Exposure to prolonged controllable or uncontrollable stress affects GABAergic function in sub-regions of the hippocampus and the amygdala

Sex differences in anxiety and threat avoidance in GAD65 knock-out mice

Anxiety disorders have been linked to a disbalance of excitation and inhibition in a network of brain structures comprising frontal cortical regions, the amygdala and the hippocampus, among others. Recent imaging studies suggest sex differences in the activation of this anxiety network during the processing of emotional information. Rodent models with genetically altered ϒ-amino butyric acid (GABA) neurotransmission allow studying the neuronal basis of such activation shifts and their relation to anxiety endophenotypes, but to date sex effects have rarely been addressed. Using mice with a null mutation of the GABA synthetizing enzyme glutamate decarboxylase 65 (GAD65-/-), we started to compare anxiety-like behavior and avoidance in male vs. female GAD65-/- mice and their wildtype littermates. In an open field, female GAD65-/- mice displayed increased activity, while male GAD65-/- mice showed an increased adaptation of anxiety-like behavior over time. GAD65-/- mice of both sexes had a higher preference for social interaction partners, which was further heightened in male mice. In male mice higher escape responses were observed during an active avoidance task. Together, female mice showed more stable emotional responses despite GAD65 deficiency. To gain insights into interneuron function in network structures controlling anxiety and threat perception, fast oscillations (10-45 Hz) were measured in ex vivo slice preparations of the anterior cingulate cortex (ACC). GAD65-/- mice of both sexes displayed increased gamma power in the ACC and a higher density of PV-positive interneurons, which are crucial for generating such rhythmic activity. In addition, GAD65-/- mice had lower numbers of somatostatin-positive interneurons in the basolateral amygdala and in the dorsal dentate gyrus especially in male mice, two key regions important for anxiety and active avoidance responses. Our data suggest sex differences in the configuration of GABAergic interneurons in a cortico-amygdala-hippocampal network controlling network activity patterns, anxiety and threat avoidance behavior.

Reducing glutamic acid decarboxylase in the dorsal dentate gyrus attenuates juvenile stress induced emotional and cognitive deficits

A high degree of regional, temporal and molecular specificity is evident in the regulation of GABAergic signaling in stress-responsive circuitry, hampering the use of systemic GABAergic modulators for the treatment of stress-related psychopathology. Here we investigated the effectiveness of local intervention with the GABA synthetic enzymes GAD65 and GAD67 in the dorsal dentate gyrus (dDG) vs ventral DG (vDG) to alleviate anxiety-like behavior and stress-induced symptoms in the rat. We induced shRNA-mediated knock down of either GAD65 or GAD67 with lentiviral vectors microinjected into the dDG or vDG of young adult male rats and examined anxiety behavior, learning and memory performance. Subsequently we tested whether reducing GAD65 expression in the dDG would also confer resilience against juvenile stress-induced behavioral and physiological symptoms in adulthood. While knock down of either isoform in the vDG increased anxiety levels in the open field and the elevated plus maze tests, the knock down of GAD65, but not GAD67, in the dDG conferred a significant reduction in anxiety levels. Strikingly, this manipulation also attenuated juvenile stress evoked anxiety behavior, cognitive and synaptic plasticity impairments. Local GABAergic circuitry in the DG plays an important and highly region-specific role in control of emotional behavior and stress responding. Reduction of GAD65 expression in the dDG appears to provide resilience to juvenile stress-induced emotional and cognitive deficits, opening a new direction towards addressing a significant risk factor for developing stress and trauma-related psychopathologies later in life.

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GAD67/65 in the dorsal/ventral dentate gyrus differentially modulate anxiety.

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Reduced GAD65 expression in the dorsal dentate gyrus supports stress resilience.

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The dorsal dentate gyrus plays a key role in stress resilience.

Brief stress impairs recognition memory through amygdalar activation in animals with medial prefrontal cortex lesions

The medial prefrontal cortex (mPFC) is thought to exert inhibitory control over stress-induced activation of the amygdala and neurocognitive effects. As evidence to support this, we examined how exposure to either a brief or prolonged stress affected on amygdalar c-Fos levels and recognition memory of animals with mPFC chemical lesions. mPFC-lesioned and sham-operated animals were subjected to either a brief 20-min restraint+20 tailshocks or a prolonged 60-min restraint+60 tailshocks. Post-stress performances in the object recognition memory and c-Fos immunoreactivity in the amygdala were then assessed. In sham-operated animals, the object recognition memory was reliably impaired following the prolonged, but not following the brief stress exposure. On the other hand, in mPFC-lesioned animals, the brief stress significantly impaired recognition memory and enhanced c-Fos expression in the amygdala. Present findings of loss of mPFC activity exacerbating stress effects provide causal evidence that the mPFC exerts inhibitory control on stress.

A Translational Paradigm to Study the Effects of Uncontrollable Stress in Humans

Theories on the aetiology of depression in humans are intimately linked to animal research on stressor controllability effects. However, explicit translations of established animal designs are lacking. In two consecutive studies, we developed a translational paradigm to study stressor controllability effects in humans. In the first study, we compared three groups of participants, one exposed to escapable stress, one yoked inescapable stress group, and a control group not exposed to stress. Although group differences indicated successful stress induction, the manipulation failed to differentiate groups according to controllability. In the second study, we employed an improved paradigm and contrasted only an escapable stress group to a yoked inescapable stress group. The final design successfully induced differential effects on self-reported perceived control, exhaustion, helplessness, and behavioural indices of adaptation to stress. The latter were examined in a new escape behaviour test which was modelled after the classic shuttle box animal paradigm. Contrary to the learned helplessness literature, exposure to uncontrollable stress led to more activity and exploration; however, these behaviours were ultimately not adaptive. We discuss the results and possible applications in light of the findings on learning and agency beliefs, inter-individual differences, and interventions aimed at improving resilience to stress-induced mental dysfunction.

Acute Stress Facilitates LTD Induction at Glutamatergic Synapses in the Hippocampal CA1 Region by Activating μ-Opioid Receptors on GABAergic Neurons

Acute stress impairs recall memory through the facilitation of long-term depression (LTD) of hippocampal synaptic transmission. The endogenous opioid system (EOS) plays essential roles in stress-related emotional and physiological responses. Specifically, behavioral studies have shown that the impairment of memory retrieval induced by stressful events involves the activation of opioid receptors. However, it is unclear whether signaling mediated by μ-opioid receptors (μRs), one of the three major opioid receptors, participates in acute stress-related hippocampal LTD facilitation. Here, we examined the effects of a single elevated platform (EP) stress exposure on excitatory synaptic transmission and plasticity at the Schaffer collateral-commissural (SC) to CA1 synapses by recording electrically evoked field excitatory postsynaptic potentials and population spikes of hippocampal pyramidal neurons in anesthetized adult mice. EP stress exposure attenuated GABAergic feedforward and feedback inhibition of CA1 pyramidal neurons and facilitated low-frequency stimulation (LFS)-induced long-term depression (LTD) at SC-CA1 glutamatergic synapses. These effects were reproduced by exogenously activating μRs in unstressed mice. The specific deletion of μRs on GABAergic neurons (μR_GABA) not only prevented the EP stress-induced memory impairment but also reversed the EP stress-induced attenuation of GABAergic inhibition and facilitation of LFS-LTD. Our results suggest that acute stress endogenously activates μR_GABA to attenuate hippocampal GABAergic signaling, thereby facilitating LTD induction at excitatory synapses and eliciting memory impairments.

Perceived Stress is Associated with Accelerated Monocyte/Macrophage Aging Trajectories in Clinically Normal Adults

OBJECTIVES

Chronic stress is associated with poorer age-related cognition, but the mechanisms of this relationship are not well understood. Aging increases expression of activated macrophages, leading to exacerbated immune responses to stressors. We examined the impact of stress and aging on macrophage-related inflammation and cognition in clinically normal adults.

METHODS

Three hundred eighty clinically normal adults were followed longitudinally (age M = 73 years; visit range: 1-8; M = 2.5 visits). Participants completed the Perceived Stress Scale, a neuropsychological battery, and blood draws. Plasma was analyzed for cytokines related to macrophage function (interleukin 6, tumor necrosis factor alpha, macrophage inflammatory protein-1 alpha, macrophage inflammatory protein-1 beta). Linear mixed-effects examined the effects of age, baseline stress, and their interaction predicting macrophage cytokines, adjusting for sex, education, and depressive symptoms. Latent growth curve models assessed the mediating role of macrophage cytokines in the relationship between age and cognition in high or low stress.

RESULTS

Baseline perceived stress interacted with age to predict macrophage cytokines longitudinally. Specifically, high-stress adults demonstrated accelerated age-related elevations in macrophage cytokines across time. Macrophage cytokines negatively tracked with executive functioning longitudinally. Macrophage cytokines mediated 19% of the relationship between age and executive function in high-stress, but not low-stress, adults.

CONCLUSIONS

Our data provide evidence of accelerated immune aging among individuals with high stress. Elevated macrophage cytokine trajectories mediated the effect of age on executive function only in individuals with high stress, suggesting these constructs may be more tightly linked in elevated stress contexts. Stress interventions are warranted to optimize immune aging, with possible downstream cognitive benefits among even clinically normal adults.

Physical enrichment enhances memory function by regulating stress hormone and brain acetylcholinesterase activity in rats exposed to restraint stress

To study the effects of stress on mental health activity is of great importance in neuropsychological studies as it may affect the lifelong performance related to brain and overall health and wellbeing of an individual. It is observed very often that exposure to stress during early life can alter the brain function which may reflect as cognitive disability. Impairment of memory is associated with increased oxidative stress which is due to enhanced production of free radicals that may lead to lipid peroxidation and disintegration of cell structure and functions. Exposure to enriched environment has shown to enhance spatial learning and memory, although the underlying mechanism covering the regulation of antioxidant capacity is limited. Here we investigated short and long term memory using Morris water maze before and after giving restraint stress procedure in rats exposed to social and physically enriched environment. Levels of malondialdehyde (MDA), activity of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), and acetylcholinesterase (AChE) in brain tissue were estimated. Plasma corticosterone was also determined after decapitation. Results demonstrated that rats pre-exposed to physical along with social enrichment showed improved short and long term memory as compared to control group. However, restraint stress exerted differential effects in socially and physically enriched groups. Reduced lipid peroxidation and decreased activity of SOD, GPx and AChE were observed in physically enriched rats subjected to stress as compared to stressed rats kept in social environment. Levels of corticosterone were also found to be significantly reduced in rats kept in physically enriched environment. This study shows the beneficial effects of environmental enrichment on learning and spatial memory by reducing oxidative stress via reducing lipid peroxidation and regulation of antioxidant enzymes in rats.

Neonatal handling enduringly decreases anxiety and stress responses and reduces hippocampus and amygdala volume in a genetic model of differential anxiety: Behavioral-volumetric associations in the Roman rat strains

The hippocampus and amygdala have been proposed as key neural structures related to anxiety. A more active hippocampus/amygdala system has been related to greater anxious responses in situations involving conflict/novelty. The Roman Low- (RLA) and High-avoidance (RHA) rat lines/strains constitute a genetic model of differential anxiety. Relative to RHA rats, RLA rats exhibit enhanced anxiety/fearfulness, augmented hippocampal/amygdala c-Fos expression following exposure to novelty/conflict, increased hippocampal neuronal density and higher endocrine responses to stress. Neonatal handling (NH) is an environmental treatment with long-lasting anxiety/stress-reducing effects in rodents. Since hippocampus and amygdala volume are supposed to be related to anxiety/fear, we hypothesized a greater volume of both areas in RLA than in RHA rats, as well as that NH treatment would reduce anxiety and the volume of both structures, in particular in the RLA strain. Adult untreated and NH-treated RHA and RLA rats were tested for anxiety, sensorimotor gating (PPI), stress-induced corticosterone and prolactin responses, two-way active avoidance acquisition and in vivo 7 T 1H-Magnetic resonance image. As expected, untreated RLA rats showed higher anxiety and post-stress hormone responses, as well as greater hippocampus and amygdala volumes than untreated RHA rats. NH decreased anxiety/stress responses, especially in RLA rats, and significantly reduced hippocampus and amygdala volumes in this strain. Dorsal striatum volume was not different between the strains nor it was affected by NH. Finally, there were positive associations (as shown by correlations, factor analysis and multiple regression) between anxiety and PPI and hippocampus/amygdala volumes.