Coping with stress: The Impact of the Hypothalamus Pituitary Adrenal (HPA) System and Neurotrophic Circuits in the Learned Helplessness Model of Depression

Animal models currently represent a viable route for gaining further insights into the mechanisms involved in the pathogenesis of particular diseases. Depression, in this respect, constitutes a major challenge since the characterization of disease-specific traits is complicated due to the multifactorial nature of the disorder. The understanding of diverse factors, e.g. neurotrophic circuits and the role of the HPA axis, which have to be considered in the pathophysiology of the disease represent a major target of behavioural animal models of depression. Working on a model such as Learned Helplessness, consequently requires careful consideration of modulating aspects to ensure representative results. This work aims at elucidating the role of recently postulated target genes of depression as well as the impact of potential distorting factors, such as housing conditions of the experimental animals. To guarantee a specific readout, which permits concrete statements regarding the role of particular target genes like BDNF, CREB, and GR, we compared both, the effects of different social and as environmental factors with regard to general and Helplessness-specific effects on behaviour. Furthermore, we confirmed the model by a pharmacological validation, simultaneously monitoring effects of the obligatory handling procedure. In studies of depression and emotionality it is important to establish standardized protocols, involving the animal’s environment, to be able to precisely assess potential sources of stress and exclude artefacts. The design and modification of animal models like the Learned Helplessness subsequently bears the advantage of not only detecting potential genetic aspects by investigating mice carrying mutations of particular target genes, e.g. the glucocorticoid receptor, in which significant differences with regard to helpless behaviour and further depressive-like parameters became evident, but also to exploit fundamental causes of depressive-like phenotypes such as stress effects. The detailed evaluation of the Learned Helplessness in mice as a model of depression suggests it as a valuable instrument to investigate mouse models for depression, like GR heterozygous animals, in which the behavioural phenotype was associated with depressive-like characteristics such as a decrease of BDNF protein and relevant physiological parameters which mimick stress, i.e. a depression-typical Dex/CRH Test and elevated corticosterone levels after restraint stress

Marble burying as compulsive behaviors in male and female mice

Marble burying is considered an, albeit controversial, animal model of the compulsive like behaviors of obsessive‑compulsive disorder (OCD). Hallmark features of OCD patients are similarities and, more prominent, differences from anxiety disorders, e.g., the absence of sex differences and resistance to spontaneous remission. We report an experiment on marble burying by male and female C57/BL6/N mice. Animals were administered either the classic anxiolytic drug, diazepam, that targets the GABA receptor or a "pure" inhibitor of the serotonin transporter, escitalopram, that has been reported to be particularly effective in OCD. A burying paradigm that more precisely mimics the human condition was used, e.g., testing in the home environment, chronic drug exposure and acknowledging individual differences by pre‑selecting for high marble burying. Results were that there were no sex differences in groups treated with drugs or in control mice. Both diazepam and escitalopram decreased numbers of marbles buried compared to vehicle‑only controls in the absence of correlated changes in anxiety. Diazepam, however, was more effective than escitalopram in suppressing MB. The conclusion is that along with serotonin, GABA is involved in regulating compulsive behaviors. The marble burying paradigm may prove more useful for pharmacological drugs tests of impulsivity or attention deficit because of the involvement of serotonin and GABA in both disorders

Mice with genetically altered glucocorticoid receptor expression show altered sensitivity for stress-induced depressive reactions

Altered glucocorticoid receptor (GR) signaling is a postulated mechanism for the pathogenesis of major depression. To mimic the human situation of altered GR function claimed for depression, we generated mouse strains that underexpress or overexpress GR, but maintain the regulatory genetic context controlling the GR gene. To achieve this goal, we used the following: (1) GR-heterozygous mutant mice (GR+/-) with a 50% GR gene dose reduction, and (2) mice overexpressing GR by a yeast artificial chromosome resulting in a twofold gene dose elevation. GR+/- mice exhibit normal baseline behaviors but demonstrate increased helplessness after stress exposure, a behavioral correlate of depression in mice. Similar to depressed patients, GR+/- mice have a disinhibited hypothalamic-pituitary-adrenal (HPA) system and a pathological dexamethasone/corticotropin-releasing hormone test. Thus, they represent a murine depression model with good face and construct validity. Overexpression of GR in mice evokes reduced helplessness after stress exposure, and an enhanced HPA system feedback regulation. Therefore, they may represent a model for a stress-resistant strain. These mouse models can now be used to study biological changes underlying the pathogenesis of depressive disorders. As a first potential molecular correlate for such changes, we identified a downregulation of BDNF protein content in the hippocampus of GR+/- mice, which is in agreement with the so-called neurotrophin hypothesis of depression

Differences in Mouse Maternal Care Behavior – Is There a Genetic Impact of the Glucocorticoid Receptor?

Depressive episodes are frequently preceded by stressful life events. Evidence from genetic association studies suggests a role for the glucocorticoid receptor (GR), an essential element in the regulation of stress responses, in the pathophysiology of the disorder. Since the stress response system is affected by pregnancy and postpartum-associated changes, it has also been implicated in the pathophysiology of postpartum depression. Using a 2×2 factorial design, we investigated whether a heterozygous deletion of GR would influence maternal care behavior in C57BL/6 and Balb/c mice, two inbred strains known to display qualitative differences in this behavior. Behavioral observation was carried out between postnatal days 1 and 7, followed by a pup retrieval test on postnatal days 7 or 8. While previously noted inter-strain differences were confirmed for different manifestations of caring behavior, self-maintenance and neglecting behaviors as well as the pup retrieval test, no strain-independent effect of the GR mutation was noted. However, an interaction between GR genotype and licking/grooming behavior was observed: it was down-regulated in heterozygous C57BL/6 mice to the level recorded for Balb/c mice. Home cage observation poses minimal disturbance of the dam and her litter as compared to more invasive assessments of dams' emotional behavior. This might be a reason for the absence of any overall effects of the GR mutation, particularly since GR heterozygous animals display a depressive-like phenotype under stressful conditions only. Still, the subtle effect we observed may point towards a role of GR in postpartum affective disorders

Handling method alters the hedonic value of reward in laboratory mice

Mice are the most widely used model species for drug discovery and scientific research. Consequently, it is important to refine laboratory procedures and practices to ensure high standards of welfare and scientific data quality. Recent studies have identified that the standard practice of handling laboratory mice by their tails increases behaviours indicative of anxiety, which can be overcome by handling mice using a tunnel. However, despite clear negative effects on mice’s behaviour, tunnel handling has yet to be widely implemented. In this study, we provide the first evidence that tail handling also reduces mice’s responses to reward. Anhedonia is a core symptom of clinical depression, and is measured in rodents by assessing how they consume a sucrose solution: depressed mice consume less sucrose and the size of their licking bouts when drinking (their ‘lick cluster sizes’) also tend to be smaller. We found that tail handled mice showed more anhedonic responses in both measures compared to tunnel handled mice, indicative of a decreased responsiveness to reward and potentially a more depressive-like state. Our findings have significant implications for the welfare of laboratory mice as well as the design and interpretation of scientific studies, particularly those investigating or involving reward

Prolonged Depression-Like Behavior Caused by Immune Challenge: Influence of Mouse Strain and Social Environment

Immune challenge by bacterial lipopolysaccharide (LPS) causes short-term behavioral changes indicative of depression. The present study sought to explore whether LPS is able to induce long-term changes in depression-related behavior and whether such an effect depends on mouse strain and social context. LPS (0.83 mg/kg) or vehicle was administered intraperitoneally to female CD1 and C57BL/6 mice that were housed singly or in groups of 4. Depression-like behavior was assessed with the forced swim test (FST) 1 and 28 days post-treatment. Group-housed CD1 mice exhibited depression-like behavior 1 day post-LPS, an effect that leveled off during the subsequent 28 days, while the behavior of singly housed CD1 mice was little affected. In contrast, singly housed C57BL/6 mice responded to LPS with an increase in depression-like behavior that was maintained for 4 weeks post-treatment and confirmed by the sucrose preference test. Group-housed C57BL/6 mice likewise displayed an increased depression-like behavior 4 weeks post-treatment. The behavioral changes induced by LPS in C57BL/6 mice were associated with a particularly pronounced rise of interleukin-6 in blood plasma within 1 day post-treatment and with changes in the dynamics of the corticosterone response to the FST. The current data demonstrate that immune challenge with LPS is able to induce prolonged depression-like behavior, an effect that depends on genetic background (strain). The discovery of an experimental model of long-term depression-like behavior after acute immune challenge is of relevance to the analysis of the epigenetic and pathophysiologic mechanisms of immune system-related affective disorders

Effect of Population Heterogenization on the Reproducibility of Mouse Behavior : A Multi-Laboratory Study

Peer reviewe

Signaling pathways responsible for the rapid antidepressant-like effects of a GluN2A-preferring NMDA receptor antagonist

In a previous study we found that the preferring GluN2A receptor antagonist, NVP-AAM077, elicited rapid antidepressant-like effects in the forced swim test that was related to the release of glutamate and serotonin in the medial prefrontal cortex. In the present work we sought to examine the duration of this behavioral effect as well as the molecular readouts involved. Our results showed that NVP-AAM077 reduced the immobility in the forced swim test 30?min and 24?h after its administration. However, this effect waned 7 days later. The rapid antidepressant-like response seems to be associated with increases in the GluA1 subunit of ?-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, mammalian target of rapamycin (mTOR) signaling, glia markers such as glial fibrillary acidic protein (GFAP) and excitatory amino acid transporter 1 (EAAT1), and a rapid mobilization of intracellular stores of brain-derived neurotrophic factor (BDNF) in the medial prefrontal cortex.Acknowledgements: M.G.-S. was recipient of a contract from the Sistema Nacional de Garantía Juvenil co-funded by the European Social Fund. We also thank Novartis for the generous gift of NVP-AAM077. This work was supported by the Instituto de Salud Carlos III, Subdirección General del Evaluación y Fomento de la Investigación (FIS Grants PI13/00038 and PI16/00217) that were co-funded by the European Regional Development Fund (‘A way to build Europe’). Funding from the Centro de Investigación Biomédica en Red de Salud Mental (CIBERSAM), Instituto de Salud Carlos III is also acknowledged