Jennifer J. Donegan

Chronic Intermittent Cold Stress Sensitizes Neuro-Immune Reactivity in the Rat Brain

Chronic stress contributes to many neuropsychiatric disorders in which the HPA axis, cognition and neuro-immune activity are dysregulated. Patients with major depression, or healthy individuals subjected to acute stress, present elevated levels of circulating pro-inflammatory markers. Acute stress also activates pro-inflammatory signals in the periphery and in the brain of rodents. However, despite the clear relevance of chronic stress to human psychopathology, the effects of prolonged stress exposure on central immune activity and reactivity have not been well characterized. Our laboratory has previously shown that, in rats, chronic intermittent cold stress (CIC stress, 4°C, 6h/day, 14 days) sensitizes the HPA response to a subsequent novel stressor, and produces deficits in a test of cognitive flexibility that is dependent upon prefrontal cortical function. We have hypothesized that CIC stress could potentially exert some of these effects by altering the neuro-immune status of the brain, leading to neuronal dysfunction. In this study, we have begun to address this question by determining whether previous exposure to CIC stress could alter the subsequent neuro-immune response to an acute immunological challenge (lipopolysaccharide, LPS) or an acute heterologous stressor (footshock). We examined the response of the pro-inflammatory cytokines, IL1β and IL6, the enzyme cyclooxygenase 2, and the chemokines, CXCL1 and MCP-1 in plasma, hypothalamus and prefrontal cortex. There was no effect of CIC stress on basal expression of these markers 24h after the termination of stress. However, CIC stress enhanced the acute induction of the pro-inflammatory cytokines, IL1β and particularly IL6, and the chemokines, CXCL1 and MCP-1, in plasma, hypothalamus and prefrontal cortex in response to LPS, and also sensitized the hypothalamic IL1β response to acute footshock. Thus, sensitization of acute pro-inflammatory responses in the brain could potentially mediate some of the CIC-dependent changes in HPA and cognitive function.

Activation of a ventral hippocampus–medial prefrontal cortex pathway is both necessary and sufficient for an antidepressant response to ketamine

A single sub-anesthetic dose of ketamine exerts rapid and sustained antidepressant effects. Here, we examined the role of the ventral hippocampus (vHipp)-medial prefrontal cortex (mPFC) pathway in ketamine’s antidepressant response. Inactivation of the vHipp with lidocaine prevented the sustained, but not acute, antidepressant-like effect of ketamine as measured by the forced swim test (FST). Moreover, optogenetic as well as pharmacogenetic specific activation of the vHipp–mPFC pathway using DREADDs (designer receptors exclusively activated by designer drugs) mimicked the antidepressant-like response to ketamine; importantly, this was pathway specific, in that activation of a vHipp to nucleus accumbens circuit did not do this. Furthermore, optogenetic inactivation of the vHipp/mPFC pathway at the time of FST completely reversed ketamine’s antidepressant response. In addition, we found that a transient increase in TrkB receptor phosphorylation in the vHipp contributes to ketamine’s sustained antidepressant response. These data demonstrate that activity in the vHipp–mPFC pathway is both necessary and sufficient for the antidepressant-like effect of ketamine.

A novel role for brain interleukin-6: Facilitation of cognitive flexibility in rat orbitofrontal cortex

Cytokines, small proteins released by the immune system to combat infection, are typically studied under inflammatory conditions. However, these molecules are also expressed in the brain in basal, nonpathological states, where they can regulate neuronal processes, such as learning and memory. However, little is known about how cytokine signaling in the brain may influence higher-order cognitive functions. Cognitive flexibility is one such executive process, mediated by the prefrontal cortex, which requires an adaptive modification of learned behaviors in response to environmental change. We explored the role of basal IL-6 signaling in the orbitofrontal cortex (OFC) in reversal learning, a form of cognitive flexibility that can be measured in the rat using the attentional set-shifting test. We found that inhibiting IL-6 or its downstream JAK/STAT signaling pathway in the OFC impaired reversal learning, suggesting that basal IL-6 and JAK/STAT signaling facilitate cognitive flexibility. Further, we demonstrated that elevating IL-6 in the OFC by adeno-associated virus-mediated gene delivery reversed a cognitive deficit induced by chronic stress, thus identifying IL-6 and the downstream JAK/STAT signaling pathway as potentially novel therapeutic targets for the treatment of stress-related psychiatric diseases associated with cognitive dysfunction.

Influence of hypothalamic IL-6/gp130 receptor signaling on the HPA axis response to chronic stress

Abnormal basal activity and stress-evoked reactivity of the hypothalamic-pituitary-adrenal (HPA) axis are often seen in depression, implicating HPA axis dysfunction as a potentially causative or exacerbating factor. Chronic stress is also a factor in depression, but it is not known what may underlie the shift from adaptive to maladaptive HPA activity over the course of chronic stress. Interleukin 6 (IL-6), a stress-inducible cytokine that signals through gp130 and IL-6Rα receptors to activate the JAK/STAT3 signaling cascade, is elevated in some subtypes of depression, and may have a modulatory effect on HPA activation, raising the possibility that IL-6 contributes to depression through effects on the HPA axis. In this study, we examined the effects of three different stress modalities, acute footshock, chronic intermittent cold (CIC) stress and chronic unpredictable stress (CUS) on IL-6 signaling in the hypothalamus. We also investigated whether IL-6 modulates the HPA response to chronic stress, by blocking IL-6 signaling in the brain during CIC stress using either a neutralizing antibody or an inhibitor of STAT3 phosphorylation. We show that IL-6 and STAT3 in the hypothalamus are activated in response to footshock and CUS. We also found that basal IL-6 signaling through the JAK/STAT3 pathway is required for the sustained CORT response to chronic, but not acute, cold stress and therefore is a potential determinant of plasticity in the HPA axis specifically during chronic stress exposure.

Stem cell-derived interneuron transplants as a treatment for schizophrenia: preclinical validation in a rodent model

An increasing literature suggests that schizophrenia is associated with a reduction in hippocampal interneuron function. Thus, we posit that stem cell-derived interneuron transplants may be an effective therapeutic strategy to reduce hippocampal hyperactivity and attenuate behavioral deficits in schizophrenia. Here we used a dual-reporter embryonic stem cell line to generate enriched populations of parvalbumin (PV)- or somatostatin (SST)-positive interneurons, which were transplanted into the ventral hippocampus of the methylazoxymethanol rodent model of schizophrenia. These interneuron transplants integrate within the existing circuitry, reduce hippocampal hyperactivity and normalize aberrant dopamine neuron activity. Further, interneuron transplants alleviate behaviors that model negative and cognitive symptoms, including deficits in social interaction and cognitive inflexibility. Interestingly, PV- and SST-enriched transplants produced differential effects on behavior, with PV-enriched populations effectively normalizing all the behaviors examined. These data suggest that the stem cell-derived interneuron transplants may represent a novel therapeutic strategy for schizophrenia.

Interleukin-6 Attenuates Serotonin 2A Receptor Signaling by Activating the JAK-STAT Pathway

The serotonin 2A (5-HT2A) receptor and the proinflammatory cytokine, interleukin-6 (IL-6), have both been implicated in psychiatric disorders. Previously, we demonstrated that these molecules both facilitate cognitive flexibility, a prefrontal cortex-mediated executive function impaired in multiple mental illnesses. In this study, we tested the hypothesis that IL-6 influences 5-HT2A receptor signaling, providing a potential mechanism by which this cytokine may influence behavior. We first demonstrated that 5-HT2A receptors and IL-6–mediated STAT3 phosphorylation colocalize in cells of the prefrontal cortex, providing the neuroanatomical substrate for a potential interaction. In the neuronally derived A1A1 cell line, which expresses both IL-6 and 5-HT2A receptors, we found that IL-6 attenuates inositol phosphate (IP) accumulation in response to the 5-HT2 agonist, 2,5-dimethoxy-4-iodoamphetamine (DOI), suggesting that IL-6 can regulate 5-HT2A receptor function. To identify the signaling pathway(s) that mediate this effect, we measured DOI-mediated IP accumulation in the presence of IL-6 and either the JAK-STAT inhibitor 124 [(9β,10α,16α,23E)-2,16,20,25-tetrahydroxy-9-methyl-19-norlanosta-1,5,23-triene-3,11,22-trione], JSI-124, or the extracellular signal-regulated kinase inhibitor, 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one (PD-98059). The IL-6 effect was blocked by JSI-124 but not PD-98059. Furthermore, silencing RNA knockdown of either JAK or STAT blocked the IL-6 effect, suggesting that IL-6–induced JAK-STAT activation can regulate 5-HT2A receptor signaling. Finally, to determine if IL-6 specifically regulates the 5-HT2A receptor system, we measured IP production mediated by another Gq-coupled receptor, bradykinin B2. IL-6 had no effect on bradykinin-mediated IP accumulation, suggesting that regulation may occur at the 5-HT2A receptor. These results may provide clues to the pathologic mechanisms underlying certain psychiatric disorders and may suggest novel therapeutic strategies for their treatment.

Hippocampal Perineuronal Nets Are Required for the Sustained Antidepressant Effect of Ketamine

Abstract Background N-methyl-D-aspartate receptor antagonists, like ketamine, produce a rapid-acting and long-lasting antidepressant effect. Although the mechanism is not completely understood, ketamine is thought to preferentially target N-methyl-D-aspartate receptors on fast-spiking parvalbumin-containing interneurons. The function of parvalbumin-containing interneurons is dependent on perineuronal nets, a specialized form of extracellular matrix that surrounds these cells. Methods Chondroitinase was used to enzymatically degrade perineuronal nets surrounding parvalbumin-containing interneurons in the ventral hippocampus, a region that is involved in the antidepressant response to ketamine. Rats were tested on the forced swim test 30 minutes and 1 week after ketamine administration. Results Thirty minutes after ketamine injection, both chondroitinase-treated and control animals had a decrease in immobility. One week later, however, the antidepressant-like response observed with ketamine was completely abolished in the chondroitinase-treated animals. Conclusion This suggests that parvalbumin interneuron function in the ventral hippocampus is essential for the sustained antidepressant effect of ketamine.

Cell-based therapies for the treatment of schizophrenia.

Schizophrenia is a devastating psychiatric disorder characterized by positive, negative and cognitive symptoms. While aberrant dopamine system function is typically associated with the positive symptoms of the disease, it is thought that this is secondary to pathology in afferent regions. Indeed, schizophrenia patients show dysregulated activity in the hippocampus and prefrontal cortex, two regions known to regulate dopamine neuron activity. These deficits in hippocampal and prefrontal cortical function are thought to result, in part, from reductions in inhibitory interneuron function in these brain regions. Therefore, it has been hypothesized that restoring interneuron function in the hippocampus and/or prefrontal cortex may be an effective treatment strategy for schizophrenia. In this article, we will discuss the evidence for interneuron pathology in schizophrenia and review recent advances in our understanding of interneuron development. Finally, we will explore how these advances have allowed us to test the therapeutic value of interneuron transplants in multiple preclinical models of schizophrenia. This article is part of a Special Issue entitled SI:StemsCellsinPsychiatry.

Embryonic stem cell transplants as a therapeutic strategy in a rodent model of autism

Autism is a neurodevelopmental disorder characterized by disruptions in three core behavioral domains: deficits in social interaction, impairments in communication, and repetitive and stereotyped patterns of behavior or thought. There are currently no drugs available for the treatment of the core symptoms of ASD and drugs that target comorbid symptoms often have serious adverse side effects, suggesting an urgent need for new therapeutic strategies. The neurobiology of autism is complex, but converging evidence suggests that ASD involves disruptions in the inhibitory GABAergic neurotransmitter system. Specifically, people with autism have a reduction in parvalbumin (PV)-containing interneurons in the PFC, leading to the suggestion that restoring interneuron function in this region may be a novel therapeutic approach for ASD. Here we used a dual-reporter embryonic stem cell line to generate enriched populations of PV-positive interneurons, which were transplanted into the medial prefrontal cortex (mPFC) of the Poly I:C rodent model of autism. PV interneuron transplants were able to decrease pyramidal cell firing in the mPFC and alleviated deficits in social interaction and cognitive flexibility. Our results suggest that restoring PV interneuron function in the mPFC may be a novel and effective treatment strategy to reduce the core symptoms of autism.

Stem cell-derived interneuron transplants functionally integrate within the existing circuitry

Stem cell-derived interneuron transplants functionally integrate within the existing circuitry