Coping with unpredictability: dopaminergic and neurotrophic responses to omission of expected reward in Atlantic salmon (Salmo salar L.)

Feeding predictability as a cognitive enrichment protects brain function and physiological status in rainbow trout: a multidisciplinary approach to assess fish welfare

Cognitive enrichment is a promising but understudied type of environmental enrichment that aims to stimulate the cognitive abilities of animals by providing them with more opportunities to interact with (namely, to predict events than can occur) and to control their environment. In a previous study, we highlighted that farmed rainbow trout can predict daily feedings after two weeks of conditioning, the highest conditioned response being elicited by the combination of both temporal and signalled predictability. In the present study, we tested the feeding predictability that elicited the highest conditioned response in rainbow trout (both temporal and signalled by bubbles, BUBBLE + TIME treatment) as a cognitive enrichment strategy to improve their welfare. We thus analysed the long-term effects of this feeding predictability condition as compared with an unpredictable feeding condition (RANDOM treatment) on the welfare of rainbow trout, including the markers in the modulation of brain function, through a multidisciplinary approach. To reveal the brain regulatory pathways and networks involved in the long-term effects of feeding predictability, we measured gene markers of cerebral activity and plasticity, neurotransmitter pathways and physiological status of fish (oxidative stress, inflammatory status, cell type and stress status). After almost three months under these predictability conditions of feeding, we found clear evidence of improved welfare in fish from BUBBLE + TIME treatment. Feeding predictability allowed for a food anticipatory activity and resulted in fewer aggressive behaviours, burst of accelerations, and jumps before mealtime. BUBBLE + TIME fish were also less active between meals, which is in line with the observed decreased expression of transcripts related to the dopaminergic system. BUBBLE + TIME fish tented to present fewer eroded dorsal fin and infections to the pathogen Flavobacterium psychrophilum. Decreased expression of most of the studied mRNA involved in oxidative stress and immune responses confirm these tendencies else suggesting a strong role of feeding predictability on fish health status and that RANDOM fish may have undergone chronic stress. Fish emotional reactivity while isolated in a novel-tank as measured by fear behaviour and plasma cortisol levels were similar between the two treatments, as well as fish weight and size. To conclude, signalled combined with temporal predictability of feeding appears to be a promising approach of cognitive enrichment to protect brain function via the physiological status of farmed rainbow trout in the long term.

Positive effects of bubbles as a feeding predictor on behaviour of farmed rainbow trout

Occupational enrichment emerges as a promising strategy for improving the welfare of farmed animals. This form of enrichment aims to stimulate cognitive abilities of animals by providing them with more opportunities to interact with and control their environment. Predictability of salient daily events, and in particular predictability of feeding, is currently one of the most studied occupational enrichment strategies and can take several forms. In fish, while temporal predictability of feeding has been widely investigated, signalled predictability (based on a signal, such as light or sound) has received little attention. Depending on the type of predictability used and the ecology of the species, the effects on fish welfare often differ. The present study aimed to determine which feeding predictability would be most appropriate for rainbow trout, the main continental farmed fish in Europe, and what the consequences might be for their welfare. We tested four feeding predictability conditions: temporal (based on time of day), signalled (based on bubble diffusion), temporal + signalled (based on time and bubble diffusion), and unpredictable (random feeding times). Behavioural and zootechnical outcomes recorded were swimming activity, aggressive behaviours, burst of accelerations, and jumps, emotional reactivity, and growth. Our results showed that rainbow trout can predict daily feedings relying on time and/or bubbles as predictors as early as two weeks of conditioning, as evidenced by their increased swimming activity before feeding or during feed omission tests, which allowed to reinforce their conditioned response. Temporal predictability alone resulted in an increase in pre-feeding aggressive behaviours, burst of accelerations, and jumps, suggesting that the use of time as the sole predictor of feedings in husbandry practices may be detrimental to fish welfare. Signalled predictability with bubbles alone resulted in fewer pre-feeding agonistic behaviours, burst of accelerations, and jumps than in the temporal predictability condition. The combination of temporal and signalled predictability elicited the highest conditioned response and the level of pre-feeding aggression behaviours, burst of accelerations and jumps tended to be lower than for temporal predictability alone. Interestingly, fish swimming activity during bubble diffusion also revealed that bubbles were highly attractive regardless of the condition. Rainbow trout growth and emotional reactivity were not affected by the predictability condition. We conclude, therefore, that the use of bubbles as a feeding predictor could represent an interesting approach to improve rainbow trout welfare in farms, by acting as both an occupational and physical enrichment.

Convergent neural correlates of prenatal exposure to air pollution and behavioral phenotypes of risk for internalizing and externalizing problems: Potential biological and cognitive pathways

Humans are ubiquitously exposed to neurotoxicants in air pollution, causing increased risk for psychiatric outcomes. Effects of prenatal exposure to air pollution on early emerging behavioral phenotypes that increase risk of psychopathology remain understudied. We review animal models that represent analogues of human behavioral phenotypes that are risk markers for internalizing and externalizing problems (behavioral inhibition, behavioral exuberance, irritability), and identify commonalities among the neural mechanisms underlying these behavioral phenotypes and the neural targets of three types of air pollutants (polycyclic aromatic hydrocarbons, traffic-related air pollutants, fine particulate matter < 2.5 µm). We conclude that prenatal exposure to air pollutants increases risk for behavioral inhibition and irritability through distinct mechanisms, including altered dopaminergic signaling and hippocampal morphology, neuroinflammation, and decreased brain-derived neurotrophic factor expression. Future studies should investigate these effects in human longitudinal studies incorporating complex exposure measurement methods, neuroimaging, and behavioral characterization of temperament phenotypes and neurocognitive processing to facilitate efforts aimed at improving long-lasting developmental benefits for children, particularly those living in areas with high levels of exposure.

Distribution of two isoforms of tryptophan hydroxylase in the brain of rainbow trout (Oncorhynchus mykiss). An in situ hybridization study

Serotonin (5-HT) is one of the principal neurotransmitters in the nervous system of vertebrates. It is initially synthesized by hydroxylation of tryptophan (Trp) by means of tryptophan hydroxylase or TPH which is the rate-limiting enzyme in the production of 5-HT. In most vertebrates, there are two isoforms of TPH present, TPH1 and TPH2, which exhibit different catalytic or substrate specificity as well as different expression domains. Studies carried out in mammals show that only tph2 is expressed in the brain whereas tph1-mRNA is primarily localized in the enterochromaffin cells and pineal gland. A large number of neurons are also considered to be serotonergic or "pseudo-serotonergic" as they accumulate and release 5-HT yet do not produce it as no amine-synthetic enzymes are expressed, yet a combination of 5-HT transporters is observed. Therefore, tph expression is considered to be the only specific marker of 5-HT-producing neurons that can discriminate true 5-HT from pseudo-serotonergic neurons. This work examined in situ hybridization to study the mRNA distribution of one paralogue for tph1 and tph2 in the central nervous system of rainbow trout. Results show a segregated expression for both paralogues that predominantly match previous immunocytochemical studies. This study thus adds valuable information to the scarce analyses focusing on the central distribution of the expression of serotonergic markers, particularly tphs, in the vertebrate brain thus characterizing the true serotonergic brain territories.

Stressor controllability modulates the stress response in fish

BACKGROUND

In humans the stress response is known to be modulated to a great extent by psychological factors, particularly by the predictability and the perceived control that the subject has of the stressor. This psychological dimension of the stress response has also been demonstrated in animals phylogenetically closer to humans (i.e. mammals). However, its occurrence in fish, which represent a divergent vertebrate evolutionary lineage from that of mammals, has not been established yet, and, if present, would indicate a deep evolutionary origin of these mechanisms across vertebrates. Moreover, the fact that psychological modulation of stress is implemented in mammals by a brain cortical top-down inhibitory control over subcortical stress-responsive structures, and the absence of a brain cortex in fish, has been used as an argument against the possibility of psychological stress in fish, with implications for the assessment of fish sentience and welfare. Here, we have investigated the occurrence of psychological stress in fish by assessing how stressor controllability modulates the stress response in European seabass (Dicentrarchus labrax).

RESULTS

Fish were exposed to either a controllable or an uncontrollable stressor (i.e. possibility or impossibility to escape a signaled stressor). The effect of loss of control (possibility to escape followed by impossibility to escape) was also assessed. Both behavioral and circulating cortisol data indicates that the perception of control reduces the response to the stressor, when compared to the uncontrollable situation. Losing control had the most detrimental effect. The brain activity of the teleost homologues to the sensory cortex (Dld) and hippocampus (Dlv) parallels the uncontrolled and loss of control stressors, respectively, whereas the activity of the lateral septum (Vv) homologue responds in different ways depending on the gene marker of brain activity used.

CONCLUSIONS

These results suggest the psychological modulation of the stress response to be evolutionary conserved across vertebrates, despite being implemented by different brain circuits in mammals (pre-frontal cortex) and fish (Dld-Dlv).

Skin swabbing is a refined technique to collect DNA from model fish species

Model fish species such as sticklebacks and zebrafish are frequently used in studies that require DNA to be collected from live animals. This is typically achieved by fin clipping, a procedure that is simple and reliable to perform but that can harm fish. An alternative procedure to sample DNA involves swabbing the skin to collect mucus and epithelial cells. Although swabbing appears to be less invasive than fin clipping, it still requires fish to be netted, held in air and handled—procedures that can cause stress. In this study we combine behavioural and physiological analyses to investigate changes in gene expression, behaviour and welfare after fin clipping and swabbing. Swabbing led to a smaller change in cortisol release and behaviour on the first day of analysis compared to fin clipping. It also led to less variability in data suggesting that fewer animals need to be measured after using this technique. However, swabbing triggered some longer term changes in zebrafish behaviour suggesting a delayed response to sample collection. Skin swabbing does not require the use of anaesthetics and triggers fewer changes in behaviour and physiology than fin clipping. It is therefore a more refined technique for DNA collection with the potential to improve fish health and welfare.

Enriched environments enhance cognition, exploratory behaviour and brain physiological functions of Sparus aurata

Environmental enrichment is considered as a recommended tool to guarantee or improve the welfare of captive fish. This study demonstrates for the first time that structural environmental enrichment enhances cognition, exploratory behaviour and brain physiological functions of gilthead seabream (Sparus aurata). Seabream was reared in groups (n = 15) during 60 days under two different treatments: enriched tanks with plant-fibre ropes (EE) or bare/non-enriched tanks (NE). Fish were then exposed to a purpose-built maze for 1 h every second day in four trials. Analysis of video recordings showed that seabream under EE conditions presented higher overall exploratory behaviour, spatial orientation and learning capability compared to seabream from NE conditions. Results from brain monoamines analyses may suggest increased recent dopaminergic activity in telencephalon, known to be involved in learning processes; and increased serotonergic activity in cerebellum, involved in the coordination of balance, movements and orientation. In addition, EE-reared fish showed increased antioxidant activity in whole brain, with no apparent oxidative damage. Structural EE seemed to induce an hormetic response on juvenile seabream, improving their welfare status during captivity. Application of this kind of physical structure might be feasible at fish farms as a passive and non-invasive tool to improve welfare of intensively cultured seabream.

Dopaminergic signaling of uncertainty and the aetiology of gambling addiction

Although there is increasing clinical recognition of behavioral addictions, of which gambling disorder is the prototype example, there is a limited understanding of the psychological properties of (non-substance-related) behaviors that enable them to become 'addictive' in a way that is comparable to drugs of abuse. According to an influential application of reinforcement learning to substance addictions, the direct effects of drugs to release dopamine can create a perpetual escalation of incentive salience. This article focusses on reward uncertainty, which is proposed to be the core feature of gambling that creates the capacity for addiction. We describe the neuro-dynamics of the dopamine response to uncertainty that may allow a similar escalation of incentive salience, and its relevance to behavioral addictions. We review translational evidence from both preclinical animal models and human clinical research, including studies in people with gambling disorder. Further, we describe the evidence for 1) the effects of the omission of expected reward as a stressor and to promote sensitization, 2) the effect of the resolution of reward uncertainty as a source of value, 3) structural characteristics of modern Electronic Gaming Machines (EGMs) in leveraging these mechanisms, 4) analogies to the aberrant salience hypothesis of psychosis for creating and maintaining gambling-related cognitive distortions. This neurobiologically-inspired model has implications for harm profiling of other putative behavioral addictions, as well as offering avenues for enhancing neurological, pharmacological and psychological treatments for gambling disorder, and harm reduction strategies for EGM design.

Effects of exposure to chronic uncertainty and a sensitizing regimen of amphetamine injections on locomotion, decision-making, and dopamine receptors in rats

Gambling disorder (GD) is a behavioral addiction that may be linked to alterations in dopamine (DA) systems. Gambling involves chronic exposure to uncertain reward, which can sensitize the activity of DA systems. Here we explored how combinations of Pavlovian and instrumental uncertainty impact DA sensitization and risky decision-making. Experiment 1: 40 rats underwent 66 uncertainty exposure (UE) sessions during which they responded for saccharin. Animal responding was reinforced according to a fixed or variable (FR/VR) ratio schedule that turned on a conditioned stimulus (CS; light), which predicted saccharin on 50% or 100% of trials. Animals responded under one of the four conditions: FR-CS100% (no uncertainty), VR-CS100%, FR-CS50%, and VR-CS50% (maximal uncertainty). DA sensitization was inferred from an enhanced locomotor response to d-amphetamine (d-AMPH; 0.5 mg/kg) challenge. The rat gambling task (rGT) was used to assess decision-making. Experiment 2: 24 rats received 5 weeks of sensitizing d-AMPH or saline doses, followed by locomotor activity and rGT testing. Experiment 3: Effects of UE and a sensitizing d-AMPH regimen on DA D1, D2, and D3 receptor binding were assessed in 44 rats using autoradiography. Compared to FR-CS100%, VR-CS100% and VR-CS50% rats displayed a greater locomotor response to d-AMPH, and VR-CS50% rats demonstrated riskier decision-making. Chronic d-AMPH-treated rats mirrored the effects of VR-CS50% groups on these two indices. Both VR-CS50% and d-AMPH-treated groups had increased striatal DA D2 receptor binding. These results suggest that chronic uncertainty exposure, similar to exposure to a sensitizing d-AMPH regimen, sensitized the function of DA systems and increased risky decision-making.

Cognitive appraisal in fish: stressor predictability modulates the physiological and neurobehavioural stress response in sea bass

The role of cognitive factors in triggering the stress response is well established in humans and mammals (aka cognitive appraisal theory) but very seldom studied in other vertebrate taxa. Predictability is a key factor of the cognitive evaluation of stimuli. In this study, we tested the effects of stressor predictability on behavioral, physiological and neuromolecular responses in the European sea bass (Dicentrarchus labrax). Groups of four fish were exposed to a predictable (signalled) or unpredictable (unsignalled) stressor. Stressor predictability elicited a lower behavioural response and reduced cortisol levels. Using the expression of immediate early genes (c-fos, egr-1, bdnf and npas4) as markers of neuronal activity, we monitored the activity of three sea bass brain regions known to be implicated in stressor appraisal: the dorsomedian telencephalon, Dm (putative homologue of the pallial amygdala); and the dorsal (Dld) and ventral (Dlv) subareas of the dorsolateral telencephalon (putative homologue of the hippocampus). The activity of both the Dm and Dlv significantly responded to stressor predictability, suggesting an evolutionarily conserved role of these two brain regions in information processing related to stressor appraisal. These results indicate that stressor predictability plays a key role in the activation of the stress response in a teleost fish, hence highlighting the role of cognitive processes in fish stress.

Physiological responses during acute stress recovery depend on stress coping style in European sea bass, Dicentrarchus labrax

Individual stress coping style (reactive, intermediate and proactive) was determined in 3 groups of 120 pit tagged European seabass using the hypoxia avoidance test. The same three groups (no change in social composition) were then reared according to the standards recommended for this species. Then, 127 days later, individuals initially characterized as reactive, intermediate or proactive were submitted to an acute confinement stress for 30 min. Blood samples were taken to measure plasma cortisol levels 30 min (Stress30) or 150 min (Stress150) after the end of the confinement stress. Individuals were then sacrificed to sample the telencephalon in order to measure the main monoamines and their catabolites (at Stress30 only). Individuals from Stress150 were sampled for whole brain for a transcriptomic analysis. The main results showed that reactive individuals had a lower body mass than intermediate individuals which did not differ from proactive individuals. The physiological cortisol response did not differ between coping style at Stress30 but at Stress150 when intermediate and proactive individuals had recovered pre stress levels, reactive individuals showed a significant higher level illustrating a modulation of stress recovery by coping style. Serotonin turnover ratio was higher in proactive and reactive individuals compared to intermediate individuals and a significant positive correlation was observed with cortisol levels whatever the coping style. Further, the confinement stress led to a general increase in the serotonin turnover comparable between coping styles. Stress150 had a significant effect on target mRNA copy number (Gapdh mRNA copy number decreased while ifrd1 mRNA copy number increased) and such changes tended to depend upon coping style.

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.

Dopamine D1 receptor activation leads to object recognition memory in a coral reef fish

Object recognition memory is the ability to identify previously seen objects and is an adaptive mechanism that increases survival for many species throughout the animal kingdom. Previously believed to be possessed by only the highest order mammals, it is now becoming clear that fish are also capable of this type of memory formation. Similar to the mammalian hippocampus, the dorsolateral pallium regulates distinct memory processes and is modulated by neurotransmitters such as dopamine. Caribbean bicolour damselfish (Stegastes partitus) live in complex environments dominated by coral reef structures and thus likely possess many types of complex memory abilities including object recognition. This study used a novel object recognition test in which fish were first presented two identical objects, then after a retention interval of 10 min with no objects, the fish were presented with a novel object and one of the objects they had previously encountered in the first trial. We demonstrate that the dopamine D₁-receptor agonist (SKF 38393) induces the formation of object recognition memories in these fish. Thus, our results suggest that dopamine-receptor mediated enhancement of spatial memory formation in fish represents an evolutionarily conserved mechanism in vertebrates.

How do individuals cope with stress? Behavioural, physiological and neuronal differences between proactive and reactive coping styles in fish

Despite the use of fish models to study human mental disorders and dysfunctions, knowledge of regional telencephalic responses in non-mammalian vertebrates expressing alternative stress coping styles is poor. As perception of salient stimuli associated with stress coping in mammals is mainly under forebrain limbic control, we tested region-specific forebrain neural (i.e. mRNA abundance and monoamine neurochemistry) and endocrine responses under basal and acute stress conditions for previously characterised proactive and reactive Atlantic salmon. Reactive fish showed a higher degree of the neurogenesis marker proliferating cell nuclear antigen (pcna) and dopamine activity under basal conditions in the proposed hippocampus homologue (Dl) and higher post-stress plasma cortisol levels. Proactive fish displayed higher post-stress serotonergic signalling (i.e. higher serotonergic activity and expression of the 5-HT_1A receptor) in the proposed amygdala homologue (Dm), increased expression of the neuroplasticity marker brain-derived neurotropic factor (bdnf) in both Dl and the lateral septum homologue (Vv), as well as increased expression of the corticotropin releasing factor 1 (crf₁ ) receptor in the Dl, in line with active coping neuro-profiles reported in the mammalian literature. We present novel evidence of proposed functional equivalences in the fish forebrain with mammalian limbic structures.

Stimulation of dopamine D₁ receptor improves learning capacity in cooperating cleaner fish

Accurate contextual decision-making strategies are important in social environments. Specific areas in the brain are tasked to process these complex interactions and generate correct follow-up responses. The dorsolateral and dorsomedial parts of the telencephalon in the teleost fish brain are neural substrates modulated by the neurotransmitter dopamine (DA), and are part of an important neural circuitry that drives animal behaviour from the most basic actions such as learning to search for food, to properly choosing partners and managing decisions based on context. The Indo-Pacific cleaner wrasse Labroides dimidiatus is a highly social teleost fish species with a complex network of interactions with its 'client' reef fish. We asked if changes in DA signalling would affect individual learning ability by presenting cleaner fish two ecologically different tasks that simulated a natural situation requiring accurate decision-making. We demonstrate that there is an involvement of the DA system and D1 receptor pathways on cleaners' natural abilities to learn both tasks. Our results add significantly to the growing literature on the physiological mechanisms that underlie and facilitate the expression of cooperative abilities.

Dopamine disruption increases negotiation for cooperative interactions in a fish

Humans and other animals use previous experiences to make behavioural decisions, balancing the probabilities of receiving rewards or punishments with alternative actions. The dopaminergic system plays a key role in this assessment: for instance, a decrease in dopamine transmission, which is signalled by the failure of an expected reward, may elicit a distinct behavioural response. Here, we tested the effect of exogenously administered dopaminergic compounds on a cooperative vertebrate's decision-making process, in a natural setting. We show, in the Indo-Pacific bluestreak cleaner wrasse Labroides dimidiatus, that blocking dopamine receptors in the wild induces cleaners to initiate more interactions with and to provide greater amounts of physical contact to their client fish partners. This costly form of tactile stimulation using their fins is typically used to prolong interactions and to reconcile with clients after cheating. Interestingly, client jolt rate, a correlate of cheating by cleaners, remained unaffected. Thus, in low effective dopaminergic transmission conditions cleaners may renegotiate the occurrence and duration of the interaction with a costly offer. Our results provide first evidence for a prominent role of the dopaminergic system in decision-making in the context of cooperation in fish.

Differences in inhibitory avoidance, cortisol and brain gene expression in TL and AB zebrafish

Recently, we established an inhibitory avoidance paradigm in Tupfel Long-Fin (TL) zebrafish. Here, we compared task performance of TL fish and fish from the AB strain; another widely used strain and shown to differ genetically and behaviourally from TL fish. Whole-body cortisol and telencephalic gene expression related to stress, anxiety and fear were measured before and 2 h post-task. Inhibitory avoidance was assessed in a 3-day paradigm: fish learn to avoid swimming from a white to a black compartment where a 3V-shock is given: day 1 (first shock), day 2 (second shock) and day 3 (no shock, sampling). Tupfel Long-Fin fish rapidly learned to avoid the black compartment and showed an increase in avoidance-related spatial behaviour in the white compartment across days. In contrast, AB fish showed no inhibitory avoidance learning. AB fish had higher basal cortisol levels and expression levels of stress-axis related genes than TL fish. Tupfel Long-Fin fish showed post-task learning-related changes in cortisol and gene expression levels, but these responses were not seen in AB fish. We conclude that AB fish show higher cortisol levels and no inhibitory avoidance than TL fish. The differential learning responses of these Danio strains may unmask genetically defined risks for stress-related disorders.

Increased reactivity and monoamine dysregulation following stress in triploid Atlantic salmon (Salmo salar)

Artificial triploid salmonids are sterile and therefore commercially bred to prevent genetic interactions between wild and domestic fish strains. The full biological effects of having an extra chromosome set are largely unknown, but triploids are considered to be more sensitive to sub-optimal environmental conditions and to be stressed by the presence of diploid conspecifics. Brain serotonergic and dopaminergic activity are known to regulate the stress response in vertebrates, but monoamine systems in diploid and triploid fish have yet to be compared. Here we study monoamine neurochemistry in the telencephalon and brain stem of juvenile diploid and triploid Atlantic salmon (Salmo salar) in response to stress (unstressed vs stressed individuals) and holding (separate- vs mixed-ploidy) conditions. Both diploids and triploids showed an increase in serotonergic activity following stress, but the increase was significantly greater in the telencephalon of triploids compared to diploids. Furthermore, while telencephalic dopaminergic activity was significantly increased in diploids following stress, there was no response in triploids. Holding conditions had a significant effect on dopaminergic activity in the brain stem of diploids only, with lower values in mixed- compared to separate-ploidy conditions. These results suggest artificially produced triploids experience increased reactivity and monoaminergic dysregulation following stress that may impede their welfare and performance.

Feeding motivation as a personality trait in Nile tilapia (Oreochromis niloticus): role of serotonergic neurotransmission

Consistent individual variation in behaviour and physiology (i.e. animal personality or coping style) has emerged as a central topic in many biological disciplines. Yet, underlying mechanisms of crucial personality traits like feeding behaviour in novel environments remain unclear. Comparative studies, however, reveal a strong degree of evolutionary conservation of neural mechanisms controlling such behaviours throughout the vertebrate lineage. Previous studies have indicated duration of stress-induced anorexia as a consistent individual characteristic in teleost fishes. This study aims to determine to what degree brain 5-hydroxytryptamine (5-HT, serotonin) activity pertains to this aspect of animal personality, as a correlate to feed anticipatory behaviour and recovery of feed intake after transfer to a novel environment. Crucial to the definition of animal personality, a strong degree of individual consistency in different measures of feeding behaviour (feeding latency and feeding score), was demonstrated. Furthermore, low serotonergic activity in the hypothalamus was highly correlated with a personality characterized by high feeding motivation, with feeding motivation represented as an overall measure incorporating several behavioural parameters in a Principle Component Analyses (PCA). This study thus confirms individual variation in brain 5-HT neurotransmission as a correlate to complex behavioural syndromes related to feeding motivation.

Frustrative reward omission increases aggressive behaviour of inferior fighters

Animals use aggressive behaviour to gain access to resources, and individuals adjust their behaviour relative to resource value and own resource holding potential (RHP). Normally, smaller individuals have inferior fighting abilities compared with larger conspecifics. Affective and cognitive processes can alter contest dynamics, but the interaction between such effects and that of differing RHPs has not been adjudged. We investigated effects of omission of expected reward (OER) on competing individuals with contrasting RHPs. Small and large rainbow trout (Oncorhynchus mykiss) were conditioned to associate a light with reward. Thereafter, the reward was omitted for half of the fish prior to a contest between individuals possessing a 36-40% difference in RHP. Small control individuals displayed submissive behaviour and virtually no aggression. By contrast, small OER individuals were more aggressive, and two out of 11 became socially dominant. Increased aggression in small OER individuals was accompanied by increased serotonin levels in the dorsomedial pallium (proposed amygdala homologue), but no changes in limbic dopamine neurochemistry were observed in OER-exposed individuals. The behavioural and physiological response to OER in fish indicates that frustration is an evolutionarily conserved affective state. Moreover, our results indicate that aggressive motivation to reward unpredictability affects low RHP individuals strongest.