Social isolation-induced increase in NMDA receptors in the hippocampus exacerbates emotional dysregulation in mice

Elevated GABAergic neurotransmission prevents chronic intermittent ethanol induced hyperexcitability of intrinsic and extrinsic inputs to the ventral subiculum of female rats

With the recent rise in the rate of alcohol use disorder (AUD) in women, the historical gap between men and women living with this condition is narrowing. While there are many commonalities in how men and women are impacted by AUD, an accumulating body of evidence is revealing sex-dependent adaptations that may require distinct therapeutic approaches. Preclinical rodent studies are beginning to shed light on sex differences in the effects of chronic alcohol exposure on synaptic activity in a number of brain regions. Prior studies from our laboratory revealed that, while withdrawal from chronic intermittent ethanol (CIE), a commonly used model of AUD, increased excitability in the ventral hippocampus (vHC) of male rats, this same treatment had the opposite effect in females. A follow-up study not only expanded on the synaptic mechanisms of these findings in male rats, but also established a CIE-dependent increase in the excitatory-inhibitory (E-I) balance of a glutamatergic projection from the basolateral amygdala to vHC (BLA-vHC). This pathway modulates anxiety-like behavior and could help explain the comorbid occurrence of anxiety disorders in individuals suffering from AUD. The present study sought to conduct a similar analysis of CIE effects on both synaptic mechanisms in the vHC and adaptations in the BLA-vHC pathway of female rats. Our findings indicate that CIE increases the strength of inhibitory neurotransmission in the vHC and that this sex-specific adaptation blocks, or at least delays, the increases in intrinsic vHC excitability and BLA-vHC synaptic transmission observed in males. Our findings establish the BLA-vHC pathway and the vHC as important circuitry to consider for future studies directed at identifying sex-dependent therapeutic approaches to AUD.

Astrocyte neuronal metabolic coupling in the anterior cingulate cortex of mice with inflammatory pain

Chronic pain is a major global concern, with at least 1 in 5 people suffering from chronic pain worldwide. Mounting evidence indicates that neuroplasticity of the anterior cingulate cortex (ACC) is a critical step in the development of chronic pain. Previously, we found that chronic pain and fear learning are both associated with enhanced neuronal excitability and cause similar neuroplasticity-related gene expression changes in the ACC of male mice. However, neuroplasticity, imposes large metabolic demands. In the brain, neurons have the highest energy needs and interact with astrocytes, which extract glucose from blood, mobilize glycogen, and release lactate in response to neuronal activity. Here, we use chronic and continuous inflammatory pain models in female and male mice to investigate the involvement of astrocyte-neuronal lactate shuttling (ANLS) in the ACC of female and male mice experiencing inflammatory pain. We found that ANLS in the mouse ACC promotes the development of chronic inflammatory pain, and expresses sex specific patterns of activation. Specifically, whereas both male and female mice show similar levels of chronic pain hypersensitivity, only male mice show sustained increases in lactate levels. Accordingly, chronic pain alters the expression levels of proteins involved in lactate metabolism and shuttling in a sexually dimorphic manner. We found that disrupting astrocyte-neuronal lactate shuttling in the ACC prior to inflammatory injury prevents the development of pain hypersensitivity in female and male mice, but only reduces temporary pain in male mice. Furthermore, using a transgenic mouse model (itga1-null mice) that displays a naturally occurring form of spontaneous osteoarthritis (OA), a painful inflammatory pain condition, we found that whereas both female and male mice develop OA, only male mice show increases in mechanisms involved in astrocyte-neuronal lactate shuttling. Our findings thus indicate that there are sex differences in astrocyte-neuronal metabolic coupling in the mouse ACC during chronic pain development.

The dorsal raphe-to-ventral hippocampal projection modulates reactive aggression through 5-HT1B receptors

Maladaptive reactive aggression is a core symptom of neuropsychiatric disorders such as schizophrenia. While uncontrolled aggression dampens societal safety, there is a limited understanding of the neural regulation involved in reactive aggression and its treatment. High levels of aggression have been linked to low serotonin (5-HT) levels. Additionally, post-weaning socially isolated (SI) mice exhibit outbursts of aggression following encountering acute stress, and hyperactivated ventral hippocampus (vHip) involves this stress-provoked escalated aggression. Here, we investigated the potential role of the raphe nucleus projecting to the vHip in modulating aggressive behavior. Chemogenetically activating the dorsal raphe nucleus (DRN) soma projecting the vHip or DRN nerve terminals in the vHip reduced reactive aggression. The reduction of attack behavior was abolished by the pretreatment of 5-HT1B receptor antagonist SB-224289. However, activating the median raphe nucleus (MRN)-to-vHip pathway ameliorated depression-like behavior but did not affect reactive aggression. DRN→vHip activation suppressed the vHip downstream area, the ventromedial hypothalamus (VMH), which is a core aggression area. Intra-vHip infusion of 5-HT1B receptor agonists (anpirtoline, CP-93129) suppressed reactive aggression and decreased c-Fos levels in the vHip neurons projecting to the VMH, suggesting an inhibition mechanism. Our findings indicate that activating the DRN projecting to the vHip is sufficient to inhibit reactive aggression in a 5-HT1B receptor-dependent manner. Thus, targeting 5-HT1B receptor could serve as a promising therapeutic approach to ameliorate symptoms of reactive aggression.

Elevated GABAergic neurotransmission prevents chronic intermittent ethanol induced hyperexcitability of intrinsic and extrinsic inputs to the ventral subiculum of female rats

With the recent rise in the rate of alcohol use disorder (AUD) in women, the historical gap between men and women living with this condition is narrowing. While there are many commonalities in how men and women are impacted by AUD, an accumulating body of evidence is revealing sex-dependent adaptations that may require distinct therapeutic approaches. Preclinical rodent studies are beginning to shed light on sex differences in the effects of chronic alcohol exposure on synaptic activity in a number of brain regions. Prior studies from our laboratory revealed that, while withdrawal from chronic intermittent ethanol (CIE), a commonly used model of AUD, increased excitability in the ventral hippocampus (vHC) of male rats, this same treatment had the opposite effect in females. A follow-up study not only expanded on the synaptic mechanisms of these findings in male rats, but also established a CIE-dependent increase in the excitatory-inhibitory (E-I) balance of a glutamatergic projection from the basolateral amygdala to vHC (BLA-vHC). This pathway modulates anxiety-like behavior and could help explain the comorbid occurrence of anxiety disorders in individuals suffering from AUD. The present study sought to conduct a similar analysis of CIE effects on both synaptic mechanisms in the vHC and adaptations in the BLA-vHC pathway of female rats. Our findings indicate that CIE increases the strength of inhibitory neurotransmission in the vHC and that this sex-specific adaptation blocks, or at least delays, the increases in intrinsic vHC excitability and BLA-vHC synaptic transmission observed in males. Our findings establish the BLA-vHC pathway and the vHC as important circuitry to consider for future studies directed at identifying sex-dependent therapeutic approaches to AUD.

Chronic ingestion of soy peptide supplementation reduces aggressive behavior and abnormal fear memory caused by juvenile social isolation

Juvenile loneliness is a risk factor for psychopathology in later life. Deprivation of early social experience due to peer rejection has a detrimental impact on emotional and cognitive brain function in adulthood. Accumulating evidence indicates that soy peptides have many positive effects on higher brain function in rodents and humans. However, the effects of soy peptide use on juvenile social isolation are unknown. Here, we demonstrated that soy peptides reduced the deterioration of behavioral and cellular functions resulting from juvenile socially-isolated rearing. We found that prolonged social isolation post-weaning in male C57BL/6J mice resulted in higher aggression and impulsivity and fear memory deficits at 7 weeks of age, and that these behavioral abnormalities, except impulsivity, were mitigated by ingestion of soy peptides. Furthermore, we found that daily intake of soy peptides caused upregulation of postsynaptic density 95 in the medial prefrontal cortex and phosphorylation of the cyclic adenosine monophosphate response element binding protein in the hippocampus of socially isolated mice, increased phosphorylation of the adenosine monophosphate-activated protein kinase in the hippocampus, and altered the microbiota composition. These results suggest that soy peptides have protective effects against juvenile social isolation-induced behavioral deficits via synaptic maturation and cellular functionalization.

Systemic histone deacetylase inhibition ameliorates the aberrant responses to acute stress in socially isolated male mice

Adverse experiences in early life can induce maladaptive responses to acute stress in later life. Chronic social isolation during adolescence is an early life adversity that can precipitate stress-related psychiatric disorders. We found that male mice after 8 weeks of adolescent social isolation (SI) have markedly increased aggression after being exposed to 2 h of restraint stress (RS), which was accompanied by a significant increase of AMPA receptor- and NMDA receptor-mediated synaptic transmission in prefrontal cortex (PFC) pyramidal neurons of SIRS males. Compared to group-housed counterparts, SIRS males exhibited a significantly decreased level of histone H3 acetylation in PFC. Systemic administration of class I histone deacetylase inhibitors, romidepsin or MS-275, ameliorated the aggressive behaviour, as well as general social interaction deficits, of SIRS males. Electrophysiological recordings also found normalization of PFC glutamatergic currents by romidepsin treatment of SIRS male mice. These results revealed an epigenetic mechanism and intervention avenue for aggression induced by chronic social isolation. KEY POINTS: Adolescent chronic social isolation can precipitate stress-related psychiatric disorders. A significant increase of glutamatergic transmission is found in the prefrontal cortex (PFC) of socially isolated male mice exposed to an acute stress (SIRS). Treatment with class I histone deacetylase (HDAC) inhibitors ameliorates the aggressive behaviour and social interaction deficits of SIRS males, and normalizes glutamatergic currents in PFC neurons. It provides an epigenetic mechanism and intervention avenue for aberrant stress responses induced by chronic social isolation.

Disruption of Cdk5-GluN2B complex by a small interfering peptide attenuates social isolation-induced escalated intermale attack behavior and hippocampal oxidative stress in mice

Social isolation has emerged as a significant issue during the COVID-19 pandemic that can adversely impact human mental health and potentially lead to pathological aggression. Given the lack of effective therapeutic interventions for aggressive behavior, alternative approaches are necessary. In this study, we utilized a genetic method combined with a pharmacological approach to identify and demonstrate the crucial role of Cdk5 in escalated intermale attack behavior induced by 2-week social isolation. Moreover, we developed a small peptide that effectively disrupts the interaction between Cdk5 and GluN2B, given the known involvement of this complex in various neuropsychiatric disorders. Administration of the peptide, either systemically or via intrahippocampal injection, significantly reduced oxidative stress in the hippocampus and attenuated intermale attack behavior induced by 2-week social isolation. These findings highlight the previously unknown role of the hippocampal Cdk5-GluN2B complex in social isolation-induced aggressive behavior in mice and propose the peptide as a promising therapeutic strategy for regulating attack behavior and oxidative stress.

Blue light at night produces stress-evoked heightened aggression by enhancing brain-derived neurotrophic factor in the basolateral amygdala

Light is an underappreciated mood manipulator. People are often exposed to electronic equipment, which results in nocturnal blue light exposure in modern society. Light pollution drastically shortens the night phase of the circadian rhythm. Preclinical and clinical studies have reported that nocturnal light exposure can influence mood, such as depressive-like phenotypes. However, the effects of blue light at night (BLAN) on other moods and how it alters mood remain unclear. Here, we explored the impact of BLAN on stress-provoked aggression in male Sprague‒Dawley rats, focusing on its influence on basolateral amygdala (BLA) activity. Resident-intruder tests, extracellular electrophysiological recordings, and enzyme-linked immunosorbent assays were performed. The results indicated that BLAN produces stress-induced heightened aggressive and anxiety-like phenotypes. Moreover, BLAN not only potentiates long-term potentiation and long-term depression in the BLA but also results in stress-induced elevation of brain-derived neurotrophic factor (BDNF), mature BDNF, and phosphorylation of tyrosine receptor kinase B expression in the BLA. Intra-BLA microinfusion of BDNF RNAi, BDNF neutralizing antibody, K252a, and rapamycin blocked stress-induced heightened aggressive behavior in BLAN rats. In addition, intra-BLA application of BDNF and 7,8-DHF caused stress-induced heightened aggressive behavior in naïve rats. Collectively, these results suggest that BLAN results in stress-evoked heightened aggressive phenotypes, which may work by enhancing BLA BDNF signaling and synaptic plasticity. This study reveals that nocturnal blue light exposure may have an impact on stress-provoked aggression. Moreover, this study provides novel insights into the BLA BDNF-dependent mechanism underlying the impact of the BLAN on mood.

Social isolation leads to mild social recognition impairment and losses in brain cellularity

Chronic social stress is a significant risk factor for several neuropsychiatric disorders, mainly major depressive disorder (MDD). In this way, patients with clinical depression may display many symptoms, including disrupted social behavior and anxiety. However, like many other psychiatric diseases, MDD has a very complex etiology and pathophysiology. Because social isolation is one of the multiple depression-inducing factors in humans, this study aims to understand better the link between social stress and MDD using an animal model based on social isolation after weaning, which is known to produce social stress in mice. We focused on cellular composition and white matter integrity to establish possible links with the abnormal social behavior that rodents isolated after weaning displayed in the three-chamber social approach and recognition tests. We used the isotropic fractionator method to assess brain cellularity, which allows us to robustly estimate the number of oligodendrocytes and neurons in dissected brain regions. In addition, diffusion tensor imaging (DTI) was employed to analyze white matter microstructure. Results have shown that post-weaning social isolation impairs social recognition and reduces the number of neurons and oligodendrocytes in important brain regions involved in social behavior, such as the anterior neocortex and the olfactory bulb. Despite the limitations of animal models of psychological traits, evidence suggests that behavioral impairments observed in patients might have similar biological underpinnings.

Visualizing traumatic stress-induced structural plasticity in a medial amygdala pathway using mGRASP

Traumatic stress has been shown to contribute to persistent behavioral changes, yet the underlying neural pathways are not fully explored. Structural plasticity, a form of long-lasting neural adaptability, offers a plausible mechanism. To scrutinize this, we used the mGRASP imaging technique to visualize synaptic modifications in a pathway formed between neurons of the posterior ventral segment of the medial amygdala and ventrolateral segment of the ventromedial hypothalamus (MeApv-VmHvl), areas we previously showed to be involved in stress-induced excessive aggression. We subjected mice (7-8 weeks of age) to acute stress through foot shocks, a reliable and reproducible form of traumatic stress, and compared synaptic changes to control animals. Our data revealed an increase in synapse formation within the MeApv-VmHvl pathway post-stress as evidenced by an increase in mGRASP puncta and area. Chemogenetic inhibition of CaMKIIα-expressing neurons in the MeApv during the stressor led to reduced synapse formation, suggesting that the structural changes were driven by excitatory activity. To elucidate the molecular mechanisms, we administered the NMDAR antagonist MK-801, which effectively blocked the stress-induced synaptic changes. These findings suggest a strong link between traumatic stress and enduring structural changes in an MeApv-VmHvl neural pathway. Furthermore, our data point to NMDAR-dependent mechanisms as key contributors to these synaptic changes. This structural plasticity could offer insights into persistent behavioral consequences of traumatic stress, such as symptoms of PTSD and social deficits.

Early life stress and altered social behaviors: A perspective across species

Childhood and adolescent affiliations guide how individuals engage in social relationships throughout their lifetime and adverse experiences can promote biological alterations that facilitate behavioral maladaptation. Indeed, childhood victims of abuse are more likely to be diagnosed with conduct or mood disorders which are both characterized by altered social engagement. A key domain particularly deserving of attention is aggressive behavior, a hallmark of many disorders characterized by deficits in reward processing. Animal models have been integral in identifying both the short- and long-term consequences of stress exposure and suggest that whether it is disruption to parental care or social isolation, chronic exposure to early life stress increases corticosterone, changes the expression of neurotransmitters and neuromodulators, and facilitates structural alterations to the hypothalamus, hippocampus, and amygdala, influencing how these brain regions communicate with other reward-related substrates. Herein, we describe how adverse early life experiences influence social behavioral outcomes across a wide range of species and highlight the long-term biological mechanisms that are most relevant to maladaptive aggressive behavior.

Betaine prevents cognitive dysfunction by suppressing hippocampal microglial activation in chronic social isolated male mice

Chronic social isolation (SI) stress, which became more prevalent during the COVID-19 pandemic, contributes to abnormal behavior, including mood changes and cognitive impairment. Known as a functional nutrient, betaine has potent antioxidant and anti-inflammatory properties in vivo. However, whether betaine can alleviate the abnormal behavior induced by chronic SI in mice remains unknown. In this study, we investigated the efficacy of betaine in the treatment of behavioral changes and its underlying mechanism. Three-week-old male mice were randomly housed for 8 weeks in either group housing (GH) or SI. The animals were divided into normal saline-treated GH, normal saline-treated SI, and betaine-treated SI groups in the sixth week. The cognitive and depression-like behavior was determined in the eighth week. We found that long-term betaine administration improved cognitive behavior in SI mice but failed to prevent depression-like behavior. Moreover, long-term betaine administration inhibited hippocampal microglia over-activation and polarized microglia toward the M2 phenotype, which effectively inhibited the expression of inflammatory factors in SI mice. Finally, the protective effect of betaine treatment in SI mice might not be due to altered activity of the hypothalamic-pituitary-adrenal axis. Collectively, our findings reveal that betaine can improve SI-induced cognitive impairment, thus providing an alternative natural source for the prevention of memory loss caused by SI or loneliness.

The role of social isolation stress in escalated aggression in rodent models

Anti-social behavior and violence are major public health concerns. Globally, violence contributes to more than 1.6 million deaths each year. Previous studies have reported that social rejection or neglect exacerbates aggression. In rodent models, social isolation stress is used to demonstrate the adverse effects of social deprivation on physiological, endocrinological, immunological, and behavioral parameters, including aggressive behavior. This review summarizes recent rodent studies on the effect of social isolation stress during different developmental periods on aggressive behavior and the underlying neural mechanisms. Social isolation during adulthood affects the levels of neurosteroids and neuropeptides and increases aggressive behavior. These changes are ethologically relevant for the adaptation to changes in local environmental conditions in the natural habitats. Chronic deprivation of social interaction after weaning, especially during the juvenile to adolescent periods, leads to the disruption of the development of appropriate social behavior and the maladaptive escalation of aggressive behavior. The understanding of neurobiological mechanisms underlying social isolation-induced escalated aggression will aid in the development of therapeutic interventions for escalated aggression.

Early Life Adversity and Neuropsychiatric Disease: Differential Outcomes and Translational Relevance of Rodent Models

It is now well-established that early life adversity (ELA) predisposes individuals to develop several neuropsychiatric conditions, including anxiety disorders, and major depressive disorder. However, ELA is a very broad term, encompassing multiple types of negative childhood experiences, including physical, sexual and emotional abuse, physical and emotional neglect, as well as trauma associated with chronic illness, family separation, natural disasters, accidents, and witnessing a violent crime. Emerging literature suggests that in humans, different types of adverse experiences are more or less likely to produce susceptibilities to certain conditions that involve affective dysfunction. To investigate the driving mechanisms underlying the connection between experience and subsequent disease, neuroscientists have developed several rodent models of ELA, including pain exposure, maternal deprivation, and limited resources. These studies have also shown that different types of ELA paradigms produce different but somewhat overlapping behavioral phenotypes. In this review, we first investigate the types of ELA that may be driving different neuropsychiatric outcomes and brain changes in humans. We next evaluate whether rodent models of ELA can provide translationally relevant information regarding links between specific types of experience and changes in neural circuits underlying dysfunction.

Promises and Pitfalls of NMDA Receptor Antagonists in Treating Violent Aggression

Treatment options for chronically aggressive individuals remain limited despite recent medical advances. Traditional pharmacological agents used to treat aggression, such as atypical antipsychotics, have limited efficacy and are often replete with dangerous side effects. The non-competitive NMDAR antagonists ketamine and memantine are promising alternatives, but their effects appear to be highly dependent on dosage, context, and personal experience. Importantly, these drugs can increase aggression when combined with substances of abuse or during periods of heightened stress. This is likely due to mechanistic differences operating at specific synapses under different contexts. Previous findings from our lab and others have shown that early life stress, substance abuse, and attack experience promote aggression through NMDAR-dependent synaptic plasticity within aggression-related brain circuits. Ketamine and memantine affect these types of aggression in opposite ways. This has led us to propose that ketamine and memantine oppositely affect aggression brought on by early life stress, substance abuse, or attack experience through opposite effects on NMDAR-dependent synaptic plasticity. This would account for the persistent effects of these drugs on aggression and suggest they could be leveraged as a more long-lasting treatment option. However, a more thorough examination of the effects of ketamine and memantine on cellular and synaptic function will be necessary for responsible administration. Additionally, because the effects of ketamine and memantine are highly dependent on prior drug use, traumatic stress, or a history of aggressive behavior, we propose a more thorough medical evaluation and psychiatric assessment will be necessary to avoid possible adverse interactions with these drugs.

Opposing effects of NMDA receptor antagonists on early life stress-induced aggression in mice

Rates of childhood trauma are high amongst violent offenders who frequently recidivate. Few clinical options are available to treat excessive and recurring violent aggression associated with childhood trauma. Those that do exist are largely ineffective and often replete with side effects. One promising pharmacological target is the glutamate binding N-methyl- d-aspartate receptor (NMDAR). Clinically available NMDAR antagonists have proven successful in mitigating violent and aggressive behavior associated with a host of psychiatric diseases and have both immediate and long-term effects on nervous system function and behavior. This study examined the impact of three NMDAR antagonists on long-lasting aggression brought on by early-life stress: MK-801, memantine, and ketamine. We find that social isolation early in adolescence followed by acute traumatic stress in the form of noncontingent foot shock (FS) late in adolescence works in tandem to promote long-lasting excessive aggression in mice when measured 1 week later. Systemic injections of MK-801 and memantine 30 min before FS suppressed the long-lasting attack behavior induced by our early life stress induction protocol. Systemic injections of ketamine, on the other hand, significantly enhanced the long-lasting attack behavior when injected before FS. These findings indicate that MK-801, memantine, and ketamine have distinct and opposing effects on early life stress-induced aggression, suggesting these drugs may be mechanistically distinct. This study identifies memantine as a promising pharmacological treatment for aggressive behavior associated with early life stress and demonstrates the need for greater care when using glutamate receptor antagonists to treat aggression.

Fluoxetine ameliorates Alzheimer's disease progression and prevents the exacerbation of cardiovascular dysfunction of socially isolated depressed rats through activation of Nrf2/HO-1 and hindering TLR4/NLRP3 inflammasome signaling pathway

Depression is a risk factor for Alzheimer's (AD) and cardiovascular diseases (CVD). Therefore, depression treatment restricts its deteriorating effects on mood, memory and CV system. Fluoxetine is the most widely used antidepressant drug, it has neuroprotective effect through its antioxidant/anti-inflammatory properties. The current study investigated for the first-time the cross link between depression, AD and CVD besides, role of fluoxetine in mitigating such disorders. Depression was induced in rats by social isolation (SI) for 12 weeks, AlCL3 (70 mg/kg/day, i.p.) was used to induce AD which was administered either in SI or normal control (NC) grouped rats starting at 8th week till the end of the experiment, fluoxetine (10 mg/kg/day, p.o) treatment also was started at 8th week. SI and AD showed a statistically significant deteriorated effect on behavioral, neurochemical and histopathological analysis which was exaggerated when two disorder combined than each alone. Fluoxetine treatment showed protective effect against SI, AD and prevents exacerbation of CVD. Fluoxetine improved animals' behavior, increased brain monoamines, BDNF besides increased antioxidant defense mechanism of SOD, TAC contents and increased protein expression of Nrf2/HO-1 with significant decrease of AChE activity, β-amyloid, Tau protein, MDA, TNF-α, IL1β contents as well as decreased protein expression of NF-kB, TLR4, NLRP3 and caspase1. It also showed cardioprotective effects as it improved lipid profile with pronounced decrease of cardiac enzymes of CK-MB, troponin and MEF2. In conclusion, fluoxetine represents as a promising drug against central and peripheral disorders through its anti-inflammatory/antioxidant effects via targeting antioxidant Nrf2/HO-1 and hindering TLR4/NLRP3 inflammasome signaling pathways.

The Microbiota-Gut Axis in Premature Infants: Physio-Pathological Implications

Intriguing evidence is emerging in regard to the influence of gut microbiota composition and function on host health from the very early stages of life. The development of the saprophytic microflora is conditioned by several factors in infants, and peculiarities have been found for babies born prematurely. This population is particularly exposed to a high risk of infection, postnatal antibiotic treatment, feeding difficulties and neurodevelopmental disabilities. To date, there is still a wide gap in understanding all the determinants and the mechanism behind microbiota disruption and its influence in the development of the most common complications of premature infants. A large body of evidence has emerged during the last decades showing the existence of a bidirectional communication axis involving the gut microbiota, the gut and the brain, defined as the microbiota–gut–brain axis. In this context, given that very few data are available to demonstrate the correlation between microbiota dysbiosis and neurodevelopmental disorders in preterm infants, increasing interest has arisen to better understand the impact of the microbiota–gut–brain axis on the clinical outcomes of premature infants and to clarify how this may lead to alternative preventive, diagnostic and therapeutic strategies. In this review, we explored the current evidence regarding microbiota development in premature infants, focusing on the effects of delivery mode, type of feeding, environmental factors and possible influence of the microbiota–gut–brain axis on preterm clinical outcomes during their hospital stay and on their health status later in life.

Neurotransmitter and Intestinal Interactions: Focus on the Microbiota-Gut-Brain Axis in Irritable Bowel Syndrome

Irritable bowel syndrome (IBS) is a functional gastrointestinal disorder of unknown etiology. IBS is caused by a disruption in the gut-brain axis. Given the importance of the gut microbiota in maintaining local and systemic homeostasis of immunity, endocrine, and other physiological processes, the microbiota-gut-brain axis has been proposed as a key regulator in IBS. Neurotransmitters have been shown to affect blood flow regulation, intestinal motility, nutrient absorption, the gastrointestinal immune system, and the microbiota in recent studies. It has the potential role to play a function in the pathophysiology of the gastrointestinal and neurological systems. Transmitters and their receptors, including 5-hydroxytryptamine, dopamine, γ-aminobutyric acid, and histamine, play an important role in IBS, especially in visceral sensitivity and gastrointestinal motility. Studies in this field have shed light on revealing the mechanism by which neurotransmitters act in the pathogenesis of IBS and discovering new therapeutic strategies based on traditional pharmacological approaches that target the nervous system or novel therapies that target the microbiota.

Anger management: Mechanisms of glutamate receptor-mediated synaptic plasticity underlying animal aggression

Excessive and recurring violent aggression is a serious concern for society and a symptom of many psychiatric diseases. Substance abuse, attack experience, and social and traumatic stress increase vulnerability to developing this type of aggression. Glutamate receptors are an intriguing target for long-term treatment. This review will assess the importance of glutamate receptors and glutamatergic pathways in aggression, focusing on the role of glutamate receptor-mediated synaptic plasticity in experience-dependent long-lasting aggression. By synthesizing what is known about glutamatergic systems in aggression, it is hoped more effective treatments can be developed.

Social Stress and Aggression in Murine Models

Throughout life, animals engage in a variety of social interactions ranging from the affiliative mother-offspring interaction and juvenile play to aggressive conflict. Deprivation of the appropriate social interaction during early development is stressful and disrupts the development of appropriate social behaviors and emotional responses later in life. Additionally, agonistic encounters can induce stress responses in both dominant and subordinate individuals. This review focuses on the social stress that escalates aggressive behavior of animals and discusses the known neurobiological and physiological mechanisms underlying the link between social stress and aggression. Social instigation, a brief exposure to a rival without physical contact, induces aggressive arousal in dominant animals and escalates aggressive behaviors in the following agonistic encounter. Furthermore, the experience of winning an aggressive encounter is known to be as rewarding as addictive drugs, and the experience of repeatedly winning induces addiction-like behavioral and neurobiological changes and leads to abnormal aggressive behaviors. Social isolation stress in early development from neonatal to juvenile and adolescent periods also affects aggressive behavior, but these effects largely depend on the strain, sex, and species as well as the stage of development in which isolation stress is experienced. In conclusion, understanding neurobiological mechanisms underlying the link between social stress and aggression will provide an important insight for the development of more effective and tolerable treatments for maladaptive aggression in humans.

Chronic social defeat stress impairs goal-directed behavior through dysregulation of ventral hippocampal activity in male mice

Chronic stress is a risk factor for a variety of psychiatric disorders, including depression. Although impairments to motivated behavior are a major symptom of clinical depression, little is known about the circuit mechanisms through which stress impairs motivation. Furthermore, research in animal models for depression has focused on impairments to hedonic aspects of motivation, whereas patient studies suggest that impairments to appetitive, goal-directed motivation contribute significantly to motivational impairments in depression. Here, we characterized goal-directed motivation in repeated social defeat stress (R-SDS), a well-established mouse model for depression in male mice. R-SDS impaired the ability to sustain and complete goal-directed behavior in a food-seeking operant lever-press task. Furthermore, stress-exposed mice segregated into susceptible and resilient subpopulations. Interestingly, susceptibility to stress-induced motivational impairments was unrelated to stress-induced social withdrawal, another prominent effect of R-SDS in mouse models. Based on evidence that ventral hippocampus (vHP) modulates sustainment of goal-directed behavior, we monitored vHP activity during the task using fiber photometry. Successful task completion was associated with suppression of ventral hippocampal neural activity. This suppression was diminished after R-SDS in stress-susceptible but not stress-resilient mice. The serotonin selective reuptake inhibitor (SSRI) escitalopram and ketamine both normalized vHP activity during the task and restored motivated behavior. Furthermore, optogenetic vHP inhibition was sufficient to restore motivated behavior after stress. These results identify vHP hyperactivity as a circuit mechanism of stress-induced impairments to goal-directed behavior and a putative biomarker that is sensitive to antidepressant treatments and that differentiates susceptible and resilient individuals.

Targeting GluN2B/NO Pathway Ameliorates Social Isolation-Induced Exacerbated Attack Behavior in Mice

Exacerbated attack behavior has a profound socioeconomic impact and devastating social consequences; however, there is no satisfactory clinical management available for an escalated attack behavior. Social isolation (SI) is widespread during this pandemic and may exert detrimental effects on mental health, such as causing heightened attack behavior. To explore the therapeutic approaches that alleviate the SI-induced heightened attack behavior, we utilized pharmacological methods targeting the GluN2B/NO signaling pathway during the attack behavior. Ifenprodil and TAT-9C peptide targeting GluN2B showed that the inhibition of GluN2B mitigated the SI-induced escalated attack behavior and the SI-induced aberrant nitric oxide (NO) level in the brain. Additionally, the potentiation of the NO level by L-arginine reversed the effects of the inhibition of GluN2B. Moreover, we showed that high doses of L-NAME and 7-NI and subeffective doses of L-NAME in combination with ifenprodil or TAT-9C or subeffective doses of 7-NI plus ifenprodil or TAT-9C all decreased the SI-induced escalated attack behavior and reduced the NO level, further supporting the idea that GluN2B/NO signaling is a crucial modulator of the escalated attack behavior.

Regulation of stress-provoked aggressive behavior using endocannabinoids

Reactive impulsive aggression is characterized by outbursts of rage and violence when subjects encounter threatening stressful events. Although impulsive aggression and violence create a high-cost burden on health and society, relatively little is known about treatment. Early adolescent social isolation (SI) alters brain development and functions. It induces hyper-excitatory in the ventral hippocampus (vHip) to promote acute stress-provoked outbursts of aggression, referred to as impulsive aggression, in mouse models. Cannabinoid type 1 receptors (CB1Rs) act on presynaptic sites and suppress neurotransmitter release into synapses. Given that CB1R activation inhibits neurotransmitter releases and modulates excitatory network activity, we tested the hypothesis that CB1R activation reduces impulsive aggression in SI mice through decreasing excitatory activity in the vHip. Here, we report that CB1R agonists, WIN-552122 (WIN) or arachidonylcyclopropylamide (ACPA), ameliorated acute stress-provoked attack behavior in the resident-intruder test without affecting general locomotion activity. Increasing endocannabinoids (eCBs) by inhibiting degradation enzymes in the vHip reduced impulsive aggression, and the effect was blunted by administration of AM251, a CB1R antagonist. Acute stress in SI mice induced c-Fos expression, a marker of neuronal activation, on vHip neurons projecting to the ventromedial hypothalamus (VMH), a well-known brain area that controls attack behavior. eCB augmentation inhibited c-Fos expression in VMH-projecting vHip neurons surrounded by CB1Rs. These results suggest that enhancing eCB signaling in order to activate CB1Rs suppresses impulsive aggression via suppressing vHip→VMH neural activity and point to a role of CB1R activation in ameliorating impulsive aggression in adults who have had adverse experiences during early adolescence.

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Early adolescent social isolation (SI) promotes impulsive aggression.

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Treatment of CB1R agonists reduces impulsive aggression in SI mice.

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Increasing endocannabinoids (eCBs) to activate CB1Rs reduces impulsive aggression.

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Augmenting eCBs suppresses stress-provoked neuronal activation in the hippocampus.

Chronic Ethanol Exposures Leads to a Negative Affective State in Female Rats That Is Accompanied by a Paradoxical Decrease in Ventral Hippocampus Excitability

Alcohol use disorder (AUD) differentially impacts men and women and a growing body of evidence points to sex-dependent adaptations in a number of brain regions. In a prior study, we explored the effect of a chronic intermittent ethanol exposure (CIE) model of AUD on neuronal and molecular adaptations in the dorsal and ventral domains of the hippocampus (dHC and vHC, respectively) in male rats. We found the vHC to be particularly sensitive to CIE, showing an increase in neuronal excitability and synaptic proteins associated with augmented excitation. These findings were accompanied by a CIE-dependent increase in anxiety-like behaviors. To explore sex-dependent adaptations in the hippocampus, we conducted a similar study in female rats. CIE-treated female rats showed a relatively modest increase in anxiety-like behaviors along with a robust increase in depressive-like measures. Despite both sexes showing clear evidence of a negative affective state following CIE, the vHC of females showed a decrease, rather than an increase, in neuronal excitability. In line with the reduced sensitivity to neural adaptations in the dHC of male rats, we were unable to identify any functional changes in the dHC of females. The functional changes of the vHC in female rats could not be explained by altered expression levels of a number of proteins typically associated with changes in neuronal excitability. Taken together, these findings point to sex as a major factor in CIE-dependent hippocampal adaptations that should be explored further to better understand possible gender differences in the etiology and treatment of AUD.

Targeting the dysfunction of glutamate receptors for the development of novel antidepressants

Increasing evidence indicates that dysfunction of glutamate receptors is involved in the pathophysiology of major depressive disorder (MDD). Although accumulating efforts have been made to elucidate the applications and mechanisms underlying antidepressant-like effects of ketamine, a non-selective antagonist of N-methyl-d-aspartate receptor (NMDAR), the role of specific glutamate receptor subunit in regulating depression is not completely clear. The current review aims to discuss the relationships between glutamate receptor subunits and depressive-like behaviors. Research literatures were searched from inception to July 2020. We summarized the alterations of glutamate receptor subunits in patients with MDD and animal models of depression. Animal behaviors in response to dysfunction of glutamate receptor subunits were also surveyed. To fully understand mechanisms underlying antidepressant-like effects of modulators targeting glutamate receptors, we discussed effects of each glutamate receptor subunit on serotonin system, synaptic plasticity, neurogenesis and neuroinflammation. Finally, we collected most recent clinical applications of glutamate receptor modulators and pointed out the limitations of these candidates in the treatment of MDD.

Pain-induced aggression and changes in social behavior in mice

The effects of neuropathic, formalin, and acetic acid-induced visceral pain were investigated on the social and aggressive behaviors in the Swiss male mice. Neuropathic pain was induced by tibial nerve transection (TNT). Also, somatic and visceral pain was conducted by intraplantar injection of diluted formalin (1%, 20 μl) and intraperitoneal administration of acetic acid (0.6%, 200 μl), respectively. Fourteen and twenty one days after the TNT surgery, and also, 1 and 7 days following formalin and acetic acid administration, the three-chambered test was used to determine sociability and preference for social novelty and resident/intruder test was used for the evaluation of the aggressive behaviors. In the sociability phase of the three-chambered test, all the three models of pain did not change the animal's sociability. However, in the social novelty preference phase, the animals in pain showed deficits in social novelty preference by a significant increase in the time spent with the familiar mice compared to the control groups. Also, animals in pain significantly showed more aggressive behaviors like biting and clinching and have much less attack latency in comparison to the control groups. Pain-induced changes in the social novelty preference and aggressive behaviors continued in the neuropathic group until the end of the experiment. However, 7 days following the induction of both formalin and visceral pain, animals' social memory, and aggression almost returned to the standard value. These results suggest that long-lasting pain could lead to social memory impairment and increase aggressive behaviors in mice.

A Handful of Details to Ensure the Experimental Reproducibility on the FORCED Running Wheel in Rodents: A Systematic Review

A well-documented method and experimental design are essential to ensure the reproducibility and reliability in animal research. Experimental studies using exercise programs in animal models have experienced an exponential increase in the last decades. Complete reporting of forced wheel and treadmill exercise protocols would help to ensure the reproducibility of training programs. However, forced exercise programs are characterized by a poorly detailed methodology. Also, current guidelines do not cover the minimum data that must be included in published works to reproduce training programs. For this reason, we have carried out a systematic review to determine the reproducibility of training programs and experimental designs of published research in rodents using a forced wheel system. Having determined that most of the studies were not detailed enough to be reproducible, we have suggested guidelines for animal research using FORCED exercise wheels, which could also be applicable to any form of forced exercise.

NMDA receptors promote hippocampal sharp-wave ripples and the associated coactivity of CA1 pyramidal cells

Hippocampal sharp-wave ripples (SWRs) support the reactivation of memory representations, relaying information to neocortex during "offline" and sleep-dependent memory consolidation. While blockade of NMDA receptors (NMDAR) is known to affect both learning and subsequent consolidation, the specific contributions of NMDAR activation to SWR-associated activity remain unclear. Here, we combine biophysical modeling with in vivo local field potential (LFP) and unit recording to quantify changes in SWR dynamics following inactivation of NMDAR. In a biophysical model of CA3-CA1 SWR activity, we find that NMDAR removal leads to reduced SWR density, but spares SWR properties such as duration, cell recruitment and ripple frequency. These predictions are confirmed by experiments in which NMDAR-mediated transmission in rats was inhibited using three different NMDAR antagonists, while recording dorsal CA1 LFP. In the model, loss of NMDAR-mediated conductances also induced a reduction in the proportion of cell pairs that co-activate significantly above chance across multiple events. Again, this prediction is corroborated by dorsal CA1 single-unit recordings, where the NMDAR blocker ketamine disrupted correlated spiking during SWR. Our results are consistent with a framework in which NMDA receptors both promote activation of SWR events and organize SWR-associated spiking content. This suggests that, while SWR are short-lived events emerging in fast excitatory-inhibitory networks, slower network components including NMDAR-mediated currents contribute to ripple density and promote consistency in the spiking content across ripples, underpinning mechanisms for fine-tuning of memory consolidation processes.

Effect of Aggressive Experience in Female Syrian Hamsters on Glutamate Receptor Expression in the Nucleus Accumbens

Our social relationships determine our health and well-being. In rodent models, there is now strong support for the rewarding properties of aggressive or assertive behaviors to be critical for the expression and development of adaptive social relationships, buffering from stress and protecting from the development of psychiatric disorders such as depression. However, due to the false belief that aggression is not a part of the normal repertoire of social behaviors displayed by females, almost nothing is known about the neural mechanisms mediating the rewarding properties of aggression in half the population. In the following study, using Syrian hamsters as a well-validated and translational model of female aggression, we investigated the effects of aggressive experience on the expression of markers of postsynaptic structure (PSD-95, Caskin I) and excitatory synaptic transmission (GluA1, GluA2, GluA4, NR2A, NR2B, mGluR1a, and mGluR5) in the nucleus accumbens (NAc), caudate putamen and prefrontal cortex. Aggressive experience resulted in an increase in PSD-95, GluA1 and the dimer form of mGluR5 specifically in the NAc 24 h following aggressive experience. There was also an increase in the dimer form of mGluR1a 1 week following aggressive experience. Aggressive experience also resulted in an increase in the strength of the association between these postsynaptic proteins and glutamate receptors, supporting a common mechanism of action. In addition, 1 week following aggressive experience there was a positive correlation between the monomer of mGluR5 and multiple AMPAR and NMDAR subunits. In conclusion, we provide evidence that aggressive experience in females results in an increase in the expression of postsynaptic density, AMPARs and group I metabotropic glutamate receptors, and an increase in the strength of the association between postsynaptic proteins and glutamate receptors. This suggests that aggressive experience may result in an increase in excitatory synaptic transmission in the NAc, potentially encoding the rewarding and behavioral effects of aggressive interactions.

Race, Ethnicity, Family Socioeconomic Status, and Children's Hippocampus Volume

Introduction

The hippocampus has a significant role in memory, learning, and cognition. Although hippocampal size is highly susceptible to family socioeconomic status (SES) and associated stress, very little is known on racial and ethnic group differences in the effects of SES indicators on hippocampus volume among American children.

Purpose

This study explored the multiplicative effects of race, ethnicity, and family SES on hippocampus volume among American children.

Methods

Using data from the Adolescent Brain Cognitive Development (ABCD), we analyzed the functional Magnetic Resonance Imaging (fMRI) data of 9390 9-10 years old children. The main outcome was hippocampus volume. The predictor was parental education. Subjective family SES was the independent variable. Age, sex, and marital status were the covariates. Racial and ethnic group membership were the moderators. To analyze the data, we used regression models.

Results

High subjective family SES was associated with larger hippocampus volume. This effect was significantly larger for Whites than Black families.

Conclusions

The effect of subjective family SES on children's hippocampus volume is weaker in Black than White families.

The Ventral Hippocampus Controls Stress-Provoked Impulsive Aggression through the Ventromedial Hypothalamus in Post-Weaning Social Isolation Mice

Impulsively aggressive individuals may suddenly attack others when under stress, but the neural circuitry underlying stress-provoked aggression is poorly understood. Here, we report that acute stress activates ventral hippocampus (vHip) neurons to induce attack behavior in post-weaning socially isolated mice. Chemogenetic inhibition of vHip neural activity blunts stress-provoked attack behavior, whereas chemogenetic activation promotes it. The activation of cell bodies in vHip neurons projecting into the ventromedial hypothalamus (VMH) induces attack behavior, suggesting that the vHip-VMH projection contributes to impulsive aggression. Furthermore, optogenetic inhibition of vHip glutamatergic neurons blocks stress-provoked attacks, whereas optogenetic activation of vHip glutamatergic neurons drives attack behavior. These results show direct evidence that vHip-VMH neural circuitry modulates attack behavior in socially isolated mice.

Family Income Mediates the Effect of Parental Education on Adolescents' Hippocampus Activation During an N-Back Memory Task

Introduction: Hippocampus, a medial temporal lobe structure, has significant implications in memory formation and learning. Although hippocampus activity is believed to be affected by socioeconomic status (SES), limited knowledge exists on which SES indicators influence hippocampus function. Purpose: This study explored the separate and combined effects of three SES indicators, namely parental education, family income, and neighborhood income, on adolescents’ hippocampus activation during an N-Back memory task. As some of the effects of parental education may be through income, we also tested if the effect of parental education on hippocampus activation during our N-Back memory task is mediated by family or neighborhood income. Methods: The Adolescent Brain Cognitive Development (ABCD) study is a national multi-center investigation of American adolescents’ brain development. Functional magnetic resonance imaging (fMRI) data of a total sample of 3067 9–10-year-old adolescents were used. The primary outcome was left- hippocampus activation during the N-Back memory task (mean beta weight for N-Back run 1 2 back versus 0 back contrast in left hippocampus). The independent variable was parental education. Family income and neighborhood income were two possible mediators. Age, sex, and marital status were the covariates. To test mediation, we used hierarchical linear regression models first without and then with our mediators. Full mediation was defined according to Kenny. The Sobel test was used to confirm statistical mediation. Results: In the absence of family and neighborhood income in the model, higher parental educational attainment was associated with lower level of left hippocampus activation during the N-Back memory task. This effect was significant while age, sex, and marital status were controlled. The association between parental educational attainment and hippocampus activation during the N-Back memory task was no more significant when we controlled for family and neighborhood income. Instead, family income was associated with hippocampus activation during the N-Back memory task. These findings suggested that family income fully mediates the effect of parental educational attainment on left hippocampus activation during the N-Back memory task. Conclusions: The effect of parental educational attainment on adolescents’ hippocampus activation during an N-Back memory task is fully explained by family income. That means low family income is why adolescents with low-educated parents show highlighted hippocampus activation during an N-Back memory task. Given the central role of the hippocampus in learning and memory and as income is a modifiable factor by tax and economic policies, income-redistribution policies, fair taxation, and higher minimum wage may have implications for promotion of adolescent equality and social justice. There is a need to focus on family-level economic needs across all levels of neighborhood income.

Traumatic Stress Induces Prolonged Aggression Increase through Synaptic Potentiation in the Medial Amygdala Circuits

Traumatic stress can lead to heightened aggression which may be a symptom of psychiatric diseases such as PTSD and intermittent explosive disorder. The medial amygdala (MeA) is an evolutionarily conserved subnucleus of the amygdala that regulates attack behavior and behavioral responses to stressors. The precise contribution of the MeA in traumatic stress-induced aggression, however, requires further elucidation. In this study, we used foot shock to induce traumatic stress in mice and examine the mechanisms of prolonged aggression increase associated with it. Foot shock causes a prolonged increase in aggression that lasts at least one week. In vivo electrophysiological recordings revealed that foot shock induces potentiation of synapses formed between the MeA and the ventromedial hypothalamus (VmH) and bed nucleus of the stria terminalis (BNST). This synaptic potentiation lasts at least one week. Induction of synaptic depotentiation with low-frequency photostimulation (LFPS) immediately after foot shock suppresses the prolonged aggression increase without affecting non-aggressive social behavior, anxiety-like and depression-like behaviors, or fear learning. These results show that potentiation of the MeA-VmH and MeA-BNST circuits is essential for traumatic stress to cause a prolonged increase in aggression. These circuits may be potential targets for the development of therapeutic strategies to treat the aggression symptom associated with psychiatric diseases.

Potentiation of Divergent Medial Amygdala Pathways Drives Experience-Dependent Aggression Escalation

Heightened aggression can be serious concerns for the individual and society at large and are symptoms of many psychiatric illnesses, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression increase, however, are poorly understood. Here we find that prior attack experience leading to an increase in aggressive behavior, known as aggression priming, activates neurons within the posterior ventral segment of the medial amygdala (MeApv). Optogenetic stimulation of MeApv using a synaptic depression protocol suppresses aggression priming, whereas high-frequency stimulation enhances aggression, mimicking attack experience. Interrogation of the underlying neural circuitry revealed that the MeApv mediates aggression priming via synaptic connections with the ventromedial hypothalamus (VmH) and bed nucleus of the stria terminalis (BNST). These pathways undergo NMDAR-dependent synaptic potentiation after attack. Furthermore, we find that the MeApv-VmH synapses selectively control attack duration, whereas the MeApv-BNST synapses modulate attack frequency, both with no effect on social behavior. Synaptic potentiation of the MeApv-VmH and MeApv-BNST pathways contributes to increased aggression induced by traumatic stress, and weakening synaptic transmission at these synapses blocks the effect of traumatic stress on aggression. These results reveal a circuit and synaptic basis for aggression modulation by experience that can be potentially leveraged toward clinical interventions.SIGNIFICANCE STATEMENT Heightened aggression can have devastating social consequences and may be associated with psychiatric disorders, such as post-traumatic stress disorder. The circuit and synaptic mechanisms underlying experience-induced aggression escalation, however, are poorly understood. Here we identify two aggression pathways between the posterior ventral segment of the medial amygdala and its downstream synaptic partners, the ventromedial hypothalamus and bed nucleus of the stria terminalis that undergo synaptic potentiation after attack and traumatic stress to enhance aggression. Notably, weakening synaptic transmission in these circuits blocks aggression priming, naturally occurring aggression, and traumatic stress-induced aggression increase. These results illustrate a circuit and synaptic basis of aggression modulation by experience, which can be potentially targeted for clinical interventions.

Tryptophan Metabolites Along the Microbiota-Gut-Brain Axis: An Interkingdom Communication System Influencing the Gut in Health and Disease

The ‘microbiota-gut-brain axis’ plays a fundamental role in maintaining host homeostasis, and different immune, hormonal, and neuronal signals participate to this interkingdom communication system between eukaryota and prokaryota. The essential aminoacid tryptophan, as a precursor of several molecules acting at the interface between the host and the microbiota, is fundamental in the modulation of this bidirectional communication axis. In the gut, tryptophan undergoes 3 major metabolic pathways, the 5-HT, kynurenine, and AhR ligand pathways, which may be directly or indirectly controlled by the saprophytic flora. The importance of tryptophan metabolites in the modulation of the gastrointestinal tract is suggested by several preclinical and clinical studies; however, a thorough revision of the available literature has not been accomplished yet. Thus, this review attempts to cover the major aspects on the role of tryptophan metabolites in host-microbiota cross-talk underlaying regulation of gut functions in health conditions and during disease states, with particular attention to 2 major gastrointestinal diseases, such as irritable bowel syndrome (IBS) and inflammatory bowel disease (IBD), both characterized by psychiatric disorders. Research in this area opens the possibility to target tryptophan metabolism to ameliorate the knowledge on the pathogenesis of both diseases, as well as to discover new therapeutic strategies based either on conventional pharmacological approaches or on the use of pre- and probiotics to manipulate the microbial flora.

MicroRNA-206 Regulates Stress-Provoked Aggressive Behaviors in Post-weaning Social Isolation Mice

When facing stressful conditions, some people tend to be impulsively aggressive whereas others are not. However, the causes and underlying mechanisms remain elusive. It has been reported that acute stress induces outbursts of aggression in post-weaning social isolation (SI) mice but not in group housing (GH) mice. Here we report epigenetic regulation of impulsive aggression in SI mice. At post-natal day 21, mice were randomly assigned to GH or SI groups. We found that SI mice exhibited a higher level of microRNA 206 (miR-206) compared with GH mice. Intra-hippocampal injection of AM206, an antagomir of miR-206, decreased stress-induced attack behavior in SI mice and increased BDNF expression. Moreover, BDNF expression was required for AM206 effects on the reduction of aggression. On the other hand, miR-206 overexpression in GH mice induced attack behavior. Intranasal administration of AM206 rather than a scramble control significantly reduced attack behavior and depression-like behavior in SI mice. Our results suggest that miR-206 mediates development of maladaptive impulsive aggression in early life adversity and that its antagomir could potentially be a therapeutic target against stress-exacerbated aggressive behavior.

Social experience and sex-dependent regulation of aggression in the lateral septum by extrasynaptic δGABAA receptors

RATIONALE

Understanding the neurobiological mechanisms mediating dominance and competitive aggression is essential to understanding the development and treatment of various psychiatric disorders. Previous research suggests that these mechanisms are both sexually differentiated and influenced substantially by social experience. In numerous species, GABA_A receptors in the lateral septum have been shown to play a significant role in aggression in males. However, very little is known about the role of these GABA_A receptors in female aggression, the role of social experience on GABA_A receptor-mediated aggression, or the roles of different GABA_A subtypes in regulating aggression.

OBJECTIVES

Thus, in the following set of experiments, we determined the role of social experience in modulating GABA_A receptor-induced aggression in both male and female Syrian hamsters, with a particular focus on the GABA_A receptor subtype mediating these effects.

RESULTS

Activation of GABA_A receptors in the dorsal lateral septum increased aggression in both males and females. Social housing, however, significantly decreased the ability of GABA_A receptor activation to induce aggression in males but not females. No significant differences were observed in the effects of GABA_A receptor activation in dominant versus subordinate group-housed hamsters. Finally, examination of potential GABA_A receptor subtype specificity revealed that social housing decreased the ratio of δ extrasynaptic to γ₂ synaptic subunit GABA_A receptor mRNA expression in the anterior dorsal lateral septum, while activation of δ extrasynaptic, but not γ₂ synaptic, GABA_A receptors in the dorsal lateral septum increased aggression.

CONCLUSIONS

These data suggest that social experience can have profound effects on the neuronal mechanisms mediating aggression, especially in males, and that δ extrasynaptic GABA_A receptors may be an important therapeutic target in disorders characterized by high levels of aggression.

Life-Course Contribution of Prenatal Stress in Regulating the Neural Modulation Network Underlying the Prepulse Inhibition of the Acoustic Startle Reflex in Male Alzheimer's Disease Mice

The prepulse inhibition (PPI) of the acoustic startle reflex (ASR), as an index of sensorimotor gating, is one of the most extensively used paradigms in the field of neuropsychiatric disorders. Few studies have examined how prenatal stress (PS) regulates the sensorimotor gating during the lifespan and how PS modifies the development of amyloid-beta (Aβ) pathology in brain areas underlying the PPI formation. We followed alternations in corticosterone levels, learning and memory, and the PPI of the ASR measures in APPNL-G-F/NL-G-F offspring of dams exposed to gestational noise stress. In-depth quantifications of the Aβ plaque accumulation were also performed at 6 months. The results indicated an age-dependent deterioration of sensorimotor gating, long-lasting PS-induced abnormalities in PPI magnitudes, as well as deficits in spatial memory. The PS also resulted in a higher Aβ aggregation predominantly in brain areas associated with the PPI modulation network. The findings suggest the contribution of a PS-induced hypothalamic-pituitary-adrenal (HPA) axis hyperactivity in regulating the PPI modulation substrates leading to the abnormal development of the neural protection system in response to disruptive stimuli. The long-lasting HPA axis dysregulation appears to be the major underlying mechanism in precipitating the Aβ deposition, especially in brain areas contributed to the PPI modulation network.

The Role of the N-Methyl-D-Aspartate Receptors in Social Behavior in Rodents

The appropriate display of social behaviors is essential for the well-being, reproductive success and survival of an individual. Deficits in social behavior are associated with impaired N-methyl-D-aspartate (NMDA) receptor-mediated neurotransmission. In this review, we describe recent studies using genetically modified mice and pharmacological approaches which link the impaired functioning of the NMDA receptors, especially of the receptor subunits GluN1, GluN2A and GluN2B, to abnormal social behavior. This abnormal social behavior is expressed as impaired social interaction and communication, deficits in social memory, deficits in sexual and maternal behavior, as well as abnormal or heightened aggression. We also describe the positive effects of pharmacological stimulation of the NMDA receptors on these social deficits. Indeed, pharmacological stimulation of the glycine-binding site either by direct stimulation or by elevating the synaptic glycine levels represents a promising strategy for the normalization of genetically-induced, pharmacologically-induced or innate deficits in social behavior. We emphasize on the importance of future studies investigating the role of subunit-selective NMDA receptor ligands on different types of social behavior to provide a better understanding of the underlying mechanisms, which might support the development of selective tools for the optimized treatment of disorders associated with social deficits.

Rigor and reproducibility in rodent behavioral research

Behavioral neuroscience research incorporates the identical high level of meticulous methodologies and exacting attention to detail as all other scientific disciplines. To achieve maximal rigor and reproducibility of findings, well-trained investigators employ a variety of established best practices. Here we explicate some of the requirements for rigorous experimental design and accurate data analysis in conducting mouse and rat behavioral tests. Novel object recognition is used as an example of a cognitive assay which has been conducted successfully with a range of methods, all based on common principles of appropriate procedures, controls, and statistics. Directors of Rodent Core facilities within Intellectual and Developmental Disabilities Research Centers contribute key aspects of their own novel object recognition protocols, offering insights into essential similarities and less-critical differences. Literature cited in this review article will lead the interested reader to source papers that provide step-by-step protocols which illustrate optimized methods for many standard rodent behavioral assays. Adhering to best practices in behavioral neuroscience will enhance the value of animal models for the multiple goals of understanding biological mechanisms, evaluating consequences of genetic mutations, and discovering efficacious therapeutics.

Ketamine metabolite (2R,6R)-hydroxynorketamine enhances aggression via periaqueductal gray glutamatergic transmission

(2R,6R)-hydroxynorketamine (HNK), a metabolite of ketamine, has recently been suggested to be a potent antidepressant for treating animal depression and has rapid-onset and long-lasting action through potentiating glutamatergic transmission. However, its other effects are still unclear. In the present study, we tested the effects of (2R,6R)-HNK on offensive aggression. A resident-intruder (RI) test was used as the main model to test elements of offensive aggression, including threats and bites. Electrophysiological recordings in the ventrolateral periaqueductal gray (vlPAG) were used to measure the functions of glutamatergic synaptic transmission. A single systemic injection of (2R,6R)-HNK, but not (2S,6S)-HNK, increased elements of offensive aggression, including threats and bites, in a dose-dependent manner with long-lasting action. Moreover, (2R,6R)-HNK increased the input-output curve, the AMPA-mediated current, and the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) and decreased the paired-pulse ratio (PPR) in the vlPAG. Furthermore, intra-vlPAG application of (2R,6R)-HNK increased aggressive and biting behaviors, which were abolished by an intra-vlPAG pretreatment with the AMPA receptors antagonist, CNQX. Notably, the intra-vlPAG CNQX pretreatment eliminated systemic (2R,6R)-HNK-enhanced aggressive and biting behaviors. The results of this suggest that (2R,6R)-HNK evokes offensive aggression by increasing vlPAG glutamatergic transmission. Although (2R,6R)-HNK is currently suggested to be effective for treating depression, its side effect of increasing offensive aggression should be a subject of concern in future drug development and therapy.

Glutamatergic Signaling Along The Microbiota-Gut-Brain Axis

A complex bidirectional communication system exists between the gastrointestinal tract and the brain. Initially termed the “gut-brain axis” it is now renamed the “microbiota-gut-brain axis” considering the pivotal role of gut microbiota in maintaining local and systemic homeostasis. Different cellular and molecular pathways act along this axis and strong attention is paid to neuroactive molecules (neurotransmitters, i.e., noradrenaline, dopamine, serotonin, gamma aminobutyric acid and glutamate and metabolites, i.e., tryptophan metabolites), sustaining a possible interkingdom communication system between eukaryota and prokaryota. This review provides a description of the most up-to-date evidence on glutamate as a neurotransmitter/neuromodulator in this bidirectional communication axis. Modulation of glutamatergic receptor activity along the microbiota-gut-brain axis may influence gut (i.e., taste, visceral sensitivity and motility) and brain functions (stress response, mood and behavior) and alterations of glutamatergic transmission may participate to the pathogenesis of local and brain disorders. In this latter context, we will focus on two major gut disorders, such as irritable bowel syndrome and inflammatory bowel disease, both characterized by psychiatric co-morbidity. Research in this area opens the possibility to target glutamatergic neurotransmission, either pharmacologically or by the use of probiotics producing neuroactive molecules, as a therapeutic approach for the treatment of gastrointestinal and related psychiatric disorders.

Assessment of Improvement in Oxidative Stress Indices with Resocialization in Memory Retrieval in Y-Maze in Male Rats

Introduction:

Memory deficit is an important issue in some psychiatric diseases either as a primary symptom or as a comorbid symptom. Factors that determine the decline or improvement of memory are an important subject to reduce the severity of these diseases.

Methods and materials:

In this study, 32 male Sprague-Dawley rats were randomly divided into 4 experimental groups: social (control), isolation, resocialization for 3 days, and resocialization for 7 days. Isolation occurred for 14 days. Resocialization groups were resocialized for 3 or 7 days after isolation. In the social group, there was no intervention with normal socializing among the rats. In the isolation group, rats were isolated with no resocialization. In all 4 groups, after performing the Y-maze, the rats’ brains were removed to assess oxidative stress status in the hippocampus and prefrontal cortex.

Results:

Y-maze performance improved after 3 and 7 days of resocialization. However, oxidative stress status for malondialdehyde, glutathione and nitrite/nitrate returned to normal levels except in 2 experiments after 7 days of resocialization. In addition, in 2 experiments, just glutathione in the prefrontal cortex and nitrite/nitrate in the hippocampus after 3 days of resocialization improved.

Conclusions:

A return to normal levels in all types of antioxidant markers in the resocialization groups is not the only factor for improving memory deficits resulting from isolation. Resocialization may also be activating other regulatory mechanisms besides an antioxidant defense.

Putative Inflammatory Sensitive Mechanisms Underlying Risk or Resilience to Social Stress

It has been well recognized that exposure to stress can lead to the onset of psychosocial disorders such as depression. While there are a number of antidepressant therapies currently available and despite producing immediate neurochemical alterations, they require weeks of continuous use in order to exhibit antidepressant efficacy. Moreover, up to 30% of patients do not respond to typical antidepressants, suggesting that our understanding of the pathophysiology underlying stress-induced depression is still limited. In recent years inflammation has become a major focus in the study of depression as several clinical and preclinical studies have demonstrated that peripheral and central inflammatory mediators, including interleukin (IL)-1β, are elevated in depressed patients. Moreover, it has been suggested that inflammation and particularly neuroinflammation may be a direct and immediate link in the emergence of stress-induced depression due to the broad neural and glial effects that are elicited by proinflammatory cytokines. Importantly, individual differences in inflammatory reactivity may further explain why certain individuals exhibit differing susceptibility to the consequences of stress. In this review article, we discuss sources of individual differences such as age, sex and coping mechanisms that are likely sources of distinct changes in stress-induced neuroimmune factors and highlight putative sources of exaggerated neuroinflammation in susceptible individuals. Furthermore, we review the current literature of specific neural and glial mechanisms that are regulated by stress and inflammation including mitochondrial function, oxidative stress and mechanisms of glutamate excitotoxicity. Taken together, the impetus for this review is to move towards a better understanding of mechanisms regulated by inflammatory cytokines and chemokines that are capable of contributing to the emergence of depressive-like behaviors in susceptible individuals.