Antidepressant-like cognitive and behavioral effects of acute ketamine administration associated with plasticity in the ventral hippocampus to medial prefrontal cortex pathway

Ketamine differentially affects implicit and explicit memory processes in rats

RATIONALE

Ketamine, a non-competitive NMDA receptor antagonist, produces antidepressant effects at subanesthetic doses. The therapeutic effect, however, is often accompanied by cognitive side effects, including memory impairments. Yet, the specific effects of ketamine on different processes of implicit and explicit memory remain to be elucidated.

OBJECTIVES

We examined the effect of an antidepressant dose of ketamine (10 mg/kg, IP) on the encoding, retrieval, and modulation processes of fear memory and spatial memory in adult Wistar rats.

METHODS

Ketamine was administered before the fear acquisition, retrieval, or extinction procedures in a Pavlovian fear conditioning task. In another set of experiments, it was administered before the training, probe trial, or reversal training phases of the Morris Water Maze (MWM).

RESULTS

The antidepressant dose of ketamine partially impaired fear extinction when administered before the acquisition or retrieval. In contrast, it facilitated memory modulation and decreased the escape latency in the first day of reversal training in the MWM when administered before the training or reversal training sessions. Encoding or retrieval performance in either type of memory was not affected.

CONCLUSIONS

These findings show that ketamine does not impair the acquisition or retrieval processes of cued fear or spatial memory; but exerts differential effects on memory modulation of these implicit and explicit memory paradigms, by disrupting fear extinction and facilitating reversal spatial learning.

Maternal separation after postnatal day 10 induces increase in depression-like behavior with decrease in hippocampal dendritic spines, but no change in anxiety-like behavior in male rats

Neurodevelopment has a "sensitive period" during which the brain is highly sensitive to experience. In this study, we used maternal separation (MS) models of male Long-Evans rats to examine whether sensitivity to stress changes after postnatal day (PND) 10, when dendritic spine density begins to increase rapidly in the CA (Cornu Ammonis)1 region of the hippocampus. We assigned littermates to three groups: early maternal separation group (EMS: MS during PND 1-9), late maternal separation group (LMS: MS during PND 10-20), and control group. During adulthood (PND 56-75, which strictly corresponds to young adulthood), LMS showed increased depression-like behaviors and decreased dendritic spine density in the CA1 hippocampal region; however, EMS did not show any such changes. Accordingly, littermates at PND 10-20 have a greater vulnerability to MS than those at PND 1-9. These findings suggest that dendritic spine formation in the hippocampus is an important factor in determining sensitivity to MS.

Neural circuits mediating chronic stress: Implications for major depressive disorder

Major depressive disorder (MDD), also known as depression, is a prevalent mental disorder that leads to severe disease burden worldwide. Over the past two decades, significant progress has been made in understanding the pathogenesis and developing novel treatments for MDD. Among the complicated etiologies of MDD, chronic stress is a major risk factor. Exploring the underlying brain circuit mechanisms of chronic stress regulation has been an area of active research for recent years. A growing body of preclinical and clinical research has revealed that abnormalities in the brain circuits are closely associated with failures in coping with stress in depressed individuals. Nevertheless, neural circuit mechanisms underlying chronic stress processing and the onset of depression remain a major puzzle. Here, we review recent literature focusing on circuit- and cell-type-specific dissection of depression-like behaviors in chronic stress-related animal models of MDD and outline the key questions.

A projection from the medial prefrontal cortex to the lateral septum modulates coping behavior on the shock-probe test

Effective coping plays an important role in preventing stress-induced neuropsychiatric conditions. The ventromedial prefrontal cortex (vmPFC) has been associated with active, adaptive coping in humans and rodents. Chronic or severe stress has been shown to induce a maladaptive shift from active to passive coping behavior; however, the neural circuits for effective coping strategies remain unclear. In the current study, we demonstrated that neurons in the infralimbic (IL) subregion of rat vmPFC that project to the lateral septum (LS) were recruited by exposure to the shock probe in the shock-probe defensive burying (SPDB) test. Both chemogenetic inhibition of LS-projecting neurons in the IL and optogenetic inhibition of glutamatergic IL terminals in the LS selectively suppressed active burying responses in the SPDB test in non-stressed rats. In contrast, chemogenetic activation of the IL-LS pathway effectively reversed the shift from active coping to passive immobility in the SPDB test induced by chronic unpredictable stress (CUS). These results indicate that top-down regulation of the LS by a projection from the IL cortex is necessary for an active, adaptive behavioral coping response, and is sufficient to restore active coping that has been compromised by chronic stress. More broadly, these results point to the IL-to-LS circuit as a potential substrate underlying maladaptive shifts from active to passive coping behavior that are often associated with stress-related neuropsychiatric disorders.

Effects of chronic stress on cognitive function - From neurobiology to intervention

Exposure to chronic stress contributes considerably to the development of cognitive impairments in psychiatric disorders such as depression, generalized anxiety disorder (GAD), obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and addictive behavior. Unfortunately, unlike mood-related symptoms, cognitive impairments are not effectively treated by available therapies, a situation in part resulting from a still incomplete knowledge of the neurobiological substrates that underly cognitive domains and the difficulty in generating interventions that are both efficacious and safe. In this review, we will present an overview of the cognitive domains affected by stress with a specific focus on cognitive flexibility, behavioral inhibition, and working memory. We will then consider the effects of stress on neuronal correlates of cognitive function and the factors which may modulate the interaction of stress and cognition. Finally, we will discuss intervention strategies for treatment of stress-related disorders and gaps in knowledge with emerging new treatments under development. Understanding how cognitive impairment occurs during exposure to chronic stress is crucial to make progress towards the development of new and effective therapeutic approaches.

GluN2B on Adult-Born Granule Cells Modulates (R,S)-Ketamine's Rapid-Acting Effects in Mice

BACKGROUND

Standard antidepressant treatments often take weeks to reach efficacy and are ineffective for many patients. (R,S)-ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, has been shown to be a rapid-acting antidepressant and to decrease depressive symptoms within hours of administration. While previous studies have shown the importance of the GluN2B subunit of the NMDA receptor on interneurons in the medial prefrontal cortex, no study to our knowledge has investigated the influence of GluN2B-expressing adult-born granule cells.

METHODS

Here, we examined whether (R,S)-ketamine's efficacy depends on adult-born hippocampal neurons using a genetic strategy to selectively ablate the GluN2B subunit of the NMDA receptor from Nestin+ cells in male and female mice, tested across an array of standard behavioral assays.

RESULTS

We report that in male mice, GluN2B expression on 6-week-old adult-born neurons is necessary for (R,S)-ketamine's effects on behavioral despair in the forced swim test and on hyponeophagia in the novelty suppressed feeding paradigm, as well on fear behavior following contextual fear conditioning. In female mice, GluN2B expression is necessary for effects on hyponeophagia in novelty suppressed feeding. These effects were not replicated when ablating GluN2B from 2-week-old adult-born neurons. We also find that ablating neurogenesis increases fear expression in contextual fear conditioning, which is buffered by (R,S)-ketamine administration.

CONCLUSIONS

In line with previous studies, these results suggest that 6-week-old adult-born hippocampal neurons expressing GluN2B partially modulate (R,S)-ketamine's rapid-acting effects. Future work targeting these 6-week-old adult-born neurons may prove beneficial for increasing the efficacy of (R,S)-ketamine.

Neuron-Microglia Interaction is Involved in Anti-inflammatory Response by Vagus Nerve Stimulation in the Prefrontal Cortex of Rats Injected with Polyinosinic:Polycytidylic Acid

Injection of polyinosinic:polycytidylic acid (poly(I:C)) into experimental animals induces neuroimmunological responses and thus has been used for the study of neurological disorders such as anxiety, depression, and chronic fatigue. Here, we investigated the effects of vagus nerve stimulation (VNS) on poly(I:C)-induced neuroinflammation and associated behavioral consequences in rats. The microglia in the prefrontal cortex (PFC) displayed the activated form of morphology in poly(I:C)-injected rats and changed to a normal shape after acute VNS (aVNS). Production of phospho-NF-κB, phospho-IκB, IL-1β, and cleaved caspase 3 was elevated by poly(I:C) and downregulated by aVNS. In contrast, phospho-Akt levels were decreased by poly(I:C) and increased by aVNS. Neuronal production of fractalkine (CX3CL1) in the PFC was markedly reduced by poly(I:C), but recovered by aVNS. Fractalkine interaction with its receptor CX3CR1 was highly elevated by VNS. We further demonstrated that the pharmacological blockade of CX3CR1 activity counteracted the production of IL-1β, phospho-Akt, and cleaved form of caspase 3 that was modulated by VNS, suggesting the anti-inflammatory effects of fractalkine-CX3CR1 signaling as a mediator of neuron-microglia interaction. Behavioral assessments of pain and temperature sensations by von Frey and hot/cold plate tests showed significant improvement by chronic VNS (cVNS) and forced swimming and marble burying tests revealed that the depressive-like behaviors caused by poly(I:C) injection were rescued by cVNS. We also found that the recognition memory which was impaired by poly(I:C) administration was improved by cVNS. This study suggests that VNS may play a role in regulating neuroinflammation and somatosensory and cognitive functions in poly(I:C)-injected animals.

Divergent Effects of Ketamine and the Serotoninergic Psychedelic 2,5-Dimethoxy-4-Iodoamphetamine on Hippocampal Plasticity and Metaplasticity

Introduction

Serotonergic psychedelics and ketamine produce rapid and long-lasting symptomatic relief in multiple psychiatric disorders. Evidence suggests that despite having distinct molecular targets, both drugs may exert therapeutic benefit via their pro-neuroplastic effects. Following treatment with ketamine or serotonergic psychedelics, patients are reported to be more open to behavioral change, which is leveraged for psychotherapy-assisted reframing of narratives of the self. This period of enhanced behavioral change is postulated to be supported by a post-treatment window of enhanced neural plasticity, but evidence for such 'metaplastic' effects is limited. In this study, we tested for neural plasticity and metaplasticity in murine hippocampus.

Methods

Brain slices were obtained from C57BL/6J mice 24 hours after treatment (intraperitoneal injection) with saline, ketamine, or the serotonergic psychedelic 2,5-Dimethoxy-4-iodoamphetamine (DOI). Extracellular fiber volleys (FVs) and field excitatory postsynaptic potentials (fEPSPs) were recorded in stratum radiatum of CA1 in response to stimulation of Schaffer collateral fibers before and after induction of short-term and long-term potentiation (STP, LTP).

Results

Before LTP induction, responses differed across treatment groups (F1,2 = 5.407, p = 0.00665), with fEPSPs enhanced in slices from DOI-treated animals (p = 0.0182), but not ketamine-treated animals (p = 0.9786), compared to saline. There were no treatment effects on LT (F1,2 = 0.6, p = 0.516), but there were on STP (F1,2 =, p = 0.0167), with enhanced STP in DOI-treated (p = 0.0352) but not ketamine-treated (p = 0.9999) animals compared to saline. A presynaptic component to the mechanism for the DOI effects was suggested by (1) significantly enhanced FV amplitudes (F1,2 = 3.17, p = 0.049) in DOI-treated (p = 0.0457) but not ketamine-treated animals compared to saline (p = 0.8677); and (2) enhanced paired pulse ratios (F1,2 = 3.581, p = 0.0339) in slices from DOI-treated (p= 0.0257) but not ketamine-treated animals (p = 0.4845) compared to saline.

Conclusions

DOI, but not ketamine, induced significant neuroplastic and metaplastic effects at hippocampal CA1 synapses 24 hours after treatment, likely in part via a presynaptic mechanism.

A novel framework for ketamine-assisted couple therapy

Intimate relationship distress is prevalent and is associated with poorer health, mental health, and mortality outcomes. Evidence-based couple therapies target cognitive, behavioral, and emotional processes that underlie relationship dysfunction. Increasing research and clinical evidence supports the efficacy of ketamine-assisted psychotherapy (KAP) for addressing clinical mental health concerns, including depression, anxiety disorders, posttraumatic stress disorder, and more. The purported mechanisms of KAP are also likely to improve psychosocial and relational functioning for patients and may be useful for supporting change mechanisms in couple therapy. This paper reviews the current evidence for therapeutic ketamine and KAP and outlines how the mechanisms of ketamine therapy may also augment the cognitive, behavioral, and emotional interventions in the most commonly used evidence-based couple therapies. Key mechanisms include increased neuroplasticity, changes in functional connectivity, adaptive dissociation, decreased inhibition, and reduced avoidance. Given the reciprocal interaction between relationship dysfunction and mental health problems, ketamine may also help alleviate relationship distress by directly treating clinical mental health symptoms. We then outline a proposed framework for ketamine-assisted couple therapy, addressing the application of KAP preparation, dosing, and integration to a dyadic intervention framework in a way that can be applied to different couple therapy modalities. This clinical framework for couples' KAP may be useful for clinicians and researchers working to improve the efficacy of couple therapy, particularly when one or both partners has accompanying mental health concerns.

Immediate and long-term electrophysiological biomarkers of antidepressant-like behavioral effects after subanesthetic ketamine and medial prefrontal cortex deep brain stimulation treatment

Introduction

Both ketamine (KET) and medial prefrontal cortex (mPFC) deep brain stimulation (DBS) are emerging therapies for treatment-resistant depression, yet our understanding of their electrophysiological mechanisms and biomarkers is incomplete. This study investigates aperiodic and periodic spectral parameters, and the signal complexity measure sample entropy, within mPFC local field potentials (LFP) in a chronic corticosterone (CORT) depression model after ketamine and/or mPFC DBS.

Methods

Male rats were intraperitoneally administered CORT or vehicle for 21 days. Over the last 7 days, animals receiving CORT were treated with mPFC DBS, KET, both, or neither; then tested across an array of behavioral tasks for 9 days.

Results

We found that the depression-like behavioral and weight effects of CORT correlated with a decrease in aperiodic-adjusted theta power (5-10 Hz) and an increase in sample entropy during the administration phase, and an increase in theta peak frequency and a decrease in the aperiodic exponent once the depression-like phenotype had been induced. The remission-like behavioral effects of ketamine alone correlated with a post-treatment increase in the offset and exponent, and decrease in sample entropy, both immediately and up to eight days post-treatment. The remission-like behavioral effects of mPFC DBS alone correlated with an immediate decrease in sample entropy, an immediate and sustained increase in low gamma (20-50 Hz) peak width and aperiodic offset, and sustained improvements in cognitive function. Failure to fully induce remission-like behavior in the combinatorial treatment group correlated with a failure to suppress an increase in sample entropy immediately after treatment.

Conclusion

Our findings therefore support the potential of periodic theta parameters as biomarkers of depression-severity; and periodic low gamma parameters and cognitive measures as biomarkers of mPFC DBS treatment efficacy. They also support sample entropy and the aperiodic spectral parameters as potential cross-modal biomarkers of depression severity and the therapeutic efficacy of mPFC DBS and/or ketamine. Study of these biomarkers is important as objective measures of disease severity and predictive measures of therapeutic efficacy can be used to personalize care and promote the translatability of research across studies, modalities, and species.

Ketamine's Amelioration of Fear Extinction in Adolescent Male Mice Is Associated with the Activation of the Hippocampal Akt-mTOR-GluA1 Pathway

Fear-related disorders, including post-traumatic stress disorder (PTSD), and anxiety disorders are pervasive psychiatric conditions marked by persistent fear, stemming from its dysregulated acquisition and extinction. The primary treatment for these disorders, exposure therapy (ET), relies heavily on fear extinction (FE) principles. Adolescence, a vulnerable period for developing psychiatric disorders, is characterized by neurobiological changes in the fear circuitry, leading to impaired FE and increased susceptibility to relapse following ET. Ketamine, known for relieving anxiety and reducing PTSD symptoms, influences fear-related learning processes and synaptic plasticity across the fear circuitry. Our study aimed to investigate the effects of ketamine (10 mg/kg) on FE in adolescent male C57 BL/6 mice at the behavioral and molecular levels. We analyzed the protein and gene expression of synaptic plasticity markers in the hippocampus (HPC) and prefrontal cortex (PFC) and sought to identify neural correlates associated with ketamine's effects on adolescent extinction learning. Ketamine ameliorated FE in the adolescent males, likely affecting the consolidation and/or recall of extinction memory. Ketamine also increased the Akt and mTOR activity and the GluA1 and GluN2A levels in the HPC and upregulated BDNF exon IV mRNA expression in the HPC and PFC of the fear-extinguished mice. Furthermore, ketamine increased the c-Fos expression in specific brain regions, including the ventral HPC (vHPC) and the left infralimbic ventromedial PFC (IL vmPFC). Providing a comprehensive exploration of ketamine's mechanisms in adolescent FE, our study suggests that ketamine's effects on FE in adolescent males are associated with the activation of hippocampal Akt-mTOR-GluA1 signaling, with the vHPC and the left IL vmPFC as the proposed neural correlates.

Effects of vortioxetine on hippocampal-related cognitive impairment induced in rats by androgen deprivation as a model of prostate cancer treatment

Advances in prostate cancer treatment have significantly improved survival, but quality of life for survivors remains an under-studied area of research. Androgen deprivation therapy (ADT) is a foundational treatment for advanced prostate cancer and is used as an adjuvant for prolonged periods in many high-risk, localized tumors. More than half of patients treated with ADT experience debilitating cognitive impairments in domains such as spatial learning and working memory. In this study, we investigated the effects of androgen deprivation on hippocampal-mediated cognition in rats. Vortioxetine, a multimodal antidepressant, has been shown to improve cognition in depressed patients. Thus, we also tested the potential efficacy of vortioxetine in restoring impaired cognition after ADT. We further investigated mechanisms that might contribute to these effects, measuring changes in the circuitry and gene expression within the dorsal hippocampus. ADT via surgical castration induced impairments in visuospatial cognition on the novel object location test and attenuated afferent-evoked local field potentials recorded in the CA1 region of the dorsal hippocampus. Chronic dietary administration of vortioxetine effectively reversed these deficits. Castration significantly altered gene expression in the hippocampus, whereas vortioxetine had little effect. Pathway analysis revealed that androgen depletion altered pathways related to synaptic plasticity. These results suggest that the hippocampus may be vulnerable to ADT, contributing to cognitive impairment in prostate cancer patients. Further, vortioxetine may be a candidate to improve cognition in patients who experience cognitive decline after androgen deprivation therapy for prostate cancer and may do so by restoring molecular and circuit-level plasticity-related mechanisms compromised by ADT.

Ventral Hippocampal Input to Infralimbic Cortex Is Necessary for the Therapeutic-Like Effects of Extinction in Stressed Rats

BACKGROUND

Posttraumatic stress disorder is characterized by deficits in cognitive flexibility related to dysfunction of the medial prefrontal cortex (mPFC). Exposure therapy can effectively reverse these deficits. Fear extinction in rodents bears similarity to exposure therapy. Extinction reverses chronic stress-induced deficits in cognitive flexibility on the attentional set-shifting test (AST), an mPFC-mediated process. This therapeutic effect requires activity of pyramidal neurons and brain derived neurotrophic factor (BDNF) signaling in infralimbic cortex (IL). However, the circuit mechanisms governing BDNF-mediated plasticity initiated by extinction in IL are unknown. The ventral hippocampus (vHipp) plays a role in regulating IL activity during extinction, and plasticity in vHipp is necessary for extinction memory consolidation. Therefore, we investigated the role of vHipp input to IL in the effects of extinction in reversing stress-induced cognitive deficits.

METHODS

vHipp input to IL was silenced using a Gi-Designer Receptors Exclusively Activated by Designer Drugs (DREADD) via local infusion of clozapine-N-oxide (CNO) into IL before extinction. A day later, rats were tested on AST. In a separate experiment, we tested whether vHipp input to the IL induces BDNF signaling to exert therapeutic effects. We activated the vHipp using a Gq-DREADD, and injected an anti-BDNF neutralizing antibody into IL. Rats were tested on the AST 24 hours later.

RESULTS

Silencing the vHipp input to IL prevented the beneficial effects of extinction in reversing stress-induced cognitive deficits. Activating vHipp input to IL in the absence of extinction was sufficient to reverse stress-induced deficits in set-shifting. The beneficial effects were blocked by local infusion of a neutralizing anti-BDNF antibody into IL.

CONCLUSIONS

vHipp-driven BDNF signaling in IL is critical for extinction to counteract the deleterious cognitive effects of chronic stress.

Epigenetic embedding of childhood adversity: mitochondrial metabolism and neurobiology of stress-related CNS diseases

This invited article ad memoriam of Bruce McEwen discusses emerging epigenetic mechanisms underlying the long and winding road from adverse childhood experiences to adult physiology and brain functions. The conceptual framework that we pursue suggest multidimensional biological pathways for the rapid regulation of neuroplasticity that utilize rapid non-genomic mechanisms of epigenetic programming of gene expression and modulation of metabolic function via mitochondrial metabolism. The current article also highlights how applying computational tools can foster the translation of basic neuroscience discoveries for the development of novel treatment models for mental illnesses, such as depression to slow the clinical manifestation of Alzheimer's disease. Citing an expression that many of us heard from Bruce, while "It is not possible to roll back the clock," deeper understanding of the biological pathways and mechanisms through which stress produces a lifelong vulnerability to altered mitochondrial metabolism can provide a path for compensatory neuroplasticity. The newest findings emerging from this mechanistic framework are among the latest topics we had the good fortune to discuss with Bruce the day before his sudden illness when walking to a restaurant in a surprisingly warm evening that preluded the snowstorm on December 18th, 2019. With this article, we wish to celebrate Bruce's untouched love for Neuroscience.

Effect of chronic unpredictable stress in female Wistar-Kyoto rats subjected to progesterone withdrawal: Relevance for Premenstrual Dysphoric Disorder neurobiology

Premenstrual Dysphoric Disorder (PMDD) is related to an abrupt drop in progesterone and impairments in the HPA axis that cause anxiety. Suffering persons report higher daily-life stress and anxiety proneness that may contribute to developing PMDD, considered a chronic stress-related disorder. Here, we explored the effect of chronic unpredictable stress (CUS) in rats subjected to progesterone withdrawal (PW) and evaluated gene expression of HPA axis activation in the stress-vulnerable Wistar-Kyoto (WKY) rat strain that is prone to anxiety. Ovariectomized WKY rats were randomly assigned to CUS or Standard-housed conditions (SHC) for 30 days. To induce PW, animals received 2 mg/kg of progesterone on day 25th for 5 days; 24 h later, they were tested using the anxiety-like burying behavior test (BBT). After behavioral completion, rats were euthanized, and brains were extracted to measure Crh (PVN) and Nr3c1 (hippocampus) mRNA. Blood corticosterone and vasopressin levels were determined. Results showed that PW exacerbated anxiety-like behaviors through passive coping in CUS-WKY. PW decreased Crh-PVN mRNA and the Nr3c1-hippocampal mRNA expression in SHC. CUS decreased Crh-PVN mRNA compared to SHC, and no further changes were observed by PW or BBT exposure. CUS reduced Nr3c1-hippocampal gene expression compared to SHC animals, and lower Nr3c1 mRNA was detected due to BBT. The PW increased corticosterone in SHC and CUS rats; however, CUS blunted corticosterone when combined with PW+BBT and similarly occurred in vasopressin concentrations. Chronic stress blunts the response of components of the HPA axis regulation when PW and BBT (systemic and psychogenic stressors, respectively) are presented. This response may facilitate less adaptive behaviors through passive coping in stress-vulnerable subjects in a preclinical model of premenstrual anxiety.

Novel ketamine and zinc treatment for anorexia nervosa and the potential beneficial interactions with the gut microbiome

Anorexia nervosa (AN) is a severe illness with diverse aetiological and maintaining contributors including neurobiological, metabolic, psychological, and social determining factors. In addition to nutritional recovery, multiple psychological and pharmacological therapies and brain-based stimulations have been explored; however, existing treatments have limited efficacy. This paper outlines a neurobiological model of glutamatergic and γ-aminobutyric acid (GABA)-ergic dysfunction, exacerbated by chronic gut microbiome dysbiosis and zinc depletion at a brain and gut level. The gut microbiome is established early in development, and early exposure to stress and adversity contribute to gut microbial disturbance in AN, early dysregulation to glutamatergic and GABAergic networks, interoceptive impairment, and inhibited caloric harvest from food (e.g., zinc malabsorption, competition for zinc ions between gut bacteria and host). Zinc is a key part of glutamatergic and GABAergic networks, and also affects leptin and gut microbial function; systems dysregulated in AN. Low doses of ketamine in conjunction with zinc, could provide an efficacious combination to act on NMDA receptors and normalise glutamatergic, GABAergic and gut function in AN.

The Effects of Acute and Repeated Administration of Ketamine on Memory, Behavior, and Plasma Corticosterone Levels in Female Mice

Ketamine is an anesthetic drug that has recently been approved for the treatment of treatment-resistant depression. Females are diagnosed with Major Depressive Disorder at higher rates than males, yet most of the pre-clinical research on ketamine has been conducted in male subjects. Additionally, the literature on the acute and long-term behavioral and cognitive effects of ketamine shows conflicting results. It is important to examine the acute and long-term cognitive and behavioral effects of ketamine exposure at lower sub-anesthetic doses, as the recreational use of the drug at higher doses is associated with cognitive and memory impairments. The current study examined the effects of acute and repeated ketamine exposure on anxiety-like behavior, novel object recognition memory, depression-like behavior, and plasma corticosterone levels in 20 adult female C57BL/6J mice. Mice were exposed acutely or repeatedly for 10 consecutive days to saline or 15 mg/kg ketamine and behavior was measured in the open field test, novel object recognition test, and the Porsolt forced swim test. Plasma corticosterone levels were measured following behavioral testing. Acute ketamine exposure decreased locomotor activity and increased anxiety-like behavior in the open field test compared to controls, while repeated ketamine exposure impaired memory in the novel object recognition test. There were no effects of acute or repeated ketamine exposure on depression-like behavior in the Porsolt forced swim test or on plasma corticosterone levels. These findings suggest that a subanesthetic dose of ketamine alters behavior and cognition in female mice and the effects are dependent on the duration of exposure.

Activity in a prefrontal-periaqueductal gray circuit overcomes behavioral and endocrine features of the passive coping stress response

The question of how the brain links behavioral and biological features of defensive responses has remained elusive. The importance of this problem is underscored by the observation that behavioral passivity in stress coping is associated with elevations in glucocorticoid hormones, and each may carry risks for susceptibility to a host of stress-related diseases. Past work implicates the medial prefrontal cortex (mPFC) in the top-down regulation of stress-related behaviors; however, it is unknown whether such changes have the capacity to buffer against the longer-lasting biological consequences associated with aversive experiences. Using the shock probe defensive burying test in rats to naturalistically measure behavioral and endocrine features of coping, we observed that the active behavioral component of stress coping is associated with increases in activity along a circuit involving the caudal mPFC and midbrain dorsolateral periaqueductal gray (PAG). Optogenetic manipulations of the caudal mPFC-to-dorsolateral PAG pathway bidirectionally modulated active (escape and defensive burying) behaviors, distinct from a rostral mPFC-ventrolateral PAG circuit that instead limited passive (immobility) behavior. Strikingly, under conditions that biased rats toward a passive coping response set, including exaggerated stress hormonal output and increased immobility, excitation of the caudal mPFC-dorsolateral PAG projection significantly attenuated each of these features. These results lend insight into how the brain coordinates response features to overcome passive coping and may be of importance for understanding how activated neural systems promote stress resilience.

Role of Orbitofrontal Cortex and Differential Effects of Acute and Chronic Stress on Motor Impulsivity Measured With 1-Choice Serial Reaction Time Test in Male Rats

BACKGROUND

Deficits in motor impulsivity, that is, the inability to inhibit a prepotent response, are frequently observed in psychiatric conditions. Several studies suggest that stress often correlates with higher impulsivity. Among the brain areas affected by stress, the orbitofrontal cortex (OFC) is notable because of its role in impulse control. OFC subregions with unique afferent and efferent circuitry play distinct roles in impulse control, yet it is not clear what OFC subregions are engaged during motor impulsivity tasks.

METHODS

In this study we used a rodent test of motor impulsivity, the 1-choice serial reaction time test, to explore activation of OFC subregions either during a well-learned motor impulsivity task or in a challenge task with a longer wait time that increases premature responding. We also examined the effects of acute inescapable stress, chronic intermittent cold stress and chronic unpredictable stress on motor impulsivity.

RESULTS

Fos expression increased in the lateral OFC and agranular insular cortex during performance in both the mastered and challenge conditions. In the ventral OFC, Fos expression increased only during challenge, and within the medial OFC, Fos was not induced in either condition. Inescapable stress produced a transient effect on premature responses in the mastered task, whereas chronic intermittent cold stress and chronic unpredictable stress altered premature responses in both conditions in ways specific to each stressor.

CONCLUSIONS

These results suggest that different OFC subregions have different roles in motor impulse control, and the effects of stress vary depending on the nature and duration of the stressor.

Adjunct treatment with ketamine enhances the therapeutic effects of extinction learning after chronic unpredictable stress

Post-traumatic stress disorder (PTSD) is a debilitating illness characterized by dysfunction in the medial prefrontal cortex (mPFC). Although both pharmacological and cognitive behavioral interventions have shown some promise at alleviating symptoms, high attrition and persistence of treatment-resistant symptoms pose significant challenges that remain unresolved. Specifically, prolonged exposure therapy, a gold standard intervention to treat PTSD, has high dropout rates resulting in many patients receiving less than a fully effective course of treatment. Administering pharmacological treatments together with behavioral psychotherapies like prolonged exposure may offer an important avenue for enhancing therapeutic efficacy sooner, thus reducing the duration of treatment and mitigating the impact of attrition. In this study, using extinction learning as a rat model of exposure therapy, we hypothesized that administering ketamine as an adjunct treatment together with extinction will enhance the efficacy of extinction in reversing stress-induced deficits in set shifting, a measure of cognitive flexibility. Results showed that combining a sub-effective dose of ketamine with a shortened, sub-effective extinction protocol fully reversed stress-induced cognitive set-shifting deficits in both male and female rats. These effects may be due to shared molecular mechanisms between extinction and ketamine, such as increased neuronal plasticity in common circuitry (e.g., hippocampus-mPFC), or increased BDNF signaling. This work suggests that fast-acting drugs, such as ketamine, can be effectively used in combination with behavioral interventions to reduce treatment duration and potentially mitigate the impact of attrition. Future work is needed to delineate other pharmacotherapies that may complement the effects of extinction via shared or independent mechanisms.

Translational control by ketamine and its implications for comorbid cognitive deficits in depressive disorders

Ketamine has shown antidepressant effects in patients with major depressive disorder (MDD) resistant to first-line treatments and approved for use in this patient population. Ketamine induces several forms of synaptic plasticity, which are proposed to underlie its antidepressant effects. However, the molecular mechanism of action directly responsible for ketamine's antidepressant effects remains under active investigation. It was recently demonstrated that the effectors of the mammalian target of rapamycin complex 1 (mTORC1) signalling pathway, namely, eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2 (4E-BP1 and 4E-BP2), are central in mediating ketamine-induced synaptic plasticity and behavioural antidepressant-like effect. 4E-BPs are a family of messenger ribonucleic acid (mRNA) translation repressors inactivated by mTORC1. We observed that their expression in inhibitory interneurons mediates ketamine's effects in the forced swim and novelty suppressed feeding tests and the long-lasting inhibition of GABAergic neurotransmission in the hippocampus. In addition, another effector pathway that regulates translation elongation downstream of mTORC1, the eukaryotic elongation factor 2 kinase (eEF2K), has been implicated in ketamine's behavioural effects. We will discuss how ketamine's rapid antidepressant effect depends on the activation of neuronal mRNA translation through 4E-BP1/2 and eEF2K. Furthermore, given that these pathways also regulate cognitive functions, we will discuss the evidence of ketamine's effect on cognitive function in MDD. Overall, the data accrued from pre-clinical research have implicated the mRNA translation pathways in treating mood symptoms of MDD. However, it is yet unclear whether the pro-cognitive potential of subanesthetic ketamine in rodents also engages these pathways and whether such an effect is consistently observed in the treatment-resistant MDD population.

Psilocybin therapy increases cognitive and neural flexibility in patients with major depressive disorder

Psilocybin has shown promise for the treatment of mood disorders, which are often accompanied by cognitive dysfunction including cognitive rigidity. Recent studies have proposed neuropsychoplastogenic effects as mechanisms underlying the enduring therapeutic effects of psilocybin. In an open-label study of 24 patients with major depressive disorder, we tested the enduring effects of psilocybin therapy on cognitive flexibility (perseverative errors on a set-shifting task), neural flexibility (dynamics of functional connectivity or dFC via functional magnetic resonance imaging), and neurometabolite concentrations (via magnetic resonance spectroscopy) in brain regions supporting cognitive flexibility and implicated in acute psilocybin effects (e.g., the anterior cingulate cortex, or ACC). Psilocybin therapy increased cognitive flexibility for at least 4 weeks post-treatment, though these improvements were not correlated with the previously reported antidepressant effects. One week after psilocybin therapy, glutamate and N-acetylaspartate concentrations were decreased in the ACC, and dFC was increased between the ACC and the posterior cingulate cortex (PCC). Surprisingly, greater increases in dFC between the ACC and PCC were associated with less improvement in cognitive flexibility after psilocybin therapy. Connectome-based predictive modeling demonstrated that baseline dFC emanating from the ACC predicted improvements in cognitive flexibility. In these models, greater baseline dFC was associated with better baseline cognitive flexibility but less improvement in cognitive flexibility. These findings suggest a nuanced relationship between cognitive and neural flexibility. Whereas some enduring increases in neural dynamics may allow for shifting out of a maladaptively rigid state, larger persisting increases in neural dynamics may be of less benefit to psilocybin therapy.

Cognitive Behavioral Therapy to Sustain the Antidepressant Effects of Ketamine in Treatment-Resistant Depression: A Randomized Clinical Trial

INTRODUCTION

Ketamine has emerged as a rapid-acting antidepressant. While ongoing treatment can prevent relapse, concerns exist regarding long-term exposure.

OBJECTIVE

We conducted a randomized trial to examine the feasibility and efficacy of cognitive behavioral therapy (CBT) following intravenous ketamine in treatment-resistant depression (TRD).

METHODS

Subjects with TRD were recruited and treated with 6 intravenous infusions of ketamine over 3 weeks. Subjects who experienced a clinical response (≥50% improvement in depression severity) were then randomized to receiving CBT or treatment as usual (TAU) for an additional 14 weeks, using a sequential treatment model.

RESULTS

Of the 42 patients who signed consent, 28 patients achieved a response and were randomized to CBT or TAU. When measured using the Montgomery-Asberg Depression Rating Scale (primary outcome measure), the effect size at the end of the study was moderate (Cohen d = 0.65; 95% CI -0.55 to 1.82), though the group-by-time interaction effect was not significant. There was a significant group-by-time interaction as measured by the Quick Inventory of Depressive Symptomatology (F = 4.58; p = 0.033), favoring a greater sustained improvement in the CBT group. This corresponded to a moderate-to-large effect size of the Cohen d = 0.71 (95% CI -0.30 to 1.70) at the end of the study (14 weeks following the last ketamine infusion). In a subset of patients (N = 20) who underwent cognitive testing using the emotional N-back assessments before and after ketamine, ketamine responders showed improvement in the accuracy of emotional N-back (t[8] = 2.33; p < 0.05) whereas nonresponders did not (t[10] <1; p ns).

CONCLUSIONS

This proof-of-concept study provides preliminary data indicating that CBT may sustain the antidepressant effects of ketamine in TRD. Further study and optimization of this treatment approach in well-powered clinical trials is recommended.

Fluoroethylnormemantine, a Novel NMDA Receptor Antagonist, for the Prevention and Treatment of Stress-Induced Maladaptive Behavior

BACKGROUND

Major depressive disorder is a common, recurrent illness. Recent studies have implicated the NMDA receptor in the pathophysiology of major depressive disorder. (R,S)-ketamine, an NMDA receptor antagonist, is an effective antidepressant but has numerous side effects. Here, we characterized a novel NMDA receptor antagonist, fluoroethylnormemantine (FENM), to determine its effectiveness as a prophylactic and/or antidepressant against stress-induced maladaptive behavior.

METHODS

Saline, memantine (10 mg/kg), (R,S)-ketamine (30 mg/kg), or FENM (10, 20, or 30 mg/kg) was administered before or after contextual fear conditioning in 129S6/SvEv mice. Drug efficacy was assayed using various behavioral tests. Protein expression in the hippocampus was quantified with immunohistochemistry or Western blotting. In vitro radioligand binding was used to assay drug binding affinity. Patch clamp electrophysiology was used to determine the effect of drug administration on glutamatergic activity in ventral hippocampal cornu ammonis 3 (vCA3) 1 week after injection.

RESULTS

Given after stress, FENM decreased behavioral despair and reduced perseverative behavior. When administered after re-exposure, FENM facilitated extinction learning. As a prophylactic, FENM attenuated learned fear and decreased stress-induced behavioral despair. FENM was behaviorally effective in both male and female mice. (R,S)-ketamine, but not FENM, increased expression of c-fos in vCA3. Both (R,S)-ketamine and FENM attenuated large-amplitude AMPA receptor-mediated bursts in vCA3, indicating a common neurobiological mechanism for further study.

CONCLUSIONS

Our results indicate that FENM is a novel drug that is efficacious when administered at various times before or after stress. Future work will further characterize FENM's mechanism of action with the goal of clinical development.

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.

A single administration of ascorbic acid rapidly reverses depressive-like behavior and hippocampal synaptic dysfunction induced by corticosterone in mice

Ketamine is the prototype for glutamate-based fast-acting antidepressants. The establishment of ketamine-like drugs is still a challenge and ascorbic acid has emerged as a candidate. This study investigated the ascorbic acid's ability to induce a fast antidepressant-like response and to improve hippocampal synaptic markers in mice subjected to chronic corticosterone (CORT) administration. CORT was administered for 21 days, followed by a single administration of ascorbic acid (1 mg ∕Kg, p.o.), ketamine (1 mg ∕Kg, i.p.) or fluoxetine (10 mg ∕Kg, p.o.) in mice. Depressive-like behavior, hippocampal synaptic proteins immunocontent, dendrite spines density in the dentate gyrus (DG) were analyzed 24 h following treatments. The administration of ascorbic acid or ketamine, but not fluoxetine, counteracted CORT-induced depressive-like behavior in the tail suspension test (TST). CORT administration reduced PSD-95, GluA1, and synapsin (synaptic markers) immunocontent, and these alterations were reversed by ascorbic acid or ketamine, but only ketamine reversed the CORT-induced reduction on GluA1 immunocontent. In the ventral and dorsal DG, CORT decreased filopodia-, thin- and stubby-shaped spines, while ascorbic acid and ketamine abolished this alteration only in filopodia spines. Ascorbic acid and ketamine increased mushroom-shaped spines density in ventral and dorsal DG. Therefore, the results show that a single administration of ascorbic acid, in a way similar to ketamine, rapidly elicits an antidepressant-like response and reverses hippocampal synaptic deficits caused by CORT, an effect associated with increased levels of synaptic proteins and dendritic remodeling.

Impact of impaired glucose metabolism on responses to a psychophysical stressor: modulation by ketamine

RATIONALE

There is evidence that hypoglycemia, a metabolic stressor, can negatively impact mood and motivation, and can interact with other stressors to potentiate their effects on behavior and physiology.

OBJECTIVES/METHODS

The current study in male Sprague-Dawley rats explored the interaction between impaired glucose metabolism induced by 0, 200, or 300 mg/kg 2-deoxy-D-glucose (2-DG) and a psychophysical stressor induced by forced swimming stress (FSS; 6 sessions, 10 min/session). The endpoints of interest were blood glucose levels, progressive behavioral immobility, and saccharin preference (2-bottle choice test). Furthermore, it was investigated whether pre-treatment with 0, 10, or 20 mg/kg ketamine could modify the interaction between 2-DG and FSS on these endpoints.

RESULTS

It was found that 2-DG increased blood glucose levels equally in all experimental groups, accelerated the immobile response to FSS, and suppressed saccharin preference 1 week following termination of stress exposure. As well, pre-treatment with ketamine blocked the effects of combined 2-DG and FSS on immobility and saccharin preference without affecting blood glucose levels and produced an anti-immobility effect that was observed during a drug-free test swim 1 week following administration.

CONCLUSIONS

Overall, these findings demonstrate that impaired glucose metabolism can potentiate the effects of a psychophysical stressor, and that this interaction can be modulated pharmacologically by ketamine.

Translating the immediate effects of S-Ketamine using hippocampal subfield analysis in healthy subjects-results of a randomized controlled trial

Antidepressant doses of ketamine rapidly facilitate synaptic plasticity and modify neuronal function within prefrontal and hippocampal circuits. However, most studies have demonstrated these effects in animal models and translational studies in humans are scarce. A recent animal study showed that ketamine restored dendritic spines in the hippocampal CA1 region within 1 h of administration. To translate these results to humans, this randomized, double-blind, placebo-controlled, crossover magnetic resonance imaging (MRI) study assessed ketamine's rapid neuroplastic effects on hippocampal subfield measurements in healthy volunteers. S-Ketamine vs. placebo data were analyzed, and data were also grouped by brain-derived neurotrophic factor (BDNF) genotype. Linear mixed models showed that overall hippocampal subfield volumes were significantly larger (p = 0.009) post ketamine than post placebo (LS means difference=0.008, standard error=0.003). Post-hoc tests did not attribute effects to specific subfields (all p > 0.05). Trend-wise volumetric increases were observed within the left hippocampal CA1 region (p = 0.076), and trend-wise volumetric reductions were obtained in the right hippocampal-amygdaloid transition region (HATA) (p = 0.067). Neither genotype nor a genotype-drug interaction significantly affected the results (all p > 0.7). The study provides evidence that ketamine has short-term effects on hippocampal subfield volumes in humans. The results translate previous findings from animal models of depression showing that ketamine has pro-neuroplastic effects on hippocampal structures and underscore the importance of the hippocampus as a key region in ketamine's mechanism of action.

Ketamine reversed short-term memory impairment and depressive-like behavior in animal model of Parkinson's disease

The most common features of Parkinson's disease (PD) are motor impairments, but many patients also present depression and memory impairment. Ketamine, an N-methyl-d-aspartate (NMDA) receptor antagonist, has been shown to be effective in patients with treatment-resistant major depression. Thus, the present study evaluated the action of ketamine on memory impairment and depressive-like behavior in an animal model of PD. Male Wistar rats received a bilateral infusion of 6 μg/side 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNc). Short-term memory was evaluated by the social recognition test, and depressive-like behaviors were evaluated by the sucrose preference and forced swimming tests (FST). Drug treatments included vehicle (i.p., once a week); ketamine (5, 10 and 15 mg/kg, i.p., once a week); and imipramine (20 mg/kg, i.p., daily). The treatments were administered 21 days after the SNc lesion and lasted for 28 days. The SNc lesion impaired short-term social memory, and all ketamine doses reversed the memory impairment and anhedonia (reduction of sucrose preference) induced by 6-OHDA. In the FST, 6-OHDA increased immobility, and all doses of ketamine and imipramine reversed this effect. The anti-immobility effect of ketamine was associated with an increase in swimming but not in climbing, suggesting a serotonergic effect. Ketamine and imipramine did not reverse the 6-OHDA-induced reduction in tyrosine hydroxylase immunohistochemistry in the SNc. In conclusion, ketamine reversed depressive-like behaviors and short-term memory impairment in rats with SNc bilateral lesions, indicating a promising profile for its use in PD patients.

Increased use of ketamine for the treatment of depression: Benefits and concerns

Ketamine was initially used as an anesthetic which could induce cognitive impairment and psychomimetic effects. In initial randomized controlled trials (RCTs) that mostly included a small sample size and were investigator-initiated, ketamine reportedly exerted antidepressant effects 1 to 2 h after a single intravenous infusion in patients with major depressive episodes, particularly treatment-resistant depression (TRD). Interest in ketamine was reported in systematic reviews and meta-analyses, however, many were primarily focused on the rapid onset of ketamine effects without equal attention to its safety and tolerability. Furthermore, several meta-analyses were based on many duplicated RCTs. The initial trends emphasized the clinical utility of ketamine as an antidepressant. The development of esketamine nasal spray by a pharmaceutical company led to an RCT with a large sample size and segmented therapeutic strategy, which provided results applicable to patients with TRD in the real-world clinical environment. However, possible effects of ketamine on cognitive function have not yet been investigated in RCTs. In numerous studies, chronic, recreational use of ketamine reportedly substantially impaired cognitive function in most domains. Although results of several human and animal studies indicated the therapeutic use of ketamine for treatment of depression did not induce cognitive impairment, this issue should be further investigated. Based on the current knowledge about ketamine, future antidepressants are expected to be glutamatergic drugs without ketamine-like adverse events (e.g., psychomimetic symptoms and cognitive impairment), but having only ketamine-like therapeutic properties (e.g., rapid antidepressants effects without time lag).

Mechanisms associated with the antidepressant-like effects of L-655,708

Previous research has demonstrated that selective modulation of hippocampal transmission by systemic administration of an α5-GABA_A receptor negative allosteric modulator, L-655,708, reproduces the sustained antidepressant-like (AD-like) effect of R,S-ketamine in the absence of any psychotomimetic or abuse-related effects. Pharmacological, electrophysiological (whole-cell patch clamp), and behavioral approaches were used to examine the mechanisms by which L-655,708 produces plasticity within the hippocampus that accounts for its sustained AD-like effect in rats. Inhibitors of either transcription or translation prevented the sustained AD-like effect of L-655,708. Unlike R,S-ketamine, L-655,708 did not cause an increase in the phosphorylation of the receptor for BDNF, TrkB, in the ventral hippocampus (vHipp) 30 or 60 min after its administration nor did administration of the TrkB inhibitor, K252a, directly into the vHipp, block the sustained AD-like effect of L-655,708. Similar to previous results with R,S-ketamine, administration of L-655,709 increased levels of GluA1 in the mPFC and, blockade of such receptors by direct administration of NBQX into the mPFC blocked the sustained AD-like effect of L-655,708. Patch-clamp recordings of ventral CA1 pyramidal cells 24 h after a single systemic administration of L-655,708 revealed a significant increase in input resistance, which resulted in an approximately two-fold increase in action potential frequency. These experiments indicate that the sustained AD-like effects of L-655,708 require protein synthesis and plasticity of GluA1 glutamate receptors in the mPFC. The drug also caused changes in GABA_A receptor gating properties in the vHipp with resultant changes in ventral CA1 that indirectly increases neuronal excitability. Such effects likely contribute to its sustained AD-like activity.

Neurocognitive impact of ketamine treatment in major depressive disorder: A review on human and animal studies

BACKGROUND

Most recent evidence support a rapid and sustained antidepressant effect of subanesthetic dose of intravenous ketamine in patients with major depressive disorder (MDD). However, clinical and animal studies investigating the effects of intravenous ketamine on specific functional domains disrupted by depression reported conflicting results. Therefore, the aim of this review is to provide an overview of the recent findings exploring the cognitive effects of ketamine in depression.

METHODS

After a bibliographic search on PubMed, Medline and PsycInfo, we retrieved 11 original studies meeting our research criteria, 7 in humans with MDD or Treatment Resistant Disorder and 4 using rats models for depression.

RESULTS

Overall the results showed that a) ketamine reduced activation and normalized connectivity measures of several brain regions related to depressive behaviors and reversed deficits in cognitive flexibility and coping response strategy in rats with depressive features, and b) ketamine leads to a no significant impairment on neurocognitive functions in most of the studies, with only three studies observing improvements in speed of processing, verbal learning, sustained attention and response control, verbal and working memory.

LIMITATIONS

The methodological heterogeneity, in terms of neuropsychological tests used and cognitive domain explored, of the studies included.

CONCLUSIONS

Most of the studies included showed no significant cognitive impairments in MDD patients after ketamine treatment. Furthermore, the results of the fMRI studies considered suggest that ketamine may have a normalizing effect on brain functions during attentional and emotional processing in MDD patients. However, further studies are needed to confirm these preliminary evidences.

Anorexia nervosa, zinc deficiency and the glutamate system: The ketamine option

Anorexia nervosa (AN) is a severe, biological brain disorder with significant medical risks and a tenacious development over time. Unfortunately, few treatments show efficacy in people with AN although numerous therapies including pharmacological have been explored. Zinc deficiency has been implicated in AN and zinc is important in a large range of processes in the brain. In particular, it is an allosteric modulator of NMDA receptors - the maintenance of zinc levels within a normal, narrow range is essential for glutamatergic functioning. Chronic zinc deficiency increases neuronal stores of calcium and reduces direct modulation of NMDA receptors which collectively lead to overactivation and upregulation of NMDA receptors. This may facilitate pathologically high levels of glutamate, calcium influx and subsequent excitotoxicity, which can disrupt synaptogenesis and synaptic plasticity. While studies of zinc supplementation in AN have shown some promise, the efficacy of this treatment is limited. This may be due to AN illness chronicity and the significant changes already made, as well as a reduced potency of zinc to inhibit NMDA receptors in a pathological state. Thus, we propose that the safe (at low doses) yet more potent NMDA receptor antagonist, ketamine, may act to normalise a perturbed glutamatergic system and increase synaptogenesis in the short term. This 'kickstart' via ketamine could then allow zinc supplementation and other forms of treatment to enhance recovery in AN.

Ketamine contributes to the alteration of Ca2+ transient evoked by behavioral tests in the prelimbic area of mPFC: A study on chronic CORT-induced depressive mice

Recent studies have showed that ketamine is a rapid and efficient antidepressant, but the mechanism of its antidepressant effect is not fully clear. It is still lack of the research investigating the relation between depressive-like behaviors and neuronal activities in specific brain area after administration of ketamine in vivo. Medial prefrontal cortex (mPFC) involved in the pathogenesis of depression. As a result of effective assessments after behavioral test, most studies lack of direct evidence of the relation between efficacy and the activity of specific brain area. Therefore, we used fiber photometry to explore the alteration of Ca²⁺ transient in the prelimbic (PrL) area of mPFC during behavioral tests in freely moving mice. Our results showed that the chronic corticosterone (CORT) protocol induced depressive-like behaviors. Administration of ketamine reversed these effects. The activation of Ca²⁺ transients was associated with some behaviors during behavioral tests. Struggling, rearing and exploring evoked strong Ca²⁺ transients, but moving and grooming did not. The Ca²⁺ transients amplitude reductions of struggling, rearing and exploring induced by CORT were reversed by ketamine. The results indicated that ketamine ameliorated depressive-like behaviors via mediating neural activation in PrL.

Ketamine and its metabolite, (2R,6R)-HNK, restore hippocampal LTP and long-term spatial memory in the Wistar-Kyoto rat model of depression

Accumulating evidence implicates dysregulation of hippocampal synaptic plasticity in the pathophysiology of depression. However, the effects of ketamine on synaptic plasticity and their contribution to its mechanism of action as an antidepressant, are still unclear. We investigated ketamine's effects on in vivo dorsal hippocampal (dHPC) synaptic plasticity and their role in mediating aspects of antidepressant activity in the Wistar-Kyoto (WKY) model of depression. dHPC long-term potentiation (LTP) was significantly impaired in WKY rats compared to Wistar controls. Importantly, a single low dose (5 mg/kg, ip) of ketamine or its metabolite, (2R,6R)-HNK, rescued the LTP deficit in WKY rats at 3.5 h but not 30 min following injection, with residual effects at 24 h, indicating a delayed, sustained facilitatory effect on dHPC synaptic plasticity. Consistent with the observed dHPC LTP deficit, WKY rats exhibited impaired hippocampal-dependent long-term spatial memory as measured by the novel object location recognition test (NOLRT), which was effectively restored by pre-treatment with both ketamine or (2R,6R)-HNK. In contrast, in WKYs, which display abnormal stress coping, ketamine, but not (2R,6R)-HNK, had rapid and sustained effects in the forced swim test (FST), a commonly used preclinical screen for antidepressant-like activity. The differential effects of (2R,6R)-HNK observed here reveal a dissociation between drug effects on FST immobility and dHPC synaptic plasticity. Therefore, in the WKY rat model, restoring dHPC LTP was not correlated with ketamine's effects in FST, but importantly, may have contributed to the reversal of hippocampal-dependent cognitive deficits, which are critical features of clinical depression. Our findings support the theory that ketamine may reverse the stress-induced loss of connectivity in key neural circuits by engaging synaptic plasticity processes to "reset the system".

Overlap in the neural circuitry and molecular mechanisms underlying ketamine abuse and its use as an antidepressant

Ketamine, a dissociative anesthetic and psychedelic compound, has revolutionized the field of psychopharmacology by showing robust, and rapid-acting antidepressant activity in patients suffering from major depressive disorder (MDD), suicidal tendencies, and treatment-resistant depression (TRD). Ketamine's efficacy, however, is transient, and patients must return to the clinic for repeated treatment as they experience relapse. This is cause for concern because ketamine is known for its abuse liability, and repeated exposure to drugs of abuse often leads to drug abuse/dependence. Though the mechanism(s) underlying its antidepressant activity is an area of current intense research, both clinical and preclinical evidence shows that ketamine's effects are mediated, at least in part, by molecular adaptations resulting in long-lasting synaptic changes in mesolimbic brain regions known to regulate natural and drug reward. This review outlines our limited knowledge of ketamine's neurobiological and biochemical underpinnings mediating its antidepressant effects and correlates them to its abuse potential. Depression and addiction share overlapping neural circuitry and molecular mechanisms, and though speculative, repeated use of ketamine for the treatment of depression could lead to the development of substance use disorder/addiction, and thus should be tempered with caution. There is much that remains to be known about the long-term effects of ketamine, and our lack of understanding of neurobiological mechanisms underlying its antidepressant effects is a clear limiting factor that needs to be addressed systematically before using repeated ketamine in the treatment of depressed patients.

Neuroplasticity in cognitive and psychological mechanisms of depression: an integrative model

Chronic stress and depressive-like behaviors in basic neuroscience research have been associated with impairments of neuroplasticity, such as neuronal atrophy and synaptic loss in the medial prefrontal cortex (mPFC) and hippocampus. The current review presents a novel integrative model of neuroplasticity as a multi-domain neurobiological, cognitive, and psychological construct relevant in depression and other related disorders of negative affect (e.g., anxiety). We delineate a working conceptual model in which synaptic plasticity deficits described in animal models are integrated and conceptually linked with human patient findings from cognitive science and clinical psychology. We review relevant reports including neuroimaging findings (e.g., decreased functional connectivity in prefrontal-limbic circuits), cognitive deficits (e.g., executive function and memory impairments), affective information processing patterns (e.g., rigid, negative biases in attention, memory, interpretations, and self-associations), and patient-reported symptoms (perseverative, inflexible thought patterns; inflexible and maladaptive behaviors). Finally, we incorporate discussion of integrative research methods capable of building additional direct empirical support, including using rapid-acting treatments (e.g., ketamine) as a means to test this integrative model by attempting to simultaneously reverse these deficits across levels of analysis.

Ketamine: Leading us into the future for development of antidepressants

Numerous randomized double-blind clinical trials have consistently shown that that a single intravenous administration of a subanesthetic dose of ketamine to treatment-resistant depressed patients significantly improved depressive symptomatology rapidly, within two hours, with the effect lasting up to seven days. Despite its very promising effects, ketamine has long been associated with potential for abuse as it can cause psychotropic side effects, such as hallucinations, false beliefs, and severe impairments in judgment and other cognitive processes. Consequently, within the last two decades preclinical research has been carried out aimed at understanding its mechanisms of action and the brain circuits involved in ketamine's antidepressant effects, both of which are discussed in this review. Furthermore, with the hippocampus being a key target for ketamine's beneficial antidepressant effects, we and others have begun to examine behavioral and neurochemical effects of drugs that act selectively on the hippocampus due to the preferential location of their receptor targets. Such drugs are negative allosteric modulators (NAMs) and positive allosteric modulator (PAM) of the α5-GABA_A receptor. Such compounds are discussed within the framework of how lessons learned with ketamine point to novel classes of drugs, targeting the GABAergic system, that can recapitulate the antidepressant effects of ketamine without its adverse effects.

Repeated ketamine injections in synergy with antidepressants for treating refractory depression: A case showing 6-month improvement

WHAT IS KNOWN AND OBJECTIVE

Some patients with refractory depression who fail to respond to rapid injection of standard-dose ketamine are injected with high doses, but the safety and efficacy of this practice are unclear.

CASE DESCRIPTION

A 57-year-old woman with refractory depression whose symptoms did not improve after 20-seconds intravenous injection of 0.5 mg/kg ketamine went into remission following eight, 1-minute intravenous injections of 1 mg/kg ketamine delivered over a 4-week period. By 6-month follow-up, no significant adverse events had occurred and cognitive function had improved.

WHAT IS NEW AND CONCLUSION

High-dose intravenous injections of ketamine may stably improve depressive symptoms and cognitive function in patients with refractory depression who do not respond to rapid intravenous injection of standard-dose ketamine. The high-dose treatment appears to be associated with only mild side effects.

Rodent ketamine depression-related research: Finding patterns in a literature of variability

Discovering that the anesthetic drug ketamine has rapidly acting antidepressant effects in many individuals with major depression is one of the most important findings in clinical psychopharmacology in recent decades. The initial report of these effects in human subjects was based on a foundation of rodent preclinical studies carried out in the 1990s, and subsequent investigation has included both further studies in individuals with depression, as well as reverse translational experiments in animal models, especially rodents. While there is general agreement in the rodent literature that ketamine has rapidly-acting, and generally sustained, antidepressant-like properties, there are also points of contention across studies, including the precise mechanism of action of this drug. In this review, we briefly summarize prominent yet variable findings regarding the mechanism of action. We also discuss a combination of similarities and variances in the rodent literature in the antidepressant-like effects of ketamine as a function of dose, species and strain, test, stressor, and presumably sex of the experimenter. We then present previously unpublished mouse strain comparison data suggesting that subanesthetic ketamine does not have robust antidepressant-like properties in unstressed animals, and may actually promote depression-like behavior, in contrast to widely reported findings. We conclude that the data best support the notion of ketamine action principally via NMDA receptor antagonism, transiently boosting glutamatergic (and possibly other) signaling in diverse brain circuits. We also suggest that future studies should address in greater detail the extent to which antidepressant-like properties of this drug are stress-sensitive, in an effort to better model major depression present in humans.

Vortioxetine reverses medial prefrontal cortex-mediated cognitive deficits in male rats induced by castration as a model of androgen deprivation therapy for prostate cancer

RATIONALE

Androgen deprivation therapy (ADT) is an effective treatment for prostate cancer, but induces profound cognitive impairment. Little research has addressed mechanisms underlying these deficits or potential treatments. This is an unmet need to improve quality of life for prostate cancer survivors.

OBJECTIVES

We investigated mechanisms of cognitive impairment after ADT in rats and potential utility of the multimodal serotonin-targeting drug, vortioxetine, to improve the impairment, as vortioxetine has specific efficacy against cognitive impairment in depression.

METHODS

Male Sprague-Dawley rats were surgically castrated. Vortioxetine (28 mg/kg/day) was administered in the diet. The attentional set-shifting test was used to assess medial prefrontal cortex (mPFC) executive function. Afferent-evoked field potentials were recorded in the mPFC of anesthetized rats after stimulating the ventral hippocampus (vHipp) or medial dorsal thalamus (MDT). Gene expression changes were assessed by microarray. Effects of vortioxetine on growth of prostate cancer cells were assessed in vitro.

RESULTS

ADT impaired cognitive set shifting and attenuated responses evoked in the mPFC by the vHipp afferent, but not the MDT. Both the cognitive impairment and attenuated vHipp-evoked responses were reversed by chronic vortioxetine treatment. Preliminary investigation of gene expression in the mPFC indicates that factors involved in neuronal plasticity and synaptic transmission were down-regulated by castration and up-regulated by vortioxetine in castrated animals. Vortioxetine neither altered the growth of prostate cancer cells in vitro nor interfered with the antiproliferative effects of the androgen antagonist, enzalutamide.

CONCLUSIONS

These results suggest that vortioxetine may be useful in mitigating cognitive impairment associated with ADT for prostate cancer.

Differences between ketamine's short-term and long-term effects on brain circuitry in depression

Ketamine acts as a rapid clinical antidepressant at 25 min after injection with effects sustained for 7 days. As dissociative effects emerging acutely after injection are not entirely discernible from therapeutic action, we aimed to dissect the differences between short-term and long-term response to ketamine to elucidate potential imaging biomarkers of ketamine's antidepressant effect. We used a genetical model of depression, in which we bred depressed negative cognitive state (NC) and non-depressed positive cognitive state (PC) rat strains. Four parallel rat groups underwent stress-escape testing and a week later received either S-ketamine (12 NC, 13 PC) or saline (12 NC, 12 PC). We acquired resting-state functional magnetic resonance imaging time series before injection and at 30 min and 48 h after injection. Graph analysis was used to calculate brain network properties. We identified ketamine's distinct action over time in a qualitative manner. The rapid response entailed robust and strain-independent topological modifications in cognitive, sensory, emotion, and reward-related circuitry, including regions that exhibited correlation of connectivity metrics with depressive behavior, and which could explain ketamine's dissociative and antidepressant properties. At 48 h ketamine had mainly strain-specific action normalizing habenula, midline thalamus, and hippocampal connectivity measures in depressed rats. As these nodes mediate cognitive flexibility impaired in depression, action within this circuitry presumably reflects ketamine's procognitive effects induced only in depressed patients. This finding is especially valid, as our model represents cognitive aspects of depression. These empirically defined circuits explain ketamine's distinct action over time and might serve as translational imaging correlates of antidepressant response in preclinical testing.

Reciprocal interactions across and within multiple levels of monoamine and cortico-limbic systems in stress-induced depression: A systematic review

The monoamine hypothesis of depression, namely that the reduction in synaptic serotonin and dopamine levels causes depression, has prevailed in past decades. However, clinical and preclinical studies have identified various cortical and subcortical regions whose altered neural activities also regulate depressive-like behaviors, independently from the monoamine system. Our systematic review indicates that neural activities of specific brain regions and associated neural circuitries are adaptively altered after chronic stress in a specific direction, such that the neural activity in the infralimbic cortex, lateral habenula and amygdala is upregulated, whereas the neural activity in the prelimbic cortex, hippocampus and monoamine systems is downregulated. The altered neural activity dynamics between monoamine systems and cortico-limbic systems are reciprocally interwoven at multiple levels. Furthermore, depressive-like behaviors can be experimentally reversed by counteracting the altered neural activity of a specific neural circuitry at multiple brain regions, suggesting the importance of the reciprocally interwoven neural networks in regulating depressive-like behaviors. These results promise for reshaping altered neural activity dynamics as a therapeutic strategy for treating depression.

A Rodent Model of Exposure Therapy: The Use of Fear Extinction as a Therapeutic Intervention for PTSD

The symptoms of post-traumatic stress disorder (PTSD) include cognitive impairment related to medial prefrontal cortical dysfunction. Indeed, a deficit of cognitive flexibility, i.e., an inability to modify previously learned thoughts and behaviors based on changes in the environment, may underlie many of the other symptoms of PTSD, such as changes in mood, hyper-arousal, intrusive thoughts, exaggerated and over-generalized fear, and avoidance behavior. Cognitive-behavioral therapies target the cognitive dysfunction observed in PTSD patients, training them to recalibrate stress-related perceptions, interpretations and responses. Preclinically, the extinction of conditioned fear bears resemblance to one form of cognitive therapy, exposure therapy, whereby an individual learns, through repeated exposure to a fear-provoking stimulus in a safe environment, that the stimulus no longer signals imminent threat, and their fear response is suppressed. In this review article, we highlight recent findings from our lab using fear extinction as a preclinical model of exposure therapy in rodents exposed to chronic unpredictable stress (CUS). We specifically focus on the therapeutic effects of extinction on stress-compromised set-shifting as a measure of cognitive flexibility, and active vs. passive coping behavior as a measure of avoidance. Finally, we discuss mechanisms involving activity and plasticity in the medial prefrontal cortex (mPFC) necessary for the therapeutic effects of extinction on cognitive flexibility and active coping.

Chronic treatment with galantamine rescues reversal learning in an attentional set-shifting test after experimental brain trauma

Approximately 10 million new cases of traumatic brain injury (TBI) are reported each year worldwide with many of these injuries resulting in higher order cognitive impairments. Galantamine (GAL), an acetylcholine esterase inhibitor (AChEI) and positive allosteric modulator of nicotinic acetylcholine receptors (nAChRs), has been reported to ameliorate cognitive deficits after clinical TBI. Previously, we demonstrated that controlled cortical impact (CCI) injury to rats resulted in significant executive function impairments as measured by the attentional set-shifting test (AST), a complex cognitive task analogous to the Wisconsin Card Sorting Test (WCST). We hypothesized that chronic administration of GAL would normalize performance on the AST post-TBI. Isoflurane-anesthetized adult male rats were subjected to moderate CCI (2.8 mm tissue deformation at 4 m/s) or sham injury. Rats were then randomized into one of three treatment groups (i.e., 1 mg/kg GAL, 2 mg/kg GAL, or 1 mL/kg saline vehicle; VEH) or their respective sham controls. GAL or VEH was administered intraperitoneally daily commencing 24 hours post-surgery and until AST testing at 4 weeks post-injury. The AST data revealed significant impairments in the first reversal stage after TBI, seen as increased trials to reach criterion and elevated total errors (p < 0.05). These behavioral flexibility deficits were equally normalized by the administration of both doses of GAL (p < 0.05). Additionally, the higher dose of GAL (2 mg/kg) also significantly reduced cortical lesion volume compared to TBI + VEH controls (p < 0.05). In summary, daily GAL administration provides an efficacious treatment for cognitive deficits and histological recovery after experimental brain trauma. Clinically, these findings are promising considering robust results were attained using a pharmacotherapy already used in the clinic to treat mild dementia.

Optogenetic stimulation of medial prefrontal cortex Drd1 neurons produces rapid and long-lasting antidepressant effects

Impaired function in the medial prefrontal cortex (mPFC) contributes to depression, and the therapeutic response produced by novel rapid-acting antidepressants such as ketamine are mediated by mPFC activity. The mPFC contains multiple types of pyramidal cells, but it is unclear whether a particular subtype mediates the rapid antidepressant actions of ketamine. Here we tested two major subtypes, Drd1 and Drd2 dopamine receptor expressing pyramidal neurons and found that activating Drd1 expressing pyramidal cells in the mPFC produces rapid and long-lasting antidepressant and anxiolytic responses. In contrast, photostimulation of Drd2 expressing pyramidal cells was ineffective across anxiety-like and depression-like measures. Disruption of Drd1 activity also blocked the rapid antidepressant effects of ketamine. Finally, we demonstrate that stimulation of mPFC Drd1 terminals in the BLA recapitulates the antidepressant effects of somatic stimulation. These findings aid in understanding the cellular target neurons in the mPFC and the downstream circuitry involved in rapid antidepressant responses.

Ventral CA3 Activation Mediates Prophylactic Ketamine Efficacy Against Stress-Induced Depressive-like Behavior

BACKGROUND

We previously reported that a single injection of ketamine prior to stress protects against the onset of depressive-like behavior and attenuates learned fear. However, the molecular pathways and brain circuits underlying ketamine-induced stress resilience are still largely unknown.

METHODS

Here, we tested whether prophylactic ketamine administration altered neural activity in the prefrontal cortex and/or hippocampus. Mice were injected with saline or ketamine (30 mg/kg) 1 week before social defeat. Following behavioral tests assessing depressive-like behavior, mice were sacrificed and brains were processed to quantify ΔFosB expression. In a second set of experiments, mice were stereotaxically injected with viral vectors into ventral CA3 (vCA3) in order to silence or overexpress ΔFosB prior to prophylactic ketamine administration. In a third set of experiments, ArcCreER^T2 mice, a line that allows for the indelible labeling of neural ensembles activated by a single experience, were used to quantify memory traces representing a contextual fear conditioning experience following prophylactic ketamine administration.

RESULTS

Prophylactic ketamine administration increased ΔFosB expression in the ventral dentate gyrus and vCA3 of social defeat mice but not of control mice. Transcriptional silencing of ΔFosB activity in vCA3 inhibited prophylactic ketamine efficacy, while overexpression of ΔFosB mimicked and occluded ketamine's prophylactic effects. In ArcCreER^T2 mice, ketamine administration altered memory traces representing the contextual fear conditioning experience in vCA3 but not in the ventral dentate gyrus.

CONCLUSIONS

Our data indicate that prophylactic ketamine may be protective against a stressor by altering neural activity, specifically the neural ensembles representing an individual stressor in vCA3.

Prefrontal cortex executive processes affected by stress in health and disease

Prefrontal cortical executive functions comprise a number of cognitive capabilities necessary for goal directed behavior and adaptation to a changing environment. Executive dysfunction that leads to maladaptive behavior and is a symptom of psychiatric pathology can be instigated or exacerbated by stress. In this review we survey research addressing the impact of stress on executive function, with specific focus on working memory, attention, response inhibition, and cognitive flexibility. We then consider the neurochemical pathways underlying these cognitive capabilities and, where known, how stress alters them. Finally, we review work exploring potential pharmacological and non-pharmacological approaches that can ameliorate deficits in executive function. Both preclinical and clinical literature indicates that chronic stress negatively affects executive function. Although some of the circuitry and neurochemical processes underlying executive function have been characterized, a great deal is still unknown regarding how stress affects these processes. Additional work focusing on this question is needed in order to make progress on developing interventions that ameliorate executive dysfunction.