Brendan D. Hare

Exercise-Associated Changes in the Corticosterone Response to Acute Restraint Stress: Evidence for Increased Adrenal Sensitivity and Reduced Corticosterone Response Duration

Exercise promotes stress resistance and is associated with reduced anxiety and reduced depression in both humans and in animal models. Despite the fact that dysfunction within the hypothalamic pituitary adrenal (HPA) axis is strongly linked to both anxiety and depressive disorders, the evidence is mixed as to how exercise alters the function of the HPA axis. Here we demonstrate that 4 weeks of voluntary wheel running was anxiolytic in C57BL/6J mice and resulted in a shorter time to peak corticosterone (CORT) and a more rapid decay of CORT following restraint stress. Wheel running was also associated with increased adrenal size and elevated CORT following systemic administration of adrenocorticotropic hormone. Finally, the HPA-axis response to peripheral or intracerebroventricular administration of dexamethasone did not suggest that wheel running increases HPA-axis negative feedback through GR-mediated mechanisms. Together these findings suggest that exercise may promote stress resilience in part by insuring a more rapid and shortened HPA response to a stressor thus affecting overall exposure to the potentially negative effects of more sustained HPA-axis activation.

GLYX-13 Produces Rapid Antidepressant Responses with Key Synaptic and Behavioral Effects Distinct from Ketamine

GLYX-13 is a putative NMDA receptor modulator with glycine-site partial agonist properties that produces rapid antidepressant effects, but without the psychotomimetic side effects of ketamine. Studies were conducted to examine the molecular, cellular, and behavioral actions of GLYX-13 to further characterize the mechanisms underlying the antidepressant actions of this agent. The results demonstrate that a single dose of GLYX-13 rapidly activates the mTORC1 pathway in the prefrontal cortex (PFC), and that infusion of the selective mTORC1 inhibitor rapamycin into the medial PFC (mPFC) blocks the antidepressant behavioral actions of GLYX-13, indicating a requirement for mTORC1 similar to ketamine. The results also demonstrate that GLYX-13 rapidly increases the number and function of spine synapses in the apical dendritic tuft of layer V pyramidal neurons in the mPFC. Notably, GLYX-13 significantly increased the synaptic responses to hypocretin, a measure of thalamocortical synapses, compared with its effects on 5-HT responses, a measure of cortical-cortical responses mediated by the 5-HT2A receptor. Behavioral studies further demonstrate that GLYX-13 does not influence 5-HT2 receptor induced head twitch response or impulsivity in a serial reaction time task (SRTT), whereas ketamine increased responses in both tests. In contrast, both GLYX-13 and ketamine increased attention in the SRTT task, which is linked to hypocretin–thalamocortical responses. The differences in the 5-HT2 receptor synaptic and behavioral responses may be related to the lack of psychotomimetic side effects of GLYX-13 compared with ketamine, whereas regulation of the hypocretin responses may contribute to the therapeutic benefits of both rapid acting antidepressants.

Rapid Acting Antidepressants in Chronic Stress Models: Molecular and Cellular Mechanisms:

Stress-associated disorders, including depression and anxiety, impact nearly 20% of individuals in the United States. The social, health, and economic burden imposed by stress-associated disorders requires in depth research efforts to identify suitable treatment strategies. Traditional medications (e.g., selective serotonin reuptake inhibitors, monoamine oxidase inhibitors) have significant limitations, notably a time lag for therapeutic response that is compounded by low rates of efficacy. Excitement over ketamine, a rapid acting antidepressant effective in treatment resistant patients, is tempered by transient dissociative and psychotomimetic effects, as well as abuse potential. Rodent stress models are commonly used to produce behavioral abnormalities that resemble those observed in stress-associated disorders. Stress models also produce molecular and cellular morphological changes in stress sensitive brain regions, including the prefrontal cortex and hippocampus that resemble alterations observed in depression. Rapid acting antidepressants such as ketamine can rescue stress-associated morphological and behavioral changes in rodent models. Here, we review the literature supporting a role for rapid acting antidepressants in opposing the effects of stress, and summarize research efforts seeking to elucidate the molecular, cellular, and circuit level targets of these agents.

Persistent Increase in Microglial RAGE Contributes to Chronic Stress–Induced Priming of Depressive-like Behavior

BACKGROUND Chronic stress-induced inflammatory responses occur in part via danger-associated molecular pattern (DAMP) molecules, such as high mobility group box 1 protein (HMGB1), but the receptor(s) underlying DAMP signaling have not been identified. METHODS Microglia morphology and DAMP signaling in enriched rat hippocampal microglia were examined during the development and expression of chronic unpredictable stress (CUS)-induced behavioral deficits, including long-term, persistent changes after CUS. RESULTS The results show that CUS promotes significant morphological changes and causes robust upregulation of HMGB1 messenger RNA in enriched hippocampal microglia, an effect that persists for up to 6 weeks after CUS exposure. This coincides with robust and persistent upregulation of receptor for advanced glycation end products (RAGE) messenger RNA, but not toll-like receptor 4 in hippocampal microglia. CUS also increased surface expression of RAGE protein on hippocampal microglia as determined by flow cytometry and returned to basal levels 5 weeks after CUS. Importantly, exposure to short-term stress was sufficient to increase RAGE surface expression as well as anhedonic behavior, reflecting a primed state that results from a persistent increase in RAGE messenger RNA expression. Further evidence for DAMP signaling in behavioral responses is provided by evidence that HMGB1 infusion into the hippocampus was sufficient to cause anhedonic behavior and by evidence that RAGE knockout mice were resilient to stress-induced anhedonia. CONCLUSIONS Together, the results provide evidence of persistent microglial HMGB1-RAGE expression that increases vulnerability to depressive-like behaviors long after chronic stress exposure.

Prior Stress Interferes With the Anxiolytic Effect of Exercise in C57BL/6J Mice

Recent reports demonstrate that the beneficial effects of voluntary exercise may be sensitive to stress prior to and during the wheel access period. Here, a variate stress procedure is used with socially isolated mice for 7 days prior to the introduction of running wheels to assess the impact of prior and concurrent stress on the anxiolytic effect of exercise. Following stress exposure, functioning or nonfunctioning running wheels were introduced into stressed and unstressed group-housed control cages. Following 3 weeks of wheel access, the anxiolytic effect of exercise was assessed using acoustic startle, stress-induced hyperthermia, and a challenge with the anxiogenic drug metachlorophenylpiperazine (mCPP). Variate stress was demonstrated to interfere with normal weight gain. Further, exercise was not anxiolytic in stressed mice. Consistent with previous reports unstressed exercising mice demonstrated reduced acoustic startle, attenuated stress induced hyperthermia, and a blunted increase in startle following mCPP administration when compared with unstressed sedentary controls. Stressed exercising mice were indistinguishable from stressed sedentary and unstressed sedentary controls on each anxiety measure. Although running distance varied between individual mice, the distance run did not predict the level of anxiety on any measure. These findings suggest that prior and ongoing stress delays or prevents the anxiolytic effect of exercise without affecting exercise itself.

Molecular and Cellular Effects of Traumatic Stress: Implications for PTSD

Purpose of ReviewPosttraumatic stress disorder (PTSD) is characterized by hyperarousal and recurrent stressful memories after an emotionally traumatic event. Extensive research has been conducted to identify the neurobiological determinants that underlie the pathophysiology of PTSD. In this review, we examine evidence regarding the molecular and cellular pathophysiology of PTSD focusing on two primary brain regions: the vmPFC and the amygdala.Recent FindingsThis discussion includes a review of the molecular alterations related to PTSD, focusing mainly on changes to glucocorticoid receptor signaling. We also examine postmortem gene expression studies that have been conducted to date and the molecular changes that have been observed in peripheral blood studies of PTSD patients. Causal, mechanistic evidence is difficult to obtain in human studies, so we also review preclinical models of PTSD.SummaryIntegration of peripheral blood and postmortem studies with preclinical models of PTSD has begun to reveal the molecular changes occurring in patients with PTSD. These findings indicate that the pathophysiology of PTSD includes disruption of glucocorticoid signaling and inflammatory systems and occurs at the level of altered gene expression. We will assess the impact of these findings on the future of PTSD molecular research.

Two Weeks of Variable Stress Increases Gamma-H2AX Levels in the Mouse Bed Nucleus of the Stria Terminalis

Recent reports demonstrate that DNA damage is induced, and rapidly repaired, in circuits activated by experience. Moreover, stress hormones are known to slow DNA repair, suggesting that prolonged stress may result in persistent DNA damage. Prolonged stress is known to negatively impact physical and mental health; however, DNA damage as a factor in stress pathology has only begun to be explored. Histone H2A-X phosphorylated at serine 139 (γH2AX) is a marker of DNA double-strand breaks (DSB), a type of damage that may lead to cell death if unrepaired. We hypothesized that a 14-day period of variable stress exposure sufficient to alter anxiety-like behavior in male C57BL/6J mice would produce an increase in γH2AX levels in the bed nucleus of the stria terminalis (BNST), a region implicated in anxiety and stress regulation. We observed that 14 days of variable stress, but not a single stress exposure, was associated with increased levels of γH2AX 24 h after termination of the stress paradigm. Further investigation found that phosphorylation levels of a pair of kinases associated with the DNA damage response, glycogen synthase kinase 3 β (GSK3β) and p38 mitogen-activated protein kinase (MAPK) were also elevated following variable stress. Our results suggest that unrepaired DNA DSBs and/or repetitive attempted repair may represent an important component of the allostatic load that stress places on the brain.

Failure to Inactivate Nuclear GSK3β by Ser 389 -Phosphorylation Leads to Focal Neuronal Death and Prolonged Fear Response

GSK3β plays an essential role in promoting cell death and is emerging as a potential target for neurological diseases. Understanding the mechanisms that control neuronal GSK3β is critical. A ubiquitous mechanism to repress GSK3β involves Akt-mediated phosphorylation of Ser9. Here we show that phosphorylation of GSK3β on Ser389 mediated by p38 MAPK specifically inactivates nuclear GSK3β in the cortex and hippocampus. Using GSK3β Ser389 to Ala mutant mice, we show that failure to inactivate nuclear GSK3β by Ser389 phosphorylation causes neuronal cell death in subregions of the hippocampus and cortex. Although this focal neuronal death does not impact anxiety/depression-like behavior or hippocampal-dependent spatial learning, it leads to an amplified and prolonged fear response. This phenotype is consistent with some aspects of post-traumatic stress disorder (PTSD). Our studies indicate that inactivation of nuclear GSK3β by Ser389 phosphorylation plays a key role in fear response, revealing new potential therapeutic approaches to target PTSD.