:000-000, 2017. © 2017 Wiley Periodicals, Inc. Anat Rec, 301:88-110, 2018. © 2017 Wiley Periodicals, Inc.

Expression of regulatory genes in the embryonic brain of a lizard and implications for understanding pallial organization and evolution

The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9, and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six‐part model of pallial divisions. The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels. Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1‐expressing/Lhx9‐negative area at the front of the telencephalon, resembling the avian hyperpallium. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. These data open new venues for understanding the organization and evolution of the pallium.

Endogenous orienting in the archer fish

The literature has long emphasized the neocortex's role in volitional processes. In this work, we examined endogenous orienting in an evolutionarily older species, the archer fish, which lacks neocortex-like cells. We used Posner's classic endogenous cuing task, in which a centrally presented, spatially informative cue is followed by a target. The fish responded to the target by shooting a stream of water at it. Interestingly, the fish demonstrated a human-like "volitional" facilitation effect: their reaction times to targets that appeared on the side indicated by the precue were faster than their reaction times to targets on the opposite side. The fish also exhibited inhibition of return, an aftermath of orienting that commonly emerges only in reflexive orienting tasks in human participants. We believe that this pattern demonstrates the acquisition of an arbitrary connection between spatial orienting and a nonspatial feature of a centrally presented stimulus in nonprimate species. In the literature on human attention, orienting in response to such contingencies has been strongly associated with volitional control. We discuss the implications of these results for the evolution of orienting, and for the study of volitional processes in all species, including humans.

Three urocortins in medaka: identification and spatial expression in the central nervous system

The urocortin (UCN) group of neuropeptides includes urocortin 1/sauvagine/urotensin 1 (UTS1), urocortin 2 (UCN2) and urocortin 3 (UCN3). In recent years, evidence has accumulated showing that UCNs play pivotal roles in mediating stress response and anxiety in mammals. Evidence has also emerged regarding the evolutionary conservation of UCNs in vertebrates, but very little information is available about UCNs in non-mammalian vertebrates. Indeed, at present, there are no reports of the empirical identification of ucn2 in non-mammalian vertebrates or of the distribution of ucn2 and ucn3 expression in the adult central nervous system (CNS) of these animals. To gain insight into the evolutionary nature of UCNs in vertebrates, we cloned uts1, ucn2 and ucn3 in a teleost fish, medaka and examined the spatial expression of these genes in the adult brain and spinal cord. Although all known UCN2 genes except those in rodents have been reported to likely lack the necessary structural features to produce a functional pre-pro-protein, all three UCN genes in medaka, including ucn2, displayed all of these features, suggesting their functionality. The three UCN genes exhibited distinct spatial expression patterns in the medaka brain: uts1 was primarily expressed in broad regions of the dorsal telencephalon, ucn2 was expressed in restricted regions of the thalamus and brainstem and ucn3 was expressed in discrete nuclei throughout many regions of the brain. We also found that these genes were all expressed throughout the medaka spinal cord, each with a distinct spatial pattern. Given that many of these regions have been implicated in stress responses and anxiety, the three UCNs may serve distinct physiological roles in the medaka CNS, including those involved in stress and anxiety, as shown in the mammalian CNS.

BDNF Expression in Larval and Adult Zebrafish Brain: Distribution and Cell Identification

Brain-derived neurotrophic factor (BDNF), a member of the neurotrophin family, has emerged as an active mediator in many essential functions in the central nervous system of mammals. BDNF plays significant roles in neurogenesis, neuronal maturation and/or synaptic plasticity and is involved in cognitive functions such as learning and memory. Despite the vast literature present in mammals, studies devoted to BDNF in the brain of other animal models are scarse. Zebrafish is a teleost fish widely known for developmental genetic studies and is emerging as model for translational neuroscience research. In addition, its brain shows many sites of adult neurogenesis allowing higher regenerative properties after traumatic injuries. To add further knowledge on neurotrophic factors in vertebrate brain models, we decided to determine the distribution of bdnf mRNAs in the larval and adult zebrafish brain and to characterize the phenotype of cells expressing bdnf mRNAs by means of double staining studies. Our results showed that bdnf mRNAs were widely expressed in the brain of 7 days old larvae and throughout the whole brain of mature female and male zebrafish. In adults, bdnf mRNAs were mainly observed in the dorsal telencephalon, preoptic area, dorsal thalamus, posterior tuberculum, hypothalamus, synencephalon, optic tectum and medulla oblongata. By combining immunohistochemistry with in situ hybridization, we showed that bdnf mRNAs were never expressed by radial glial cells or proliferating cells. By contrast, bdnf transcripts were expressed in cells with neuronal phenotype in all brain regions investigated. Our results provide the first demonstration that the brain of zebrafish expresses bdnf mRNAs in neurons and open new fields of research on the role of the BDNF factor in brain mechanisms in normal and brain repairs situations.

Fish do not feel pain and its implications for understanding phenomenal consciousness

Phenomenal consciousness or the subjective experience of feeling sensory stimuli is fundamental to human existence. Because of the ubiquity of their subjective experiences, humans seem to readily accept the anthropomorphic extension of these mental states to other animals. Humans will typically extrapolate feelings of pain to animals if they respond physiologically and behaviourally to noxious stimuli. The alternative view that fish instead respond to noxious stimuli reflexly and with a limited behavioural repertoire is defended within the context of our current understanding of the neuroanatomy and neurophysiology of mental states. Consequently, a set of fundamental properties of neural tissue necessary for feeling pain or experiencing affective states in vertebrates is proposed. While mammals and birds possess the prerequisite neural architecture for phenomenal consciousness, it is concluded that fish lack these essential characteristics and hence do not feel pain.

What can fish brains tell us about visual perception?

Fish are a complex taxonomic group, whose diversity and distance from other vertebrates well suits the comparative investigation of brain and behavior: in fish species we observe substantial differences with respect to the telencephalic organization of other vertebrates and an astonishing variety in the development and complexity of pallial structures. We will concentrate on the contribution of research on fish behavioral biology for the understanding of the evolution of the visual system. We shall review evidence concerning perceptual effects that reflect fundamental principles of the visual system functioning, highlighting the similarities and differences between distant fish groups and with other vertebrates. We will focus on perceptual effects reflecting some of the main tasks that the visual system must attain. In particular, we will deal with subjective contours and optical illusions, invariance effects, second order motion and biological motion and, finally, perceptual binding of object properties in a unified higher level representation.

Who would have thought that 'Jaws' also has brains? Cognitive functions in elasmobranchs

Adaptation of brain structures, function and higher cognitive abilities most likely have contributed significantly to the evolutionary success of elasmobranchs, but these traits remain poorly studied when compared to other vertebrates, specifically mammals. While the pallium of non-mammalian vertebrates lacks the mammalian neocortical organization responsible for all cognitive abilities of mammals, several behavioural and neuroanatomical studies in recent years have clearly demonstrated that elasmobranchs, just like teleosts and other non-mammalian vertebrates, can nonetheless solve a multitude of cognitive tasks. Sharks and rays can learn and habituate, possess spatial memory; can orient according to different orientation strategies, remember spatial and discrimination tasks for extended periods of time, use tools; can imitate and learn from others, distinguish between conspecifics and heterospecifics, discriminate between either visual objects or electrical fields; can categorize visual objects and perceive illusory contours as well as bilateral symmetry. At least some neural correlates seem to be located in the telencephalon, with some pallial regions matching potentially homologous areas in other vertebrates where similar functions are being processed. Results of these studies indicate that the assessed cognitive abilities in elasmobranchs are as well developed as in teleosts or other vertebrates, aiding them in fundamental activities such as food retrieval, predator avoidance, mate choice and habitat selection.

The shark Chiloscyllium griseum can orient using turn responses before and after partial telencephalon ablation

This study assessed spatial memory and orientation strategies in Chiloscyllium griseum. In the presence of visual landmarks, six sharks were trained in a fixed turn response. Group 1 started from two possible compartments approaching two goal locations, while group 2 started from and approached only one location, respectively. The learning criterion was reached within 9 ± 5.29 (group 1) and 8.3 ± 3.51 sessions (group 2). Transfer tests revealed that sharks had applied a direction strategy, possibly in combination with some form of place learning. Without visual cues, sharks relied solely on the former. To identify the underlying neural substrate(s), telencephalic were lesioned and performance compared before and after surgery. Ablation of the dorsal and medial pallia only had an effect on one shark (group 1), indicating that the acquisition and retention of previously gained knowledge were unaffected in the remaining four individuals. Nonetheless, the shark re-learned the task. In summary, C. griseum can utilize fixed turn responses to navigate to a goal; there is also some evidence for the use of external visual landmarks while orienting. Probably, strategies can be used alone or in combination. Neither the dorsal nor medial pallium seems to be responsible for the acquisition and processing of egocentric information.

The pallium and mind/behavior relationships in teleost fishes

Three interrelated pallial areas mediate behaviors reflective of the cognitive and emotional aspects of the teleost mind. The dorsocentral area (Dc) has specific associations with both of the other pallial areas and projects to major lower sensorimotor centers. While Dc generally functions as an output or modulatory component of the pallium, it probably also has integrative features important for certain behaviors. The dorsolateral region (Dl) has dorsal (Dld) and ventral (Dlv) divisions. In association with the dorsal part of Dc, Dld processes visual information via a 'tectal loop' which is hypertrophied in certain coral reef species. The region also receives afferents related to other modalities. Functionally, Dld resembles the tetrapod sensory neocortex. Anatomical and behavioral data (i.e. involvement in spatial and temporal learning) strongly suggest that Dlv is homologous to the tetrapod hippocampus. The dorsal part of the dorsomedial area (Dmd) processes acoustic, lateral line, gustatory, and multimodal information. It has reciprocal connections with Dld such that the Dmd and Dld together can be considered the teleost nonolfactory 'sensory pallium'. Behavioral studies indicate that Dmd creates the 'fear' necessary for defense/escape and avoidance behaviors and controls several components of species-typical sexual and aggressive behavior (responsiveness, behavioral sequencing, and aspects of social cognition). While the functional results generally support the anatomical evidence that Dmd is homologous to the tetrapod amygdala, a case can also be made that Dmd has 'sensory neocortex-like' features. Understanding the interrelationships of Dl, Dmd, and Dc seems a necessary 'next step' in the identification of the neural processes responsible for mental experiences such as those of a unified sensory experience (Umwelt) or of feelings of discomfort versus well-being.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.