Chronic social stress induces peripheral and central immune activation, blunted mesolimbic dopamine function, and reduced reward-directed behaviour in mice

Male and female mice respectively form stronger social aversive memories with same and different sex conspecifics

Mice offer a wealth of opportunities for investigating brain circuits regulating multiple behaviors, largely due to their genetic tractability. Social behaviors are of translational relevance, considering both mice and humans are highly social mammals, and disruptions in human social behavior are key symptoms of myriad neuropsychiatric disorders. Stresses related to social experiences are particularly influential in the severity and maintenance of neuropsychiatric disorders like anxiety disorders, and trauma and stressor-related disorders. Yet, induction and study of social stress in mice is disproportionately focused on males, influenced heavily by their natural territorial nature. Conspecific-elicited stress (i.e., defeat), while ethologically relevant, is quite variable and predominantly specific to males, making rigorous and sex-inclusive studies challenging. In pursuit of a controllable, consistent, high throughput, and sex-inclusive paradigm for eliciting social stress, we have discovered intriguing sex-specific social aversions that are dependent upon the sex of both experimental and conspecific mice. Specifically, we trained male and female F1 129S1/SvlmJ × C57BL/6J mice to associate (via classical conditioning) same or different sex C57BL/6J conspecifics with a mild, aversive stimulus. Upon subsequent testing for social interaction 24 h later, we found that males socially conditioned better to male conspecifics by exhibiting reduced social interaction, whereas females socially conditioned better to male conspecifics. Serum corticosterone levels inversely corresponded to social avoidance after different sex, but not same sex, conditioning, suggesting corticosterone-mediated arousal could influence cross sex interactions. While our paradigm has further optimization ahead, these current findings reveal why past pursuits to develop same sex female social stress paradigms may have met with limited success. Future research should expand investigation of utilizing male mouse conspecifics to instigate social stress across sexes.

Chronic stress deficits in reward behaviour co-occur with low nucleus accumbens dopamine activity during reward anticipation specifically

Whilst reward pathologies are major and common in stress-related neuropsychiatric disorders, their neurobiology and treatment are poorly understood. Imaging studies in human reward pathology indicate attenuated BOLD activity in nucleus accumbens (NAc) coincident with reward anticipation but not reinforcement; potentially, this is dopamine (DA) related. In mice, chronic social stress (CSS) leads to reduced reward learning and motivation. Here, DA-sensor fibre photometry is used to investigate whether these behavioural deficits co-occur with altered NAc DA activity during reward anticipation and/or reinforcement. In CSS mice relative to controls: (1) Reduced discriminative learning of the sequence, tone-on + appetitive behaviour = tone-on + sucrose reinforcement, co-occurs with attenuated NAc DA activity throughout tone-on and sucrose reinforcement. (2) Reduced motivation during the sequence, operant behaviour = tone-on + sucrose delivery + sucrose reinforcement, co-occurs with attenuated NAc DA activity at tone-on and typical activity at sucrose reinforcement. (3) Reduced motivation during the sequence, operant behaviour = appetitive behaviour + sociosexual reinforcement, co-occurs with typical NAc DA activity at female reinforcement. Therefore, in CSS mice, low NAc DA activity co-occurs with low reward anticipation and could account for deficits in learning and motivation, with important implications for understanding human reward pathology.

Higher exploratory and vigilant behaviors related to higher central dopaminergic activities of Formosan wood mice (Apodemus semotus) in light-dark exploration tests

Formosan wood mice (Apodemus semotus) are endemic rodents in Taiwan. Recently Formosan wood mice exhibit similar locomotor behaviors in the laboratory environment as in the field environment has shown. Contemporaneously, Formosan wood mice have higher moving distances of and central dopaminergic (DAergic) activities than C57BL/6 mice in behavioral test. This study tried to compare the behavioral responses between male Formosan wood mice and male C57BL/6 mice in the light-dark exploration tests. We also measured the levels of DA and 3,4-dihydroxyphenylacetic acid (DOPAC), the primary metabolite of DA, to assess the dopaminergic activity of the medial prefrontal cortex, striatum, and nucleus accumbens. Our data show that Formosan wood mice revealed higher exploration and central DAergic activities than did C57BL/6 mice in the light-dark exploration tests, and diazepam (an anxiolytics) treatment reduced the exploratory activity and central dopaminergic activities in Formosan wood mice, but not in C57BL/6 mice. After repeated exposure to light-dark exploration tests, the latency to dark zone was increased, and the duration in light zone as well as the central DAergic activity were decreased in C57BL/6 mice. This study provides comparative findings; Formosan wood mice showed the higher exploratory activities than C57BL/6 mice did, and their central DAergic activities were related to the behavioral responses in these two mice. This could potentially shed light on the reasons behind the prevalence of higher exploration and central dopaminergic activities. Using Formosan wood mice as a model to study human diseases related to hyperactivity adds significant value to the potential research.

The regulation of social factors on anxiety and microglial activity in nucleus accumbens of adolescent male mice: Influence of social interaction strategy

BACKGROUND

Adolescence is a period characterized by a high vulnerability to emotional disorders, which are modulated by biological, psychological, and social factors. However, the underlying mechanisms remain poorly understood.

METHODS

Combining physical or emotional social defeat stress (PS and ES) and pair or isolation rearing conditions, we investigated the effects of stress type and social support on emotional behavior and central immune molecules in adolescent mice, including anxiety, social fear, and social interaction strategies, as well as changes in microglia-specific molecules (ionized calcium-binding adaptor molecule 1 (Iba1) and a cluster of differentiation molecule 11b (CD11b)) in the medial prefrontal cortex (mPFC), hippocampus (HIP), amygdala (AMY), and nucleus accumbens (NAc).

RESULTS

Mice exposed to both physical stress and isolated rearing condition exhibited the highest levels of anxiety, social fear, and microglial CD11b expression in the NAc. In terms of social support, pair-housing with siblings ameliorated social fear and NAc molecular changes in ES mice, but not in PS mice. The reason for the differential benefit from social support was attributed to the fact that ES mice exhibited more active and less passive social strategies in social environment compared to PS mice. Further, the levels of stress-induced social fear were positively associated with the expression of microglial CD11b in the NAc.

CONCLUSION

These findings offer extensive evidence regarding the intricate effects of multiple social factors on social anxiety and immune alteration in the NAc of adolescent mice. Additionally, they suggest potential behavioral and immune intervention strategies for anxiety-related disorders in adolescents.

Chronic social stress in early life can predispose mice to antisocial maltreating behavior

Purpose

In our previous study, we developed an assay system to evaluate antisocial maltreating behavior of conspecific mice using a perpetrator-victim paradigm. We also generated a mouse model for the maltreating behavior by mimicking child maltreatment or abuse. Here, we further investigate the antisocial behavior using anti-aggressive and antipsychotic drugs.

Methods

Model mice sequentially subjected to maternal separation (MS), social defeat (SD), and social isolation (SI) in that order (MS/SD/SI model) were subjected to a maltreating behavioral task. The MS/SD/SI mice were treated with oxytocin (OXY), clozapine (CLZ), haloperidol (HAL), and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). Western blotting and enzyme-linked immunosorbent assay were used for protein analysis.

Results

A substantial portion of the MS/SD/SI model mice (46% of males and 40% of females) showed a higher number of nose pokes than the control. OXY or 8-OH-DPAT treatment reduced the high number of nose pokes by the MS/SD/SI mice, whereas HAL increased it. CLZ did not affect the number of nose pokes by the MS/SD/SI mice. Interestingly, although the OXY level in the MS/SD/SI mice was similar to that in the control, the amount of OXY receptor was lower in the MS/SD/SI mice. The amount of 5-HT1A receptor was also decreased in the MS/SD/SI mice.

Conclusion

Chronic social stress in childhood might predispose a mouse to antisocial behavior. Our maltreating behavior assay system, including the MS/SD/SI model, is a good animal system for research on and drug screening for brain disorders associated with antisocial or psychotic behavior.

Mouse Model of Chronic Social Stress-Induced Excessive Pavlovian Aversion Learning-Memory

Increased experience of aversive stimuli/events is a psychological-neurobiological state of major importance in psychiatry. It occurs commonly in generalized anxiety disorder, post-traumatic stress disorder, and major depression. A sustained period of exposure to threat (chronic stressor) is a common risk factor, and a major symptom is generalized excessive perception of, and reactivity to, aversive stimuli. In rodents, Pavlovian aversion learning and memory (PAL, PAM), quantified in terms of the conditioned defensive behavior freezing, is an extensively studied behavioral paradigm, and well understood in terms of underlying neural circuitry. In mice, chronic social stress (CSS) is a 15-day resident-intruder paradigm in which C57BL/6 adult males are exposed continuously and distally to dominant-aggressive CD-1 male mice (sustained threat) interspersed with a brief daily period of proximal attack (acute threat). To ensure that physical wounding is minimized, proximal attacks are limited to 30 to 60 s/day and lower incisor teeth of CD-1 mice are blunted. Control (comparison) mice are maintained in littermate pairs. The CSS and CD-1 mice are maintained in distal contact during subsequent behavioral testing. For PAL, CSS and control (CON) mice are placed in a conditioning chamber (context) and exposed to a tone [conditioned stimulus (CS)] and mild, brief foot shock [unconditioned stimulus (US)]. For PAM, mice are placed in the same context and presented with CS repetitions. The CSS mice acquire (learn) and express (memory) a higher level of freezing than CON mice, indicating that CSS leads to generalized hypersensitivity to aversion, i.e., chronic social aversion leads to increased aversion salience of foot shock. Distinctive features of the model include the following: high reproducibility; rare, mild wounding only; male specificity; absence of "susceptible" vs "resilient" subgroups; behavioral effects dependent on continued presence of CD-1 mice; and preclinical validation of novel compounds for normalizing aversion hypersensitivity with accurate feedforward prediction of efficacy in human patients. © 2024 The Authors. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: Chronic social stress (CSS) Basic Protocol 2: Pavlovian aversion learning and memory (PALM).

The role of sociability in social instability stress: Behavioral, neuroendocrine and monoaminergic effects

Extensive literature has reported a link between social stress and mental health. In this complex relationship, individual strategies for coping with social stress are thought to have a possible modulating effect, with sociability being a key factor. Despite the higher incidence of affective disorders in females and sex-related neurochemical differences, female populations have been understudied. The aim of the present study was, therefore, to analyze the behavioral, neuroendocrine, and neurochemical effects of stress in female OF1 mice, paying special attention to social connectedness (female mice with high vs low sociability). To this end, subjects were exposed to the Chronic Social Instability Stress (CSIS) model for four weeks. Although female mice exposed to CSIS had increased arousal, there was no evidence of depressive-like behavior. Neither did exposure to CSIS affect corticosterone levels, although it did increase the MR/GR ratio by decreasing GR expression. Female mice exposed to CSIS had higher noradrenaline and dopamine levels in the hippocampus and striatum respectively, with a lower monoaminergic turnover, resulting in an increased arousal. CSIS increased serotonin levels in both the hippocampus and striatum. Similarly, CSIS was found to reduce kynurenic acid, 3-HK, and IDO and iNOS enzyme levels in the hippocampus. Interestingly, the observed decrease in IDO synthesis and the increased serotonin and dopamine levels in the striatum were only found in subjects with high sociability. These highly sociable female mice also had significantly lower levels of noradrenaline in the striatum after CSIS application. Overall, our model has produced neuroendocrine and neurochemical but not behavioral changes, so it has not allowed us to study sociability in depth. Therefore, a model that induces both molecular and behavioral phenotypes should be applied to determine the role of sociability.

Effects of the COVID-19 Pandemic on Neural Responses to Reward: A Quasi-experiment

BACKGROUND

The COVID-19 pandemic has been a prolonged period of stress due to social isolation, illness, death, and other major life disruptions. Neural reward sensitivity, essential for healthy functioning, may become reduced under major naturalistic stressors, though few studies have examined this. The present study sought to test whether neural responses to rewards were significantly blunted by the stress of the pandemic.

METHODS

We compared 2 groups of young adult participants, who completed a monetary reward task while an electroencephalogram was recorded, at 2 time points, 1 to 3 years apart. Our measure of reward sensitivity was the reward positivity (RewP), a neural marker enhanced to gain relative to loss feedback. The magnitude of the RewP is sensitive to stress exposure and can prospectively predict depression. The pre-pandemic group (n = 41) completed both time points before the pandemic, while the pandemic group (n = 39) completed the baseline visit before the pandemic and the follow-up visit during its second year.

RESULTS

The pandemic group reported having experienced significant stressors over the course of the pandemic. We did not observe a significant decrease in the RewP from baseline to follow-up in the pre-pandemic group. In contrast, in the pandemic group, the RewP was significantly blunted at the follow-up visit to the extent that it no longer distinguished gain from loss feedback.

CONCLUSIONS

These results suggest that prolonged naturalistic stressors can result in adaptations in neural responses to rewards. Our findings also highlight a possible mechanism linking stress to the development of depression.

Stress to inflammation and anhedonia: Mechanistic insights from preclinical and clinical models

Anhedonia, as evidenced by impaired pleasurable response to reward, reduced reward motivation, and/or deficits in reward-related learning, is a common feature of depression. Such deficits in reward processing are also an important clinical target as a risk factor for depression onset. Unfortunately, reward-related deficits remain difficult to treat. To address this gap and inform the development of effective prevention and treatment strategies, it is critical to understand the mechanisms that drive impairments in reward function. Stress-induced inflammation is a plausible mechanism of reward deficits. The purpose of this paper is to review evidence for two components of this psychobiological pathway: 1) the effects of stress on reward function; and 2) the effects of inflammation on reward function. Within these two areas, we draw upon preclinical and clinical models, distinguish between acute and chronic effects of stress and inflammation, and address specific domains of reward dysregulation. By addressing these contextual factors, the review reveals a nuanced literature which might be targeted for additional scientific inquiry to inform the development of precise interventions.

Gut microbiota: Candidates for a novel strategy for ameliorating sleep disorders

The aim of this review was to evaluate the feasibility of treating sleep disorders using novel gut microbiota intervention strategies. Multiple factors can cause sleep disorders, including an imbalance in the gut microbiota. Studies of the microbiome-gut-brain axis have revealed bidirectional communication between the central nervous system and gut microbes, providing a more comprehensive understanding of mood and behavioral regulatory patterns. Changes in the gut microbiota and its metabolites can stimulate the endocrine, nervous, and immune systems, which regulate the release of neurotransmitters and alter the activity of the central nervous system, ultimately leading to sleep disorders. Here, we review the main factors affecting sleep, discuss possible pathways and molecular mechanisms of the interaction between sleep and the gut microbiota, and compare common gut microbiota intervention strategies aimed at improving sleep physiology.

Inflammation in the pathogenesis of depression: a disorder of neuroimmune origin

There are several hypotheses concerning the underlying pathophysiological mechanisms of major depression, which centre largely around adaptive changes in neuronal transmission and plasticity, neurogenesis, and circuit and regional connectivity. The immune and endocrine systems are commonly implicated in driving these changes. An intricate interaction of stress hormones, innate immune cells and the actions of soluble mediators of immunity within the nervous system is described as being associated with the symptoms of depression. Bridging endocrine and immune processes to neurotransmission and signalling within key cortical and limbic brain circuits are critical to understanding depression as a disorder of neuroimmune origins. Emergent areas of research include a growing recognition of the adaptive immune system, advances in neuroimaging techniques and mechanistic insights gained from transgenic animals. Elucidation of glial-neuronal interactions is providing additional avenues into promising areas of research, the development of clinically relevant disease models and the discovery of novel therapies. This narrative review focuses on molecular and cellular mechanisms that are influenced by inflammation and stress. The aim of this review is to provide an overview of our current understanding of depression as a disorder of neuroimmune origin, focusing on neuroendocrine and neuroimmune dysregulation in depression pathophysiology. Advances in current understanding lie in pursuit of relevant biomarkers, as the potential of biomarker signatures to improve clinical outcomes is yet to be fully realised. Further investigations to expand biomarker panels including integration with neuroimaging, utilising individual symptoms to stratify patients into more homogenous subpopulations and targeting the immune system for new treatment approaches will help to address current unmet clinical need.

Stress deficits in reward behaviour are associated with and replicated by dysregulated amygdala-nucleus accumbens pathway function in mice

Reduced reward interest/learning and reward-to-effort valuation are distinct, common symptoms in neuropsychiatric disorders for which chronic stress is a major aetiological factor. Glutamate neurons in basal amygdala (BA) project to various regions including nucleus accumbens (NAc). The BA-NAc neural pathway is activated by reward and aversion, with many neurons being monovalent. In adult male mice, chronic social stress (CSS) leads to reduced discriminative reward learning (DRL) associated with decreased BA-NAc activity, and to reduced reward-to-effort valuation (REV) associated, in contrast, with increased BA-NAc activity. Chronic tetanus toxin BA-NAc inhibition replicates the CSS-DRL effect and causes a mild REV reduction, whilst chronic DREADDs BA-NAc activation replicates the CSS effect on REV without affecting DRL. This study provides evidence that stress disruption of reward processing involves the BA-NAc neural pathway; the bi-directional effects implicate opposite activity changes in reward (learning) neurons and aversion (effort) neurons in the BA-NAc pathway following chronic stress.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

The neuroimmunology of social-stress-induced sensitization

Myriad clinical findings provide links between chronic stressors, inflammation, and mood disorders. Furthermore, traumatic or chronic exposure to psychological stressors may promote stress sensitization, in which individuals have long-term complications, including increased vulnerability to subsequent stressors. Post-traumatic stress disorder (PTSD) is a clinically relevant example of stress sensitization. PTSD alters neuronal circuitry and mood; however, the mechanisms underlying long-term stress sensitization within this disorder are unclear. Rodent models of chronic social defeat recapitulate several key physiological, immunological, and behavioral responses associated with psychological stress in humans. Repeated social defeat (RSD) uniquely promotes the convergence of neuronal, central inflammatory (microglial), and peripheral immune (monocyte) pathways, leading to prolonged anxiety, social withdrawal, and cognitive impairment. Moreover, RSD promotes stress sensitization, in which mice are highly sensitive to subthreshold stress exposure and recurrence of anxiety weeks after the cessation of stress. Therefore, the purpose of this Review is to discuss the influence of social-defeat stress on the immune system that may underlie stress sensitization within three key cellular compartments: neurons, microglia, and monocytes. Delineating the mechanisms of stress sensitization is critical in understanding and treating conditions such as PTSD.

Chronic social stress during early development is involved in antisocial maltreatment behavior in mice

Purpose

Early-life stress can cause brain inflammation and affect social behavior in adulthood. In humans, maltreated (abused or neglected) children often exhibit antisocial behavior, including violent and sadistic behavior, in adulthood. However, it is unknown whether maltreatment behavior occurs in rodents. Here, we developed an assay system to evaluate conspecific maltreatment behavior in the mouse.

Methods

To assess maltreatment behavior, we devised a two-chamber apparatus separated by a transparent partition, in which one chamber was provided with a nose-poking hole that would trigger foot shocks onto the other. Lidocaine was used to inhibit neural activity in vivo. Brain oscillations were investigated by electroencephalograph. Enzyme-linked immunosorbent assay was used for protein assay. The mouse model was sequentially subjected to maternal separation (MS), social defeat (SD), and social isolation (SI) in that order (MS/SD/SI model).

Results

Inactivation of the anterior cingulate cortex and medial prefrontal cortex increased the level of nose-poking. Maltreatment behavior provoked changes in oxytocin, corticosterone, and brain-derived neurotrophic factor levels. MS/SD/SI mice exhibited more sustained nose-poking behavior during the experiment, resulting in increased foot shocks to the mouse in the opposite chamber. Abnormal brain oscillations were observed in the MS/SD/SI mice.

Conclusion

The MS/SD/SI model and maltreatment-behavior assay may be useful not only to study the relationship between social stress in childhood and antisocial behavior in adulthood, but also for study of etiology, pathology, or treatment for brain disorders, such as psychopathy.

Repeated social defeat stress exacerbates lipopolysaccharide-induced behavioural deficits in mice: ameliorative role of Chrysophyllum albidum fruit extract through anti-neuroinflammation, antioxidant and neurochemical balance

Development of neuropsychiatric disorder is associated with stress-related increase in pro-inflammatory cytokines. Chrysophyllum albidum fruit is an edible tropical fruit containing vitamins and phenolic compounds, well known for their anti-inflammatory and antioxidant activities. This study was designed to investigate the neuroprotective effect of C. albidum fruit extract (CAFE) on stress and lipopolysaccharide (LPS)-induced behavioral and neurochemical impairments in mice. Male Swiss mice were divided into 6 groups (n = 6). Groups 1-3 were orally treated daily for 14 days with normal saline (0.1 mL/10 g), CAFE (100 mg/kg) and Ferulic acid (FA, 10 mg/kg), and left in home cage as controls. Groups 4-6 were treated similarly but subjected to repeated social defeat (RSD) stress using the resident-intruder model from days 1-14. The RSD-animals were injected with LPS (125 µg/kg, i.p) 60 min after each RSD session from days 8-14. Neurobehavioral functions: locomotor, cognitive and anxiety-like behaviors were assessed 24 h after the last treatment. Pro-inflammatory cytokines (IL-1β, IL-6 and TNF-α), dopamine, acetylcholinesterase, glutamic acid decarboxylase (GAD), malondialdehyde, nitrites, and reduced glutathione (GSH) were determined in brain tissue. CAFE significantly attenuated RSD and LPS-induced hypolocomotion, cognitive impairment and anxiety-like behavior when compared to the control. Treatment with CAFE also significantly reversed the negative effects of RSD and LPS on pro-inflammatory cytokines, dopamine, acetylcholinesterase, GAD, and oxidative-nitrosative stress levels. The findings clearly indicated that Chrysophyllum albidum fruit demonstrated neuroprotective effects and can play a key role in mitigating against chronic stress and inflammation linked to neuropsychiatric disorders.

Effects of chronic social stress on oligodendrocyte proliferation-maturation and myelin status in prefrontal cortex and amygdala in adult mice

Stress-related neuropsychiatric disorders present with excessive processing of aversive stimuli. Whilst underlying pathophysiology remains poorly understood, within- and between-regional changes in oligodendrocyte (OL)-myelination status in anterior cingulate cortex and amygdala (ACC-AMY network) could be important. In adult mice, a 15-day chronic social stress (CSS) protocol leads to increased aversion responsiveness, accompanied by increased resting-state functional connectivity between, and reduced oligodendrocyte- and myelin-related transcript expression within, medial prefrontal cortex and amygdala (mPFC-AMY network), the analog of the human ACC-AMY network. In the current study, young-adult male C57BL/6 mice underwent CSS or control handling (CON). To assess OL proliferation-maturation, mice received 5-ethynyl-2'-deoxyuridine via drinking water across CSS/CON and brains were collected on day 16 or 31. In mPFC, CSS decreased the density of proliferative OL precursor cells (OPCs) at days 16 and 31. CSS increased mPFC myelin basic protein (MBP) integrated density at day 31, as well as increasing myelin thickness as determined using transmission electron microscopy, at day 16. In AMY, CSS increased the densities of total CC1+ OLs (day 31) and CC1+/ASPA+ OLs (days 16 and 31), whilst decreasing the density of proliferative OPCs at days 16 and 31. CSS was without effect on AMY MBP content and myelin thickness, at days 16 and 31. Therefore, CSS impacts on the OL lineage in mPFC and AMY and to an extent that, in mPFC at least, leads to increased myelination. This increased myelination could contribute to the excessive aversion learning and memory that occur in CSS mice and, indeed, human stress-related neuropsychiatric disorders.

Comprehensive Preclinical Assessment of Sensory, Functional, Motivational-Affective, and Neurochemical Outcomes in Neuropathic Pain: The Case of the Sigma-1 Receptor

Chronic pain remains a major health problem and is currently facing slow drug innovation. New drug treatments should address not only the sensory-discriminative but also functional and motivational-affective components of chronic pain. In a mouse model of neuropathic pain induced by partial sciatic nerve ligation (PSNL), we analyzed sensory and functional-like outcomes by hindpaw mechanical stimulation and automated gait analysis (CatWalk). We characterized over time a reward-seeking task based on diminished motivation for natural reinforcers (anhedonic-like behavior). To differentiate the appetitive ("wanting") and consummatory ("liking") aspects of motivational behavior, we quantified the latency and number of approaches to eat white chocolate, as well as the eating duration and amount consumed. We explored a putative chronic pain-induced dysregulation of monoamine function by measuring monoamine levels in the nucleus accumbens (NAc), a well-known brain reward area. Finally, we investigated the role of sigma-1 receptor (σ₁R) modulation, a nonopioid target, in these multiple dimensions by genetic deletion and pharmacological dose-response studies. After 6 weeks, PSNL increased the approach latency and reduced the consumption of white chocolate in 20-25% of the mice, while around 50-60% had one or the other parameter affected independently. After 10 weeks, sham-operated mice also displayed anhedonic-like behavior. PSNL was associated with reduced extracellular baseline dopamine and increased norepinephrine in the NAc and with a suppression of increased dopamine and serotonin efflux in response to the rewarding stimulus. Genetic and pharmacological blockade of σ₁R relieved these multiple alterations in nerve-injured mice. We comprehensively describe sensory, functional, and depression-like impairment of key components of motivated behavior associated with nerve injury. We provide a neurochemical substrate for the depressed mesocorticolimbic reward processing in chronic pain, with a potentially increased translational value. Our results also highlight σ₁R for the therapeutic intervention of neuropathic pain.

Chronic social stress during early development elicits unique behavioral changes in adulthood

Purpose

Chronic social stress is known to induce inflammation in the brain, and early-life stress affects the brain and social behavior in adulthood. To study the relationship between social stress in childhood development and social behavior in adulthood, we subjected mice to a sequential early-life social stresses and characterized their adult behavioral phenotypes.

Methods

C57BL/6 mice were sequentially subjected to maternal separation (MS), social defeat (SD), and social isolation (SI) in that order. The body weights of the MS/SD/SI mice were measured. Behavioral tasks related to anxiety, depression, locomotion, learning/memory, and repetitive/compulsive-like behavior were conducted. Social behaviors suggesting sociability, social interaction, aggression, and social fear were investigated.

Results

MS/SD/SI mice weighed less than the control mice. At 7 and 8 weeks of age. These mice displayed normal behaviors in anxiety-, depression-, and learning/memory-related tasks, but they exhibited increased locomotor activity and a low level of repetitive/compulsive-like behavior. Notably, they exhibited increased social interaction, impaired empathy-related fear, reduced predator fear, and increased defensive aggressiveness.

Conclusion

Social stress during childhood development resulted in behavioral alterations, and MS/SD/SI mice generated by mimicking child abuse or maltreatment showed unique abnormalities in social behaviors. MS/SD/SI mice might be useful not only to study the relationship between social stress and brain inflammation but also psychosocial behaviors observed in individuals with brain disorders, such as psychopaths.

CCR2 monocytes repair cerebrovascular damage caused by chronic social defeat stress

Immune surveillance of the brain plays an important role in health and disease. Peripheral leukocytes patrol blood-brain barrier interfaces, and after injury, monocytes cross the cerebrovasculature and follow a pattern of pro- and anti-inflammatory activity leading to tissue repair. We have shown that chronic social defeat (CSD) causes scattered vasculature disruptions. Here, we assessed CCR2⁺ monocyte trafficking to the vascular injury sites in Ccr2^wt/rfp reporter mice both during CSD and one week following CSD cessation. We found that CSD for 14 days induced microhemorrhages where plasma fibrinogen leaked into perivascular spaces, but it did not affect the distribution or density of CCR2^rfp+ monocytes in the brain. However, after recovery from CSD, many vascularly adhered CCR2⁺ cells were detected, and gene expression of the CCR2 chemokine receptor ligands CCL7 and CCL12, but not CCL2, was elevated in endothelial cells. Adhered CCR2⁺ cells were mostly the non-classical, anti-inflammatory Ly6C^lo type, and they phagocytosed fibrinogen in perivascular spaces. In CCR2-deficient Ccr2^rfp/rfp mice, fibrinogen levels remained elevated in recovery. Fibrinogen infused intracerebroventricularly induced CCR2⁺ cells to adhere to the vasculature and phagocytose perivascular fibrinogen in Ccr2^wt/rfp but not Ccr2^rfp/rfp mice. Depletion of monocytes with clodronate liposomes during CSD recovery prevented fibrinogen clearance and blocked behavioral recovery. We hypothesize that peripheral CCR2⁺ monocytes are not elevated in the brain on day 14 at the end of CSD and do not contribute to its behavioral effects at that time, but in recovery following cessation of stress, they enter the brain and exert restorative functions mediating vascular repair and normalization of behavior.

Cyclophosphamide and epirubicin induce high apoptosis in microglia cells while epirubicin provokes DNA damage and microglial activation at sub-lethal concentrations

Chemotherapy Related Cognitive Impairment (CRCI), also called chemobrain, diminishes cancer patient's life quality. Breast cancer (BC) patients have been described to be importantly affected, however, the mechanism leading to CRCI has not been fully elucidated. Recent research proposes microglia as the main architect of CRCI, thus dysregulations in these cells could trigger CRCI. The aim of this research was to evaluate the effects of two drugs commonly used against breast cancer, cyclophosphamide (CTX) and epirubicin (EPI), on the microglia cell line SIM-A9, using the BC cell line, 4T1, as a control. Our results show that CTX and EPI decrease microglia-cell viability and increase cell death on a concentration-dependent manner, being 5 and 2 times more cytotoxic to microglia cell line than to breast cancer 4T1cells, respectively. Both chemotherapies induce cell cycle arrest and a significant increase in p53, p16 and γ-H2AX in breast cancer and microglia cells. Furthermore, mitochondrial membrane potential (ΔΨm) diminishes as cell death increases, and both chemotherapies induce reactive oxygen species (ROS) production on SIM-A9 and 4T1. Moreover, caspase activation increases with treatments and its pharmacological blockade inhibits CTX and EPI induced-cell death. Finally, low concentrations of CTX and EPI induce γ-H2AX, and EPI induces cytokine release, NO production and Iba-1 overexpression. These findings indicate that microglia cells are more sensitive to CTX and EPI than BC cells and undergo DNA damage and cell cycle arrest at very low concentrations, moreover EPI induces microglia activation and a pro-inflammatory profile.

Evidence for Effects of Extracellular Vesicles on Physical, Inflammatory, Transcriptome and Reward Behaviour Status in Mice

Immune-inflammatory activation impacts extracellular vesicles (EVs), including their miRNA cargo. There is evidence for changes in the EV miRNome in inflammation-associated neuropsychiatric disorders. This mouse study investigated: (1) effects of systemic lipopolysaccharide (LPS) and chronic social stress (CSS) on plasma EV miRNome; and (2) physiological, transcriptional, and behavioural effects of peripheral or central delivered LPS-activated EVs in recipient mice. LPS or CSS effects on the plasma EV miRNome were assessed by using microRNA sequencing. Recipient mice received plasma EVs isolated from LPS-treated or SAL-treated donor mice or vehicle only, either intravenously or into the nucleus accumbens (NAc), on three consecutive days. Bodyweight, spleen or NAc transcriptome and reward (sucrose) motivation were assessed. LPS and CSS increased the expression of 122 and decreased expression of 20 plasma EV miRNAs, respectively. Peripheral LPS-EVs reduced bodyweight, and both LPS-EVs and SAL-EVs increased spleen expression of immune-relevant genes. NAc-infused LPS-EVs increased the expression of 10 immune-inflammatory genes. Whereas motivation increased similarly across test days in all groups, the effect of test days was more pronounced in mice that received peripheral or central LPS-EVs compared with other groups. This study provides causal evidence that increased EV levels impact physiological and behavioural processes and are of potential relevance to neuropsychiatric disorders.

Somatostatin receptor 4 agonism normalizes stress-related excessive amygdala glutamate release and Pavlovian aversion learning and memory in rodents

Background

Excessive processing of aversive life events is a major pathology in stress-related anxiety and depressive disorders. Current pharmacological treatments have rather nonspecific mechanisms of action. Somatostatin is synthesized and released as an inhibitory co-neurotransmitter by specific GABA (gamma-aminobutyric acid) interneurons, and one of its receptors, SSTR4 (somatostatin receptor 4), is localized in brain regions involved in adaptive aversion processing and implicated in negative valence neuropathology, including the amygdala.

Methods

Rat and mouse experiments were conducted to investigate effects of specific SSTR4 agonism on neurobehavioral aversion processing, including any normalization of stress-related hyperresponsiveness. A mouse experiment to investigate stress and SSTR4 agonism effects on reward processing was also conducted.

Results

In male rats (n = 5–10/group) fitted with glutamate biosensors in basolateral amygdala, SSTR4 agonism attenuated glutamate release to restraint stress in control rats and particularly in rats previously exposed to chronic corticosterone. In male mice (n = 10–18/group), SSTR4 agonism dose-dependently attenuated Pavlovian tone/footshock learning and memory measured as freezing behavior, in both control mice and mice exposed to chronic social stress, which induces excessive Pavlovian aversion learning and memory. Specificity of SSTR4 agonism effects to aversion learning/memory was demonstrated by absence of effects on discriminative reward (sucrose) learning/memory in both control mice and mice exposed to chronic social stress; SSTR4 agonism did increase reward-to-effort valuation in a dose-dependent manner and in both control mice and mice exposed to chronic social stress, which attenuates reward motivation.

Conclusions

These neuropsychopharmacological findings add substantially to the preclinical proof-of-concept evidence for SSTR4 agonism as a treatment in anxiety and depressive disorders.

Social disruption-induced stress pre-exposure aggravates, while the presence of conspecifics diminishes, acetic acid-induced writhing

RATIONALE AND OBJECTIVE

This study was undertaken to assess the modulating effects of (1) pre-exposure to repeated social disruption and (2) group testing on writhing associated with visceral pain induced by intraperitoneal administration of acetic acid.

MATERIALS AND METHODS

Six consecutive days of social disruption were used to prime for stress, while group testing referred to 3 mouse cage-mates receiving the acetic acid-induced writhing test as a group.

RESULTS

Social disruption-induced stress-pre-exposed mice displayed a greater number acid-induced writhes compared to mice not receiving the pre-exposure. However, mice displayed fewer acid-induced writhes in a triad group vs. individually, suggesting group-mediated writhing-reducing effects. Likewise, group testing prevented the stress pre-exposure escalation in acid-induced writhes. Additional studies revealed that the stress-pre-exposed mice had increased expression in accumbal TRPV1 receptors. Systemic (0.25 mg/kg) and bilateral intra-accumbal (0.2 ng/0.2 µl/side) administration of SB366791, a TRPV1 receptor antagonist, reliably prevented the stress pre-exposure escalation in acid-induced writhing; SB366791 treatment alone did not affect acid-induced writhing, stress pre-exposure anxiety-like behavior, or the group testing effects. Furthermore, lower neuronal activation was found in the medial septal nucleus in group vs. individual tested mice. Intra-medial septum (0.2 µg/0.5 µl) infusion with bicuculline, a GABAA receptor antagonist, effectively prevented group-mediated writhing-reducing effects, but not individual acid-induced writhing effects.

CONCLUSIONS

These findings suggest that social disruption-induced stress pre-exposure may upregulate accumbal TRPV1 receptor expression and consequently aggravate acid-induced writhing. Group testing prevents such stress pre-exposure escalation of acid-induced writhing most likely by strengthening the GABAergic inhibition on local neural activity in the medial septum.

HIV-Associated Apathy/Depression and Neurocognitive Impairments Reflect Persistent Dopamine Deficits

Individuals living with human immunodeficiency virus type 1 (HIV-1) are often plagued by debilitating neurocognitive impairments and affective alterations;the pathophysiology underlying these deficits likely includes dopaminergic system dysfunction. The present review utilized four interrelated aims to critically examine the evidence for dopaminergic alterations following HIV-1 viral protein exposure. First, basal dopamine (DA) values are dependent upon both brain region andexperimental approach (i.e., high-performance liquid chromatography, microdialysis or fast-scan cyclic voltammetry). Second, neurochemical measurements overwhelmingly support decreased DA concentrations following chronic HIV-1 viral protein exposure. Neurocognitive impairments, including alterations in pre-attentive processes and attention, as well as apathetic behaviors, provide an additional line of evidence for dopaminergic deficits in HIV-1. Third, to date, there is no compelling evidence that combination antiretroviral therapy (cART), the primary treatment regimen for HIV-1 seropositive individuals, has any direct pharmacological action on the dopaminergic system. Fourth, the infection of microglia by HIV-1 viral proteins may mechanistically underlie the dopamine deficit observed following chronic HIV-1 viral protein exposure. An inclusive and critical evaluation of the literature, therefore, supports the fundamental conclusion that long-term HIV-1 viral protein exposure leads to a decreased dopaminergic state, which continues to persist despite the advent of cART. Thus, effective treatment of HIV-1-associated apathy/depression and neurocognitive impairments must focus on strategies for rectifying decreases in dopamine function.

Microbiota and sleep: awakening the gut feeling

Various lifestyle and environmental factors are known to influence sleep. Increasingly, evidence points to a role for the microbiota in regulating brain and behaviour. This article explores how the microbiota-gut-brain axis affects sleep directly and indirectly. We summarize the possible molecular mechanisms underlying sleep-microbiome interactions and discuss how various factors interact with the gut microbiota to influence sleep. Furthermore, we present the current evidence of alterations of the microbiota-gut-brain axis in various sleep disorders and pathologies where comorbid sleep disturbances are common. Since manipulating the gut microbiota could potentially improve sleep, we outline ways in which this can be achieved.

Reactivating a positive feedback loop VTA-BLA-NAc circuit associated with positive experience ameliorates the attenuated reward sensitivity induced by chronic stress

Both genetic predisposition and life events, particularly life stress, are thought to increase the risk for depression. Reward sensitivity appears to be attenuated in major depressive disorder (MDD), suggesting deficits in reward processing in these patients. We identified the VTA-BLA-NAc circuit as being activated by sex reward, and the VTA neurons that respond to sex reward are mostly dopaminergic. Acute or chronic reactivation of this circuit ameliorates the reward insensitivity induced by chronic restraint stress. Our histological and electrophysiological results show that the VTA neuron subpopulation responding to restraint stress, predominantly GABAergic neurons, inhibits the responsiveness of VTA dopaminergic neurons to reward stimuli, which is probably the mechanism by which stress modulates the reward processing neural circuits and subsequently disrupts reward-related behaviours. Furthermore, we found that the VTA-BLA-NAc circuit is a positive feedback loop. Blocking the projections from the BLA to the NAc associated with sex reward increases the excitability of VTA GABAergic neurons and decreases the excitability of VTA dopaminergic neurons, while activating this pathway decreases the excitability of VTA GABAergic neurons and increases the excitability of VTA dopaminergic neurons, which may be the cellular mechanism by which the VTA-BLA-NAc circuit associated with sex reward ameliorates the attenuated reward sensitivity induced by chronic stress.

Involvement of the dopamine system in the effect of chronic social isolation during adolescence on social behaviors in male C57 mice

In the early stage of life, experiencing social isolation can generate long-lasting deleterious effects on behaviors and brain development. However, the effects of chronic social isolation during adolescence on social behaviors and its underlying neurobiological mechanisms remain unclear. The present study found that four weeks of social isolation during adolescence impaired social recognition ability in the three-chamber test and five-trial social recognition test, and increased aggressive-like behaviors, but reduced environmental exploration, as showed in the social interaction test. Chronic social isolation decreased levels of dopamine D2 receptor in the shell of the nucleus accumbens (NAcc) and medial prefrontal cortex. It also reduced TH in the NAcc. Using in vivo fiber photometry, it was also found that isolated mice displayed a reduction in NAcc shell activity upon exploring unfamiliar social stimuli. An injection of a 100 ng dose of the D2R agonist quinpirole into the shell of the NAcc reversed behavioral abnormalities induced by chronic social isolation. These data suggest that the dopamine system is involved in alterations in social behaviors induced by chronic social isolation. This finding sheds light on the mechanism underlying abnormalities in social behavior induced by adolescent chronic social isolation and provides a promising target to treat mental diseases relevant to social isolation.

Region- and receptor-specific effects of chronic social stress on the central serotonergic system in mice

Serotonin (5-HT), via its receptors expressed in discrete brain regions, modulates aversion and reward processing and is implicated in various psychiatric disorders including depression. Stressful experiences affect central serotonergic activity and act as a risk factor for depression; this can be modelled preclinically. In adult male C57BL/6J mice, 15-day chronic social stress (CSS) leads to depression-relevant behavioural states, including increased aversion and reduced reward sensitivity. Based on this evidence, here we investigated CSS effects on 5-HT1A, 5-HT2A, and 5-HT2C receptor binding in discrete brain regions using in vitro quantitative autoradiography with selective radioligands. In addition, mRNA expression of Htr1a, 2a, 2c and Slc6a4 (5-HT transporter) was measured by quantitative PCR. Relative to controls, the following effects were observed in CSS mice: 5-HT1A receptor binding was markedly increased in the dorsal raphe nucleus (136%); Htr1a mRNA expression was increased in raphe nuclei (19%), medial prefrontal cortex (35%), and hypothalamic para- and periventricular nuclei (21%) and ventral medial nucleus (38%). 5-HT2A receptor binding was decreased in the amygdala (48%) and ventral tegmental area (60%); Htr2a mRNA expression was increased in the baso-lateral amygdala (116%). 5-HT2C receptor binding was decreased in the dorsal raphe nucleus (42%). Slc6a4 mRNA expression was increased in the raphe (59%). The present findings add to the translational evidence that chronic social stress impacts on the central serotonergic system in a region- and receptor-specific manner, and that this altered state of the serotonergic system contributes to stress-induced dysfunctions in emotional processing.

Antidepressant-like Effects of Degraded Porphyran Isolated from Porphyra haitanensis

INTRODUCTION

Degraded porphyran is a bioactive polysaccharide extracted from Porphyra haitanensis (P. haitanensis). According to the previous studies, it produced anti-inflammatory activity, but little is known about its effects on depression.

METHODS AND RESULTS

As inflammation is one of the critical factors involved in the development of depression, this study aims to elucidate the potential antidepressant-like effects of degraded porphyran. The results show that acute porphyran treatment decreased the immobility time in despair tests. In addition, subchronic porphyran administration reverses depressive-like behaviors in lipopolysaccharide (LPS)-treated mice. Meanwhile, porphyran inhibits NF-κB/NLRP3 signaling, proinflammatory cytokine release, and microglial activation in the hippocampus. Moreover, chronic porphyran treatment activates hippocampal brain derived neurotrophic factor (BDNF)/TrkB/ERK/CREB signaling pathway in chronic unpredictable mild stress (CUMS) in mice. As a result, neurogenesis and spinogenesis are maintained.

CONCLUSIONS

The findings of the present study indicate that degraded porphyran intake provides a potential strategy for depression treatment, which is mediated by the inhibition of neuroinflammation and the enhancement of neurogenesis and spinogenesis in the central nervous systems.

Characterization of proinflammatory markers in the ventral tegmental area across mouse models of chronic stress

Despite the high prevalence of major depressive disorder (MDD), understanding of the biological underpinnings remains limited. Rodent models suggest that changes in activity and output of dopamine (DA) neurons in the ventral tegmental area (VTA) are important for depressive-like phenotypes. Additionally, brain inflammatory processes are thought to contribute to MDD pathology and inflammation in the VTA has been linked to changes in VTA DA neuronal activity. Thus, we sought to determine whether there is increased inflammatory signaling in the VTA following forms of chronic stress that induce depressive-like symptoms. First, we subjected male mice to either physical or vicarious chronic social defeat stress (CSDS), paradigms known to induce long-term depressive-like behavior and changes in VTA signaling. Second, we subjected male and female mice to subchronic variable stress (SCVS), a paradigm that induces depressive-like behavior only in female mice. We then isolated mRNA from the VTA and assessed proinflammatory gene regulation via RT-PCR. Our results show that physical, but not vicarious, CSDS increases interleukin 1β (IL-1β) mRNA expression and this inversely correlates with social interaction score. In contrast, IL-1β expression was unchanged in male or female mice following SCVS. No significant increases in VTA ionized calcium binding adapter molecule 1 (Iba1) and glial fibrillary acidic protein (GFAP) immunochemistry were detected following CSDS that would be indicative of a robust inflammatory response. In conclusion, we show that chronic stressors distinctively alter expression of proinflammatory genes in the VTA and changes may depend on the severity and time-course of the stress exposure.

Dysregulation of brain dopamine systems in major depressive disorder

Major depressive disorder (MDD or depression) is a debilitating neuropsychiatric syndrome with genetic, epigenetic, and environmental contributions. Depression is one of the largest contributors to chronic disease burden; it affects more than one in six individuals in the United States. A wide array of cellular and molecular modifications distributed across a variety of neuronal processes and circuits underlie the pathophysiology of depression-no established mechanism can explain all aspects of the disease. MDD suffers from a vast treatment gap worldwide, and large numbers of individuals who require treatment do not receive adequate care. This mini-review focuses on dysregulation of brain dopamine (DA) systems in the pathophysiology of MDD and describing new cellular targets for potential medication development focused on DA-modulated micro-circuits. We also explore how neurodevelopmental factors may modify risk for later emergence of MDD, possibly through dopaminergic substrates in the brain.

A Load to Find Clinically Useful Biomarkers for Depression

Depression is heterogeneous and complex disease with diverse symptoms. Its neurobiological underpinning is still not completely understood. For now, there are still no validated, easy obtainable, clinically useful noninvasive biomarker(s) or biomarker panel that will be able to confirm a diagnosis of depression, its subtypes and improve diagnostic procedures. Future multimodal preclinical and clinical research that involves (epi)genetic, molecular, cellular, imaging, and other studies is necessary to advance our understanding of the role of monoamines, GABA, HPA axis, neurotrophins, metabolome, and glycome in the pathogenesis of depression and their potential as diagnostic, prognostic, and treatment response biomarkers. These studies should be focused to include the first-episode depression and antidepressant drug-naïve patients with large sample sizes to reduce variability in different biological and clinical parameters. At present, metabolomics study revealed with high precision that a neurometabolite panel consisting of plasma metabolite biomarkers (GABA, dopamine, tyramine, kynurenine) might represent clinically useful biomarkers of MDD.

Basomedial amygdala activity in mice reflects specific and general aversion uncontrollability

Learning adaptive behaviour to control aversion is a major brain function. Detecting the absence of control is also important, although chronic uncontrollable aversion can impact maladaptively on stimulus processing in general. The mouse basomedial amygdala (BMA) contributes to aversion processing with high BMA activity associated with active behavioural responding. The overall aim of the present study was to investigate the associations between aversion (un)controllability, BMA activity and behaviour. Fibre photometry of GCaMP6‐expressing BMA neuron populations was applied in freely behaving adult male mice during exposure to mild electrical shocks, and effects of specific or general (un)controllability were investigated. In a discrete learned helplessness (LH) effect paradigm, mice underwent discrete sessions of pre‐exposure to either escapable shock (ES) or inescapable shock (IES) followed by an escape test. IES mice acquired fewer escape attempts than ES mice, and this co‐occurred with higher aversion‐related BMA activity in the IES group. After 30 days, ES and IES mice were allocated equally to either chronic social stress (CSS)—exposure to continuous uncontrollable social aversion—or control handling (CON), and on days 5 and 15 underwent an IES session. CSS mice made fewer escape attempts than CON mice, and this was now associated with lower aversion‐related BMA activity in the CSS group. These findings suggest that mouse BMA activity is higher when discrete aversion is uncontrollable but becomes lower following chronic uncontrollable aversion exposure. Therefore, BMA activity could be a neural marker of adaptive and maladaptive states consequent to specific and general uncontrollability, respectively.

The Psychological Impact of COVID-19 Pandemic on People With Multiple Sclerosis

Objective: The coronavirus disease 2019 (COVID-19) has radically changed the world in a few weeks. Italy has been one of the first and most affected countries with more than 30,000 deaths up to now. Public health measures as quarantine or national lockdown are necessary to limit the spread of infectious diseases, but it is unsurprising that depriving people of their liberty has negative psychological effects. This is especially the case for people with chronic diseases, including neurological conditions like multiple sclerosis (MS). People with MS (PwMS) have a higher burden of neuropsychiatric comorbidities and are known to undertake maladaptive coping strategies in stress conditions. The aim of the present study is to investigate the impact of COVID-19 pandemic lockdown on mental health of an Italian cohort of PwMS in comparison with healthy controls (HCs).

Methods: A total of 60 PwMS and 50 HCs (chosen among patients' cohabitants) were asked to answer a Web-based survey. This survey inquired about the impact of COVID-19 on patient's quality of life, job, and daily routine. Mood, fatigue, and sleep quality were evaluated using the Beck Depression Inventory II (BDI-II), the Generalized Anxiety Disease 7 (GAD-7), the Fatigue Severity Scale (FSS), and the Pittsburgh Sleep Quality Index (PSQI).

Results: Overall, patients had higher scores of BDI, FSS, and PSQI, and these differences were statistically significant (p < 0.05). When we looked at the subscores of the BDI, we detected a statistically significant difference for the neurovegetative part—that concerns with sleep, appetite, sex, and quality of sleep (p < 0.05). One out of five patients reported new symptoms or worsening of known symptom, in particular, sensory disturbances, and fatigue. However, no symptoms were severe enough to require hospitalization. When we looked for correlations among variables, we found that there was a significant relationship between unemployment and BDI total score, GAD-7, and PSQI in MS group. The presence of new symptoms or the worsening of symptoms positively related to FSS and to PSQI.

Discussion: We identified that the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pandemic had a significant impact on the psychological status of patients with MS. Compared with the general population, PwMS presented a higher burden of depressive symptoms, a worse sleep quality and perceived an increase in fatigue level, one of the most disabling symptoms of MS. The COVID-19 epidemic poses a challenge to psychological resilience. More studies are warranted to better understand the long-term consequences of the pandemic on mental health of vulnerable people during the disease outbreaks.

The novel atypical dopamine transport inhibitor CT-005404 has pro-motivational effects in neurochemical and inflammatory models of effort-based dysfunctions related to psychopathology

Depressed individuals suffer from effort-related motivational symptoms such as anergia and fatigue, which are resistant to treatment with many common antidepressants. While drugs that block dopamine transport (DAT) reportedly have positive motivational effects, DAT inhibitors such as cocaine and amphetamines produce undesirable side effects. Thus, there is a need to develop and characterize novel atypical DAT inhibitors with unique and selective binding profiles. Rodent effort-based choice tasks provide useful models of motivational dysfunctions. With these tasks, animals choose between a high-effort instrumental action leading to highly valued reinforcement vs. a low effort/low reward option. The present studies focused on the initial characterization of a novel atypical DAT inhibitor, CT-005404, which binds to DAT with high selectivity relative to serotonin and norepinephrine transport, and produces long-term elevations of extracellular DA. CT-005404 was assessed for its ability to attenuate the effort-related motivational effects of the DA depleting agent tetrabenazine and the pro-inflammatory cytokine interleukin-1β (IL-1β) using a fixed ratio 5/chow feeding choice test. Tetrabenazine (1.0 mg/kg i.p.) shifted choice behavior, decreasing lever pressing and increasing chow intake. IL-1β (4.0 μg/kg i.p.) also decreased lever pressing. CT-005404 was co-administered (7.5-30.0 mg/kg p.o.) with either tetrabenazine or IL-1β, and the 15.0 and 30.0 mg/kg doses significantly reversed the effects of tetrabenazine and IL-1β. CT-005404 administered alone produced a dose-related increase in lever pressing in rats tested on a progressive ratio/chow feeding choice task. Atypical DAT inhibitors such as CT-005404 offer potential as a new avenue for drug treatment of motivational dysfunctions in humans.

The Impact of Chronic Mild Stress and Agomelatine Treatment on the Expression Level and Methylation Status of Genes Involved in Tryptophan Catabolic Pathway in PBMCs and Brain Structures

Depression is the serious mental disorder. Previous studies suggest that the development mechanism of depression may be associated with disorders of the tryptophan catabolic pathway (TRYCAT). Thus, this study investigates the effect of agomelatine treatment on the expression and methylation status of genes involved in TRYCAT in the brain and blood of rats exposed to a chronic mild stress (CMS). Separate groups of rats were exposed to CMS for two or seven weeks; the second group received vehicle or agomelatine for five weeks. After completion of both stress conditions and treatment, the expression levels of messenger RNA (mRNA) and protein, as well as the methylation status of promoters, were measured in peripheral blood mononuclear cells (PBMCs) and in brain structures with the use of TaqMan Gene Expression Assay, Western blot, and methylation-sensitive high-resolution melting techniques. In PBMCs, Kmo mRNA expression increased in the group after CMS, while this effect was normalized by agomelatine therapy. In brain, KatI and KatII expression changed following CMS exposure. Moreover, CMS decreased the methylation status of the second Tdo2 promoter in the amygdala. Protein expression of Tph1, Tph2, Ido1, and KatII changed in the group after CMS and agomelatine administration, most prominently in the basal ganglia, cerebral cortex, hippocampus, and amygdala. The results indicate that CMS and agomelatine affect the mRNA and protein expression, as well as the methylation of promoters of genes involved in the tryptophan catabolic pathway.

Chronic social stress in mice alters energy status including higher glucose need but lower brain utilization

Chronic stress leads to changes in energy status and is a major risk factor for depression, with common symptoms of reductions in body weight and effortful motivation for reward. Indeed, stress-induced disturbed energy status could be a major aetio-pathogenic factor for depression. Improved understanding of these putative inter-relationships requires animal model studies of effects of stress on both peripheral and central energy-status measures and determinants. Here we conducted a study in mice fed on a standard low-fat diet and exposed to either 15-day chronic social stress (CSS) or control handling (CON). Relative to CON mice, CSS mice had attenuated body weight maintenance/gain despite consuming the same amount of food and expending the same amount of energy at any given body weight. The low weight of CSS mice was associated with less white and brown adipose tissues, and with a high respiratory exchange ratio consistent with increased dependence on glucose as energy substrate. Basal plasma insulin was low in CSS mice and exogenous glucose challenge resulted in a relatively prolonged elevation of blood glucose. With regard to hunger and satiety hormones, respectively, CSS mice had higher levels of acylated ghrelin in plasma and of ghrelin receptor gene expression in ventromedial hypothalamus and lower levels of plasma leptin, relative to CON mice. However, whilst CSS mice displayed this constellation of peripheral changes consistent with increases in energy need and glucose utilization relative to CON mice, they also displayed attenuated uptake of [¹⁸F]FDG in brain tissue specifically. Reduced brain glucose utilization in CSS mice could contribute to the reduced effortful motivation for reward in the form of sweet-tasting food that we have reported previously for CSS mice. It will now be important to utilize this model to further understanding of the mechanisms via which chronic stress can increase energy need but decrease brain glucose utilization and how this relates to regional and cellular changes in neural circuits for reward processing relevant to depression.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

Formosan wood mice (Apodemus semotus) exhibit more exploratory behaviors and central dopaminergic activities than C57BL/6 mice in the open field test

Three-quarters of the lands in Taiwan are over 1000 m above sea level. Formosan wood mice (Apodemus semotus), also called Taiwanese field mice, are largely found at altitudes of 1400 ~ 3700 m and are the dominant rodents in these areas. Notably, Formosan wood mice show high levels of exploratory behaviors, not only in the wild but also in laboratory situations. Therefore, in this study, we examined the behavioral responses and central dopaminergic activities of male C57BL/6J mice and Formosan wood mice in the open field test. Dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid were used as indices of dopaminergic activities. Formosan wood mice showed higher levels of exploration and locomotor activity than C57BL/6J mice in the open field test. Higher central dopaminergic activities in the nucleus accumbens, striatum, and medial prefrontal cortex were found in Formosan wood mice than in C57BL/6J mice in the open field test. Higher levels of locomotion and central dopaminergic activities in Formosan wood mice were consistent after two exposures to the open field test; however, dramatic decreases in levels of locomotion and central dopaminergic activities in C57BL/6J mice were found after two exposures to the open field test. The present study found that Formosan wood mice exhibited higher levels of locomotor activity and exploration and central dopaminergic activities than C57BL/6J mice after one or two exposures to the open field test.

Microglial activation contributes to depressive-like behavior in dopamine D3 receptor knockout mice

We previously demonstrated that the dopamine D3 receptor (D3R) inhibitor, NGB2904, increases susceptibility to depressive-like symptoms, elevates pro-inflammatory cytokine expression, and alters brain-derived neurotrophic factor (BDNF) levels in mesolimbic dopaminergic regions, including the medial prefrontal cortex (mPFC), nucleus accumbens (NAc), and ventral tegmental area (VTA) in mice. The mechanisms by which D3R inhibition affects neuroinflammation and onset of depression remain unclear. Here, using D3R-knockout (D3RKO) and congenic wild-type C56BL/6 (WT) mice, we demonstrated that D3RKO mice displayed depressive-like behaviors, increased tumornecrosisfactor-α (TNF-α), interleukin-1β (IL-1β), and IL-6 levels, and altered BDNF expression in selected mesolimbic dopaminergic regions. D3R expression was localized to astrocytes or microglia in the mPFC, NAc, and VTA in WT mice. D3RKO mice exhibited a large number of Iba1-labelled microglia in the absence of glial fibrillary acidic protein (GFAP)-labelled astrocytes in mesolimbic dopaminergic brain areas. Inhibition or ablation of microglia by minocycline (25 mg/kg and 50 mg/kg) or PLX3397 (40 mg/kg) treatment ameliorated depressive-like symptoms, alterations in pro-inflammatory cytokine levels, and BDNF expression in the indicated brain regions in D3RKO mice. Minocycline therapy alleviated the increase in synaptic density in the NAc in D3RKO mice. These findings suggest that microglial activation in selected mesolimbic reward regions affects depressive-like behaviors induced by D3R deficiency.

Molecular neuro-biological and systemic health benefits of achieving dopamine homeostasis in the face of a catastrophic pandemic (COVID- 19): A mechanistic exploration

In the face of the global pandemic of COVID 19, approaching 1.75 Million infected worldwide (4/12/2020) and associated mortality (over 108, 000 as of 4/12/2020) as well-as other catastrophic events including the opioid crisis, a focus on brain health seems prudent [1] (https://www.coronavirus.gov). This manuscript reports on the systemic benefits of restoring and achieving dopamine homeostasis to reverse and normalize thoughts and behaviors of Reward Deficiency Syndrome (RDS) dysfunctional conditions and their effects on behavioral physiology; function of reward genes; and focuses on digestive, immune, eye health, and the constellation of symptomatic behaviors. The role of nutrigenomic interventions on restoring normal brain functions and its benefits on these systems will be discussed. We demonstrate that modulation of dopamine homeostasis using nutrigenomic dopamine agonists, instead of pharmaceutical interventions, is achievable. The allied interlinking with diverse chronic diseases and disorders, roles of free radicals and incidence of anaerobic events have been extensively highlighted. In conjunction, the role of dopamine in aspects of sleep, rapid eye movement and waking are extensively discussed. The integral aspects of food indulgence, the influence of taste sensations, and gut-brain signaling are also discussed along with a special emphasis on ocular health. The detailed mechanistic insight of dopamine, immune competence and the allied aspects of autoimmune disorders are also highlighted. Finally, the integration of dopamine homeostasis utilizing a patented gene test and a research-validated nutrigenomic intervention are presented. Overall, a cutting-edge nutrigenomic intervention could prove to be a technological paradigm shift in our understanding of the extent to which achieving dopamine homeostasis will benefit overall health.

Suicide Has Many Faces, So Does Ketamine: a Narrative Review on Ketamine's Antisuicidal Actions

PURPOSE OF REVIEW

Suicidal behaviours are a challenge for a medical system and public health, partly due to the current lack of evidence-based, effective, rapid tools for suicidal crisis management. Ketamine and its enantiomer esketamine have raised hopes regarding this issue in the recent years. However, their efficacy in suicidal behaviours and mechanisms for it remain a topic of debate.

RECENT FINDINGS

Subanesthetic ketamine doses rapidly, albeit transiently decrease suicidal ideation, with effects emerging within an hour and persisting up to a week. Current evidence points to various and not necessarily exclusive mechanisms for ketamine's antisuicidal action, including effects on neuroplasticity, inflammation, reward system and pain processing. Ketamine rapidly decreases suicidal ideation, but whether it leads to meaningful clinical outcomes past 1 week is unclear. Multiple putative mechanisms drive ketamine's antisuicidal action. Future studies will have to show long-term ketamine treatment outcomes and further elucidate its mechanisms of action.

Neuropharmacology of the mesolimbic system and associated circuits on social hierarchies

Most socially living species are organized hierarchically, primarily based on individual differences in social dominance. Dominant individuals typically gain privileged access to important resources, such as food, mating partners and territories, whereas submissive conspecifics are often devoid of such benefits. The benefits associated with a high social status provide a strong incentive to become dominant. Importantly, motivational- and reward-related processes are regulated, to a large extent, by the mesolimbic system. Consequently, several studies point to a key role for the mesolimbic system in social hierarchy formation. This review summarizes the growing body of literature that implicates the mesolimbic system, and associated neural circuits, on social hierarchies. In particular, we discuss the neurochemical and pharmacological studies that have highlighted the contributions of the mesolimbic system and associated circuits including dopamine signaling through the D1 or D2 receptors, GABAergic neurotransmission, the androgen receptor system, and mitochondria and bioenergetics. Given that low social status has been linked to the emergence of anxiety- and depressive-like disorders, a greater understanding of the neurochemistry underlying social dominance could be of tremendous benefit for the development of pharmacological treatments to dysfunctions in social behaviors. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Exploratory and agile behaviors with central dopaminergic activities in open field tests in Formosan wood mice (Apodemus semotus)

Taiwan is a mountainous island, and nearly 75% of its lands are 1000 m above sea level. Formosan wood mice, Apodemus semotus, are endemic rodents and are broadly distributed at altitudes between 1400 m and 3700 m in Taiwan. Interestingly, Formosan wood mice show similar locomotor activity in the laboratory as they do in the wild. Hence, we are interested in studying whether exploratory behaviors and central dopaminergic activity are changed in the open field test. We used male C57BL/6J mice as the control, comparing their behavioral responses in the open field, step-down inhibitory avoidance discrimination and novel object recognition tests with those of male Formosan wood mice. We also examined dopamine and its major metabolite 3,4-dihydroxyphenylacetic acid in the medial prefrontal cortex, striatum and nucleus accumbens. In open field tests, Formosan wood mice revealed higher levels of locomotion and exploration than C57BL/6J mice. Learning and memory performance in the novel object recognition test was similar in both Formosan wood mice and C57BL/6J mice, but more agile responses in the inhibitory avoidance discrimination task were found in Formosan wood mice. There was no difference in behavioral responses in the open field test between new second-generation Formosan wood mice and Formosan wood mice that were inbred for more than ten generations. After repeated exposure to the open field test, high levels of locomotion and exploration as well as central dopaminergic activities were markedly persistent in Formosan wood mice, but these activities were significantly reduced in C57BL/6J mice. Diazepam (anxiolytic) treatment reduced the higher exploratory activity and central dopaminergic activities in Formosan wood mice, but this treatment had no effect in C57BL/6J mice. This study provides comparative findings, as two phylogenetically related species showed differences in behavioral responses.

Making Sense of Rodent Models of Anhedonia

A markedly reduced interest or pleasure in activities previously considered pleasurable is a main symptom in mood disorder and psychosis and is often present in other psychiatric disorders and neurodegenerative diseases. This condition can be labeled as "anhedonia," although in its most rigorous connotation the term refers to the lost capacity to feel pleasure that is one aspect of the complex phenomenon of processing and responding to reward. The responses to rewarding stimuli are relatively easy to study in rodents, and the experimental conditions that consistently and persistently impair these responses are used to model anhedonia. To this end, long-term exposure to environmental aversive conditions is primarily used, and the resulting deficits in reward responses are often accompanied by other deficits that are mainly reminiscent of clinical depressive symptoms. The different components of impaired reward responses induced by environmental aversive events can be assessed by different tests or protocols that require different degrees of time allocation, technical resources, and equipment. Rodent models of anhedonia are valuable tools in the study of the neurobiological mechanisms underpinning impaired behavioral responses and in the screening and characterization of drugs that may reverse these behavioral deficits. In particular, the antianhedonic or promotivational effects are relevant features in the spectrum of activities of drugs used in mood disorders or psychosis. Thus, more than the model, it is the choice of tests that is crucial since it influences which facets of anhedonia will be detected and should be tuned to the purpose of the study.

Chronic Social Stress Leads to Reduced Gustatory Reward Salience and Effort Valuation in Mice

Pathology of reward processing is a major clinical feature of stress-related neuropsychiatric disorders including depression. Several dimensions of reward processing can be impacted, including reward valuation/salience, learning, expectancy and effort valuation. To establish the causal relationships between stress, brain changes, and reward processing pathologies, valid animal models are essential. Here, we present mouse experiments investigating behavioral effects of chronic social stress (CSS) in association learning tests of gustatory reward salience and effort valuation. The reward salience test (RST) comprised Pavlovian pairing of a tone with gustatory reward. The effort valuation test (EVT) comprised operant responding for gustatory reinforcement on a progressive ratio schedule (PRS). All testing was conducted with mice at 100% baseline body weight (BBW). In one experiment, mice underwent 15-day CSS or control handling (CON) and testing was conducted using sucrose pellets. In the RST on days 16–17, CSS mice made fewer feeder responses and had a longer tone response latency, than CON mice. In a shallow EVT on days 19–20, CSS mice attained a lower final ratio than CON mice. In a second CSS experiment, mice underwent CSS or CON and testing was conducted with chocolate pellets and in the presence of standard diet (low effort/low reward). In the RST on days 16–18, CSS mice made fewer feeder responses and had a longer tone response latency, than CON mice. In a steep EVT on days 19–20, CSS and CON mice attained less pellets than in the RST, and CSS mice attained a lower final ratio than CON mice. At day 21, blood levels of glucose and the satiety adipokine leptin were similar in CSS and CON mice. Therefore, CSS leads to consistent reductions in reward salience and effort valuation in tests based on association learning. These reward pathology models are being applied to identify the underlying neurobiology and putative molecular targets for therapeutic pharmacology.