Neurobiology of chronic mild stress: parallels to major depression

Exposure to chronic mild stress prevents kappa opioid-mediated reinstatement of cocaine and nicotine place preference

Stress increases the risk of drug abuse, causes relapse to drug seeking, and potentiates the rewarding properties of both nicotine and cocaine. Understanding the mechanisms by which stress regulates the rewarding properties of drugs of abuse provides valuable insight into potential treatments for drug abuse. Prior reports have demonstrated that stress causes dynorphin release, activating kappa opioid receptors (KOR) in monoamine circuits resulting in both potentiation and reinstatement of cocaine and nicotine conditioned place preference. Here we report that kappa opioid-dependent reinstatement of cocaine and nicotine place preference is reduced when the mice are exposed to a randomized chronic mild stress (CMS) regime prior to training in a conditioned place preference-reinstatement paradigm. The CMS schedule involves seven different stressors (removal of nesting for 24 h, 5 min forced swim stress at 15°C, 8 h food and water deprivation, damp bedding overnight, white noise, cage tilt, and disrupted home cage lighting) rotated over a 3-week period. This response is KOR-selective, as CMS does not protect against cocaine or nicotine drug-primed reinstatement. This protection from reinstatement is also observed following sub-chronic social defeat stress, where each mouse is placed in an aggressor mouse home cage for a period of 20 min over 5 days. In contrast, a single acute stressor resulted in a potentiation of KOR-induced reinstatement, as previously reported. Prior studies have shown that stress alters sensitivity to opioids and prior stress can influence the pharmacodynamics of the opioid receptor system. Together, these findings suggest that exposure to different forms of stress may cause a dysregulation of kappa opioid circuitry and that changes resulting from mild stress can have protective and adaptive effects against drug relapse.

Effects of curcumin on glucose metabolism in the brains of rats subjected to chronic unpredictable stress: a 18 F-FDG micro-PET study

Background

Chronic unpredictable stress (CUS) can cause behavioral and physiological abnormalities that are important to the prediction of symptoms of depression that may be associated with cerebral glucose metabolic abnormalities. Curcumin showed potential antidepressant effects, but whether or not it can reverse cerebral functional abnormalities and so ameliorate depression remains unknown.

Methods

To investigate the effects of curcumin on brain activity in CUS rats, rats were subjected to 3 weeks of CUS and then treated with curcumin orally at a dose of 40 mg/kg/day for one month. ¹⁸ F fluorodeoxyglucose (¹⁸ F-FDG)-micro positron emission tomography (micro-PET) neuroimaging was used to detect changes in cerebral metabolism. Body weight, sucrose preference, and open field tests were used to record depressive behaviors during CUS and after curcumin treatment.

Results

Three weeks of CUS significantly decreased body weight, sucrose preference, sucrose consumption, total distance travelling, and the number of rearing events. It also induced metabolic alterations in several parts of the brain, showing increased glucose metabolism in the right hemisphere. After curcumin treatment for one month, sucrose preference, sucrose consumption, total distance travelling, and the number of rearing events returned to normal levels. Curcumin treatment also induced strong deactivation of the left primary auditory cortex and activation of amygdalohippocampal cortex.

Conclusion

Curcumin was found to ameliorate the abnormalities in the behavior and brain glucose metabolism caused by CUS, which may account for its antidepressive effects.

Lamotrigine positively affects the development of psychiatric comorbidity in epileptic animals, while psychiatric comorbidity aggravates seizures

Several clinical and preclinical studies have focused on the relationship between epilepsy and psychological disturbances. Although behavior in some experimental models of epilepsy has been studied, only few of them can be considered as models of epilepsy and mood disorder comorbidity. Since several models of epilepsy or psychiatric disorders are already available, we wondered whether a mixture of the two could experimentally represent a valid alternative to study such comorbidity. Here, we present a possible experimental protocol to study drug effects and physiopathogenesis of psychiatric comorbidity in epileptic animals. Pentylentetrazol-kindled animals were subjected to the chronic mild stress (CMS) procedure; furthermore, we tested the effects of chronic lamotrigine treatment on the development of comorbidity. We found that epileptic-depressed animals showed more pronounced behavioral alterations in comparison to other mice groups, indicating that kindled animals develop more pronounced CMS-induced behavioral alterations than nonepileptic mice; lamotrigine was able to prevent the development of comorbidities such as anxiety, depression-like behavior, and memory impairment.

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

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

Transient increase in alcohol self-administration following a period of chronic exposure to corticosterone

Stressful life events and chronic stressors have been associated with escalations in alcohol drinking. Stress exposure leads to the secretion of glucocorticoids (cortisol in the human; corticosterone (CORT) in the rodent). To model a period of heightened elevations in CORT, the present work assessed the effects of chronic exposure to the stress hormone CORT on alcohol self-administration. Male Long Evans rats were trained to self-administer a sweetened alcohol solution (2% sucrose/15% alcohol) resulting in moderate levels of daily alcohol intake (0.5-0.7 g/kg). Following stable baseline operant self-administration, rats received CORT in the drinking water for 7 days. A transient increase in alcohol self-administration was observed on the first self-administration session following CORT exposure, and behavior returned to control levels by the second session. Control experiments determined that this increase in alcohol self-administration was specific to alcohol, unrelated to general motor activation, and functionally dissociated from decreased CORT levels at the time of testing. These results indicate that repeated exposure to heightened levels of stress hormone (e.g., as may be experienced during stressful episodes) has the potential to lead to exacerbated alcohol intake in low to moderate drinkers. Given that maladaptive drinking patterns, such as escalated alcohol drinking following stressful episodes, have the potential to put an individual at risk for future drinking disorders, utilization of this model will be important for examination of neuroadaptations that occur as a consequence of CORT exposure in order to better understand escalated drinking following stressful episodes in nondependent individuals.

Age of stress exposure modulates the immediate and sustained effects of repeated stress on corticolimbic cannabinoid CB₁ receptor binding in male rats

Chronic stress is known to modulate cannabinoid CB1 receptor binding densities in corticolimbic structures, in a region-dependent manner; however, the ontogeny of these changes and the degree to which they recover following exposure to stress have yet to be determined. To this extent, we examined both the immediate and sustained effects (following a 40-day recovery period) of a repeated restraint stress paradigm (30-min restraint/day for 10 days) on CB1 receptor binding in the prefrontal cortex (PFC), hippocampus and amygdala in both adolescent (stress onset at post-natal day [PND] 35) and adult (stress onset at PND 75) male Sprague-Dawley rats. Consistent with previous reports, we found that repeated stress in adult rats resulted in an increase in CB1 receptor binding in the PFC, a reduction in CB1 receptor binding in the hippocampus and no effect in the amygdala. Interestingly, adolescent rats exposed to repeated restraint stress did not show any change in hippocampal CB1 receptor density, but exhibited an upregulation of CB1 receptor binding in both the PFC and amygdala. In adults, a 40-day recovery period resulted in a normalization of CB1 receptor binding in the PFC, and surprisingly a pronounced upregulation of CB1 receptor binding in the hippocampus, possibly indicative of a rebound effect. Adolescents similarly exhibited this rebound increase in hippocampal CB1 receptor binding, despite a lack in immediate downregulation following repeated restraint. Of particular interest, adolescents exposed to stress were found to have a sustained downregulation of prefrontocortical CB1 receptors in adulthood, which may relate to some of the reported sustained behavioral effects of stress in adolescence. Collectively, these data indicate that the effects of chronic stress on cannabinoid CB1 receptor binding are modulated by the age of stress exposure and period of recovery following the cessation of stress.

An update on mindfulness meditation as a self-help treatment for anxiety and depression

In recent years, complementary and alternative medicine (CAM) treatments have increased in popularity. This is especially true for treatments that are related to exercise and mindfulness-based interventions (MBIs) in the treatment of both mental and physical illness. MBIs, such as Mindfulness-based Cognitive Therapy (MBCT) and Mindfulness-Based Stress Reduction (MBSR), which are derived from ancient Buddhist and Yoga philosophies, have become popular treatments in contemporary psychotherapy. While there is growing evidence that supports the role of these interventions in relapse prevention, little is known about the role that MBIs play in the treatment of acute symptoms of depression and anxiety. Even less is known about the importance of specific components of MBIs (eg, mindfulness meditation [MM]) and the overall impact that these interventions have on the experience or expression of psychological distress. Moreover, few studies have rigorously evaluated the dose-response relationship that is required to effect positive symptom change and the mechanisms of change that are responsible for observed improvements. This review will define meditation and mindfulness, discuss the relationship between stress and health and how MM relates to therapeutically engaging the relaxation response, and review the empirical findings that are related to the efficacy of MM in the treatment of depression and anxiety symptoms. Given the paucity of research that examines the applications of these treatments in clinical populations, the limitations of applying these findings to clinical samples will be mentioned. A brief review of the issues related to the possible mechanisms of change and the dose-response relationship regarding MBIs, particularly MM, will be provided. Finally, limitations of the extant literature and future directions for further exploration of this topic will be offered.

Upregulation of CB₁ receptor binding in the ventromedial prefrontal cortex promotes proactive stress-coping strategies following chronic stress exposure

Accumulating evidence has revealed that dysregulation of the endocannabinoid system could contribute to the development of major depression. Studies carried out post-mortem in depressed suicide victims have revealed increased CB(1) receptor binding site density in the prefrontal cortex (PFC). Accordingly, exposure of rodents to chronic unpredictable stress (CUS) results in phenotypic changes that mirror those of human depression, including increased CB(1) receptor binding site density in the PFC. Our goal in these studies was to examine the effects of CUS on the density of CB(1) receptor binding sites in the rodent medial PFC and to explore the role of this alteration in the behavioral changes invoked by CUS. Rodents exposed to CUS exhibited increased CB(1) receptor maximal binding site density (B(max)) within the ventromedial PFC, but not the dorsomedial PFC. To determine whether this change in the ventromedial PFC is an adaptive response, or alternatively, a consequence of chronic stress that contributes to the adoption of passive coping, we examined whether local CB(1) receptor blockade within the ventromedial PFC following CUS would significantly alter behaviors in the forced swim test (FST). CUS exposure significantly increased passive coping in the FST, and this was further augmented by discrete ventromedial PFC microinfusions of the CB(1) receptor antagonist AM251 prior to swim stress. Moreover, local CB(1) receptor blockade reduced active coping responses in CUS-exposed rats. These findings suggest that the increase in CB(1) receptor B(max) observed in the ventromedial PFC of rodents exposed to CUS maintains proactive coping strategies following chronic stress exposure.