Vortioxetine Reverses Impairment of Visuospatial Memory and Cognitive Flexibility Induced by Degarelix as a Model of Androgen Deprivation Therapy in Rats.

INTRODUCTION: Androgen deprivation therapy (ADT) is a mainstay treatment for prostate cancer, but many patients experience cognitive impairment in domains mediated by the medial prefrontal cortex (mPFC) and hippocampus. Prostate cancer typically occurs in older patients (>65 years). As age is often accompanied by cognitive decline, it may impact the efficacy of any treatment aimed at restoring cognitive impairment induced by ADT. Vortioxetine, a multimodal antidepressant that improves cognition in depression, has been shown to be efficacious in elderly patients. Therefore, vortioxetine may improve cognition in older patients who experience cognitive decline after ADT. METHODS: Young (3 months) and middle-aged (13 months) rats were used to investigate the influence of age on treating ADT-induced cognitive decline. As our previous studies used surgical castration, we tested if vortioxetine would reverse cognitive deficits associated with more translationally relevant chemical castration using degarelix. Vortioxetine was given in the diet for 21 days. Animals underwent behavioral testing to assess visuospatial memory mediated by the hippocampus and cognitive flexibility mediated by the mPFC. We also investigated changes in afferent-evoked responses in these regions in middle-aged rats. RESULTS: Degarelix induced impairments in both visuospatial memory and cognitive flexibility that were reversed by vortioxetine. Vortioxetine also rescued afferent-evoked responses in the mPFC and hippocampus. However, modest age-related reductions in baseline visuospatial memory limited our ability to detect further decreases induced by degarelix in middle-aged rats due to a floor effect. CONCLUSION: These results suggest that vortioxetine may be a treatment option for older prostate cancer patients who experience cognitive decline after ADT.

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.

Heterogeneous complement and microglia activation mediates stress-induced synapse loss.

Spatially heterogeneous synapse loss is a characteristic of many psychiatric and neurological disorders, but the underlying mechanisms are unclear. Here, we show that spatially-restricted complement activation mediates stress-induced heterogeneous microglia activation and synapse loss localized to the upper layers of the mouse medial prefrontal cortex (mPFC). Single cell RNA sequencing also reveals a stress-associated microglia state marked by high expression of the apolipoprotein E gene (ApoE high ) localized to the upper layers of the mPFC. Mice lacking complement component C3 are protected from stress-induced layer-specific synapse loss, and the ApoE high microglia population is markedly reduced in the mPFC of these mice. Furthermore, C3 knockout mice are also resilient to stress-induced anhedonia and working memory behavioral deficits. Our findings suggest that region-specific complement and microglia activation can contribute to the disease-specific spatially restricted patterns of synapse loss and clinical symptoms found in many brain diseases.

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.

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.

Infralimbic BDNF signaling is necessary for the beneficial effects of extinction on set shifting in stressed rats.

Current pharmacotherapies for posttraumatic stress disorder (PTSD) and major depressive disorder (MDD) are ineffective for many patients, and often do not restore cognitive dysfunction associated with these disorders. Behavioral therapies, such as exposure therapy, can be effective for treatment-resistant patients. The mechanisms underlying exposure therapy are not well-understood. Fear extinction as an intervention after chronic stress can model the beneficial effects of exposure therapy in rats. Extinction requires neuronal activity and protein synthesis in the infralimbic (IL) cortex for its beneficial effects. We hypothesized that extinction requires Brain-Derived Neurotrophic Factor (BDNF) activity in the IL cortex to reverse stress-induced cognitive flexibility impairments. Extinction learning reversed set-shifting deficits induced by Chronic Unpredictable Stress (CUS), tested 24 h after extinction. Blocking BDNF signaling in the IL cortex during extinction by local administration of a neutralizing antibody prevented the beneficial effects of extinction on set shifting after stress. Extinction induced activation of the BDNF TrkB receptor, and signaling pathways associated with BDNF (Akt and Erk). Administration of exogenous BDNF into IL cortex in the absence of extinction was sufficient to reverse the effects of stress on set shifting. The effects of extinction were prevented by blocking either Erk or Akt signaling in the IL cortex, whereas the effects of exogenous BDNF were dependent on Erk, but not Akt, signaling. Our observations suggest that BDNF-Erk signaling induced by extinction underlies plastic changes that can reverse or counteract the effects of chronic stress in the IL cortex.

Optogenetically-induced long term depression in the rat orbitofrontal cortex ameliorates stress-induced reversal learning impairment.

Cognitive flexibility is a higher-order executive function that requires plasticity in neuronal circuits of the prefrontal cortex. Deficits in cognitive flexibility are prominent in a variety of psychiatric disorders, such as major depression, obsessive-compulsive disorder, and posttraumatic stress disorder. Chronic stress induces deficits in cognitive flexibility, perhaps through effects on plasticity, but the mechanism is not well understood. Previous work has demonstrated that stress reduces activity and dendritic elaboration in the medial prefrontal cortex (mPFC). In contrast, stress appears to increase dendritic elaboration in the orbitofrontal cortex (OFC). This suggests that there may be a differential effect of stress on plasticity in different prefrontal cortical areas. To test this hypothesis, we examined the effects of inducing plasticity optogenetically in the OFC on reversal learning, an OFC-mediated form of cognitive flexibility, in stressed and non-stressed rats. Inducing opto-LTD in the projection from mediodorsal thalamus to OFC ameliorated reversal learning deficits in rats exposed to chronic intermittent cold (CIC) stress. Additionally, we found that inducing opto-LTP in non-stressed rats produced deficits in reversal learning similar to those seen in rats after CIC stress. Finally, CIC stress produced complex subregion-specific changes in dendritic material and spine subtype composition in the OFC. These results indicate that the effects of stress on plasticity in the OFC are distinct from those in the mPFC, and that the PFC should therefore not be treated as a homogenous region in studying either stress effects or potential treatments for stress-related psychiatric disorders.

Bidirectional Optogenetically-Induced Plasticity of Evoked Responses in the Rat Medial Prefrontal Cortex Can Impair or Enhance Cognitive Set-Shifting.

Chronic stress compromises cognition, including executive function mediated in the medial prefrontal cortex (mPFC). To investigate mechanisms underlying these processes, we use chronic unpredictable stress (CUS), which reduces activity in the mPFC and impairs cognitive set-shifting, a measure of cognitive flexibility in laboratory rats. It has been shown that CUS attenuates the local electrical field potential response evoked in the mPFC by stimulation of the ascending excitatory afferent from the mediodorsal thalamus (MDT). Thus, in this study, to investigate the role that such changes in afferent-evoked responsivity of the mPFC might play in the cognitive deficits induced by CUS, we used optogenetics to directly induce plastic changes in the thalamic-mPFC afferent pathway. Glutamatergic neurons in the MDT were virally-induced to express the ChETA variant of channelrhodopsin. Then, to first validate the optogenetic induction of plasticity, long-term depression (LTD) or long-term potentiation (LTP) were induced by laser stimulation of ChETA-expressing terminals in the mPFC of anesthetized rats. In subsequent experiments, induction of opto-LTD in awake animals produced set-shifting deficits similar to those induced by CUS. By contrast, inducing opto-LTP in rats that had received prior CUS treatment corrected the stress-induced deficit in set-shifting. These results suggest that stress-induced plasticity in the thalamic-mPFC pathway is sufficient to produce stress-induced cognitive deficits, and may represent a novel target for effective therapeutic intervention to correct cognitive impairment in stress-related psychiatric disorders.

Ciliary neurotrophic factor signaling in the rat orbitofrontal cortex ameliorates stress-induced deficits in reversal learning.

Deficits in cognitive flexibility, i.e. the ability to modify behavior in response to changes in the environment, are present in several psychiatric disorders and are often refractory to treatment. However, improving treatment response has been hindered by a lack of understanding of the neurobiology of cognitive flexibility. Using a rat model of chronic stress (chronic intermittent cold stress, CIC) that produces selective deficits in reversal learning, a form of cognitive flexibility dependent on orbitofrontal cortex (OFC) function, we have previously shown that JAK2 signaling is required for optimal reversal learning. In this study we explore the molecular basis of those effects. We show that, within the OFC, CIC stress reduces the levels of phosphorylated JAK2 and of ciliary neurotrophic factor (CNTF), a promoter of neuronal survival and an activator of JAK2 signaling, and that neutralizing endogenous CNTF with an intra-OFC microinjection of a specific antibody is sufficient to produce reversal-learning deficits similar to stress. Intra-OFC delivery of recombinant CNTF to CIC-stressed rats, at a dose that induces JAK2 and Akt but not STAT3 or ERK, ameliorates reversal-learning deficits, and Akt blockade prevents the positive effects of CNTF. Further analysis revealed that CNTF may exert its beneficial effects by inhibiting GSK3ß, a substrate of Akt and a regulator of protein degradation. We also revealed a novel mechanism of CNTF action through modulation of p38/Mnk1/eIF4E signaling. This cascade controls translation of select mRNAs, including those encoding several plasticity-related proteins. Thus, we suggest that CNTF-driven JAK2 signaling corrects stress-induced reversal learning deficits by modulating the steady-state levels of plasticity-related proteins in the OFC.

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.

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.

Shock-probe Defensive Burying Test to Measure Active versus Passive Coping Style in Response to an Aversive Stimulus in Rats.

Maladaptive avoidance behaviors are seen in many stress-related psychiatric illnesses. Patients with these illnesses favor passive, avoidant coping strategies rather than adaptive, active coping strategies. Preclinically, coping strategy can be measured in rats using the shock-probe defensive burying test, wherein rats receive a shock from an electrified probe inserted into a test cage that mimics their home cage environment, and behavioral output (immobility or burying) is recorded for 15 min following the shock. Immobility in response to the perceived threat of the shock-probe, associated with elevated stress hormone levels, is regarded as a passive, maladaptive coping strategy. In opposition, burying the probe is associated with lower stress hormone levels and is considered an active, adaptive coping style. In rats, chronic stress induces a shift from active to passive coping in this test (i.e., proportionally less burying and more immobility), modeling the avoidant symptoms presented across many stress-related psychiatric illnesses. The stress-induced shifts in coping style and overall behavioral reactivity to the shock-probe provide a unique and well-validated measure of not only an anxiety-like behavioral response but also coping strategy selection in rat models of psychiatric illness.

Ketamine Corrects a Deficit in Reversal Learning Caused by Chronic Intermittent Cold Stress in Female Rats.

Background: Individuals with stress-related psychiatric disorders exhibit deficits in cognitive flexibility. We have shown that chronic intermittent cold stress induces deficits in reversal learning, a form of cognitive flexibility mediated in the orbitofrontal cortex, that was reversed by ketamine in male rats. Such effects have not been tested in females. In this study, we examined effects of chronic intermittent cold stress and ketamine on reversal learning in females. Methods: Female Sprague-Dawley rats underwent 14 days of chronic intermittent cold and 3 days later received an injection of ketamine (10 mg/kg, i.p.). They were tested on reversal learning 24 hours post-injection. A separate cohort of female rats underwent 14 days of chronic intermittent cold. Three days later they received ketamine and were killed 2 hours post-injection for measurement of the synaptic marker PSD95 in orbitofrontal cortex. Results: Chronic intermittent cold induced a reversal learning deficit in females comparable with that seen in males, which was corrected by ketamine. Moreover, chronic intermittent cold increased PSD95 expression in orbitofrontal cortex, but this increase was not seen in rats receiving ketamine. Conclusions: Chronic intermittent cold stress and ketamine altered reversal learning in female rats similar to effects seen in males. Further, chronic intermittent cold increased PSD95 in orbitofrontal cortex of female rats, indicative of synaptic dysregulation. This effect was attenuated after ketamine administration.

Activity in the Ventral Medial Prefrontal Cortex Is Necessary for the Therapeutic Effects of Extinction in Rats.

Poor response and high relapse rates remain problematic in the treatment of stress-related psychiatric disorders such as depression and post-traumatic stress disorder. Although mechanisms of pharmacotherapies are intensely studied, little is known about mechanisms of behavioral therapy that could inform improved treatments. We have previously demonstrated the therapeutic effects of extinction learning as a behavioral intervention modeling exposure therapy in rats. In the present study, we tested the hypothesis that activity in the ventral medial prefrontal cortex (vmPFC) during extinction is necessary for its therapeutic effects. The inhibitory Gi-coupled designer receptor exclusively activated by designer drug CaMKIIα-hM4Di was expressed in vmPFC before administering chronic unpredictable stress (CUS). vmPFC projection neurons were then inhibited during extinction treatment by administering clozapine-N-oxide. Coping behavior and cognitive flexibility were assessed 24 h later on the shock-probe defensive burying test and attentional set-shifting test, respectively. Replicating previous results, extinction reversed the CUS-induced deficits in coping behavior and cognitive flexibility. Inhibiting vmPFC during extinction blocked these therapeutic effects. Further, increasing vmPFC activity with the excitatory Gq-coupled designer receptor exclusively activated by designer drug hM3Dq 24 h before testing was sufficient to reverse the CUS-induced deficits. CUS reduced mPFC responsivity, assessed by measuring afferent-evoked field potentials in the mPFC, and this reduction was reversed by extinction treatment 24 h before testing. These results demonstrate the necessity of vmPFC activity in the therapeutic effects of extinction as a model of exposure therapy, and suggest that increased vmPFC activity induced by extinction is sufficient to produce lasting plastic changes that underlie its beneficial effects.SIGNIFICANCE STATEMENT Stress-related psychiatric disorders remain poorly treated. Psychotherapies can be effective, but their mechanisms remain unknown, hindering progress toward improved treatment. We used a rat model of behavioral therapy to identify potential targets for enhancing treatment. Fear extinction as a therapeutic behavioral intervention reversed stress-induced cognitive dysfunction and passive coping in rats, modeling components of stress-related psychiatric disease. Extinction also reversed stress-induced attenuation of mPFC responsivity. The therapeutic effects were prevented by blocking activity of glutamatergic neurons in the mPFC during extinction, and were mimicked by inducing activity in lieu of extinction. Thus, activity and plasticity in the mPFC underlie the beneficial effects of extinction on cognitive flexibility and coping behavior compromised by stress, and could be targets to enhance behavioral therapy.

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.

Deficits in cognitive flexibility induced by chronic unpredictable stress are associated with impaired glutamate neurotransmission in the rat medial prefrontal cortex.

Deficits in cognitive flexibility, the ability to modify behavior in response to changes in the environment, contribute to the onset and maintenance of stress-related neuropsychiatric illnesses, such as depression. Cognitive flexibility depends on medial prefrontal cortex (mPFC) function, and in depressed patients, cognitive inflexibility is associated with hypoactivity and decreased glutamate receptor expression in the mPFC. Rats exposed to chronic unpredictable stress (CUS) exhibit compromised mPFC function on the extradimensional (ED) set-shifting task of the attentional set-shifting test. Moreover, CUS-induced ED deficits are associated with dendritic atrophy and decreased glutamate receptor expression in the mPFC. This evidence suggests that impaired glutamate signaling may underlie stress-induced deficits in cognitive flexibility. To test this hypothesis, we first demonstrated that blocking NMDA or AMPA receptors in the mPFC during ED replicated CUS-induced deficits in naïve rats. Secondly, we found that expression of activity-regulated cytoskeleton-associated protein (Arc) mRNA, a marker of behaviorally induced glutamate-mediated plasticity, was increased in the mPFC following ED. We then showed that CUS compromised excitatory afferent activation of the mPFC following pharmacological stimulation of the mediodorsal thalamus (MDT), indicated by a reduced induction of c-fos expression. Subsequently, in vivo recordings of evoked potentials in the mPFC indicated that CUS impaired afferent activation of the mPFC evoked by MDT stimulation, but not the ventral hippocampus. Lastly, glutamate microdialysis showed that CUS attenuated the acute stress-evoked increase in extracellular glutamate in the mPFC. Together, these results demonstrate that CUS-induced ED deficits are associated with compromised glutamate neurotransmission in the mPFC.

Chronic Vortioxetine Treatment Reduces Exaggerated Expression of Conditioned Fear Memory and Restores Active Coping Behavior in Chronically Stressed Rats.

Background: Stress is a risk factor for depression and anxiety disorders, disrupting neuronal processes leading to exaggerated fear and compromised coping behaviors. Current antidepressants are only partially effective. Vortioxetine, a novel multimodal antidepressant, is a serotonin transporter inhibitor; 5-HT3, 5-HT7, and 5-HT1D receptor antagonist; 5-HT1B partial agonist; and 5-HT1A agonist. We have shown that chronic dietary vortioxetine administration reversed stress-induced deficits in cognitive flexibility. In the present studies, we investigated the generality of vortioxetine's effects on other stress-related behavioral changes after different types of chronic stress. Methods: In experiment 1, rats were fear-conditioned by pairing a tone with footshock, then exposed to chronic plus acute prolonged stress. In experiment 2, rats were exposed to chronic unpredictable stress. In both experiments, beginning on day 4 of chronic stress, vortioxetine was given in the diet (24 mg/kg/d). In experiment 1, effects of vortioxetine were tested on stress-induced changes in retention and extinction of cue-conditioned fear, and in experiment 2, on coping behavior on the shock probe defensive burying test after chronic stress. Results: Chronic stress exaggerated the expression of conditioned fear memory. Vortioxetine restored fear memory to control levels and rendered extinction in stressed rats comparable with that in controls. In experiment 2, chronic unpredictable stress caused a shift from active to passive coping behavior, and vortioxetine restored active coping. Conclusions: Vortioxetine reduced exaggerated expression of conditioned fear and restored adaptive coping behavior following 2 different types of chronic stress, adding to the evidence of its therapeutic potential in the management of depression and anxiety disorders.

Ketamine Corrects Stress-Induced Cognitive Dysfunction through JAK2/STAT3 Signaling in the Orbitofrontal Cortex.

Deficits in cognitive flexibility are prominent in stress-related psychiatric disorders, including depression. Ketamine has rapid antidepressant efficacy, but it is unknown if ketamine improves cognitive symptoms. In rats, 2 weeks chronic intermittent cold (CIC) stress impairs reversal learning, a form of cognitive flexibility mediated by the orbitofrontal cortex (OFC) that we have used previously to model cognitive dysfunction in depression. We have shown that activating JAK2/STAT3 signaling in the OFC rescued the CIC stress-induced reversal learning deficit. Thus, in the present study we determined whether ketamine also corrects the stress-induced reversal learning deficit, and if JAK2/STAT3 signaling is involved in this effect. A single injection of ketamine (10 mg/kg, i.p.) 24 h prior to testing rescued the CIC stress-induced reversal learning deficit. CIC stress decreased JAK2 phosphorylation in the OFC, and ketamine restored pJAK2 levels within 2 h post injection. The JAK2 inhibitor AG490 given systemically or into the OFC at the time of ketamine injection prevented its beneficial effect on reversal learning. We then tested the role of JAK2/STAT3 in ketamine-induced plasticity in the OFC. Ketamine depressed local field potentials evoked in the OFC by excitatory thalamic afferent stimulation, and this was prevented by JAK2 inhibition in the OFC. Further, in both the OFC and primary cortical neurons in culture, ketamine increased expression of the neural plasticity-related protein Arc, and this was prevented by JAK2 inhibition. These results suggest that the JAK2/STAT3 signaling pathway is a novel mechanism by which ketamine exerts its therapeutic effects on stress-induced cognitive dysfunction in the OFC.

Therapeutic Effects of Extinction Learning as a Model of Exposure Therapy in Rats.

Current treatments for stress-related psychiatric disorders, such as depression and posttraumatic stress disorder (PTSD), are inadequate. Cognitive behavioral psychotherapies, including exposure therapy, are an alternative to pharmacotherapy, but the neurobiological mechanisms are unknown. Preclinical models demonstrating therapeutic effects of behavioral interventions are required to investigate such mechanisms. Exposure therapy bears similarity to extinction learning. Thus, we investigated the therapeutic effects of extinction learning as a behavioral intervention to model exposure therapy in rats, testing its effectiveness in reversing chronic stress-induced deficits in cognitive flexibility and coping behavior that resemble dimensions of depression and PTSD. Rats were fear-conditioned by pairing a tone with footshock, and then exposed to chronic unpredictable stress (CUS) that induces deficits in cognitive set-shifting and active coping behavior. They then received an extinction learning session as a therapeutic intervention by repeated exposure to the tone with no shock. Effects on cognitive flexibility and coping behavior were assessed 24 h later on the attentional set-shifting test or shock-probe defensive burying test, respectively. Extinction reversed the CUS-induced deficits in cognitive flexibility and coping behavior, and increased phosphorylation of ribosomal protein S6 in the medial prefrontal cortex (mPFC) of stress-compromised rats, suggesting a role for activity-dependent protein synthesis in the therapeutic effect. Inhibiting protein synthesis by microinjecting anisomycin into mPFC blocked the therapeutic effect of extinction on cognitive flexibility. These results demonstrate the utility of extinction as a model by which to study mechanisms underlying exposure therapy, and suggest these mechanisms involve protein synthesis in the mPFC, the further study of which may identify novel therapeutic targets.