Chronic stress alters inhibitory networks in the medial prefrontal cortex of adult mice

Stress-Induced Neuron Remodeling Reveals Differential Interplay Between Neurexin and Environmental Factors in Caenorhabditis elegans

Neurexins are neuronal adhesion molecules important for synapse maturation, function, and plasticity. Neurexins have been genetically associated with neurodevelopmental disorders, including autism spectrum disorders (ASDs) and schizophrenia, but can have variable penetrance and phenotypic severity. Heritability studies indicate that a significant percentage of risk for ASD and schizophrenia includes environmental factors, highlighting a poorly understood interplay between genetic and environmental factors. The singular Caenorhabditis elegans ortholog of human neurexins, nrx-1, controls experience-dependent morphologic remodeling of a GABAergic neuron in adult males. Here, I show remodeling of this neuron's morphology in response to each of three environmental stressors (nutritional, heat, or genotoxic stress) when applied specifically during sexual maturation. Increased outgrowth of axon-like neurites following adolescent stress is the result of an altered morphologic plasticity in adulthood. Despite remodeling being induced by each of the three stressors, only nutritional stress affects downstream behavior and is dependent on neurexin/nrx-1 Heat or genotoxic stress in adolescence does not alter behavior despite inducing GABAergic neuron remodeling, in a neurexin/nrx-1 independent fashion. Starvation-induced remodeling is also dependent on neuroligin/nlg-1, the canonical binding partner for neurexin/nrx-1, and the transcription factors FOXO/daf-16 and HSF1/hsf-1hsf-1 and daf-16, in addition, each have unique roles in remodeling induced by heat and UV stress. The differential molecular mechanisms underlying GABAergic neuron remodeling in response to different stressors, and the disparate effects of stressors on downstream behavior, are a paradigm for understanding how genetics, environmental exposures, and plasticity may contribute to brain dysfunction in ASDs and schizophrenia.

"Braking" the Prefrontal Cortex: The Role of Glucocorticoids and Interneurons in Stress Adaptation and Pathology

The medial prefrontal cortex (mPFC) receives information regarding stimuli and appropriately orchestrates neurophysiological, autonomic, and behavioral responses to stress. The cellular and neurochemical heterogeneity of the mPFC and its projections are key to fine-tuning of stress responses and adaptation. Output of the mPFC is mediated by glutamatergic pyramidal neurons whose activity is coordinated by an intricate network of interneurons. Excitatory/inhibitory (E/I) balance in the mPFC is critical for appropriate responsiveness to stress, and E/I imbalance occurs in numerous neuropsychiatric disorders that co-occur with chronic stress. Moreover, there is mounting data suggesting that chronic stress may precipitate E/I imbalance. This review will provide information regarding the cellular and anatomical makeup of the mPFC and discuss the impact of acute and chronic stress in adulthood and early life on interneuron function, with implications for E/I balance affecting functional connectivity. Specifically, the review will highlight the importance of interneuron type, connectivity, and location (both layer- and subregion-specific). The discussion of local mPFC networks will focus on stress context, including stressor duration (acute vs. chronic) and timing (early life vs. adulthood), as these factors have significant implications for the interpretation of experiments and mPFC E/I balance. Indeed, interneurons appear to play a prominent role in prefrontal adaptation, and a better understanding of the interactions between stress and interneuron function may yield insight to the transition from adaptation to pathology. Ultimately, determining the mechanisms mediating adaptive versus pathologic plasticity will promote the development of novel treatments for neuropsychiatric disorders related to prefrontal E/I imbalance.

Neurovascular Coupling under Chronic Stress Is Modified by Altered GABAergic Interneuron Activity

Neurovascular coupling (NVC), the interaction between neural activity and vascular response, ensures normal brain function by maintaining brain homeostasis. We previously reported altered cerebrovascular responses during functional hyperemia in chronically stressed animals. However, the underlying neuronal-level changes associated with those hemodynamic changes remained unclear. Here, using in vivo and ex vivo experiments, we investigate the neuronal origins of altered NVC dynamics under chronic stress conditions in adult male mice. Stimulus-evoked hemodynamic and neural responses, especially beta and gamma-band local field potential activity, were significantly lower in chronically stressed animals, and the NVC relationship, itself, had changed. Further, using acute brain slices, we discovered that the underlying cause of this change was dysfunction of neuronal nitric oxide synthase (nNOS)-mediated vascular responses. Using FISH to check the mRNA expression of several GABAergic subtypes, we confirmed that only nNOS mRNA was significantly decreased in chronically stressed mice. Ultimately, chronic stress impairs NVC by diminishing nNOS-mediated vasodilation responses to local neural activity. Overall, these findings provide useful information in understanding NVC dynamics in the healthy brain. More importantly, this study reveals that impaired nNOS-mediated NVC function may be a contributory factor in the progression of stress-related diseases.

SIGNIFICANCE STATEMENT The correlation between neuronal activity and cerebral vascular dynamics is defined as neurovascular coupling (NVC), which plays an important role for meeting the metabolic demands of the brain. However, the impact of chronic stress, which is a contributory factor of many cerebrovascular diseases, on NVC is poorly understood. We therefore investigated the effects of chronic stress on impaired neurovascular response to sensory stimulation and their underlying mechanisms. Multimodal approaches, from in vivo hemodynamic imaging and electrophysiology to ex vivo vascular imaging with pharmacological treatment, patch-clamp recording, FISH, and immunohistochemistry revealed that chronic stress-induced dysfunction of nNOS-expressing interneurons contributes to NVC impairment. These findings will provide useful information to understand the role of nNOS interneurons in NVC in normal and pathological conditions.

Fluoxetine-induced dematuration of hippocampal neurons and adult cortical neurogenesis in the common marmoset

The selective serotonin reuptake inhibitor fluoxetine (FLX) is widely used to treat depression and anxiety disorders. Chronic FLX treatment reportedly induces cellular responses in the brain, including increased adult hippocampal and cortical neurogenesis and reversal of neuron maturation in the hippocampus, amygdala, and cortex. However, because most previous studies have used rodent models, it remains unclear whether these FLX-induced changes occur in the primate brain. To evaluate the effects of FLX in the primate brain, we used immunohistological methods to assess neurogenesis and the expression of neuronal maturity markers following chronic FLX treatment (3 mg/kg/day for 4 weeks) in adult marmosets (n = 3 per group). We found increased expression of doublecortin and calretinin, markers of immature neurons, in the hippocampal dentate gyrus of FLX-treated marmosets. Further, FLX treatment reduced parvalbumin expression and the number of neurons with perineuronal nets, which indicate mature fast-spiking interneurons, in the hippocampus, but not in the amygdala or cerebral cortex. We also found that FLX treatment increased the generation of cortical interneurons; however, significant up-regulation of adult hippocampal neurogenesis was not observed in FLX-treated marmosets. These results suggest that dematuration of hippocampal neurons and increased cortical neurogenesis may play roles in FLX-induced effects and/or side effects. Our results are consistent with those of previous studies showing hippocampal dematuration and increased cortical neurogenesis in FLX-treated rodents. In contrast, FLX did not affect hippocampal neurogenesis or dematuration of interneurons in the amygdala and cerebral cortex.

Prefrontal excitatory/inhibitory balance in stress and emotional disorders: Evidence for over-inhibition

Chronic stress-induced emotional disorders like anxiety and depression involve imbalances between the excitatory glutamatergic system and the inhibitory GABAergic system in the prefrontal cortex (PFC). However, the precise nature and trajectory of excitatory/inhibitory (E/I) imbalances in these conditions is not clear, with the literature reporting glutamatergic and GABAergic findings that are at times contradictory and inconclusive. Here we propose and discuss the hypothesis that chronic stress-induced emotional dysfunction involves hypoactivity of the PFC due to increased inhibition. We will also discuss E/I imbalances in the context of sex differences. In this review, we will synthesize research about how glutamatergic and GABAergic systems are perturbed by chronic stress and in related emotional disorders like anxiety and depression and propose ideas for reconciling contradictory findings in support of the hypothesis of over-inhibition. We will also discuss evidence for how aspects of the GABAergic system such as parvalbumin (PV) cells can be targeted therapeutically for reinstating activity and plasticity in the PFC and treating stress-related disorders.

Acute stress-induced change in polysialic acid levels mediated by sialidase in mouse brain

Stress is an important environmental factor influencing human behaviour and causing several mental disorders. Alterations in the structure of polysialic acid (polySia/PSA) due to genetic alterations in ST8SIA2, which encodes a polySia-synthesizing enzyme, are related to certain mental disorders. However, whether stress as an environmental factor leads to changes in polySia structure is unknown. Here we studied the effects of acute stress on polySia expression and found reductions in both the quantity and quality of polySia in the olfactory bulb and prefrontal cortex, even with short-term exposure to acute stress. The use of inhibitors for sialidase, microglia and astrocytes revealed that these declines were due to a transient action of sialidase from microglia and astrocytes in the olfactory bulb and prefrontal cortex, respectively. These data suggest that sialidase dynamically regulates polySia expression in a brain region-specific manner.

Chronic Stress Modulates Interneuronal Plasticity: Effects on PSA-NCAM and Perineuronal Nets in Cortical and Extracortical Regions

Chronic stress has an important impact on the adult brain. However, most of the knowledge on its effects is focused on principal neurons and less on inhibitory neurons. Consequently, recent reports have begun to describe stress-induced alterations in the structure, connectivity and neurochemistry of interneurons. Some of these changes appear to be mediated by certain molecules particularly associated to interneurons, such as the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) and components of the perineuronal nets (PNN), specialized regions of the extracellular matrix. These plasticity-related molecules modulate interneuronal structure and connectivity, particularly of parvalbumin expressing basket interneurons, both during development and adult life. These inhibitory neurons are specially affected after chronic stress and in some stress-related disorders, in which the expression of PSA-NCAM and certain components of PNN are also altered. For these reasons we have decided to study PSA-NCAM, PNN and parvalbumin expressing interneurons after 10 days of chronic restraint stress, a time point in which its behavioral consequences are starting to appear. We have focused initially on the medial prefrontal cortex (mPFC), basolateral amygdala (BLA) and hippocampus, regions affected by stress and stress-related psychiatric diseases, but we have also explored the habenula and the thalamic reticular nucleus (TRN) due to the important presence of PNN and their relationship with certain disorders. PSA-NCAM expression was increased by stress in the stratum lacunosum-moleculare of CA1. Increases in parvalbumin immunoreactive cells were detected in the mPFC and the BLA, but were not accompanied by increases in the number of parvalbumin expressing perisomatic puncta on the somata of principal neurons. The number of PNN was also increased in the mPFC and the habenula, although habenular PNN were not associated to parvalbumin cells. Increased expression of parvalbumin and components of PNN were also detected in the TRN after chronic restraint stress, revealing for the first time substantial effects on this region. Our study shows that, even a short chronic stress protocol, can induce consistent changes in interneuronal plasticity-related molecules in cortical and extracortical regions, which may represent initial responses of inhibitory circuits to counteract the effects of this aversive experience.

Medial prefrontal cortex neuropeptide Y modulates binge-like ethanol consumption in C57BL/6J mice

Neuropeptide Y (NPY) signaling via limbic NPY1 and 2 receptors (NPY1R and NPY2R, respectively) is known to modulate binge-like ethanol consumption in rodents. However, the role of NPY signaling in the medial prefrontal cortex (mPFC), which provides top-down modulation of the limbic system, is unknown. Here, we used "drinking-in-the-dark" (DID) procedures in C57BL/6J mice to address this gap in the literature. First, the impact of DID on NPY immunoreactivity (IR) was assessed in the mPFC. Next, the role of NPY1R and NPY2R signaling in the mPFC on ethanol consumption was evaluated through site-directed pharmacology. Chemogenetic inhibition of NPY1R+ neurons in the mPFC was performed to further evaluate the role of this population. To determine the potential role of NPY1R+ neurons projecting from the mPFC to the basolateral amygdala (BLA) this efferent population was selectively silenced. Three, 4-day cycles of DID reduced NPY IR in the mPFC. Intra-mPFC activation of NPY1R and antagonism of NPY2R resulted in decreased binge-like ethanol intake. Silencing of mPFC NPY1R+ neurons overall, and specifically NPY1R+ neurons projecting to the BLA, significantly reduced binge-like ethanol intake. We provide novel evidence that (1) binge-like ethanol intake reduces NPY levels in the mPFC; (2) activation of NPY1R or blockade of NPY2R reduces binge-like ethanol intake; and (3) chemogenetic inhibition of NPY1R+ neurons in the mPFC and NPY1R+ mPFC neurons projecting to the BLA blunts binge-like drinking. These observations provide the first direct evidence that NPY signaling in the mPFC modulates binge-like ethanol consumption.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Cortical GABAergic Dysfunction in Stress and Depression: New Insights for Therapeutic Interventions

Major depressive disorder (MDD) is a debilitating illness characterized by neuroanatomical and functional alterations in limbic structures, notably the prefrontal cortex (PFC), that can be precipitated by exposure to chronic stress. For decades, the monoaminergic deficit hypothesis of depression provided the conceptual framework to understand the pathophysiology of MDD. However, accumulating evidence suggests that MDD and chronic stress are associated with an imbalance of excitation-inhibition (E:I) within the PFC, generated by a deficit of inhibitory synaptic transmission onto principal glutamatergic neurons. MDD patients and chronically stressed animals show a reduction in GABA and GAD67 levels in the brain, decreased expression of GABAergic interneuron markers, and alterations in GABA_A and GABA_B receptor levels. Moreover, genetically modified animals with deletion of specific GABA receptors subunits or interneuron function show depressive-like behaviors. Here, we provide further evidence supporting the role of cortical GABAergic interneurons, mainly somatostatin- and parvalbumin-expressing cells, required for the optimal E:I balance in the PFC and discuss how the malfunction of these cells can result in depression-related behaviors. Finally, considering the relatively low efficacy of current available medications, we review new fast-acting pharmacological approaches that target the GABAergic system to treat MDD. We conclude that deficits in cortical inhibitory neurotransmission and interneuron function resulting from chronic stress exposure can compromise the integrity of neurocircuits and result in the development of MDD and other stress-related disorders. Drugs that can establish a new E:I balance in the PFC by targeting the glutamatergic and GABAergic systems show promising as fast-acting antidepressants and represent breakthrough strategies for the treatment of depression.

Effects of the Genetic Depletion of Polysialyltransferases on the Structure and Connectivity of Interneurons in the Adult Prefrontal Cortex

Polysialic acid (polySia) is a complex sugar that in the nervous system appears mainly as a posttranslational modification of the neural cell adhesion molecule (NCAM). PolySia plays important roles during brain development, but also in its plasticity during adulthood. Two polysialyltransferases (polyST), ST8SIA2 and ST8SIA4, are involved in the synthesis and attachment of polySia. Both polyST are relevant for developmental migration of cortical interneurons and their establishment in the prefrontal cortex (PFC). In contrast, only ST8SIA4 appears to be important for the structural plasticity of a subpopulation of cortical interneurons in the adult. Interestingly, ST8SIA2 and NCAM are candidate genes for schizophrenia, a disorder in which interneuronal circuits are altered. However, there is still no data on the effects of polyST depletion on the dendritic structure or the connectivity of cortical interneurons. Here, we studied the contribution of each polyST on these parameters in the medial PFC (mPFC) of polyST knock-out mice with GAD67-GFP-labeled interneurons. Genetic depletion of ST8SIA4, but not ST8SIA2, resulted in a decrease in the complexity of the dendritic arbor of interneurons. In contrast, ablation of either of the two polyST induced a decrease in the density of parvalbumin (PV) expressing perisomatic puncta on pyramidal neurons. Thus, the depletion of each polyST results in similar impairments of not only developmental migration but also efferent synaptic connectivity of interneurons. In contrast, the loss of ST8SIA4 has a unique effect on dendritic structure, hence on afferent connectivity, suggesting differential and independent contributions of each polyST to neuritogenesis and synaptogenesis.

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

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

Neurobiological Mechanisms of Stress Resilience and Implications for the Aged Population

BACKGROUND

Stress is a common reaction to an environmental adversity, but a dysregulation of the stress response can lead to psychiatric illnesses such as major depressive disorder (MDD), post-traumatic stress disorder (PTSD), and anxiety disorders. Yet, not all individuals exposed to stress will develop psychiatric disorders; those with enhanced stress resilience mechanisms have the ability to adapt successfully to stress without developing persistent psychopathology. Notably, the potential to enhance stress resilience in at-risk populations may prevent the onset of stress-induced psychiatric disorders. This novel idea has prompted a number of studies probing the mechanisms of stress resilience and how it can be manipulated.

METHODS

Here, we review the neurobiological factors underlying stress resilience, with particular focus on the serotoninergic (5-HT), glutamatergic, and γ-Aminobutyric acid (GABA) systems, as well as the hypothalamic-pituitary axis (HPA) in rodents and in humans. Finally, we discuss stress resiliency in the context of aging, as the likelihood of mood disorders increases in older adults.

RESULTS

Interestingly, increased resiliency has been shown to slow aging and improved overall health and quality of life. Research in the neurobiology of stress resilience, particularly throughout the aging process, is a nascent, yet, burgeoning field.

CONCLUSION

Overall, we consider the possible methods that may be used to induce resilient phenotypes, prophylactically in at-risk populations, such as in military personnel or in older MDD patients. Research in the mechanisms of stress resilience may not only elucidate novel targets for antidepressant treatments, but also provide novel insight about how to prevent these debilitating disorders from developing.

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.

Difference in Perseverative Errors during a Visual Attention Task with Auditory Distractors in Alpha-9 Nicotinic Receptor Subunit Wild Type and Knock-Out Mice

The auditory efferent system is a neural network that originates in the auditory cortex and projects to the cochlear receptor through olivocochlear (OC) neurons. Medial OC neurons make cholinergic synapses with outer hair cells (OHCs) through nicotinic receptors constituted by α9 and α10 subunits. One of the physiological functions of the α9 nicotinic receptor subunit (α9-nAChR) is the suppression of auditory distractors during selective attention to visual stimuli. In a recent study we demonstrated that the behavioral performance of alpha-9 nicotinic receptor knock-out (KO) mice is altered during selective attention to visual stimuli with auditory distractors since they made less correct responses and more omissions than wild type (WT) mice. As the inhibition of the behavioral responses to irrelevant stimuli is an important mechanism of the selective attention processes, behavioral errors are relevant measures that can reflect altered inhibitory control. Errors produced during a cued attention task can be classified as premature, target and perseverative errors. Perseverative responses can be considered as an inability to inhibit the repetition of an action already planned, while premature responses can be considered as an index of the ability to wait or retain an action. Here, we studied premature, target and perseverative errors during a visual attention task with auditory distractors in WT and KO mice. We found that α9-KO mice make fewer perseverative errors with longer latencies than WT mice in the presence of auditory distractors. In addition, although we found no significant difference in the number of target error between genotypes, KO mice made more short-latency target errors than WT mice during the presentation of auditory distractors. The fewer perseverative error made by α9-KO mice could be explained by a reduced motivation for reward and an increased impulsivity during decision making with auditory distraction in KO mice.

Characterization of GABAergic marker expression in the chronic unpredictable stress model of depression

Evidence continues to build suggesting that the GABAergic neurotransmitter system is altered in brains of patients with major depressive disorder. However, there is little information available related to the extent of these changes or the potential mechanisms associated with these alterations. As stress is a well-established precipitant to depressive episodes, we sought to explore the impact of chronic stress on GABAergic interneurons. Using western blot analyses and quantitative real-time PCR (qPCR) we assessed the effects of five-weeks of chronic unpredictable stress (CUS) exposure on the expression of GABA-synthesizing enzymes (GAD₆₅ and GAD₆₇), calcium-binding proteins (calbindin (CB), parvalbumin (PV) and calretinin (CR)), and neuropeptides co-expressed in GABAergic neurons (somatostatin (SST), neuropeptide Y (NPY), vasoactive intestinal peptide (VIP) and cholecystokinin (CCK)) in the prefrontal cortex (PFC) and hippocampus (HPC) of rats. We also investigated the effects of corticosterone (CORT) and dexamethasone (DEX) exposure on these markers in vitro in primary cortical and hippocampal cultures. We found that CUS induced significant reductions of GAD₆₇ protein levels in both the PFC and HPC of CUS-exposed rats, but did not detect changes in GAD₆₅ protein expression. Similar protein expression changes were found in vitro in cortical neurons. In addition, our results provide clear evidence of reduced markers of interneuron population(s), namely SST and NPY, in the PFC, suggesting these cell types may be selectively vulnerable to chronic stress. Together, this work highlights that chronic stress induces regional and cell type-selective effects on GABAergic interneurons in rats. These findings provide additional supporting evidence that stress-induced GABA neuron dysfunction and cell vulnerability play critical roles in the pathophysiology of stress-related illnesses, including major depressive disorder.

Early Social Isolation Stress and Perinatal NMDA Receptor Antagonist Treatment Induce Changes in the Structure and Neurochemistry of Inhibitory Neurons of the Adult Amygdala and Prefrontal Cortex

The exposure to aversive experiences during early life influences brain development and leads to altered behavior. Moreover, the combination of these experiences with subtle alterations in neurodevelopment may contribute to the emergence of psychiatric disorders, such as schizophrenia. Recent hypotheses suggest that imbalances between excitatory and inhibitory (E/I) neurotransmission, especially in the prefrontal cortex and the amygdala, may underlie their etiopathology. In order to understand better the neurobiological bases of these alterations, we studied the impact of altered neurodevelopment and chronic early-life stress on these two brain regions. Transgenic mice displaying fluorescent excitatory and inhibitory neurons, received a single injection of MK801 (NMDAR antagonist) or vehicle solution at postnatal day 7 and/or were socially isolated from the age of weaning until adulthood (3 months old). We found that anxiety-related behavior, brain volume, neuronal structure, and the expression of molecules related to plasticity and E/I neurotransmission in adult mice were importantly affected by early-life stress. Interestingly, many of these effects were potentiated when the stress paradigm was applied to mice perinatally injected with MK801 ("double-hit" model). These results clearly show the impact of early-life stress on the adult brain, especially on the structure and plasticity of inhibitory networks, and highlight the double-hit model as a valuable tool to study the contribution of early-life stress in the emergence of neurodevelopmental psychiatric disorders, such as schizophrenia.

Changes in the Prefrontal Glutamatergic and Parvalbumin Systems of Mice Exposed to Unpredictable Chronic Stress

The prefrontal cortex (PFC) is highly sensitive to the effects of stress, a known risk factor of mood disorders including anxiety and depression. Abnormalities in PFC functioning have been well described in humans displaying stress-induced depressive symptoms, and hypoactivity of the PFC is now recognized to be a key feature of the depressed brain. However, little is known about the causes and mechanisms leading to this altered prefrontal functional activity in the context of stress-related mood disorders. We previously showed that unpredictable chronic mild stress (UCMS) in mice increases prefrontal expression of parvalbumin (PV), an activity-dependent calcium-binding albumin protein expressed in a specific subtype of GABAergic neurons, highlighting a potential mechanism through which chronic stress leads to hypofunction of the PFC. In this study, we aimed to investigate the mechanisms by which chronic stress alters the prefrontal GABA system. We hypothesized that chronic stress-induced enhancement of glutamatergic transmission in the PFC is a crucial contributing factor to changes within the prefrontal GABAergic and, specifically, PV system. BALB/c male and female mice were exposed to daily handling (control) or 2 or 4 weeks of UCMS. Female mice displayed a more severe altered phenotype than males, as shown by increased anxiety- and depressive-like behaviors and deficits in PFC-dependent cognitive abilities, particularly after exposure to 2 weeks of UCMS. This behavioral phenotype was paralleled by a large increase in prefrontal PV messenger RNA (mRNA) and number of PV-expressing neurons, supporting our previous findings. We further showed that the expression of pre- and postsynaptic markers of glutamatergic transmission (VGlut1 presynaptic terminals and pERK1/2, respectively) onto PV neurons was increased by 2 weeks of UCMS in a sex-specific manner; this was associated with sex-specific changes in the mRNA expression of the NR2B subunit of the NMDA receptor. These findings provide evidence of increased glutamatergic transmission onto prefrontal PV neurons, particularly in female mice, which could potentially contribute to their increased PV expression and the extent of their behavioral impairment following UCMS. Finally, our analysis of activity of subcortical regions sending glutamatergic afferents to the PFC reveals that glutamatergic neurons from the basolateral amygdala might be specifically involved in UCMS-induced changes in prefrontal glutamatergic transmission.

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.

Reduced interneuronal dendritic arborization in CA1 but not in CA3 region of mice subjected to chronic mild stress

INTRODUCTION

Chronic stress induces dendritic atrophy and decreases spine density in excitatory hippocampal neurons, although there is also ample evidence indicating that the GABAergic system is altered in the hippocampus after this aversive experience. Chronic stress causes dendritic remodeling both in excitatory neurons and interneurons in the medial prefrontal cortex and the amygdala.

METHODS

In order to know whether it also has an impact on the structure and neurotransmission of hippocampal interneurons, we have analyzed the dendritic arborization, spine density, and the expression of markers of inhibitory synapses and plasticity in the hippocampus of mice submitted to 21 days of mild restrain stress. The analyses were performed in GIN mice, a strain that displays EGFP-labeled interneurons.

RESULTS

We observed a significant decrease in the dendritic arborization of interneurons in the CA1 region, which did not occur in those in CA3. We found neither changes in dendritic spine density in these regions nor alterations in the number of EGFP-positive interneurons. Nevertheless, the expression of glutamic acid decarboxylase 67 was reduced in different layers of CA1 and CA3 regions of the hippocampus. No significant changes were found in the expression of the polysialylated form of the neural cell adhesion molecule (PSA-NCAM) or synaptophysin.

CONCLUSIONS

Chronic stress reduces the interneuronal dendritic arborization in CA1 region of the hippocampus but not in CA3.

Sex-Dependent Anti-Stress Effect of an α5 Subunit Containing GABAA Receptor Positive Allosteric Modulator

Rationale: Current first-line treatments for stress-related disorders such as major depressive disorder (MDD) act on monoaminergic systems and take weeks to achieve a therapeutic effect with poor response and low remission rates. Recent research has implicated the GABAergic system in the pathophysiology of depression, including deficits in interneurons targeting the dendritic compartment of cortical pyramidal cells.

Objectives: The present study evaluates whether SH-053-2’F-R-CH3 (denoted “α5-PAM”), a positive allosteric modulator selective for α5-subunit containing GABA_A receptors found predominantly on cortical pyramidal cell dendrites, has anti-stress effects.

Methods: Female and male C57BL6/J mice were exposed to unpredictable chronic mild stress (UCMS) and treated with α5-PAM acutely (30 min prior to assessing behavior) or chronically before being assessed behaviorally.

Results: Acute and chronic α5-PAM treatments produce a pattern of decreased stress-induced behaviors (denoted as “behavioral emotionality”) across various tests in female, but not in male mice. Behavioral Z-scores calculated across a panel of tests designed to best model the range and heterogeneity of human symptomatology confirmed that acute and chronic α5-PAM treatments consistently produce significant decreases in behavioral emotionality in several independent cohorts of females. The behavioral responses to α5-PAM could not be completely accounted for by differences in drug brain disposition between female and male mice. In mice exposed to UCMS, expression of the Gabra5 gene was increased in the frontal cortex after acute treatment and in the hippocampus after chronic treatment with α5-PAM in females only, and these expression changes correlated with behavioral emotionality.

Conclusion: We showed that acute and chronic positive modulation of α5 subunit-containing GABA_A receptors elicit anti-stress effects in a sex-dependent manner, suggesting novel therapeutic modalities.

The co-expression of GluN2B subunits of the NMDA receptors and glucocorticoid receptors after chronic restraint stress in low and high anxiety rats

The aim of this study was to assess the mechanisms underlying behavioural differences between high- (HR) and low- (LR) anxiety rats, selected according to their behaviour in the contextual fear test (i.e., the duration of the freezing response was used as a discriminating variable), after a chronic restraint procedure (21days, 3h daily). We analysed the expression of the GluN2B subunits of the NMDA and glucocorticoid receptors (GRs) in selected brain structures (immunofluorescence). Following chronic restraint stress in the HR rats, we observed a decrease in the expression of the GRs and GluN2B subunits of the NMDA receptor in the prefrontal cortical areas and the hippocampus compared to the HR-control and the LR-restraint groups. These effects coincided with an increase in passive depressive-like behaviour in the Porsolt test of the HR rats. Moreover, in the hippocampus, the HR-restraint animals demonstrated decreased glutamate levels and a decreased glutamate/glutamine ratio compared to the LR-restraint rats. Furthermore, the HR-restraint group had increased GRs/GluN2B subunits colocalisation in the basolateral amygdala (BLA) compared to the HR-control and the LR-restraint rats. The present results suggest that in HR rats exposed to chronic restraint stress, the hippocampal and cortical glutamatergic system components are changed. These effects could have a negative influence on the feedback mechanisms regulating the hypothalamic-pituitary-adrenal axis as well as on the behavioural processes expressed as depressive-like symptoms.

Chronic Stress Increases Prefrontal Inhibition: A Mechanism for Stress-Induced Prefrontal Dysfunction

BACKGROUND

Multiple neuropsychiatric disorders, e.g., depression, are linked to imbalances in excitatory and inhibitory neurotransmission and prefrontal cortical dysfunction, and are concomitant with chronic stress.

METHODS

We used electrophysiologic (n = 5-6 animals, 21-25 cells/group), neuroanatomic (n = 6-8/group), and behavioral (n = 12/group) techniques to test the hypothesis that chronic stress increases inhibition of medial prefrontal cortex (mPFC) glutamatergic output neurons.

RESULTS

Using patch clamp recordings from infralimbic mPFC pyramidal neurons, we found that chronic stress selectively increases the frequency of miniature inhibitory postsynaptic currents with no effect on amplitude, which suggests that chronic stress increases presynaptic gamma-aminobutyric acid release. Elevated gamma-aminobutyric acid release under chronic stress is accompanied by increased inhibitory appositions and terminals onto glutamatergic cells, as assessed by both immunohistochemistry and electron microscopy. Furthermore, chronic stress decreases glucocorticoid receptor immunoreactivity specifically in a subset of inhibitory neurons, which suggests that increased inhibitory tone in the mPFC after chronic stress may be caused by loss of a glucocorticoid receptor-mediated brake on interneuron activity. These neuroanatomic and functional changes are associated with impairment of a prefrontal-mediated behavior. During chronic stress, rats initially make significantly more errors in the delayed spatial win-shift task, an mPFC-mediated behavior, which suggests a diminished impact of the mPFC on decision making.

CONCLUSIONS

Taken together, the data suggest that chronic stress increases synaptic inhibition onto prefrontal glutamatergic output neurons, limiting the influence of the prefrontal cortex in control of stress reactivity and behavior. Thus, these data provide a mechanistic link among chronic stress, prefrontal cortical hypofunction, and behavioral dysfunction.

Adult microbiota-deficient mice have distinct dendritic morphological changes: differential effects in the amygdala and hippocampus

Increasing evidence implicates the microbiota in the regulation of brain and behaviour. Germ-free mice (GF; microbiota deficient from birth) exhibit altered stress hormone signalling and anxiety-like behaviours as well as deficits in social cognition. Although the mechanisms underlying the ability of the gut microbiota to influence stress responsivity and behaviour remain unknown, many lines of evidence point to the amygdala and hippocampus as likely targets. Thus, the aim of this study was to determine if the volume and dendritic morphology of the amygdala and hippocampus differ in GF versus conventionally colonized (CC) mice. Volumetric estimates revealed significant amygdalar and hippocampal expansion in GF compared to CC mice. We also studied the effect of GF status on the level of single neurons in the basolateral amygdala (BLA) and ventral hippocampus. In the BLA, the aspiny interneurons and pyramidal neurons of GF mice exhibited dendritic hypertrophy. The BLA pyramidal neurons of GF mice had more thin, stubby and mushroom spines. In contrast, the ventral hippocampal pyramidal neurons of GF mice were shorter, less branched and had less stubby and mushroom spines. When compared to controls, dentate granule cells of GF mice were less branched but did not differ in spine density. These findings suggest that the microbiota is required for the normal gross morphology and ultrastructure of the amygdala and hippocampus and that this neural remodelling may contribute to the maladaptive stress responsivity and behavioural profile observed in GF mice.

Chronic stress affects the number of GABAergic neurons in the orbitofrontal cortex of rats

Cortical GABAergic dysfunctions have been documented by clinical studies in major depression. We used here an animal model for depression and investigated whether long-term stress exposure can affect the number of GABAergic neurons in the orbitofrontal cortex (OFC). Adult male rats were subjected to 7-weeks of daily stress exposure and behaviorally phenotyped as anhedonic or stress-resilient animals. GABAergic interneurons were identified by immunohistochemistry and systematically quantified. We analyzed calbindin-(CB), calretinin-(CR), cholecystokinin-(CCK), parvalbumin-(PV), neuropeptide Y-(NPY) and somatostatin-positive (SST+) neurons in the following specific subareas of the OFC: medial orbital (MO), ventral orbital (VO), lateral orbital (LO) and dorsolateral orbital (DLO) cortex. For comparison, we also analyzed the primary motor cortex (M1) as a non-limbic cortical area. Stress had a pronounced effect on CB+ neurons and reduced their densities by 40-50% in the MO, VO and DLO. Stress had no effect on CCK+, CR+, PV+, NPY+ and SST+ neurons in any cortical areas. None of the investigated GABAergic neurons were affected by stress in the primary motor cortex. Interestingly, in the stress-resilient animals, we observed a significantly increased density of CCK+ neurons in the VO. NPY+ neuron densities were also significantly different between the anhedonic and stress-resilient rats, but only in the LO. Our present data demonstrate that chronic stress can specifically reduce the density of calbindin-positive GABAergic neurons in the orbitofrontal cortex and suggest that NPY and CCK expression in the OFC may relate to the stress resilience of the animals.

Social play in juvenile hamsters alters dendritic morphology in the medial prefrontal cortex and attenuates effects of social stress in adulthood

Social play is a fundamental aspect of behavioral development in many species. Social play deprivation in rats alters dendritic morphology in the ventromedial prefrontal cortex (vmPFC) and we have shown that this brain region regulates responses to social defeat stress in Syrian hamsters. In this study, we tested whether play deprivation during the juvenile period disrupts dendritic morphology in the prefrontal cortex and potentiates the effects of social defeat stress. At weaning, male hamsters were either group-housed with peers or pair-housed with their mother, with whom they do not play. In adulthood, animals received acute social defeat stress or no-defeat control treatment. The hamsters were then tested for a conditioned defeat response in a social interaction test with a novel intruder, and were also tested for social avoidance of a familiar opponent. Brains were collected for Golgi-Cox staining and analysis of dendritic morphology in the infralimbic (IL), prelimbic (PL), and orbitofrontal cortex (OFC). Play-deprived animals showed an increased conditioned defeat response and elevated avoidance of a familiar opponent compared with play-exposed animals. Furthermore, play-deprived animals showed increased total length and branch points in apical dendrites of pyramidal neurons in the IL and PL cortices, but not in the OFC. These findings suggest that social play deprivation in juvenile hamsters disrupts neuronal development in the vmPFC and increases vulnerability to the effects of social stress in adulthood. Overall, these results suggest that social play is necessary for the natural dendritic pruning process during adolescence and promotes coping with stress in adulthood. (PsycINFO Database Record

Brain glutamic acid decarboxylase-67kDa alterations induced by magnesium treatment in olfactory bulbectomy and chronic mild stress models in rats

BACKGROUND

The preclinical results indicate that magnesium, an N-methyl-d-aspartate receptor (NMDAR) blocker has anxiolytic and antidepressant-like activity. One of the mechanisms involved in these activities is modulation of glutamate, γ-aminobutyric acid (GABA) system. Based on this, the aim of the present study was to investigate the effect of magnesium on the level of glutamic acid decarboxylase-67kDa (GAD-67) in the different brain areas in the chronic mild stress (CMS) and olfactory bulbectomy (OB) models of depression in rats.

METHODS

Magnesium (15mg/kg) was administered intraperitonealy once daily for 14 days in the OB model and for 35 days in the CMS model. 24h after the last dose, the prefrontal cortex (PFC), hippocampus and amygdala were collected and the GAD-67 protein level was determined by the western blotting method.

RESULTS

In the OB model, chronic magnesium treatment normalized decreased by OB protein level of GAD-67 in PFC. CMS did not influence the GAD-67 protein level, however magnesium increased GAD-67 protein expression in amygdala and PFC of stress rats when compared to vehicle-treated stress group. OB or CMS models as well as magnesium treatment did not affect GAD-67 protein level in the hippocampus.

CONCLUSIONS

Obtained results indicate that the antidepressant-like activity of magnesium in CMS and OB models of depression is associated with an enhanced expression of GAD-67 in the PFC and amygdala.

Orbitofrontal cortex 5-HT2A receptor mediates chronic stress-induced depressive-like behaviors and alterations of spine density and Kalirin7

Neuroimaging studies show that patients with major depression have reduced volume of the orbitofrontal cortex (OFC). Although the serotonin (5-HT) 2A receptor, which is abundant in the OFC, has been implicated in depression, the underlying mechanisms in the development of stress-induced depression remain unclear. Kalirin-7 (Kal7) is an essential component of mature excitatory synapses for maintaining dendritic spines density, size and synaptic functions. The aim of this study was to investigate the role of orbitofrontal 5-HT and 5-HT2A receptors in depressive-like behaviors and their associations with Kal7 and dendritic spines using chronic unpredictable mild stress (CUMS), an established animal model of depression. CUMS had no effect on the levels of 5-HT or the 5-HT2A receptor in the OFC. However, CUMS or microinjection of the 5-HT2A/2C receptor agonist (±)-1-(2, 5-Dimethoxy-4-iodophenyl)- 2-aminopropane hydrochloride (DOI, 5 μg/0.5 μL) into the OFC induced depressive-like behaviors, including anhedonia in the sucrose preference test and behavioral despair in the tail suspension test, a significant reduction in body weight gain and locomotor activity in the open field test, which were accompanied by decreased expression of Kal7 and PSD95 as well as decreased density of dendritic spines in the OFC. These alterations induced by CUMS were reversed by pretreatment with the 5-HT2A receptor antagonist Ketanserin (Ket, 5 μg/0.5 μL into the OFC). These results suggest that CUMS alters structural plasticity through activation of the orbital 5-HT2A receptor and is associated with decreased expression of Kal7, thereby resulting in depressive-like behaviors in rats, suggesting an important role of Kal7 in the OFC in depression.

Dendritic Spines in Depression: What We Learned from Animal Models

Depression, a severe psychiatric disorder, has been studied for decades, but the underlying mechanisms still remain largely unknown. Depression is closely associated with alterations in dendritic spine morphology and spine density. Therefore, understanding dendritic spines is vital for uncovering the mechanisms underlying depression. Several chronic stress models, including chronic restraint stress (CRS), chronic unpredictable mild stress (CUMS), and chronic social defeat stress (CSDS), have been used to recapitulate depression-like behaviors in rodents and study the underlying mechanisms. In comparison with CRS, CUMS overcomes the stress habituation and has been widely used to model depression-like behaviors. CSDS is one of the most frequently used models for depression, but it is limited to the study of male mice. Generally, chronic stress causes dendritic atrophy and spine loss in the neurons of the hippocampus and prefrontal cortex. Meanwhile, neurons of the amygdala and nucleus accumbens exhibit an increase in spine density. These alterations induced by chronic stress are often accompanied by depression-like behaviors. However, the underlying mechanisms are poorly understood. This review summarizes our current understanding of the chronic stress-induced remodeling of dendritic spines in the hippocampus, prefrontal cortex, orbitofrontal cortex, amygdala, and nucleus accumbens and also discusses the putative underlying mechanisms.

Short-term repeated corticosterone administration enhances glutamatergic but not GABAergic transmission in the rat motor cortex

It has been demonstrated that stress impairs performance of skilled reaching and walking tasks in rats due to the action of glucocorticoids involved in the stress response. Skilled reaching and walking are controlled by the primary motor cortex (M1); however, it is not known whether stress-related impairments in skilled motor tasks are related to functional and/or structural alterations within the M1. We studied the effects of single and repeated injections of corticosterone (twice daily for 7 days) on spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) recorded from layer II/III pyramidal neurons in ex vivo slices of the M1, prepared 2 days after the last administration of the hormone. We also measured the density of dendritic spines on pyramidal cells and the protein levels of selected subunits of AMPA, NMDA, and GABA_A receptors after repeated corticosterone administration. Repeatedly administered corticosterone induced an increase in the frequency but not in the amplitude of sEPSCs, while a single administration had no effect on the recorded excitatory currents. The frequency and amplitude of sIPSCs as well as the excitability of pyramidal cells were changed neither after single nor after repeated corticosterone administration. Treatment with corticosterone for 7 days did not modify the density of dendritic spines on pyramidal neurons. Corticosterone influenced neither the protein levels of GluA1, GluA2, GluN1, GluN2A, and GluN2B subunits of glutamate receptors nor those of α₁, β₂, and γ₂ subunits of the GABA_A receptor. The increase in sEPSCs frequency induced by repeated corticosterone administration faded out within 7 days. These data indicate that prolonged administration of exogenous corticosterone selectively and reversibly enhances glutamatergic, but not GABAergic transmission in the rat motor cortex. Our results suggest that corticosterone treatment results in an enhancement of spontaneous glutamate release from presynaptic terminals in the M1 and thereby uncovers a potential mechanism underlying stress-induced motor functions impairment.

Stress weakens prefrontal networks: molecular insults to higher cognition

A variety of cognitive disorders are worsened by stress exposure and involve dysfunction of the newly evolved prefrontal cortex (PFC). Exposure to acute, uncontrollable stress increases catecholamine release in PFC, reducing neuronal firing and impairing cognitive abilities. High levels of noradrenergic α1-adrenoceptor and dopaminergic D1 receptor stimulation activate feedforward calcium-protein kinase C and cyclic AMP-protein kinase A signaling, which open potassium channels to weaken synaptic efficacy in spines. In contrast, high levels of catecholamines strengthen the primary sensory cortices, amygdala and striatum, rapidly flipping the brain from reflective to reflexive control of behavior. These mechanisms are exaggerated by chronic stress exposure, where architectural changes lead to persistent loss of PFC function. Understanding these mechanisms has led to the successful translation of prazosin and guanfacine for treating stress-related disorders. Dysregulation of stress signaling pathways by genetic insults likely contributes to PFC deficits in schizophrenia, while age-related insults initiate interacting vicious cycles that increase vulnerability to Alzheimer's degeneration.

Molecular aspects of age-related cognitive decline: the role of GABA signaling

Alterations in inhibitory interneurons contribute to cognitive deficits associated with several psychiatric and neurological diseases. Phasic and tonic inhibition imparted by γ-aminobutyric acid (GABA) receptors regulates neural activity and helps to establish the appropriate network dynamics in cortical circuits that support normal cognition. This review highlights basic science demonstrating that inhibitory signaling is altered in aging, and discusses the impact of age-related shifts in inhibition on different forms of memory function, including hippocampus-dependent spatial reference memory and prefrontal cortex (PFC)-dependent working memory. The clinical appropriateness and tractability of select therapeutic candidates for cognitive aging that target receptors mediating inhibition are also discussed.

The medial prefrontal cortex: coordinator of autonomic, neuroendocrine and behavioural responses to stress

Responding to real or potential threats in the environment requires the coordination of autonomic, neuroendocrine and behavioural processes to promote adaptation and survival. These diverging systems necessitate input from the limbic forebrain to integrate and modulate functional output in accordance with contextual demand. In the present review, we discuss the potential role of the medial prefrontal cortex (mPFC) as a coordinator of behavioural and physiological stress responses across multiple temporal and contextual domains. Furthermore, we highlight converging evidence from rodent and human research indicating the necessity of the mPFC for modulating physiological energetic systems to mobilise or limit energetic resources as needed to ultimately promote behavioural adaptation in the face of stress. We review the literature indicating that glucocorticoids act as one of the primary messengers in the reallocation of energetic resources having profound effects locally within the mPFC, as well as shaping how the mPFC acts within a network of brain structures to modulate responses to stress. Finally, we discuss how both rodent and human studies point toward a critical role of the mPFC in the coordination of anticipatory responses to stress and why this distinction is an important one to make in stress neurobiology.

Hepatocyte nuclear factor 4 alpha is a key factor related to depression and physiological homeostasis in the mouse brain

Major depressive disorder (MDD) is a common psychiatric disorder that involves marked disabilities in global functioning, anorexia, and severe medical comorbidities. MDD is associated with not only psychological and sociocultural problems, but also pervasive physical dysfunctions such as metabolic, neurobiological and immunological abnormalities. Nevertheless, the mechanisms underlying the interactions between these factors have yet to be determined in detail. The aim of the present study was to identify the molecular mechanisms responsible for the interactions between MDD and dysregulation of physiological homeostasis, including immunological function as well as lipid metabolism, coagulation, and hormonal activity in the brain. We generated depression-like behavior in mice using chronic mild stress (CMS) as a model of depression. We compared the gene expression profiles in the prefrontal cortex (PFC) of CMS and control mice using microarrays. We subsequently categorized genes using two web-based bioinformatics applications: Ingenuity Pathway Analysis and The Database for Annotation, Visualization, and Integrated Discovery. We then confirmed significant group-differences by analyzing mRNA and protein expression levels not only in the PFC, but also in the thalamus and hippocampus. These web tools revealed that hepatocyte nuclear factor 4 alpha (Hnf4a) may exert direct effects on various genes specifically associated with amine synthesis, such as genes involved in serotonin metabolism and related immunological functions. Moreover, these genes may influence lipid metabolism, coagulation, and hormonal activity. We also confirmed the significant effects of Hnf4a on both mRNA and protein expression levels in the brain. These results suggest that Hnf4a may have a critical influence on physiological homeostasis under depressive states, and may be associated with the mechanisms responsible for the interactions between MDD and the dysregulation of physiological homeostasis in humans.

GABAergic control of depression-related brain states

The GABAergic deficit hypothesis of major depressive disorders (MDDs) posits that reduced γ-aminobutyric acid (GABA) concentration in brain, impaired function of GABAergic interneurons, altered expression and function of GABA(A) receptors, and changes in GABAergic transmission dictated by altered chloride homeostasis can contribute to the etiology of MDD. Conversely, the hypothesis posits that the efficacy of currently used antidepressants is determined by their ability to enhance GABAergic neurotransmission. We here provide an update for corresponding evidence from studies of patients and preclinical animal models of depression. In addition, we propose an explanation for the continued lack of genetic evidence that explains the considerable heritability of MDD. Lastly, we discuss how alterations in GABAergic transmission are integral to other hypotheses of MDD that emphasize (i) the role of monoaminergic deficits, (ii) stress-based etiologies, (iii) neurotrophic deficits, and (iv) the neurotoxic and neural circuit-impairing consequences of chronic excesses of glutamate. We propose that altered GABAergic transmission serves as a common denominator of MDD that can account for all these other hypotheses and that plays a causal and common role in diverse mechanistic etiologies of depressive brain states and in the mechanism of action of current antidepressant drug therapies.

Chronic stress alters the dendritic morphology of callosal neurons and the acute glutamate stress response in the rat medial prefrontal cortex

We have previously reported that interhemispheric regulation of medial prefrontal cortex (PFC)-mediated stress responses is subserved by glutamate (GLU)- containing callosal neurons. Evidence of chronic stress-induced dendritic and spine atrophy among PFC pyramidal neurons led us to examine how chronic restraint stress (CRS) might alter the apical dendritic morphology of callosal neurons and the acute GLU stress responses in the left versus right PFC. Morphometric analyses of retrogradely labeled, dye-filled PFC callosal neurons revealed hemisphere-specific CRS-induced dendritic retraction; whereas significant dendritic atrophy occurred primarily within the distal arbor of left PFC neurons, it was observed within both the proximal and distal arbor of right PFC neurons. Overall, CRS also significantly reduced spine densities in both hemispheres with the greatest loss occurring among left PFC neurons, mostly at the distal extent of the arbor. While much of the overall decrease in dendritic spine density was accounted by the loss of thin spines, the density of mushroom-shaped spines, despite being fewer in number, was halved. Using microdialysis we found that, compared to controls, basal PFC GLU levels were significantly reduced in both hemispheres of CRS animals and that their GLU response to 30 min of tail-pinch stress was significantly prolonged in the left, but not the right PFC. Together, these findings show that a history of chronic stress alters the dendritic morphology and spine density of PFC callosal neurons and suggest a mechanism by which this might disrupt the interhemispheric regulation of PFC-mediated responses to subsequent stressors.

Functional and structural remodeling of glutamate synapses in prefrontal and frontal cortex induced by behavioral stress

Increasing evidence has shown that the pathophysiology of neuropsychiatric disorders, including mood disorders, is associated with abnormal function and regulation of the glutamatergic system. Consistently, preclinical studies on stress-based animal models of pathology showed that glucocorticoids and stress exert crucial effects on neuronal excitability and function, especially in cortical and limbic areas. In prefrontal and frontal cortex, acute stress was shown to induce enhancement of glutamate release/transmission dependent on activation of corticosterone receptors. Although the mechanisms whereby stress affects glutamate transmission have not yet been fully understood, it was shown that synaptic, non-genomic action of corticosterone is required to increase the readily releasable pool of glutamate vesicles, but is not sufficient to enhance transmission in prefrontal and frontal cortex. Slower, partly genomic mechanisms are probably necessary for the enhancement of glutamate transmission induced by stress. Combined evidence has suggested that the changes in glutamate release and transmission are responsible for the dendritic remodeling and morphological changes induced by stress and it has been argued that sustained alterations of glutamate transmission may play a key role in the long-term structural/functional changes associated with mood disorders in patients. Intriguingly, modifications of the glutamatergic system induced by stress in the prefrontal cortex seem to be biphasic. Indeed, while the fast response to stress suggests an enhancement in the number of excitatory synapses, synaptic transmission and working memory, long-term adaptive changes - including those consequent to chronic stress - induce opposite effects. Better knowledge of the cellular effectors involved in this biphasic effect of stress may be useful to understand the pathophysiology of stress-related disorders, and open new paths for the development of therapeutic approaches.

Chronic stress induces prolonged suppression of the P2X7 receptor within multiple regions of the hippocampus: a cumulative threshold spectra analysis

A number of studies have identified that mutations in the P2X7 receptor occur with a significantly higher incidence in individuals with major depression. Consistent with these findings, a number of preclinical studies have identified that mice in which the P2X7 receptor has been deleted exhibit a higher level of resilience-like behaviour to acutely aversive situations. At present, however, no studies have examined changes in P2X7 receptor expression in otherwise healthy animals exposed to persistently stressful situations. This is significant as several lines of evidence have demonstrated that it is exposure to persistently aversive, rather than acutely aversive, situations that is associated with the emergence of mood disturbance. Accordingly, the objective of the current study was to examine whether chronic exposure to restraint stress was associated with alterations in the expression of P2X7 within the hippocampal formation. The study involved three principal groups: acute stress (1 session), chronic stress (21 sessions, 1 per day) and a chronic stress with recovery group (21 sessions, 1 per day followed by 7days of no stress) and appropriate control groups. The results of the analysis indicate that all forms of stress, regardless of the duration, provoked a reduction in P2X7 receptor expression. Comparative analysis on normalised data indicated that the magnitude of the P2X7 reduction was significantly greater in the chronic stress relative to the acute stress group. We additionally found that there was a gradual rebound in P2X7 expression, in two of nine regions examined, in animals that were allowed to recover for 7days following the final stress session. Collectively, these findings provide the first evidence that exposure to chronic restraint stress produces a pronounced and relatively persistent suppression of the P2X7 receptor within the hippocampus.

Chronic stress reduces the number of GABAergic interneurons in the adult rat hippocampus, dorsal-ventral and region-specific differences

Major depressive disorder is a common and complex mental disorder with unknown etiology. GABAergic dysfunction is likely to contribute to the pathophysiology since disrupted GABAergic systems are well documented in depressed patients. Here we studied structural changes in the hippocampal GABAergic network using the chronic mild stress (CMS) model, as one of the best validated animal models for depression. Rats were subjected to 9 weeks of daily stress and behaviorally characterized using the sucrose consumption test into anhedonic and resilient animals based on their response to stress. Different subtypes of GABAergic interneurons were visualized by immunohistochemistry using antibodies for parvalbumin (PV), calretinin (CR), calbindin (CB), cholecystokinin (CCK), somatostatin (SOM), and neuropeptide Y (NPY). We used an unbiased quantification method to systematically count labeled cells in different subareas of the dorsal and ventral hippocampus. Chronic stress reduced the number of specific interneurons in distinct hippocampal subregions significantly. PV+ and CR+ neurons were reduced in all dorsal subareas, whereas in the ventral part only the CA1 was affected. Stress had the most pronounced effect on the NPY+ and SOM+ cells and reduced their number in almost all dorsal and ventral subareas. Stress had no effect on the CCK+ and CB+ interneurons. In most cases the effect of stress was irrespective to the behavioral phenotype. However, in a few specific areas the number of SOM+, NPY+, and CR+ neurons were significantly reduced in anhedonic animals compared to the resilient group. Overall, these data clearly demonstrate that chronic stress affects the structural integrity of specific GABAergic neuronal subpopulations and this should also affect the functioning of these hippocampal GABAergic networks.

Chronic fluoxetine treatment alters the structure, connectivity and plasticity of cortical interneurons

Novel hypotheses suggest that antidepressants, such as the selective serotonin reuptake inhibitor fluoxetine, induce neuronal structural plasticity, resembling that of the juvenile brain, although the underlying mechanisms of this reopening of the critical periods still remain unclear. However, recent studies suggest that inhibitory networks play an important role in this structural plasticity induced by fluoxetine. For this reason we have analysed the effects of a chronic fluoxetine treatment in the hippocampus and medial prefrontal cortex (mPFC) of transgenic mice displaying eGFP labelled interneurons. We have found an increase in the expression of molecules related to critical period plasticity, such as the polysialylated form of the neural cell adhesion molecule (PSA-NCAM), GAD67/65 and synaptophysin, as well as a reduction in the number of parvalbumin expressing interneurons surrounded by perineuronal nets. We have also described a trend towards decrease in the perisomatic inhibitory puncta on pyramidal neurons in the mPFC and an increase in the density of inhibitory puncta on eGFP interneurons. Finally, we have found that chronic fluoxetine treatment affects the structure of interneurons in the mPFC, increasing their dendritic spine density. The present study provides evidence indicating that fluoxetine promotes structural changes in the inhibitory neurons of the adult cerebral cortex, probably through alterations in plasticity-related molecules of neurons or the extracellular matrix surrounding them, which are present in interneurons and are known to be crucial for the development of the critical periods of plasticity in the juvenile brain.

Glucocorticoid actions on synapses, circuits, and behavior: implications for the energetics of stress

Environmental stimuli that signal real or potential threats to homeostasis lead to glucocorticoid secretion by the hypothalamic-pituitary-adrenocortical (HPA) axis. Glucocorticoids promote energy redistribution and are critical for survival and adaptation. This adaptation requires the integration of multiple systems and engages key limbic-neuroendocrine circuits. Consequently, glucocorticoids have profound effects on synaptic physiology, circuit regulation of stress responsiveness, and, ultimately, behavior. While glucocorticoids initiate adaptive processes that generate energy for coping, prolonged or inappropriate glucocorticoid secretion becomes deleterious. Inappropriate processing of stressful information may lead to energetic drive that does not match environmental demand, resulting in risk factors for pathology. Thus, dysregulation of the HPA axis may promote stress-related illnesses (e.g. depression, PTSD). This review summarizes the latest developments in central glucocorticoid actions on synaptic, neuroendocrine, and behavioral regulation. Additionally, these findings will be discussed in terms of the energetic integration of stress and the importance of context-specific regulation of glucocorticoids.

Corticolimbic catecholamines in stress: a computational model of the appraisal of controllability

Appraisal of a stressful situation and the possibility to control or avoid it is thought to involve frontal-cortical mechanisms. The precise mechanism underlying this appraisal and its translation into effective stress coping (the regulation of physiological and behavioural responses) are poorly understood. Here, we propose a computational model which involves tuning motivational arousal to the appraised stressing condition. The model provides a causal explanation of the shift from active to passive coping strategies, i.e. from a condition characterised by high motivational arousal, required to deal with a situation appraised as stressful, to a condition characterised by emotional and motivational withdrawal, required when the stressful situation is appraised as uncontrollable/unavoidable. The model is motivated by results acquired via microdialysis recordings in rats and highlights the presence of two competing circuits dominated by different areas of the ventromedial prefrontal cortex: these are shown having opposite effects on several subcortical areas, affecting dopamine outflow in the striatum, and therefore controlling motivation. We start by reviewing published data supporting structure and functioning of the neural model and present the computational model itself with its essential neural mechanisms. Finally, we show the results of a new experiment, involving the condition of repeated inescapable stress, which validate most of the model's predictions.

Neuropathology of stress

Environmental challenges are part of daily life for any individual. In fact, stress appears to be increasingly present in our modern, and demanding, industrialized society. Virtually every aspect of our body and brain can be influenced by stress and although its effects are partly mediated by powerful corticosteroid hormones that target the nervous system, relatively little is known about when, and how, the effects of stress shift from being beneficial and protective to becoming deleterious. Decades of stress research have provided valuable insights into whether stress can directly induce dysfunction and/or pathological alterations, which elements of stress exposure are responsible, and which structural substrates are involved. Using a broad definition of pathology, we here review the "neuropathology of stress" and focus on structural consequences of stress exposure for different regions of the rodent, primate and human brain. We discuss cytoarchitectural, neuropathological and structural plasticity measures as well as more recent neuroimaging techniques that allow direct monitoring of the spatiotemporal effects of stress and the role of different CNS structures in the regulation of the hypothalamic-pituitary-adrenal axis in human brain. We focus on the hypothalamus, hippocampus, amygdala, nucleus accumbens, prefrontal and orbitofrontal cortex, key brain regions that not only modulate emotions and cognition but also the response to stress itself, and discuss disorders like depression, post-traumatic stress disorder, Cushing syndrome and dementia.

Transcranial electrical stimulation modifies the neuronal response to psychosocial stress exposure

Stress is a constant characteristic of everyday life in our society, playing a role in triggering several chronic disorders. Therefore, there is an ongoing need to develop new methods in order to manage stress reactions. The regulatory function of right medial-prefrontal cortex (mPFC) is frequently reported by imaging studies during psychosocial stress situations. Here, we examined the effects of inhibitory and excitatory preconditioning stimulation via cathodal and anodal transcranial direct current stimulation (tDCS) on psychosocial stress related behavioral indicators and physiological factors, including the cortisol level in the saliva and changes in brain perfusion. Twenty minutes real or sham tDCS was applied over the right mPFC of healthy subjects before the performance of the Trier Social Stress Test (TSST). Regional cerebral blood flow (rCBF) was measured during stimulation and after TSST, using pseudo-continuous arterial spin labeling (pCASL). Comparing the effect of the different stimulation conditions, during anodal stimulation we found higher rCBF in the right mPFC, compared to the sham and in the right amygdala, superior PFC compared to the cathodal condition. Salivary cortisol levels showed a decrease in the anodal and increase in cathodal groups after completion of the TSST. The behavioral stress indicators indicated the increase of stress level, however, did not show any significant differences among groups. In this study we provide the first insights into the neuronal mechanisms mediating psychosocial stress responses by prefrontal tDCS.