Disruption of the Glutamate-Glutamine Cycle Involving Astrocytes in an Animal Model of Depression for Males and Females

The cycle of stress: A systematic review of the impact of chronic psychological stress models on the rodent estrous cycle

Stress is known to impair reproduction through interactions between the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes. However, while it is well accepted that stress can alter estrous cycle regularity, a key indicator of female's HPG axis function, effects of different types of psychological stress have been inconsistent. This systematic review evaluated the impact of rodent models of psychological stress on estrous cyclicity, while reporting biological parameters pertaining to HPA or HPG axis function assessed within these studies. We performed a systematic database search and included articles that implemented a psychological stress model in rodents and reported estrous cyclicity for at least two cycles after initiation of stress. Of the 32 studies included, 62.5% reported post-stress alterations to estrous cyclicity, with Chronic Mild Stress (CMS) models showing the most conclusive effects. Twenty-five studies measured HPG or HPA axis markers, with cycle disruptions being commonly observed in parallel with altered estradiol and increased corticosterone levels. Our review highlights gaps in reporting estrous cyclicity assessments and makes recommendations to improve comparability between studies.

Astrocytic ALKBH5 in stress response contributes to depressive-like behaviors in mice

Epigenetic mechanisms bridge genetic and environmental factors that contribute to the pathogenesis of major depression disorder (MDD). However, the cellular specificity and sensitivity of environmental stress on brain epitranscriptomics and its impact on depression remain unclear. Here, we found that ALKBH5, an RNA demethylase of N6-methyladenosine (m6A), was increased in MDD patients' blood and depression models. ALKBH5 in astrocytes was more sensitive to stress than that in neurons and endothelial cells. Selective deletion of ALKBH5 in astrocytes, but not in neurons and endothelial cells, produced antidepressant-like behaviors. Astrocytic ALKBH5 in the mPFC regulated depression-related behaviors bidirectionally. Meanwhile, ALKBH5 modulated glutamate transporter-1 (GLT-1) m6A modification and increased the expression of GLT-1 in astrocytes. ALKBH5 astrocyte-specific knockout preserved stress-induced disruption of glutamatergic synaptic transmission, neuronal atrophy and defective Ca2+ activity. Moreover, enhanced m6A modification with S-adenosylmethionine (SAMe) produced antidepressant-like effects. Our findings indicate that astrocytic epitranscriptomics contribute to depressive-like behaviors and that astrocytic ALKBH5 may be a therapeutic target for depression.

Chronic social defeat stress induces the down-regulation of the Nedd4L-GLT-1 ubiquitination pathway in the prefrontal cortex of mice

Stressful life events contribute to the onset of major depressive disorder (MDD). We recently demonstrated abnormalities in ubiquitination in the pathophysiology of MDD. However, the underlying molecular mechanisms remain unclear. We investigated the involvement of the ubiquitination system-mediated glutamatergic dysfunction in social impairment induced by chronic social defeat stress (CSDS). Adult C57BL/6J mice were exposed to aggressor ICR male mice for 10 consecutive days. Social impairment was induced by CSDS in the social interaction test 1 days after the last stress exposure. In terms of brain microdialysis, CSDS reduced depolarization-evoked glutamate release in the prefrontal cortex (PFC), which was reversed by a glutamate transporter 1 (GLT-1) inhibitor. Interestingly, the expression of ubiquitinated, but not total GLT-1, was decreased in the PFC of mice exposed to CSDS. The expression of neural precursor cells expressing developmentally downregulated gene 4-like (Nedd4L: E3 ligase for GLT-1), and ubiquitin-conjugating enzyme E2D2 (Ube2d2: E2 ubiquitin-conjugating enzyme for Nedd4L) was also reduced in CSDS mice. Furthermore, the downregulation of the Nedd4L-GLT-1 ubiquitination pathway decreased SIT ratio, but up-regulation increased it even in non-CSDS mice. Taken together, the decrease in GLT-1 ubiquitination may reduce the release of extracellular glutamate induced by high-potassium stimulation, which may lead to social impairment, while we could not find differences in GLT-1 ubiquitination between susceptible and resistant CSDS mice. In conclusion, GLT-1 ubiquitination could play a crucial role in the pathophysiology of MDD and is an attractive target for the development of novel antidepressants.

Neuroinflammation and Post-Stroke Depression: Focus on the Microglia and Astrocytes

Post-stroke depression (PSD), a frequent and disabling complication of stroke, has a strong impact on almost thirty percent of stroke survivors. The pathogenesis of PSD is not completely clear so far. Neuroinflammation following stroke is one of underlying mechanisms that involves in the pathophysiology of PSD and plays an important function in the development of depression and is regarded as a sign of depression. During the neuroinflammation after ischemic stroke onset, both astrocytes and microglia undergo a series of morphological and functional changes and play pro-inflammatory or anti-inflammatory effect in the pathological process of stroke. Importantly, astrocytes and microglia exert dual roles in the pathological process of PSD due to the phenotypic transformation. We summarize the latest evidence of neuroinflammation involving in PSD in this review, focus on the phenotypic transformation of microglia and astrocytes following ischemic stroke and reveal the dual roles of both microglia and astrocytes in the PSD via modulating the neuroinflammation.

Astrocytes in Post-Stroke Depression: Roles in Inflammation, Neurotransmission, and Neurotrophin Signaling

Post-stroke depression (PSD) is a frequent and disabling complication of stroke that affects up to one-third of stroke survivors. The pathophysiology of PSD involves multiple mechanisms, including neurochemical, neuroinflammatory, neurotrophic, and neuroplastic changes. Astrocytes are a type of glial cell that is plentiful and adaptable in the central nervous system. They play key roles in various mechanisms by modulating neurotransmission, inflammation, neurogenesis, and synaptic plasticity. This review summarizes the latest evidence of astrocyte involvement in PSD from human and animal studies, focusing on the alterations of astrocyte markers and functions in relation to monoamine neurotransmitters, inflammatory cytokines, brain-derived neurotrophic factor, and glutamate excitotoxicity. We also discuss the potential therapeutic implications of targeting astrocytes for PSD prevention and treatment. Astrocytes could be new candidates for antidepressant medications and other interventions that aim to restore astrocyte homeostasis and function in PSD. Astrocytes could be new candidates for antidepressant medications and other interventions that aim to restore astrocyte homeostasis and function in PSD.

Intrusive thinking: Circuit and synaptic mechanisms of a transdiagnostic psychiatric symptom

Spontaneous thought is an adaptive cognitive process that can produce novel and insightful thought sequences useful in guiding future behavior. In many psychiatric disorders, spontaneous thinking becomes intrusive and uncontrolled, and can trigger symptoms such as craving, repetitive negative thinking and trauma-related memories. We link studies using clinical imaging and rodent modeling towards understanding the neurocircuitry and neuroplasticity of intrusive thinking. We propose a framework in which drugs or stress change the homeostatic set point of brain reward circuitry, which then impacts subsequent plasticity induced by drug/stress conditioned cues (metaplastic allostasis). We further argue for the importance of examining not only the canonical pre- and postsynapse, but also the adjacent astroglial protrusions and extracellular matrix that together form the tetrapartite synapse and that plasticity throughout the tetrapartite synapse is necessary for cue-induced drug or stress behaviors. This analysis reveals that drug use or trauma cause long-lasting allostatic brain plasticity that sets the stage for subsequent drug/trauma-associated cues to induce transient plasticity that can lead to intrusive thinking.

Neurovascular adaptations modulating cognition, mood, and stress responses

The neurovascular unit (NVU) is a dynamic center for substance exchange between the blood and the brain, making it an essential gatekeeper for central nervous system (CNS) homeostasis. Recent evidence supports a role for the NVU in modulating brain function and cognition. In addition, alterations in NVU processes are observed in response to stress, although the mechanisms via which they can affect mood and cognitive functions remain elusive. Here, we summarize recent studies of neurovascular regulation of emotional processes and cognitive function, including under stressful conditions. We also highlight relevant RNA-sequencing (RNA-seq) databases aiming to profile the NVU along with innovative tools to study and manipulate NVU function that can be exploited in the context of cognition and stress research throughout development, aging, or brain disorders.

Is depression the missing link between inflammatory mediators and cancer?

Patients with cancer are at greater risk of developing depression in comparison to the general population and this is associated with serious adverse effects, such as poorer quality of life, worse prognosis and higher mortality. Although the relationship between depression and cancer is now well established, a common underlying pathophysiological mechanism between the two conditions is yet to be elucidated. Existing theories of depression, based on monoamine neurotransmitter system dysfunction, are insufficient as explanations of the disorder. Recent advances have implicated neuroinflammatory mechanisms in the etiology of depression and it has been demonstrated that inflammation at a peripheral level may be mirrored centrally in astrocytes and microglia serving to promote chronic levels of inflammation in the brain. Three major routes to depression in cancer in which proinflammatory mediators are implicated, seem likely. Activation of the kynurenine pathway involving cytokines, increases tryptophan catabolism, resulting in diminished levels of serotonin which is widely acknowledged as being the hallmark of depression. It also results in neurotoxic effects on brain regions thought to be involved in the evolution of major depression. Proinflammatory mediators also play a crucial role in impairing regulatory glucocorticoid mediated feedback of the hypothalamic-pituitary-adrenal axis, which is activated by stress and considered to be involved in both depression and cancer. The third route is via the glutamatergic pathway, whereby glutamate excitotoxicity may lead to depression associated with cancer. A better understanding of the mechanisms underlying these dysregulated and other newly emerging pathways may provide a rationale for therapeutic targeting, serving to improve the care of cancer patients.

Alterations in Astrocytic Regulation of Excitation and Inhibition by Stress Exposure and in Severe Psychopathology

Dysregulation of excitatory and inhibitory signaling is commonly observed in major psychiatric disorders, including schizophrenia, depression, and bipolar disorder, and is often targeted by psychological and pharmacological treatment methods. The balance of excitation and inhibition is highly sensitive to severe psychological stress, one of the strongest risk factors for psychiatric disorders. The role of astrocytes in regulating excitatory and inhibitory signaling is now widely recognized; however, the specific involvement of astrocytes in the context of psychiatric disorders with a history of significant stress exposure remains unclear. In this review, we summarize how astrocytes regulate the balance of excitation and inhibition in the context of stress exposure and severe psychopathology, with a focus on the PFC, a brain area highly implicated in psychopathology. We first focus on preclinical models to demonstrate that the duration of stress (particularly acute vs chronic stress) is key to shaping astrocyte function and downstream behavior. We then provide a hypothesis for how astrocytes are involved in stress-associated cortical signaling imbalance, discuss how this directly contributes to phenotypes of psychopathologies, and provide suggestions for future research. We highlight that astrocytes are a key target to understand and treat the dysregulation of cortical signaling associated with stress-related psychiatric disorders.

Impairment of glutamate homeostasis in the nucleus accumbens core underpins cross-sensitization to cocaine following chronic restraint stress

Though the facilitating influence of stress on drug abuse is well documented, the mechanisms underlying this interaction have yet to be fully elucidated. The present study explores the neurobiological mechanisms underpinning the sensitized response to the psychomotor-stimulating effects of cocaine following chronic restraint stress (CRS), emphasizing the differential contribution of both subcompartments of the nucleus accumbens (NA), the core (NAcore) and shell (NAshell), to this phenomenon. Adult male Wistar rats were restrained for 2 h/day for 7 days and, 2 weeks after the last stress exposure (day 21), all animals were randomly assigned to behavioral, biochemical or neurochemical tests. Our results demonstrated that the enduring CRS-induced increase in psychostimulant response to cocaine was paralleled by an increase of extracellular dopamine levels in the NAcore, but not the NAshell, greater than that observed in the non-stress group. Furthermore, we found that CRS induced an impairment of glutamate homeostasis in the NAcore, but not the NAshell. Its hallmarks were increased basal extracellular glutamate concentrations driven by a CRS-induced downregulation of GLT-1, blunted glutamate levels in response to cocaine and postsynaptic structural remodeling in pre-stressed animals. In addition, ceftriaxone, a known GLT-1 enhancer, prevented the CRS-induced GLT-1 downregulation, increased basal extracellular glutamate concentrations and changes in structural plasticity in the NAcore as well as behavioral cross-sensitization to cocaine, emphasizing the biological importance of GLT-1 in the comorbidity between chronic stress exposure and drug abuse. A future perspective concerning the paramount relevance of the stress-induced disruption of glutamate homeostasis as a vulnerability factor to the development of stress and substance use disorders during early life or adulthood of descendants is provided.

The intersection of astrocytes and the endocannabinoid system in the lateral habenula: on the fast-track to novel rapid-acting antidepressants

Despite attaining significant advances toward better management of depressive disorders, we are still facing several setbacks. Developing rapid-acting antidepressants with sustained effects is an aspiration that requires thinking anew to explore possible novel targets. Recently, the lateral habenula (LHb), the brain's "anti-reward system", has been shown to go awry in depression in terms of various molecular and electrophysiological signatures. Some of the presumed contributors to such observed aberrations are astrocytes. These star-shaped cells of the brain can alter the firing pattern of the LHb, which keeps the activity of the midbrain's aminergic centers under tight control. Astrocytes are also integral parts of the tripartite synapses, and can therefore modulate synaptic plasticity and leave long-lasting changes in the brain. On the other hand, it was discovered that astrocytes express cannabinoid type 1 receptors (CB₁R), which can also take part in long-term plasticity. Herein, we recount how the LHb of a depressed brain deviates from the "normal" one from a molecular perspective. We then try to touch upon the alterations of the endocannabinoid system in the LHb, and cast the idea that modulation of astroglial CB₁R may help regulate habenular neuronal activity and synaptogenesis, thereby acting as a new pharmacological tool for regulation of mood and amelioration of depressive symptoms.

Ceftriaxone as a Novel Therapeutic Agent for Hyperglutamatergic States: Bridging the Gap Between Preclinical Results and Clinical Translation

Dysregulation of glutamate homeostasis is a well-established core feature of neuropsychiatric disorders. Extracellular glutamate concentration is regulated by glutamate transporter 1 (GLT-1). The discovery of a beta-lactam antibiotic, ceftriaxone (CEF), as a safe compound with unique ability to upregulate GLT-1 sparked the interest in testing its efficacy as a novel therapeutic agent in animal models of neuropsychiatric disorders with hyperglutamatergic states. Indeed, more than 100 preclinical studies have shown the efficacy of CEF in attenuating the behavioral manifestations of various hyperglutamatergic brain disorders such as ischemic stroke, amyotrophic lateral sclerosis (ALS), seizure, Huntington’s disease, and various aspects of drug use disorders. However, despite rich and promising preclinical data, only one large-scale clinical trial testing the efficacy of CEF in patients with ALS is reported. Unfortunately, in that study, there was no significant difference in survival between placebo- and CEF-treated patients. In this review, we discussed the translational potential of preclinical efficacy of CEF based on four different parameters: (1) initiation of CEF treatment in relation to induction of the hyperglutamatergic state, (2) onset of response in preclinical models in relation to onset of GLT-1 upregulation, (3) mechanisms of action of CEF on GLT-1 expression and function, and (4) non-GLT-1-mediated mechanisms for CEF. Our detailed review of the literature brings new insights into underlying molecular mechanisms correlating the preclinical efficacy of CEF. We concluded here that CEF may be clinically effective in selected cases in acute and transient hyperglutamatergic states such as early drug withdrawal conditions.

St. John's wort (Hypericum perforatum) and depression: what happens to the neurotransmitter systems?

St. John's wort (Hypericum perforatum) is a herbaceous plant containing many bioactive molecules including naphthodianthrones, phloroglucinol derivatives, flavonoids, bioflavonoids, proanthocyanidins, and chlorogenic acid. Evidence has shown the therapeutic effects of St. John's wort and especially its two major active components, hyperforin and hypericin, on different psychiatric and mood disorders such as posttraumatic stress disorder (PTSD), attention-deficit hyperactivity disorder (ADHD), obsessive-compulsive disorder (OCD), and anxiety disorders. St. John's wort also induces antidepressant effects. In this review study, we aimed to discuss the role of St. John's wort in modulating depression, with respect to the role of different neurotransmitter systems in the brain. We discussed changes in the neurotransmitter levels in depression, and following use of St. John's wort. It was concluded that changes in the function and level of neurotransmitters in depression are complex. Also, St. John's wort can induce inconsistent effects on neurotransmitter levels. We also found that glutamate and acetylcholine may be the most important neurotransmitters to study in future works, because the function of both neurotransmitters in depression is unclear. In addition, St. John's wort induces a dualistic modulation on the activity of cholinergic signaling, which can be an interesting topic for future studies.

Neurotransmitter and Related Metabolic Profiling in the Nucleus Accumbens of Chronic Unpredictable Mild Stress-Induced Anhedonia-Like Rats

Major depressive disorder (MDD) is a serious mental disorder that affects many people. The neurotransmitter deficiency hypothesis has been the crux of much research on the treatment of depression. Anhedonia, as a core symptom, was closely associated with altered levels of 5-hydroxytryptamine (5-HT), dopamine (DA), and diverse types of glutamate (Glu) receptors in the nucleus accumbens (NAc). However, there were no reports showing how Glu changed in the NAc, and there were other unreported molecules involved in modulating stress-induced anhedonia. Thus, we investigated changes in neurotransmitters and their related metabolites in GABAergic, serotonergic and catecholaminergic pathways in the NAc of a rat model of chronic unpredictable mild stress- (CUMS-) induced anhedonia-like behavior. Then, liquid chromatography-tandem mass spectrometry (LC-MS/MS) was employed to detect target neurotransmitters and related metabolites in the NAc. Finally, the Western blot was used to assess the expression of key enzymes and receptors. Here, we found that the 5-HT level in anhedonia-susceptible (Sus) rats was increased while the Glu level decreased. DA did not show a significant change among CUMS rats. Correspondingly, we detected a reduction in monoamine oxidase-A (MAOA) and Glu receptor 1 levels in anhedonia-Sus rats while Glu receptor 2 (GluR2) and NMDA2B levels were increased in anhedonia-resilient (Res) rats. We also found that the levels of glutamine (Gln), kynurenic acid (Kya), histamine (HA), L-phenylalanine (L-Phe), and tyramine (Tyra) were changed after CUMS. These alterations in neurotransmitters may serve as a new insight into understanding the development of anhedonia-like behavior in depression.

Beyond the neuron: Role of non-neuronal cells in stress disorders

Stress disorders are leading causes of disease burden in the U.S. and worldwide, yet available therapies are fully effective in less than half of all individuals with these disorders. Although to date, much of the focus has been on neuron-intrinsic mechanisms, emerging evidence suggests that chronic stress can affect a wide range of cell types in the brain and periphery, which are linked to maladaptive behavioral outcomes. Here, we synthesize emerging literature and discuss mechanisms of how non-neuronal cells in limbic regions of brain interface at synapses, the neurovascular unit, and other sites of intercellular communication to mediate the deleterious, or adaptive (i.e., pro-resilient), effects of chronic stress in rodent models and in human stress-related disorders. We believe that such an approach may one day allow us to adopt a holistic "whole body" approach to stress disorder research, which could lead to more precise diagnostic tests and personalized treatment strategies. Stress is a major risk factor for many psychiatric disorders. Cathomas et al. review new insight into how non-neuronal cells mediate the deleterious effects, as well as the adaptive, protective effects, of stress in rodent models and human stress-related disorders.

The Regulation of Glutamate Transporter 1 in the Rapid Antidepressant-Like Effect of Ketamine in Mice

Accumulating evidence suggests that glutamate clearance plays a critical role in the pathophysiology and treatment of depression. Preclinical and clinical studies have demonstrated that ketamine provides an immediate and sustained antidepressant effect. However, the precise mechanism of its action remains to be elucidated. Glutamate transporter 1 (GLT1) participates in glutamate clearance; therefore, we hypothesized that GLT1 may play an important role in the antidepressant effect of ketamine. In this study, we determined that GLT1 inhibition blocks the antidepressant-like properties of ketamine and alters the phosphorylation of the mammalian target of rapamycin (mTOR) in the prefrontal cortex (PFC). Our results show that pretreatment with dihydrokainic acid (DHK), a GLT1 inhibitor, alleviated the antidepressant-like effect of ketamine, and decreased the level of phosphorylated mTOR (pmTOR) in mice (which is normally upregulated by ketamine). In addition, inhibition of α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptor and L-type voltage-dependent calcium channel (L-VDCC) significantly abolished the antidepressant-like effect of ketamine. Moreover, inhibition of L-VDCC significantly blocked the upregulation of GLT1 and BDNF in the PFC of mice. The inhibition of the AMPA receptor only significantly alleviated BDNF. Our results provide insight into the role of GLT1 as the critical presynaptic molecule participating in the pathophysiological mechanism of depression and contributing to the antidepressant-like effect of ketamine. In addition, our study confirms that both AMPA receptor and L-VDCC are crucial factors in the immediate antidepressant-like effect of ketamine.

Minocycline prevents chronic restraint stress-induced vulnerability to developing cocaine self-administration and associated glutamatergic mechanisms: a potential role of microglia

Stressful experience-induced cocaine-related behaviors are associated with a significant impairment of glutamatergic mechanisms in the Nucleus Accumbens core (NAcore). The hallmarks of disrupted glutamate homeostasis following restraint stress are the enduring imbalance of glutamate efflux after a cocaine stimulus and increased basal concentrations of extracellular glutamate attributed to GLT-1 downregulation in the NAcore. Glutamate transmission is tightly linked to microglia functioning. However, the role of microglia in the biological basis of stress-induced addictive behaviors is still unknown. By using minocycline, a potent inhibitor of microglia activation with anti-inflammatory properties, we determined whether microglia could aid chronic restraint stress (CRS)-induced glutamate homeostasis disruption in the NAcore, underpinning stress-induced cocaine self-administration. In this study, adult male rats were restrained for 2 h/day for seven days (day 1-7). From day 16 until completing the experimental protocol, animals received a vehicle or minocycline treatment (30 mg/Kg/12h i.p.). On day 21, animals were assigned to microscopic, biochemical, neurochemical or behavioral studies. We confirm that the CRS-induced facilitation of cocaine self-administration is associated with enduring GLT-1 downregulation, an increase of basal extracellular glutamate and postsynaptic structural plasticity in the NAcore. These alterations were strongly related to the CRS-induced reactive microglia and increased TNF-α mRNA and protein expression, since by administering minocycline, the impaired glutamate homeostasis and the facilitation of cocaine self-administration were prevented. Our findings are the first to demonstrate that minocycline suppresses the CRS-induced facilitation of cocaine self-administration and glutamate homeostasis disruption in the NAcore. A role of microglia is proposed for the development of glutamatergic mechanisms underpinning stress-induced vulnerability to cocaine addiction.

Animal Models of Anxiety and Depression: Incorporating the Underlying Mechanisms of Sex Differences in Macroglia Biology

Animal models have been utilized to explore the mechanisms by which mood disorders develop. Ethologically based stress paradigms are used to induce behavioral responses consistent with those observed in humans suffering from anxiety and depression. While mood disorders are more often diagnosed in women, animal studies are more likely to be carried out in male rodents. However, understanding the mechanisms behind anxiety- and depressive-like behaviors in both sexes is necessary to increase the predictive and construct validity of the models and identify therapeutic targets. To understand sex differences following stress, we must consider how all cell types within the central nervous system are influenced by the neuroendocrine system. This review article discusses the effects of stress and sex steroids on the macroglia: astrocytes and oligodendrocytes. Glia are involved in shaping the synapse through the regulation of neurotransmitter levels and energy resources, making them essential contributors to neural dynamics following stress. As the role of glia in neuromodulation has become more apparent, studies exploring the mechanisms by which glia are altered by stress and steroids will provide insight into sex differences in animal models. These insights will facilitate the optimization of animal models of psychiatric disorders and development of future therapeutic targets.

Quantum energy levels of glutamate modulate neural biophotonic signals

Glutamate is the most abundant excitatory neurotransmitter in the brain, and it plays an essential and important role in neural functions. Current studies have shown that glutamate can induce neural biophotonic activity and transmission, which may involve the mechanism of photon quantum brain; however, it is unclear whether such a mechanism follows the principle of quantum mechanics. Here we show that the action of glutamate on its receptors leads to a decrease in its quantum energy levels, and glutamate then partially or completely loses its function to further induce the biophotonic activity in mouse brain slices. The reduced quantum energy levels of glutamate can be restored by direct-current electrical discharges and the use of energy transfer of chloroplast photosynthesis; hence, the quantum energy recovered glutamate can again induce significant biophotonic activity. Furthermore, the changes in quantum energy levels of glutamate are related to the exchange and transfer of electron energy on its active hydrogen atom. These findings suggest that the glutamate-induced neural biophotonic signals may be involved in the transfer of the quantum energy levels of glutamate, which implies a quantum mechanism of neurotransmitter action.

Glutamate in Male and Female Sexual Behavior: Receptors, Transporters, and Steroid Independence

The survival of animal species predicates on the success of sexual reproduction. Neurotransmitters play an integral role in the expression of these sexual behaviors in the brain. Here, we review the role of glutamate in sexual behavior in rodents and non-rodent species for both males and females. These encompass the release of glutamate and correlations with glutamate receptor expression during sexual behavior. We then present the effects of glutamate on sexual behavior, as well as the effects of antagonists and agonists on different glutamate transporters and receptors. Following that, we discuss the potential role of glutamate on steroid-independent sexual behavior. Finally, we demonstrate the interaction of glutamate with other neurotransmitters to impact sexual behavior. These sexual behavior studies are crucial in the development of novel treatments of sexual dysfunction and in furthering our understanding of the complexity of sexual diversity. In the past decade, we have witnessed the burgeoning of novel techniques to study and manipulate neuron activity, to decode molecular events at the single-cell level, and to analyze behavioral data. They pose exciting avenues to gain further insight into future sexual behavior research. Taken together, this work conveys the essential role of glutamate in sexual behavior.

Glutamine-to-glutamate ratio in the nucleus accumbens predicts effort-based motivated performance in humans

Substantial evidence implicates the nucleus accumbens in motivated performance, but very little is known about the neurochemical underpinnings of individual differences in motivation. Here, we applied ¹H magnetic resonance spectroscopy (¹H-MRS) at ultra-high-field in the nucleus accumbens and inquired whether levels of glutamate (Glu), glutamine (Gln), GABA or their ratios predict interindividual differences in effort-based motivated task performance. Given the incentive value of social competition, we also examined differences in performance under self-motivated or competition settings. Our results indicate that higher accumbal Gln-to-Glu ratio predicts better overall performance and reduced effort perception. As performance is the outcome of multiple cognitive, motor and physiological processes, we applied computational modeling to estimate best-fitting individual parameters related to specific processes modeled with utility, effort and performance functions. This model-based analysis revealed that accumbal Gln-to-Glu ratio specifically relates to stamina; i.e., the capacity to maintain performance over long periods. It also indicated that competition boosts performance from task onset, particularly for low Gln-to-Glu individuals. In conclusion, our findings provide novel insights implicating accumbal Gln and Glu balance on the prediction of specific computational components of motivated performance. This approach and findings can help developing therapeutic strategies based on targeting metabolism to ameliorate deficits in effort engagement.

Astrocytic Glutamate Transporter 1 (GLT1) Deficiency Reduces Anxiety- and Depression-Like Behaviors in Mice

Glutamatergic dysregulation is known to contribute to altered emotional regulation. Astrocytic glutamate transporter 1 (GLT1) is responsible for the majority of glutamate clearance from synapse. However, the role of astrocytic GLT1 in affective processes such as anxiety- and depression-like behavior is not fully understood. Here, we found that astrocytic GLT1 deficient mice entered more frequently, and spent more time in the open arms of elevated plus maze without difference in overall distance traveled in the open field, nor were there any metabolic changes observed in the metabolic chamber compared to wildtype mice. Moreover, mice lacking astrocytic GLT1 exhibited less immobile time and moved greater area in the tail suspension test. Similarly, in the forced swim test, they showed less immobile time and moved greater area. In addition, we found that astrocytic GLT1 deficiency reduced freezing responses in the fear contextual and cued tests. Taken together, our findings suggest that astrocytic GLT1 deficiency decreases anxiety and depression-like behaviors.

EAAT2 as a Research Target in Bipolar Disorder and Unipolar Depression: A Systematic Review

Glutamate is implicated in the neuropathology of both major depressive disorder and bipolar disorder. Excitatory amino acid transporter 2 (EAAT2) is the major glutamate transporter in the mammalian brain, removing glutamate from the synaptic cleft and transporting it into glia for recycling. It is thereby the principal regulator of extracellular glutamate levels and prevents neuronal excitotoxicity. EAAT2 is a promising target for elucidating the mechanisms by which the glutamate-glutamine cycle interacts with neuronal systems in mood disorders. Forty EAAT2 studies (published January 1992-January 2018) were identified via a systematic literature search. The studies demonstrated that chronic stress/steroids were most commonly associated with decreased EAAT2. In rodents, EAAT2 inhibition worsened depressive behaviors. Human EAAT2 expression usually decreased in depression, with some regional brain differences. Fewer data have been collected regarding the roles and regulation of EAAT2 in bipolar disorder. Future directions for research include correlating EAAT2 and glutamate levels in vivo, elucidating genetic variability and epigenetic regulation, clarifying intracellular protein and pharmacologic interactions, and examining EAAT2 in different bipolar mood states. As part of a macromolecular complex within glia, EAAT2 may contribute significantly to intracellular signaling, energy regulation, and cellular homeostasis. An enhanced understanding of this system is needed.

Sex differences in the glutamate system: Implications for addiction

Clinical and preclinical research have identified sex differences in substance use and addiction-related behaviors. Historically, substance use disorders are more prevalent in men than women, though this gap is closing. Despite this difference, women appear to be more susceptible to the effects of many drugs and progress to substance abuse treatment more quickly than men. While the glutamate system is a key regulator of addiction-related behaviors, much of the work implicating glutamate signaling and glutamatergic circuits has been conducted in men and male rodents. An increasing number of studies have identified sex differences in drug-induced glutamate alterations as well as sex and estrous cycle differences in drug seeking behaviors. This review will describe sex differences in the glutamate system with an emphasis on implications for substance use disorders, highlighting the gaps in our current understanding of how innate and drug-induced alterations in the glutamate system may contribute to sex differences in addiction-related behaviors.

Sleep Disturbance in Bipolar Disorder: Neuroglia and Circadian Rhythms

The worldwide prevalence of sleep disorders is approximately 50%, with an even higher occurrence in a psychiatric population. Bipolar disorder (BD) is a severe mental illness characterized by shifts in mood and activity. The BD syndrome also involves heterogeneous symptomatology, including cognitive dysfunctions and impairments of the autonomic nervous system. Sleep abnormalities are frequently associated with BD and are often a good predictor of a mood swing. Preservation of stable sleep-wake cycles is therefore a key to the maintenance of stability in BD, indicating the crucial role of circadian rhythms in this syndrome. The symptom most widespread in BD is insomnia, followed by excessive daytime sleepiness, nightmares, difficulty falling asleep or maintaining sleep, poor sleep quality, sleep talking, sleep walking, and obstructive sleep apnea. Alterations in the structure or duration of sleep are reported in all phases of BD. Understanding the role of neuroglia in BD and in various aspects of sleep is in nascent state. Contributions of the different types of glial cells to BD and sleep abnormalities are discussed in this paper.

A Systematic Search and Mapping Review of Studies on Intracerebral Microdialysis of Amino Acids, and Systematized Review of Studies on Circadian Rhythms

Background:

Microdialysis can be used to measure amino acids in the extracellular space in vivo, based on the principle of diffusion. Variations in experimental set-up result in variations in baseline levels of the compounds measured. Variations may also be due to circadian rhythms.

Method:

We systematically searched and mapped the literature on all studies reporting baseline microdialysis measurements of histamine and the amino acids asparagine, aspartate, GABA, glutamate, glutamine, glycine, proline and taurine. We fully reviewed the studies describing circadian rhythms for histamine and the selected amino acids.

Results:

We retrieved 2331 papers describing baseline measurements of one or more of the compounds of interest. We provide a numerical summary and lists of the publications by compound. We retrieved 11 references describing studies on the circadian rhythms of the compounds of interest. Aspartate, glutamate and histamine are generally higher during the dark than during the light phase in nocturnal rodents. For glutamine, no rhythmicity was observed. For GABA, the results were too inconsistent to generalise. For asparagine, glycine, proline and taurine, insufficient data are available.

Conclusion:

The literature on intracerebral microdialysis measurements of the amino acids is vast, but certain primary studies are still warranted. Future systematic reviews on the individual compounds can shed light on the effects of experimental variations on baseline concentrations.

Social Defeat Stress, Sex, and Addiction-Like Behaviors

Social confrontation is a form of social interaction in animals where two conspecific individuals confront each other in dispute over territory, during the formation of hierarchies, and during breeding seasons. Typically, a social confrontation involves a prevailing individual and a yielding individual. The prevailing individual often exhibits aggressive postures and launches attacks, whereas the yielding individual often adopts postures of defeat. The yielding or defeated animals experience a phenomenon known as social defeat stress, in which they show exaggerated stress as well as autonomic and endocrine responses that cause impairment of both the brain and body. In laboratory settings, one can reliably generate social defeat stress by allowing a naïve (or already defeated) animal to intrude into a home cage in which its resident has already established a territory or is nursing. This resident-intruder paradigm has been widely used in both males and females to study mechanisms in the brain that underlie the stress responses. Stress has profound effects on drug reward for cocaine, methamphetamine, alcohol, and opioids. Particularly, previous experiences with social defeat can exaggerate subsequent addiction-like behaviors. The extent of these addiction-like behaviors depends on the intensity, duration, frequency, and intermittency of the confrontation episodes. This chapter describes four types of social defeat stress: acute, repeated, intermittent, and chronic. Specifically, it focuses on social defeat stress models used in laboratories to study individual, sex, and animal strain differences in addiction-like behaviors.

OGT-related mitochondrial motility is associated with sex differences and exercise effects in depression induced by prenatal exposure to glucocorticoids

BACKGROUND

Prenatal exposure to glucocorticoids (GCs) has been found to trigger abnormal behaviors and deleterious neurological effects on offspring both in animals and in humans. The sex differences in depression have been replicated in numerous studies across cultures, persisting throughout the reproductive years. As an X-linked gene in rodents and in humans, O-GlcNAc transferase (OGT) may provide a novel perspective for the sex differences in depression.

METHODS

In the last third of pregnancy (gestational day 14-21), rats were subcutaneously administered either 0.13mg/kg dexamethasone-21-phosphate disodium salt (0.1mg/kg DEX) or vehicle (0.9% saline) once a day for 7 days. Adolescent (4 weeks) offspring were then trained in a swimming program or not.

RESULTS

Here we found that adult offspring rats exposed to DEX prenatally exhibited sex-specific depression-like behaviors, males being more vulnerable than females. Swimming exercise ameliorated the above-mentioned depressive syndromes, which may be a compensatory effect for male disadvantage suffering from prenatal stress. Furthermore, the effects of prenatal DEX exposure and swimming exercise on depression were associated with OGT-related mitochondrial motility, including PINK1/Parkin pathway and AKT/GSK3β pathway.

LIMITATIONS

Representative kymographs of mitochondrial motility were not detected and no causal effects were obtained by OGT gene overexpression or gene knockout in this study.

CONCLUSIONS

Our results provide a new perspective for better understanding sex differences and exercise effects in depression and may offer new mechanism-based therapeutic targets for depression.

Sex-dependent mental illnesses and mitochondria

The prevalence of some mental illnesses, including major depression, anxiety-, trauma-, and stress-related disorders, some substance use disorders, and later onset of schizophrenia, is higher in women than men. While the higher prevalence in women could simply be explained by socioeconomic determinants, such as income, social status, or cultural background, extensive studies show sex differences in biological, pharmacokinetic, and pharmacological factors contribute to females' vulnerability to these mental illnesses. In this review, we focus on estrogens, chronic stress, and neurotoxicity from behavioral, pharmacological, biological, and molecular perspectives to delineate the sex differences in these mental illnesses. Particularly, we investigate a possible role of mitochondrial function, including biosynthesis, bioenergetics, and signaling, on mediating the sex differences in psychiatric disorders.