Social Stress Engages Neurochemically-Distinct Afferents to the Rat Locus Coeruleus Depending on Coping Strategy

Noradrenergic Regulation of the Medial Prefrontal Cortex Mediates Stress Coping in Postpartum Female Mice

The prevalence of depression in women increases during the postpartum period. We previously reported that subchronic exposure to social stress decreased passive coping in postpartum female mice. This study aimed to investigate whether noradrenaline regulation might regulate coping styles in mice. We first determined whether a different type of stress, subchronic physical stress, decreases passive coping in postpartum females. Postpartum female, virgin female, and male mice were exposed to subchronic restraint stress (restraint stress for 4 h for 5 consecutive days). Subchronic restraint stress decreased passive coping in postpartum females but not in virgin females and males in the forced swim and tail suspension tests. We next examined the neuronal mechanism by which subchronic stress decreases passive coping in postpartum female mice. Neuronal activity and expression of noradrenergic receptors in the medial prefrontal cortex (mPFC) were analyzed using immunohistochemistry and reverse transcription-quantitative polymerase chain reaction, respectively. The mPFC was manipulated using chemogenetics, knockdown, or an α2A adrenergic receptor (AR) antagonist. Immunohistochemistry revealed that subchronic restraint stress increased glutamatergic neuron activation in the mPFC via forced swim stress and decreased α2A AR expression in postpartum females. Chemogenetic activation of glutamatergic neurons in the mPFC, knockdown of α2AAR in the mPFC, and the α2A AR receptor antagonist atipamezole treatment decreased passive coping in postpartum females. Subchronic restraint stress decreased passive coping in postpartum females by increasing glutamatergic neuron activity in the mPFC through α2A AR attenuation. The noradrenergic regulation of the mPFC may be a new target for treating postpartum depression.

Aged females unilaterally hypersensitize, lack descending inhibition, and overexpress alpha1D adrenergic receptors in a murine posttraumatic chronic pain model

ABSTRACT

Vulnerability to chronic pain is found to depend on age and sex. Most patients with chronic pain are elderly women, especially with posttraumatic pain after bone fracture that prevails beyond the usual recovery period and develops into a complex regional pain syndrome (CRPS). There, a distal bone fracture seems to initiate a pathophysiological process with unknown mechanism. To investigate whether sex, age, and alpha adrenergic receptors also contribute to a CRPS-like phenotype in animals, we performed experiments on tibia-fractured mice. Those mice commonly are resilient to the development of a CRPS-like phenotype. However, we found them to be vulnerable to long-lasting pain after distal bone fracture when they were of old age. These mice expressed mechanical and thermal hypersensitivity, as well as weight-bearing and autonomic impairment following bone trauma, which persisted over 3 months. Site-specific and body side-specific glycinergic and α1D-noradrenergic receptor expression in the spinal cord and the contralateral locus coeruleus were misbalanced. Aged female tibia-fractured mice lost descending noradrenergic inhibition and displayed enhanced spinal activity on peripheral pressure stimuli. Together, changes in the noradrenergic, hence, glycinergic system towards excitation in the pain pathway-ascending and descending-might contribute to the development or maintenance of long-lasting pain. Conclusively, changes in the noradrenergic system particularly occur in aged female mice after trauma and might contribute to the development of long-lasting pain. Our data support the hypothesis that some patients with chronic pain would benefit from lowering the adrenergic/sympathetic tone or antagonizing α1(D).

The development of compulsive coping behaviour is associated with a downregulation of Arc in a Locus Coeruleus neuronal ensemble

Some compulsive disorders have been considered to stem from the loss of control over coping strategies, such as displacement. However, the cellular mechanisms involved in the acquisition of coping behaviours and their subsequent compulsive manifestation in vulnerable individuals have not been elucidated. Considering the role of the locus coeruleus (LC) noradrenaline-dependent system in stress and related excessive behaviours, we hypothesised that neuroplastic changes in the LC may be associated with the acquisition of an adjunctive polydipsic water drinking, a prototypical displacement behaviour, and the ensuing development of compulsion in vulnerable individuals. Thus, male Sprague Dawley rats were characterised for their tendency, or not, to develop compulsive polydipsic drinking in a schedule-induced polydipsia (SIP) procedure before their fresh brains were harvested. A new quantification tool for RNAscope assays revealed that the development of compulsive adjunctive behaviour was associated with a low mRNA copy number of the plasticity marker Arc in the LC which appeared to be driven by specific adaptations in an ensemble of tyrosine hydroxylase (TH)+, zif268- neurons. This ensemble was specifically engaged by the expression of compulsive adjunctive behaviour, not by stress, because its functional recruitment was not observed in individuals that no longer had access to the water bottle before sacrifice, while it consistently correlated with the levels of polydipsic water drinking only when it had become compulsive. Together these findings suggest that downregulation of Arc mRNA levels in a population of a TH+/zif268- LC neurons represents a signature of the tendency to develop compulsive coping behaviours.

Locus Coeruleus-Noradrenergic Neurons Regulate Stress Coping During Subchronic Exposure to Social Threats: A Characteristic Feature in Postpartum Female Mice

Stress-coping strategies have been implicated in depression. The control of stress coping may improve the symptom and higher prevalence of depression during the postpartum period in women. However, the neuronal mechanisms underlying stress coping remain to be fully elucidated in postpartum women. In this study, we examined how locus coeruleus-noradrenergic (LC-NA) neurons, which have been associated with both stress coping and depression, regulate changes in coping style induced by subchronic exposure to unfamiliar male mice as a social threat in postpartum female mice. In contrast to virgin females, dams exposed to unfamiliar males daily for four consecutive days showed reduced immobility duration in the forced swim test, indicating that exposure to unfamiliar males decreased passive stress coping in dams. Exposure to unfamiliar males also decreased sucrose palatability in the sucrose preference test and suppressed the crouching behavior in the maternal care test but did not affect anxiety-like behavior in the hole-board test in dams. In fiber photometry analyses, LC-NA neurons showed differential activity between dams and virgin females in response to unfamiliar males. Chemogenetic inhibition of LC-NA neurons during exposure to unfamiliar males prevented the social threat-induced decrease in immobility duration in the forced swim test in dams. Furthermore, inhibition or activation of LC-NA neurons exacerbated crouching behavior in dams. These results indicate that LC-NA neurons regulate the social threat-induced decrease in passive stress coping and relieve social threat-induced inhibition of maternal care in postpartum female mice.

Neural signals implicated in the processing of appetitive and aversive events in social and non-social contexts

In 2014, we participated in a special issue of Frontiers examining the neural processing of appetitive and aversive events. Specifically, we reviewed brain areas that contribute to the encoding of prediction errors and value versus salience, attention and motivation. Further, we described how we disambiguated these cognitive processes and their neural substrates by using paradigms that incorporate both appetitive and aversive stimuli. We described a circuit in which the orbitofrontal cortex (OFC) signals expected value and the basolateral amygdala (BLA) encodes the salience and valence of both appetitive and aversive events. This information is integrated by the nucleus accumbens (NAc) and dopaminergic (DA) signaling in order to generate prediction and prediction error signals, which guide decision-making and learning via the dorsal striatum (DS). Lastly, the anterior cingulate cortex (ACC) is monitoring actions and outcomes, and signals the need to engage attentional control in order to optimize behavioral output. Here, we expand upon this framework, and review our recent work in which within-task manipulations of both appetitive and aversive stimuli allow us to uncover the neural processes that contribute to the detection of outcomes delivered to a conspecific and behaviors in social contexts. Specifically, we discuss the involvement of single-unit firing in the ACC and DA signals in the NAc during the processing of appetitive and aversive events in both social and non-social contexts.

Delta Opioid Receptors and Enkephalinergic Signaling within Locus Coeruleus Promote Stress Resilience

The noradrenergic nucleus locus coeruleus is a key component of the stress circuitry of the brain. During stress, the neuropeptide corticotropin-releasing factor (CRF) is secreted onto LC, increasing LC output and norepinephrine concentration in the brain, which is thought to promote anxiety-like behavior. LC is also innervated by several structures that synthesize and release the endogenous opioid peptide enkephalin onto LC upon stressor termination. While the role of CRF neurotransmission within LC in mediating anxiety-like behavior and the behavioral response to stress has been well characterized, the role of enkephalinergic signaling at LC-expressed δ-opioid receptors has been comparatively understudied. We have previously shown that acute stressor exposure increases LC activity and anxiety-like behavior for at least one week. Here, we extend these findings by showing that these effects may be mediated at least in part through stress-induced downregulation of DORs within LC. Furthermore, overexpression of DORs in LC blocks the effects of stress on both LC firing properties and anxiety-like behavior. In addition, intra-LC infusions of enkephalin blocked stress-induced freezing behavior and promoted conditioned place preference. These findings indicate that enkephalinergic neurotransmission at DORs within LC is an important component of the behavioral response to stress and may drive reward-related behavior as well.

The Role of the Locus Coeruleus in Pain and Associated Stress-Related Disorders

The locus coeruleus (LC)-noradrenergic system is the main source of noradrenaline in the central nervous system and is involved intensively in modulating pain and stress-related disorders (e.g., major depressive disorder and anxiety) and in their comorbidity. However, the mechanisms involving the LC that underlie these effects have not been fully elucidated, in part owing to the technical difficulties inherent in exploring such a tiny nucleus. However, novel research tools are now available that have helped redefine the LC system, moving away from the traditional view of LC as a homogeneous structure that exerts a uniform influence on neural activity. Indeed, innovative techniques such as DREADDs (designer receptors exclusively activated by designer drugs) and optogenetics have demonstrated the functional heterogeneity of LC, and novel magnetic resonance imaging applications combined with pupillometry have opened the way to evaluate LC activity in vivo. This review aims to bring together the data available on the efferent activity of the LC-noradrenergic system in relation to pain and its comorbidity with anxiodepressive disorders. Acute pain triggers a robust LC stress response, producing spinal cord-mediated endogenous analgesia while promoting aversion, vigilance, and threat detection through its ascending efferents. However, this protective biological system fails in chronic pain, and LC activity produces pain facilitation, anxiety, increased aversive memory, and behavioral despair, acting at the medulla, prefrontal cortex, and amygdala levels. Thus, the activation/deactivation of specific LC projections contributes to different behavioral outcomes in the shift from acute to chronic pain.

Central Taurine Attenuates Hyperthermia and Isolation Stress Behaviors Augmented by Corticotropin-Releasing Factor with Modifying Brain Amino Acid Metabolism in Neonatal Chicks

The objective of this study was to determine the effects of centrally administered taurine on rectal temperature, behavioral responses and brain amino acid metabolism under isolation stress and the presence of co-injected corticotropin-releasing factor (CRF). Neonatal chicks were centrally injected with saline, 2.1 pmol of CRF, 2.5 μmol of taurine or both taurine and CRF. The results showed that CRF-induced hyperthermia was attenuated by co-injection with taurine. Taurine, alone or with CRF, significantly decreased the number of distress vocalizations and the time spent in active wakefulness, as well as increased the time spent in the sleeping posture, compared with the saline- and CRF-injected chicks. An amino acid chromatographic analysis revealed that diencephalic leucine, isoleucine, tyrosine, glutamate, asparagine, alanine, β-alanine, cystathionine and 3-methylhistidine were decreased in response to taurine alone or in combination with CRF. Central taurine, alone and when co-administered with CRF, decreased isoleucine, phenylalanine, tyrosine and cysteine, but increased glycine concentrations in the brainstem, compared with saline and CRF groups. The results collectively indicate that central taurine attenuated CRF-induced hyperthermia and stress behaviors in neonatal chicks, and the mechanism likely involves the repartitioning of amino acids to different metabolic pathways. In particular, brain leucine, isoleucine, cysteine, glutamate and glycine may be mobilized to cope with acute stressors.

High-fat diet attenuates morphine withdrawal effects on sensory-evoked locus coeruleus norepinephrine neural activity in male obese rats

Objective: These experiments sought to characterize the effects of obesity propensity and obesogenic diet on locus coeruleus (LC) norepinephrine (NE) activity and determine the effects of obesity on LC neural responses to morphine withdrawal.Methods: In vivo single-unit LC electrophysiological activity was measured in obese prone (OP) and obese resistant (OR) male SD rats following high-fat (HFD: 45% fat) or low-fat (LFD; 10% fat) feeding. A separate cohort of LFD and HFD rats underwent in vivo LC recording on day 3 of spontaneous morphine withdrawal following an escalation dose paradigm (5-15 mg/kg; SQ twice daily).Results: OP (LFD: 34 cells/7 rats; HFD: 32 cells/6 rats) had higher spontaneous and tonic activity, and lower sensory-evoked activity compared with OR (LFD: 31 cells/6 rats; HFD: 41 cells/7 rats). Interacting effect of diet x strain status was observed on signal-to-noise ratio with OR-LFD having higher ratio than OP-LFD and OP-HFD. Morphine treatment decreased body weights. Withdrawal increased sensory-evoked rate in LFD (morphine; 20 cells/10 rats; saline 24 cells/6 rats) but not HFD (saline: 22 cells/7 rats; morphine: 21 cells/5 rats) rats. In a separate group of age-matched SD rats, a similar weight loss (5-7%) in response to the morphine did not alter sensory-evoked rate but decreased signal-to-noise ratio (Control: 22 cells/8 rats; Weight-matched: 23 cells/8 rats).Discussion: Taken together, our findings suggest that obesity and diet alter the sensory-evoked LC-NE neural responses, which could have implication for emotional stress and opioid-withdrawal behaviors.

Locus Coeruleus Norepinephrine in Learned Behavior: Anatomical Modularity and Spatiotemporal Integration in Targets

The locus coeruleus (LC), a small brainstem nucleus, is the primary source of the neuromodulator norepinephrine (NE) in the brain. The LC receives input from widespread brain regions, and projects throughout the forebrain, brainstem, cerebellum, and spinal cord. LC neurons release NE to control arousal, but also in the context of a variety of sensory-motor and behavioral functions. Despite its brain-wide effects, much about the role of LC-NE in behavior and the circuits controlling LC activity is unknown. New evidence suggests that the modular input-output organization of the LC could enable transient, task-specific modulation of distinct brain regions. Future work must further assess whether this spatial modularity coincides with functional differences in LC-NE subpopulations acting at specific times, and how such spatiotemporal specificity might influence learned behaviors. Here, we summarize the state of the field and present new ideas on the role of LC-NE in learned behaviors.

The different hormonal system during exercise stress coping in horses

The review discusses the hormonal changes during exercise stress. The exercise generally produces a rise of adrenaline (A), noradrenaline (NA), adrenocorticotropic hormone (ACTH), cortisol, glucagon, growth hormone, arginine vasopressine, etc., and a drop of insulin. The hormonal events during reestablishment of homeostasis due to exercise stress can be divided into a catabolic phase, with decreased tolerance of effort, and reversible biochemical, hormonal and immunological changes, and an anabolic phase, with a higher adaptive capacity, and enhanced performance. The two main hormonal axes activated in the catabolic phase are sympathetic–adrenal–medullary system and hypothalamic-pituitary-adrenal (HPA) axis, while in the anabolic phase, growth hormone-insulin-like factor I axis, and gonadal axes. The hormonal responses during exercise and recovery can be regarded as regulatory and integrated endocrine responses. The increase of catecholamines and ACTH is dependent on the intensity of exercise; a marked increase in plasma A occurs during exercises with high emotional content. The response of cortisol is correlated with the duration of exercise, while the effect of exercise duration on b-endorphin changes is highly dependent on the type of exercise performed. Cortisol and b-endorphin changes usually occur in phase, but not during exercises with high emotional content. Glucocorticoids and iodothyronines are involved in meeting immediate energy demands, and a model of functional interactions between HPA axis and hypothalamic-pituitary-thyroid axis during exercise stress is proposed. A modulation of coping responses to different energy demanding physical activities required for sport activities could be hypothesized. This review supports the proposed regulation of hypophysiotropic TRHergic neurons as metabolic integrators during exercise stress. Many hormonal systems (ghrelin, leptin, glucose, insulin, and cortisol) are activated to control substrate mobilizations and utilization. The cardiovascular homeostasis, the fluid and electrolyte balance during exercise are highly dependent on vasoactive hormones (antidiuretic hormone, atrial natriuretic peptide, renin–angiotensin–aldosterone, and prostaglandins) control.

Redefining Noradrenergic Neuromodulation of Behavior: Impacts of a Modular Locus Coeruleus Architecture

The locus coeruleus (LC) is a seemingly singular and compact neuromodulatory nucleus that is a prominent component of disparate theories of brain function due to its broad noradrenergic projections throughout the CNS. As a diffuse neuromodulatory system, noradrenaline affects learning and decision making, control of sleep and wakefulness, sensory salience including pain, and the physiology of correlated forebrain activity (ensembles and networks) and brain hemodynamic responses. However, our understanding of the LC is undergoing a dramatic shift due to the application of state-of-the-art methods that reveal a nucleus of many modules that provide targeted neuromodulation. Here, we review the evidence supporting a modular LC based on multiple levels of observation (developmental, genetic, molecular, anatomical, and neurophysiological). We suggest that the concept of the LC as a singular nucleus and, alongside it, the role of the LC in diverse theories of brain function must be reconsidered.

Endocannabinoids, stress signaling, and the locus coeruleus-norepinephrine system

The endocannabinoid (eCB) system has been implicated in a variety of physiological functions due to abundant expression of its receptors and endogenous ligands in the central nervous system. Substantial progress has been made in understanding how the eCB system influences the brain norepinephrine (NE) system, an important neurochemical target in the continued development of new therapies for stress-induced psychiatric disorders. We, and others, have characterized the neuroanatomical, biochemical and pharmacological effects of cannabinoid receptor modulation on brain noradrenergic circuitry and defined how molecular elements of the eCB system are positioned to directly impact the locus coeruleus (LC)-prefrontal cortex pathway, a neural circuit well recognized for contributing to symptoms of hyperarousal, a key pathophysiological feature of stress-related disorders. We also described molecular and electrophysiological properties of LC noradrenergic neurons and NE release in the medial prefrontal cortex under conditions of cannabinoid type 1 receptor deletion. Finally, we identified how stress influences cannabinoid modulation of the coeruleo-cortical pathway. A number of significant findings emerged from these studies that will be summarized in the present review and have important implications for clinical studies targeting the eCB system in the treatment of stress-induced psychiatric disorders.

Targeting opioid dysregulation in depression for the development of novel therapeutics

Since the serendipitous discovery of the first class of modern antidepressants in the 1950's, all pharmacotherapies approved by the Food and Drug Administration for major depressive disorder (MDD) have shared a common mechanism of action, increased monoaminergic neurotransmission. Despite the widespread availability of antidepressants, as many as 50% of depressed patients are resistant to these conventional therapies. The significant length of time required to produce meaningful symptom relief with these medications, 4-6 weeks, indicates that other mechanisms are likely involved in the pathophysiology of depression which may yield more viable targets for drug development. For decades, no viable candidate target with a different mechanism of action to that of conventional therapies proved successful in clinical studies. Now several exciting avenues for drug development are under intense investigation. One of these emerging targets is modulation of endogenous opioid tone. This review will evaluate preclinical and clinical evidence pertaining to opioid dysregulation in depression, focusing on the role of the endogenous ligands endorphin, enkephalin, dynorphin, and nociceptin/orphanin FQ (N/OFQ) and their respective receptors, mu (MOR), delta (DOR), kappa (KOR), and the N/OFQ receptor (NOP) in mediating behaviors relevant to depression and anxiety. Finally, putative opioid based antidepressants that are under investigation in clinical trials, ALKS5461, JNJ-67953964 (formerly LY2456302 and CERC-501) and BTRX-246040 (formerly LY-2940094) will be discussed. This review will illustrate the potential therapeutic value of targeting opioid dysregulation in developing novel therapies for MDD.

Reciprocal interactions across and within multiple levels of monoamine and cortico-limbic systems in stress-induced depression: A systematic review

The monoamine hypothesis of depression, namely that the reduction in synaptic serotonin and dopamine levels causes depression, has prevailed in past decades. However, clinical and preclinical studies have identified various cortical and subcortical regions whose altered neural activities also regulate depressive-like behaviors, independently from the monoamine system. Our systematic review indicates that neural activities of specific brain regions and associated neural circuitries are adaptively altered after chronic stress in a specific direction, such that the neural activity in the infralimbic cortex, lateral habenula and amygdala is upregulated, whereas the neural activity in the prelimbic cortex, hippocampus and monoamine systems is downregulated. The altered neural activity dynamics between monoamine systems and cortico-limbic systems are reciprocally interwoven at multiple levels. Furthermore, depressive-like behaviors can be experimentally reversed by counteracting the altered neural activity of a specific neural circuitry at multiple brain regions, suggesting the importance of the reciprocally interwoven neural networks in regulating depressive-like behaviors. These results promise for reshaping altered neural activity dynamics as a therapeutic strategy for treating depression.

Neurochemically distinct circuitry regulates locus coeruleus activity during female social stress depending on coping style

Stress-related psychiatric diseases are nearly twice as prevalent in women compared to men. We recently showed in male rats that the resident-intruder model of social stress differentially engages stress-related circuitry that regulates norepinephrine-containing neurons of the locus coeruleus (LC) depending on coping strategy as determined by the latency to assume a defeat posture. Here, we determined whether this social stress had similar effects in female rats. LC afferents were retrogradely labeled with Fluorogold (FG) and rats had one or five daily exposures to an aggressive resident. Sections through the nucleus paragigantocellularis (PGi), a source of enkephalin (ENK) afferents to the LC, and central nucleus of the amygdala (CeA), a source of corticotropin-releasing factor (CRF) afferents to the LC, were processed for immunocytochemical detection of c-fos, a marker of neuronal activity, FG and ENK or CRF. Like male rats, female rats defeated with a relatively short latency (SL) in response to a single resident-intruder exposure and showed significant c-fos activation of LC neurons, PGi-ENK LC afferents, and CeA-CRF-LC afferents. With repeated exposure, some rats exhibited a long latency to defeat (LL). LC neurons and CeA-CRF-LC afferents were activated in SL rats compared to control and LL, whereas PGi-ENK LC afferents were not. Conversely, in LL rats, PGi-ENK LC and CeA-CRF-LC afferents were activated compared to controls but not LC neurons. CRF type 1 receptor (CRF1) and µ-opioid receptor (MOR) expression levels in LC were decreased in LL rats. Finally, electron microscopy showed a relative increase in MOR on the plasma membrane of LL rats and a relative increase in CRF1 on the plasma membrane of SL rats. Together, these results suggest that as is the case for males, social stress engages divergent circuitry to regulate the LC in female rats depending on coping strategy, with a bias towards CRF influence in more subordinate rats and opioid influence in less subordinate rats.

Social defeat stress and escalation of cocaine and alcohol consumption: Focus on CRF

Both the ostensibly aversive effects of unpredictable episodes of social stress and the intensely rewarding effects of drugs of abuse activate the mesocorticolimbic dopamine systems. Significant neuroadaptations in interacting stress and reward neurocircuitry may underlie the striking connection between stress and substance use disorders. In rodent models, recurring intermittent exposure to social defeat stress appears to produce a distinct profile of neuroadaptations that translates most readily to the repercussions of social stress in humans. In the present review, preclinical rodent models of social defeat stress and subsequent alcohol, cocaine or opioid consumption are discussed with regard to: (1) the temporal pattern of social defeat stress, (2) male and female protocols of social stress-escalated drug consumption, and (3) the neuroplastic effects of social stress, which may contribute to escalated drug-taking. Neuroadaptations in corticotropin-releasing factor (CRF) and CRF modulation of monoamines in the ventral tegmental area and the bed nucleus of the stria terminalis are highlighted as potential mechanisms underlying stress-escalated drug consumption. However, the specific mechanisms that drive CRF-mediated increases in dopamine require additional investigation as do the stress-induced neuroadaptations that may contribute to the development of compulsive patterns of drug-taking.

A Comprehensive Overview on Stress Neurobiology: Basic Concepts and Clinical Implications

Stress is recognized as an important issue in basic and clinical neuroscience research, based upon the founding historical studies by Walter Canon and Hans Selye in the past century, when the concept of stress emerged in a biological and adaptive perspective. A lot of research after that period has expanded the knowledge in the stress field. Since then, it was discovered that the response to stressful stimuli is elaborated and triggered by the, now known, stress system, which integrates a wide diversity of brain structures that, collectively, are able to detect events and interpret them as real or potential threats. However, different types of stressors engage different brain networks, requiring a fine-tuned functional neuroanatomical processing. This integration of information from the stressor itself may result in a rapid activation of the Sympathetic-Adreno-Medullar (SAM) axis and the Hypothalamus-Pituitary-Adrenal (HPA) axis, the two major components involved in the stress response. The complexity of the stress response is not restricted to neuroanatomy or to SAM and HPA axes mediators, but also diverge according to timing and duration of stressor exposure, as well as its short- and/or long-term consequences. The identification of neuronal circuits of stress, as well as their interaction with mediator molecules over time is critical, not only for understanding the physiological stress responses, but also to understand their implications on mental health.

Persistent Stress-Induced Neuroplastic Changes in the Locus Coeruleus/Norepinephrine System

Neural plasticity plays a critical role in mediating short- and long-term brain responses to environmental stimuli. A major effector of plasticity throughout many regions of the brain is stress. Activation of the locus coeruleus (LC) is a critical step in mediating the neuroendocrine and behavioral limbs of the stress response. During stressor exposure, activation of the hypothalamic-pituitary-adrenal axis promotes release of corticotropin-releasing factor in LC, where its signaling promotes a number of physiological and cellular changes. While the acute effects of stress on LC physiology have been described, its long-term effects are less clear. This review will describe how stress changes LC neuronal physiology, function, and morphology from a genetic, cellular, and neuronal circuitry/transmission perspective. Specifically, we describe morphological changes of LC neurons in response to stressful stimuli and signal transduction pathways underlying them. Also, we will review changes in excitatory glutamatergic synaptic transmission in LC neurons and possible stress-induced modifications of AMPA receptors. This review will also address stress-related behavioral adaptations and specific noradrenergic receptors responsible for them. Finally, we summarize the results of several human studies which suggest a link between stress, altered LC function, and pathogenesis of posttraumatic stress disorder.

Female psychopharmacology matters! Towards a sex-specific psychopharmacology

There is increasing recognition that women have a higher prevalence of certain psychiatric illnesses, and a differential treatment response and course of illness compared to men. Additionally, clinicians deal with a number of disorders like premenstrual syndrome, premenstrual dysphoric disorder, and postpartum depression, which affect women specifically and for which treatment and biological pathways are still unclear. In this article we highlight recent research which suggests that different biological mechanisms may underlie sex differences in responsiveness to stress. Sex differences are evident at the receptor level; where the corticotropin-releasing factor receptor shows differential coupling to adaptor proteins in males and females. The neuropeptide oxytocin also shows sex-specific effects in a range of social behaviors. It may act as a biomarker in post-traumatic stress disorder where sex differences are evident. Studies in women using hormonal contraception show that some of these oxytocin-mediated effects are likely influenced by sex hormones. In female rats rapid changes in circulating progesterone levels are associated with exaggerated behavioral responses to mild stress and blunted responses to benzodiazepines that could be prevented by acute treatment with low-dose fluoxetine. Perceived barriers in research on women have hindered progress. The development of a sex-specific psychopharmacology as a basis for translating this type of research into clinical practice is vital to improve treatment outcomes for women.

The anti-inflammatory mechanism of antidepressants - SSRIs, SNRIs

The cytokine theory of depression no longer brings about any doubts. Experiments and research studies conducted in the last ten years have confirmed that both physical and psychological (emotional) stress increases the likelihood of occurrence of mental disorders (including depressive disorders) owing to the action of a series of hormonal and biochemical mechanisms. Selective serotonin reuptake inhibitors (SSRI) as well as serotonin and norepinephrine reuptake inhibitors (SNRIs) are some of the most commonly applied drugs in the world during pharmacotherapy of recurrent depressive disorder. The underestimated anti-inflammatory and anti-oxidative effect may be one of the potential mechanisms of action of the preparations mentioned above. The detailed specificity of action of this mechanism still remains unknown. The aim of our work will be to perform a review of contemporary literature in order to present the latest scientific reports regarding the anti-inflammatory effects of SSRIs and SNRIs. The mechanism of anti-inflammatory action may serve as a possible explanation for the efficacy of antidepressants from the groups of SSRIs and SNRIs.

Contribution of amygdala CRF neurons to chronic pain

We investigated the role of amygdala corticotropin-releasing factor (CRF) neurons in the perturbations of descending pain inhibition caused by neuropathic pain. Forced swim increased the tail-flick response latency in uninjured mice, a phenomenon known as stress-induced analgesia (SIA) but did not change the tail-flick response latency in mice with neuropathic pain caused by sciatic nerve constriction. Neuropathic pain also increased the expression of CRF in the central amygdala (CeAmy) and ΔFosB in the dorsal horn of the spinal cord. Next, we injected the CeAmy of CRF-cre mice with cre activated AAV-DREADD (Designer Receptors Exclusively Activated by Designer Drugs) vectors. Activation of CRF neurons by DREADD/Gq did not affect the impaired SIA but inhibition of CRF neurons by DREADD/Gi restored SIA and decreased allodynia in mice with neuropathic pain. The possible downstream circuitry involved in the regulation of SIA was investigated by combined injections of retrograde cre-virus (CAV2-cre) into the locus ceruleus (LC) and cre activated AAV-diphtheria toxin (AAV-FLEX-DTX) virus into the CeAmy. The viral injections were followed by a sciatic nerve constriction ipsilateral or contralateral to the injections. Ablation of amygdala projections to the LC on the side of injury but not on the opposite side, completely restored SIA, decreased allodynia and decreased ΔFosB expression in the spinal cord in mice with neuropathic pain. The possible lateralization of SIA impairment to the side of injury was confirmed by an experiment in which unilateral inhibition of the LC decreased SIA even in uninjured mice. The current view in the field of pain research attributes the process of pain chronification to abnormal functioning of descending pain inhibition. Our results demonstrate that the continuous activity of CRF neurons brought about by persistent pain leads to impaired SIA, which is a symptom of dysregulation of descending pain inhibition. Therefore, an over-activation of amygdala CRF neurons is very likely an important contributing factor for pain chronification.

Locus coeruleus to basolateral amygdala noradrenergic projections promote anxiety-like behavior

Increased tonic activity of locus coeruleus noradrenergic (LC-NE) neurons induces anxiety-like and aversive behavior. While some information is known about the afferent circuitry that endogenously drives this neural activity and behavior, the downstream receptors and anatomical projections that mediate these acute risk aversive behavioral states via the LC-NE system remain unresolved. Here we use a combination of retrograde tracing, fast-scan cyclic voltammetry, electrophysiology, and in vivo optogenetics with localized pharmacology to identify neural substrates downstream of increased tonic LC-NE activity in mice. We demonstrate that photostimulation of LC-NE fibers in the BLA evokes norepinephrine release in the basolateral amygdala (BLA), alters BLA neuronal activity, conditions aversion, and increases anxiety-like behavior. Additionally, we report that β-adrenergic receptors mediate the anxiety-like phenotype of increased NE release in the BLA. These studies begin to illustrate how the complex efferent system of the LC-NE system selectively mediates behavior through distinct receptor and projection-selective mechanisms.

DOI:http://dx.doi.org/10.7554/eLife.18247.001

Enkephalins: Endogenous Analgesics with an Emerging Role in Stress Resilience

Psychological stress is a state of mental or emotional strain or tension that results from adverse or demanding circumstances. Chronic stress is well known to induce anxiety disorders and major depression; it is also considered a risk factor for Alzheimer's disease. Stress resilience is a positive outcome that is associated with preserved cognition and healthy aging. Resilience presents psychological and biological characteristics intrinsic to an individual conferring protection against the development of psychopathologies in the face of adversity. How can we promote or improve resilience to chronic stress? Numerous studies have proposed mechanisms that could trigger this desirable process. The roles of enkephalin transmission in the control of pain, physiological functions, like respiration, and affective disorders have been studied for more than 30 years. However, their role in the resilience to chronic stress has received much less attention. This review presents the evidence for an emerging involvement of enkephalin signaling through its two associated opioid receptors, μ opioid peptide receptor and δ opioid peptide receptor, in the natural adaptation to stressful lifestyles.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

The protective effects of resveratrol on social stress-induced cytokine release and depressive-like behavior

Social stress is a risk factor for psychiatric disorders, however only a subset of the population is susceptible while others remain resilient. Inflammation has been linked to the pathogenesis of psychosocial disorders in humans and may underlie these individual differences. Using a resident-intruder paradigm capable of revealing individual differences in coping behavior and inflammatory responses, the present study determined if resveratrol (RSV; 0, 10, 30mg/kg/day) protected against persistent stress-induced inflammation in socially defeated rats. Furthermore, the antidepressant efficacy of RSV was evaluated using the sucrose preference test. Active coping rats were characterized by more time spent in upright postures and increased defeat latencies versus passive coping rats. Five days after defeat, flow cytometry revealed enhanced stimulation of proinflammatory proteins (IL-β, TNF-α) in spleen cells of passive rats as compared to active coping and controls, an effect that was blocked by both doses of RSV. Furthermore, only passive coping rats exhibited increased proinflammatory proteins (IL-1β, TNF-α, GM-CSF) in the locus coeruleus (LC), a noradrenergic brain region implicated in depression. Notably, only 30mg/kg RSV blocked LC neuroinflammation and importantly, was the only dose that blocked anhedonia. Alternatively, while stress had minimal impact on resting cytokines in the dorsal raphe (DR), RSV dose-dependently reduced DR cytokine expression. However, this did not result in changes in indoleamine 2,3-dioxygenase activity or serotonin levels. Taken together, these data suggest that social stress-induced depressive-like behavior evident in passive coping rats may be driven by stress-induced neuroinflammation and highlight natural anti-inflammatory agents to protect against social stress-related consequences.

Inhibition of vasopressin V1a receptors in the medioventral bed nucleus of the stria terminalis has sex- and context-specific anxiogenic effects

Vasopressin V1a receptors (V1aR) are thought to contribute to the pathophysiology of psychiatric disorders such as anxiety and depression, sparking interest in V1aR as a therapeutic target. Although the global effects of V1aR have been documented, less is known about the specific neural circuits mediating these effects. Moreover, few studies have examined context-specific V1aR function in both males and females. By using the California mouse, we first studied the effects of sex and social defeat stress on V1aR binding in the forebrain. In females but not males, V1aR binding in the bed nucleus of the stria terminalis (BNST) was negatively correlated to social interaction behavior. In females stress also increased V1aR binding in the nucleus accumbens (NAc). Infusions of V1aR antagonist in to the medioventral BNST (BNSTmv) had anxiogenic effects only in animals naïve to defeat. For males, inhibition of V1aR in BNSTmv had anxiogenic effects in social and nonsocial contexts, but for females, anxiogenic effects were limited to social contexts. In stressed females, inhibition of V1aR in the NAc shell had no effect on social interaction behavior, but had an anxiogenic effect in an open field test. These data suggest that V1aR in BNSTmv have anxiolytic and prosocial effects in males, and that in females, prosocial and anxiolytic effects of V1aR appear to be mediated independently by receptors in the BNSTmv and NAc shell, respectively. These findings suggest that males have more overlap in neural circuits modulating anxiety in social and nonsocial contexts than females.

Individual differences in the locus coeruleus-norepinephrine system: Relevance to stress-induced cardiovascular vulnerability

Repeated exposure to psychosocial stress is a robust sympathomimetic stressor and as such has adverse effects on cardiovascular health. While the neurocircuitry involved remains unclear, the physiological and anatomical characteristics of the locus coeruleus (LC)-norepinephrine (NE) system suggest that it is poised to contribute to stress-induced cardiovascular vulnerability. A major theme throughout is to review studies that shed light on the role that the LC may play in individual differences in vulnerability to social stress-induced cardiovascular dysfunction. Recent findings are discussed that support a unique plasticity in afferent regulation of the LC, resulting in either excitatory or inhibitory input to the LC during establishment of different stress coping strategies. This contrasting regulation of the LC by either afferent regulation, or distinct differences in stress-induced neuroinflammation would translate to differences in cardiovascular regulation and may serve as the basis for individual differences in the cardiopathological consequences of social stress. The goal of this review is to highlight recent developments in the interplay between the LC-NE and cardiovascular systems during repeated stress in an effort to advance therapeutic treatments for the development of stress-induced cardiovascular vulnerability.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Ascending mechanisms of stress integration: Implications for brainstem regulation of neuroendocrine and behavioral stress responses

In response to stress, defined as a real or perceived threat to homeostasis or well-being, brain systems initiate divergent physiological and behavioral processes that mobilize energy and promote adaptation. The brainstem contains multiple nuclei that engage in autonomic control and reflexive responses to systemic stressors. However, brainstem nuclei also play an important role in neuroendocrine responses to psychogenic stressors mediated by the hypothalamic-pituitary-adrenocortical axis. Further, these nuclei integrate neuroendocrine responses with stress-related behaviors, significantly impacting mood and anxiety. The current review focuses on the prominent brainstem monosynaptic inputs to the endocrine paraventricular hypothalamic nucleus (PVN), including the periaqueductal gray, raphe nuclei, parabrachial nuclei, locus coeruleus, and nucleus of the solitary tract (NTS). The NTS is a particularly intriguing area, as the region contains multiple cell groups that provide neurochemically-distinct inputs to the PVN. Furthermore, the NTS, under regulatory control by glucocorticoid-mediated feedback, integrates affective processes with physiological status to regulate stress responding. Collectively, these brainstem circuits represent an important avenue for delineating interactions between stress and health.

Neuroinflammation at the interface of depression and cardiovascular disease: Evidence from rodent models of social stress

A large body of evidence has emerged linking stressful experiences, particularly from one's social environment, with psychiatric disorders. However, vast individual differences emerge in susceptibility to developing stress-related pathology which may be due to distinct differences in the inflammatory response to social stress. Furthermore, depression is an independent risk factor for cardiovascular disease, another inflammatory-related disease, and results in increased mortality in depressed patients. This review is focused on discussing evidence for stress exposure resulting in persistent or sensitized inflammation in one individual while this response is lacking in others. Particular focus will be directed towards reviewing the literature underlying the impact that neuroinflammation has on neurotransmitters and neuropeptides that could be involved in the pathogenesis of comorbid depression and cardiovascular disease. Finally, the theme throughout the review will be to explore the notion that stress-induced inflammation is a key player in the high rate of comorbidity between psychosocial disorders and cardiovascular disease.

The brain norepinephrine system, stress and cardiovascular vulnerability

Chronic exposure to psychosocial stress has adverse effects on cardiovascular health, however the stress-sensitive neurocircuitry involved remains to be elucidated. The anatomical and physiological characteristics of the locus coeruleus (LC)-norepinephrine (NE) system position it to contribute to stress-induced cardiovascular disease. This review focuses on cardiovascular dysfunction produced by social stress and a major theme highlighted is that differences in coping strategy determine individual differences in social stress-induced cardiovascular vulnerability. The establishment of different coping strategies and cardiovascular vulnerability during repeated social stress has recently been shown to parallel a unique plasticity in LC afferent regulation, resulting in either excitatory or inhibitory input to the LC. This contrasting regulation of the LC would translate to differences in cardiovascular regulation and may serve as the basis for individual differences in the cardiopathological consequences of social stress. The advances described suggest new directions for developing treatments and/or strategies for decreasing stress-induced cardiovascular vulnerability.