Behavioral, physiological, and neuroendocrine stress responses and differential sensitivity to diazepam in two Wistar rat lines selectively bred for high- and low-anxiety-related behavior

Diazepam effects on anxiety-related defensive behavior of male and female high and low open-field activity inbred mouse strains

Open-field activity is a commonly used measure of anxiety-related behavior in rodents. The inbred High and Low Activity strains of mice, selected for extreme differences in open-field activity, have been used as a genetic model of anxiety-related behaviors. These selected strains have been thoroughly studied through extensive behavioral testing, quantitative trait locus (QTL) mapping, whole-genome sequencing, and RNA sequencing, to uncover phenotypic and genotypic differences related to anxiety-related behavior. However, the effects of anxiolytic drugs on anxiety-related behavior in these strains have not been studied previously. This study allowed us to expand on previous findings to further characterize the anxiety-related behavior of these unique strains, using an anxiolytic drug. The goal of this study was to determine whether the treatment of adult male and female High Activity (low anxiety) and Low Activity (high anxiety) mice with diazepam, an agonist at the benzodiazepine allosteric site on the GABAA receptor and a drug commonly prescribed to treat anxiety disorders in humans, led to decreases in anxiety-like defensive behavioral responses as assessed in the open-field test (OFT) and elevated plus-maze (EPM). We tested the effects of three doses of diazepam (0, 0.5, 1.0, 3.0 mg/kg, i.p.), given 30 min before behavioral testing to one High Activity strain (H2) and two Low Activity strains (L1 and L2). There was an anxiolytic effect of diazepam observed in the High Activity strain, with more entries into the open arms of the elevated plus-maze, an effect similar to that seen in common mouse strains. However, the only anxiolytic effect of diazepam seen in the Low Activity strains was a reduction in stretch attend posture (SAP). Low Activity strains also displayed freezing behavior in both the OFT and EPM. The combination of the observed freezing behavior, that was not reduced by diazepam, and the reduction in SAP seen with diazepam, suggests a more complex phenotype that includes a component of innate fear in addition to anxiety-related risk assessment behaviors. Since fear and anxiety are distinguishable traits, and both contribute to human anxiety disorders, these results provide novel insight about interpretation of previous genetic and phenotypic differences observed between the High and Low Activity strains.

Selective breeding of rats for high (HAB) and low (LAB) anxiety-related behaviour: A unique model for comorbid depression and social dysfunctions

Animal models of selective breeding for extremes in emotionality are a strong experimental approach to model psychopathologies. They became indispensable in order to increase our understanding of neurobiological, genetic, epigenetic, hormonal, and environmental mechanisms contributing to anxiety disorders and their association with depressive symptoms or social deficits. In the present review, we extensively discuss Wistar rats selectively bred for high (HAB) and low (LAB) anxiety-related behaviour on the elevated plus-maze. After 30 years of breeding, we can confirm the prominent differences between HAB and LAB rats in trait anxiety, which are accompanied by consistent differences in depressive-like, social and cognitive behaviours. We can further confirm a single nucleotide polymorphism in the vasopressin promotor of HAB rats causative for neuropeptide overexpression, and show that low (or high) anxiety and fear levels are unlikely due to visual dysfunctions. Thus, HAB and LAB rats continue to exist as a reliable tool to study the multiple facets underlying the pathology of high trait anxiety and its comorbidity with depression-like behaviour and social dysfunctions.

Domestication and social environment modulate fear responses in young chickens

Domesticated species differ from their wild ancestors in a mosaic of traits. Classical domestication theories agree that reactivity to fear and stress is one of the main traits affected. Domesticated species are expected to be less fear and stress prone to than their wild counterparts. To test this hypothesis, we compared the behavioural responses of White Leghorn (WL) chicks to their wild counterparts, Red Junglefowl (RJF) chicks in risk-taking situations. In order to obtain food, the chicks faced an unknown and potentially harmful object at the presence or absence of a social partner. We found that according to our predictions, RJF were more stressed and fearful of the object than the WL. Still, RJF were more explorative than WL. Additionally, the presence of a social partner reduced the fear response in both, but had a stronger effect on RJF. Finally, WL were more food orientated than the RJF. Our results confirmed classical domestication hypotheses of downregulation of the stress system and importance of the social partner in domesticated farm chicken.

Fast antidepressant action of ketamine in mouse models requires normal VGLUT1 levels from prefrontal cortex neurons

The NMDA antagonist ketamine demonstrated a fast antidepressant activity in treatment-resistant depression. Pre-clinical studies suggest that de novo synthesis of the brain-derived neurotrophic factor (BDNF) in the PFC might be involved in the rapid antidepressant action of ketamine. Applying a genetic model of impaired glutamate release, this study aims to further identify the molecular mechanisms that could modulate antidepressant action and resistance to treatment. To that end, mice knocked-down for the vesicular glutamate transporter 1 (VGLUT1+/-) were used. We analyzed anhedonia and helpless behavior as well as the expression of the proteins linked to glutamate transmission in the PFC of mice treated with ketamine or the reference antidepressant reboxetine. Moreover, we analyzed the acute effects of ketamine in VGLUT1+/- mice pretreated with chronic reboxetine or those that received a PFC rescue expression of VGLUT1. Chronic reboxetine rescued the depressive-like phenotype of the VGLUT1+/- mice. In addition, it enhanced the expression of the proteins linked to the AMPA signaling pathway as well as the immature form of BDNF (pro-BDNF). Unlike WT mice, ketamine had no effect on anhedonia or pro-BDNF expression in VGLUT1+/- mice; it also failed to decrease phosphorylated eukaryote elongation factor 2 (p-eEF2). Nevertheless, we found that reboxetine administered as pretreatment or PFC overexpression of VGLUT1 did rescue the antidepressant-like activity of acute ketamine in the mice. Our results strongly suggest that not only do PFC VGLUT1 levels modulate the rapid-antidepressant action of ketamine, but also highlight a possible mechanism for antidepressant resistance in some patients.

Dissecting operant alcohol self-administration using saccharin-fading procedure

BACKGROUND

Although alcohol use disorder is a complex human pathology, the use of animal models represents an opportunity to study some aspects of this pathology. One of the most used paradigms to study the voluntary alcohol consumption in rodents is operant self-administration (OSA).

AIMS

In order to facilitate the performance of this paradigm, we aim to describe some critical steps of OSA under a saccharin-fading procedure.

MATERIAL & METHODS

We used 40 male Wistar rats to study the process of acquiring the operant response through a saccharin-fading procedure under a fixed ratio (FR1) schedule of reinforcement. Next, we analyze the alcohol introduction and concentration increase, the effect of an alcohol deprivation, and the analogy between this paradigm with the Drinking in the Dark-Multiple Scheduled Access paradigm.

RESULTS

During alcohol concentration increase, animals reduced their lever presses in accordance with the increase in alcohol concentration. On the contrary, the consumption measured in g·kg^-1 BW showed a great stability. The lever presses pattern within operant session changes with the introduction of different alcohol concentrations: at higher alcohol concentrations, animals tended to accumulate most of their presses in the initial period of the session.

DISCUSSION

We show the utility of fading with low concentrations of saccharin and the evolution of the operant response through the different concentrations of alcohol.

CONCLUSION

Taken together, our results aimed to dissect the acquisition and maintenance of OSA behavior as well as other related variables, to facilitate the understanding and performance of this paradigm.

Pharmacological and Physiological Correlates of the Bidirectional Fear Phenotype of the Carioca Rats and Other Bidirectionally Selected Lines

The Carioca rat lines originated from the selective bidirectional breeding of mates displaying extreme defense responses to contextual conditioned fear. After three generations, two distinct populations could be distinguished: the Carioca High- and Low-conditioned Freezing rats, CHF, and CLF, respectively. Later studies identified strong anxiety-like behaviors in the CHF line, while indications of impulsivity and hyperactivity were prominent in the CLF animals. The present review details the physiological and pharmacological-related findings obtained from these lines. The results discussed here point towards a dysfunctional fear circuitry in CHF rats, including alterations in key brain structures and the serotoninergic system. Moreover, data from these animals highlight important alterations in the stress-processing machinery and its associated systems, such as energy metabolism and antioxidative defense. Finally, evidence of an alteration in the dopaminergic pathway in CLF rats is also debated. Thus, accumulating data gathered over the years, place the Carioca lines as significant animal models for the study of psychiatric disorders, especially fear-related ones like anxiety.

Challenges in the use of animal models and perspectives for a translational view of stress and psychopathologies

The neurobiology and development of treatments for stress-related neuropsychiatric disorders rely heavily on animal models. However, the complexity of these disorders makes it difficult to model them entirely, so only specific features of human psychopathology are emulated and these models should be used with great caution. Importantly, the effects of stress depend on multiple factors, like duration, context of exposure, and individual variability. Here we present a review on pre-clinical studies of stress-related disorders, especially those developed to model posttraumatic stress disorder, major depression, and anxiety. Animal models provide relevant evidence of the underpinnings of these disorders, as long as face, construct, and predictive validities are fulfilled. The translational challenges faced by scholars include reductionism and anthropomorphic/anthropocentric interpretation of the results instead of a more naturalistic and evolutionary understanding of animal behavior that must be overcome to offer a meaningful model. Other limitations are low statistical power of analysis, poor evaluation of individual variability, sex differences, and possible conflicting effects of stressors depending on specific windows in the lifespan.

Bidirectional genetic selection of behaviors involved in social interaction of Wistar rats

Bidirectional selection is a procedure in which an arbitrary characteristic is chosen as a selection criterion and animals exhibiting more of this characteristic are bred in one group and animals exhibiting less are bred in another group. The procedure is repeated along generations until the selected characteristic becomes stable, resulting in two strains that are opposite in relation to the chosen characteristic. The present study aimed at selectively breeding rats exhibiting either a high or a low tendency to socialize by using the proximity test. We tested male and female Wistar rats in a square open field with a communicating birdcage, separated by a grid, containing a co-specific rat and coupled on the outside. Subjects that remained more time in front of the birdcage, interacting with the co-specific rat were bred in a group considered of high sociability (SOC+). Likewise, subjects that remained little time in front of the birdcage, with little interaction with the co-specific rat, were bred in a second group considered of low sociability (SOC-). By the 10th generation, the bidirectional selection resulted in SOC+ rats that spent a large amount of time in front of the cage sniffing and rearing in interaction with the co-specific rat and spent less time in the corners, exploring and grooming. It also resulted in SOC- rats that spent a small amount of time in front of the cage sniffing and rearing in interaction with the co-specific rat and spent more time in the corners and used most of their time grooming.

Endocannabinoids, cannabinoids and the regulation of anxiety

Cannabis has been used for hundreds of years, with its ability to dampen feelings of anxiety often reported as a primary reason for use. Only recently has the specific role cannabinoids play in anxiety been thoroughly investigated. Here we discuss the body of evidence describing how endocannabinoids and exogenous cannabinoids are capable of regulating the generation and termination of anxiety states. Disruption of the endogenous cannabinoid (eCB) system following genetic manipulation, pharmacological intervention or stress exposure reliably leads to the generation of an anxiety state. On the other hand, upregulation of eCB signaling is capable of alleviating anxiety-like behaviors in multiple paradigms. When considering exogenous cannabinoid administration, cannabinoid receptor 1 (CB1) agonists have a biphasic, dose-dependent effect on anxiety such that low doses are anxiolytic while high doses are anxiogenic, a phenomenon that is evident in both rodent models and humans. Translational studies investigating a loss of function mutation in the gene for fatty acid amide hydrolase, the enzyme responsible for metabolizing AEA, have also shown that AEA signaling regulates anxiety in humans. Taken together, evidence reviewed here has outlined a convincing argument for cannabinoids being powerful regulators of both the manifestation and amelioration of anxiety symptoms, and highlights the therapeutic potential of targeting the eCB system for the development of novel classes of anxiolytics. This article is part of the special issue on 'Cannabinoids'.

Individual differences in the neuroendocrine response of male rats to emotional stressors are not trait-like and strongly depend on the intensity of the stressors

Biological response to stressors is critical to understand stress-related pathologies and vulnerability to psychiatric diseases. It is assumed that we can identify trait-like characteristics in biological responsiveness by testing subjects in a particular stressful situation, but there is scarce information on this issue. We then studied, in a normal outbred population of adult male rats (n = 32), the response of well-characterized stress markers (ACTH, corticosterone and prolactin) to different types of stressors: two novel environments (open-field, OF1 and OF2), an elevated platform (EP), forced swim (SWIM) and immobilization (IMO). Based on both plasma ACTH and prolactin levels, the OF1 was the lowest intensity situation, followed by the OF2 and the EP, then SWIM and finally IMO. When correlations between the individual responses to the different stressors were studied, the magnitude of the correlations was most dependent on the similarities in intensity rather than on other characteristics of stressors, with good correlations between similar intensity stressors and no correlations at all were found between stressors markedly differing in intensity. In two additional confirmatory experiments (n = 37 and n = 20) with HPA hormones, we observed good correlation between the response to restraint and IMO, which were close in intensity, and no correlation between OF1 and SWIM. The present results suggest that individual neuroendocrine response to a particular stressor does not predict the response to another stressor greatly differing in intensity, thus precluding characterization of low or high responsive individuals to any stressor in a normal population. The present data have important implications for human studies.

Recovering from depression with repetitive transcranial magnetic stimulation (rTMS): a systematic review and meta-analysis of preclinical studies

Repetitive transcranial magnetic stimulation (rTMS) has gained growing interest for the treatment of major depression (MDD) and treatment-resistant depression (TRD). Most knowledge on rTMS comes from human studies as preclinical application has been problematic. However, recent optimization of rTMS in animal models has laid the foundations for improved translational studies. Preclinical studies have the potential to help identify optimal stimulation protocols and shed light on new neurobiological-based rationales for rTMS use. To assess existing evidence regarding rTMS effects on depressive-like symptoms in rodent models, we conducted a comprehensive literature search in accordance with PRISMA guidelines (PROSPERO registration number: CRD42019157549). In addition, we conducted a meta-analysis to determine rTMS efficacy, performing subgroup analyses to examine the impact of different experimental models and neuromodulation parameters. Assessment of the depressive-like phenotype was quite homogeneous whilst rTMS parameters among the 23 included studies varied considerably. Most studies used a stress-induced model. Overall, results show a largely beneficial effect of active rTMS compared to sham stimulation, as reflected in the statistically significant recovery of both helplessness (SDM 1.34 [1.02;1.66]) and anhedonic (SDM 1.87 [1.02;2.72]) profiles. Improvement of the depressive-like phenotype was obtained in all included models and independently of rTMS frequency. Nonetheless, these results have limited predictive value for TRD patients as only antidepressant-sensitive models were used. Extending rTMS studies to other MDD models, corresponding to distinct endophenotypes, and to TRD models is therefore crucial to test rTMS efficacy and to develop cost-effective protocols, with the potential of yielding faster clinical responses in MDD and TRD.

Individual differences in inflammatory and oxidative mechanisms of stress-related mood disorders

Emotional stress leads to the development of peripheral disorders and is recognized as a modifiable risk factor for psychiatric disorders, particularly depression and anxiety. However, not all individuals develop the negative consequences of emotional stress due to different stress coping strategies and resilience to stressful stimuli. In this review, we discuss individual differences in coping styles and the potential mechanisms that contribute to individual vulnerability to stress, such as parameters of the immune system and oxidative state. Initial differences in inflammatory and oxidative processes determine resistance to stress and stress-related disorders via the alteration of neurotransmitter content in the brain and biological fluids. Differences in coping styles may serve as possible predictors of resistance to stress and stress-related disorders, even before stressful conditions. The investigation of natural variabilities in stress resilience may allow the development of new methods for preventive medicine and the personalized treatment of stress-related conditions.

New neurons restore structural and behavioral abnormalities in a rat model of PTSD

Post-traumatic stress disorder (PTSD) has been associated with anxiety, memory impairments, enhanced fear, and hippocampal volume loss, although the relationship between these changes remain unknown. Single-prolonged stress (SPS) is a model for PTSD combining three forms of stress (restraint, swim, and anesthesia) in a single session that results in prolonged behavioral effects. Using pharmacogenetic ablation of adult neurogenesis in rats, we investigated the role of new neurons in the hippocampus in the long-lasting structural and behavioral effects of SPS. Two weeks after SPS, stressed rats displayed increased anxiety-like behavior and decreased preference for objects in novel locations regardless of the presence or absence of new neurons. Chronic stress produced by daily restraint for 2 or 6 hr produced similar behavioral effects that were also independent of ongoing neurogenesis. At a longer recovery time point, 1 month after SPS, rats with intact neurogenesis had normalized, showing control levels of anxiety-like behavior. However, GFAP-TK rats, which lacked new neurons, continued to show elevated anxiety-like behavior and enhanced serum corticosterone response to anxiogenic experience. Volume loss in ventral CA1 region of the hippocampus paralleled increases in anxiety-like behavior, occurring in all rats exposed to SPS at the early time point and only rats lacking adult neurogenesis at the later time point. In chronic stress experiments, volume loss occurred broadly throughout the dentate gyrus and CA1 after 6-hr daily stress but was not apparent in any hippocampal subregion after 2-hr daily stress. No effect of SPS was seen on cell proliferation in the dentate gyrus, but the survival of young neurons born a week after stress was decreased. Together, these data suggest that new neurons are important for recovery of normal behavior and hippocampal structure following a strong acute stress and point to the ventral CA1 region as a potential key mediator of stress-induced anxiety-like behavior.

Sex matters: females in proestrus show greater diazepam anxiolysis and brain-derived neurotrophin factor- and parvalbumin-positive neurons than males

In humans and animal models, sex differences are reported for anxiety-like behavior and response to anxiogenic stimuli. In the current work, we studied anxiety-like behavior and response to the prototypical anti-anxiety drug, diazepam. We used 6th generation outbred lines of adult Long Evans rats with high and low anxiety-like behavior phenotypes to investigate the impact of proestrus on the baseline and diazepam-induced behavior. At three doses of diazepam (0, 0.1, and 1.0 mg/kg, i.p.), we measured anxiogenic responses on the elevated plus maze of adult male and female rats. We assessed parvalbumin and brain-derived neurotrophin protein levels in forebrain and limbic structures implicated in anxiety/stress using immunohistochemistry. At baseline, we saw significant differences between anxiety lines, with high anxiety lines displaying less time on the open arms of the elevated plus maze, and less open arm entries, regardless of sex. During proestrus, high anxiety females showed less anxiety-like behavior at 0.1 mg/kg, while low anxiety females displayed less anxiety-like behavior at 0.1 and 1.0 doses, relative to males. Brain-derived neurotrophin protein was elevated in females in the medial prefrontal cortex and central amygdala, while parvalbumin-immunoreactive cells were greater in males in the medial prefrontal cortex. Parvalbumin-positive cells in high anxiety females were higher in CA2 and dentate gyrus relative to males from the same line. In sum, when tested in proestrus, females showed greater anxiolytic effects of diazepam relative to males, and this correlated with increases in neurotrophin and parvalbumin neuron density in corticolimbic structures.

Stress-induced hyperthermia and hypothermia

Stress affects core body temperature (T_c). Many kinds of stress induce transient, monophasic hyperthermia, which diminishes gradually if the stressor is terminated. Stronger stressors produce a longer-lasting effect. Repeated/chronic stress induces anticipatory hyperthermia, reduces diurnal changes in T_c, or slightly increases T_c throughout the day. Animals that are exposed to chronic stress or a cold environment exhibit an enhanced hyperthermic response to a novel stress. These changes persist for several days after cessation of stress exposure. In contrast, long-lasting inescapable stress sometimes induces hypothermia. In healthy humans, psychologic stress induces slight increases in T_c, which are within the normal range of T_c or just above it. Some individuals, however, develop extremely high T_c (up to 41°C) when they are exposed to emotional events or show persistent low-grade high T_c (37-38°C) during or after chronic stress situations. In addition to the nature of the stressor itself, such stress-induced thermal responses are modulated by sex, age, ambient temperature, cage mates, past stressful experiences and cold exposure, and coping. Stress-induced hyperthermia is driven by mechanisms distinct from infectious fever, which requires inflammatory mediators. However, both stress and infection activate the dorsomedial hypothalamus-rostral medullary raphe region-sympathetic nerve axis to increase T_c.

Anxiolytic effects of buspirone and MTEP in the Porsolt Forced Swim Test

Traditionally, a reduction in floating behavior or immobility in the Porsolt forced swim test is employed as a predictor of anti-depressant efficacy. However, over the past several years, our studies of alcohol withdrawal-induced negative affect consistently indicate the coincidence of increased anxiety-related behaviors on various behavioral tests with reduced immobility in the forced swim test. Further, this behavioral profile correlates with increased mGlu5 protein expression within limbic brain regions. As the role for mGlu5 in anxiety is well established, we hypothesized that the reduced immobility exhibited by alcohol-withdrawn mice when tested in the forced swim test might reflect anxiety, possibly a hyper-reactivity to the acute swim stressor. Herein, we evaluated whether or not the decreased forced swim test immobility during alcohol withdrawal responds to systemic treatment with a behaviorally effective dose of the prototypical anxiolytic, buspirone (5 mg/kg). We also determined the functional relevance of the withdrawal-induced increase in mGlu5 expression for forced swim test behavior by comparing the effects of buspirone to a behaviorally effective dose of the mGlu5 negative allosteric modulator MTEP (3 mg/kg). Adult male C57BL/6J mice were subjected to a 14-day, multi-bottle, binge-drinking protocol that elicits hyper-anxiety and increases glutamate-related protein expression during early withdrawal. Control animals received only water. At 24-h withdrawal, animals from each drinking condition were subdivided into groups and treated with an intraperitoneal injection of buspirone, MTEP, or vehicle, 30 min prior to the forced swim test. Drug effects on general locomotor activity were also assessed. As we reported previously, alcohol-withdrawn animals exhibited significantly reduced immobility in the forced swim test compared to water controls. Both buspirone and MTEP significantly increased immobility in alcohol-withdrawn animals, with a modest increase also seen in water controls. No significant group differences were observed for locomotor activity, indicating that neither anxiolytic was sedating. These results provide predictive validity for increased swimming/reduced immobility in the forced swim test as a model of anxiety and provide novel evidence in favor of mGlu5 inhibition as an effective therapeutic strategy for treating hyper-anxiety during alcohol withdrawal.

Autonomic changes induced by provocative motion in rats bred for high (HAB) and low (LAB) anxiety-related behavior: Paradoxical responses in LAB animals

In humans, associations between anxiety and nausea (including motion-induced) are reported but the underlying mechanisms are not known. Hypothermia is proposed to be an index of nausea in rats. Utilising hypothermia and heart rate as outcome measures we investigated the response to provocative motion in rats selectively bred for high (HAB) and low (LAB) anxiety-related behaviors and in non-selected (NAB) rats to further elucidate the potential relationship between hypothermia and nausea-like state. Core temperature and electrocardiogram were monitored in each group (n=10 per group) using telemetry, with or without circular motion (40min; 0.75Hz) and vehicle or diazepam (2mg/kg, i.p.) pre-treatment. Heart rate and time- and frequency-domain parameters of heart rate variability were derived from the electrocardiogram. There was no baseline difference in core temperature between the three groups (mean 38.0±0.1°C), but HAB animals had a significantly lower resting heart rate (330±7bpm) compared to LAB (402±5bpm) and NAB (401±9bpm). Animals in all groups exhibited hypothermia during motion (HAB: 36.3±0.1°C; NAB: 36.4±0.1°C; LAB: 34.9±0.2°C) with the magnitude (area under the curve, AUC) of the response during 40-min motion being greater in LAB compared to NAB and HAB rats, and this was also the case for the motion-induced bradycardia. Diazepam had minimal effects on baseline temperature and heart rate in all groups, but significantly reduced the hypothermia response (AUC) to motion in all groups by ~30%. Breeding for extremes in anxiety-related behavior unexpectedly selects animals with low trait anxiety that have enhanced bradycardia and hypothermic responses to motion; consequently, this animal model appears to be not suitable for exploring relationships between anxiety and autonomic correlates of nausea. Thermal and cardiovascular responses to motion were little different between HAB and NAB rats indicating that either hypothermia is not an index of a nausea-like state in rats, or that the positive correlation between anxiety and nausea demonstrated in humans does not exist in rats. The mechanism underlying the enhanced physiological responses in LAB requires more detailed study and may provide a novel model to investigate factors modulating motion sensitivity.

Relaxin' the brain: a case for targeting the nucleus incertus network and relaxin-3/RXFP3 system in neuropsychiatric disorders

Relaxin‐3 has been proposed to modulate emotional–behavioural functions such as arousal and behavioural activation, appetite regulation, stress responses, anxiety, memory, sleep and circadian rhythm. The nucleus incertus (NI), in the midline tegmentum close to the fourth ventricle, projects widely throughout the brain and is the primary site of relaxin‐3 neurons. Over recent years, a number of preclinical studies have explored the function of the NI and relaxin‐3 signalling, including reports of mRNA or peptide expression changes in the NI in response to behavioural or pharmacological manipulations, effects of lesions or electrical or pharmacological manipulations of the NI, effects of central microinfusions of relaxin‐3 or related agonist or antagonist ligands on physiology and behaviour, and the impact of relaxin‐3 gene deletion or knockdown. Although these individual studies reveal facets of the likely functional relevance of the NI and relaxin‐3 systems for human physiology and behaviour, the differences observed in responses between species (e.g. rat vs. mouse), the clearly identified heterogeneity of NI neurons and procedural differences between laboratories are some of the factors that have prevented a precise understanding of their function. This review aims to draw attention to the current preclinical evidence available that suggests the relevance of the NI/relaxin‐3 system to the pathology and/or symptoms of certain neuropsychiatric disorders and to provide cognizant directions for future research to effectively and efficiently uncover its therapeutic potential.

Linked Articles

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc

Effects of gonadectomy and serotonin depletion on inter-individual differences in anxiety-like behaviour in male Wistar rats

Previous studies in Wistar rats suggest inter-individual differences in anxiety-like behaviour as assessed using the elevated plus maze (EPM), both between sexes and among males, to be abolished by serotonin depletion. To shed further light on the influence of sex steroids and serotonin - and on the interplay between the two - on proneness for EPM-assessed anxiety in males, outbred Wistar rats were divided into those with high and low anxiety, respectively, and exposed to gonadectomy or sham operation followed by administration of a serotonin synthesis inhibitor, para-chlorophenylalanine, or saline. Whereas gonadectomy enhanced anxiety-like behaviour in low anxiety rats so that these no longer differed in this regard from the high anxiety group, serotonin depletion reversed this effect, and also reduced anxiety in the low anxiety group regardless of gonadal state. A previously observed association between high anxiety-like behaviour and high expression of the serotonin-synthesizing enzyme tryptophan hydroxylase 2 (Tph2) in the raphe was confirmed in sham-operated animals but absent in gonadectomised rats, an ANCOVA revealing a significant interactive effect of baseline anxiety and gonadal state on Tph2 expression. It is suggested that androgens may contribute to upholding inter-individual differences in anxiety-like behaviour in male rats by interacting with serotonergic neurotransmission.

Prevalence and influence of cys407* Grm2 mutation in Hannover-derived Wistar rats: mGlu2 receptor loss links to alcohol intake, risk taking and emotional behaviour

Modulation of metabotropic glutamate 2 (mGlu2) receptor function has huge potential for treating psychiatric and neurological diseases. Development of drugs acting on mGlu2 receptors depends on the development and use of translatable animal models of disease. We report here a stop codon mutation at cysteine 407 in Grm2 (cys407*) that is common in some Wistar rats. Therefore, researchers in this field need to be aware of strains with this mutation. Our genotypic survey found widespread prevalence of the mutation in commercial Wistar strains, particularly those known as Han Wistar. Such Han Wistar rats are ideal for research into the separate roles of mGlu2 and mGlu3 receptors in CNS function. Previous investigations, unknowingly using such mGlu2 receptor-lacking rats, provide insights into the role of mGlu2 receptors in behaviour. The Grm2 mutant rats, which dominate some selectively bred lines, display characteristics of altered emotionality, impulsivity and risk-related behaviours and increased voluntary alcohol intake compared with their mGlu2 receptor-competent counterparts. In addition, the data further emphasize the potential therapeutic role of mGlu2 receptors in psychiatric and neurological disease, and indicate novel methods of studying the role of mGlu2 and mGlu3 receptors. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Role of single prolonged stress in acquisition of alcohol conditioned place preference in rats

AIMS

Previous studies showed that exposure to certain types of stressors enhance the rewarding effects of many drugs of abuse, including alcohol; however, no systematic study has investigated the role of single prolonged stress (SPS) in acquisition of alcohol conditioned place preference (CPP). The purpose of this study was to examine whether SPS would facilitate the acquisition of alcohol CPP in rats.

MAIN METHODS

Male Sprague-Dawley rats were randomly assigned to either SPS exposure condition or no exposure condition. Freezing behavior and Elevated plus maze (EPM) were employed to evaluate PTSD-like symptoms induced by SPS. Further, using unbiased procedure, CPP conditioning was conducted with alcohol (2g/kg).

KEY FINDINGS

SPS significantly enhanced freezing behavior of rats, decreased percentages (%) of both time spent and number of entry into the open arms, and facilitated the acquisition of alcohol CPP without inhibiting rats' activity.

SIGNIFICANCE

Our findings suggest that SPS plays an important role in alcohol dependence, and CPP paradigm with SPS may be useful for exploring the rewarding mechanism of alcohol with regard to the interaction between alcohol and post-traumatic stress disorder (PTSD).

Effect of periconceptional nutrition on the growth, behaviour and survival of the neonatal lamb

Periconceptional nutrition (PCN) can influence foetal hypothalamo-pituitary adrenal (HPA) axis function and alter cortisol secretion with possible consequences for maturation and growth of major organs, gestation length and behaviour. We examined effects of PCN on phenotype and survival of the neonatal lamb in 466 Merino ewes allocated to treatments providing 70%, 100% and 150% respectively, of maintenance requirements for 17 days prior and 6 days after insemination. Gestation length and birth weight for lambs in PCN treatment groups was similar (P > 0.05) but low PCN decreased the size of the neonate (crown-rump-length and metacarpal length P < 0.05). A subset of lambs euthanased at 5 days of age further showed that low PCN decreased the amount of peri-renal fat (P < 0.05) and increased liver mass (P < 0.05) while high PCN increased neck thymus and ovary mass (P < 0.05). Neonatal lambs from low PCN ewes returned faster to their mothers after release (P < 0.05) and contacted the udder in the shortest time (P < 0.05). Significant interactions between PCN treatment and sex (P < 0.05) and between PCN treatment and ewe age (P < 0.05) were also observed for time lambs took to follow the ewe. Survival of lambs was similar but potential differences may have been masked by favourable weather conditions. In conclusion, this study provides evidence of significant changes in lamb growth and development dependent on PCN and, for the first time, links these changes with significant changes in behaviour of the neonate. The impact of these effects on lamb survival and potential reproductive capacity of female offspring remains to be determined.

Differences in Anxiety-Like Behavior within a Batch of Wistar Rats Are Associated with Differences in Serotonergic Transmission, Enhanced by Acute SRI Administration, and Abolished By Serotonin Depletion

BACKGROUND

The anxiety-reducing effect of long-term administration of serotonin reuptake inhibitors is usually seen only in subjects with anxiety disorders, and such patients are also abnormally inclined to experience a paradoxical anxiety-enhancing effect of acute serotonin reuptake inhibition. These unique responses to serotonin reuptake inhibitors in anxiety-prone subjects suggest, as do genetic association studies, that inter-individual differences in anxiety may be associated with differences in serotonergic transmission.

METHODS

The one-third of the animals within a batch of Wistar rats most inclined to spend time on open arms in the elevated plus maze were compared with the one-third most inclined to avoid them with respect to indices of brain serotonergic transmission and how their behavior was influenced by serotonin-modulating drugs.

RESULTS

"Anxious" rats displayed higher expression of the tryptophan hydroxylase-2 gene and higher levels of the tryptophan hydroxylase-2 protein in raphe and also higher levels of serotonin in amygdala. Supporting these differences to be important for the behavioral differences, serotonin depletion obtained by the tryptophan hydroxylase-2 inhibitor p-chlorophenylalanine eliminated them by reducing anxiety in "anxious" but not "non-anxious" rats. Acute administration of a serotonin reuptake inhibitor, paroxetine, exerted an anxiety-enhancing effect in "anxious" but not "non-anxious" rats, which was eliminated by long-term pretreatment with another serotonin reuptake inhibitor, escitalopram.

CONCLUSIONS

Differences in an anxiogenic impact of serotonin, which is enhanced by acute serotonin reuptake inhibitor administration, may contribute to differences in anxiety-like behavior amongst Wistar rats.

Adult neurogenesis: beyond learning and memory

New neurons continue to be generated in the dentate gyrus throughout life, providing this region of the hippocampus with exceptional structural plasticity, but the function of this ongoing neurogenesis is unknown. Inhibition of adult neurogenesis produces some behavioral impairments that suggest a role for new neurons in learning and memory; however, other behavioral changes appear inconsistent with this function. A review of studies investigating the function of the hippocampus going back several decades reveals many ideas that seem to converge on a critical role for the hippocampus in stress response and emotion. These potential hippocampal functions provide new avenues for investigating the behavioral functions of adult neurogenesis. And, conversely, studies in animals lacking adult neurogenesis, which are likely to have more limited and more specific impairments than are seen with lesions, may provide valuable new insights into the function of the hippocampus. A complete understanding of the function of the hippocampus must explain its role in emotion and the relationship between its emotional and memory functions.

Sex differences in diazepam effects and parvalbumin-positive GABA neurons in trait anxiety Long Evans rats

In clinical populations, prevalence rates for a number of anxiety disorders differ between males and females and gonadal hormones are thought to contribute to these differences. While these hormones have been shown to modulate the anxiolytic effects of the benzodiazepine agonist diazepam in some models, findings are inconsistent. Here, we tested for sex differences in response to anxiogenic stimuli following a 30-min diazepam (1.0mg/kg) pre-treatment in male and female rats showing high (HAn) and low (LAn) anxiety-like behavior on the elevated plus maze. Acute diazepam administration resulted in decreased anxiety-like behavior only in HAn males as demonstrated by a significant increase in percent open arm time in the elevated plus maze (EPM). Immunohistochemical analysis for parvalbumin (PV; a calcium-binding protein that selectively stains GABAergic neurons) in central amygdala (CeA), caudate putamen (CPu) and the hippocampus indicated the number of GABAergic interneurons in these areas differed across sex and anxiety trait. In the CPu, females had significantly more PV-immunoreactive (IR) cells than males, and LAn females had greater PV-IR neurons than HAn females. In the CeA, males displayed an increased number of PV-IR neurons compared to females, with no differences found between LAn and HAn. Further, trait differences were evident in the CA2 region of the hippocampus, regardless of sex. Taken together, these data suggest that gonadal hormones and trait anxiety may influence the sensitivity to the anti-anxiety effects of diazepam and these differences may be due in part to the distribution of GABA-containing interneurons.

Sex-dependent effects of an early life treatment in rats that increases maternal care: vulnerability or resilience?

Early life stress (ELS) in rodents has profound long-term effects that are partially mediated by changes in maternal care. ELS not only induces “detrimental” effects in adulthood, increasing psychopathology, but also promotes resilience to further stressors. In Long-Evans rats, we evaluated a combination of two procedures as a model of ELS: restriction of bedding during the first post-natal days and exposure to a “substitute” mother. The maternal care of biological and “substitute” mothers was measured. The male and female offspring were evaluated during adulthood in several contexts. Anxiety was measured by the elevated plus-maze (EPM), acoustic startle response (ASR) and forced swim test (FST). In other group of animals, novelty-seeking was measured (activity in an inescapable novel environment, preference for novel environments and exploration of novel objects). Plasmatic ACTH and corticosterone in basal conditions and in response to stress were also measured. Cognitive impulsivity was assessed by a delay-discounting paradigm, and impulsive action, attention and compulsive-like behavior by a five choice serial reaction time task (5CSRTT). ELS decreased pup body weight and increased the care of the biological mother; however, the “substitute” mother did not exhibit overt maltreatment. A mixture of “detrimental” and “beneficial” effects was shown. In the 5CSRTT, attention was impaired in both genders, and in females, ELS increased compulsive-like behavior. Novel object exploration was only increased by ELS in males, but the preference for novel spaces decreased in both genders. Baseline anxiety (EPM and ASR) and recognition memory were not affected. Unexpectedly, ELS decreased the ACTH response to novelty and swim stress and increased active coping in the FST in both genders. Cognitive impulsivity was decreased only in females, but impulsive action was not affected. The enhancement in maternal care may “buffer” the effects of ELS in a context-dependent manner.

Stress resilience: a low-anxiety genotype protects male mice from the consequences of chronic psychosocial stress

Chronic psychosocial stress is a risk factor for the development of affective as well as somatic disorders. However, vulnerability to adverse stress effects varies between individuals, with previous negative life events along with genetic predisposition playing a major role. In support, we previously showed that the consequences of chronic psychosocial stress induced by chronic subordinate colony housing (CSC, 19 days) can be amplified by pre-exposing mice to repeated maternal separation during early life. To test the significance of the genetic predisposition on the effects of CSC, mice selectively bred for high (mHAB) and low (mLAB) anxiety-related behavior and nonselected CD1 mice (mNAB) were exposed to CSC in the present study. In confirmation of our previous results, CSC mice of both mHAB and mNAB lines displayed chronic stress-related symptoms including increased adrenal weight, decreased adrenal in vitro ACTH sensitivity, lower plasma corticosterone to ACTH ratio, and increased interferon-γ secretion from isolated mesenteric lymph node cells compared with single-housed controls of the respective line. However, the CSC-induced anxiogenic effect found in mNAB was not confirmed in mHAB mice, possibly due to a ceiling effect in these highly anxious mice. Interestingly, mHAB were not more vulnerable to CSC than mNAB mice, whereas mLAB mice were resilient to CSC as indicated by all of the above mentioned parameters assessed. Taken together, our findings indicate that the genetic predisposition, in this case the innate anxiety of an individual, affects vulnerability to chronic psychosocial stress, with a low-anxiety phenotype mediating resilience to both affective and somatic consequences of CSC.

Individual differences and the characterization of animal models of psychopathology: a strong challenge and a good opportunity

Despite the development of valuable new techniques (i.e., genetics, neuroimage) for the study of the neurobiological substrate of psychiatric diseases, there are strong limitations in the information that can be gathered from human studies. It is thus critical to develop appropriate animal models of psychiatric diseases to characterize their putative biological bases and the development of new therapeutic strategies. The present review tries to offer a general perspective and several examples of how individual differences in animals can contribute to explain differential susceptibility to develop behavioral alterations, but also emphasizes methodological problems that can lead to inappropriate or over-simplistic interpretations. A critical analysis of the approaches currently used could contribute to obtain more reliable data and allow taking full advantage of new and sophisticated technologies. The discussion is mainly focused on anxiety-like and to a lower extent on depression-like behavior in rodents.

Different patterns of respiration in rat lines selectively bred for high or low anxiety

In humans, there is unequivocal evidence of an association between anxiety states and altered respiratory function. Despite this, the link between anxiety and respiration has been poorly evaluated in experimental animals. The primary objective of the present study was to investigate the hypothesis that genetic lines of rats that differ largely in their anxiety level would display matching alterations in respiration. To reach this goal, respiration was recorded in high-anxiety behavior (HAB, n = 10) and low-anxiety behavior (LAB, n = 10) male rats using whole-body plethysmography. In resting state, respiratory rate was higher in HABs (85 ± 2 cycles per minute, cpm) than LABs (67 ± 2 cpm, p<0.05). During initial testing into the plethysmograph and during a restraint test, HAB rats spent less time at high-frequency sniffing compared to LAB rats. In addition, HAB rats did not habituate in terms of respiratory response to repetitive acoustic stressful stimuli. Finally, HAB rats exhibited a larger incidence of sighs during free exploration of the plethysmograph and under stress conditions. We conclude that: i) HAB rats showed respiratory changes (elevated resting respiratory rate, reduced sniffing in novel environment, increased incidence of sighs, and no habituation of the respiratory response to repetitive stimuli) that resemble those observed in anxious and panic patients, and ii) respiratory patterns may represent a promising way for assessing anxiety states in preclinical studies.

High and abnormal forms of aggression in rats with extremes in trait anxiety--involvement of the dopamine system in the nucleus accumbens

A better neurobiological understanding of high and abnormal aggression based on adequate animal models is essential for novel therapy and prevention. Selective breeding of rats for extremes in anxiety-related behavior resulted in two behavioral phenotypes with high and abnormal forms of aggression. Rats bred for low anxiety-related behavior (LAB) consistently show highest levels of aggression and little social investigation in the resident-intruder (RI) test, compared with non-selected low-aggressive (NAB) rats. High anxiety-related (HAB) rats also show higher levels of aggression than NAB rats, but to a lesser extent than LAB rats. Accordingly, extremes in inborn anxiety in both directions are linked to an increased aggression level. Further, both LAB and HAB, but not NAB males, display abnormal aggression (attacks towards vulnerable body parts, females or narcotized males), which is particularly prominent in LABs. Also, only in LAB rats, the nucleus accumbens (NAc) was found to be strongly activated in response to the RI test as reflected by increased c-fos and zif268 mRNA expression, and higher local dopamine release compared with NAB males, without differences in local dopamine receptor binding. Consequently, local pharmacological manipulation by infusion of an anesthetic (lidocaine, 20 μg/μl) or a dopamine D2 (haloperidol, 10 ng/μl), but not D1 (SCH-23390 10 ng/μl), receptor antagonist significantly reduced high aggression in LAB rats. Thus, LAB rats are an adequate model to study high and abnormal aggression. In LAB males, this is likely to be linked to hyper-activation of the reward system, as found in psychopathic patients. Specifically, activation of the accumbal dopamine system is likely to underlie the high aggression observed in LAB rats.

The biology of fear- and anxiety-related behaviors

Anxiety is a psychological, physiological, and behavioral state induced in animals and humans by a threat to well-being or survival, either actual or potential. It is characterized by increased arousal, expectancy, autonomic and neuroendocrine activation, and specific behavior patterns. The function of these changes is to facilitate coping with an adverse or unexpected situation. Pathological anxiety interferes with the ability to cope successfully with life challenges. Vulnerability to psychopathology appears to be a consequence of predisposing factors (or traits), which result from numerous gene-environment interactions during development (particularly during the perinatal period) and experience (life events), in this review, the biology of fear and anxiety will be examined from systemic (brain-behavior relationships, neuronal circuitry, and functional neuroanatomy) and cellular/molecular (neurotransmitters, hormones, and other biochemical factors) points of view, with particular reference to animal models. These models have been instrumental in establishing the biological correlates of fear and anxiety, although the recent development of noninvasive investigation methods in humans, such as the various neuroimaging techniques, certainly opens new avenues of research in this field. Our current knowledge of the biological bases of fear and anxiety is already impressive, and further progress toward models or theories integrating contributions from the medical, biological, and psychological sciences can be expected.

What can we know from pituitary-adrenal hormones about the nature and consequences of exposure to emotional stressors?

Exposure to stress induces profound physiological and behavioral changes in the organisms and some of these changes may be important regarding stress-induced pathologies and animal models of psychiatric diseases. Consequences of stress are dependent on the duration of exposure to stressors (acute, chronic), but also of certain characteristics such as intensity, controllability, and predictability. If some biological variables were able to reflect these characteristics, they could be used to predict negative consequences of stress. Among the myriad of physiological changes caused by stress, only a restricted number of variables appears to reflect the intensity of the situation, mainly plasma levels of ACTH and adrenaline. Peripheral hypothalamic-pituitary-adrenal (HPA) hormones (ACTH and corticosterone) are also able to reflect fear conditioning. In contrast, the activation of the HPA axis is not consistently related to anxiety as evaluated by classical tests such as the elevated plus-maze. Similarly, there is no consistent evidence about the sensitivity of the HPA axis to psychological variables such as controllability and predictability, despite the fact that: (a) lack of control over aversive stimuli can induce behavioral alterations not seen in animals which exert control, and (b) animals showed clear preference for predictable versus unpredictable stressful situations. New studies are needed to re-evaluate the relationship between the HPA axis and psychological stress characteristics using ACTH instead of corticosterone and taking advantages of our current knowledge about the regulation of this important stress system.

Selectively bred rodents as models of depression and anxiety

Stress related diseases such as depression and anxiety have a high degree of co morbidity, and represent one of the greatest therapeutic challenges for the twenty-first century. The present chapter will summarize existing rodent models for research in psychiatry, mimicking depression- and anxiety-related diseases. In particular we will highlight the use of selective breeding of rodents for extremes in stress-related behavior. We will summarize major behavioral, neuroendocrine and neuronal parameters, and pharmacological interventions, assessed in great detail in two rat model systems: The Flinders Sensitive and Flinders Resistant Line rats (FSL/FRL model), and rats selectively bred for high (HAB) or low (LAB) anxiety related behavior (HAB/LAB model). Selectively bred rodents also provide an excellent tool in order to study gene and environment interactions. Although it is generally accepted that genes and environmental factors determine the etiology of mental disorders, precise information is limited: How rigid is the genetic disposition? How do genetic, prenatal and postnatal influences interact to shape adult disease? Does the genetic predisposition determine the vulnerability to prenatal and postnatal or adult stressors? In combination with modern neurobiological methods, these models are important to elucidate the etiology and pathophysiology of anxiety and affective disorders, and to assist in the development of new treatment paradigms.

Magnesium deficiency induces anxiety and HPA axis dysregulation: modulation by therapeutic drug treatment

Preclinical and some clinical studies suggest a relationship between perturbation in magnesium (Mg²⁺) homeostasis and pathological anxiety, although the underlying mechanisms remain largely unknown. Since there is evidence that Mg²⁺ modulates the hypothalamic-pituitary adrenal (HPA) axis, we tested whether enhanced anxiety-like behaviour can be reliably elicited by dietary Mg²⁺ deficiency and whether Mg²⁺ deficiency is associated with altered HPA axis function. Compared with controls, Mg²⁺ deficient mice did indeed display enhanced anxiety-related behaviour in a battery of established anxiety tests. The enhanced anxiety-related behaviour of Mg²⁺ deficient mice was sensitive to chronic desipramine treatment in the hyponeophagia test and to acute diazepam treatment in the open arm exposure test. Mg²⁺ deficiency caused an increase in the transcription of the corticotropin releasing hormone in the paraventricular hypothalamic nucleus (PVN), and elevated ACTH plasma levels, pointing to an enhanced set-point of the HPA axis. Chronic treatment with desipramine reversed the identified abnormalities of the stress axis. Functional mapping of neuronal activity using c-Fos revealed hyper-excitability in the PVN of anxious Mg²⁺ deficient mice and its normalisation through diazepam treatment. Overall, the present findings demonstrate the robustness and validity of the Mg²⁺ deficiency model as a mouse model of enhanced anxiety, showing sensitivity to treatment with anxiolytics and antidepressants. It is further suggested that dysregulations in the HPA axis may contribute to the hyper-emotionality in response to dietary induced hypomagnesaemia.

This article is part of a Special Issue entitled ‘Anxiety and Depression’.

Animal models of depression and anxiety: What do they tell us about human condition?

While modern neurobiology methods are necessary they are not sufficient to elucidate etiology and pathophysiology of affective disorders and develop new treatments. Achievement of these goals is contingent on applying cutting edge methods on appropriate disease models. In this review, the authors present four rodent models with good face-, construct-, and predictive-validity: the Flinders Sensitive rat line (FSL); the genetically "anxious" High Anxiety-like Behavior (HAB) line; the serotonin transporter knockout 5-HTT(-/-) rat and mouse lines; and the post-traumatic stress disorder (PTSD) model induced by exposure to predator scent, that they have employed to investigate the nature of depression and anxiety.

Behavioral and endocrine consequences of simultaneous exposure to two different stressors in rats: interaction or independence?

Although behavioral and endocrine consequences of acute exposure to stressors have been extensively studied, little is known about how simultaneous exposure to two different stressors interacts to induce short- and long-term effects. In the present experiment we studied this interaction in adult male rats exposed to cat fur odor (impregnated cloth) or immobilization on boards either separately or simultaneously. We reasoned that exposure to the odor of a potential predator while immobilized, may potentiate its negative consequences as compared to exposure to only one of the stressors. Exposure to cat odor elicited the expected reduction of activity and avoidance of the area where the impregnated cloth was located. The endocrine response (plasma levels of ACTH and corticosterone, as a measure of the hypothalamic-pituitary-adrenal axis, HPA) was markedly greater after immobilization than after cat fur odor and no additive effects were found by simultaneous exposure to both stressors. Cat odor, but not immobilization, increased anxiety-like behavior as evaluated in the elevated plus-maze 7 days after the stressors, with no evidence of enhanced HPA activation. In addition, cat odor exposure resulted in long-lasting (8 days later) fear conditioning to the box containing a clean cloth, which was reflected by hypoactivity, avoidance of the cloth area and enhanced HPA activation. All these effects were similarly observed in rats exposed simultaneously to cat odor and immobilization. In rats only exposed to immobilization, only some weak behavioral signs of fear conditioning were found, but HPA activation in response to the context paired to immobilization was enhanced to the same extent as in cat odor-exposed animals, supporting a certain degree of endocrine conditioning. The present results did not reveal important behavioral interactions between the two stressors when animals experienced both simultaneously, whereas some interactions were found regarding HPA activation. Theoretical implications are discussed.

Altered GABA transmission in a mouse model of increased trait anxiety

Anxiety disorders are the most prevalent central nervous system diseases imposing a high social burden to our society. Emotional processing is particularly controlled by GABA-ergic transmission in the amygdala. Using in situ hybridization and immunohistochemistry we now investigated changes in the expression of GABA synthesizing enzymes (GAD65 and GAD67), GABA_A (α1–5, β1–3, γ1–2) and GABA_B receptor subunits (GBBR1, GBBR2) in amygdaloid nuclei of high anxiety-related behavior (HAB) mice in comparison to mice selected for normal anxiety-related behavior (NAB). Levels of GAD65 and GAD67 mRNAs and protein, as well as those of GABA were increased in the amygdala of HAB mice. Relative to NAB controls, mRNA expression of the GABA_A receptor subunits β1, β2 and γ2 was specifically increased in the basolateral amygdala of HAB mice while transcription of α5 and γ1 subunits was reduced in the central and medial amygdala. On the protein level, increases in β2 and γ2 subunit immunoreactivities were evident in the basolateral amygdala of HAB mice. No change in GABA_B receptor expression was observed. These findings point towards an imbalanced GABA-ergic neurotransmission in the amygdala of HAB mice. On the other hand, FosB, a marker for neuronal activity, was increased in principal neurons of the basolateral amygdala in HAB mice, reflecting activation of excitatory neurons, possibly as a consequence of reduced GABA-ergic tonic inhibition through α5 and γ1 containing receptors. Ultimately these mechanisms may lead to the compensatory activation of GABA transmission, as indicated by the increased expression of GAD65/67 in HAB mice.

Temperament influences endotoxin-induced changes in rectal temperature, sickness behavior, and plasma epinephrine concentrations in bulls

This study was designed to determine the influence of temperament on endotoxin-induced changes in body temperature, sickness behavior, and stress hormone concentrations in cattle. Brahman bulls were selected based on temperament score measured 28 d prior to weaning. In dwelling recording devices were used to monitor rectal temperature, and jugular catheters were used to collect blood samples to determine cortisol and epinephrine concentrations before and after LPS administration (0.5 μg/kg body weight). Temperamental bulls had the lowest peak rectal temperature and sickness behavior scores relative to the Calm and Intermediate bulls. Prior to the administration of LPS, Temperamental bulls had greater cortisol and epinephrine concentrations than Calm or Intermediate bulls. Cortisol concentrations increased following LPS administration but were not affected by temperament. Epinephrine concentrations peaked 1 h after LPS administration in Calm bulls. Temperamental bulls did not exhibit an epinephrine response to LPS challenge. These data demonstrate that the temperament of calves can modulate the physiological, behavioral, and endocrine responses of pre-pubertal Brahman bulls to endotoxin challenge. Specifically, temperament differentially affected the rectal temperature, sickness behavior and epinephrine, but not cortisol, responses to LPS challenge.

Evaluation of light/dark cycle in anxiety- and depressive-like behaviors after regular treatment with methylphenidate hydrochloride in rats of different ages

OBJECTIVE: Methylphenidate hydrochloride is the most widely used medication for treatment and management of attention-deficit hyperactivity disorder. However, the chronic effects of methylphenidate hydrochloride on anxiety- and depressive-like rat behaviors remain poorly investigated. In this context, the present study evaluated the effects of treatment with methylphenidate hydrochloride on anxiety- and depressive-like behaviors using young and adult rats during the light and the dark cycle. METHOD: Male Wistar rats (25 or 60 days old) received a once-daily (in either the light or dark cycle) methylphenidate hydrochloride (2mg/kg) or saline intraperitoneal injection for 28 days. We performed elevated plus maze and forced swimming test two hours after the last injection. RESULTS: The light/dark cycle was a significant factor in the anxiety-like behaviors; however, no significant interaction between all three factors (cycle, age and methylphenidate hydrochloride) was found. Nevertheless, we observed a nominally significant interaction between the light/ dark cycle and age in the forced swimming test. CONCLUSION: Our results have shown that age and the light/dark cycle are more significant modulators of anxiety- and depressive-like behaviors than methylphenidate hydrochloride treatment.

The role of the alpha 2A-adrenoceptor in mouse stress-coping behaviour

Acute stress is known to impair memory functions in both men and laboratory rodents. In human the alpha 2A-adrenoceptor system is known to play a critical role in regulating acute neuropsychological stress responses and, ultimately, stress-coping behaviour. In search for neurobiological and central nervous mechanisms behind these behaviours we investigated if the alpha 2A-adrenoceptor is involved in these mechanisms in mice. Phenotypical differences between the A/J and C57BL/6J (B6) mouse inbred strains were evaluated in previous studies. These data showed significant strain differences in various motivational systems (anxiety, exploration, locomotion, memory etc.). From the literature it is known that chromosome 19 contains the gene for the adrenergic alpha 2A receptor that is thought to be involved in emotional behaviours, among others anxiety-related avoidance behaviour and arousal. We investigated if this pathway could possibly be involved in avoidance/arousal susceptibility by applying an agonist (dexmedetomidine) and an antagonist (atipamezole) of the alpha 2A-adrenoceptor to male mice from a consomic strain (C57BL/6J-Chr 19(A)/NaJ, abbreviated to CSS19=anxious), and the corresponding donor (A/J=anxious) and host (B6=non-anxious) strains. The mice were tested in the modified hole board (mHB) test which allows for the assessment of a variety of behavioural patterns by use of only one test. In addition, a forced swimming test (FST) was conducted to test for stress-coping behaviour. Results of the behavioural testing in the mHB-test showed significant strains differences and strain-specific treatment effects for parameters describing anxiety-related endophenotypes. The FST revealed effects of dexmedetomidine and atipamezole on stress-coping behaviour. In conclusion, the involvement of the alpha 2A-adrenoceptor, located on mouse chromosome 19, on anxiety-related behaviour remains unclear and will possibly not play a main role in the development of anxiety-related behaviour in mice. However, we could show involvement of this receptor in stress-coping behaviour in mice.

Aggression and anxiety: social context and neurobiological links

Psychopathologies such as anxiety- and depression-related disorders are often characterized by impaired social behaviours including excessive aggression and violence. Excessive aggression and violence likely develop as a consequence of generally disturbed emotional regulation, such as abnormally high or low levels of anxiety. This suggests an overlap between brain circuitries and neurochemical systems regulating aggression and anxiety. In this review, we will discuss different forms of male aggression, rodent models of excessive aggression, and neurobiological mechanisms underlying male aggression in the context of anxiety. We will summarize our attempts to establish an animal model of high and abnormal aggression using rats selected for high (HAB) vs. low (LAB) anxiety-related behaviour. Briefly, male LAB rats and, to a lesser extent, male HAB rats show high and abnormal forms of aggression compared with non-selected (NAB) rats, making them a suitable animal model for studying excessive aggression in the context of extremes in innate anxiety. In addition, we will discuss differences in the activity of the hypothalamic–pituitary–adrenal axis, brain arginine vasopressin, and the serotonin systems, among others, which contribute to the distinct behavioural phenotypes related to aggression and anxiety. Further investigation of the neurobiological systems in animals with distinct anxiety phenotypes might provide valuable information about the link between excessive aggression and disturbed emotional regulation, which is essential for understanding the social and emotional deficits that are characteristic of many human psychiatric disorders.

Individual differences in the sensitivity to serotonergic drugs: a pharmacobehavioural approach using rats selected on the basis of their response to novelty

RATIONALE

The mechanisms underlying individual differences in the response to serotonergic drugs are poorly understood. Rat studies may contribute to our knowledge of the neuronal substrates that underlie these individual differences.

OBJECTIVES

A pharmacobehavioural study was performed to assess individual differences in the sensitivity to serotonergic drugs in rats that were selected based on their response to a novel environment.

METHODS

Low responders (LR) and high responders (HR) to novelty rats were tested on the elevated T-maze following systemic injections of increasing doses of various serotonergic agents. The duration of avoidance of the open arms was scored for five trials.

RESULTS

The duration of avoidance behaviour was larger in saline-treated LR rats compared to saline-treated HR rats. The 5-HT1A agonist 8-OH-DPAT and the 5-HT2 agonists mCPP and DOI decreased the duration of avoidance behaviour in LR rats, but increased it in HR rats. The 5-HT3 agonist SR57227A and the 5-HT releaser/reuptake inhibitor d-fenfluramine increased the duration of avoidance behaviour in both types of rat. However, higher doses of SR57227A were required to alter avoidance behaviour in HR than in LR rats. The onset of the effects of SR57227A, d-fenfluramine and WAY100635 was faster in LR than in HR rats. The described effects were receptor specific. A model explaining the data is presented.

CONCLUSIONS

These data demonstrate that LR and HR rats differ in their sensitivity to serotonergic drugs that act at 5-HT3, 5-HT2 and 5-HT1A receptors. The implications of these individual differences for individual-specific treatment of substance abuse are briefly discussed.