ANIMAL BEHAVIOR AND WELL-BEING SYMPOSIUM: Interaction between coping style/personality, stress, and welfare: Relevance for domestic farm animals

This paper will argue that understanding animal welfare and the individual vulnerability to stress-related disease requires a fundamental understanding of functional individual variation as it occurs in nature as well as the underlying neurobiology and neuroendocrinology. Ecological studies in feral populations of mice, fish, and birds start to recognize the functional significance of phenotypes that individually differ in their behavioral and neuroendocrine response to environmental challenge. Recent studies indicate that the individual variation within a species may buffer the species for strong fluctuations in the natural habitat. Similarly, evolutionary ancient behavioral trait characteristics have now been identified in a range of domestic farm animals including cattle, pigs, and horses. Individual variation in behavior can be summarized in a 3-dimensional model with coping style, emotionality, and sociality as independent dimensions. These dimensions can be considered trait characteristics that are stable over time and across situations within the individual. This conceptual model has several consequences. First, the coping style dimension is strongly associated with differential stress vulnerability. Social stress studies show that proactive individuals are resilient under stable environmental conditions but vulnerable when outcome expectancies are violated. Reactive individuals are, in fact, rather flexible and seem to adapt more easily to a changing environment. A second consequence relates to genetics and breeding. Genetic selection for one trait usually implies selection for other traits as well. It is discussed that a more balanced breeding program that takes into account biologically functional temperamental traits will lead to more robust domestic farm animals. Finally, the relationship between temperamental traits, animal production, fitness, and welfare is discussed.

Stress and Adaptation

Animal models have contributed considerably to the current understanding of mechanisms underlying the role of stress in health and disease. Despite the progress made already, much more can be made by more carefully exploiting animals' and humans' shared biology, using ecologically relevant models. This allows a fundamental analysis of factors modulating individual adaptive capacity and hence individual vulnerability to disease. This article highlights an emerging scientific approach that uses a framework of interpretation that is more biologically oriented than previous approaches, to evaluate both the adaptive and maladaptive nature of the stress response in relation to existing environmental demands.

Adolescent social defeat disturbs adult aggression-related impulsivity in wild-type rats

Adolescence is generally considered as a developmental period during which adverse social experiences may have lasting consequences in terms of an increased vulnerability to affective disorders. This study aimed at determining the individual susceptibility to adolescent social stress using a rat model. We used rats of the Wild-type Groningen strain, which are characterized by a broad variation in adult levels of aggression and impulsivity. We hypothesized that experience of social defeat in adolescence results in heightened aggression and impulsivity levels in adulthood. In contrast to our expectation, adolescent social defeat did not lead to a difference in the average adult level of aggression and impulsivity, but the significant correlation between offensive aggression and impulsivity found in control animals was not present in animals defeated during adolescence.

Aggression and aspects of impulsivity in wild-type rats

Aggression is closely related to impulsive behavior both in humans and in animals. To avoid potential negative consequences, aggressive behavior is kept in control by strong inhibitory mechanisms. Failure of these inhibitory mechanisms results in violent behavior. In the present experiments, we investigated whether aggressive behavior is related to impulsive behavior. Furthermore, we investigated if violent behavior can be distinguished from "normal" aggressive behavior in terms of impulsivity levels. We used rats of the wild-type Groningen strain, rats of this strain differ widely in their level of offensive aggression expressed toward an unfamiliar intruder male, ranging from no aggression at all to very high levels of intense and sometimes violent behavior. Violent behavior was displayed by some of the animals that were given repeated winning experience. We used behavioral performance in an unpredictable operant conditioning paradigm for food reinforcement (variable interval 15) and performance in a differential-reinforcement of low rate (DRL-60s) responding as determinants for impulsivity. We predicted that offensive aggression is correlated with behavioral flexibility measured by the VI-15 procedure and that aggressive behavior is characterized by low behavioral inhibition on the DRL task. In addition we expected that violent animals would be characterized by extremely low levels of behavioral inhibition on the DRL task. We showed that the level of offensive aggression indeed positively correlated with VI-15 performance. In addition, we showed that behavioral performance on the DRL procedure is similar in low and high aggressive rats. However, violent animals can be dissociated by a lower efficiency of lever pressing on a DRL-60s schedule of reinforcement.

Chronic enhancement of brain oxytocin levels causes enduring anti-aggressive and pro-social explorative behavioral effects in male rats

Oxytocin (OXT) has been implicated in the regulation of social behaviors, including intermale offensive aggression. Recently, we showed that acute enhancement of brain OXT levels markedly suppressed offensive aggression and increased social exploration in resident rats confronted with an intruder in their home territory. Moreover, a different responsivity to the exogenous OXTergic manipulation was observed among individuals based on their baseline aggression. In this study we aimed at evaluating the behavioral response to chronically enhancing or attenuating central OXT levels, and at scrutinizing whether the trait-aggression moderates the treatment-induced behavioral changes. To this end, resident male wild-type Groningen rats were continuously (via osmotic minipumps) intracerebroventricularly infused with synthetic OXT or a selective OXT receptor (OXTR) antagonist for 7days. Changes in behavior were assessed performing a resident-intruder test before and at the end of the treatment period, as well as after 7days of withdrawal. Chronic infusion of OXT was found to selectively suppress aggression and enhance social exploration. Chronic blockage of OXTRs instead increased introductory aggressive behavior (i.e. lateral threat), yet without affecting the total duration of the aggression. The magnitude of the anti-aggressive changes correlated positively with the level of baseline aggression. Interestingly, OXT-induced behavioral changes persisted 7days after cessation of the treatment. In conclusion, these findings provide further evidence that enhanced functional activity of the central OXTergic system decreases social offensive aggression while it increases social explorative behavior. The data also indicate that chronically enhancing brain OXT levels may cause enduring anti-aggressive and pro-social explorative behavioral effects.

Antiaggressive activity of central oxytocin in male rats

RATIONALE

A substantial body of research suggests that the neuropeptide oxytocin promotes social affiliative behaviors in a wide range of animals including humans. However, its antiaggressive action has not been unequivocally demonstrated in male laboratory rodents.

OBJECTIVE

Our primary goal was to examine the putative serenic effect of oxytocin in a feral strain (wild type Groningen, WTG) of rats that generally show a much broader variation and higher levels of intermale aggression than commonly used laboratory strains of rats.

METHODS

Resident animals were intracerebroventricularly (icv) administered with different doses of synthetic oxytocin and oxytocin receptor antagonist, alone and in combination, in order to manipulate brain oxytocin functioning and to assess their behavioral response to an intruder.

RESULTS

Our data clearly demonstrate that acute icv administered oxytocin produces dose-dependent and receptor-selective changes in social behavior, reducing aggression and potentiating social exploration. These antiaggressive effects are stronger in the more offensive rats. On the other hand, administration of an oxytocin receptor antagonist tends to increase (nonsignificantly) aggression only in low-medium aggressive animals.

CONCLUSIONS

These results suggest that transiently enhancing brain oxytocin function has potent antiaggressive effects, whereas its attenuation tends to enhance aggressiveness. In addition, a possible inverse relationship between trait aggression and endogenous oxytocinergic signaling is revealed. Overall, this study emphasizes the importance of brain oxytocinergic signaling for regulating intermale offensive aggression. This study supports the suggestion that oxytocin receptor agonists could clinically be useful for curbing heightened aggression seen in a range of neuropsychiatric disorders like antisocial personality disorder, autism, and addiction.

The resident-intruder paradigm: a standardized test for aggression, violence and social stress

This video publication explains in detail the experimental protocol of the resident-intruder paradigm in rats. This test is a standardized method to measure offensive aggression and defensive behavior in a semi natural setting. The most important behavioral elements performed by the resident and the intruder are demonstrated in the video and illustrated using artistic drawings. The use of the resident intruder paradigm for acute and chronic social stress experiments is explained as well. Finally, some brief tests and criteria are presented to distinguish aggression from its more violent and pathological forms.

Poor sleep as a potential causal factor in aggression and violence

Clinical observations suggest that sleep problems may be a causal factor in the development of reactive aggression and violence. In this review we give an overview of existing literature on the relation between poor sleep and aggression, irritability, and hostility. Correlational studies are supporting such a relationship. Although limited in number, some studies suggest that treatment of sleep disturbances reduces aggressiveness and problematic behavior. In line with this is the finding that sleep deprivation actually increases aggressive behavior in animals and angriness, short-temperedness, and the outward expression of aggressive impulses in humans. In most people poor sleep will not evoke actual physical aggression, but certain individuals, such as forensic psychiatric patients, may be particularly vulnerable to the emotional dysregulating effects of sleep disturbances. The relation between sleep problems and aggression may be mediated by the negative effect of sleep loss on prefrontal cortical functioning. This most likely contributes to loss of control over emotions, including loss of the regulation of aggressive impulses to context-appropriate behavior. Other potential contributing mechanisms connecting sleep problems to aggression and violence are most likely found within the central serotonergic and the hypothalamic-pituitary-adrenal-axis. Individual variation within these neurobiological systems may be responsible for amplified aggressive responses induced by sleep loss in certain individuals. It is of great importance to identify the individuals at risk, since recognition and adequate treatment of their sleep problems may reduce aggressive and violent incidents.

The acute glucocorticoid stress response does not differentiate between rewarding and aversive social stimuli in rats

The mere presence of elevated plasma levels of corticosterone is generally regarded as evidence of compromised well-being. However, environmental stimuli do not necessarily need to be of a noxious or adverse nature to elicit activation of the stress response systems. In the present study, the physiological and neuroendocrine responses to repeated social stimuli that can be regarded as emotional opposites, i.e. social defeat and sexual behavior, were compared. Similar corticosterone responses were observed in animals confronted for the first time with either a highly aggressive male intruder or a receptive female, but a decrease was noticed in defeated rats tested during a third interaction. Only if animals are being physically attacked does the corticosterone response remain similar to the one observed during sexual behavior. In addition, the number of activated cells in the parvocellular hypothalamic paraventricular nucleus, as visualized by c-Fos immunocytochemistry, shows no difference between rats 1h after the third exposure to defeat or sex. Finally, biotelemetric recordings of heart rate, body temperature and locomotor activity show a robust response to both social stimuli that is generally, however, higher in animals being confronted with a receptive female. The data clearly indicate that acute plasma corticosterone levels are not reflecting the emotional valence of a salient stimulus. The magnitude of the response seems to be a direct reflection of the behavioral activity and hence of the metabolic requirements of activated tissues. Next to its direct metabolic role, acute increases in plasma corticosterone will have neurobiological and behavioral effects that largely depend on the neural circuitry that is activated by the stimulus that triggered its release.

Social Defeat during Adolescence and Adulthood Differentially Induce BDNF-Regulated Immediate Early Genes

Stressful life events generally enhance the vulnerability for the development of human psychopathologies such as anxiety disorders and depression. The incidence rates of adult mental disorders steeply rises during adolescence in parallel with a structural and functional reorganization of the neural circuitry underlying stress reactivity. However, the mechanisms underlying susceptibility to stress and manifestation of mental disorders during adolescence are little understood. We hypothesized that heightened sensitivity to stress during adolescence reflects age-dependent differences in the expression of activity-dependent genes involved in synaptic plasticity. Therefore, we compared the effect of social stress during adolescence with social stress in adulthood on the expression of a panel of genes linked to induction of long-term potentiation (LTP) and brain-derived neurotrophic factor (BDNF) signaling. We show that social defeat during adolescence and adulthood differentially regulates expression of the immediate early genes BDNF, Arc, Carp, and Tieg1, as measured by qPCR in tissue lysates from prefrontal cortex, nucleus accumbens, and hippocampus. In the hippocampus, mRNA levels for all four genes were robustly elevated following social defeat in adolescence, whereas none were induced by defeat in adulthood. The relationship to coping style was also examined using adult reactive and proactive coping rats. Gene expression levels of reactive and proactive animals were similar in the prefrontal cortex and hippocampus. However, a trend toward a differential expression of BDNF and Arc mRNA in the nucleus accumbens was detected. BDNF mRNA was increased in the nucleus accumbens of proactive defeated animals, whereas the expression level in reactive defeated animals was comparable to control animals. The results demonstrate striking differences in immediate early gene expression in response to social defeat in adolescent and adult rats.

Male Wistar rats are more susceptible to lasting social anxiety than Wild-type Groningen rats following social defeat stress during adolescence

Adolescence is an important period for the development of adult social competences. Social stress during adolescence may contribute not only to an inadequate social development but also to adult vulnerability to social anxiety. There seems to be a clear individual differentiation, however, in the vulnerability to the long-term negative consequences of social stress. The current study further explores this individual vulnerability and is aimed at the influence of social stress during adolescence on adult social anxiety and its context specificity. Rats from different strains (Wistar and Wild-type Groningen rats) were exposed to the resident-intruder paradigm five times during 10 min each in the period between postnatal day 45 and 58. Three and 7 weeks later, the animals were re-exposed to the context in the presence of either a dominant male or an anestrous female behind a wire mesh screen. Wistar rats that were socially defeated spent less time exploring the social stimulus in comparison with socially defeated Wild-type rats and their non-defeated controls. We conclude that the stressed Wistar rat shows signs of generalized social anxiety indicating that the Wistar rat can be considered as a vulnerable phenotype to effects of adolescent social stress.

Does the early social environment affect structure and consistency of personality in wild-type male's rat?

Animal personality has been extensively studied from a functional and evolutionary point of view. Less attention has been paid to the development of personality, its phenotypic plasticity, and the influence of manipulation of early environmental factors. Here we describe the effects of manipulating the sex ratio of the litter, at postnatal day (pnd) 3, in wild-type rats, on personality traits in adulthood. We measured the treatment effects on aggression, defensive burying, and open field behavior at pnd 90 and 120, as well as on their contextual generality, and stability over time (differential and structural consistency). Main effects of litter composition were found on open field behavior at pnd 120 but not on the other behaviors. Since correlations between behaviors changed over time irrespective of the specific treatment, whereas in previous studies on unmanipulated litters this was not the case we suggest that early handling may disrupt adult personality traits. Overall the data indicate that personality is less stable over time that often assumed, having both proximate and ultimate implications.

Physiological and hormonal responses to novelty exposure in rats are mainly related to ongoing behavioral activity

Stress research has been dominated by a circular type of reasoning that occurrence of a stress response is bad. Consequently, the stimulus is often interpreted as stressful in terms of aversiveness involving uncontrollability and unpredictability, which may have maladaptive and pathological consequences. However, the hypothalamic-pituitary-adrenal (HPA) axis and sympathico-adrenomedullary (SAM) system are not only activated in response of the organism to challenges, but also prepare and support the body for behavior. Therefore, a considerable part of the physiological and hormonal responses to a certain situation can be a direct reflection of the metabolic requirements for the normal ongoing behavioral activity, rather than of the stressful nature. In order to clarify this, behavioral, physiological, hormonal and electroencephalographic (EEG) responses to novel cage exposure were studied in male Sprague-Dawley rats. Forced confrontation with a novel cage has been interpreted as a psychological and aversive stressor. However, this interpretation is simply based on the occurrence of a stress response. This study aimed at detailed analysis of the time course of the novelty-induced responses. Different parameters were measured simultaneously in freely moving rats, which allowed correlational comparisons. Hereto, radio telemetry using a small implantable transmitter combined with permanent catheters and an automated blood sampling system was used. A camera placed above the cage allowed behavioral observations. The results show that novelty exposure induced significant increases in locomotor activity, heart rate, blood pressure and plasma corticosterone together with a complete lack of sleep as compared to the undisturbed control situation. The latency to reach significance and the duration of responses varied across parameters but all had recovered within 30min after termination of novelty. The behavioral activity (locomotor activity and EEG wakefulness duration) response pattern was significantly correlated with that of heart rate, blood pressure and plasma corticosterone. Behavioral observations showed mainly explorative behavior in response to novelty. Therefore, the present results indicate that the novelty-induced physiological and hormonal responses are closely related to the ongoing, mainly explorative behavioral activity induced by novelty. An interpretation in terms of metabolic support of ongoing behavior seems to be more appropriate than the frequently used stress interpretation. The present study also emphasizes the added value of simultaneous assessment of behavioral, physiological and hormonal parameters under controlled, non-confounding conditions.

Coping styles and behavioural flexibility: towards underlying mechanisms

A coping style (also termed behavioural syndrome or personality) is defined as a correlated set of individual behavioural and physiological characteristics that is consistent over time and across situations. This relatively stable trait is a fundamental and adaptively significant phenomenon in the biology of a broad range of species, i.e. it confers differential fitness consequences under divergent environmental conditions. Behavioural flexibility appears to be an important underlying attribute or feature of the coping style that might explain consistency across situations. Proactive coping is characterized by low flexibility expressed as rather rigid, routine-like behavioural tendencies and reduced impulse control (behavioural inhibition) in operant conditioning paradigms. This article summarizes some of the evidence that individual differentiation in behavioural flexibility emerges as a function of underlying variability in the activation of a brain circuitry that includes the prefrontal cortex and its key neurochemical signalling pathways (e.g. dopaminergic and serotonergic input). We argue that the multidimensional nature of animal personality and the terminology used for the various dimensions should reflect the differential pattern of activation of the underlying neuronal network and the behavioural control function of its components. Accordingly, unravelling the molecular mechanisms that give rise to individual differences in the coping style will be an important topic in biobehavioural neurosciences, ecology and evolutionary biology.

Stress revisited: a critical evaluation of the stress concept

With the steadily increasing number of publications in the field of stress research it has become evident that the conventional usage of the stress concept bears considerable problems. The use of the term 'stress' to conditions ranging from even the mildest challenging stimulation to severely aversive conditions, is in our view inappropriate. Review of the literature reveals that the physiological 'stress' response to appetitive, rewarding stimuli that are often not considered to be stressors can be as large as the response to negative stimuli. Analysis of the physiological response during exercise supports the view that the magnitude of the neuroendocrine response reflects the metabolic and physiological demands required for behavioural activity. We propose that the term 'stress' should be restricted to conditions where an environmental demand exceeds the natural regulatory capacity of an organism, in particular situations that include unpredictability and uncontrollability. Physiologically, stress seems to be characterized by either the absence of an anticipatory response (unpredictable) or a reduced recovery (uncontrollable) of the neuroendocrine reaction. The consequences of this restricted definition for stress research and the interpretation of results in terms of the adaptive and/or maladaptive nature of the response are discussed.

A time for learning and a time for sleep: the effect of sleep deprivation on contextual fear conditioning at different times of the day

STUDY OBJECTIVES

Sleep deprivation negatively affects memory consolidation, especially in the case of hippocampus-dependent memories. Studies in rodents have shown that 5 hours of sleep deprivation immediately following footshock exposure selectively impairs the formation of a contextual fear memory. In these studies, both acquisition and subsequent sleep deprivation were performed in the animals' main resting phase. However, in everyday life, subjects most often learn during their active phase.

DESIGN

Here we examined the effects of sleep deprivation on memory consolidation for contextual fear in rats when the task was performed at different times of the day, particularly, at the beginning of the resting phase or right before the onset of the active phase.

MEASUREMENTS AND RESULTS

Results show that sleep deprivation immediately following training affects consolidation of contextual fear, independent of time of training. However, in the resting phase memory consolidation was impaired by 6 hours of posttraining sleep deprivation, whereas, in the active phase, the impairment was only seen after 12 hours of sleep deprivation. Since rats sleep at least twice as much during the resting phase compared with the active phase, these data suggest that the effect of sleep deprivation depends on the amount of sleep that was lost. Also, control experiments show that effects of sleep deprivation were not related to the amount of stimulation the animals received and were therefore not likely an indirect effect of the sleep-deprivation method.

CONCLUSION

These results support the notion that sleep immediately following acquisition, independent of time of day, promotes memory consolidation and that sleep deprivation may disrupt this process depending on the amount of sleep that is lost.

Agomelatine reverses the decrease in hippocampal cell survival induced by chronic mild stress

The antidepressant agomelatine is a MT(1)/MT(2) receptor agonist and 5-HT(2C) antagonist. Its antidepressant activity is proposed to result from the synergy between these sets of receptors. Agomelatine-induced changes in the brain have been reported under basal conditions. Yet, little is known about its effects in the brain exposed to chronic stress as a risk factor for major depressive disorder. Recently, we described agomelatine-induced changes on neuronal activity and adult neurogenesis in the hippocampus of rats subjected to chronic footshock stress. In order to better characterize the actions of agomelatine in the stress-compromised brain, here we investigated its effects on hippocampal neurogenesis in the chronic mild stress (CMS) model. Adult male rats were subjected to various mild stressors for 5 weeks, and treated with agomelatine during the last 3 weeks of the stress period. The sucrose preference test was performed weekly to measure anhedonia, and the marble burying test was carried out at the end of the experiment to assess anxiety-like behavior. In our model, the CMS paradigm did not change sucrose preference; however, it increased marble burying behavior, indicating enhanced anxiety. Interestingly, this stress model differentially affected distinct stages of the neurogenesis process. Whereas CMS did not influence the rate of hippocampal cell proliferation, it significantly decreased the newborn cell survival and doublecortin expression in the dentate gyrus. Importantly, treatment with agomelatine completely normalized stress-affected cell survival and partly reversed reduced doublecortin expression. Taken together, these data show that agomelatine has beneficial effects on hippocampal neurogenesis in the CMS paradigm.

Sleep deprivation impairs contextual fear conditioning and attenuates subsequent behavioural, endocrine and neuronal responses

Sleep deprivation (SD) affects hippocampus-dependent memory formation. Several studies in rodents have shown that brief SD immediately following a mild foot shock impairs consolidation of contextual fear memory as reflected in a reduced behavioural freezing response during re-exposure to the shock context later. In the first part of this study, we examined whether this reduced freezing response is accompanied by an attenuated fear-induced activation of the hypothalamic-pituitary-adrenal (HPA) axis. Results show that 6h of SD immediately following the initial shock results in a diminished adrenal corticosterone (CORT) response upon re-exposure to the shock context the next day. In the second part, we established whether the attenuated freezing response in SD animals is associated with reduced activation of relevant brain areas known to be involved in the retrieval and expression of fear memory. Immunohistochemical analysis of brain slices showed that the normal increase in phosphorylation of the transcription factor 3',5'-cyclic AMP response-element binding protein (CREB) upon re-exposure to the shock context was reduced in SD animals in the CA1 region of the hippocampus and in the amygdala. In conclusion, brief SD impairs the consolidation of contextual fear memory. Upon re-exposure to the context, this is reflected in a diminished behavioural freezing response, an attenuated HPA axis response and a reduction of the normal increase of phosphorylated CREB expression in the hippocampus and amygdala.

Testosterone decrease does not play a major role in the suppression of hippocampal cell proliferation following social defeat stress in rats

Stress of social defeat in rodents is known to have a strong and long-lasting effect on brain, physiology and behavior, which bears similarities with certain human stress related psychopathologies. Previous experiments in this lab showed that social defeat stress suppresses testosterone secretion and causes a lasting desensitization of the serotonergic 5-HT(1A) receptors. Testosterone supplementation in socially stressed tree shrews prevented a decrease in hippocampal 5-HT(1A) receptor binding. These receptors are hypothesized to play an important role in neurogenesis in this brain structure. We designed the present experiment to test if social defeat reduces hippocampal cell proliferation and neurogenesis in rats and if testosterone supplementation can prevent this reduction. The results indicate that repeated social defeat stress on 5 successive days induces a significant drop in plasma testosterone levels in male rats and suppresses hippocampal cell proliferation 24h and 3weeks after the end of the stress period. Testosterone supplementation prevented the social stress induced drop in plasma testosterone levels. The hormone supplementation also reduced the negative effect of stress on hippocampal BrdU labeling at 3weeks post-defeat. This effect was, however, rather weak and was caused by the tendency of the hormone in itself to suppress proliferation and the failure to fully recover the proliferation rate. Survival of dentate gyrus cells that either proliferated prior to the stress period or 24h after the last defeat was not affected by the social defeats. Thus the stress-induced lowering of hippocampal cell proliferation is not likely to be caused by transient inhibition of testosterone secretion during social stress.

Neuroendocrinology of coping styles: towards understanding the biology of individual variation

Individual variation in behavior and physiology is a widespread and ecologically functional phenomenon in nature in virtually all vertebrate species. Due to domestication of laboratory animals, studies may suffer from a strong selection bias. This paper summarizes behavioral, neuroendocrine and neurobiological studies using the natural individual variation in rats and mice. Individual behavioral characteristics appear to be consistent over time and across situations. The individual variation has at least two dimensions in which the quality of the response to a challenging condition (coping style) is independent from the quantity of that response (stress reactivity). The neurobiology reveals important differences in the homeostatic control of the serotonergic neuron and the neuropeptides vasopressin and oxytocin in relation to coping style. It is argued that a careful exploitation of the broad natural and biologically functional individual variation in behavior and physiology may help in developing better animal models for understanding individual disease vulnerability.

The vicious cycle towards violence: focus on the negative feedback mechanisms of brain serotonin neurotransmission

Violence can be defined as a form of escalated aggressive behavior that is expressed out of context and out of inhibitory control, and apparently has lost its adaptive function in social communication. Little is known about the social and environmental factors as well as the underlying neurobiological mechanisms involved in the shift of normal adaptive aggression into violence. In an effort to model the harmful acts of aggression and violence in humans, we recently (re)developed an animal model that is focused on engendering uncontrolled forms of maladaptive aggressive behavior in laboratory-bred feral rats and mice. We show that certain (8-12%) constitutionally aggressive individuals gradually develop, over the course of repetitive exposures to victorious social conflicts, escalated (short-latency, high-frequency and ferocious attacks), persistent (lack of attack inhibition by defeat/submission signals and perseverance of the aggressive attack-biting bout), indiscriminating (attacking female and anesthetized male intruders) and injurious (enhanced vulnerable-body region attacks and inflicted wounding) forms of offensive aggression. Based on the neurobiological results obtained using this model, a revised view is presented on the key role of central serotonergic (auto)regulatory mechanisms in this transition of normal aggression into violence.

Inhibition of hippocampal cell proliferation by methotrexate in rats is not potentiated by the presence of a tumor

Methotrexate is a widely used cytostatic in chemotherapy cocktails for the treatment of cancer but is associated with cognitive impairment. Previous animal studies indicated that methorexate decreases hippocampal cell proliferation, which might contribute to the observed cognitive impairment. However, clinical studies have shown that cognitive impairment can also be noticed in some cancer patients before any systemic treatment is initiated. We aim in the present study to discern whether hippocampal cell proliferation is negatively affected by tumor growth and if the presence of a tumor amplifies the effects of methotrexate. Buffalo rats were subcutaneously injected with PBS or Morris Hepatoma 7777 cells to induce a tumor. Two weeks after this injection the animals received an intraperitoneal injection of methotrexate or saline. Three weeks later hippocampal cell proliferation was quantified using immunohistochemical staining. Treatment with Morris Hepatoma 7777 cells decreased the number of proliferating cells as compared to control animals. An overall tumor effect was absent mainly because methotrexate treatment significantly decreased cell proliferation with no differences between animals with or without a tumor. Neither methotrexate nor the tumor induced pica behavior. These findings indicate that although the presence of a tumor reduces hippocampal cell proliferation it does not affect the negative effect of methotrexate on this plasticity marker. Since sickness behavior is not induced by methotrexate or tumor presence it does not play a role in the development of cognitive deficits. This study further indicates that the effects of methotrexate on brain and behavior can be studied in healthy animals.

Violent phenotype in SAL mice is inflexible and fixed in adulthood

Violence was shown to be qualitatively different from functional hyper-aggression in mice selected for high aggression namely Short Attack Latency (SAL), Turku Aggressive (TA) and North Carolina (NC900) strains. This study aimed at investigating whether this adulthood violent phenotype as seen previously in the SAL mice is fixed and hence behaviorally inflexible right from day 1 of the experiment or consequential, i.e., subject to gradual change from functional aggression to violence. The functionally hyper-aggressive strains namely TA and NC900 strains served as controls for the study. Methodologically, behavioral (in)flexibility was studied using the overall sequential structure of agonistic behavior. In particular, intra-individual variations in the overall agonistic behavior as well as offensive, pre- and post-offensive behavior transitions, directly related to the resident-intruder interactions were investigated. The SAL mice showed the least intra-individual variation in their overall sequential agonistic structure as well as a fixed offense-oriented agonistic behavior of highest magnitude when compared with the other strains. Additionally, the pre- and post- offensive transitions were most salient in the functionally hyper-aggressive TA and NC900 strains, whereas virtually absent in the SAL mice. Thus, the violent behavior of the adult SAL mice is behaviorally inflexible or fixed, whereas the functionally hyper-aggressive behavior of the adult TA and NC900 mice is behaviorally flexible and constantly adaptive to the opponent behavior, over 3 days of repeated resident-intruder interaction.

Delineation of violence from functional aggression in mice: an ethological approach

The present study aims at delineating violence from aggression, using genetically selected high (SAL, TA, NC900) and low (LAL, TNA NC100) aggressive mouse strains. Unlike aggression, violence lacks intrinsic control, environmental constraints as well as functional endpoints. Conventional measures namely latency, frequency and duration were used initially to accomplish the objective of delineation using the above strains. However, these quantitative measures fail to reveal further details beyond the magnitude of differential aggression, especially within the high aggressive mouse strains. Hence, it was necessary to analyze further, the behavioral sequences that make up the agonistic encounter. Novel measures such as threat/(attack + chase) (T/AC) and offense/withdrawal (O/W) ratios, context dependency and first-order Markov chain analysis were used for the above purpose. Our present analyses reveal clear qualitative behavioral differences between the three high aggressive selection strains based on the following facets namely structure and context in an agonistic interaction. Structure refers to a detailed study of the agonistic interaction components (ritualistic display, offense and sensitivity to the opponent submission cues) between any two subjects (inter-male interaction for the present study). Context refers to the capacity to identify an opponent by nature of its state (free moving/anesthetized), sex and the environment (home/neutral territory). NC900 displayed context dependency and structurally a rich repertoire of agonistic interaction components with an opponent. SAL failed to show discrimination and its inter-male agonistic behavior is restricted to a repetitive and an opponent-insensitive pattern of attack and chase. TA was comparable to SAL in terms of the structure but sensitive to context variables. Thus, SAL seems to display a violent form of aggressive behavior, while NC900 display 'functional' hyperaggression against a docile opponent in an inter-male agonistic interaction.

Is hyper-aggressiveness associated with physiological hypoarousal? A comparative study on mouse lines selected for high and low aggressiveness

Aggressiveness is often considered a life-long, persistent personality trait and is therefore expected to have a consistent neurobiological basis. Recent meta-analyses on physiological correlates of aggression and violence suggest that certain aggression-related psychopathologies are associated with low functioning of the hypothalamo-pituitary-adrenal (HPA) axis and autonomic nervous system (ANS). We tested this hypothesis in mice selected for high and low aggressiveness by measuring baseline plasma corticosterone levels and, via radiotelemetry, heart rate and core body temperature. The radiotelemetric recordings were made for 48 h under baseline undisturbed conditions and for 90 min after a handling stressor. Consistent with the hypoarousal hypothesis of violence, we found lower resting heart rates in two out of the three highly aggressive selection lines. In contrast, body temperature during the active phase, as another ANS-regulated physiological parameter, was higher in two out of three highly aggressive lines. The handling-induced tachycardiac and hyperthermic responses were similar across the six mouse lines except for the most docile and obese line, which showed a blunted reactivity. Besides significant differences between strains, no differences in plasma corticosterone levels were found between the high- and low-aggressive phenotypes. These results are discussed in relation to the different types of aggression (normal versus pathological) exhibited by the three highly aggressive lines. We conclude that while high trait-like aggressiveness is generally associated with a higher active phase core body temperature, only animals that express pathological forms of aggression are characterized by a low resting heart rate.

Coping style and immunity in animals: making sense of individual variation

Predicting the individual vulnerability to immune mediated disease is one of the main challenges of modern biomedical research. However, the question of individual behavioral and physiological characteristics that might predict this vulnerability has been subject of research and debate for a long time. This paper will argue that animal models aimed at individual vulnerability should consider the biological function of variation in nature. An increasing number of studies show the ecological significance of variation within a species. Based on behavioral studies in several vertebrate species two coping style can be distinguished. Variation in coping style appears to play a role in the population dynamics and the evolutionary fitness of the species. Coping styles are reflected in a stable differentiation in the behavioral and physiological stress responsiveness over time and across situations. Based on the observations that the individual level of offensive aggressive behavior (i.e., the tendency to defend the home territory) is strongly related to the way animals react to various other environmental challenges, it is argued that the individual's level of offensiveness is an important indicator and component of a more trait-like behavioral and physiological response pattern (coping style) to environmental demands. The coping style of aggressive animals is principally aimed at a (pro)active prevention or manipulation of a stressor whereas the non-aggressive individuals tend to passively accept or react to it. Proactive coping is associated with high sympathetic reactivity to stressors whereas the more passive or reactive coping style generally has a higher HPA axis reactivity. In view of the immune modulating nature of these major neuroendocrine stress systems, one might expect that coping styles will be reflected in a differential vulnerability to immune mediated disease as well. Indeed, several studies have demonstrated such a relationship, indicating that the functional variation in coping style and related neuroendocrine stress reactivity, as it occurs in nature, might be a good standard for studies aimed at understanding individual vulnerability. This is in agreement with more recent views that also in humans stress reactivity may be the best predictor for the individual vulnerability to immune mediated diseases. This asks for a more fundamental and translational approach of individual disease vulnerability based on a common biological basis of individual differentiation in behavior and physiology in humans and animals.

Aggressive and non-aggressive personalities differ in oxidative status in selected lines of mice (Mus musculus)

Mice selected for aggression and coping (long attack latency (LAL), reactive coping strategy; short attack latency (SAL), pro-active coping strategy) are a useful model for studying the physiological background of animal personalities. These mice also show a differential stress responsiveness, especially in terms of hypothalamic-pituitary-adrenal axis reactivity, to various challenges. Since the stress response can increase the production of reactive oxygen species, we predicted that the basic oxidative status of the lines could differ. We found that LAL showed higher serum antioxidant capacity (OXY) than SAL, while no differences emerged for reactive oxygen metabolites (ROMs) or the balance between ROMs and OXY, reflecting oxidative stress. Moreover, the lines showed inverse relationships between ROMs or OXY and body mass corrected for age. The results indicate that variation in oxidative status is heritable and linked to personality. This suggests that different animal personalities may be accompanied by differences in oxidative status, which may predict differences in longevity.

Long-lasting suppression of hippocampal cell proliferation and impaired cognitive performance by methotrexate in the rat

Methotrexate (MTX) is a cytostatic agent widely used in combination with other agents as adjuvant chemotherapy for breast cancer and is associated with cognitive impairment as a long-term side effect in some cancer patients. This paper aimed to identify a neurobiological mechanism possibly responsible for this cognitive impairment using an animal model. The first study explored the hypothesis that MTX reduces neuronal cell proliferation. A dose-dependent long-lasting decrease in hippocampal cell proliferation was shown with Ki-67 immunocytochemistry, following a single intravenous injection of MTX (37.5-300 mg/kg). Animals treated with MTX also showed a dose-dependent transient decrease in body weight gain. In the second study, the effect of MTX (250 mg/kg) on two spatial learning tasks was examined. Animals treated with MTX learned the Morris water maze task adequately; however, these animals showed a longer latency time to cross the platform location in the probe trial, reflecting an impairment of spatial memory function. In the novel object recognition task, animals treated with MTX failed to distinguish a novel object from a familiar one, indicating a decrease in the comparator function of the hippocampus. Our studies indicated that, in the rat, MTX has a dose-dependent negative effect on hippocampal cell proliferation, and on cognitive behavior. These findings suggest that adverse effects of certain cytotoxic agents on hippocampal cell proliferation may have a potential contributory role in cognitive impairment observed in humans after chemotherapy.

Development of violence in mice through repeated victory along with changes in prefrontal cortex neurochemistry

Recent reviews on the validity of rodent aggression models for human violence have addressed the dimension of pathological, maladaptive, violent forms of aggression in male rodent aggressive behaviour. Among the neurobiological mechanisms proposed for the regulation of aggressive behaviour in its normal and pathological forms, serotonin plays a major role. However, the results on the detailed mechanism are still confusing and controversial, mainly because of difficulties in extrapolating from rodent to human psychopathological behaviour. Our aim was to investigate the involvement of serotonin in pathological aggression. We subjected mice genetically selected for high (SAL, TA, NC900 lines) and low (LAL, TNA, NC100) aggression levels to a repeated resident-intruder experience (RRI mice) or to handling as a control procedure (CTR mice). Pathological aggression parameters we recorded were aggression towards females and lack of communication between the resident and its opponent. In the same mice, we measured the monoamine levels in the prefrontal cortex, a brain region strongly involved in the regulation of motivated behaviour. Our results show that SAL mice augmented their proneness to attack and showed the most pathological phenotype, with disregard of the opponent's sex, high territorial behavioural patterns, and low sensitivity to signals of subordination. In contrast, TA and NC900 augmented their proneness to attack and low discrimination of the opponent's signals, without showing offence towards females. After repeated resident-intruder experience, serotonin levels in the prefrontal cortex were significantly lower in SAL than in LAL whereas dopamine turnover was significantly higher, compared to CTR mice. Serotonin turnover was significantly reduced in all RRI mice, with no strain differences. Noradrenaline was significantly lower in aggressive mice of the TA and NC900 lines compared to their low-aggressive counterparts, with no effect of the repeated resident-intruder experience. We conclude that social experience changes prefrontal cortex neurochemistry and elicits pathologically aggressive phenotypes.

Individual Variation in Coping with Stress: A Multidimensional Approach of Ultimate and Proximate Mechanisms

Ecological studies on feral populations of mice, fish and birds elucidate the functional significance of phenotypes that differ individually in their behavioral and neuroendocrine response to environmental challenge. Within a species, the capacity to cope with environmental challenges largely determines individual survival in the natural habitat. Recent studies indicate that individual variation within a species may buffer the species for strong fluctuations in the natural habitat. A conceptual framework will be presented that is based on the view that individual variation in aggressive behavior can be considered more generally as a variation in actively coping with environmental challenges. Highly aggressive individuals adopt a proactive coping style whereas low levels of aggression indicate a more passive or reactive style of coping. Coping styles have now been identified in a range of species and can be considered as trait characteristics that are stable over time and across situations. The dimension of coping style seems to be independent of an emotionality dimension. Hence, in the analysis of the proximate mechanisms of stress and adaptation, one has to consider the possibility that the mechanisms which determine the type of stress response might be independent from those underlying the magnitude of the response. The two coping styles differ in a number of important neurobiological and neuroendocrine systems. For example, proactive males differ significantly from reactive males in the homeostatic control of serotonergic activity resulting in completely opposite dose response relationships of various serotonergic drugs. The results so far show that proactive coping is characterized by a strong inhibitory control of the 5-HT neuron via its somatodendritic 5-HT(1A) autoreceptor. It is hypothesized that the regulation of serotonin release is causally related to coping style rather than emotionality. Understanding the functional individual variation as it occurs in nature and the underlying neurobiology and neuroendocrinology is fundamental in understanding individual vulnerability to stress related disease.

Differential effects of stress on adult hippocampal cell proliferation in low and high aggressive mice

Male wild house mice selected for a long (LAL) or a short (SAL) latency to attack a male intruder generally show opposing behavioural coping responses to environmental challenges. LAL mice, unlike SAL mice, adapt to novel challenges with a highly reactive hypothalamic-pituitary-adrenal axis and show an enhanced expression of markers for hippocampal plasticity. The present study aimed to test the hypothesis that these features of the more reactive LAL mice are reflected in parameters of hippocampal cell proliferation. The data show that basal cell proliferation in the subgranular zone (SGZ) of the dentate gyrus, assessed by the endogenous proliferation marker Ki-67, is lower in LAL than in SAL mice. Furthermore, application of bromodeoxyuridine (BrdU) over 3 days showed an almost two-fold lower cell proliferation rate in the SGZ in LAL versus SAL mice. Exposure to forced swimming resulted, 24 h later, in a significant reduction in BrdU + cell numbers in LAL mice, whereas cell proliferation was unaffected by this stressor in SAL mice. Plasma corticosterone and dentate gyrus glucocorticoid receptor levels were higher in LAL than in SAL mice. However, no differences between the SAL and LAL lines were found for hippocampal NMDA receptor binding. In conclusion, the data suggest a relationship between coping responses and hippocampal cell proliferation, in which corticosterone may be one of the determinants of line differences in cell proliferation responses to environmental challenges.

Social stress during adolescence in Wistar rats induces social anxiety in adulthood without affecting brain monoaminergic content and activity

Adolescence has been described as an important period to acquire social competences required for adult life. It has been suggested that early stress experiences could affect the development of the brain at different levels. These changes in the brain during adolescence may be related with the development of psychopathologies such as depression and social anxiety in adulthood. In the first experiment, we examined long-term effects of repeated social stress during adolescence on adult social approach-avoidance behavior. For that purpose, adolescent male Wistar rats were exposed twice at postnatal day (Pnd) 45 and Pnd48 to the resident-intruder paradigm followed by three times psychosocial threat with the same resident. Three weeks after the last psychosocial threat experience the animals were behaviorally tested in a social approach-avoidance test. Socially stressed animals spent less time in the interaction zone with an unfamiliar male adult rat. These data suggest that animals exposed to social stress during adolescence show a higher level of social anxiety in adulthood. In the second experiment, we investigated whether these long-term effects of social stress during adolescence on behavior draw a parallel with changes in brain monoamine content, biosynthesis and turnover. Using the same experimental design as in the first experiment, HPLC analysis of various brain regions showed that there were no differences in monoamine content, monoamine biosynthesis and monoamines activity in the prefrontal cortex, hippocampus, hypothalamus and striatum in adulthood. These results indicate that long-lasting changes in social behavior following social stress during adolescence are not accompanied by changes in brain monoamine content, biosynthesis and turnover.

Differential role of the 5-HT1A receptor in aggressive and non-aggressive mice: An across-strain comparison

Differential role of the 5-HT(1A) receptor in aggressive and non-aggressive mice: an across-strain comparison. PHYSIOL BEHAV 00(0) 000-000, 2006. According to the serotonin (5-HT)-deficiency hypothesis of aggression, highly aggressive individuals are characterized by low brain 5-HT neurotransmission. Key regulatory mechanisms acting on the serotonergic neuron involve the activation of the somatodendritic inhibitory 5-HT(1A) autoreceptor (short feedback loop) and/or the activation of postsynaptic 5-HT(1A) receptors expressed on neurons in cortico-limbic areas (long feedback loop). In this study, we examined whether low serotonin neurotransmission is associated with enhanced 5-HT(1A) (auto)receptor activity in highly aggressive animals. Male mice (SAL-LAL, TA-TNA, NC900-NC100) obtained through different artificial-selection breeding programs for aggression were observed in a resident-intruder test. The prefrontal cortex level of 5-HT and its metabolite 5-HIAA were determined by means of HPLC. The activity of the 5-HT(1A) receptors was assessed by means of the hypothermic response to the selective 5-HT(1A) agonists S-15535 (preferential autoreceptor agonist) and 8-OHDPAT (full pre- and postsynaptic receptor agonist). Highly aggressive mice had lower serotonin levels in the prefrontal cortex and two out of three aggressive strains had higher 5-HT(1A) (auto)receptor sensitivity. The results strengthen the validity of the serotonin-deficiency hypothesis of aggression and suggest that chronic exaggerated activity of the 5-HT(1A) receptor may be a causative link in the neural cascade of events leading to 5-HT hypofunction in aggressive individuals.

A new animal welfare concept based on allostasis

Animal welfare is an increasing issue of public concern and debate. As a result, many countries are reconsidering the way animal welfare is embedded in the legislation and rules for housing and care of animals. This requires general agreement of what animal welfare is. Unfortunately, the current science of animal welfare is less scientific than what has been claimed. In our view, it is overly guided by anthropocentric thinking about how animals ought to be handled and neglects the latest concept of physiology: 'The Allostasis Concept'. Allostasis, which means stability through change, has the potential to replace homeostasis as the core model of physiological regulation. Not constancy or freedoms, but capacity to change is crucial to good physical and mental health and good animal welfare. Therefore, not homeostasis but allostasis is at the basis of our new animal welfare concept. This paper is aimed at a broader scientific discussion of animal welfare that includes knowledge from the latest scientific developments in neurobiology and behavioral physiology, and generates views that are extremely relevant for the animal welfare discussion.

Do similar neural systems subserve aggressive and sexual behaviour in male rats? Insights from c-Fos and pharmacological studies

It is a common belief that male aggressive and sexual behaviour share many of the underlying neurobiological, neurological, pharmacological and neuroendocrine mechanisms. Therefore, we studied brain activation patterns in male rat after performance of aggressive and sexual behaviour and compared serotonergic pharmacology in the same paradigms to delineate possible similarities and differences. Patterns of Fos-immunoreactivity induced by aggressive and sexual encounters of Wild-type male Brown Norway rats were studied to localise the commonly activated (functionally shared) parts of the circuitry, and the specific (functionally different) parts of the neuronal circuitry. Some brain areas (caudal medial preoptic area and medial amygdala) were commonly activated, but other areas (e.g. posterodorsal parts of the medial amygdala, rostral preoptic and premammillary hypothalamus) showed remarkably specific differences in neural activation. 5-HT(1A) receptor agonists inhibit aggressive, but stimulate male sexual behaviour, whereas 5-HT(1B) receptor agonists inhibit both types of behaviour. Selective serotonin reuptake inhibitors share comparable inhibitory effects in aggression and sexual behaviour, although only at relatively high doses. We propose that separate hard-wired neural systems exist in the brain for aggressive and sexual behaviours, modulated via hierarchically 'higher-level' brain areas that are involved in the integration (gating) of the behavioural outcome of an organism.

5-HT1A and 5-HT1B receptor agonists and aggression: a pharmacological challenge of the serotonin deficiency hypothesis

More than any other brain neurotransmitter system, the indolamine serotonin (5-HT) has been linked to aggression in a wide and diverse range of species, including humans. The nature of this linkage, however, is not simple and it has proven difficult to unravel the precise role of this amine in the predisposition for and execution of aggressive behavior. The dogmatic view that 5-HT inhibits aggression has dominated both pharmacological research strategies to develop specific and effective novel drug treatments that reduce aggressive behavior and the pharmacological mechanistic interpretation of putative serenic drug effects. Our studies on brain serotonin and aggression in feral wild-type rats using the resident-intruder paradigm have challenged this so-called serotonin deficiency hypothesis of aggressive behavior. The well-known fact that certain 5-HT(1A/1B) receptor agonists potently and specifically reduce aggressive behavior without motor slowing and sedative effects is only consistent with this hypothesis under the assumption that the agonist mainly acts on the postsynaptic 5-HT(1A/1B) receptor sites. However, systemic injections of anti-aggressive doses of 5-HT(1A) and (1B) agonists robustly decrease brain 5-HT release due to their inhibitory actions at somatodendritic and terminal autoreceptors, respectively. The availability of the novel benzodioxopiperazine compound S-15535, which acts in vivo as a preferential agonist of the somatodendritic 5-HT(1A) auto-receptor and as an antagonist (weak partial agonist) at postsynaptic 5-HT(1A) receptors, allows for a pharmacological analysis of the exact site of action of this anti-aggressive effect. It was found that, similar to other prototypical full and partial 5-HT(1A) and/or 5-HT(1B) receptor agonists like repinotan, 8-OHDPAT, ipsapirone, buspirone, alnespirone, eltoprazine, CGS-12066B and CP-93129, also S-15535 very effectively reduced offensive aggressive behavior. Unlike the other ligands, however, a remarkable degree of behavioral specificity was observed after treatment with S-15535, in that the anti-aggressive effects were not accompanied by inhibiting (like other 5-HT(1A) receptor agonist with moderate to high efficacy at postsynaptic 5-HT(1A) receptors) or enhancing (like agonists with activity at 5-HT(1B) receptors and alnespirone) non-aggressive motor behaviors (e.g., social exploration, ambulation, rearing, and grooming) beyond the range of undrugged animals with corresponding levels of aggression. The involvement of 5-HT(1A) and/or 5-HT(1B) receptors in the anti-aggressive actions of these drugs was convincingly confirmed by showing that the selective 5-HT(1A) receptor antagonist WAY-100635 and/or the 5-HT(1B) receptor antagonist GR-127935, while inactive when given alone, effectively attenuated/prevented these actions. Furthermore, combined administration of S-15535 with either alnespirone or CGS-42066B elicited a clear additive effect, indicated by a left-ward shift in their dose-effect curves, providing further support for presynaptic sites of action (i.e., inhibitory somatodendritic 5-HT(1A) and terminal 5-HT(1B) autoreceptors). These findings strongly suggest that the specific anti-aggressive effects of 5-HT(1A) and 5-HT(1B) receptor agonists are predominantly based on reduction rather than enhancement of 5-HT neurotransmission during the combative social interaction. Apparently, normal display of offensive aggressive behavior is positively related to brief spikes in serotonergic activity, whereas an inverse relationship probably exists between tonic 5-HT activity and abnormal forms of aggression only.