Avoidance response in goldfish: emotional and temporal involvement of medial and lateral telencephalic pallium

Cloning and expression of tachykinins and their association with kisspeptins in the brains of zebrafish

The tachykinins are a family of neuropeptides, including substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), that are encoded by the tac1 (SP and NKA) or tac2/3 (NKB) genes. Tachykinins are widely distributed in the central nervous system and have roles as neurotransmitters and/or neuromodulators. Recent studies in mammals have demonstrated the coexpression of NKB and kisspeptin and their comodulatory roles over the control of reproduction. We have recently identified two kisspeptin-encoding genes, kiss1 and kiss2, in teleosts. However, such relationship between tachykinins and kisspeptins has not been demonstrated in non-mammalian species. To determine the involvement of tachykinins in the reproduction in teleosts, we identified tac1 and two tac2 (tac2a and tac2b) sequences in the zebrafish genome using in silico data mining. Zebrafish tac1 encodes SP and NKA, whereas the tac2 sequences encode NKB and an additional peptide homologous to NKB (NKB-related peptide). Digoxigenin in situ hybridization in the brain of zebrafish showed tac1 mRNA-containing cells in the olfactory bulb, telencephalon, preoptic region, hypothalamus, mesencephalon, and rhombencephalon. The zebrafish tac2a mRNA-containing cells were observed in the preoptic region, habenula, and hypothalamus, whereas the tac2b mRNA-containing cells were predominantly observed in the dorsal telencephalic area. Furthermore, we examined the coexpression of tachykinins and two kisspeptin genes in the brain of zebrafish. Dual fluorescent in situ hybridization showed no coexpression of tachykinins mRNA with kisspeptins mRNA in hypothalamic nuclei or the habenula. These results suggest the presence of independent pathways for kisspeptins and NKB neurons in the brain of zebrafish.

Fear, anxiety, and control in the zebrafish

Emotional responses are triggered by environmental signals and involve profound changes at multiple levels, from molecular to behavior. Much has been learnt about two emotions, fear and anxiety, by studying mammalian models. In particular, neural circuits and the corresponding molecular mechanisms essential for the learning and retention of fear, as well as the activation of anxiety, are well known. In contrast, little is known about how these emotions are terminated. The zebrafish is a newcomer to the world of emotion research. A number of assays for fear and anxiety now exist, but the underlying neural circuitry is largely undefined. Recent experiments, however, appear to provide a hint as to how anxiety is downregulated. In particular, they point to an essential role for a circuit involving the posterior septum, medial habenula, and interpeduncular nucleus. This evolutionarily conserved circuit may fulfill a similar function in mammals.

Current Emotion Research in Behavioral Neuroscience: The Role(s) of the Amygdala

Substantial advances in our understanding of the neural bases of emotional processing have been made over the past decades. Overall, studies in humans and other animals highlight the key role of the amygdala in the detection and evaluation of stimuli with affective value. Nonetheless, contradictory findings have been reported, especially in terms of the exact role of this structure in the processing of different emotions, giving rise to different neural models of emotion. For instance, although the amygdala has traditionally been considered as exclusively involved in fear (and possibly anger), more recent work suggests that it may be important for processing other types of emotions, and even nonemotional information. A review of the main findings in this field is presented here, together with some of the hypotheses that have been put forward to interpret this literature and explain its inconsistencies.

From omics to drug metabolism and high content screen of natural product in zebrafish: a new model for discovery of neuroactive compound

The zebrafish (Danio rerio) has recently become a common model in the fields of genetics, environmental science, toxicology, and especially drug screening. Zebrafish has emerged as a biomedically relevant model for in vivo high content drug screening and the simultaneous determination of multiple efficacy parameters, including behaviour, selectivity, and toxicity in the content of the whole organism. A zebrafish behavioural assay has been demonstrated as a novel, rapid, and high-throughput approach to the discovery of neuroactive, psychoactive, and memory-modulating compounds. Recent studies found a functional similarity of drug metabolism systems in zebrafish and mammals, providing a clue with why some compounds are active in zebrafish in vivo but not in vitro, as well as providing grounds for the rationales supporting the use of a zebrafish screen to identify prodrugs. Here, we discuss the advantages of the zebrafish model for evaluating drug metabolism and the mode of pharmacological action with the emerging omics approaches. Why this model is suitable for identifying lead compounds from natural products for therapy of disorders with multifactorial etiopathogenesis and imbalance of angiogenesis, such as Parkinson's disease, epilepsy, cardiotoxicity, cerebral hemorrhage, dyslipidemia, and hyperlipidemia, is addressed.

Effect of MK-801-induced impairment of inhibitory avoidance learning in zebrafish via inactivation of extracellular signal-regulated kinase (ERK) in telencephalon

N-Methyl-D-aspartate (NMDA) receptors are implicated in a wide range of complex behavioral functions, including cognitive activity. Numerous studies have shown that using the repetitive administration of a noncompetitive NMDA receptor antagonist, MK-801, induces amnesia in rodents. In this study, the effect of a subchronic MK-801 treatment on the cognitive function of zebrafish was evaluated using a novel inhibitory avoidance task. First, we established a new system to investigate the inhibitory avoidance learning of zebrafish where they were trained to refrain from swimming from a shallow compartment to a deep compartment in order to avoid electric shock. Second, we found that blocking NMDA receptors by MK-801 could significantly attenuate the inhibitory avoidance behavior of the zebrafish and alter the telencephalic extracellular signal-regulated kinase (ERK) phosphorylation level 90 min after the inhibitory avoidance training. These results suggest that the formation of long-term emotional memory is possibly mediated by ERK activation in the telencephalon of zebrafish.

Ontogeny of the cocaine- and amphetamine-regulated transcript (CART) neuropeptide system in the brain of zebrafish, Danio rerio

The cocaine- and amphetamine-regulated transcript (CART) peptidergic system is involved in processing diverse neuronal functions in adult animals, including energy metabolism. Although CART is widely distributed in the brain of a range of vertebrates, the ontogeny of this system has not been explored. The CART-immunoreactive system in the zebrafish central nervous system (CNS) was studied across developmental stages until adulthood. The peptide is expressed as early as 24 hours post fertilization and establishes itself in several discrete areas of the brain and spinal cord as development progresses. The trends in CART ontogeny suggest that it may be involved in the establishment of commissural tracts, typically expressing early but subsequently decaying. CART elements are commonly overrepresented in diverse sensory areas like the olfactory, photic, and acoustico-mechanosensory systems, perhaps indicating a role for the peptide in sensory perception. Key neuroendocrine centers, like the preoptic area, hypothalamus, and pituitary, conspicuously show CART innervations, suggesting functions analogous to those demonstrated in other chordates. Uniquely, the epiphysis also appears to employ CART as a neurotransmitter. The entopeduncular nucleus is a major CART-containing group in the adult teleost forebrain that may participate in glucose sensing. This region responds to glucose in the 15-day larvae, suggesting that the energy status sensing CART circuits is active early in development. The pattern of CART expression in zebrafish suggests conserved evolutionary trends among vertebrate species. Developmental expression profiling reveals putative novel functions and establishes zebrafish as a model to investigate CART function in physiology and development.

Adult neurogenesis in the brain of the Mozambique tilapia, Oreochromis mossambicus

Although the generation of new neurons in the adult nervous system ('adult neurogenesis') has been studied intensively in recent years, little is known about this phenomenon in non-mammalian vertebrates. Here, we examined the generation, migration, and differentiation of new neurons and glial cells in the Mozambique tilapia (Oreochromis mossambicus), a representative of one of the largest vertebrate taxonomic orders, the perciform fish. The vast majority of new cells in the brain are born in specific proliferation zones of the olfactory bulb; the dorsal and ventral telencephalon; the periventricular nucleus of the posterior tuberculum, optic tectum, and nucleus recessi lateralis of the diencephalon; and the valvula cerebelli, corpus cerebelli, and lobus caudalis of the cerebellum. As shown in the olfactory bulb and the lateral part of the valvula cerebelli, some of the young cells migrate from their site of origin to specific target areas. Labeling of mitotic cells with the thymidine analog 5-bromo-2'-deoxyuridine, combined with immunostaining against the neuron-specific marker protein Hu or against the astroglial marker glial fibrillary acidic protein demonstrated differentiation of the adult-born cells into both neurons and glia. Taken together, the present investigation supports the hypothesis that adult neurogenesis is an evolutionarily conserved vertebrate trait.

Stimulation of the lateral division of the dorsal telencephalon induces synaptic plasticity in the medial division of adult zebrafish

In ray-finned fishes, the lateral (Dl) and medial (Dm) division of the dorsal telencephalon are important in learning and memory formation. Tract-tracing studies revealed that neural connections are formed between these regions via afferent Dl fibers projecting to the Dm. However, research analyzing Dl-Dm synaptic transmission is scant. We have used electrophysiological techniques to assess neurotransmission and synaptic plasticity in projections from the Dl to the Dm in zebrafish. We demonstrate that electrical stimulation of the Dl division evoked a negative field potential in the Dm division that could be inhibited by application of the AMPA/kainate receptor antagonist, CNQX (5μM). Pairs of stimuli, when delivered at brief inter-pulse intervals (IPI), elicited paired pulse facilitation (PPF). Long-term potentiation (LTP), induced through the application of three trains of high frequency stimulation (HFS; 100Hz for 1s), lasted for more than 1h and could be inhibited with DL-AP5 (40μM), an N-methyl-d-aspartate (NMDA) receptor antagonist. Our results suggest that the intratelencephalic connection between Dl and Dm may play an important role in the synaptic plasticity of the zebrafish brain. It also provides a new electrophysiological model for studying the neural mechanisms underlying learning and memory in zebrafish.

Neural plasticity is affected by stress and heritable variation in stress coping style

Here we use a comparative model to investigate how behavioral and physiological traits correlate with neural plasticity. Selection for divergent post-stress cortisol levels in rainbow trout (Oncorhynchus mykiss) has yielded low- (LR) and high responsive (HR) lines. Recent reports show low behavioral flexibility in LR compared to HR fish and we hypothesize that this divergence is caused by differences in neural plasticity. Genes involved in neural plasticity and neurogenesis were investigated by quantitative PCR in brains of LR and HR fish at baseline conditions and in response to two different stress paradigms: short-term confinement (STC) and long-term social (LTS) stress. Expression of proliferating cell nuclear antigen (PCNA), neurogenic differentiation factor (NeuroD) and doublecortin (DCX) was generally higher in HR compared to LR fish. STC stress led to increased expression of PCNA and brain-derived neurotrophic factor (BDNF) in both lines, whereas LTS stress generally suppressed PCNA and NeuroD expression while leaving BDNF expression unaltered. These results indicate that the transcription of neuroplasticity-related genes is associated with variation in coping style, while also being affected by STC - and LTS stress in a biphasic manner. A higher degree of neural plasticity in HR fish may provide the substrate for enhanced behavioral flexibility.

Linking fearfulness and coping styles in fish

Consistent individual differences in cognitive appraisal and emotional reactivity, including fearfulness, are important personality traits in humans, non-human mammals, and birds. Comparative studies on teleost fishes support the existence of coping styles and behavioral syndromes also in poikilothermic animals. The functionalist approach to emotions hold that emotions have evolved to ensure appropriate behavioral responses to dangerous or rewarding stimuli. Little information is however available on how evolutionary widespread these putative links between personality and the expression of emotional or affective states such as fear are. Here we disclose that individual variation in coping style predicts fear responses in Nile tilapia Oreochromis niloticus, using the principle of avoidance learning. Fish previously screened for coping style were given the possibility to escape a signalled aversive stimulus. Fearful individuals showed a range of typically reactive traits such as slow recovery of feed intake in a novel environment, neophobia, and high post-stress cortisol levels. Hence, emotional reactivity and appraisal would appear to be an essential component of animal personality in species distributed throughout the vertebrate subphylum.

Circuit neuroscience in zebrafish

A central goal of modern neuroscience is to obtain a mechanistic understanding of higher brain functions under healthy and diseased conditions. Addressing this challenge requires rigorous experimental and theoretical analysis of neuronal circuits. Recent advances in optogenetics, high-resolution in vivo imaging, and reconstructions of synaptic wiring diagrams have created new opportunities to achieve this goal. To fully harness these methods, model organisms should allow for a combination of genetic and neurophysiological approaches in vivo. Moreover, the brain should be small in terms of neuron numbers and physical size. A promising vertebrate organism is the zebrafish because it is small, it is transparent at larval stages and it offers a wide range of genetic tools and advantages for neurophysiological approaches. Recent studies have highlighted the potential of zebrafish for exhaustive measurements of neuronal activity patterns, for manipulations of defined cell types in vivo and for studies of causal relationships between circuit function and behavior. In this article, we summarize background information on the zebrafish as a model in modern systems neuroscience and discuss recent results.

Secondary neurogenesis and telencephalic organization in zebrafish and mice: a brief review

Most zebrafish neurodevelopmental studies have focused on the embryo, which is characterized by primary neurogenesis of mostly transient neurons. Secondary neurogenesis becomes dominant in the hatching larva, when major brain parts are established and begin to differentiate. This developmental period allows for a comparative analysis of zebrafish brain organization with amniotes at equivalent stages of neurogenesis. Within a particular time window, the early forebrains of mice (Embyronic stage [E] 12.5/13.5 days [d]) and zebrafish (3 d) reveal highly comparable expression patterns of genes involved in neurogenesis, for example proneural and other transcription factors (Neurogenin1, NeuroD, Mash1/Zashla and Pax6). Further topological correspondences are seen in the expression of LIM and homeobox genes, such as Lhx6/7, Tbr2 and Dlx2a. When this analysis is extended to gamma-aminobutyric acid/glutamic acid decarboxylase (GABA/GAD) cell patterns during this critical time window, an astonishing degree of similarity between the two species is again seen, for example regarding the presence of GABA/GAD cells in the subpallium, with the pallium only starting to be invaded by such cells from the subpallium. Furthermore, the expression of proneural and other genes correlates with GABA cell patterns (e.g. Mash1/Zash1a gene expression in GABA-positive and Neurogenin1/NeuroD in GABA-negative telencephalic regions) in mice and zebrafish. Data from additional vertebrates, such as Xenopus, are also highly consistent with this analysis. Therefore, the vertebrate forebrain appears to undergo a phylotypic stage of secondary neurogenesis, characterized by regionally separated GABAergic (inhibitory) versus glutamatergic (excitatory) cell production sites, which are obscured later in development by tangential migration. This period is highly advantageous for molecular neuroanatomical cross-species comparisons.

Dorsomedial pallium lesions impair taste aversion learning in goldfish

The present work shows that the dorsomedial telencephalic pallium of teleost fish, proposed as homologous to the amygdala of mammals, is involved in taste aversion learning (TAL). To analyze the behavioral properties of TAL in goldfish, in Experiment 1, we used a delayed procedure similar to that employed with mammals, which consists of the presentation of two flavors on different days, one followed by lithium chloride and the other by saline, both after a 10-min delay. The results showed that goldfish developed a strong aversion to the gustatory stimulus followed by visceral discomfort and that, as in mammals, this learning was rapidly acquired, highly flexible and maintained for a long time. Experiment 2 showed that dorsomedial pallium lesions and the ablation of the telencephalic lobes impaired the acquisition of taste aversion in goldfish, whereas damage to the dorsolateral pallium (hippocampus homologue) or cerebellar corpus did not produce significant changes in this learning. Experiment 3 showed that these TAL deficits were not due to a lesion-related disruption of taste discrimination; goldfish with telencephalon ablation were able to learn to distinguish between the two tested flavors in a differential conditioning procedure. These functional data demonstrate that the dorsomedial pallium in teleosts is, like the amygdala, an essential component of the telencephalon-dependent taste aversion memory system and provide further support concerning the homology between both structures.

Neural and hormonal mechanisms of reproductive-related arousal in fishes

The major classes of chemicals and brain pathways involved in sexual arousal in mammals are well studied and are thought to be of an ancient, evolutionarily conserved origin. Here we discuss what is known of these neurochemicals and brain circuits in fishes, the oldest and most species-rich group of vertebrates from which tetrapods arose over 350 million years ago. Highlighted are case studies in vocal species where well-delineated sensory and motor pathways underlying reproductive-related behaviors illustrate the diversity and evolution of brain mechanisms driving sexual motivation between (and within) sexes. Also discussed are evolutionary insights from the neurobiology and reproductive behavior of elasmobranch fishes, the most ancient lineage of jawed vertebrates, which are remarkably similar in their reproductive biology to terrestrial mammals.

Identification of a brain center whose activity discriminates a choice behavior in zebrafish

The ability to make choices and carry out appropriate actions is critical for individual survival and well-being. Choice behaviors, from hard-wired to experience-dependent, have been observed across the animal kingdom. Although differential engagement of sensory neuronal pathways is a known mechanism, neurobiological substrates in the brain that underlie choice making downstream of sensory perception are not well understood. Here, we report a behavioral paradigm in zebrafish in which a half-light/half-dark visual image evokes an innate choice behavior, light avoidance. Neuronal activity mapping using the immediate early gene c-fos reveals the engagement of distinct brain regions, including the medial zone of the dorsal telencephalic region (Dm) and the dorsal nucleus of the ventral telencephalic area (Vd), the teleost anatomical homologs of the mammalian amygdala and striatum, respectively. In animals that were subjected to the identical sensory stimulus but displayed little or no avoidance, strikingly, the Dm and Vd were not engaged, despite similar levels of activation in the brain nuclei involved in visual processing. Based on these findings and previous connectivity data, we propose a neural circuitry model in which the Dm serves as a brain center, the activity of which predicates this choice behavior in zebrafish.

The dorsal pallium in zebrafish, Danio rerio (Cyprinidae, Teleostei)

Zebrafish as a neurogenetic model system depends on the correct neuroanatomical understanding of its brain organization. Here, we address the unresolved question regarding a possible zebrafish homologue of the dorsal pallial division, the region that in mammals gives rise to the isocortex. Analyzing the distributions of nicotine adenine dinucleotide phosphate diphorase (NADPHd) activity and parvalbumin in the anterior zebrafish telencephalon, we show that against previous assumptions the central (Dc) zone possesses its own germinative region in the dorsal proliferative zone. We define the central (Dc) zone as topologically corresponding to the dorsal pallial division of other vertebrates (mammalian isocortex). In addition, we confirm through BrdU-labeling experiments that the posterior (Dp) zone is formed by radial migration and homologous to the mammalian piriform cortex. Based on our results, we propose a new developmental and organizational model of the zebrafish pallium-one which is the result of a complex outward-inward folding.

Effects of stress and motivation on performing a spatial task

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

Variation of telencephalon morphology of the threespine stickleback (Gasterosteus aculeatus) in relation to inferred ecology

We tested the hypothesis that increased telencephalon size has evolved in threespine stickleback fish (Gasterosteus aculeatus) from structurally complex habitats using field-caught samples from one sea-run (ancestral) and 18 ecologically diverse freshwater (descendant) populations. Freshwater habitats ranged from shallow, structurally complex lakes with benthic-foraging stickleback (benthics), to deeper, structurally simple lakes in which stickleback depend more heavily on plankton for prey (generalists). Contrary to our expectations, benthics had smaller telencephala than generalists, but the shape of the telencephalon of the sea-run and benthic populations were more convex laterally. Convex telencephalon shape may indicate enlargement of the dorsolateral region, which is homologous with the tetrapod hippocampus. Telencephalon morphology is also sexually dimorphic, with larger, less convex telencephala in males. Freshwater stickleback from structurally complex habitats have retained the ancestral telencephalon morphology, but populations that feed more in open habitats on plankton have evolved larger, laterally concave telencephala.

Effects of local anesthesia of the cerebellum on classical fear conditioning in goldfish

BACKGROUND

Besides the amygdala, of which emotion roles have been intensively studied, the cerebellum has also been demonstrated to play a critical role in simple classical fear conditioning in both mammals and fishes. In the present study, we examined the effect of local administration of the anesthetic agent lidocaine into the cerebellum on fear-related, classical heart-rate conditioning in goldfish.

METHODS

The effects of microinjection of the anesthetic agent lidocaine into the cerebellum on fear conditioning were investigated in goldfish. The fear conditioning paradigm was delayed classical conditioning with light as a conditioned stimulus and electric shock as an unconditioned stimulus; cardiac deceleration (bradycardia) was the conditioned response.

RESULTS

Injecting lidocaine into the cerebellum had no effect on the base heart rate, an arousal/orienting response to the novel stimulus (i.e., the first presentation of light), or an unconditioned response to electric shock. However, lidocaine injection greatly impaired acquisition of conditioned bradycardia. Lidocaine injection 60 min before the start of the conditioning procedure showed no effect on acquisition of conditioned bradycardia, indicating that the effect of lidocaine was reversible.

CONCLUSIONS

The present results further confirm the idea that the cerebellum in teleost fish, as in mammals, is critically involved in classical fear conditioning.

Selective involvement of the goldfish lateral pallium in spatial memory

The involvement of the main pallial subdivisions of the teleost telencephalic pallium in spatial cognition was evaluated in a series of three experiments. The first two compared the effects of lesions selective to the lateral (LP), medial (MP) and dorsal (DP) telencephalic pallium of goldfish, on the retention and the reversal learning of a spatial constancy task which requires the use of allocentric or relational strategies. The results showed that LP lesions produced a selective impairment on the capability of goldfish to solve the spatial task previously learned and on the reversal learning of the same procedure, whereas MP and DP lesions did not produce observable deficits. The third experiment evaluated, by means of the AgNOR stain, learning-dependent changes of the neuronal transcription activity in the pallium of goldfish trained in the spatial constancy task or in a cue version of the same procedure, which only differed on their spatial cognition demands. The results revealed that training in the spatial task produced an increment in the transcriptive activity which was selective to the neurons of the ventral lateral pallium, as indicated by increases in the size of the nucleolar organizing region (NOR), the nucleolar organelles associated with the synthesis of ribosomal proteins. In contrast, training in the cue version did not produced observable changes. These data, revealing a striking functional similarity between the lateral telencephalic pallium of the teleost fish and the amniote hippocampus, provide additional evidence regarding the homology of both structures.

Adult neurogenesis and neuronal regeneration in the brain of teleost fish

Whereas adult neurogenesis appears to be a universal phenomenon in the vertebrate brain, enormous differences exist in neurogenic potential between "lower" and "higher" vertebrates. Studies in the gymnotiform fish Apteronotus leptorhynchus and in zebrafish have indicated that the relative number of new cells, as well as the number of neurogenic sites, are at least one, if not two, orders of magnitude larger in teleosts than in mammals. In teleosts, these neurogenic sites include brain regions homologous to the mammalian hippocampus and olfactory bulb, both of which have consistently exhibited neurogenesis in all species examined thus far. The source of the new cells in the teleostean brain are intrinsic stem cells that give rise to both glial cells and neurons. In several brain regions, the young cells migrate, guided by radial glial fibers, to specific target areas where they integrate into existing neural networks. Approximately half of the new cells survive for the rest of the fish's life, whereas the other half are eliminated through apoptotic cell death. A potential mechanism regulating development of the new cells is provided by somatic genomic alterations. The generation of new cells, together with elimination of damaged cells through apoptosis, also enables teleost fish rapid and efficient neuronal regeneration after brain injuries. Proteome analysis has identified a number of proteins potentially involved in the individual regenerative processes. Comparative analysis has suggested that differences between teleosts and mammals in the growth of muscles and sensory organs are key to explain the differences in adult neurogenesis that evolved during phylogenetic development of the two taxa.

How do social dominance and social information influence reproduction and the brain?

How does living in a social environment influence the brain? In particular, we ask the following questions: How do animals perceive and use social information? How does the perception of social information influence the reproductive system? Where is this represented in the brain? We present a model system in which these questions can be addressed, focusing on the brain's role in integrating information. In the social fish, Astatotilapia burtoni (Haplochromis), the relationship between social status and gonadotropin-releasing hormone (GnRH1) has been well established. Change in status results in numerous changes in the physiology of A. burtoni at every level of organization. Social status can regulate reproduction via the hypothalamic-pituitary-gonadal (HPG) axis. GnRH1 is used by the brain to signal the pituitary about reproductive state so reproductive control depends on regulation of this signaling peptide. In this fish, social dominance is tightly coupled to fertility. Here, we have exploited this link to understand the regulatory systems from circulating hormones, brain volume to gene expression.

Differential distribution of SK channel subtypes in the brain of the weakly electric fish Apteronotus leptorhynchus

Calcium signals in vertebrate neurons can induce hyperpolarizing membrane responses through the activation of Ca(2+)-activated potassium channels. Of these, small conductance (SK) channels regulate neuronal responses through the generation of the medium after-hyperpolarization (mAHP). We have previously shown that an SK channel (AptSK2) contributes to signal processing in the electrosensory system of Apteronotus leptorhynchus. It was shown that for pyramidal neurons in the electrosensory lateral line lobe (ELL), AptSK2 expression selectively decreases responses to low-frequency signals. The localization of all the SK subunits throughout the brain of Apteronotus then became of substantial interest. We have now cloned two additional SK channel subunits from Apteronotus and determined the expression patterns of all three AptSK subunits throughout the brain. In situ hybridization experiments have revealed that, as in mammalian systems, the AptSK1 and 2 channels showed a partially overlapping expression pattern, whereas the AptSK3 channel was expressed in different brain areas. The AptSK1 and 2 channels were the primary subunits found in the major electrosensory processing areas. Immunohistochemistry further revealed distinct compartmentalization of the AptSK1 and 2 channels in the ELL. AptSK1 was localized to the apical dendrites of pyramidal neurons, whereas AptSK2 channels are primarily somatic. The distinct expression patterns of all three AptSK channels may reflect subtype-specific contributions to neuronal function, and the high homology between subtypes from a number of species suggests that the functional roles for each channel subtype are conserved from early vertebrate evolution.

Social Regulation of Neurogenesis in Teleosts

Salmonid fishes such as the rainbow trout (Oncorhynchus mykiss) are frequently used to study behavioral and neuroendocrine effects of socially induced stress. A predictable aggressive response to territorial intrusion, a well described neuroanatomy, and many essential similarities in the stress response in fishes and other vertebrates are among the advantages of this comparative model. One conspicuous difference when compared to mammals, however, is that in teleost fish and other non-mammalian vertebrates, neurogenesis persists into adulthood to a much higher degree. Very little is known about the functional significance of individual differences in the rate of brain cell proliferation in fish, or whether structural changes in the fish brain are influenced by the social environment. In this paper we discuss the observation that brain cell proliferation is reduced in subordinate fish, focusing in particular on whether such individual variation reflects a difference in coping style or is indeed a response to social interactions.

Learning in cod (Gadus morhua): long trace interval retention

Basic knowledge about learning capacities and awareness in fish is lacking. In this study we investigated which temporal gaps Atlantic cod could tolerate between two associated events, using an appetitive trace-conditioning paradigm with blinking light as conditioned stimulus (CS) and dry fish food as unconditioned stimulus (US). CS-US presentations were either temporally overlapping (delay conditioning, CS duration 24 s, interstimulus interval 12 s) or separated by 20, 60, or 120 s (trace conditioning, CS duration 12 s) or 2 h (control, CS duration 12 s). The percentage of fish in the feeding area increased strongly during CS presentation in all delay, 20 s, and 60 s trace groups and in one out of two 120 s trace groups, but not in the control groups. In the 20 and 60 s trace procedures, the fish crowded together in the small feeding area during the trace interval, showing strong anticipatory behaviour. In all the conditioned groups, the fish responded to the CS within eight trials, demonstrating rapid learning. At 88 and 70 days after the end of the conditioning experiments, the delay and 20 s trace groups, respectively, were presented the CS six times at 2-h intervals without reward. All groups responded to the light signal, demonstrating memory retention after at least 3 months. This study demonstrates that Atlantic cod has an impressively good ability to associate two time-separated events and long time retention of learnt associations.

New neurons for the injured brain: mechanisms of neuronal regeneration in adult teleost fish

In contrast to mammals, teleost fish exhibit an enormous potential to continuously produce new neurons in many areas of the adult brain, and to regenerate neural tissue after brain injury. The regenerative capability of the teleost fish brain is based upon a series of well-orchestrated individual processes, including: elimination of damaged cells by apoptosis, removal of cellular debris by the action of microglia/macrophages, proliferation of endogenous neural precursor cells, radial glia-mediated migration of their progeny to the site of the lesion, neuronal differentiation, promotion of cellular survival, and integration of the new neurons into existing neural circuits. Combination of a well-defined cerebellar lesion paradigm with differential proteome analysis has demonstrated that identification of the multitude of proteins mediating the regenerative potential of the adult fish brain is feasible in the foreseeable future. A molecular understanding of brain regeneration in fish could help investigators to define novel strategies to stimulate endogenous neural precursor cells in the mammalian brain to undergo neurogenesis, thus forming the basis of a neuronal replacement therapy for brain injury or neurodegenerative diseases.

Distribution of NADPH-diaphorase and nitric oxide synthase reactivity in the central nervous system of the goldfish (Carassius auratus)

The nitrergic system has been inferred from cells positive to nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry and/or to the neuronal isoform of nitric oxide synthase (nNOS) immunohistochemistry in different species of vertebrates. The aim of the present work was to systematically study the distribution of cell producing nitric oxide in the goldfish (Carassius auratus) brain. To reach this goal, we firstly studied co-localization for NADPHd and nNOS techniques and demonstrated an extensive double labeling. Then, we studied the distribution through the brain by the two separate methods and found labeled cells widely distributed in brain and spinal cord. In the telencephalon, such cells were in both dorsal and ventral areas. In the diencephalon, the cells were found in some nuclei of the preoptic area and hypothalamus, habenula, pretectum, and dorsal and ventral thalamic regions. In the midbrain, cells were observed in the optic tectum, torus longitudinalis, and tegmental nuclei. In the rhombencephalon, cells were found in the cerebellum, the reticular formation, the locus coeruleus, the raphe nuclei, and the nuclei of the cranial nerves. Labeled cells were also observed in the gray area of the spinal cord. Cognizing that a direct comparison of the present results with those reported in other vertebrates is not clear-cut because of homologies; we conclude that the nitrergic system is roughly similar from fish to mammals.

Generation and long-term persistence of new neurons in the adult zebrafish brain: A quantitative analysis

Zebrafish, like other teleosts, are distinguished by their enormous potential to produce new neurons in many parts of the adult brain. By labeling S-phase cells with the thymidine analog 5-bromo-2'-deoxyuridine (BrdU), quantitative analysis demonstrated that, on average, 6000 new cells were generated in the entire adult brain within any 30 min period. This corresponds to roughly 0.06% of the total number of brain cells. Part of these cells underwent a second round of cell division a few days after their generation so that 10 days post-BrdU administration, when the cells have exited the mitotic cycle, approximately 10,000 BrdU-labeled cells were present in the entire brain. At post-BrdU survival times of 446-656 days, on average 4600 BrdU-labeled cells were found, suggesting that approximately 46% of the cells present at 10 days persisted in the adult zebrafish brain. Combination of BrdU-labeling of mitotic cells with immunostaining against Hu showed that roughly 47% of the BrdU-labeled cells that persisted in the brain expressed this neuronal marker protein. Taken together, the results of this investigation demonstrate that at least half of the cells generated in the adult zebrafish brain develop into neurons and are likely to persist for the rest of the fish's life.

Anxiety from a phylogenetic perspective: is there a qualitative difference between human and animal anxiety?

A phylogenetic approach to anxiety is proposed. The different facets of human anxiety and their presence at different levels of the phylum are examined. All organisms, including unicellular such as protozoan, can display a specific reaction to danger. The mechanisms enabling the appraisal of harmful stimuli are fully present in insects. In higher invertebrates, fear is associated with a specific physiological response. In mammals, anxiety is accompanied by specific cognitive responses. The expression of emotions diversifies in higher vertebrates, only primates displaying facial expressions. Finally, autonoetic consciousness, a feature essential for human anxiety, appears only in great apes. This evolutive feature parallels the progress in the complexity of the logistic systems supporting it (e.g., the vegetative and central nervous systems). The ability to assess one's coping potential, the diversification of the anxiety responses, and autonoetic consciousness seem relevant markers in a phylogenetic perspective.

Environmental Complexity and Social Organization Sculpt the Brain in Lake Tanganyikan Cichlid Fish

Complex brains and behaviors have occurred repeatedly within vertebrate classes throughout evolution. What adaptive pressures drive such changes? Both environmental and social features have been implicated in the expansion of select brain structures, particularly the telencephalon. East African cichlid fishes provide a superb opportunity to analyze the social and ecological correlates of neural phenotypes and their evolution. As a result of rapid, recent, and repeated radiations, there are hundreds of closely-related species available for study, with an astonishing diversity in habitat preferences and social behaviors. In this study, we present quantitative ecological, social, and neuroanatomical data for closely-related species from the (monophyletic) Ectodini clade of Lake Tanganyikan cichlid fish. The species differed either in habitat preference or social organization. After accounting for phylogeny with independent contrasts, we find that environmental and social factors differentially affect the brain, with environmental factors showing a broader effect on a range of brain structures compared to social factors. Five out of seven of the brain measures show a relationship with habitat measures. Brain size and cerebellar size are positively correlated with species number (which is correlated with habitat complexity); the medulla and olfactory bulb are negatively correlated with habitat measures. The telencephalon shows a trend toward a positive correlation with rock size. In contrast, only two brain structures, the telencephalon and hypothalamus, are correlated with social factors. Telencephalic size is larger in monogamous species compared to polygamous species, as well as with increased numbers of individuals; monogamy is also associated with smaller hypothalamic size. Our results suggest that selection or drift can act independently on different brain regions as the species diverge into different habitats and social systems.

Evolutionary background for stress-coping styles: relationships between physiological, behavioral, and cognitive traits in non-mammalian vertebrates

Reactions to stress vary between individuals, and physiological and behavioral responses tend to be associated in distinct suites of correlated traits, often termed stress-coping styles. In mammals, individuals exhibiting divergent stress-coping styles also appear to exhibit intrinsic differences in cognitive processing. A connection between physiology, behavior, and cognition was also recently demonstrated in strains of rainbow trout (Oncorhynchus mykiss) selected for consistently high or low cortisol responses to stress. The low-responsive (LR) strain display longer retention of a conditioned response, and tend to show proactive behaviors such as enhanced aggression, social dominance, and rapid resumption of feed intake after stress. Differences in brain monoamine neurochemistry have also been reported in these lines. In comparative studies, experiments with the lizard Anolis carolinensis reveal connections between monoaminergic activity in limbic structures, proactive behavior in novel environments, and the establishment of social status via agonistic behavior. Together these observations suggest that within-species diversity of physiological, behavioral and cognitive correlates of stress responsiveness is maintained by natural selection throughout the vertebrate sub-phylum.