Deep brain stimulation in treatment-resistant depression in mice: comparison with the CRF1 antagonist, SSR125543

The impact of stress on the behavior of C57BL/6 mice with liver injury: a comparative study

Introduction

Depressive-like behavior has been shown to be associated with liver damage. This study aimed to evaluate the impact of three different models of depression on the behavior of mice with liver injury.

Methods

During the 4 weeks of methionine/choline deficiency diet (MCD), adult C57BL/6 mice were randomly divided into four groups: MCD (no stress protocol, n = 6), chronic unpredictable mild stress (CUMS, n = 9), acute and repeated forced swim stress [aFSS (n = 9) and rFSS (n = 9)].

Results

All depression protocols induced increased anhedonia and anxiety-like behavior compared to baseline and had no impact on the severity of liver damage, according to ultrasonography. However, different protocols evoked different overall behavior patterns. After the depressive-like behavior induction protocols, animals subjected to aFSS did not exhibit anxiety-like behavior differences compared to MCD animals, while mice subjected to CUMS showed additional weight loss compared to FSS animals. All tested protocols for inducing depressive-like behavior decreased the short-term memory of mice with liver damage, as assessed by the novel object recognition test (NORT).

Discussion

Our results show that the use of all protocols seems to generate different levels of anxiety-like behavior, but only the depressive-like behavior induction procedures associate additional anhedonia and memory impairment in mice with liver injury.

Continuous deep brain stimulation of the nucleus accumbens reduces food intake but does not affect body weight in mice fed a high-fat diet

Obesity is an enormous health problem, and many patients do not respond to any of the available therapies. Deep brain stimulation (DBS) is currently investigated as a potential treatment for morbid obesity. In this study, we tested the hypothesis that high-frequency DBS targeting the nucleus accumbens (NAc) shell region reduces food intake and weight gain in mice fed a high-fat diet. We implanted male C57BL/6J mice with bilateral electrodes and a head-mounted microstimulator enabling continuous stimulation for up to 5 weeks. In successfully operated animals (n = 9 per group, high-frequency vs. sham stimulation), we investigated immediate and long-term stimulation effects on metabolic and behavioral phenotypes. Here we show that stimulation acutely induced a transient reduction in energy expenditure and locomotor activity but did not significantly affect spontaneous food intake, social interaction, anxiety or exploratory behaviors. In contrast, continuous stimulation over 5 weeks led to a decrease in food intake and thigmotaxis (the tendency to stay near walls in an open lit arena). However, chronic stimulation did not substantially change weight gain in mice fed a high-fat diet. Our results do not support the use of continuous high-frequency NAc shell DBS as a treatment for obesity. However, DBS can alter obesity-related parameters with differing short and long-term effects. Therefore, future research should employ time and context-sensitive experimental designs to assess the potential of DBS for clinical translation in this area.

Galactic Cosmic Irradiation Alters Acute and Delayed Species-Typical Behavior in Male and Female Mice

Exposure to space galactic cosmic radiation is a principal consideration for deep space missions. While the effects of space irradiation on the nervous system are not fully known, studies in animal models have shown that exposure to ionizing radiation can cause neuronal damage and lead to downstream cognitive and behavioral deficits. Cognitive health implications put humans and missions at risk, and with the upcoming Artemis missions in which female crew will play a major role, advance critical analysis of the neurological and performance responses of male and female rodents to space radiation is vital. Here, we tested the hypothesis that simulated Galactic Cosmic Radiation (GCRSim) exposure disrupts species-typical behavior in mice, including burrowing, rearing, grooming, and nest-building that depend upon hippocampal and medial prefrontal cortex circuitry. Behavior comprises a remarkably well-integrated representation of the biology of the whole animal that informs overall neural and physiological status, revealing functional impairment. We conducted a systematic dose-response analysis of mature (6-month-old) male and female mice exposed to either 5, 15, or 50 cGy 5-ion GCRSim (H, Si, He, O, Fe) at the NASA Space Radiation Laboratory (NSRL). Behavioral performance was evaluated at 72 h (acute) and 91-days (delayed) postradiation exposure. Specifically, species-typical behavior patterns comprising burrowing, rearing, and grooming as well as nest building were analyzed. A Neuroscore test battery (spontaneous activity, proprioception, vibrissae touch, limb symmetry, lateral turning, forelimb outstretching, and climbing) was performed at the acute timepoint to investigate early sensorimotor deficits postirradiation exposure. Nest construction, a measure of neurological and organizational function in rodents, was evaluated using a five-stage Likert scale 'Deacon' score that ranged from 1 (a low score where the Nestlet is untouched) to 5 (a high score where the Nestlet is completely shredded and shaped into a nest). Differential acute responses were observed in females relative to males with respect to species-typical behavior following 15 cGy exposure while delayed responses were observed in female grooming following 50 cGy exposure. Significant sex differences were observed at both timepoints in nest building. No deficits in sensorimotor behavior were observed via the Neuroscore. This study revealed subtle, sexually dimorphic GCRSim exposure effects on mouse behavior. Our analysis provides a clearer understanding of GCR dose effects on species typical, sensorimotor and organizational behaviors at acute and delayed timeframes postirradiation, thereby setting the stage for the identification of underlying cellular and molecular events.

The Evolving Role of Animal Models in the Discovery and Development of Novel Treatments for Psychiatric Disorders

Historically, animal models have been routinely used in the characterization of novel chemical entities (NCEs) for various psychiatric disorders. Animal models have been essential in the in vivo validation of novel drug targets, establishment of lead compound pharmacokinetic to pharmacodynamic relationships, optimization of lead compounds through preclinical candidate selection, and development of translational measures of target occupancy and functional target engagement. Yet, with decades of multiple NCE failures in Phase II and III efficacy trials for different psychiatric disorders, the utility and value of animal models in the drug discovery process have come under intense scrutiny along with the widespread withdrawal of the pharmaceutical industry from psychiatric drug discovery. More recently, the development and utilization of animal models for the discovery of psychiatric NCEs has undergone a dynamic evolution with the application of the Research Domain Criteria (RDoC) framework for better design of preclinical to clinical translational studies combined with innovative genetic, neural circuitry-based, and automated testing technologies. In this chapter, the authors will discuss this evolving role of animal models for improving the different stages of the discovery and development in the identification of next generation treatments for psychiatric disorders.

Tetrahydrobiopterin modulates the behavioral neuroinflammatory response to an LPS challenge in mice

Tetrahydrobiopterin (BH4) is a necessary cofactor for the synthesis of monoamines from essential amino-acids, phenylalanine, tyrosine and tryptophan. The BH4 synthesis pathway is induced by inflammatory factors but highly regulated processes maintain levels in a physiological range. However, BH4 activity can be durably altered in inflammation-related pathologies, such as certain types of depression, potentially involving impairment of dopaminergic neurotransmission. The purpose of this study was to investigate the response of the brain BH4 pathway to the inflammatory stimulus induced by lipopolysaccharide (LPS) in mice. Brain expression of genes related to BH4 synthesis, levels of BH4, changes in L-aromatic amino acid precursors of monoamines and dopamine levels were determined. As secondary aim, the effect of acute BH4 supply under the inflammatory challenge was tested on these parameters and on the expression of inflammatory cytokines. Mice were also submitted to the sucrose preference test and to the open-field in order to asses hedonic and locomotor responses to LPS, in addition to their modulation by BH4 supply. The LPS challenge resulted in decreased striatal DA levels and increased Phenylalanine/Tyrosine ratio, suggesting reduced BH4 activity. BH4 supply was effective to increase striatal BH4 levels, to restore the LPS-induced decreased in DA levels in striatum and to dampen the LPS-induced expression of inflammatory cytokines. At the behavioral level, BH4 supply was able to restore the loss of locomotor response to amphetamine in the LPS treated mice, suggesting a modulation of the dopaminergic neurotransmission. These data suggest that BH4 can be considered as a potential add-on molecule, helping to maintain or restore dopaminergic neurotransmission in neuroinflammatory conditions..

CRF-R1 Antagonist Treatment Exacerbates Circadian Corticosterone Secretion under Chronic Stress, but Preserves HPA Feedback Sensitivity

Despite promising initial reports, corticotropin-releasing factor receptor type-1 (CRF-R1) antagonists have mostly failed to display efficacy in clinical trials for anxiety or depression. Rather than broad-spectrum antidepressant/anxiolytic-like drugs, they may represent an ‘antistress’ solution for single stressful situations or for patients with chronic stress conditions. However, the impact of prolonged CRF-R1 antagonist treatments on the hypothalamic–pituitary–adrenal (HPA) axis under chronic stress conditions remained to be characterized. Hence, our study investigated whether a chronic CRF-R1 antagonist (crinecerfont, formerly known as SSR125543, 20 mg·kg⁻¹·day⁻¹ ip, 5 weeks) would alter HPA axis basal circadian activity and negative feedback sensitivity in mice exposed to either control or chronic stress conditions (unpredictable chronic mild stress, UCMS, 7 weeks), through measures of fecal corticosterone metabolites, plasma corticosterone, and dexamethasone suppression test. Despite preserving HPA axis parameters in control non-stressed mice, the 5-week crinercerfont treatment improved the negative feedback sensitivity in chronically stressed mice, but paradoxically exacerbated their basal corticosterone secretion nearly all along the circadian cycle. The capacity of chronic CRF-R1 antagonists to improve the HPA negative feedback in UCMS argues in favor of a potential therapeutic benefit against stress-related conditions. However, the treatment-related overactivation of HPA circadian activity in UCMS raise questions about possible physiological outcomes with long-standing treatments under ongoing chronic stress.

Insufficiency of ventral hippocampus to medial prefrontal cortex transmission explains antidepressant non-response

BACKGROUND

There is extensive evidence that antidepressant drugs restore normal brain function by repairing damage to ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC). While the damage is more extensive in hippocampus, the evidence of treatments, such as deep brain stimulation, suggests that functional changes in prefrontal cortex may be more critical. We hypothesized that antidepressant non-response may result from an insufficiency of transmission from vHPC to mPFC.

METHOD

Antidepressant non-responsive Wistar Kyoto (WKY) rats were subjected to chronic mild stress (CMS), then treated with chronic daily administration of the antidepressant drug venlafaxine (VEN) and/or repeated weekly optogenetic stimulation (OGS) of afferents to mPFC originating from vHPC or dorsal HPC (dHPC).

RESULTS

As in many previous studies, CMS decreased sucrose intake, open-arm entries on the elevated plus maze (EPM), and novel object recognition (NOR). Neither VEN nor vHPC-mPFC OGS alone was effective in reversing the effects of CMS, but the combination of chronic VEN and repeated OGS restored normal behaviour on all three measures. dHPC-mPFC OGS restored normal behaviour in the EPM and NOR test irrespective of concomitant VEN treatment, and had no effect on sucrose intake.

CONCLUSIONS

The synergism between VEN and vHPC-mPFC OGS supports the hypothesis that the antidepressant non-responsiveness of WKY rats results from a failure of antidepressant treatment fully to restore transmission in the vHPC-mPFC pathway.

Perspectives for therapy of treatment-resistant depression

A high proportion of depressed patients fail to respond to antidepressant drug treatment. Treatment-resistant depression (TRD) is a major challenge for the psychopharmacology of mood disorders. Only in the past decade have novel treatments, including deep brain stimulation (DBS) and ketamine, been discovered that provide rapid and sometimes prolonged relief to a high proportion of TRD sufferers. In this review, we consider the current status of TRD from four perspectives: the challenge of developing an appropriate regulatory framework for novel rapidly acting antidepressants; the efficacy of non-pharmacological somatic therapies; the development of an animal model of TRD and its use to understand the neural basis of antidepressant non-response; and the potential for rapid antidepressant action from targets (such as 5-HT_1A receptors) beyond the glutamate receptor.

AMPA receptors mediate the pro-cognitive effects of electrical and optogenetic stimulation of the medial prefrontal cortex in antidepressant non-responsive Wistar-Kyoto rats

BACKGROUND

The chronic mild stress (CMS) procedure is a widely used animal model of depression, and its application in Wistar-Kyoto (WKY) rats has been validated as a model of antidepressant-refractory depression. While not responding to chronic treatment with antidepressant drugs, WKY rats do respond to acute deep brain stimulation (DBS) of the medial prefrontal cortex (mPFC). In antidepressant-responsive strains there is evidence suggesting a role for AMPA subtype of glutamate receptor in the action mechanism of both antidepressants and DBS.

METHODS

Animals were subjected to CMS for 6 to 8 weeks; sucrose intake was monitored weekly and novel object recognition (NOR) test was conducted following recovery from CMS. Wistars were treated chronically with venlafaxine (VEN), while WKY were treated acutely with either DBS, optogenetic stimulation (OGS) of virally-transduced (AAV5-hSyn-ChR2-EYFP) mPFC or ventral hippocampus, or acute intra-mPFC injection of the AMPA receptor positive allosteric modulator CX-516. The AMPA receptor antagonist NBQX was administered, at identical sites in mPFC, immediately following the exposure trial in the NOR.

RESULTS

Sucrose intake and NOR were suppressed by CMS, and restored by VEN in Wistars and by DBS, OGS, or CX-516 in WKY. However, OGS of the ventral hippocampal afferents to mPFC was ineffective. A low dose of NBQX selectively blocked the procognitive effect of VEN, DBS and OGS.

CONCLUSIONS

These results suggest that activation of AMPA receptors in the mPFC represents a common pathway for the antidepressant effects of both conventional (VEN) and novel (DBS, OGS) antidepressant modalities, in both antidepressant responsive (Wistar) and antidepressant-resistant (WKY) rats.

The kappa opioid receptor antagonist aticaprant reverses behavioral effects from unpredictable chronic mild stress in male mice

RATIONALE

Major depressive disorder is a leading cause of disability worldwide and is likely precipitated by chronic stress. Although many antidepressants are currently available, these drugs require weeks to months of daily administration before reduction of symptoms occurs and many patients remain treatment-resistant despite several courses of treatment. There is a pressing need for new treatments for stress-related disorders. Kappa opioid receptors (KORs) are a promising new therapeutic target for major depressive disorder and anhedonia because acute KOR blockade prevents many effects of stress in rodents.

OBJECTIVES

The following study assessed whether repeated treatment with the selective KOR antagonist aticaprant (also known as JNJ-67953964, and previously LY-2456302 and CERC-501) was effective in reversing behaviors in rodents following exposure to unpredictable chronic mild stress (UCMS).

METHODS

Adult male C57BL/6J mice were exposed to 4 weeks of UCMS. After 3 weeks of stress, aticaprant (10 mg/kg) was administered daily for 11 treatments. Behavioral assessments included the sucrose preference test, nesting, forced swim test, hot plate test, light-dark test, and social interaction test.

RESULTS

Aticaprant significantly reversed stress-induced deficits produced by UCMS on the SPT, nesting, FST, and hot plate test. The effects of aticaprant persisted through a stress and treatment recovery period. Aticaprant was not effective at reversing behavioral effects caused by stress in the light-dark and social interaction tests.

CONCLUSIONS

The results support further study of the role of KORs in regulating circuits related to reward, self-care, and cognition when they are disrupted by chronic stress. They are also consistent with the clinical development of aticaprant as a therapeutic for stress-related disorders targeted at anhedonia, such as depression and post-traumatic stress disorder.

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.

Chronic stress induces significant gene expression changes in the prefrontal cortex alongside alterations in adult hippocampal neurogenesis

Adult hippocampal neurogenesis is involved in stress-related disorders such as depression, posttraumatic stress disorders, as well as in the mechanism of antidepressant effects. However, the molecular mechanisms involved in these associations remain to be fully explored. In this study, unpredictable chronic mild stress in mice resulted in a deficit in neuronal dendritic tree development and neuroblast migration in the hippocampal neurogenic niche. To investigate molecular pathways underlying neurogenesis alteration, genome-wide gene expression changes were assessed in the prefrontal cortex, hippocampus and the hypothalamus alongside neurogenesis changes. Cluster analysis showed that the transcriptomic signature of chronic stress is much more prominent in the prefrontal cortex compared to the hippocampus and the hypothalamus. Pathway analyses suggested huntingtin, leptin, myelin regulatory factor, methyl-CpG binding protein and brain-derived neurotrophic factor as the top predicted upstream regulators of transcriptomic changes in the prefrontal cortex. Involvement of the satiety regulating pathways (leptin) was corroborated by behavioural data showing increased food reward motivation in stressed mice. Behavioural and gene expression data also suggested circadian rhythm disruption and activation of circadian clock genes such as Period 2. Interestingly, most of these pathways have been previously shown to be involved in the regulation of adult hippocampal neurogenesis. It is possible that activation of these pathways in the prefrontal cortex by chronic stress indirectly affects neuronal differentiation and migration in the hippocampal neurogenic niche via reciprocal connections between the two brain areas.

Sex differences in the modulation of mouse nest building behavior by kappa opioid receptor signaling

Emerging evidence suggests that females are less sensitive than males to the effects of kappa opioid receptor (KOR) ligands across multiple behavioral measures. The effects of the KOR agonist U50,488 and the KOR antagonist aticaprant were assessed on nest building behavior, an ethologically relevant indicator of overall well-being and affect, in adult male and female C57BL/6J mice. Females required a higher dose of U50,488 to suppress nesting, and a higher dose of aticaprant to restore U50,488-induced impairment of nesting. Females also required a higher dose of aticaprant to decrease immobility scores in the forced swim test. Pretreatment with the estrogen receptor modulator tamoxifen, at a dose which blocked estrogen receptors, augmented the effect of U50,488 on nesting in female mice, suggesting that estrogen receptors play a key role in attenuating the effects of KOR ligands in female mice. Together, these results suggest that females are less sensitive to KOR mediation, requiring a higher dose to achieve comparable results to males. This behavioral sensitivity, as measured by nesting, may be mediated by estrogen receptors. Together these studies highlight the importance of comparing sex differences in response to KOR regulation on behaviors related to affective states.

A Custom Made Electrode Construct and Reliable Implantation Method That Allows for Long-Term Bilateral Deep Brain Stimulation in Mice

Objectives

The underlying mechanisms behind the therapeutic and side effects of deep brain stimulation (DBS) need further investigation. The utilization of transgenic mouse lines is a suitable approach to better understand the cellular and network effects of DBS. However, not many bilateral DBS studies have been conducted in mice. This might be due to a lack of commercially available bilateral DBS constructs.

Materials and Methods

We developed an approach to perform repetitive long‐term DBS in freely moving mice. In this study, we implanted an in‐house custom‐made DBS construct containing two bipolar concentric electrodes to target the subthalamic nucleus (STN) bilaterally. Subsequently, we stimulated half of the animals with clinically relevant parameters three to five times a week with a duration of 20 min for ten weeks. Several behavioral tests were conducted of which the open field test (OFT) is shown to validate the reliability of this electrode construct and implantation method. Furthermore, we performed fiber photometry measurements to show the acute effect of STN‐DBS on serotonin network activity in the dorsal raphe nucleus.

Results

Repetitive DBS and long‐term behavioral testing were performed without complications. STN‐DBS resulted in an increase of the distance traveled in the OFT and a reduction of calcium levels in serotonergic neurons of the dorsal raphe nucleus. None of the mice had lost their electrodes and postmortem evaluation of the tissue showed accurate targeting of the STN without excessive gliosis.

Conclusion

The DBS electrode construct and implantation method described can be used for long‐term DBS studies to further investigate the mechanisms underlying DBS.

GPR56/ADGRG1 is associated with response to antidepressant treatment

It remains unclear why many patients with depression do not respond to antidepressant treatment. In three cohorts of individuals with depression and treated with serotonin-norepinephrine reuptake inhibitor (N = 424) we show that responders, but not non-responders, display an increase of GPR56 mRNA in the blood. In a small group of subjects we also show that GPR56 is downregulated in the PFC of individuals with depression that died by suicide. In mice, we show that chronic stress-induced Gpr56 downregulation in the blood and prefrontal cortex (PFC), which is accompanied by depression-like behavior, and can be reversed by antidepressant treatment. Gpr56 knockdown in mouse PFC is associated with depressive-like behaviors, executive dysfunction and poor response to antidepressant treatment. GPR56 peptide agonists have antidepressant-like effects and upregulated AKT/GSK3/EIF4 pathways. Our findings uncover a potential role of GPR56 in antidepressant response.

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.

Animal models of major depression: drawbacks and challenges

Major depression is a leading contributor to the global burden of disease. This situation is mainly related to the chronicity and/or recurrence of the disorder, and to poor response to antidepressant therapy. Progress in this area requires valid animal models. Current models are based either on manipulating the environment to which rodents are exposed (during the developmental period or adulthood) or biological underpinnings (i.e. gene deletion or overexpression of candidate genes, targeted lesions of brain areas, optogenetic control of specific neuronal populations, etc.). These manipulations can alter specific behavioural and biological outcomes that can be related to different symptomatic and pathophysiological dimensions of major depression. However, animal models of major depression display substantial shortcomings that contribute to the lack of innovative pharmacological approaches in recent decades and which hamper our capabilities to investigate treatment-resistant depression. Here, we discuss the validity of these models, review putative models of treatment-resistant depression, major depression subtypes and recurrent depression. Furthermore, we identify future challenges regarding new paradigms such as those proposing dimensional rather than categorical approaches to depression.

The role of prefrontal cortex dopamine D2 and D3 receptors in the mechanism of action of venlafaxine and deep brain stimulation in animal models of treatment-responsive and treatment-resistant depression

AIMS

The Wistar-Kyoto rat has been validated as an animal model of treatment-resistant depression. Here we investigated a role of dopamine D2 and D3 receptors in the ventro-medial prefrontal cortex in the mechanism of action of deep brain stimulation in Wistar-Kyoto rats and venlafaxine in Wistar rats.

METHODS

Wistar or Wistar-Kyoto rats were exposed chronically to chronic mild stress. Wistar rats were treated chronically with venlafaxine (10 mg/kg) beginning after two weeks of chronic mild stress; Wistar-Kyoto rats received two sessions of deep brain stimulation before behavioural tests. L-742,626 (1 µg), a D2 receptor agonist, or 7-OH DPAT (3 µg), a D3 receptor antagonist, were infused into the ventro-medial prefrontal cortex immediately following the exposure trial in the Novel Object Recognition Test, and discrimination between novel and familiar object was tested one hour later.

RESULTS

Chronic mild stress decreased sucrose intake and impaired memory consolidation; these effects were reversed by venlafaxine in Wistar rats and deep brain stimulation in Wistar-Kyoto rats. In control animals, L-742,626 and 7-OH DPAT also impaired memory consolidation. In Wistar rats, venlafaxine reversed the effect of L-742,626 in controls, but not in the chronic mild stress group, and venlafaxine did not reverse the effect of 7-OH DPAT in either group. In Wistar-Kyoto rats, deep brain stimulation reversed the effect of both L-742,626 and 7-OH DPAT in both control and chronic mild stress groups.

CONCLUSIONS

We conclude that the action of venlafaxine to reverse the impairment of memory consolidation caused by chronic mild stress in Wistar rats involves D2 receptors in the ventro-medial prefrontal cortex; but the effect of deep brain stimulation to reverse the same effect in Wistar-Kyoto rats does not.

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.

Deep brain stimulation for treatment-resistant depression: an integrative review of preclinical and clinical findings and translational implications

Although deep brain stimulation (DBS) is an established treatment choice for Parkinson's disease (PD), essential tremor and movement disorders, its effectiveness for the management of treatment-resistant depression (TRD) remains unclear. Herein, we conducted an integrative review on major neuroanatomical targets of DBS pursued for the treatment of intractable TRD. The aim of this review article is to provide a critical discussion of possible underlying mechanisms for DBS-generated antidepressant effects identified in preclinical studies and clinical trials, and to determine which brain target(s) elicited the most promising outcomes considering acute and maintenance treatment of TRD. Major electronic databases were searched to identify preclinical and clinical studies that have investigated the effects of DBS on depression-related outcomes. Overall, 92 references met inclusion criteria, and have evaluated six unique DBS targets namely the subcallosal cingulate gyrus (SCG), nucleus accumbens (NAc), ventral capsule/ventral striatum or anterior limb of internal capsule (ALIC), medial forebrain bundle (MFB), lateral habenula (LHb) and inferior thalamic peduncle for the treatment of unrelenting TRD. Electrical stimulation of these pertinent brain regions displayed differential effects on mood transition in patients with TRD. In addition, 47 unique references provided preclinical evidence for putative neurobiological mechanisms underlying antidepressant effects of DBS applied to the ventromedial prefrontal cortex, NAc, MFB, LHb and subthalamic nucleus. Preclinical studies suggest that stimulation parameters and neuroanatomical locations could influence DBS-related antidepressant effects, and also pointed that modulatory effects on monoamine neurotransmitters in target regions or interconnected brain networks following DBS could have a role in the antidepressant effects of DBS. Among several neuromodulatory targets that have been investigated, DBS in the neuroanatomical framework of the SCG, ALIC and MFB yielded more consistent antidepressant response rates in samples with TRD. Nevertheless, more well-designed randomized double-blind, controlled trials are warranted to further assess the efficacy, safety and tolerability of these more promising DBS targets for the management of TRD as therapeutic effects have been inconsistent across some controlled studies.

Fluoxetine induces paradoxical effects in C57BL6/J mice: comparison with BALB/c mice

The C57BL6/J mouse is the most commonly used strain in genetic investigations and behavioural tests. However, only a few studies have used C57BL6/J mice to assess the effects of antidepressant compounds. We carried out a study to compare the behavioural effects of fluoxetine (FLX) in a model of depression in two mice strains: C57BL6/J and BALB/c. We used an 8-week unpredictable chronic mild stress (UCMS) protocol during which FLX was administered (15 mg/kg, oral) from the third week to the end of the protocol. We found that UCMS induced degradation of the coat state in the two strains. Moreover, as expected, we observed that FLX elicited antidepressant-like effects in the BALB/c mice by reducing the coat state deterioration and the latency of grooming in splash test. However, in the C57BL6/J mice, it did not induce this action, but instead triggered an opposite effect: an increased sniffing latency in the novelty suppression of feeding test. We conclude that FLX exerts a paradoxical effect in the C57Bl6/J strain. This observation is consistent with some clinical features of hyper-reactivity to FLX observed in humans. Therefore, the UCMS protocol used in C57Bl6/J mice could be a good model to study the mechanisms of the paradoxical effects caused by selective serotonin reuptake inhibitors.

Divergent anomaly in mesocorticolimbic dopaminergic circuits might be associated with different depressive behaviors, an animal study

BACKGROUND

The mesocorticolimbic dopamine system, which originates from the ventral tegmental area (VTA) and projects primarily to the prefrontal cortex (PFC), olfactory tubercle (OT), nucleus accumbens (NAc), dorsal striatum (ST), and the amygdala (AMy), plays a pivotal role in determining individual motivation and sensitivity to rewards, namely, anhedonia. Not all depressive individuals exhibited anhedonia, thus, it is natural to speculate that the heterogenous manifestations of depression might be related to the mesocorticolimbic dopamine system. Maternal deprivation (MD) and chronic unpredictable stress (CUPS) are two well-established depressogenic stressors, and they were proven to induce different depressive phenotypes.

METHODS

The depressive and anxiety-like behaviors of MD and CUPS-treated rats were measured by classical behavioral tests including open field, forced swimming, and sucrose preference test. The expression of D1-5 dopamine receptors and DAT mRNA and protein in the mesocorticolimbic dopamine system of rats exposed to MD and CUPS were measured by real-time PCR and Western blot, respectively.

RESULTS

Severe anhedonia was observed in MD but not CUPS rats. Divergent expression of D1 and D2 receptors and DAT mRNA and protein in the mesocorticolimbic dopamine system were found between MD and CUPS rats. Significant correlations between different depressive behaviors and D1-/D2-like receptors and DAT protein levels in the mesocorticolimbic dopamine system were observed.

CONCLUSION

Different depressive behaviors of rats such as anhedonia, passive coping behavior, and declined exploratory interest might be related to divergent dopaminergic pathways. Anhedonia is associated with the dysfunction of VTA-NAc and VTA-OT dopaminergic pathways, the passive coping behavior is related to the dysregulation of VTA-PFC and VTA-AMy pathways, and individual exploratory interest is associated with abnormal activity of VTA-PFC and VTA-ST pathways.

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.

The acute social defeat stress and nest-building test paradigm: A potential new method to screen drugs for depressive-like symptoms

Psychosocial stress can cause mental conditions such as depression in humans. To develop drug therapies for the treatment of depression, it is necessary to use animal models of depression to screen drug candidates that exhibit anti-depressive effects. Unfortunately, the present methods of drug screening for antidepressants, the forced-swim test and tail-suspension test, are limiting factors in drug discovery because they are not based on the constructive validity of objective phenotypes in depression. Previously, we discovered that the onset of nest building is severely delayed in mice exposed to subchronic mild social defeat stress (sCSDS). Therefore, a novel paradigm combining acute social defeat stress (ASDS) and the nest-building test (SNB) were established for the efficient screening of drugs for depressive-like symptoms. Since ASDS severely delayed the nest-building process as shown in chronically social defeated mice, we sought to rescue the delayed nest-building behavior in ASDS mice. Injecting a specific serotonin 2a receptor antagonist (SR-46349B), the nest-building deficit exhibited by ASDS mice was partially rescued. On the other hand, a selective serotonin reuptake inhibitor (fluoxetine) did not rescue the nest-building deficit in ASDS mice. Therefore, we conclude that the SNB paradigm is an another potential behavioral method for screening drugs for depressive-like symptoms including attention deficit, anxiety, low locomotion, and decreased motivation.

Effects of acute alcohol withdrawal on nest building in mice selectively bred for alcohol withdrawal severity

Nest building has been used to assess thermoregulatory behavior and positive motivational states in mice. There are known genetic influences on ethanol withdrawal severity as well as individual/thermoregulatory nest building. Withdrawal Seizure-Prone (WSP-1, WSP-2) and Withdrawal Seizure-Resistant (WSR-1, WSR-2) mice were selectively bred for high vs low handling-induced convulsion (HIC) severity, respectively, during withdrawal from chronic ethanol vapor inhalation. They also differ in HIC severity during withdrawal from an acute, 4g/kg ethanol injection. In our initial study, withdrawal from an acute dose of ethanol dose-dependently impaired nest building over the initial 24h of withdrawal in genetically segregating Withdrawal Seizure Control (WSC) mice. In two further studies, acute ethanol withdrawal suppressed nest building for up to two days in WSP-1 females. Deficits in nest building from ethanol were limited to the initial 10h of withdrawal in WSR-1 females and to the initial 24h of withdrawal in WSP-1 and WSR-1 males. Effects of ethanol on nest building for up to two days were found in WSP-2 and WSR-2 mice of both sexes. Nest building deficits in female mice from the first replicate could not be explained by a general decrease in locomotor behavior. These results suggest that nest building is a novel behavioral phenotype for indexing the severity of acute ethanol withdrawal, and that genes contributing to this trait differ from those affecting acute withdrawal HIC severity.

Nest building is a novel method for indexing severity of alcohol withdrawal in mice

Withdrawal after chronic ethanol (EtOH) affects body temperature, goal-directed behavior and motor function in mice and increases general central nervous system excitability. Nest-building tests have been used to assay these states but to this point have not been employed as measures of EtOH withdrawal severity. We first refined nest-scoring methods using a genetically heterogeneous stock of mice (HS/Npt). Mice were then made physically dependent following three days of chronic EtOH vapor inhalation to produce average blood EtOH concentrations (BECs) of 1.89 mg/mL. EtOH withdrawal affected the progression of nest building over time when mice were tested 2-4 days after removal from three days of chronic exposure to EtOH. In a separate group of mice, chronic EtOH vapor inhalation (BECs 1.84 mg/mL) suppressed nest building over days 1-2 but not days 2-3 of withdrawal. In a following experiment, EtOH withdrawal dose-dependently slowed recovery of nest building for up to 32 h. Finally, we determined that long-lasting nest-building deficits extend to mice undergoing withdrawal from a high dose (4 g/kg) of acute EtOH. Sex differences for nest building were absent following EtOH exposure. In mice naïve to EtOH treatments, male mice had lower pre-test body temperatures and increased nest scores across a two-day testing period compared to females. These results suggest that nest building can be used to assess chronic and acute EtOH withdrawal severity in mice.

Treatment-resistant depression: are animal models of depression fit for purpose?

BACKGROUND

Resistance to antidepressant drug treatment remains a major health problem. Animal models of depression are efficient in detecting effective treatments but have done little to increase the reach of antidepressant drugs. This may be because most animal models of depression target the reversal of stress-induced behavioural change, whereas treatment-resistant depression is typically associated with risk factors that predispose to the precipitation of depressive episodes by relatively low levels of stress. Therefore, the search for treatments for resistant depression may require models that incorporate predisposing factors leading to heightened stress responsiveness.

METHOD

Using a diathesis-stress framework, we review developmental, genetic and genomic models against four criteria: (i) increased sensitivity to stress precipitation of a depressive behavioural phenotype, (ii) resistance to chronic treatment with conventional antidepressants, (iii) a good response to novel modes of antidepressant treatment (e.g. ketamine; deep brain stimulation) that are reported to be effective in treatment-resistant depression and (iv) a parallel to a known clinical risk factor.

RESULTS

We identify 18 models that may have some potential. All require further validation. Currently, the most promising are the Wistar-Kyoto (WKY) and congenital learned helplessness (cLH) rat strains, the high anxiety behaviour (HAB) mouse strain and the CB1 receptor knockout and OCT2 null mutant mouse strains.

CONCLUSION

Further development is needed to validate models of antidepressant resistance that are fit for purpose. The criteria used in this review may provide a helpful framework to guide research in this area.

Rodent models of treatment-resistant depression

Major depression is a prevalent and debilitating disorder and a substantial proportion of patients fail to reach remission following standard antidepressant pharmacological treatment. Limited efficacy with currently available antidepressant drugs highlights the need to develop more effective medications for treatment- resistant patients and emphasizes the importance of developing better preclinical models that focus on treatment- resistant populations. This review discusses methods to adapt and refine rodent behavioral models that are predictive of antidepressant efficacy to identify populations that show reduced responsiveness or are resistant to traditional antidepressants. Methods include separating antidepressant responders from non-responders, administering treatments that render animals resistant to traditional pharmacological treatments, and identifying genetic models that show antidepressant resistance. This review also examines pharmacological and non-pharmacological treatments regimes that have been effective in refractory patients and how some of these approaches have been used to validate animal models of treatment-resistant depression. The goals in developing rodent models of treatment-resistant depression are to understand the neurobiological mechanisms involved in antidepressant resistance and to develop valid models to test novel therapies that would be effective in patients that do not respond to traditional monoaminergic antidepressants.

Antidepressant-like effects of cortical deep brain stimulation coincide with pro-neuroplastic adaptations of serotonin systems

BACKGROUND

Cortical deep brain stimulation (DBS) is a promising therapeutic option for treatment-refractory depression, but its mode of action remains enigmatic. Serotonin (5-HT) systems are engaged indirectly by ventromedial prefrontal cortex (vmPFC) DBS. Resulting neuroplastic changes in 5-HT systems could thus coincide with the long-term therapeutic activity of vmPFC DBS.

METHODS

We tested this hypothesis by evaluating the antidepressant-like activity of vmPFC DBS in the chronic social defeat stress (CSDS) model of depression (n = 8-13 mice/group). Circuit-wide activation induced by vmPFC DBS was mapped with c-Fos immunolabeling. The effects of chronic vmPFC DBS on the physiology and morphology of genetically identified 5-HT cells from the dorsal raphe nucleus (DRN) were examined with whole-cell recording, somatodendritic three-dimensional reconstructions and morphometric analyses of presynaptic boutons along 5-HT axons.

RESULTS

Acute DBS drove c-Fos expression locally in the vmPFC and in several distal monosynaptically connected regions, including the DRN. Chronic DBS reversed CSDS-induced social avoidance, restored the disrupted balance of excitatory/inhibitory inputs onto 5-HT neurons, and reversed 5-HT hypoexcitability observed after CSDS. Furthermore, vmPFC DBS reversed CSDS-induced arborization of 5-HT dendrites in the DRN and increased the size and density of 5-HT presynaptic terminals in the dentate gyrus and vmPFC.

CONCLUSIONS

We validate a new preclinical paradigm to examine cellular mechanisms underlying the antidepressant-like activity of vmPFC DBS and identify dramatic circuit-mediated cellular adaptations that coincide with this treatment. These neuroplastic changes of 5-HT neurons might contribute to the progressive mood improvements reported in patients treated with chronic courses of cortical DBS.

Emotional regulatory function of receptor interacting protein 140 revealed in the ventromedial hypothalamus

Receptor-interacting protein (RIP140) is a transcription co-regulator highly expressed in macrophages to regulate inflammatory and metabolic processes. However, its implication in neurological, cognitive and emotional conditions, and the cellular systems relevant to its biological activity within the central nervous system are currently less clear. A transgenic mouse line with macrophage-specific knockdown of RIP140 was generated (MΦRIPKD mice) and brain-region specific RIP140 knockdown efficiency evaluated. Mice were subjected to a battery of tests, designed to evaluate multiple behavioral domains at naïve or following site-specific RIP140 re-expression. Gene expression analysis assessed TNF-α, IL-1β, TGF-1β, IL1-RA and neuropeptide Y (NPY) expression, and in vitro studies examined the effects of macrophage's RIP140 on astrocytes' NPY production. We found that RIP140 expression was dramatically reduced in macrophages within the ventromedial hypothalamus (VMH) and the cingulate cortex of MΦRIPKD mice. These animals exhibited increased anxiety- and depressive-like behaviors. VMH-targeted RIP140 re-expression in MΦRIPKD mice reversed its depressive- but not its anxiety-like phenotype. Analysis of specific neurochemical changes revealed reduced astrocytic-NPY expression within the hypothalamus of MΦRIPKD mice, and in vitro analysis confirmed that conditioned medium of RIP140-silnenced macrophage culture could no longer stimulate NPY production from astrocytes. The current study revealed an emotional regulatory function of macrophage-derived RIP140 in the VMH, and secondary dysregulation of NPY within hypothalamic astrocyte population, which might be associated with the observed behavioral phenotype of MΦRIPKD mice. This study highlights RIP140 as a novel target for the development of potential therapeutic and intervention strategies for emotional regulation disorders.

Innovative drugs to treat depression: did animal models fail to be predictive or did clinical trials fail to detect effects?

Over recent decades, encouraging preclinical evidence using rodent models pointed to innovative pharmacological targets to treat major depressive disorder. However, subsequent clinical trials have failed to show convincing results. Two explanations for these rather disappointing results can be put forward, either animal models of psychiatric disorders have failed to predict the clinical effectiveness of treatments or clinical trials have failed to detect the effects of these new drugs. A careful analysis of the literature reveals that both statements are true. Indeed, in some cases, clinical efficacy has been predicted on the basis of inappropriate animal models, although the contrary is also true, as some clinical trials have not targeted the appropriate dose or clinical population. On the one hand, refinement of animal models requires using species that have better homological validity, designing models that rely on experimental manipulations inducing pathological features, and trying to model subtypes of depression. On the other hand, clinical research should consider carefully the results from preclinical studies, in order to study these compounds at the correct dose, in the appropriate psychiatric nosological entity or symptomatology, in relevant subpopulations of patients characterized by specific biomarkers. To achieve these goals, translational research has to strengthen the dialogue between basic and clinical science.