Connections of the lateral and medial divisions of the goldfish telencephalic pallium

Adult sex change leads to extensive forebrain reorganization in clownfish

BACKGROUND

Sexual differentiation of the brain occurs in all major vertebrate lineages but is not well understood at a molecular and cellular level. Unlike most vertebrates, sex-changing fishes have the remarkable ability to change reproductive sex during adulthood in response to social stimuli, offering a unique opportunity to understand mechanisms by which the nervous system can initiate and coordinate sexual differentiation.

METHODS

This study explores sexual differentiation of the forebrain using single nucleus RNA-sequencing in the anemonefish Amphiprion ocellaris, producing the first cellular atlas of a sex-changing brain.

RESULTS

We uncover extensive sex differences in cell type-specific gene expression, relative proportions of cells, baseline neuronal excitation, and predicted inter-neuronal communication. Additionally, we identify the cholecystokinin, galanin, and estrogen systems as central molecular axes of sexual differentiation. Supported by these findings, we propose a model of sexual differentiation in the conserved vertebrate social decision-making network spanning multiple subtypes of neurons and glia, including neuronal subpopulations within the preoptic area that are positioned to regulate gonadal differentiation.

CONCLUSIONS

This work deepens our understanding of sexual differentiation in the vertebrate brain and defines a rich suite of molecular and cellular pathways that differentiate during adult sex change in anemonefish.

Label-free multiphoton imaging reveals volumetric shifts across development in sensory-related brain regions of a miniature transparent vertebrate

Animals integrate information from different sensory modalities as they mature and perform increasingly complex behaviors. This may parallel differential investment in specific brain regions depending on the demands of changing sensory inputs. To investigate developmental changes in the volume of canonical sensory integration brain regions, we used third harmonic generation imaging for morphometric analysis of forebrain and midbrain regions from 5 to 90 days post fertilization (dpf) in Danionella dracula , a transparent, miniature teleost fish whose brain is optically accessible throughout its lifespan. Relative to whole brain volume, increased volume or investment in telencephalon, a higher order sensory integration center, and torus longitudinalis (TL), a midbrain visuomotor integration center, is relatively consistent from 5 to 30 dpf, until it increases at 60 dpf, followed by another increase at 90 dpf, as animals reach adulthood. In contrast, investment in midbrain optic tectum (TeO), a retinal-recipient target, progressively decreases from 30-90 dpf, whereas investment is relatively consistent across all stages for the midbrain torus semicircularis (TS), a secondary auditory and mechanosensory lateral line center, and the olfactory bulb (OB), a direct target of the olfactory epithelium. In sum, increased investment in higher order integration centers (telencephalon, TL) occurs as juveniles reach adulthood and exhibit more complex cognitive tasks, whereas investment in modality-dominant regions occurs in earlier stages (TeO) or is relatively consistent across development (TS, OB). Complete optical access throughout Danionella 's lifespan provides a unique opportunity to investigate how changing brain structure over development correlates with changes in connectivity, microcircuitry, or behavior.

Does a Vertebrate Morphotype of Pallial Subdivisions Really Exist?

BACKGROUND

Comparative neuroanatomists have long sought to determine which part of the pallium in nonmammals is homologous to the mammalian neocortex. A number of similar connectivity patterns across species have led to the idea that the basic organization of the vertebrate brain is relatively conserved; thus, efforts of the last decades have been focused on determining a vertebrate "morphotype" - a model comprising the characteristics believed to have been present in the last common ancestor of all vertebrates.

SUMMARY

The endeavor to determine the vertebrate morphotype has been riddled with controversies due to the extensive morphological diversity of the pallium among vertebrate taxa. Nonetheless, most proposed scenarios of pallial homology are variants of a common theme where the vertebrate pallium is subdivided into subdivisions homologous to the hippocampus, neocortex, piriform cortex, and amygdala, in a one-to-one manner. We review the rationales of major propositions of pallial homology and identify the source of the discrepancies behind different hypotheses. We consider that a source of discrepancies is the prevailing assumption that there is a single "morphotype of the pallial subdivisions" throughout vertebrates. Instead, pallial subdivisions present in different taxa probably evolved independently in each lineage.

KEY MESSAGES

We encounter discrepancies when we search for a single morphotype of subdivisions across vertebrates. These discrepancies can be resolved by considering that several subdivisions within the pallium were established after the divergence of the different lineages. The differences of pallial organization are especially remarkable between actinopterygians (including teleost fishes) and other vertebrates. Thus, the prevailing notion of a simple one-to-one homology between the mammalian and teleost pallia needs to be reconsidered.

The Dorsal Part of the Anterior Tuberal Nucleus Responds to Auditory Stimulation in Zebrafish (Danio rerio)

The zebrafish, a widely used model in neurobiology, relies on hearing in aquatic environments. Unfortunately, its auditory pathways have mainly been studied in larvae. In this study, we examined the involvement of the anterior tuberal nucleus (AT) in auditory processing in adult zebrafish. Our tract-tracing experiments revealed that the dorsal subdivision of AT is strongly bidirectionally connected to the central nucleus of the torus semicircularis (TSc), a major auditory nucleus in fishes. Immunohistochemical visualization of the ribosomal protein S6 (pS6) phosphorylation to map neural activity in response to auditory stimulation substantiated this finding: the dorsal but not the ventral part of AT responded strongly to auditory stimulation. A similar response to auditory stimulation was present in the TSc but not in the nucleus isthmi, a visual region, which we used as a control for testing if the pS6 activation was specific to the auditory stimulation. We also measured the time course of pS6 phosphorylation, which was previously unreported in teleost fish. After auditory stimulation, we found that pS6 phosphorylation peaked between 100 and 130 min and returned to baseline levels after 190 min. This information will be valuable for the design of future pS6 experiments. Our results suggest an anatomical and functional subdivision of AT, where only the dorsal part connects to the auditory network and processes auditory information.

Spatially resolved cell atlas of the teleost telencephalon and deep homology of the vertebrate forebrain

The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking variations in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely "everted" telencephalon, which has confounded comparisons of their brain regions to other vertebrates. Here we combine spatial transcriptomics and single nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the Mchenga conophorus cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell-types in the fish telencephalon and subpallial, hippocampal, and cortical cell-types in tetrapods, and find support for partial eversion of the teleost telencephalon. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.

Coordination and persistence of aggressive visual communication in Siamese fighting fish

Animals coordinate their behavior with each other during both cooperative and agonistic social interactions. Such coordination often adopts the form of "turn taking", in which the interactive partners alternate the performance of a behavior. Apart from acoustic communication, how turn taking between animals is coordinated is not well understood. Furthermore, the neural substrates that regulate persistence in engaging in social interactions are poorly studied. Here, we use Siamese fighting fish ( Betta splendens ), to study visually-driven turn-taking aggressive behavior. Using encounters with conspecifics and with animations, we characterize the dynamic visual features of an opponent and the behavioral sequences that drive turn taking. Through a brain-wide screen of neuronal activity during coordinated and persistent aggressive behavior, followed by targeted brain lesions, we find that the caudal portion of the dorsomedial telencephalon, an amygdala-like region, promotes persistent participation in aggressive interactions, yet is not necessary for coordination. Our work highlights how dynamic visual cues shape the rhythm of social interactions at multiple timescales, and points to the pallial amygdala as a region controlling engagement in such interactions. These results suggest an evolutionarily conserved role of the vertebrate pallial amygdala in regulating the persistence of emotional states.

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.

Identification of olfactory alarm substances in zebrafish

Escaping from danger is one of the most fundamental survival behaviors for animals. Most freshwater fishes display olfactory alarm reactions in which an injured fish releases putative alarm substances from the skin to notify its shoaling company about the presence of danger. Here, we identified two small compounds in zebrafish skin extract, designated as ostariopterin and daniol sulfate. Ostariopterin is a pterin derivative commonly produced in many freshwater fishes belonging to the Ostariophysi superorder. Daniol sulfate is a novel sulfated bile alcohol specifically present in the Danio species, including zebrafish. Ostariopterin and daniol sulfate activate distinct glomeruli in the olfactory bulb. Zebrafish display robust alarm reactions, composed of darting, freezing, and bottom dwelling, only when they are concomitantly stimulated with ostariopterin and daniol sulfate. These results demonstrate that the fish alarm reaction is driven through a coincidence detection mechanism of the two compounds along the olfactory neural circuitry.

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.

Genoarchitectonics of the larval zebrafish diencephalon

The brain is spatially organized into subdivisions, nuclei and areas, which often correspond to functional and developmental units. A segmentation of brain regions in the form of a consensus atlas facilitates mechanistic studies and is a prerequisite for sharing information among neuroanatomists. Gene expression patterns objectively delineate boundaries between brain regions and provide information about their developmental and evolutionary histories. To generate a detailed molecular map of the larval zebrafish diencephalon, we took advantage of the Max Planck Zebrafish Brain (mapzebrain) atlas, which aligns hundreds of transcript and transgene expression patterns in a shared coordinate system. Inspection and co-visualization of close to 50 marker genes have allowed us to resolve the tripartite prosomeric scaffold of the diencephalon at unprecedented resolution. This approach clarified the genoarchitectonic partitioning of the alar diencephalon into pretectum (alar part of prosomere P1), thalamus (alar part of prosomere P2, with habenula and pineal complex), and prethalamus (alar part of prosomere P3). We further identified the region of the nucleus of the medial longitudinal fasciculus, as well as the posterior and anterior parts of the posterior tuberculum, as molecularly distinct basal parts of prosomeres 1, 2, and 3, respectively. Some of the markers examined allowed us to locate glutamatergic, GABAergic, dopaminergic, serotoninergic, and various neuropeptidergic domains in the larval zebrafish diencephalon. Our molecular neuroanatomical approach has thus (1) yielded an objective and internally consistent interpretation of the prosomere boundaries within the zebrafish forebrain; has (2) produced a list of markers, which in sparse combinations label the subdivisions of the diencephalon; and is (3) setting the stage for further functional and developmental studies in this vertebrate brain.

Afferent and efferent connections of the secondary general visceral sensory nucleus in goldfish

The secondary general visceral sensory nucleus (SVN) receives ascending fibers from the commissural nucleus of Cajal (NCC), or the primary general visceral sensoru in the medulla oblongata of teleosts. However, the full set of fiber connections of the SVN have been studied only in the Nile tilapia. We have investigated the connections of the SVN in goldfish by tracer injection experiments to the nucleus. We paid special attention to the possible presence of spinal afferents, since the spinal cord projects to the lateral parabrachial nucleus, or the presumed homologue of SVN, in mammals. We found that the SVN indeed receives spinal projections. Spinal terminals were restricted to a region ventrolaterally adjacent to the terminal zone of NCC fibers, suggesting that the SVN can be subdivided into two subnuclei: the commissural nucleus-recipient (SVNc) and spinal-recipient (SVNsp) subnuclei. Tracer injections to the SVNc and SVNsp as well as reciprocal injections to the diencephalon revealed that both subnuclei project directly to diencephalic structures, such as the posterior thalamic nucleus and nucleus of lateral recess, although diencephalic projections of the SVNsp were rather sparse. The SVNsp appears to send fibers to more wide-spread targets in the preoptic area than the SVNc does. The SVNc projects to the telencephalon, while the SVNsp sends scarce or possibly no fibers to the telencephalon. Another notable difference was that the SVNsp gives rise to massive projections to the dorsal diencephalon (ventromedial thalamic, central posterior thalamic, and periventricular posterior tubercular nuclei). These differential connections of the subnuclei may reflect discrete functional significances of the general visceral sensory information mediated by the medulla oblongata and spinal cord.

Adult sex change leads to extensive forebrain reorganization in clownfish

Sexual differentiation of the brain occurs in all major vertebrate lineages but is not well understood at a molecular and cellular level. Unlike most vertebrates, sex-changing fishes have the remarkable ability to change reproductive sex during adulthood in response to social stimuli, offering a unique opportunity to understand mechanisms by which the nervous system can initiate and coordinate sexual differentiation. This study explores sexual differentiation of the forebrain using single nucleus RNA-sequencing in the anemonefish Amphiprion ocellaris, producing the first cellular atlas of a sex-changing brain. We uncover extensive sex differences in cell type-specific gene expression, relative proportions of cells, baseline neuronal excitation, and predicted inter-neuronal communication. Additionally, we identify the cholecystokinin, galanin, and estrogen systems as central molecular axes of sexual differentiation. Supported by these findings, we propose a model of neurosexual differentiation in the conserved vertebrate social decision-making network spanning multiple subtypes of neurons and glia, including neuronal subpopulations within the preoptic area that are positioned to regulate gonadal differentiation. This work deepens our understanding of sexual differentiation in the vertebrate brain and defines a rich suite of molecular and cellular pathways that differentiate during adult sex change in anemonefish.

Neural connections of the torus semicircularis in the adult Zebrafish

The torus semicircularis (TS) of teleosts is a key midbrain center of the lateral line and acoustic sensory systems. To characterize the TS in adult zebrafish, we studied their connections using the carbocyanine tracers applied to the TS and to other related nuclei and tracts. Two main TS nuclei, central and ventrolateral, were differentiable by their afferent connections. From central TS, (TSc) numerous toropetal cells were labeled bilaterally in several primary octaval nuclei (anterior, magnocellular, descending, and posterior octaval nuclei), in the secondary octaval nucleus, in the caudal octavolateralis nucleus, and in the perilemniscular region. In the midbrain, numerous toropetal cells were labeled in the contralateral TSc. In the diencephalon, toropetal cells labeled from the TSc were observed ipsilaterally in the medial prethalamic nucleus and the periventricular posterior tubercle nucleus. TSc toropetal neurons were also labeled bilaterally in the hypothalamic anterior tuberal nucleus (ATN) and ipsilaterally in the parvicellular preoptic nucleus but not in the telencephalon. Tracer application to the medial octavolateralis nucleus revealed contralateral projections to the ventrolateral TS (TSvl), whereas tracer application to the secondary octaval nucleus labeled fibers bilaterally in TSc and neurons in rostral TSc. The TSc sends ascending fibers to the ipsilateral lateral preglomerular region that, in turn, projects to the pallium. Application of DiI to the optic tectum labeled cells and fibers in the TSvl, whereas application of DiI to the ATN labeled cells and fibers in the TSc. These results reveal that the TSvl and TSc are mainly related with the mechanosensory lateral line and acoustic centers, respectively, and that they show different higher order connections.

Brain regions controlling courtship behavior in the bluehead wrasse

Bluehead wrasses (Thalassoma bifasciatum) follow a socially controlled mechanism of sex determination. A socially dominant initial-phase (IP) female is able to transform into a new terminal-phase (TP) male if the resident TP male is no longer present. TP males display an elaborate array of courtship behaviors, including both color changes and motor behaviors. Little is known concerning the neural circuits that control male-typical courtship behaviors. This study used glutamate iontophoresis to identify regions that may be involved in courtship. Stimulation of the following brain regions elicited diverse types of color change responses, many of which appear similar to courtship color changes: the ventral telencephalon (supracommissural nucleus of the ventral telencephalon [Vs], lateral nucleus of the ventral telencephalon [Vl], ventral nucleus of the ventral telencephalon [Vv], and dorsal nucleus of the ventral telencephalon [Vd]), parts of the preoptic area (NPOmg and NPOpc), entopeduncular nucleus, habenular nucleus, and pretectal nuclei (PSi and PSm). Stimulation of two regions in the posterior thalamus (central posterior thalamic [CP] and dorsal posterior thalamic [DP]) caused movements of the pectoral fins that are similar to courtship fluttering and vibrations. Furthermore, these responses were elicited in female IP fish, indicating that circuits for sexual behaviors typical of TP males exist in females. Immunohistochemistry results revealed regions that are more active in fish that are not courting: interpeduncular nucleus, red nucleus, and ventrolateral thalamus (VL). Taken together, we propose that the telencephalic-habenular-interpeduncular pathway plays an important role in controlling and regulating courtship behaviors in TP males; in this model, in response to telencephalic input, the habenular nucleus inhibits the interpeduncular nucleus, thereby dis-inhibiting forebrain regions and promoting the expression of courtship behaviors.

A telencephalon cell type atlas for goldfish reveals diversity in the evolution of spatial structure and cell types

Teleost fish form the largest group of vertebrates and show a tremendous variety of adaptive behaviors, making them critically important for the study of brain evolution and cognition. The neural basis mediating these behaviors remains elusive. We performed a systematic comparative survey of the goldfish telencephalon. We mapped cell types using single-cell RNA sequencing and spatial transcriptomics, resulting in de novo molecular neuroanatomy parcellation. Glial cells were highly conserved across 450 million years of evolution separating mouse and goldfish, while neurons showed diversity and modularity in gene expression. Specifically, somatostatin interneurons, famously interspersed in the mammalian isocortex for local inhibitory input, were curiously aggregated in a single goldfish telencephalon nucleus but molecularly conserved. Cerebral nuclei including the striatum, a hub for motivated behavior in amniotes, had molecularly conserved goldfish homologs. We suggest elements of a hippocampal formation across the goldfish pallium. Last, aiding study of the teleostan everted telencephalon, we describe substantial molecular similarities between goldfish and zebrafish neuronal taxonomies.

Obesity Impairs Cognitive Function with No Effects on Anxiety-like Behaviour in Zebrafish

Over the last decade, the zebrafish has emerged as an important model organism for behavioural studies and neurological disorders, as well as for the study of metabolic diseases. This makes zebrafish an alternative model for studying the effects of energy disruption and nutritional quality on a wide range of behavioural aspects. Here, we used the zebrafish model to study how obesity induced by overfeeding regulates emotional and cognitive processes. Two groups of fish (n = 24 per group) were fed at 2% (CTRL) and 8% (overfeeding-induced obesity, OIO) for 8 weeks and tested for anxiety-like behaviour using the novel tank diving test (NTDT). Fish were first tested using a short-term memory test (STM) and then trained for four days for a long-term memory test (LTM). At the end of the experiment, fish were euthanised for biometric sampling, total lipid content, and triglyceride analysis. In addition, brains (eight per treatment) were dissected for HPLC determination of monoamines. Overfeeding induced faster growth and obesity, as indicated by increased total lipid content. OIO had no effect on anxiety-like behaviour. Animals were then tested for cognitive function (learning and memory) using the aversive learning test in Zantiks AD units. Results show that both OIO and CTRL animals were able to associate the aversive stimulus with the conditioned stimulus (conditioned learning), but OIO impaired STM regardless of fish sex, revealing the effects of obesity on cognitive processes in zebrafish. Obese fish did not show a deficiency in monoaminergic transmission, as revealed by quantification of total brain levels of dopamine and serotonin and their metabolites. This provides a reliable protocol for assessing the effect of metabolic disease on cognitive and behavioural function, supporting zebrafish as a model for behavioural and cognitive neuroscience.

Spatially resolved cell atlas of the teleost telencephalon and deep homology of the vertebrate forebrain

The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking differences in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely 'everted' telencephalon, which has made it challenging to compare brain regions in fish to those in other vertebrates. Here we combine spatial transcriptomics and single-nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell populations in the fish telencephalon and subpallial, hippocampal, and cortical cell populations in tetrapods. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.

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.

Ascending Visual Pathways to the Telencephalon in Teleosts with Special Focus on Forebrain Visual Centers, Associated Neural Circuitries, and Evolution

Visual pathways to the telencephalon in teleost fishes have been studied in detail only in a few species, and their evolutionary history remained unclear. On the basis of our recent studies we propose that there were two visual pathways in the common ancestor of teleosts, while one of them became lost in acanthopterygian fishes that emerged relatively recently. Our in-depth analyses on the connections of visual centers also revealed that there are connections shared with those of mammals, and retinotopic organization of the ascending connections is maintained at least to the level of the diencephalon in the yellowfin goby. The major visual telencephalic center, or the lateral part of the dorsal telencephalon (Dl), shows considerable species differences in the number of regions and cytoarchitecture. In particular, four highly specialized compartments are noted in the Dl of gobies, and we analyzed about 100 species of teleosts to investigate the evolution of the compartments in the Dl, which indicated that four compartments emerged only in Gobiiformes, while there are fewer specialized compartments in some other percomorph lineages. We also discuss the connections of forebrain visual centers with the cerebellum and other lower brain centers and infer possible functions of the circuitries.

Afferent and efferent connections of the nucleus posterior tuberis in the firemouth cichlid, Thorichthys meeki

The nucleus posterior tuberis (NPT) in teleost fishes, also called posterior tuberal nucleus, is situated in the posterior tuberculum of the diencephalon. It is fused across the midline and densely packed with small cells, but little is known about its connections. In this study, the afferent and efferent connections of the NPT were examined by means of tracer applications of the carbocyanine dye DiI in the firemouth cichlid, Thorichthys meeki. Retrogradely labeled cell bodies were found in the corpus mamillare and nucleus periventricularis of the inferior lobe; and anterogradely labeled terminal fibers were detected in the medial zone of the dorsal telencephalon, medial part of the nucleus lateralis tuberis, dorsal posterior thalamic nucleus, torus lateralis, medial part of the nucleus diffusus of the inferior lobe, and tectum opticum. All these connections show an ipsilateral tendency. The NPT is apparently a significant relay nucleus in the diencephalon of T. meeki, and possibly involved in a variety of feedback circuits. It seems also to be part of a tecto-hypothalamo-telencephalic pathway in cichlids.

The Neuromeric/Prosomeric Model in Teleost Fish Neurobiology

The neuromeric/prosomeric model has been rejuvenated by Puelles and Rubenstein [Trends Neurosci. 1993;16(11):472-9]. Here, its application to the (teleostean) fish brain is detailed, beginning with a historical account. The second part addresses three main issues with particular interest for fish neuroanatomy and looks at the impact of the neuromeric model on their understanding. The first one is the occurrence of four early migrating forebrain areas (M1 through M4) in teleosts and their comparative interpretation. The second issue addresses the complex development and neuroanatomy of the teleostean alar and basal hypothalamus. The third topic is the vertebrate dopaminergic system, with the focus on some teleostean peculiarities. Most of the information will be coming from zebrafish studies, although the general ductus is a comparative one. Throughout the manuscript, comparative developmental and organizational aspects of the teleostean amygdala are discussed. One particular focus is cellular migration streams into the medial amygdala.

Trace classical conditioning impairment after lesion of the lateral part of the goldfish telencephalic pallium suggests a long ancestry of the episodic memory function of the vertebrate hippocampus

There is an ongoing debate on the evolutionary origin of the episodic memory function of the hippocampus. A widely accepted hypothesis claims that the hippocampus first evolved as a dedicated system for spatial navigation in ancestral vertebrates, being transformed later in phylogeny to support a broader role in episodic memory with the emergence of mammals. On the contrary, an alternative hypothesis holds that the hippocampus of ancestral vertebrates originally encoded both the spatial and temporal dimensions of relational memories since its evolutionary appearance, thus suggesting that the episodic-like memory function of the hippocampus could be the primitive condition in vertebrate forebrain evolution. The present experiment was aimed at scrutinizing these opposing hypotheses by investigating whether the hippocampal pallium of teleost fish, a vertebrate group that shares with mammals a common ancestor that lived about 400 Mya, is, like the hippocampus of mammals, essential to associate time-discontiguous events. Thus, goldfish with lesions in the ventral part of the dorsolateral pallium (Dlv), a telencephalic region considered homologous to the hippocampal pallium of land vertebrates, were trained in trace versus delay eyeblink-like classical conditioning, two learning procedures that differ only in the temporal relationships between the stimuli to be associated in memory. The results showed that hippocampal pallium lesion in goldfish severely impairs trace conditioning, but spares delay conditioning. This finding challenges the idea that navigation preceded relational memory in evolutionary appearance and suggests the possibility that a relational memory function that associates the experienced events in both the spatial and temporal dimensions could be a primitive feature of the hippocampus that pre-existed in the common ancestor of vertebrates.

The Everted Amygdala of Ray-Finned Fish: Zebrafish Makes a Case

The amygdala, a complex array of nuclei in the forebrain, controls emotions and emotion-related behaviors in vertebrates. Current research aims to understand the amygdala's evolution in ray-finned fish such as zebrafish because of the region's relevance for social behavior and human psychiatric disorders. Clear-cut molecular definitions of the amygdala and its evolutionary-developmental relationship to the one of mammals are critical for zebrafish models of affective disorders and autism. In this review, I argue that the prosomeric model and a focus on the olfactory system's organization provide ideal tools for discovering deep ancestral relationships between the emotional systems of zebrafish and mammals. The review's focus is on the "extended amygdala," which refers to subpallial amygdaloid territories including the central (autonomic) and the medial (olfactory) amygdala required for reproductive and social behaviors. Amphibians, sauropsids, and lungfish share many characteristics with the basic amygdala ground plan of mammals, as molecular and hodological studies have shown. Further exploration of the evolution of the amygdala in basally derived fish vertebrates requires researchers to test these "tetrapod-based" concepts. Historically, this has been a daunting task because the forebrains of basally derived fish vertebrates look very different from those of more familiar tetrapod ones. An extreme case are ray-finned fish (Actinopterygii) like zebrafish because their telencephalon develops through a distinct outward-growing process called eversion. To this day, scientists have struggled to determine how the everted telencephalon compares to non-actinopterygian vertebrates. Using the teleost zebrafish as a genetic model, comparative neurologists began to establish quantifiable molecular definitions that allow direct comparisons between ray-finned fish and tetrapods. In this review, I discuss how the most recent discovery of the zebrafish amygdala ground plan offers an opportunity to identify the developmental constraints of amygdala evolution and function. In addition, I explain how the zebrafish prethalamic eminence (PThE) topologically relates to the medial amygdala proper and the nucleus of the lateral olfactory tract (nLOT). In fact, I consider these previously misinterpreted olfactory structures the most critical missing evolutionary links between actinopterygian and tetrapod amygdalae. In this context, I will also explain why recognizing both the PThE and the nLOT is crucial to understanding the telencephalon eversion. Recognizing these anatomical hallmarks allows direct comparisons of the amygdalae of zebrafish and mammals. Ultimately, the new concepts of the zebrafish amygdala will overcome current dogmas and reach a holistic understanding of amygdala circuits of cognition and emotion in actinopterygians.

A versatile transcription factor: Multiple roles of orthopedia a (otpa) beyond its restricted localization in dopaminergic systems of developing and adult zebrafish (Danio rerio) brains

Many transcription factors boost neural development and differentiation in specific directions and serve for identifying similar or homologous structures across species. The expression of Orthopedia (Otp) is critical for the development of certain cell groups along the vertebrate neuraxis, for example, the medial amygdala or hypothalamic neurosecretory neurons. Therefore, the primary focus of the present study is the distribution of Orthopedia a (Otpa) in the larval and adult zebrafish (Danio rerio) brain. Since Otpa is also critical for the development of zebrafish basal diencephalic dopaminergic cells, colocalization of Otpa with the catecholamine synthesizing enzyme tyrosine hydroxylase (TH) is studied. Cellular colocalization of Otpa and dopamine is only seen in magnocellular neurons of the periventricular posterior tubercular nucleus and in the posterior tuberal nucleus. Otpa-positive cells occur in many additional structures along the zebrafish neuraxis, from the secondary prosencephalon down to the hindbrain. Furthermore, Otpa expression is studied in shh-GFP and islet1-GFP transgenic zebrafish. Otpa-positive cells only express shh in dopaminergic magnocellular periventricular posterior tubercular cells, and only colocalize with islet1-GFP in the ventral zone and prerecess caudal periventricular hypothalamic zone and the perilemniscal nucleus. The scarcity of cellular colocalization of Otpa in islet1-GFP cells indicates that the Shh-islet1 neurogenetic pathway is not active in most Otpa-expressing domains. Our analysis reveals detailed correspondences between mouse and zebrafish forebrain territories including the zebrafish intermediate nucleus of the ventral telencephalon and the mouse medial amygdala. The zebrafish preoptic Otpa-positive domain represents the neuropeptidergic supraopto-paraventricular region of all tetrapods. Otpa domains in the zebrafish basal plate hypothalamus suggest that the ventral periventricular hypothalamic zone corresponds to the otp-expressing basal hypothalamic tuberal field in the mouse. Furthermore, the mouse otp domain in the mammillary hypothalamus compares partly to our Otpa-positive domain in the prerecess caudal periventricular hypothalamic zone (Hc-a).

Organizational Conservation and Flexibility in the Evolution of Birdsong and Avian Motor Control

Birds and mammals have independently evolved complex behavioral and cognitive capabilities yet have markedly different brain structures. An open question is to what extent, despite these differences in anatomy, birds and mammals have evolved similar neural solutions to complex motor control and at what level of organization these similarities might lie. Courtship song in songbirds, a learned motor skill that is similar to the fine motor skills of many mammals including human speech, provides a powerful system in which to study the links connecting the development and evolution of cells, circuits, and behavior. Until recently, obtaining cellular-resolution views of the specialized neural circuitry that subserves birdsong was impossible due to a lack of molecular tools for songbirds. However, the ongoing revolution in cellular profiling and genomics offers unprecedented opportunities for molecular analysis in organisms that lack a traditional genetic infrastructure but have tractable, well-defined behaviors. Here, I describe recent efforts to understand the evolutionary relationships between birdsong control circuitry and mammalian neocortical circuitry using new approaches to measure gene expression in single cells. These results, combined with foundational work relating avian and mammalian brains at a range of biological levels, present an emerging view that amniote pallium evolution is a story of diverse neural circuit architectures employing conserved neuronal elements within a conserved topological framework. This view suggests that one locus of pallial neural circuit evolution lies at the intersection between the gene regulatory programs that regulate regional patterning and those that specify functional identity. Modifications to this intersection may underlie the evolution of pallial motor control in birds in general and to the evolutionary and developmental relationships of these circuits to the avian pallial amygdala.

Integrated Behavioral, Genetic and Brain Circuit Visualization Methods to Unravel Functional Anatomy of Zebrafish Amygdala

The mammalian amygdala is a complex forebrain structure consisting of a heterogeneous group of nuclei derived from the pallial and subpallial telencephalon. It plays a critical role in a broad range of behaviors such as emotion, cognition, and social behavior; within the amygdala each nucleus has a distinct role in these behavioral processes. Topological, hodological, molecular, and functional studies suggest the presence of an amygdala-like structure in the zebrafish brain. It has been suggested that the pallial amygdala homolog corresponds to the medial zone of the dorsal telencephalon (Dm) and the subpallial amygdala homolog corresponds to the nuclei in the ventral telencephalon located close to and topographically basal to Dm. However, these brain regions are broad and understanding the functional anatomy of the zebrafish amygdala requires investigating the role of specific populations of neurons in brain function and behavior. In zebrafish, the highly efficient Tol2 transposon-mediated transgenesis method together with the targeted gene expression by the Gal4-UAS system has been a powerful tool in labeling, visualizing, and manipulating the function of specific cell types in the brain. The transgenic resource combined with neuronal activity imaging, optogenetics, pharmacology, and quantitative behavioral analyses enables functional analyses of neuronal circuits. Here, we review earlier studies focused on teleost amygdala anatomy and function and discuss how the transgenic resource and tools can help unravel the functional anatomy of the zebrafish amygdala.

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.

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.

Regional synapse gain and loss accompany memory formation in larval zebrafish

Defining the structural and functional changes in the nervous system underlying learning and memory represents a major challenge for modern neuroscience. Although changes in neuronal activity following memory formation have been studied [B. F. Grewe et al., Nature 543, 670-675 (2017); M. T. Rogan, U. V. Stäubli, J. E. LeDoux, Nature 390, 604-607 (1997)], the underlying structural changes at the synapse level remain poorly understood. Here, we capture synaptic changes in the midlarval zebrafish brain that occur during associative memory formation by imaging excitatory synapses labeled with recombinant probes using selective plane illumination microscopy. Imaging the same subjects before and after classical conditioning at single-synapse resolution provides an unbiased mapping of synaptic changes accompanying memory formation. In control animals and animals that failed to learn the task, there were no significant changes in the spatial patterns of synapses in the pallium, which contains the equivalent of the mammalian amygdala and is essential for associative learning in teleost fish [M. Portavella, J. P. Vargas, B. Torres, C. Salas, Brain Res. Bull 57, 397-399 (2002)]. In zebrafish that formed memories, we saw a dramatic increase in the number of synapses in the ventrolateral pallium, which contains neurons active during memory formation and retrieval. Concurrently, synapse loss predominated in the dorsomedial pallium. Surprisingly, we did not observe significant changes in the intensity of synaptic labeling, a proxy for synaptic strength, with memory formation in any region of the pallium. Our results suggest that memory formation due to classical conditioning is associated with reciprocal changes in synapse numbers in the pallium.

The organization of the zebrafish pallium from a hodological perspective

We studied the connections (connectome) of the adult zebrafish pallium using carbocyanine dye tracing and ancillary anatomical methods. The everted zebrafish pallium (dorsal telencephalic area, D) is composed of several major zones (medial, lateral, dorsal, central, anterior, and posterior) distinguishable by their topography, cytoarchitecture, immunohistochemistry, and genoarchitecture. Our comprehensive study reveals poor interconnectivity between these pallial areas, especially between medial (Dm), lateral/dorsal (Dl, Dd), and posterior (Dp) regions. This suggests that the zebrafish pallium has dedicated modules for different neural processes. Pallial connections with extrapallial regions also show compartmental organization. Major extratelencephalic afferents come from preglomerular nuclei (to Dl, Dd, and Dm), posterior tuberal nucleus (to Dm), and lateral recess nucleus (to Dl). The subpallial (ventral, V) zones dorsal Vv, Vd, and Vs, considered homologues of the striatum, amygdala, and pallidum, are mainly afferent to Dl/Dd and Dp. Regarding the efferent pathways, they also appear characteristic of each pallial region. Rostral Dm projects to the dorsal entopeduncular nucleus. Dp is interconnected with the olfactory bulbs. The central region (Dc) defined here receives mainly projections from Dl-Dd and projects toward the pretectum and optic tectum, connections, which help to delimiting Dc. The connectome of the adult pallium revealed here complements extant studies on the neuroanatomical organization of the brain, and may be useful for neurogenetic studies performed during early stages of development. The connectome of the zebrafish pallium was also compared with the pallial connections reported in other teleosts, a large group showing high pallial diversity.

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.

Spatial Cognition in Teleost Fish: Strategies and Mechanisms

Teleost fish have been traditionally considered primitive vertebrates compared to mammals and birds in regard to brain complexity and behavioral functions. However, an increasing amount of evidence suggests that teleosts show advanced cognitive capabilities including spatial navigation skills that parallel those of land vertebrates. Teleost fish rely on a multiplicity of sensory cues and can use a variety of spatial strategies for navigation, ranging from relatively simple body-centered orientation responses to allocentric or "external world-centered" navigation, likely based on map-like relational memory representations of the environment. These distinct spatial strategies are based on separate brain mechanisms. For example, a crucial brain center for egocentric orientation in teleost fish is the optic tectum, which can be considered an essential hub in a wider brain network responsible for the generation of egocentrically referenced actions in space. In contrast, other brain centers, such as the dorsolateral telencephalic pallium of teleost fish, considered homologue to the hippocampal pallium of land vertebrates, seem to be crucial for allocentric navigation based on map-like spatial memory. Such hypothetical relational memory representations endow fish's spatial behavior with considerable navigational flexibility, allowing them, for example, to perform shortcuts and detours.

Unconventional anxiety pharmacology in zebrafish: Drugs beyond traditional anxiogenic and anxiolytic spectra

Anxiety is the most prevalent brain disorder and a common cause of human disability. Animal models are critical for understanding anxiety pathogenesis and its pharmacotherapy. The zebrafish (Danio rerio) is increasingly utilized as a powerful model organism in anxiety research and anxiolytic drug screening. High similarity between human, rodent and zebrafish molecular targets implies shared signaling pathways involved in anxiety pathogenesis. However, mounting evidence shows that zebrafish behavior can be modulated by drugs beyond conventional anxiolytics or anxiogenics. Furthermore, these effects may differ from human and/or rodent responses, as such 'unconventional' drugs may affect zebrafish behavior despite having no such profiles (or exerting opposite effects) in humans or rodents. Here, we discuss the effects of several putative unconventional anxiotropic drugs (aspirin, lysergic acid diethylamide (LSD), nicotine, naloxone and naltrexone) and their potential mechanisms of action in zebrafish. Emphasizing the growing utility of zebrafish models in CNS drug discovery, such unconventional anxiety pharmacology may provide important, evolutionarily relevant insights into complex regulation of anxiety in biological systems. Albeit seemingly complicating direct translation from zebrafish into clinical phenotypes, this knowledge may instead foster the development of novel CNS drugs, eventually facilitating innovative treatment of patients based on novel 'unconventional' targets identified in fish models.

The evolution, formation and connectivity of the anterior commissure

The anterior commissure is the most ancient of the forebrain interhemispheric connections among all vertebrates. Indeed, it is the predominant pallial commissure in all non-eutherian vertebrates, universally subserving basic functions related to olfaction and survival. A key feature of the anterior commissure is its ability to convey connections from diverse brain areas, such as most of the neocortex in non-eutherian mammals, thereby mediating the bilateral integration of diverse functions. Shared developmental mechanisms between the anterior commissure and more evolutionarily recent commissures, such as the corpus callosum in eutherians, have led to the hypothesis that the former may have been a precursor for additional expansion of commissural circuits. However, differences between the formation of the anterior commissure and other telencephalic commissures suggest that independent developmental mechanisms underlie the emergence of these connections in extant species. Here, we review the developmental mechanisms and connectivity of the anterior commissure across evolutionarily distant species, and highlight its potential functional importance in humans, both in the course of normal neurodevelopment, and as a site of plastic axonal rerouting in the absence or damage of other connections.

Neurotrophins Time Point Intervention after Traumatic Brain Injury: From Zebrafish to Human

Traumatic brain injury (TBI) remains the leading cause of long-term disability, which annually involves millions of individuals. Several studies on mammals reported that neurotrophins could play a significant role in both protection and recovery of function following neurodegenerative diseases such as stroke and TBI. This protective role of neurotrophins after an event of TBI has also been reported in the zebrafish model. Nevertheless, reparative mechanisms in mammalian brain are limited, and newly formed neurons do not survive for a long time. In contrast, the brain of adult fish has high regenerative properties after brain injury. The evident differences in regenerative properties between mammalian and fish brain have been ascribed to remarkable different adult neurogenesis processes. However, it is not clear if the specific role and time point contribution of each neurotrophin and receptor after TBI is conserved during vertebrate evolution. Therefore, in this review, I reported the specific role and time point of intervention for each neurotrophic factor and receptor after an event of TBI in zebrafish and mammals.

Calretinin immunoreactivity in the inferior lobe of the hypothalamus and associated nuclei of the firemouth cichlid, Thorichthys meeki

The distribution of the calcium-binding protein calretinin (CR) was examined by an immunohistochemical method using specific antibodies. CR is involved in the visual system, and the inferior lobe of the hypothalamus represents a multisensory integration center in cichlids. The focus of the present study was to analyze the distribution of CR immunoreactivity in a cichlid fish, the firemouth cichlid, Thorichthys meeki, for the hypothalamic inferior lobe and for the torus lateralis, nucleus glomerulosus, nucleus posterior tuberis, and corpus mamillare as associated nuclei of the hypothalamus. CR-immunoreactive (CR-ir) cell bodies were visualized in the lateral and medial part of the diffuse nucleus of the inferior lobe, ventral portion of the central nucleus of the inferior lobe, torus lateralis, nucleus glomerulosus, and nucleus posterior tuberis. CR-ir fibers could be detected in the dorsal portion of the central nucleus of the inferior lobe and corpus mamillare. The strongest labeling of CR-ir neuropil was observed in the lateral part of the diffuse nucleus of the inferior lobe, outer zone of the periventricular nucleus of the inferior lobe, torus lateralis, nucleus glomerulosus, and nucleus posterior tuberis. CR is abundantly present in the inferior lobe of the hypothalamus and associated nuclei. The role of CR in highly active processes in the inferior lobe of cichlids will be discussed.

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.

Non-thalamic origin of zebrafish sensory nuclei implies convergent evolution of visual pathways in amniotes and teleosts

Ascending visual projections similar to the mammalian thalamocortical pathway are found in a wide range of vertebrate species, but their homology is debated. To get better insights into their evolutionary origin, we examined the developmental origin of a thalamic-like sensory structure of teleosts, the preglomerular complex (PG), focusing on the visual projection neurons. Similarly to the tectofugal thalamic nuclei in amniotes, the lateral nucleus of PG receives tectal information and projects to the pallium. However, our cell lineage study in zebrafish reveals that the majority of PG cells are derived from the midbrain, unlike the amniote thalamus. We also demonstrate that the PG projection neurons develop gradually until late juvenile stages. Our data suggest that teleost PG, as a whole, is not homologous to the amniote thalamus. Thus, the thalamocortical-like projections evolved from a non-forebrain cell population, which indicates a surprising degree of variation in the vertebrate sensory systems.

Embryonic atrazine exposure and later in life behavioral and brain transcriptomic, epigenetic, and pathological alterations in adult male zebrafish

Atrazine (ATZ), a commonly used pesticide linked to endocrine disruption, cancer, and altered neurochemistry, frequently contaminates water sources at levels above the US Environmental Protection Agency's 3 parts per billion (ppb; μg/L) maximum contaminant level. Adult male zebrafish behavior, brain transcriptome, brain methylation status, and neuropathology were examined to test the hypothesis that embryonic ATZ exposure causes delayed neurotoxicity, according to the developmental origins of health and disease paradigm. Zebrafish (Danio rerio) embryos were exposed to 0 ppb, 0.3 ppb, 3 ppb, or 30 ppb ATZ during embryogenesis (1-72 h post fertilization (hpf)), then rinsed and raised to maturity. At 9 months post fertilization (mpf), males had decreased locomotor parameters during a battery of behavioral tests. Transcriptomic analysis identified altered gene expression in organismal development, cancer, and nervous and reproductive system development and function pathways and networks. The brain was evaluated histopathologically for morphometric differences, and decreased numbers of cells were identified in raphe populations. Global methylation levels were evaluated at 12 mpf, and the body length, body weight, and brain weight were measured at 14 mpf to evaluate effects of ATZ on mature brain size. No significant difference in genome methylation or brain size was observed. The results demonstrate that developmental exposure to ATZ does affect neurodevelopment and neural function in adult male zebrafish and raises concern for possible health effects in humans due to ATZ's environmental presence and persistence. Graphical abstract.

Innate chemical, but not visual, threat cues have been co-opted as unconditioned stimulus for social fear learning in zebrafish

Animals can use social information to detect threat in the environment. In particular, social learning allows animals to learn about dangers without incurring in the costs of trial-and-error learning. In zebrafish, both chemical and visual social cues elicit an innate alarm response, which consists of erratic movement followed by freezing behavior. Injured zebrafish release an alarm substance from their skin that elicits the alarm response. Similarly, the sight of conspecifics displaying the alarm response can also elicit the expression of this response in observers. In this study, we investigated if these social cues of danger can also be used by zebrafish as unconditioned stimulus (US) in learning. We found that only the chemical cue was effective in the social fear conditioning. We suggest that this differential efficacy of social cues results from the fact that the alarm cue is a more reliable indicator of threat, than the sight of an alarmed conspecific. Therefore, although multiple social cues may elicit innate responses not all have been evolutionarily co-opted to act as US in associative learning. Furthermore, the use of the expression of the immediate early genes as markers of neuronal activity showed that chemical social fear conditioning is paralleled by a differential activation of the olfactory bulbs and by a different pattern of functional connectivity across brain regions involved in olfactory processing.

Afferent and efferent connections of the nucleus prethalamicus in the yellowfin goby Acanthogobius flavimanus

The nucleus prethalamicus (PTh) receives fibers from the optic tectum and then projects to the dorsal telencephalon in the yellowfin goby Acanthogobius flavimanus. However, it remained unclear whether the PTh is a visual relay nucleus, because the optic tectum receives not only visual but also other sensory modalities. Furthermore, precise telencephalic regions receiving prethalamic input remained unknown in the goby. We therefore investigated the full set of afferent and efferent connections of the PTh by direct tracer injections into the nucleus. Injections into the PTh labeled cells in the optic tectum, ventromedial thalamic nucleus, central and medial parts of the dorsal telencephalon, and caudal lobe of the cerebellum. We found that the somata of most tecto-prethalamic neurons are present in the stratum periventriculare. Their dendrites ascend to reach the major retinorecipient layers of the tectum. The PTh is composed of two subnuclei (medial and lateral) and topographic organization was appreciated only for tectal projections to the lateral subnucleus (PTh-l), which also receives sparse retinal projections. In contrast, the medial subnucleus receives fibers only from the medial tectum. We found that the PTh projects to nine subregions in the dorsal telencephalon and four in the ventral telencephalon. Furthermore, cerebellar injections revealed that cerebello-prethalamic fibers cross the midline twice to innervate the PTh-l on both sides. The present study is the first detailed report on the full set of the connections of PTh, which suggests that the PTh relays visual information from the optic tectum to the telencephalon.

Relationship between Brain Morphology and Life History in Four Bottom-Dwelling Gobiids

In terrestrial vertebrates, the hippocampus plays a major role in spatial cognition. Recent developmental, anatomical, and histological studies suggest that the ventral region of the lateral part of the dorsal telencephalic area (Dlv) in teleost fishes is homologous to the hippocampus in terrestrial vertebrates. We hypothesized that fish species with higher spatial cognitive demands have a more highly developed Dlv compared to closely related species with relatively lower spatial cognitive demands. The fishes selected for this study were Favonigobius gymnauchen, Istigobius hoshinonis, Tridentiger trigonocephalus, and Chaenogobius annularis; all are bottom-dwelling gobiid species found in habitats that vary with respect to their spatial complexity. Volumetric analysis of the telencephalic subregions, including the Dlv, and other major brain regions showed that species from stable rocky areas had a larger Dlv than species from relatively homogenous sandy/ muddy environments. These findings support the hypothesis that the teleost Dlv is homologous to the hippocampus in terrestrial vertebrates, and that the relative development of these areas is positively correlated with spatial cognitive demand in animals.

Evolution of the Hypothalamus and Inferior Lobe in Ray-Finned Fishes

The inferior lobes are prominent bilateral brain areas in the hypothalamus of neopterygians among the ray-finned fishes. They are known as multisensory integration centers. As such, they should play a major role in fish evolution. In this study, a comparative morphometric analysis was performed. The morphology of the hypothalamus, where the inferior lobe is considered as fully developed first in Lepisosteus, was then re-examined. One hundred brains from different species of 60 families of ray-finned fishes were stained with cresyl violet and embedded in methacrylate. They were then cut on a microtome while conducting block-face imaging. The volumes were determined for the whole brain, brain areas, and nuclei. Since visual input represents a major sensory input for the inferior lobe, the nucleus glomerulosus, a visual-related nucleus in paracanthopterygian and acanthopterygian teleosts, and the tectum opticum were included in the investigations. The morphometric analysis revealed that the relative volume of the inferior lobes increases significantly from species of the Lepisosteiformes to the Tetraodontiformes. In addition, a positive correlation was detected between the relative volume of the inferior lobes and either the relative volume of the nucleus glomerulosus or the relative volume of the tectum opticum. These correlations, in combination with findings from previous hodological and behavioral studies, give rise to the speculation that the inferior lobes may be involved in higher cognitive processes and complex social interactions.

Effects of environmental enrichment on forebrain neural plasticity and survival success of stocked Atlantic salmon

Fish reared for stocking programmes are severely stimulus deprived compared with their wild conspecifics raised under natural conditions. This leads to reduced behavioural plasticity and low post-release survival of stocked fish. Environmental enrichment can have positive effects on important life skills, such as predator avoidance and foraging behaviour, but the neural mechanisms underpinning these behavioural changes are still largely unknown. In this study, juvenile Atlantic salmon (Salmo salar) were reared in an enriched hatchery environment for 7 weeks, after which neurobiological characteristics and post-release survival were compared with those of fish reared under normal hatchery conditions. Using in situ hybridization and qPCR, we quantified the expression of brain-derived neurotrophic factor (bdnf) and the neural activity marker cfos in telencephalic subregions associated with relational memory, emotional learning and stress reactivity. Aside from lower expression of bdnf in the Dlv (a region associated with relational memory) of enriched salmon, we observed no other significant effects of enrichment in the studied regions. Exposure to an enriched environment increased post-release survival during a 5 month residence in a natural river by 51%. Thus, we demonstrate that environmental enrichment can improve stocking success of Atlantic salmon parr and that environmental enrichment is associated with changes in bdnf expression in the fish's hippocampus-equivalent structure.

Cellular and Network Mechanisms May Generate Sparse Coding of Sequential Object Encounters in Hippocampal-Like Circuits

The localization of distinct landmarks plays a crucial role in encoding new spatial memories. In mammals, this function is performed by hippocampal neurons that sparsely encode an animal’s location relative to surrounding objects. Similarly, the dorsolateral pallium (DL) is essential for spatial learning in teleost fish. The DL of weakly electric gymnotiform fish receives both electrosensory and visual input from the preglomerular nucleus (PG), which has been hypothesized to encode the temporal sequence of electrosensory or visual landmark/food encounters. Here, we show that DL neurons in the Apteronotid fish and in the Carassius auratus (goldfish) have a hyperpolarized resting membrane potential (RMP) combined with a high and dynamic spike threshold that increases following each spike. Current-evoked spikes in DL cells are followed by a strong small-conductance calcium-activated potassium channel (SK)-mediated after-hyperpolarizing potential (AHP). Together, these properties prevent high frequency and continuous spiking. The resulting sparseness of discharge and dynamic threshold suggest that DL neurons meet theoretical requirements for generating spatial memory engrams by decoding the landmark/food encounter sequences encoded by PG neurons. Thus, DL neurons in teleost fish may provide a promising, simple system to study the core cell and network mechanisms underlying spatial memory.

Melatonin receptor expression in vocal, auditory, and neuroendocrine centers of a highly vocal fish, the plainfin midshipman (Porichthys notatus)

Melatonin plays a central role in entraining activity to the day-night cycle in vertebrates. Here, we investigate neuroanatomical substrates of melatonin-dependent vocal-acoustic behavior in the nocturnal and highly vocal teleost fish, the plainfin midshipman (Porichthys notatus). Using in situ hybridization (ISH) and quantitative real-time PCR (qPCR), we assess the mRNA distribution and transcript abundance of melatonin receptor subtype 1B (mel1b), shown to be important for vocalization in midshipman fish and songbirds. ISH shows robust mel1b expression in major nodes of the central vocal and auditory networks in the subpallium, preoptic area (POA), anterior hypothalamus, dorsal thalamus, posterior tuberculum, midbrain torus semicircularis and periaqueductal gray, and hindbrain. Mel1b label is also abundant in secondary targets of the olfactory, visual, and lateral line systems, as well as telencephalic regions that have been compared to the amygdala, extended amygdala, striatum, septum, and hippocampus of tetrapods. Q-PCR corroborates mel1b abundance throughout the brain and shows significant increases in the morning compared with nighttime in tissue samples inclusive of the telencephalon and POA, but remains stable in other brain regions. Plasma melatonin levels show expected increase at night. Our findings support the hypothesis that melatonin's stimulatory effects on vocal-acoustic mechanisms in midshipman is mediated, in part, by melatonin binding in vocal, auditory, and neuroendocrine centers. Together with robust mel1b expression in multiple telencephalic nuclei and sensory systems, the results further indicate an expression pattern comparable to that in birds and mammals that is indicative of melatonin's broad involvement in the modulation of physiology and behavior.