Catherine Belzung

The open field as a paradigm to measure the effects of drugs on anxiety-like behaviors: a review.

The open field is a very popular animal model of anxiety-like behavior. An overview of the literature on the action elicited by effective or putative anxiolytics in animal subjected to this procedure indicates that classical treatments such as benzodiazepine receptor full agonists or 5-HT(1A) receptor full or partial agonists elicit an anxiolytic-like effect in this procedure in most cases (approximately 2/3). However, compounds (triazolobenzodiazepines such as adinazolam and alprazolam, selective serotonin reuptake inhibitors) that have a different spectrum of therapeutic efficacy in anxiety disorders such as panic attacks, generalized anxiety disorder or obsessive-compulsive disorder were poorly effective as anxiolytics in the open field test, suggesting that this paradigm may not model features of anxiety disorders. The procedure is also relevant for the study of compounds endowed with anxiogenic effects, as such effects were detected after treatments with benzodiazepine receptor inverse agonists or with corticotropin releasing factor (CRF) receptor agonists.

Decreased GABAA-receptor clustering results in enhanced anxietyand a bias for threat cues

Patients with panic disorders show a deficit of GABAA receptors in the hippocampus, parahippocampus and orbitofrontal cortex. Synaptic clustering of GABAA receptors in mice heterozygous for the γ2 subunit was reduced, mainly in hippocampus and cerebral cortex. The γ2+/– mice showed enhanced behavioral inhibition toward natural aversive stimuli and heightened responsiveness in trace fear conditioning and ambiguous cue discrimination learning. Implicit and spatial memory as well as long-term potentiation in hippocampus were unchanged. Thus γ2+/– mice represent a model of anxiety characterized by harm avoidance behavior and an explicit memory bias for threat cues, resulting in heightened sensitivity to negative associations. This model implicates GABAA-receptor dysfunction in patients as a causal predisposition to anxiety disorders.

Differences in anxiety-related behaviours and in sensitivity to diazepam in inbred and outbred strains of mice

Abstract Rationale: Natural strain differences exist in mice for behavioural traits such as emotional reactivity. Objective: The present experiments compared the behavioural profiles of nine strains of mice (BALB/c, C57BL/6, C3H, CBA, DBA/2, NMRI, NZB, SJL, Swiss) in two models of anxiety after the administration of the benzodiazepine diazepam. Methods: The tests used were the light/dark choice task and the elevated plus-maze, two well-validated anxiolytic screening tests. Results: In vehicle-treated animals, differences on variables designed to measure anxiety-related behaviours were observed in both tests. In the light/dark test, the strains could be divided into three distinct groups: two non-reactive strains (NZB and SJL), an intermediate-reactive group (C3H, CBA, DBA/2, NMRI, C57BL/6 and Swiss), and one highly reactive strain (BALB/c). In the elevated plus-maze, SJL, NMRI, CBA and, to a lesser extent, C3H strains of mice, consistently showed low levels of anxiety-related behaviours. Intermediate levels were seen in the Swiss and BALB/c strains, and high levels of emotional reactivity were seen in C57BL/6, DBA/2 and NZB. The strain distribution between the light/dark and the elevated plus-maze tests shows similarities and differences, suggesting that each of these experimental procedures represents a different set of behaviours. Marked differences between a number of strains of mice in their sensitivity to the anxiolytic-like action of diazepam were observed in both the light/dark and the elevated plus-maze tests. Mice of the BALB/c, Swiss and, to a lesser extent, CBA and C3H strains were responsive to diazepam in both tests, although in the case of CBA mice, effects may have been contaminated by behavioural suppression. SJL mice were largely unresponsive to the action of the benzodiazepine in both tests, whereas in C57, DBA/2, NMRI and NZB mice, diazepam produced positive effects only in the elevated plus-maze. Conclusion: The finding of differential strain distributions both with and without diazepam treatment in the light/dark and the elevated plus-maze tests, indicates that not all strains of mice are suitable for investigating the effects of GABA/BZ receptor ligands. This study may thus provide a useful guide for choosing the best strain of mice for studying the pharmacology of fear-related behaviours.

Antidepressants recruit new neurons to improve stress response regulation

Recent research suggests an involvement of hippocampal neurogenesis in behavioral effects of antidepressants. However, the precise mechanisms through which newborn granule neurons might influence the antidepressant response remain elusive. Here, we demonstrate that unpredictable chronic mild stress in mice not only reduces hippocampal neurogenesis, but also dampens the relationship between hippocampus and the main stress hormone system, the hypothalamo-pituitary-adrenal (HPA) axis. Moreover, this relationship is restored by treatment with the antidepressant fluoxetine, in a neurogenesis-dependent manner. Specifically, chronic stress severely impairs HPA axis activity, the ability of hippocampus to modulate downstream brain areas involved in the stress response, the sensitivity of the hippocampal granule cell network to novelty/glucocorticoid effects and the hippocampus-dependent negative feedback of the HPA axis. Remarkably, we revealed that, although ablation of hippocampal neurogenesis alone does not impair HPA axis activity, the ability of fluoxetine to restore hippocampal regulation of the HPA axis under chronic stress conditions, occurs only in the presence of an intact neurogenic niche. These findings provide a mechanistic framework for understanding how adult-generated new neurons influence the response to antidepressants. We suggest that newly generated neurons may facilitate stress integration and that, during chronic stress or depression, enhancing neurogenesis enables a dysfunctional hippocampus to restore the central control on stress response systems, then allowing recovery.

Comparison of different behavioral test situations used in psychopharmacology for measurement of anxiety

In order to estimate the consistency of variables measured in different test situations classically used in psychopharmacology, a set of hybrid mice was confronted with five testing situations: responses to a novel object introduced in a familiar environment, responses to novel places, behavior in a light/dark choice situation, on the holeboard, and in an elevated plus maze. A principal component analysis was performed using two variables per device as active variables and 26 others as supplementary variables. The first factor was clearly due to opposition between high and low levels of neophobia. Only variables from the first two tests were strongly correlated to this factor. Variables from the holeboard and from the plus maze were highly correlated one another and with the second factor, which grouped locomotion and exploration criteria. The light/dark choice test was intermediate and seemed to be moderately related to both locomotion and neophobia. These results point out the difficulty in comparing different test devices from a psychological point of view.

Strain differences in sucrose preference and in the consequences of unpredictable chronic mild stress

Effects of unpredictable chronic mild stress (UCMS) on anhedonic-like behaviour, physical state, body weight, learning and memory were investigated in three strains of mice. These strains were chosen among 11 strains that were tested in a first experiment for their sucrose consumption and preference for sucrose solutions of different concentrations. In the second experiment, groups of mice of the CBA/H, C57BL/6 and DBA/2 strains were submitted to 7 weeks of UCMS. Measures of the sucrose consumption, the evaluation of the physical state and the measurement of body weight were weekly assessed. Following 4-week period of UCMS, sub-groups of stressed and non-stressed mice were submitted to the spontaneous alternation test in the Y-maze, and then to the water-maze test for spatial learning and memory. UCMS induced a significant decrease of the sucrose consumption in CBA/H and in C57BL/6 but not in DBA/2 mice. The UCMS effect on sucrose intake in CBA/H mice was associated with a body weight loss and a physical state degradation. Spatial learning in a water maze was not disturbed by UCMS, however, a long-term memory impairment was observed in CBA/H stressed mice during a probe test. In the Y-maze, UCMS did not modify spontaneous alternation. These results show both an anhedonic-like and an amnesic effect of UCMS in CBA/H mice. They also reveal a difference of sensitivity to UCMS according to the strain of mice.

Early life genetic, epigenetic and environmental factors shaping emotionality in rodents

Childhood trauma is known to increase risk for emotional disorders and addiction. However, little is currently understood about the neurodevelopmental basis of these effects, or how genetic and epigenetic factors interact with the environment to shape the systems subserving emotionality. In this review, we discuss the use of rodent models of early life emotional experience to study these issues in the laboratory and present some of our pertinent findings. In rats, postnatal maternal separation can produce lasting increases in emotional behavior and stressor-reactivity, together with alterations in various brain neurotransmitter systems implicated in emotionality, including corticotropin-releasing factor, serotonin, norepinephrine, and glutamate. Genetic differences between inbred mouse strains have been exploited to further study how maternal behavior affects emotional development using techniques such as cross-fostering and generation of inter-strain hybrids. Together with our own recent data, the findings of these studies demonstrate the pervasive influence of maternal and social environments during sensitive developmental periods and reveal how genetic factors determine how these early life experiences can shape brain and behavior throughout life.

Effects of unpredictable chronic mild stress on anxiety and depression-like behavior in mice.

The widely accepted stress-diathesis hypothesis of depression postulates that genetic factors contribute to biological vulnerability. Based on this concept, the unpredictable chronic mild stress (UCMS) animal model was developed. Most effects of UCMS can be reversed by antidepressant agents, illustrating a strong predictive validity. In rodents, UCMS also has good face validity as it can elicit depression-like symptoms. While abundant for rats, the UCMS literature on mice is relatively limited. Reports sometimes are contradictory, making it difficult to establish a clear profile of stress-induced depression-like behaviors in mice. As different groups often use different strains for their experiments, differential strain susceptibility to UCMS may provide at least a partial explanation of these discrepancies. Moreover, differences in testing methodology add another level of complexity. Very little is known about the role of genetic factors and their interactions with the environment in the development of stress-induced behavioral changes relevant to depression, though recent studies unequivocally demonstrated the effects of specific gene polymorphisms on stress-induced depressive symptoms, as well as the effects of stress on gene expression. In the present study, we investigated the effects of UCMS on a battery of different tests measuring anxiety and depression-like behaviors in three behaviorally and genetically distinct inbred strains. The goals of these experiments are to obtain a clearer behavioral profile of genetically/phenotypically distant mouse strains after UCMS treatment and to evaluate the limitations and strengths of the UCMS model in mice.

The free-exploratory paradigm: an effective method for measuring neophobic behaviour in mice and testing potential neophobia-reducing drugs.

When given the opportunity to choose between a novel and a familiar compartment (free-exploratory paradigm), BALB/c mice exhibited a preference for familiar places and a marked number of attempts at entry into the novel compartment followed by avoidance responses. In contrast, C57BL/6 mice showed a preference for novel places and very few avoidance responses towards novelty. When novelty was reduced by two familiar odours, fresh sawdust or urine of conspecifics, the neophobia of the BALB/c mice was reversed and the animals clearly showed a preference for the novel compartment. This experimental paradigm can be proposed as an effective animal model for investigating drugs potentially able to reduce neophobia in BALB/c mice. The effects of anxiolytics, effective in the usual animal models of “state” anxiety, were investigated in the free-exploratory paradigm which may model another type of anxiety termed by Lister (1990) “trait” anxiety. Thus, the behavioural effects of two benzodiazepine full agonists, chlordiazepoxide and diazepam, two non-benzodiazepine partial agonists at benzodiazepine receptors, Ro 19–8022 and alpidem, the 5-HT1A receptor agonist, 8-OH-DPAT, and the 5-HT, receptor antagonist, zacopride, were assessed in BALB/c and C57BL/6 mice. Chlordiazepoxide, diazepam and Ro 19–8022 completely reversed the preference of BALB/c mice for the familiar compartment, treated animals exhibiting a significant preference for novel places. In contrast, alpidem, 8-OH-DPAT and zacopride did not significantly modify their behaviour. Moreover, the same drugs did not modify the specific responses of C57BL/6 mice toward novelty. These results demonstrate that drugs which bind in a non-selective manner to heterogeneous benzodiazepine recognition sites were very effective in reducing neophobia in BALB/c mice, whereas 5-HT-interacting drugs were unable to counteract their neophobic behaviour. Thus, the free-exploratory paradigm can be proposed as an effective method for testing potential neophobia- (“trait” anxiety) reducing drugs.

5-HT1B receptor knock out — behavioral consequences

Serotonin is a neuromodulator that is involved in a number of mood disorders such as depression, anxiety and impulsive violence. In an attempt to dissect the contribution of individual 5-HT receptor subtypes to behavior, we have generated by homologous recombination, mutant mice lacking the 5-HT1B receptor. These mice did not exhibit any obvious developmental or behavioral defect. However, the hyperlocomotor effect of the 5-HT1A/1B agonist, RU 24969 was completely absent in mutant mice, indicating that this effect is mediated by 5-HT1B receptors. Moreover, when confronted with an intruder, isolated mutant mice attacked the intruder faster and more intensely than wild-type mice, suggesting an involvement of 5-HT1B receptors in the modulation of aggressive behavior. These data might be related to the fact that a class of 5-HT1 agonists, termed serenics, have anti-aggressive properties, and with the findings that certain impulsive aggressive behaviors are associated with deficits in central serotonin.