Exercise reduces the anxiogenic effects of meta-chlorophenylpiperazine: The role of 5-HT2C receptors in the bed nucleus of the stria terminalis.

Introduction: Two weeks of voluntary exercise in group-housed mice produces a reduction in anxiety-like behaviors across a number of different measures, including a reduction in the anxiety levels typically produced by the anxiogenic serotonergic drug m-chlorophenylpiperazine (mCPP), an agonist at 5-HT2C/2b receptors. We have previously demonstrated that 2-weeks of voluntary exercise blunted the anxiogenic effects of systemic mCPP, and we have also shown that mCPP infused into the bed nucleus of the stria terminalis (BNST) is anxiogenic. Here we follow up on these reports. Methods: In Experiment 1 we infused several doses of mCPP into the BNST with or without the 5-HT2C antagonist SB242084. In Experiment 2, we administered mCPP into amygdala subregions and the dorsal hippocampus to investigate site specificity. In Experiment 4 we lesioned the BNST and subsequently infused mCPP systemically, and in Experiment 4 we used RNAscope® to assess BNST 5-HT2C transcripts following wheel running. Results: BNST mCPP infusion increased acoustic startle responding, which was by 5-HT2C antagonism, while neither mCPP infused into the amygdala nor hippocampus was anxiogenic. Lesions of the BNST prevented the anxiogenic effect of systemically administered mCPP. Lastly, exercise reduced 5-HT2C transcripts in the BNST. Discussion: These results suggest that the BNST is a critical site of action for the effects of exercise on mCPP. Together these data suggest that exercise may reduce 5-HT2C receptor function in the BNST, which may, in part, explain some of the anxiolytic effects associated with wheel running.

Prefrontal cortex circuits in depression and anxiety: contribution of discrete neuronal populations and target regions.

Our understanding of depression and its treatment has advanced with the advent of ketamine as a rapid-acting antidepressant and the development and refinement of tools capable of selectively altering the activity of populations of neuronal subtypes. This work has resulted in a paradigm shift away from dysregulation of single neurotransmitter systems in depression towards circuit level abnormalities impacting function across multiple brain regions and neurotransmitter systems. Studies on the features of circuit level abnormalities demonstrate structural changes within the prefrontal cortex (PFC) and functional changes in its communication with distal brain structures. Treatments that impact the activity of brain regions, such as transcranial magnetic stimulation or rapid-acting antidepressants like ketamine, appear to reverse depression associated circuit abnormalities though the mechanisms underlying the reversal, as well as development of these abnormalities remains unclear. Recently developed optogenetic and chemogenetic tools that allow high-fidelity control of neuronal activity in preclinical models have begun to elucidate the contributions of the PFC and its circuitry to depression- and anxiety-like behavior. These tools offer unprecedented access to specific circuits and neuronal subpopulations that promise to offer a refined view of the circuit mechanisms surrounding depression and potential mechanistic targets for development and reversal of depression associated circuit abnormalities.

Ketamine increases vmPFC activity: Effects of (R)- and (S)-stereoisomers and (2R,6R)-hydroxynorketamine metabolite.

Ketamine, an NMDA receptor antagonist and fast acting antidepressant, produces a rapid burst of glutamate in the ventral medial prefrontal cortex (mPFC). Preclinical studies have demonstrated that pyramidal cell activity in the vmPFC is necessary for the rapid antidepressant response to ketamine in rodents. We sought to characterize the effects of ketamine and its stereoisomers (R and S), as well as a metabolite, (2R,6R)-hydroxynorketamine (HNK), on vmPFC activity using a genetically encoded calcium indicator (GCaMP6f). Ratiometric fiber photometry was utilized to monitor GCaMP6f fluorescence in pyramidal cells of mouse vmPFC prior to and immediately following administration of compounds. GCaMP6f signal was assessed to determine correspondance of activity between compounds. We observed dose dependent effects with (R,S)-ketamine (3-100 mg/kg), with the greatest effects on GCaMP6f activity at 30 mg/kg and lasting up to 20 min. (S)-ketamine (15 mg/kg), which has high affinity for the NMDA receptor channel produced similar effects to (R,S)-ketamine, but compounds with low NMDA receptor affinity, including (R)-ketamine (15 mg/kg) and (2R,6R)-HNK (30 mg/kg) had little or no effect on GCaMP6f activity. The initial response to administration of (R,S)-ketamine as well as (S)-ketamine is characterized by a brief period of robust GCaMP6f activation, consistent with increased activity of vmPFC pyramidal neurons. Because (2R,6R)-HNK and (R)-ketamine are reported to have antidepressant activity in rodent models the current results indicate that different initiating mechanisms lead to similar brain adaptive consequences that underlie the rapid antidepressant responses.

Optogenetic stimulation of medial prefrontal cortex Drd1 neurons produces rapid and long-lasting antidepressant effects.

Impaired function in the medial prefrontal cortex (mPFC) contributes to depression, and the therapeutic response produced by novel rapid-acting antidepressants such as ketamine are mediated by mPFC activity. The mPFC contains multiple types of pyramidal cells, but it is unclear whether a particular subtype mediates the rapid antidepressant actions of ketamine. Here we tested two major subtypes, Drd1 and Drd2 dopamine receptor expressing pyramidal neurons and found that activating Drd1 expressing pyramidal cells in the mPFC produces rapid and long-lasting antidepressant and anxiolytic responses. In contrast, photostimulation of Drd2 expressing pyramidal cells was ineffective across anxiety-like and depression-like measures. Disruption of Drd1 activity also blocked the rapid antidepressant effects of ketamine. Finally, we demonstrate that stimulation of mPFC Drd1 terminals in the BLA recapitulates the antidepressant effects of somatic stimulation. These findings aid in understanding the cellular target neurons in the mPFC and the downstream circuitry involved in rapid antidepressant responses.

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 ß (GSK3ß) 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.

Activity-Dependent Brain-Derived Neurotrophic Factor Release Is Required for the Rapid Antidepressant Actions of Scopolamine.

BACKGROUND: Brain-derived neurotrophic factor (BDNF) plays a key role in the pathophysiology and treatment of depression. Recent clinical studies demonstrate that scopolamine, a nonselective muscarinic acetylcholine receptor antagonist, produces rapid antidepressant effects in patients with depression. Rodent studies demonstrate that scopolamine increases glutamate transmission and synaptogenesis in the medial prefrontal cortex (mPFC). Here we tested the hypothesis that activity-dependent BDNF release within the mPFC is necessary for the antidepressant actions of scopolamine. METHODS: Behavioral effects of scopolamine were assessed in BDNF Val/Met knock-in mice, in which BDNF processing and release are impaired. In addition, intra-mPFC infusion of a BDNF-neutralizing antibody was performed to test the necessity of BDNF release in driving scopolamine-induced behavioral responses. Further in vivo and in vitro experiments were performed to delineate BDNF-dependent mechanisms underlying the effects of scopolamine. RESULTS: We found that BDNF Met/Met mice have attenuated responses to scopolamine and that anti-BDNF antibody infusions into the mPFC prevented the antidepressant-like behavioral effects of scopolamine. In vitro experiments show that scopolamine rapidly stimulates BDNF release and tropomyosin receptor kinase B-extracellular signal-regulated kinase signaling. Moreover, these effects require alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor activation and are blocked by neuronal silencing. Importantly, pretreatment with verapamil prevented scopolamine-induced behavioral responses and BDNF-tropomyosin receptor kinase B signaling, suggesting that these effects are dependent on activation of voltage-dependent calcium channels. CONCLUSIONS: The results identify an essential role for activity-dependent BDNF release in the rapid antidepressant effects of scopolamine. Attenuation of responses in BDNF Met mice indicates that patients with the Met allele may be less responsive to scopolamine.

Molecular and Cellular Effects of Traumatic Stress: Implications for PTSD.

PURPOSE OF REVIEW: Posttraumatic 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 FINDINGS: This 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. Integration 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.

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.

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.

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.