Presynaptic plasticity as a hallmark of rat stress susceptibility and antidepressant response

Investigating Resistance to Antidepressants in Animal Models

Major depressive disorder is one of the most prevalent psychiatric diseases, and up to 30-40% of patients remain symptomatic despite treatment. Novel therapies are sorely needed, and animal models may be used to elucidate fundamental neurobiological processes that contribute to human disease states. We conducted a systematic review of current preclinical approaches to investigating treatment resistance with the goal of describing a path forward for improving our understanding of treatment resistant depression. We conducted a broad literature search to identify studies relevant to the preclinical investigation of treatment resistant depression. We followed PRISMA (Preferred Reporting Items for Systemic Reviews and Meta-Analyses) guidelines and included all relevant studies. We identified 467 studies in our initial search. Of these studies, we included 69 in our systematic review after applying our inclusion/exclusion criteria. We identified 10 broad strategies for investigating treatment resistance in animal models. Stress hormone administration was the most commonly used model, and the most common behavioral test was the forced swim test. We systematically identified and reviewed current approaches for gaining insight into the neurobiology underlying treatment resistant depression using animal models. Each approach has its advantages and disadvantages, but all require careful consideration of their potential limitations regarding therapeutic translation. An enhanced understanding of treatment resistant depression is sorely needed given the burden of disease and lack of effective therapies.

Reduction in hippocampal GABAergic transmission in a low birth weight rat model of depression

Prenatal stress is believed to increase the risk of developing neuropsychiatric disorders, including major depression. Adverse genetic and environmental impacts during early development, such as glucocorticoid hyper-exposure, can lead to changes in the foetal brain, linked to mental illnesses developed in later life. Dysfunction in the GABAergic inhibitory system is associated with depressive disorders. However, the pathophysiology of GABAergic signalling is poorly understood in mood disorders. Here, we investigated GABAergic neurotransmission in the low birth weight (LBW) rat model of depression. Pregnant rats, exposed to dexamethasone, a synthetic glucocorticoid, during the last week of gestation, yielded LBW offspring showing anxiety- and depressive-like behaviour in adulthood. Patch-clamp recordings from dentate gyrus granule cells in brain slices were used to examine phasic and tonic GABAA receptor-mediated currents. The transcriptional levels of selected genes associated with synaptic vesicle proteins and GABAergic neurotransmission were investigated. The frequency of spontaneous inhibitory postsynaptic currents (sIPSC) was similar in control and LBW rats. Using a paired-pulse protocol to stimulate GABAergic fibres impinging onto granule cells, we found indications of decreased probability of GABA release in LBW rats. However, tonic GABAergic currents and miniature IPSCs, reflecting quantal vesicle release, appeared normal. Additionally, we found elevated expression levels of two presynaptic proteins, Snap-25 and Scamp2, components of the vesicle release machinery. The results suggest that altered GABA release may be an essential feature in the depressive-like phenotype of LBW rats.

Escitalopram reversibility of the impacts following chronic stress on central 5-HT profiles - Implications to depression and anxiety

Stress is considered a crucial determinant influencing health capacity in modern society. Long-term stress makes individuals more susceptible to mental dysfunctions, among which depression and anxiety are two major mental disorders. The success of using selective serotonin reuptake inhibitors (SSRIs) to treat these two disorders highlights the involvement of the central serotonergic (5-HT) system. Later studies suggest both presynaptic and postsynaptic 5-HT profiles should be considered for the effects of SSRIs, making it difficult to interpret the etiological and therapeutic mechanisms underlying depression and anxiety. The present study aims to examine whether the intervention of escitalopram (Es, 5 mg/kg daily for 14 days) can reverse the behavioral phenotypes of both depression-like [by sucrose preference test (SPT) and forced swim test (FST)] and anxiety-like [by avoidance latency and escape latency in elevated-T maze (ETM)] behaviors, and the brain area-dependent neurochemical changes of 5-HT profiles of the terminal regions regarding both synaptic efflux and tissue levels in rats of chronic mild stress (CMS). Our results showed that: (i) Even mild stresses when presented in an unpredictable and long-term manner, can induce both depression-like and anxiety-like behaviors. (ii) Depressive profile indexed by SPT was more sensitive to reflect the Es effect than that of FST. (iii) Es did not significantly affect the CMS-induced anxiety-like symptoms indexed by ETM. (iv) Changes in the protein expression of 5-HT1A receptors in the prefrontal cortex and hippocampus were compatible with the treatment outcome. Our results contributed to the understanding of stress-induced mood dysfunction and the involvement of central 5-HT.

The Role of Glutamate Underlying Treatment-resistant Depression

The monoamine hypothesis has significantly improved our understanding of mood disorders and their treatment by linking monoaminergic abnormalities to the pathophysiology of mood disorders. Even 50 years after the monoamine hypothesis was established, some patients do not respond to treatments for depression, including selective serotonin reuptake drugs. Accumulating evidence shows that patients with treatment-resistant depression (TRD) have severe abnormalities in the neuroplasticity and neurotrophic factor pathways, indicating that different treatment approaches may be necessary. Therefore, the glutamate hypothesis is gaining attention as a novel hypothesis that can overcome monoamine restrictions. Glutamate has been linked to structural and maladaptive morphological alterations in several brain areas associated with mood disorders. Recently, ketamine, an N-methyl-D-aspartate receptor (NMDAR) antagonist, has shown efficacy in TRD treatment and has received the U.S. Food and Drug Administration approval, revitalizing psychiatry research. However, the mechanism by which ketamine improves TRD remains unclear. In this review, we re-examined the glutamate hypothesis, bringing the glutamate system onboard to join the modulation of the monoamine systems, emphasizing the most prominent ketamine antidepressant mechanisms, such as NMDAR inhibition and NMDAR disinhibition in GABAergic interneurons. Furthermore, we discuss the animal models used in preclinical studies and the sex differences in the effects of ketamine.

Involvement of brain cell phenotypes in stress-vulnerability and resilience

Stress-related disorders' prevalence is epidemically increasing in modern society, leading to a severe impact on individuals' well-being and a great economic burden on public resources. Based on this, it is critical to understand the mechanisms by which stress induces these disorders. The study of stress made great progress in the past decades, from deeper into the hypothalamic-pituitary-adrenal axis to the understanding of the involvement of a single cell subtype on stress outcomes. In fact, many studies have used state-of-the-art tools such as chemogenetic, optogenetic, genetic manipulation, electrophysiology, pharmacology, and immunohistochemistry to investigate the role of specific cell subtypes in the stress response. In this review, we aim to gather studies addressing the involvement of specific brain cell subtypes in stress-related responses, exploring possible mechanisms associated with stress vulnerability versus resilience in preclinical models. We particularly focus on the involvement of the astrocytes, microglia, medium spiny neurons, parvalbumin neurons, pyramidal neurons, serotonergic neurons, and interneurons of different brain areas in stress-induced outcomes, resilience, and vulnerability to stress. We believe that this review can shed light on how diverse molecular mechanisms, involving specific receptors, neurotrophic factors, epigenetic enzymes, and miRNAs, among others, within these brain cell subtypes, are associated with the expression of a stress-susceptible or resilient phenotype, advancing the understanding/knowledge on the specific machinery implicate in those events.

Exploration of the Moderating Effects of Physical Activity and Early Life Stress on the Relation between Brain-Derived Neurotrophic Factor (BDNF) rs6265 Variants and Depressive Symptoms among Adolescents

Depression affects one in five persons at 18 years of age. Allele A of the brain-derived neurotrophic factor (BDNF) rs6265 is considered to be a risk factor for depression. Previous studies of the interaction between BDNF rs6265, early adversity, and/or physical activity have shown mixed results. In this study, we explored the relation between BDNF rs6265 polymorphism and childhood stress, as well as the moderating effect of physical activity in relation to depressive symptoms using binary logistic regressions and process models 1, 2 and 3 applied to data obtained at three times (waves 1, 2 and 3) from the Survey of Adolescent Life in Västmanland cohort study (SALVe). Results revealed that both childhood stress and physical activity had a moderation effect; physical activity in wave 1 with an R2 change = 0.006, p = 0.013, and the Johnson−Neyman regions of significance (RoS) below 1.259, p = 0.05 for 11.97%; childhood stress in wave 2 with the R2 change = 0.008, p = 0 002, and RoS below 1.561 with 26.71% and >4.515 with 18.20%; and a three-way interaction in wave 1 in genotype AA carriers. These results suggest that allele A is susceptible to physical activity (positive environment) and childhood stress (negative environment).

Impact of stress on inhibitory neuronal circuits, our tribute to Bruce McEwen

This manuscript is dedicated to the memory of Bruce S. McEwen, to commemorate the impact he had on how we understand stress and neuronal plasticity, and the profound influence he exerted on our scientific careers. The focus of this review is the impact of stressors on inhibitory circuits, particularly those of the limbic system, but we also consider other regions affected by these adverse experiences. We revise the effects of acute and chronic stress during different stages of development and lifespan, taking into account the influence of the sex of the animals. We review first the influence of stress on the physiology of inhibitory neurons and on the expression of molecules related directly to GABAergic neurotransmission, and then focus on specific interneuron subpopulations, particularly on parvalbumin and somatostatin expressing cells. Then we analyze the effects of stress on molecules and structures related to the plasticity of inhibitory neurons: the polysialylated form of the neural cell adhesion molecule and perineuronal nets. Finally, we review the potential of antidepressants or environmental manipulations to revert the effects of stress on inhibitory circuits.

Chronic mild stress paradigm as a rat model of depression: facts, artifacts, and future perspectives

RATIONALE

The chronic mild stress (CMS) paradigm was first described almost 40 years ago and has become a widely used model in the search for antidepressant drugs for major depression disorder (MDD). It has resulted in the publication of almost 1700 studies in rats alone. Under the original CMS procedure, the expression of an anhedonic response, a key symptom of depression, was seen as an essential feature of both the model and a depressive state. The prolonged exposure of rodents to unpredictable/uncontrollable mild stressors leads to a reduction in the intake of palatable liquids, behavioral despair, locomotor inhibition, anxiety-like changes, and vegetative (somatic) abnormalities. Many of the CMS studies do not report these patterns of behaviors, and they often fail to include consistent molecular, neuroanatomical, and physiological phenotypes of CMS-exposed animals.

OBJECTIVES

To critically review the CMS studies in rats so that conceptual and methodological flaws can be avoided in future studies.

RESULTS

Analysis of the literature supports the validity of the CMS model and its impact on the field. However, further improvements could be achieved by (i) the stratification of animals into 'resilient' and 'susceptible' cohorts within the CMS animals, (ii) the use of more refined protocols in the sucrose test to mitigate physiological and physical artifacts, and (iii) the systematic evaluation of the non-specific effects of CMS and implementation of appropriate adjustments within the behavioral tests.

CONCLUSIONS

We propose methodological revisions and the use of more advanced behavioral tests to refine the rat CMS paradigm, which offers a valuable tool for developing new antidepressant medications.

Parvalbumin interneuron alterations in stress-related mood disorders: A systematic review

Stress-related psychiatric disorders including depression involve complex cellular and molecular changes in the brain, and GABAergic signaling dysfunction is increasingly implicated in the etiology of mood disorders. Parvalbumin (PV)-expressing neurons are fast-spiking interneurons that, among other roles, coordinate synchronous neuronal firing. Mounting evidence suggests that the PV neuron phenotype is altered by stress and in mood disorders. In this systematic review, we assessed PV interneuron alterations in psychiatric disorders as reported in human postmortem brain studies and animal models of environmental stress. This review aims to 1) comprehensively catalog evidence of PV cell function in mood disorders (humans) and stress models of mood disorders (animals); 2) analyze the strength of evidence of PV interneuron alterations in various brain regions in humans and rodents; 3) determine whether the modulating effect of antidepressant treatment, physical exercise, and environmental enrichment on stress in animals associates with particular effects on PV function; and 4) use this information to guide future research avenues. Its principal findings, derived mainly from rodent studies, are that stress-related changes in PV cells are only reported in a minority of studies, that positive findings are region-, age-, sex-, and stress recency-dependent, and that antidepressants protect from stress-induced apparent PV cell loss. These observations do not currently translate well to humans, although the postmortem literature on the topic remains limited.

Harnessing the Neurobiology of Resilience to Protect the Mental Well-Being of Healthcare Workers During the COVID-19 Pandemic

Healthcare workers are at a high risk of psychological morbidity in the face of the COVID-19 pandemic. However, there is significant variability in the impact of this crisis on individual healthcare workers, which can be best explained through an appreciation of the construct of resilience. Broadly speaking, resilience refers to the ability to successfully adapt to stressful or traumatic events, and thus plays a key role in determining mental health outcomes following exposure to such events. A proper understanding of resilience is vital in enabling a shift from a reactive to a proactive approach for protecting and promoting the mental well-being of healthcare workers. Research in the past decade has identified six areas that provide promising leads in understanding the biological basis of individual variations in resilience. These are: (1) the key role played by the monoamines noradrenaline and serotonin, (2) the centrality of the hypothalamic-pituitary-adrenal axis in influencing stress vulnerability and resilience, (3) the intimate links between the immune system and stress sensitivity, (4) the role of epigenetic modulation of gene expression in influencing the stress response, (5) the role played by certain neuropeptides as a natural “brake” mechanism in the face of stress, and (6) the neurobiological mechanisms by which environmental factors, such as exercise, diet, and social support, influence resilience to subsequent life events. Though much of this research is still in its early stages, it has already provided valuable information on which strategies – including dietary changes, lifestyle modification, environmental modification, psychosocial interventions, and even pharmacological treatments – may prove to be useful in fostering resilience in individuals and groups. This paper examines the above evidence more closely, with a specific focus on the challenges faced by healthcare workers during the COVID-19 pandemic, and provides suggestions regarding how it may be translated into real-world interventions, as well as how the more tentative hypotheses advanced in this field may be tested during this critical period.

Validation of chronic mild stress in the Wistar-Kyoto rat as an animal model of treatment-resistant depression

A recent review proposed four criteria for an animal model of treatment-resistant depression (TRD): a phenotypic resemblance to a risk factor for depression; enhanced response to stress; nonresponse to antidepressant drugs and response to treatments effective in TRD, such as deep brain stimulation (DBS) of the prefrontal cortex or ketamine. Chronic mild stress (CMS) provides a valid model of depression; the Wistar-Kyoto (WKY) rat is considered to be nonresponsive to antidepressant drugs. Here, we applied CMS to WKY rats. WKY and Wistar rats were exposed to CMS, then treated with saline, imipramine, citalopram or venlafaxine. After 5 weeks of CMS and 3 weeks of drug treatment, all WKY groups were implanted unilaterally with DBS electrodes in the prefrontal cortex, and examined in sucrose intake, elevated plus maze (EPM; decreased entries and time in the open arms) and novel object recognition (decreased exploration) tests, following 2×2 h of DBS. CMS decreased sucrose intake, open arm entries on the EPM, and object recognition. Relative to Wistars, WKY rats showed evidence of increased emotionality in the EPM and novel object recognition tests, and a greater impact of CMS on body weight gain and open arm entries. Wistars responded to drug treatment with an increase in sucrose intake but WKY were nonresponsive to drug treatment on all three behavioural tests. With one exception, DBS reversed the anhedonic, anxiogenic and dyscognitive effects of CMS in all groups of WKY rats. In a further experiment, subacute ketamine (10 mg/kg) also normalized behaviour on all three tests. We conclude that WKY rats subjected to CMS meet all four criteria for a valid model of TRD, and provide a basis for studying the mechanism of action of DBS.

Hippocampal µ-opioid receptors on GABAergic neurons mediate stress-induced impairment of memory retrieval

Stressful life events induce abnormalities in emotional and cognitive behaviour. The endogenous opioid system plays an essential role in stress adaptation and coping strategies. In particular, the µ-opioid receptor (μR), one of the major opioid receptors, strongly influences memory processing in that alterations in μR signalling are associated with various neuropsychiatric disorders. However, it remains unclear whether μR signalling contributes to memory impairments induced by acute stress. Here, we utilized pharmacological methods and cell-type-selective/non-cell-type-selective μR depletion approaches combined with behavioural tests, biochemical analyses, and in vitro electrophysiological recordings to investigate the role of hippocampal μR signalling in memory-retrieval impairment induced by acute elevated platform (EP) stress in mice. Biochemical and molecular analyses revealed that hippocampal μRs were significantly activated during acute stress. Blockage of hippocampal μRs, non-selective deletion of μRs or selective deletion of μRs on GABAergic neurons (μR_GABA) reversed EP-stress-induced impairment of memory retrieval, with no effect on the elevation of serum corticosterone after stress. Electrophysiological results demonstrated that stress depressed hippocampal GABAergic synaptic transmission to CA1 pyramidal neurons, thereby leading to excitation/inhibition (E/I) imbalance in a μR_GABA-dependent manner. Pharmaceutically enhancing hippocampal GABA_A receptor-mediated inhibitory currents in stressed mice restored their memory retrieval, whereas inhibiting those currents in the unstressed mice mimicked the stress-induced impairment of memory retrieval. Our findings reveal a novel pathway in which endogenous opioids recruited by acute stress predominantly activate μR_GABA to depress GABAergic inhibitory effects on CA1 pyramidal neurons, which subsequently alters the E/I balance in the hippocampus and results in impairment of memory retrieval.

Resilience to chronic stress is associated with specific neurobiological, neuroendocrine and immune responses

Research into the molecular basis of stress resilience is a novel strategy to identify potential therapeutic strategies to treat stress-induced psychopathologies such as anxiety and depression. Stress resilience is a phenomenon which is not solely driven by effects within the central nervous system (CNS) but involves multiple systems, central and peripheral, which interact with and influence each other. Accordingly, we used the chronic social defeat stress paradigm and investigated specific CNS, endocrine and immune responses to identify signatures of stress-resilience and stress susceptibility in mice. Our results showed that mice behaviourally susceptible to stress (indexed by a reduction in social interaction behaviour) had higher plasma corticosterone levels and adrenal hypertrophy. An increase in inflammatory circulating monocytes was another hallmark of stress susceptibility. Furthermore, prefrontal cortex mRNA expression of corticotrophin-releasing factor (Crf) was increased in susceptible mice relative to resilient mice. We also report differences in hippocampal synaptic plasticity between resilient and susceptible mice. Ongoing studies will interpret the functional relevance of these signatures which could potentially inform the development of novel psychotherapeutic strategies.

Rapid antidepressant effects of deep brain stimulation of the pre-frontal cortex in an animal model of treatment-resistant depression

BACKGROUND

A significant proportion of depressed patients fail to respond to treatment with antidepressant drugs. Such patients might nonetheless respond to deep brain stimulation of the prefrontal cortex. Deep brain stimulation has also been shown to normalize behaviour in the chronic mild stress (CMS) model of depression. However, these studies have involved animals that are in general treatment responsive. Thus, this is not the ideal situation in which to investigate how deep brain stimulation is effective where antidepressant drugs are not.

AIMS

Here, we studied the behavioural effects of deep brain stimulation in treatment-resistant animals.

METHODS

Wistar rats were exposed to chronic mild stress and concurrent treatment with saline or one of three antidepressant drugs, imipramine, citalopram and venlafaxine. Individuals were selected from the CMS-exposed drug-treated groups that had failed to increase their sucrose intake by week 5 of drug treatment. All animals were then implanted with deep brain stimulation electrodes in the ventro-medial prefrontal cortex, and tested for sucrose intake and in the elevated plus maze and novel object recognition test, following 2 × 2 h of deep brain stimulation.

RESULTS

The selected drug-treated animals were found to be antidepressant-resistant in all three tests. With a single exception (sucrose intake in imipramine-treated animals), deep brain stimulation reversed the anhedonic, anxiogenic and dyscognitive effects of CMS in all four conditions, with no significant differences between saline- and drug-treated animals.

CONCLUSIONS

These data provide a proof of principle that deep brain stimulation of the prefrontal cortex can be effective in a rat model of resistance to chronic antidepressant treatment, replicating the clinical effect of deep brain stimulation in treatment-resistant depression.

Making Sense of Rodent Models of Anhedonia

A markedly reduced interest or pleasure in activities previously considered pleasurable is a main symptom in mood disorder and psychosis and is often present in other psychiatric disorders and neurodegenerative diseases. This condition can be labeled as "anhedonia," although in its most rigorous connotation the term refers to the lost capacity to feel pleasure that is one aspect of the complex phenomenon of processing and responding to reward. The responses to rewarding stimuli are relatively easy to study in rodents, and the experimental conditions that consistently and persistently impair these responses are used to model anhedonia. To this end, long-term exposure to environmental aversive conditions is primarily used, and the resulting deficits in reward responses are often accompanied by other deficits that are mainly reminiscent of clinical depressive symptoms. The different components of impaired reward responses induced by environmental aversive events can be assessed by different tests or protocols that require different degrees of time allocation, technical resources, and equipment. Rodent models of anhedonia are valuable tools in the study of the neurobiological mechanisms underpinning impaired behavioral responses and in the screening and characterization of drugs that may reverse these behavioral deficits. In particular, the antianhedonic or promotivational effects are relevant features in the spectrum of activities of drugs used in mood disorders or psychosis. Thus, more than the model, it is the choice of tests that is crucial since it influences which facets of anhedonia will be detected and should be tuned to the purpose of the study.

Long-Term Stress Disrupts the Structural and Functional Integrity of GABAergic Neuronal Networks in the Medial Prefrontal Cortex of Rats

Clinical and experimental data suggest that fronto-cortical GABAergic deficits contribute to the pathophysiology of major depressive disorder (MDD). To further test this hypothesis, we used a well characterized rat model for depression and examined the effect of stress on GABAergic neuron numbers and GABA-mediated synaptic transmission in the medial prefrontal cortex (mPFC) of rats. Adult male Wistar rats were subjected to 9-weeks of chronic mild stress (CMS) and based on their hedonic-anhedonic behavior they were behaviorally phenotyped as being stress-susceptible (anhedonic) or stress-resilient. Post mortem quantitative histopathology was used to examine the effect of stress on parvalbumin (PV)-, calretinin- (CR), calbindin- (CB), cholecystokinin- (CCK), somatostatin-(SST) and neuropeptide Y-positive (NPY+) GABAergic neuron numbers in all cortical subareas of the mPFC (anterior cingulate (Cg1), prelimbic (PrL) and infralimbic (IL) cortexes). In vitro, whole-cell patch-clamp recordings from layer II–III pyramidal neurons of the ventral mPFC was used to examine GABAergic neurotransmission. The cognitive performance of the animals was assessed in a hippocampal-prefrontal-cortical circuit dependent learning task. Stress exposure reduced the number of CCK-, CR- and PV-positive GABAergic neurons in the mPFC, most prominently in the IL cortex. Interestingly, in the stress-resilient animals, we found higher number of neuropeptide Y-positive neurons in the entire mPFC. The electrophysiological analysis revealed reduced frequencies of spontaneous and miniature IPSCs in the anhedonic rats and decreased release probability of perisomatic-targeting GABAergic synapses and alterations in GABA_B receptor mediated signaling. In turn, pyramidal neurons showed higher excitability. Anhedonic rats were also significantly impaired in the object-place paired-associate learning task. These data demonstrate that long-term stress results in functional and structural deficits of prefrontal GABAergic networks. Our findings support the concept that fronto-limbic GABAergic dysfunctions may contribute to emotional and cognitive symptoms of MDD.

Inflammation kinase PKR phosphorylates α-synuclein and causes α-synuclein-dependent cell death

Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy comprise a group of neurodegenerative diseases termed synucleinopathies. Synucleinopathie are, characterized by presence of inclusion bodies in degenerating brain cells which contain aggregated α-synuclein phosphorylated on Ser129. Although the inflammation-associated serine-threonine kinase, PKR (EIF2AK2), promotes cellular protection against infection, we demonstrate a pro-degenerative role of activated PKR in an α-synuclein-dependent cell model of multiple system atrophy, where inhibition and silencing of PKR decrease cellular degeneration. In vitro phosphorylation demonstrates that PKR can directly bind and phosphorylate monomeric and filamenteous α-synuclein on Ser129. Inhibition and knockdown of PKR reduce Ser129 phosphorylation in different models (SH-SY5Y ASYN cells, OLN-AS7 cells, primary mouse hippocampal neurons, and acute brain slices), while overexpression of constitutively active PKR increases Ser129 α-syn phosphorylation. Treatment with pre-formed α-synuclein fibrils, proteostatic stress-promoting MG-132 and known PKR activators, herpes simplex virus-1-∆ICP34.5 and LPS, as well as PKR inducer, IFN-β-1b, lead to increased levels of phosphorylated Ser129 α-synuclein that is completely blocked by simultaneous PKR inhibition. These results reveal a direct link between PKR and the phosphorylation and toxicity of α-synuclein, and they support that neuroinflammatory processes play a role in modulating the pathogenicity of α-synuclein.

A comprehensive review of genetic and epigenetic mechanisms that regulate BDNF expression and function with relevance to major depressive disorder

Major depressive disorder (MDD) is a mood disorder that affects behavior and impairs cognition. A gene potentially important to this disorder is the brain derived neurotrophic factor (BDNF) as it is involved in processes controlling neuroplasticity. Various mechanisms exist to regulate BDNF's expression level, subcellular localization, and sorting to appropriate secretory pathways. Alterations to these processes by genetic factors and negative stressors can dysregulate its expression, with possible implications for MDD. Here, we review the mechanisms governing the regulation of BDNF expression, and discuss how disease-associated single nucleotide polymorphisms (SNPs) can alter these mechanisms, and influence MDD. As negative stressors increase the likelihood of MDD, we will also discuss the impact of these stressors on BDNF expression, the cellular effect of such a change, and its impact on behavior in animal models of stress. We will also describe epigenetic processes that mediate this change in BDNF expression. Similarities in BDNF expression between animal models of stress and those in MDD will be highlighted. We will also contrast epigenetic patterns at the BDNF locus between animal models of stress, and MDD patients, and address limitations to current clinical studies. Future work should focus on validating current genetic and epigenetic findings in tightly controlled clinical studies. Regions outside of BDNF promoters should also be explored, as should other epigenetic marks, to improve identification of biomarkers for MDD.

P11 promoter methylation predicts the antidepressant effect of electroconvulsive therapy

Although electroconvulsive therapy (ECT) is among the most effective treatment options for pharmacoresistant major depressive disorder (MDD), some patients still remain refractory to standard ECT practise. Thus, there is a need for markers reliably predicting ECT non/response. In our study, we have taken a novel translational approach for discovering potential biomarkers for the prediction of ECT response. Our hypothesis was that the promoter methylation of p11, a multifunctional protein involved in both depressive-like states and antidepressant treatment responses, is differently regulated in ECT responders vs. nonresponders and thus be a putative biomarker of ECT response. The chronic mild stress model of MDD was adapted with the aim to obtain rats that are resistant to conventional antidepressant drugs (citalopram). Subsequently, electroconvulsive stimulation (ECS) was used to select responders and nonresponders, and compare p11 expression and promoter methylation. In the rat experiments we found that the gene promoter methylation and expression of p11 significantly correlate with the antidepressant effect of ECS. Next, we investigated the predictive properties of p11 promoter methylation in two clinical cohorts of patients with pharmacoresistant MDD. In a proof-of-concept clinical trial in 11 patients with refractory MDD, higher p11 promoter methylation was found in responders to ECT. This finding was replicated in an independent sample of 65 patients with pharmacoresistant MDD. This translational study successfully validated the first biomarker reliably predicting the responsiveness to ECT. Prescreening of this biomarker could help to identify patients eligible for first-line ECT treatment and also help to develop novel antidepressant treatment procedures for depressed patients resistant to all currently approved antidepressant treatments.

Hippocampal GR- and CB1-mediated mGluR5 differentially produces susceptibility and resilience to acute and chronic mild stress in rats

The molecular mechanism of individual response of susceptibility and resilience under psychological stress remains controversial and unclear. The present study aimed to explore the relationship of metabotropic glutamate receptor 5 (mGluR5) with glucocorticoid receptor (GR) or cannabinoid receptor (CB1) and further indicate the molecular mechanism of susceptibility and resilience to acute stress (AS) and chronic mild stress (CMS). Sucrose preference test and open field test were used to evaluate the response of susceptibility and resilience under stress in rats. The mRNA levels and protein expressions of mGluR5, GR, and CB1 were detected. AS induced a 35% reduction in the sucrose intake of rats, and these rats were considered as susceptible to stress; 21% of the rats showed resilience to the stress. Thirty-three percent of rats in the CMS group showed reduced sucrose water intake and were considered susceptible, while 20% of rats were considered resilient. Hippocampal mGluR5 mRNA and protein levels were increased in the susceptible rats. Pharmacological testing showed that GR was positively associated with mGluR5 in susceptible rats in the CMS group, while CB1 was negatively related to mGluR5 in susceptible rats in the AS group. The results suggested that GR and CB1 in the hippocampus might regulate mGluR5 protein and mRNA levels, which might be related to individual responses of susceptibility and resilience under AS and CMS.

Reliability of the chronic mild stress model of depression: A user survey

The chronic mild stress (CMS) model of depression is considered by many to be the animal model of depression that has the greatest validity and translational potential, but it has often been criticized for a perceived lack of reliability. The aims of this study were to establish the extent to which the procedure is reproducible, and to identify experimental variables relevant to its reliability. Because failures to replicate frequently remain unpublished, a survey methodology was used. A questionnaire was circulated to 170 labs identified from a PubMed search as having published a CMS study in the years 2010 or 2015 (with no selection in respect of the results reported). Responses were returned by 71 (42%) of the recipients, followed by further correspondence with some of them. Most of the respondents (n = 53: 75%) reported that the CMS procedure worked reliably in their hands. Of the others, 15 (21%) reported that the procedure was usually reliable, but not always (n = 9: 13%) or not for all measures (n = 6: 8%). Only three respondents (4%) reported being unable to reproduce the characteristic effects, two of whom may be using an insufficient duration of CMS exposure. A series of analyses compared the 75% of 'reliable' labs with the 25% of 'less reliable' labs on a range of experimenter, subject, stress and outcome variables. Few if any significant differences between these two samples were identified, possibly because of the small size and diversity of the 'less reliable' sample. Two other limitations of the study include the (unavoidable) omission of labs that may have worked with the model but not published their data, and the use of ad hoc measures to compare the severity of different stress regimes. The results are discussed in relation to relevant published observations. It is concluded that CMS is in fact a rather robust model, but the factors that result in a less effective implementation in a minority of laboratories remain to be firmly established.

The chronic mild stress (CMS) model of depression: History, evaluation and usage

Now 30 years old, the chronic mild stress (CMS) model of depression has been used in >1300 published studies, with a year-on-year increase rising to >200 papers in 2015. Data from a survey of users show that while a variety of names are in use (chronic mild/unpredictable/varied stress), these describe essentially the same procedure. This paper provides an update on the validity and reliability of the CMS model, and reviews recent data on the neurobiological basis of CMS effects and the mechanisms of antidepressant action: the volume of this research may be unique in providing a comprehensive account of antidepressant action within a single model. Also discussed is the use of CMS in drug discovery, with particular reference to hippocampal and extra-hippocampal targets. The high translational potential of the CMS model means that the neurobiological mechanisms described may be of particular relevance to human depression and mechanisms of clinical antidepressant action.

Chronic selective serotonin reuptake inhibition modulates endothelial dysfunction and oxidative state in rat chronic mild stress model of depression

Major depression is known to be associated with cardiovascular abnormalities, and oxidative stress has been suggested to play a role. We tested the hypothesis that antidepressant treatment reduces oxidative stress and endothelial dysfunctions in the chronic mild stress (CMS) model of depression in rats. Rats with >30% reduction in sucrose intake after 4 wk of CMS were defined in the study as CMS-susceptible and compared with unstressed controls. Sixteen CMS-susceptible and eight unstressed rats were treated during weeks 5 to 8 of the CMS protocol with escitalopram. Escitalopram-treated rats with >20% recovery in the sucrose consumption during the last 2 wk of treatment were defined as escitalopram responders. Rats that did not reach these criteria were defined as escitalopram nonresponders. In the open field test, escitalopram responders demonstrated anxiolytic effect of treatment. In mesenteric small arteries, escitalopram affected neither NO nor cyclooxygenase-1 (COX-1)-mediated vasodilation. Escitalopram potentiated endothelium-dependent hyperpolarization-like response, which was suppressed in the vehicle-treated CMS-susceptible rats and reduced COX-2-dependent relaxation, which was elevated in the vehicle-treated CMS-susceptible rats. Escitalopram did not affect blood pressure and heart rate, which were elevated in the vehicle-treated CMS-susceptible rats. Oxidative stress markers were changed in association with CMS in liver, heart, and brain. Escitalopram normalized oxidative stress markers in the majority of tissues. This study demonstrates that the antidepressant effect of escitalopram is associated with partial improvement of endothelial function in small arteries affecting COX-2 and endothelium-dependent hyperpolarization-like pathways.