Differential corticosteroid receptor regulation of mesoaccumbens dopamine efflux during the peak and nadir of the circadian rhythm: A molecular equilibrium in the midbrain?

Stressed and wired: The effects of stress on the VTA circuits underlying motivated behavior

Stress affects many brain regions, including the ventral tegmental area (VTA), which is critically involved in reward processing. Excessive stress can reduce reward-seeking behaviors but also exacerbate substance use disorders, two seemingly contradictory outcomes. Recent research has revealed that the VTA is a heterogenous structure with diverse populations of efferents and afferents serving different functions. Stress has correspondingly diverse effects on VTA neuron activity, tending to decrease lateral VTA dopamine (DA) neuron activity, while increasing medial VTA DA and GABA neuron activity. Here we review the differential effects of stress on the activity of these distinct VTA neuron populations and how they contribute to decreases in reward-seeking behavior or increases in drug self-administration.

Corticosterone in the ventral hippocampus differentially alters accumbal dopamine output in drug-naïve and amphetamine-withdrawn rats

Dysregulation in glucocorticoid stress and accumbal dopamine reward systems can alter reward salience to increase motivational drive in control conditions while contributing to relapse during drug withdrawal. Amphetamine withdrawal is associated with dysphoria and stress hypersensitivity that may be mediated, in part, by enhanced stress-induced corticosterone observed in the ventral hippocampus. Electrical stimulation of the ventral hippocampus enhances accumbal shell dopamine release, establishing a functional connection between these two regions. However, the effects of ventral hippocampal corticosterone on this system are unknown. To address this, a stress-relevant concentration of corticosterone (0.24ng/0.5 μL) or vehicle were infused into the ventral hippocampus of urethane-anesthetized adult male rats in control and amphetamine withdrawn conditions. Accumbal dopamine output was assessed with in vivo chronoamperometry. Corticosterone infused into the ventral hippocampus rapidly enhanced accumbal dopamine output in control conditions, but produced a biphasic reduction of accumbal dopamine output in amphetamine withdrawal. Selectively blocking glucocorticoid-, mineralocorticoid-, or cytosolic receptors prevented the effects of corticosterone. Overall, these results suggest that the ability of corticosterone to alter accumbal dopamine output requires cooperative activation of mineralocorticoid and glucocorticoid receptors in the cytosol, which is dysregulated during amphetamine withdrawal. These findings implicate ventral hippocampal corticosterone in playing an important role in driving neural systems involved in positive stress coping mechanisms in healthy conditions, whereas dysregulation of this system may contribute to relapse during withdrawal.

Top-down and bottom-up control of stress-coping

In this 30th anniversary issue review, we focus on the glucocorticoid modulation of limbic-prefrontocortical circuitry during stress-coping. This action of the stress hormone is mediated by mineralocorticoid receptors (MRs) and glucocorticoid receptors (GRs) that are co-expressed abundantly in these higher brain regions. Via both receptor types, the glucocorticoids demonstrate, in various contexts, rapid nongenomic and slower genomic actions that coordinate consecutive stages of information processing. MR-mediated action optimises stress-coping, whereas, in a complementary fashion, the memory storage of the selected coping strategy is promoted via GR. We highlight the involvement of adipose tissue in the allocation of energy resources to central regulation of stress reactions, point to still poorly understood neuronal ensembles in the prefrontal cortex that underlie cognitive flexibility critical for effective coping, and evaluate the role of cortisol as a pleiotropic regulator in vulnerability to, and treatment of, trauma-related psychiatric disorders.

Blockade of TrkB receptors in the nucleus accumbens prior to heterotypic stress alters corticotropin-releasing hormone (CRH), vesicular glutamate transporter 2 (vGluT2) and glucocorticoid receptor (GR) within the mesolimbic pathway

Inhibition of stress-induced elevations in brain-derived neurotrophic factor (BDNF) or its primary receptor tyrosine-related kinase B (TrkB) within the reward pathway may modulate vulnerability to anxiety and mood disorders. The current study examined the role of BDNF/TrkB signaling on biochemistry and behavior under basal conditions and following exposure to a 10-day heterotypic stress paradigm in male rats. Effects of intra-accumbal administration of TrkB antagonist ANA-12 (0.25μg/0.5μl/min) on anxiety, and expression of Trk-B, corticotropin-releasing hormone (CRH), vesicular glutamate transporter 2 (vGluT2) and glucocorticoid receptor (GR) within the mesolimbic pathway were determined. Notably, ANA-12 attenuated anxiety-like behavior in stress rats while increasing anxiety in the non-stress group in the elevated plus maze (EPM). At the neurochemical level, ANA-12 blocked the increased vGluT2 and CRH expressions in the hypothalamic PVN and basolateral amygdala in stress rats, while it enhanced vGluT2 and CRH expressions in non-stress rats. ANA-12 also showed state-dependent effects at the NAc core, attenuating TrkB-ir in non-stress rats while reversing reduced expression in stressed rats. At the cingulate cortex, ANA-12 normalized stress-induced increase in TrkB expression. Notably, ANA-12 showed region-specific effects on GR-ir at the NAc core and shell, with increased GR-ir in non-stress rats, although the drug attenuated stress-induced GR-ir expression only in the core portion of the NAc, while having no impact at the cingulate cortex. Elevated blood CORT levels post-stress was not influenced by ANA-12 treatment. Together, these findings suggest that BDNF-mediated TrkB activation exerts differential impact in regulating emotional response under basal and stress conditions.

Striatal Hypersensitivity During Stress in Remitted Individuals with Recurrent Depression

BACKGROUND

Increased sensitivity to stress and dysfunctional reward processing are two primary characteristics of major depressive disorder (MDD) that may persist after remission. Preclinical work has established the pivotal role of the striatum in mediating both stress and reward responses. Human neuroimaging studies have corroborated these preclinical findings and highlighted striatal dysfunction in MDD in response to reward but have yet to investigate striatal function during stress, in particular in individuals with recurrent depression.

METHODS

A validated mild psychological stress task involving viewing of negative stimuli during functional magnetic resonance imaging was conducted in 33 remitted individuals with a history of recurrent major depressive disorder (rMDD) and 35 matched healthy control subjects. Cortisol and anxiety levels were assessed throughout scanning. Stress-related activation was investigated in three striatal regions: caudate, nucleus accumbens, and putamen. Psychophysiologic interaction analyses probed connectivity of regions with central structures of the neural stress circuitry, such as the amygdala and hippocampus.

RESULTS

The task increased cortisol and anxiety levels, although to a greater extent in rMDD individuals than healthy control subjects. In response to the negative stimuli, rMDD individuals, but not controls, also exhibited significantly potentiated caudate, nucleus accumbens, and putamen activations and increased caudate-amygdala and caudate-hippocampus connectivity.

CONCLUSIONS

The findings highlight striatal hypersensitivity in response to a mild psychological stress in rMDD, as manifested by hyperactivation and hyperconnectivity with the amygdala and hippocampus. Striatal hypersensitivity during stress might thus constitute a trait mark of depression, providing a potential neural substrate for the interaction between stress and reward dysfunction in MDD.

Identifying candidate drivers of alcohol dependence-induced excessive drinking by assembly and interrogation of brain-specific regulatory networks

BACKGROUND

A systems biology approach based on the assembly and interrogation of gene regulatory networks, or interactomes, was used to study neuroadaptation processes associated with the transition to alcohol dependence at the molecular level.

RESULTS

Using a rat model of dependent and non-dependent alcohol self-administration, we reverse engineered a global transcriptional regulatory network during protracted abstinence, a period when relapse rates are highest. We then interrogated the network to identify master regulator genes that mechanistically regulate brain region-specific signatures associated with dependent and non-dependent alcohol self-administration. Among these, the gene coding for the glucocorticoid receptor was independently identified as a master regulator in multiple brain regions, including the medial prefrontal cortex, nucleus accumbens, central nucleus of the amygdala, and ventral tegmental area, consistent with the view that brain reward and stress systems are dysregulated during protracted abstinence. Administration of the glucocorticoid antagonist mifepristone in either the nucleus accumbens or ventral tegmental area selectively decreased dependent, excessive, alcohol self-administration in rats but had no effect on non-dependent, moderate, alcohol self-administration.

CONCLUSIONS

Our study suggests that assembly and analysis of regulatory networks is an effective strategy for the identification of key regulators of long-term neuroplastic changes within specific brain regions that play a functional role in alcohol dependence. More specifically, our results support a key role for regulatory networks downstream of the glucocorticoid receptor in excessive alcohol drinking during protracted alcohol abstinence.

Feelings about food: the ventral tegmental area in food reward and emotional eating

Overconsumption of high caloric food plays an important role in the etiology of obesity. Several factors drive such hedonic feeding. High caloric food is often palatable. In addition, when an individual is sated, stress and food-related cues can serve as potent feeding triggers. A better understanding of the neurobiological underpinnings of food palatability and environmentally triggered overconsumption would aid the development of new treatment strategies. In the current review we address the pivotal role of the mesolimbic dopamine reward system in the drive towards high caloric palatable food and its relation to stress- and cue-induced feeding. We also discuss how this system may be affected by both established and potential anti-obesity drug targets.

The implication of neuroactive steroids in Tourette's syndrome pathogenesis: A role for 5α-reductase?

Tourette's syndrome (TS) is a neurodevelopmental disorder characterised by recurring motor and phonic tics. The pathogenesis of TS is considered to reflect dysregulations in the signalling of dopamine (DA) and other neurotransmitters, which lead to excitation/inhibition imbalances in cortico-striato-thalamocortical circuits. The causes of these deficits may reflect complex gene × environment × sex (G × E × S) interactions; indeed, the disorder is markedly predominant in males, with a male-to-female prevalence ratio of approximately 4 : 1. Converging lines of evidence point to neuroactive steroids as being likely molecular candidates to account for G × E × S interactions in TS. Building on these premises, our group has begun examining the possibility that alterations in the steroid biosynthetic process may be directly implicated in TS pathophysiology; in particular, our research has focused on 5α-reductase (5αR), the enzyme catalysing the key rate-limiting step in the synthesis of pregnane and androstane neurosteroids. In clinical and preclinical studies, we found that 5αR inhibitors exerted marked anti-DAergic and tic-suppressing properties, suggesting a central role for this enzyme in TS pathogenesis. Based on these data, we hypothesise that enhancements in 5αR activity in early developmental stages may lead to an inappropriate activation of the 'backdoor' pathway for androgen synthesis from adrenarche until the end of puberty. We predict that the ensuing imbalances in steroid homeostasis may impair the signalling of DA and other neurotransmitters, ultimately resulting in the facilitation of tics and other behavioural abnormalities in TS.

The role of the glucocorticoids in developing resilience to stress and addiction

There is emerging evidence that individuals have the capacity to learn to be resilient by developing protective mechanisms that prevent them from the maladaptive effects of stress that can contribute to addiction. The emerging field of the neuroscience of resilience is beginning to uncover the circuits and molecules that protect against stress-related neuropsychiatric diseases, such as addiction. Glucocorticoids (GCs) are important regulators of basal and stress-related homeostasis in all higher organisms and influence a wide array of genes in almost every organ and tissue. GCs, therefore, are ideally situated to either promote or prevent adaptation to stress. In this review, we will focus on the role of GCs in the hypothalamic-pituitary adrenocortical axis and extra-hypothalamic regions in regulating basal and chronic stress responses. GCs interact with a large number of neurotransmitter and neuropeptide systems that are associated with the development of addiction. Additionally, the review will focus on the orexinergic and cholinergic pathways and highlight their role in stress and addiction. GCs play a key role in promoting the development of resilience or susceptibility and represent important pharmacotherapeutic targets that can reduce the impact of a maladapted stress system for the treatment of stress-induced addiction.

Deep brain stimulation for treatment-resistant depression: efficacy, safety and mechanisms of action

Deep brain stimulation (DBS), a neuromodulation therapy that has been used successfully in the treatment of symptoms associated with movement disorders, has recently undergone clinical trials for individuals suffering from treatment-resistant depression (TRD). Although the small patient numbers and open label study design limit our ability to identify optimum targets and make definitive conclusions about treatment efficacy, a review of the published research demonstrates significant reductions in depressive symptomatology and high rates of remission in a severely treatment-resistant patient group. Despite these encouraging results, an incomplete understanding of the mechanisms of action underlying the therapeutic effects of DBS for TRD is highlighted, paralleling the incomplete understanding of the neuroanatomy of mood regulation and treatment resistance. Proposed mechanisms of action include short and long-term local effects of stimulation at the neuronal level, to modulation of neural network activity.

Stress, neurotransmitters, corticosterone and body-brain integration

Stress can be defined as a brain-body reaction towards stimuli arising from the environment or from internal cues that are interpreted as a disruption of homeostasis. The organization of the response to a stressful situation involves not only the activity of different types of neurotransmitter systems in several areas of the limbic system, but also the response of neurons in these areas to several other chemicals and hormones, chiefly glucocorticoids, released from peripheral organs and glands. Thus, stress is probably the process through which body-brain integration plays a major role. Here we review first the responses to an acute stress in terms of neurotransmitters such as dopamine, acetylcholine, glutamate and GABA in areas of the brain involved in the regulation of stress responses. These areas include the prefrontal cortex, amygdala, hippocampus and nucleus accumbens and the interaction among those areas. Then, we consider the role of glucocorticoids and review some recent data about the interaction of these steroids with several neurotransmitters in those same areas of the brain. Also the actions of other substances (neuromodulators) released from peripheral organs such as the pancreas, liver or gonads (insulin, IGF-1, estrogens) are reviewed. The role of an environmental enrichment on these same responses is also discussed. Finally a section is devoted to put into perspective all these environmental-brain-body-brain interactions during stress and their consequences on aging. It is concluded that the integrative perspective framed in this review is relevant for better understanding of how the organism responds to stressful challenges and how this can be modified through different environmental conditions during the process of aging. This article is part of a Special Issue entitled: Brain Integration.

Oxidative stress and autonomic nervous system functions in restless legs syndrome

BACKGROUND

Oxidative stress has been implicated in over 100 disorders in recent years; however, the situation in restless legs syndrome (RLS) has not been studied yet.

METHODS

Fifty patients with RLS not medicated for RLS and 50 sex- and age-matched, healthy controls and controls with no pathology except mild iron deficiency or iron deficiency anaemia were enrolled. Patients with secondary RLS other than iron deficiency were excluded. Total oxidant status (TOS), total antioxidant status (TAS), oxidative stress index (OSI), arylesterase (ARE), paraoxonase (PON), stimulated paraoxonase (stim-PON), lipid hydroperoxides (LOOHs), acetyl cholinesterase (AChE) and butyryl cholinesterase (BuChE) were measured. Heart rate variability (HRV) analysis was performed.

RESULTS

TOS, ARE and AChE were increased (P = 0·018, P < 0·001 and P < 0·001, respectively), whereas LOOHs were decreased (P < 0·001) in RLS group. TAS, OSI, PON and stim-PON were comparable. Erythrocyte sedimentation rate (ESR) and mean platelet volume (MPV) were increased (P = 0·021 and P = 0·037, respectively) in RLS group. HRV triangular index (HRVi) was lower (P = 0·012) in RLS group. Other HRV parameters were similar.

CONCLUSIONS

Increased AChE and decreased LOOHs, which were influenced by increased PON1, were considered as indicators of efforts towards the protection of dopaminergic activity in central nervous system in RLS group. Increased ESR, MPV and low HRVi indicate elevated sympathetic activity in RLS group. Elevated sympathetic activity might be beneficial in relieving RLS symptoms, also causing increases in TOS. The evidence we found regarding oxidative stress and autonomic nervous system might be seminal in RLS treatment.

Genome-wide expression analysis reveals diverse effects of acute nicotine exposure on neuronal function-related genes and pathways

Previous human and animal studies demonstrate that acute nicotine exposure has complicated influences on the function of the nervous system, which may lead to long-lasting effects on the behavior and physiology of the subject. To determine the genes and pathways that might account for long-term changes after acute nicotine exposure, a pathway-focused oligoarray specifically designed for drug addiction research was used to assess acute nicotine effect on gene expression in the neuron-like SH-SY5Y cells. Our results showed that 295 genes involved in various biological functions were differentially regulated by 1 h of nicotine treatment. Among these genes, the expression changes of 221 were blocked by mecamylamine, indicating that the majority of nicotine-modulated genes were altered through the nicotinic acetylcholine receptors (nAChRs)-mediated signaling process. We further identified 14 biochemical pathways enriched among the nicotine-modulated genes, among which were those involved in neural development/synaptic plasticity, neuronal survival/death, immune response, or cellular metabolism. In the genes significantly regulated by nicotine but blocked by mecamylamine, 13 enriched pathways were detected. Nine of these pathways were shared with those enriched in the genes regulated by nicotine, including neuronal function-related pathways such as glucocorticoid receptor signaling, p38 MAPK signaling, PI3K/AKT signaling, and PTEN signaling, implying that nAChRs play important roles in the regulation of these biological processes. Together, our results not only provide insights into the mechanism underlying the acute response of neuronal cells to nicotine but also provide clues to how acute nicotine exposure exerts long-term effects on the nervous system.

Enhanced learning deficits in female rats following lifetime pb exposure combined with prenatal stress

Pb (lead) exposure and stress are co-occurring risk factors (particularly in low socioeconomic communities) that also act on common biological substrates and produce common adverse outcomes, including cognitive impairments. This study sought to determine whether lifetime Pb exposure combined with prenatal stress would enhance the cognitive deficits independently associated with each of these risk factors and to explore associated mechanisms of any observed impairments. Learning was evaluated using a multiple schedule of repeated learning and performance in female rats subjected to lifetime Pb exposure (0 or 50 ppm Pb in drinking water beginning in dams 2 months prior to breeding; blood Pb levels ∼10 μg/dl), to prenatal restraint stress on gestational days 16 and 17, or to both. Blood Pb, corticosterone levels, brain monoamines, and hippocampal nerve growth factor levels were also measured. Sequence-specific learning deficits produced by Pb, particularly the number of responses to correctly learn response sequences, were further enhanced by stress, whereas performance measures were unimpaired. Statistical analyses indicated significant relationships among corticosterone levels, frontal cortex dopamine (DA), nucleus accumbens dopamine turnover, and total responses required to learn sequences. This study demonstrates that Pb and stress can act together to produce selective and highly condition-dependent deficits in learning in female rats that may be related to glucocorticoid-mediated interactions with mesocorticolimbic regions of brain. These findings also underscore the critical need to evaluate toxicants in the context of other risk factors pertinent to human diseases and disorders.

Social isolation and stress related hormones modulate the stimulating effect of ethanol in preweanling rats

Preweanling rats are highly sensitive to the locomotor stimulation induced by relatively high ethanol doses. In adult mice this ethanol effect is modulated by stress. The goal of the present study was to analyze the role of stress and corticosterone in the stimulating effect of ethanol in preweanling rats. In Experiment 1 15-day-old rats were separated from the mother during a period of 4h in which subjects remained isolated or paired with a littermate. In a third condition pups remained in the home-cage with the dam. After this isolation period pups were given ethanol (0 or 2.5 g/kg) and were tested in a novel environment. Previous data have shown that a similar period of isolation is enough to increase corticosterone levels in preweanling rats. Experiment 2 evaluated the effect of exogenous administration of corticosterone (0, 3 or 6 mg/kg) along with ethanol, and Experiment 3 tested ethanol-mediated locomotor activation in adrenalectomized preweanling rats. The last experiment aimed to test the role of corticotropic releasing factor 1 (CRF1) receptors in locomotion induced by ethanol in isolated pups. According to our results there is a synergism between stress or corticosterone and ethanol in preweanling rats. The interaction between stress (induced by social isolation) and ethanol seems to be mediated by CRF, since blockade of CRF1 receptors cancelled the effect of ethanol in isolated pups. This study highlights the importance of considering stress as a possible intervening variable in studies evaluating ethanol effects in developing animals when maternal separation is used in the experimental procedure.