Stress and CRF gate neural activation of BDNF in the mesolimbic reward pathway

Brain region-specific roles of brain-derived neurotrophic factor in social stress-induced depressive-like behavior

Brain-derived neurotrophic factor is a key factor in stress adaptation and avoidance of a social stress behavioral response. Recent studies have shown that brain-derived neurotrophic factor expression in stressed mice is brain region-specific, particularly involving the corticolimbic system, including the ventral tegmental area, nucleus accumbens, prefrontal cortex, amygdala, and hippocampus. Determining how brain-derived neurotrophic factor participates in stress processing in different brain regions will deepen our understanding of social stress psychopathology. In this review, we discuss the expression and regulation of brain-derived neurotrophic factor in stress-sensitive brain regions closely related to the pathophysiology of depression. We focused on associated molecular pathways and neural circuits, with special attention to the brain-derived neurotrophic factor-tropomyosin receptor kinase B signaling pathway and the ventral tegmental area-nucleus accumbens dopamine circuit. We determined that stress-induced alterations in brain-derived neurotrophic factor levels are likely related to the nature, severity, and duration of stress, especially in the above-mentioned brain regions of the corticolimbic system. Therefore, BDNF might be a biological indicator regulating stress-related processes in various brain regions.

Optogenetic behavioral studies in depression research: A systematic review

Optogenetics has made substantial contributions to our understanding of the mechanistic underpinnings of depression. This systematic review employs quantitative analysis to investigate the impact of optogenetic stimulation in mice and rats on behavioral alterations in social interaction, sucrose consumption, and mobility. The review analyses optogenetic behavioral studies using standardized behavioral tests to detect behavioral changes induced via optogenetic stimulation in stressed or stress-naive mice and rats. Behavioral changes were evaluated as either positive, negative, or not effective. The analysis comprises the outcomes of 248 behavioral tests of 168 studies described in 37 articles, including negative and null results. Test outcomes were compared for each behavior, depending on the animal cohort, applied type of stimulation and the stimulated neuronal circuit and cell type. The presented synthesis contributes toward a comprehensive picture of optogenetic behavioral research in the context of depression.

Mesolimbic Neural Response Dynamics Predict Future Individual Alcohol Drinking in Mice

BACKGROUND

Individual variability in response to rewarding stimuli is a striking but understudied phenomenon. The mesolimbic dopamine system is critical in encoding the reinforcing properties of both natural reward and alcohol; however, how innate or baseline differences in the response dynamics of this circuit define individual behavior and shape future vulnerability to alcohol remain unknown.

METHODS

Using naturalistic behavioral assays, a voluntary alcohol drinking paradigm, in vivo fiber photometry, in vivo electrophysiology, and chemogenetics, we investigated how differences in mesolimbic neural circuit activity contribute to the individual variability seen in reward processing and, by proxy, alcohol drinking.

RESULTS

We first characterized heterogeneous behavioral and neural responses to natural reward and defined how these baseline responses predicted future individual alcohol-drinking phenotypes in male mice. We then determined spontaneous ventral tegmental area dopamine neuron firing profiles associated with responses to natural reward that predicted alcohol drinking. Using a dual chemogenetic approach, we mimicked specific mesolimbic dopamine neuron firing activity before or during voluntary alcohol drinking to link unique neurophysiological profiles to individual phenotype. We show that hyperdopaminergic individuals exhibit a lower neuronal response to both natural reward and alcohol that predicts lower levels of alcohol consumption in the future.

CONCLUSIONS

These findings reveal unique, circuit-specific neural signatures that predict future individual vulnerability or resistance to alcohol and expand the current knowledge base on how some individuals are able to titrate their alcohol consumption whereas others go on to engage in unhealthy alcohol-drinking behaviors.

Neuroendocrine and cellular mechanisms in stress resilience: From hormonal influence in the CNS to mitochondrial dysfunction and oxidative stress

Recent advancements in neuroendocrinology challenge the long-held belief that hormonal effects are confined to perivascular tissues and do not extend to the central nervous system (CNS). This paradigm shift, propelled by groundbreaking research, reveals that synthetic hormones, notably in anti-inflammatory medications, significantly influence steroid psychosis, behavioural, and cognitive impairments, as well as neuropeptide functions. A seminal development in this field occurred in 1968 with McEven's proposal that rodent brains are responsive to glucocorticoids, fundamentally altering the understanding of how anxiety impacts CNS functionality and leading to the identification of glucocorticosteroids and mineralocorticoids as distinct corticotropic receptors. This paper focuses on the intricate roles of the neuroendocrine, immunological, and CNS in fostering stress resilience, underscored by recent animal model studies. These studies highlight active, compensatory, and passive strategies for resilience, supporting the concept that anxiety and depression are systemic disorders involving dysregulation across both peripheral and central systems. Resilience is conceptualized as a multifaceted process that enhances psychological adaptability to stress through adaptive mechanisms within the immunological system, brain, hypothalamo-pituitary-adrenal axis, and ANS Axis. Furthermore, the paper explores oxidative stress, particularly its origin from the production of reactive oxygen species (ROS) in mitochondria. The mitochondria's role extends beyond ATP production, encompassing lipid, heme, purine, and steroidogenesis synthesis. ROS-induced damage to biomolecules can lead to significant mitochondrial dysfunction and cell apoptosis, emphasizing the critical nature of mitochondrial health in overall cellular function and stress resilience. This comprehensive synthesis of neuroendocrinological and cellular biological research offers new insights into the systemic complexity of stress-related disorders and the imperative for multidisciplinary approaches in their study and treatment.

The Formation and Function of the VTA Dopamine System

The midbrain dopamine system is a sophisticated hub that integrates diverse inputs to control multiple physiological functions, including locomotion, motivation, cognition, reward, as well as maternal and reproductive behaviors. Dopamine is a neurotransmitter that binds to G-protein-coupled receptors. Dopamine also works together with other neurotransmitters and various neuropeptides to maintain the balance of synaptic functions. The dysfunction of the dopamine system leads to several conditions, including Parkinson's disease, Huntington's disease, major depression, schizophrenia, and drug addiction. The ventral tegmental area (VTA) has been identified as an important relay nucleus that modulates homeostatic plasticity in the midbrain dopamine system. Due to the complexity of synaptic transmissions and input-output connections in the VTA, the structure and function of this crucial brain region are still not fully understood. In this review article, we mainly focus on the cell types, neurotransmitters, neuropeptides, ion channels, receptors, and neural circuits of the VTA dopamine system, with the hope of obtaining new insight into the formation and function of this vital brain region.

CRF regulates pain sensation by enhancement of corticoaccumbal excitatory synaptic transmission

Both peripheral and central corticotropin-releasing factor (CRF) systems have been implicated in regulating pain sensation. However, compared with the peripheral, the mechanisms underlying central CRF system in pain modulation have not yet been elucidated, especially at the neural circuit level. The corticoaccumbal circuit, a structure rich in CRF receptors and CRF-positive neurons, plays an important role in behavioral responses to stressors including nociceptive stimuli. The present study was designed to investigate whether and how CRF signaling in this circuit regulated pain sensation under physiological and pathological pain conditions. Our studies employed the viral tracing and circuit-, and cell-specific electrophysiological methods to label the CRF-containing circuit from the medial prefrontal cortex to the nucleus accumbens shell (mPFCCRF-NAcS) and record its neuronal propriety. Combining optogenetic and chemogenetic manipulation, neuropharmacological methods, and behavioral tests, we were able to precisely manipulate this circuit and depict its role in regulation of pain sensation. The current study found that the CRF signaling in the NAc shell (NAcS), but not NAc core, was necessary and sufficient for the regulation of pain sensation under physiological and pathological pain conditions. This process was involved in the CRF-mediated enhancement of excitatory synaptic transmission in the NAcS. Furthermore, we demonstrated that the mPFCCRF neurons monosynaptically connected with the NAcS neurons. Chronic pain increased the protein level of CRF in NAcS, and then maintained the persistent NAcS neuronal hyperactivity through enhancement of this monosynaptic excitatory connection, and thus sustained chronic pain behavior. These findings reveal a novel cell- and circuit-based mechanistic link between chronic pain and the mPFCCRF → NAcS circuit and provide a potential new therapeutic target for chronic pain.

Multiple integrated social stress induces depressive-like behavioral and neural adaptations in female C57BL/6J mice

Despite women representing most of those affected by major depression, preclinical studies have focused almost exclusively on male subjects, partially due to a lack of ideal animal paradigms. As the persistent need regarding the sex balance of neuroscience research and female-specific pathology of mental disorders surges, the establishment of natural etiology-based and systematically validated animal paradigms for depression with female subjects becomes an urgent scientific problem. This study aims to establish, characterize, and validate a "Multiple Integrated Social Stress (MISS)" model of depression in female C57BL/6J mice by manipulating and integrating daily social stressors that females are experiencing. Female C57BL/6J mice randomly experienced social competition failure in tube test, modified vicarious social defeat stress, unescapable overcrowding stress followed by social isolation on each day, for ten consecutive days. Compared with their controls, female MISS mice exhibited a relatively decreased preference for social interaction and sucrose, along with increased immobility in the tail suspension test, which could last for at least one month. These MISS mice also exhibited increased levels of blood serum corticosterone, interleukin-6 L and 1β. In the pharmacological experiment, MISS-induced dysfunctions in social interaction, sucrose preference, and tail suspension tests were amended by systematically administrating a single dose of sub-anesthetic ketamine, a rapid-onset antidepressant. Compared with controls, MISS females exhibited decreased c-Fos activation in their anterior cingulate cortex, prefrontal cortex, nucleus accumbens and some other depression-related brain regions. Furthermore, 24 h after the last exposure to the paradigm, MISS mice demonstrated a decreased center zone time in the open field test and decreased open arm time in the elevated plus-maze test, indicating anxiety-like behavioral phenotypes. Interestingly, MISS mice developed an excessive nesting ability, suggesting a likely behavioral phenotype of obsessive-compulsive disorder. These data showed that the MISS paradigm was sufficient to generate pathological profiles in female mice to mimic core symptoms, serum biochemistry and neural adaptations of depression in clinical patients. The present study offers a multiple integrated natural etiology-based animal model tool for studying female stress susceptibility.

Gabapentin improves neuropathic pain in Minamata disease model rats

BACKGROUND

Methylmercury (MeHg), the causative agent of Minamata disease, damages the cranial nervous system and causes specific sensory disturbances, especially hypoesthesia, in the extremities. However, recent reports demonstrate that patients with chronic Minamata disease conversely develop neuropathic pain in the lower extremities. Studies on our established Minamata disease model rats showed that MeHg-mediated neurodegeneration might induce neuropathic pain by over time through inducing rewiring with neuronal activation in the somatosensory cortex via microglial activation in the spinal dorsal horn.

METHODS

In this study, the effects of gabapentin, a potentially effective treatment for neuropathic pain, was evaluated using this Minamata disease model rats. To further elucidate the mechanism of its medicinal effects, histochemical and biochemical analyses of the nervous system of Minamata disease model rats were conducted.

RESULTS

Gabapentin treatment restored the reduction in the pain threshold caused by MeHg exposure in rats. Histochemical and biochemical analyses revealed that gabapentin showed no effect on MeHg-induced neurodegeneration in entire nervous system and microglial activation in the spinal dorsal horn. However, it was shown that gabapentin may reduce excessive synaptogenesis through its antagonist action on the alpha2-delta-1 subunit of calcium channels in the somatosensory cortex.

CONCLUSIONS

These results indicate that gabapentin may alleviated neuropathic pain in MeHg poisoning, as typified by Minamata disease, by reversibly modulation synaptic rewiring in the somatosensory cortex.

Limbic System Response to Psilocybin and Ketamine Administration in Rats: A Neurochemical and Behavioral Study

The pathophysiology of depression is related to the reduced volume of the hippocampus and amygdala and hypertrophy of the nucleus accumbens. The mechanism of these changes is not well understood; however, clinical studies have shown that the administration of the fast-acting antidepressant ketamine reversed the decrease in hippocampus and amygdala volume in depressed patients, and the magnitude of this effect correlated with the reduction in depressive symptoms. In the present study, we attempted to find out whether the psychedelic substance psilocybin affects neurotransmission in the limbic system in comparison to ketamine. Psilocybin and ketamine increased the release of dopamine (DA) and serotonin (5-HT) in the nucleus accumbens of naive rats as demonstrated using microdialysis. Both drugs influenced glutamate and GABA release in the nucleus accumbens, hippocampus and amygdala and increased ACh levels in the hippocampus. The changes in D2, 5-HT1A and 5-HT2A receptor density in the nucleus accumbens and hippocampus were observed as a long-lasting effect. A marked anxiolytic effect of psilocybin in the acute phase and 24 h post-treatment was shown in the open field test. These data provide the neurobiological background for psilocybin's effect on stress, anxiety and structural changes in the limbic system and translate into the antidepressant effect of psilocybin in depressed patients.

Early life stress in male mice blunts responsiveness in a translationally-relevant reward task

Early-life stress (ELS) leaves signatures upon the brain that persist throughout the lifespan and increase the risk of psychiatric illnesses including mood and anxiety disorders. In humans, myriad forms of ELS-including childhood abuse, bullying, poverty, and trauma-are increasingly prevalent. Understanding the signs of ELS, including those associated with psychiatric illness, will enable improved treatment and prevention. Here, we developed a novel procedure to model human ELS in mice and identify translationally-relevant biomarkers of mood and anxiety disorders. We exposed male mice (C57BL/6 J) to an early-life (juvenile) chronic social defeat stress (jCSDS) and examined social interaction and responsivity to reward during adulthood. As expected, jCSDS-exposed mice showed a socially avoidant phenotype in open-field social interaction tests. However, sucrose preference tests failed to demonstrate ELS-induced reductions in choice for the sweetened solution, suggesting no effect on reward function. To explore whether other tasks might be more sensitive to changes in motivation, we tested the mice in the Probabilistic Reward Task (PRT), a procedure often used in humans to study reward learning deficits associated with depressive illness. In a touchscreen PRT variant that was reverse-translated to maximize alignment with the version used in human subjects, mice exposed to jCSDS displayed significant reductions in the tendency to develop response biases for the more richly-rewarded stimulus, a hallmark sign of anhedonia when observed in humans. Our findings suggest that translationally-relevant procedures that utilize the same endpoints across species may enable the development of improved model systems that more accurately predict outcomes in humans.

HCN channel inhibitor induces ketamine-like rapid and sustained antidepressant effects in chronic social defeat stress model

Repeated, long-term (weeks to months) exposure to standard antidepressant medications is required to achieve treatment efficacy. In contrast, acute ketamine quickly improves mood for an extended time. Recent work implicates that hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are involved in mediating ketamine's antidepressant effects. In this study, we directly targeted HCN channels and achieved ketamine-like rapid and sustained antidepressant efficacy. Our in vitro electrophysiological recordings first showed that HCN inhibitor DK-AH 269 (also called cilobradine) decreased the pathological HCN-mediated current (Ih) and abnormal hyperactivity of ventral tegmental area (VTA) dopamine (DA) neurons in a depressive-like model produced by chronic social defeat stress (CSDS). Our in vivo studies further showed that acute intra-VTA or acute systemic administration of DK-AH 269 normalized social behavior and rescued sucrose preference in CSDS-susceptible mice. The single-dose of DK-AH 269, both by intra-VTA microinfusion and intraperitoneal (ip) approaches, could produce an extended 13-day duration of antidepressant-like efficacy. Animals treated with acute DK-AH 269 spent less time immobile than vehicle-treated mice during forced swim test. A social behavioral reversal lasted up to 13 days following the acute DK-AH 269 ip injection, and this rapid and sustained antidepressant-like response is paralleled with a single-dose treatment of ketamine. This study provides a novel ion channel target for acutely acting, long-lasting antidepressant-like effects.

CD200 in dentate gyrus improves depressive-like behaviors of mice through enhancing hippocampal neurogenesis via alleviation of microglia hyperactivation

BACKGROUND

Neuroinflammation and microglia play critical roles in the development of depression. Cluster of differentiation 200 (CD200) is an anti-inflammatory glycoprotein that is mainly expressed in neurons, and its receptor CD200R1 is primarily in microglia. Although the CD200-CD200R1 pathway is necessary for microglial activation, its role in the pathophysiology of depression remains unknown.

METHODS

The chronic social defeat stress (CSDS) with behavioral tests were performed to investigate the effect of CD200 on the depressive-like behaviors. Viral vectors were used to overexpress or knockdown of CD200. The levels of CD200 and inflammatory cytokines were tested with molecular biological techniques. The status of microglia, the expression of BDNF and neurogenesis were detected with immunofluorescence imaging.

RESULTS

We found that the expression of CD200 was decreased in the dentate gyrus (DG) region of mice experienced CSDS. Overexpression of CD200 alleviated the depressive-like behaviors of stressed mice and inhibition of CD200 facilitated the susceptibility to stress. When CD200R1 receptors on microglia were knocked down, CD200 was unable to exert its role in alleviating depressive-like behavior. Microglia in the DG brain region were morphologically activated after exposure to CSDS. In contrast, exogenous administration of CD200 inhibited microglia hyperactivation, alleviated neuroinflammatory response in hippocampus, and increased the expression of BDNF, which in turn ameliorated adult hippocampal neurogenesis impairment in the DG induced by CSDS.

CONCLUSIONS

Taken together, these results suggest that CD200-mediated alleviation of microglia hyperactivation contributes to the antidepressant effect of neurogenesis in dentate gyrus in mice.

The laterodorsal tegmentum-ventral tegmental area circuit controls depression-like behaviors by activating ErbB4 in DA neurons

Dopamine (DA) neurons in the ventral tegmental area (VTA) are critical to coping with stress. However, molecular mechanisms regulating their activity and stress-induced depression were not well understood. We found that the receptor tyrosine kinase ErbB4 in VTA was activated in stress-susceptible mice. Deleting ErbB4 in VTA or in DA neurons, or chemical genetic inhibition of ErbB4 kinase activity in VTA suppressed the development of chronic social defeat stress (CSDS)-induced depression-like behaviors. ErbB4 activation required the expression of NRG1 in the laterodorsal tegmentum (LDTg); LDTg-specific deletion of NRG1 inhibited depression-like behaviors. NRG1 and ErbB4 suppressed potassium currents of VTA DA neurons and increased their firing activity. Finally, we showed that acute inhibition of ErbB4 after stress attenuated DA neuron hyperactivity and expression of depression-like behaviors. Together, these observations demonstrate a critical role of NRG1-ErbB4 signaling in regulating depression-like behaviors and identify an unexpected mechanism by which the LDTg-VTA circuit regulates the activity of DA neurons.

Stress-induced plasticity of a CRH/GABA projection disrupts reward behaviors in mice

Disrupted operations of the reward circuit underlie major emotional disorders, including depression, which commonly arise following early life stress / adversity (ELA). However, how ELA enduringly impacts reward circuit functions remains unclear. We characterize a stress-sensitive projection connecting basolateral amygdala (BLA) and nucleus accumbens (NAc) that co-expresses GABA and the stress-reactive neuropeptide corticotropin-releasing hormone (CRH). We identify a crucial role for this projection in executing disrupted reward behaviors provoked by ELA: chemogenetic and optogenetic stimulation of the projection in control male mice suppresses several reward behaviors, recapitulating deficits resulting from ELA and demonstrating the pathway's contributions to normal reward behaviors. In adult ELA mice, inhibiting-but not stimulating-the projection, restores typical reward behaviors yet has little effect in controls, indicating ELA-induced maladaptive plasticity of this reward-circuit component. Thus, we discover a stress-sensitive, reward inhibiting BLA → NAc projection with unique molecular features, which may provide intervention targets for disabling mental illnesses.

Neural circuits regulating prosocial behaviors

Positive, prosocial interactions are essential for survival, development, and well-being. These intricate and complex behaviors are mediated by an amalgamation of neural circuit mechanisms working in concert. Impairments in prosocial behaviors, which occur in a large number of neuropsychiatric disorders, result from disruption of the coordinated activity of these neural circuits. In this review, we focus our discussion on recent findings that utilize modern approaches in rodents to map, monitor, and manipulate neural circuits implicated in a variety of prosocial behaviors. We highlight how modulation by oxytocin, serotonin, and dopamine of excitatory and inhibitory synaptic transmission in specific brain regions is critical for regulation of adaptive prosocial interactions. We then describe how recent findings have helped elucidate pathophysiological mechanisms underlying the social deficits that accompany neuropsychiatric disorders. We conclude by discussing approaches for the development of more efficacious and targeted therapeutic interventions to ameliorate aberrant prosocial behaviors.

Noradrenergic modulation of stress resilience

Resilience represents an active adaption process in the face of adversity, trauma, tragedy, threats, or significant sources of stress. Investigations of neurobiological mechanisms of resilience opens an innovative direction for preclinical research and drug development for various stress-related disorders. The locus coeruleus norepinephrine system has been implicated in mediating stress susceptibility versus resilience. It has attracted increasing attention over the past decades with the revolution of modern neuroscience technologies. In this review article, we first briefly go over resilience-related concepts and introduce rodent paradigms for segregation of susceptibility and resilience, then highlight recent literature that identifies the neuronal and molecular substrates of active resilience in the locus coeruleus, and discuss possible future directions for resilience investigations.

Fasudil, a ROCK inhibitor, prevents neuropathic pain in Minamata disease model rats

Methylmercury (MeHg), an environmental toxicant, is known to cause sensory impairment by inducing neurodegeneration of sensory nervous systems. However, in recent years, it has been revealed that neuropathic pain occurs in the chronic phase of MeHg poisoning, that is, in current Minamata disease patients. Our recent study using Minamata disease model rats demonstrated that MeHg-mediated neurodegeneration in the sensory nervous system may induce inflammatory microglia production in the dorsal horn of the spinal cord and subsequent somatosensory cortical rewiring, leading to neuropathic pain. We hypothesized that inhibition of the Rho-associated coiled coil-forming protein kinase (ROCK) pathway could prevent MeHg-induced neuropathic pain because the ROCK pathway is known to be involved in inducing the production of inflammatory microglia. Here, we showed for the first time that Fasudil, a ROCK inhibitor, can prevent neuropathic pain in Minamata disease model rats. In this model, Fasudil significantly suppressed nerve injury-induced inflammatory microglia production in the dorsal horn of the spinal cord and prevented subsequent somatosensory cortical rewiring. These results suggest that the ROCK pathway is involved in the onset and development of neuropathic pain in the chronic phase of Minamata disease, and that its inhibition is effective in pain prevention.

The Role of Beta-Adrenergic Receptors in Depression and Resilience

Norepinephrine is a catecholamine neurotransmitter that has been extensively implicated in the neurobiology of major depressive disorder (MDD). An accumulating body of evidence indicates that investigations into the action of norepinephrine at the synaptic/receptor level hold high potential for a better understanding of MDD neuropathology and introduce possibilities for developing novel treatments for depression. In this review article, we discuss recent advances in depression neuropathology and the effects of antidepressant medications based on preclinical and clinical studies related to beta-adrenergic receptor subtypes. We also highlight a beta-3 adrenergic receptor-involved mechanism that promotes stress resilience, through which antidepressant efficacy is achieved in both rodent models for depression and patients with major depression-an alternative therapeutic strategy that is conceptually different from the typical therapeutic approach in which treatment efficacy is achieved by reversing pathological alterations rather than by enhancing a good mechanism such as natural resilience. Altogether, in this review, we systematically describe the role of beta-adrenergic receptors in depression and stress resilience and provide a new avenue for developing a conceptually innovative treatment for depression.

Life extension factor klotho regulates behavioral responses to stress via modulation of GluN2B function in the nucleus accumbens

Klotho is a life extension factor that has the ability to regulate the function of GluN2B-containing N-methyl-D-aspartate receptors (NMDARs), whose dysfunction in the nucleus accumbens (NAc) underlies critical aspects of the pathophysiology of major depression. Here, we study the functional relevance of klotho in the pathogenesis of depression. A chronic social defeat stress paradigm, in which mice are categorized as either susceptible or unsusceptible based on their performance in a social interaction test, was used in this study. We found that the expression of klotho was largely decreased in the NAc of susceptible mice compared to control or unsusceptible mice. Genetic knockdown of klotho in the NAc induced behavioral alterations relevant to depression in naive mice, while overexpression of klotho produced an antidepressive effect in normal mice and ameliorated the behavioral responses to stress in susceptible mice. Molecularly, knockdown of klotho in the NAc resulted in selective decreases in total and synaptic GluN2B expression that were identical to those in susceptible mice. Elevation of klotho in the NAc reversed the reductions in GluN2B expressions and altered synaptic transmission and spine density in the NAc of susceptible mice. Furthermore, blockade of GluN2B with a specific antagonist abolished the beneficial effects of klotho elevation in susceptible mice. Collectively, we demonstrated that klotho in the NAc modulates behavioral responses to stress by regulating the function of GluN2B-containing NMDARs. These results reveal a novel role for klotho in the pathogenesis of depression, providing new insights into the molecular basis of major depression.

Metabotropic Glutamate Receptor 5 in Amygdala Target Neurons Regulates Susceptibility to Chronic Social Stress

BACKGROUND

Metabotropic glutamate receptor 5 (mGluR5) has been implicated in stress-related psychiatric disorders, particularly major depressive disorder. Although growing evidence supports the proresilient role of mGluR5 in corticolimbic circuitry in the depressive-like behaviors following chronic stress exposure, the underlying neural mechanisms, including circuits and molecules, remain unknown.

METHODS

We measured the c-Fos expression and probability of neurotransmitter release in and from basolateral amygdala (BLA) neurons projecting to the medial prefrontal cortex (mPFC) and to the ventral hippocampus (vHPC) after chronic social defeat stress. The role of BLA projections in depressive-like behaviors was assessed using optogenetic manipulations, and the underlying molecular mechanisms of mGluR5 and downstream signaling were investigated by Western blotting, viral-mediated gene transfer, and pharmacological manipulations.

RESULTS

Chronic social defeat stress disrupted neural activity and glutamatergic transmission in both BLA projections. Optogenetic activation of BLA projections reversed the detrimental effects of chronic social defeat stress on depressive-like behaviors and mGluR5 expression in the mPFC and vHPC. Conversely, inhibition of BLA projections of mice undergoing subthreshold social defeat stress induced a susceptible phenotype and mGluR5 reduction. These two BLA circuits appeared to act in an independent way. We demonstrate that mGluR5 overexpression in the mPFC or vHPC was proresilient while the mGluR5 knockdown was prosusceptible and that the proresilient effects of mGluR5 are mediated through distinctive downstream signaling pathways in the mPFC and vHPC.

CONCLUSIONS

These findings identify mGluR5 in the mPFC and vHPC that receive BLA inputs as a critical mediator of stress resilience, highlighting circuit-specific signaling for depressive-like behaviors.

Opposite effects of stress on effortful motivation in high and low anxiety are mediated by CRHR1 in the VTA

Individuals frequently differ in their behavioral and cognitive responses to stress. However, whether motivation is differently affected by acute stress in different individuals remains to be established. By exploiting natural variation in trait anxiety in outbred Wistar rats, we show that acute stress facilitates effort-related motivation in low anxious animals, while dampening effort in high anxious ones. This model allowed us to address the mechanisms underlying acute stress-induced differences in motivated behavior. We show that CRHR1 expression levels in dopamine neurons of the ventral tegmental area (VTA)-a neuronal type implicated in the regulation of motivation-depend on animals' anxiety, and these differences in CRHR1 expression levels explain the divergent effects of stress on both effortful behavior and the functioning of mesolimbic DA neurons. These findings highlight CRHR1 in VTA DA neurons-whose levels vary with individuals' anxiety-as a switching mechanism determining whether acute stress facilitates or dampens motivation.

Midbrain projection to the basolateral amygdala encodes anxiety-like but not depression-like behaviors

Anxiety disorders are complex diseases, and often co-occur with depression. It is as yet unclear if a common neural circuit controls anxiety-related behaviors in both anxiety-alone and comorbid conditions. Here, utilizing the chronic social defeat stress (CSDS) paradigm that induces singular or combined anxiety- and depressive-like phenotypes in mice, we show that a ventral tegmental area (VTA) dopamine circuit projecting to the basolateral amygdala (BLA) selectively controls anxiety- but not depression-like behaviors. Using circuit-dissecting ex vivo electrophysiology and in vivo fiber photometry approaches, we establish that expression of anxiety-like, but not depressive-like, phenotypes are negatively correlated with VTA → BLA dopamine neuron activity. Further, our optogenetic studies demonstrate a causal link between such neuronal activity and anxiety-like behaviors. Overall, these data establish a functional role for VTA → BLA dopamine neurons in bi-directionally controlling anxiety-related behaviors not only in anxiety-alone, but also in anxiety-depressive comorbid conditions in mice.

The Potential of KCNQ Potassium Channel Openers as Novel Antidepressants

Major depressive disorder (MDD) is a leading cause of disability worldwide and less than one-third of patients with MDD achieve stable remission of symptoms, despite currently available treatments. Although MDD represents a serious health problem, a complete understanding of the neurobiological mechanisms underlying this condition continues to be elusive. Accumulating evidence from preclinical and animal studies provides support for the antidepressant potential of modulators of KCNQ voltage-gated potassium (K⁺) channels. KCNQ K⁺ channels, through regulation of neuronal excitability and activity, contribute to neurophysiological mechanisms underlying stress resilience, and represent potential targets of drug discovery for depression. The present article focuses on the pharmacology and efficacy of KCNQ2/3 K⁺ channel openers as novel therapeutic agents for depressive disorders from initial studies conducted on animal models showing depressive-like behaviors to recent work in humans that examines the potential for KCNQ2/3 channel modulators as novel antidepressants. Data from preclinical work suggest that KCNQ-type K⁺ channels are an active mediator of stress resilience and KCNQ2/3 K⁺ channel openers show antidepressant efficacy. Similarly, evidence from clinical trials conducted in patients with MDD using the KCNQ2/3 channel opener ezogabine (retigabine) showed significant improvements in depressive symptoms and anhedonia. Overall, KCNQ channel openers appear a promising target for the development of novel therapeutics for the treatment of psychiatric disorders and specifically for MDD.

Anhedonia, Hyperkatifeia, and Negative Reinforcement in Substance Use Disorders

Drug addiction has been defined as a chronically relapsing disorder that is characterized by a compulsion to seek and take a drug or stimulus, the loss of control in limiting intake, and the emergence of a negative emotional state when access to the drug or stimulus is prevented, a component of which is anhedonia. The present review explores a heuristic framework for understanding the role of anhedonia in addiction, in which anhedonia is a key component of hyperkatifeia (conceptualized as the potentiated intensity of negative emotional/motivational symptoms during drug withdrawal) and negative reinforcement in addiction. The neural substrates that mediate such anhedonia and crosstalk between elements of hyperkatifeia that contribute to anhedonia are then explored, including crosstalk between physical pain and emotional pain systems. The present review explores current knowledge of neurochemical neurocircuitry changes that are associated with conditioned hyperkatifeia/anhedonia. The overall hypothesis is that the shift in motivation toward negative reinforcement in addiction reflects the allostatic misregulation of hedonic tone, such that drug taking makes anhedonia worse during the process of seeking temporary relief by compulsive drug taking, thereby perpetuating the addiction cycle and hedonic comorbidities that are associated with addiction.

Developmental Trajectories of Anhedonia in Preclinical Models

This chapter discusses how the complex concept of anhedonia can be operationalized and studied in preclinical models. It provides information about the development of anhedonia in the context of early-life adversity, and the power of preclinical models to tease out the diverse molecular, epigenetic, and network mechanisms that are responsible for anhedonia-like behaviors.Specifically, we first discuss the term anhedonia, reviewing the conceptual components underlying reward-related behaviors and distinguish anhedonia pertaining to deficits in motivational versus consummatory behaviors. We then describe the repertoire of experimental approaches employed to study anhedonia-like behaviors in preclinical models, and the progressive refinement over the past decade of both experimental instruments (e.g., chemogenetics, optogenetics) and conceptual constructs (salience, valence, conflict). We follow with an overview of the state of current knowledge of brain circuits, nodes, and projections that execute distinct aspects of hedonic-like behaviors, as well as neurotransmitters, modulators, and receptors involved in the generation of anhedonia-like behaviors. Finally, we discuss the special case of anhedonia that arises following early-life adversity as an eloquent example enabling the study of causality, mechanisms, and sex dependence of anhedonia.Together, this chapter highlights the power, potential, and limitations of using preclinical models to advance our understanding of the origin and mechanisms of anhedonia and to discover potential targets for its prevention and mitigation.

Decreased Serum Brain-Derived Neurotrophic Factor in Poststroke Depression: A Systematic Review and Meta-Analysis

BACKGROUNDS

There were conflicting results on the comparison of brain-derived neurotrophic factor (BDNF) levels between poststroke depression (PSD) patients and stroke patients without PSD among previous studies. Thus, we conducted this systemic review and meta-analysis to explore the alteration of serum BDNF levels in PSD.

METHODS

This study included articles from the Web of Science and PubMed databases that were published before December 2021. STATA 12.0 software was used to compute the standardized mean difference (SMD) and 95% confidence interval (CI) regarding the comparison of serum BDNF in PSD and stroke patients without PSD.

RESULTS

We collected the mean value and standard deviation (SD) of serum BDNF in PSD and stroke patients without PSD from six studies (PSD: n = 268, stroke patients without PSD: n = 425). The present meta-analysis showed decreased serum BDNF level in patients with PSD, compared to stroke patients without PSD with a random-effects model (mean value of BDNF level [PSD vs. stroke patients without PSD]: 14.106 vs. 17.995 ng/ml; SMD = -1.578; 95% CI: -2.820, -0.337; I ² = 97.8%, p-value for Q test < 0.001).

CONCLUSION

Brain-derived neurotrophic factor may work as a potential biomarker to predict the risk of PSD among stroke survivors. More large-sample clinical trials exploring the alteration of serum BDNF levels in PSD among stroke patients need to be conducted to verify this result.

Advances in optogenetic studies of depressive-like behaviors and underlying neural circuit mechanisms

BACKGROUNDS

The neural circuit mechanisms underlying depression remain unclear. Recently optogenetics has gradually gained recognition as a novel technique to regulate the activity of neurons with light stimulation. Scientists are now transferring their focus to the function of brain regions and neural circuits in the pathogenic progress of depression. Deciphering the circuitry mechanism of depressive-like behaviors may help us better understand the symptomatology of depression. However, few studies have summarized current progress on optogenetic researches into the neural circuit mechanisms of depressive-like behaviors.

AIMS

This review aimed to introduce fundamental characteristics and methodologies of optogenetics, as well as how this technique achieves specific neuronal control with spatial and temporal accuracy. We mainly summarized recent progress in neural circuit discoveries in depressive-like behaviors using optogenetics and exhibited the potential of optogenetics as a tool to investigate the mechanism and possible optimization underlying antidepressant treatment such as ketamine and deep brain stimulation.

METHODS

A systematic review of the literature published in English mainly from 2010 to the present in databases was performed. The selected literature is then categorized and summarized according to their neural circuits and depressive-like behaviors.

CONCLUSIONS

Many important discoveries have been made utilizing optogenetics. These findings support optogenetics as a powerful and potential tool for studying depression. And our comprehension to the etiology of depression and other psychiatric disorders will also be more thorough with this rapidly developing technique in the near future.

Efficacy of continuous theta burst stimulation - repetitive trancranial magnetic stimulation on the orbito frontal cortex as an adjunct to naltrexone in patients of opioid use disorder and its correlation with serum BDNF levels: a sham-controlled study

Background: Opoid use disorder (OUD) is a global illness and reduction in craving by repeatative Transcranial Magnetic Stimulation (RTMS) is one of its management approaches. Orbito-frontal Cortex is implicated in the several behavioral aspects of substance use including craving. Brain derived neurotrophic factor (BDNF) has a critical role in addictive properties of drugs of use. Previous studies have shown significant improvement in craving with RTMS and demonstrated alterations of serum BDNF levels in various substance dependent individual associated with craving. Aim: To examine the efficacy of continuous Theta Burst Stimulation RTMS (CTBS-RTMS) over the right OFC as an adjunct to Naltrexone in patients of OUD and its correlation with serum BDNF levels. Methods: Forty patients with OUD were recruited with purposive sampling. At the end of detoxification CTBS -RTMS was applied by dividing them into two equal groups as active and sham group using alternate allocation. Obsessive compulsive drug use scale (OCDUS) was applied and serum BDNF level was measured overtime till the end of CTBS-RTMS session. Data was analyzed by SPSS version 25. Results: Both groups had shown significant reduction in craving (OCDUS score) and serum BDNF from the baseline to 14th session of the RTMS. But there was no significant difference when compared between the two groups. Significant correlation was observed between serum BDNF levels overtime with different clinical variables in active group. Conclusion: The study adds to the literature in building an understanding of how rTMS could be used in reducing cravings for opioids.

Sleep-wake dynamics pre- and post-exposure to chronic social stress

An analytical approach combining the statistical distributions of the sleep-wake bouts and the Markov transition matrix is used to explain the under-examined association between the microarchitecture of the sleep-wake cycle and susceptibility to chronic social stress in C57BL/6J mice. We separated the sleep-wake transitions into distinct sleep-wake sequences, NREM↔Wake and NREM→REM→Wake, which are controlled by independent neural circuits. Our findings imply greater pull toward the wake leading to early termination and fragmentation of the sleep bouts in the light in both sleep-wake sequences pre- and post-stress. Moreover, the stability of NREM in the NREM↔Wake transition was lower, and the probability of transitioning to wake was higher in susceptible relative to resilient or stress-naïve mice pre- and post-stress. Our findings help elucidate the mechanistic interplay between sleep and mood by suggesting the potential neural underpinnings of sleep disturbances responsible the aberrant transitions of sleep-wake bouts exhibited by the stress-susceptible phenotype.

Central Nervous System Associated With Light Perception and Physiological Responses of Birds

Environmental light that animal receives (i.e., photoperiod and light intensity) has recently been shown that it affects avian central nervous system for the physiological responses to the environment by up or downregulation of dopamine and serotonin activities, and this, in turn, affects the reproductive function and stress-related behavior of birds. In this study, the author speculated on the intriguing possibility that one of the proposed avian deep-brain photoreceptors (DBPs), i.e., melanopsin (Opn4), may play roles in the dual sensory-neurosecretory cells in the hypothalamus, midbrain, and brain stem for the behavior and physiological responses of birds by light. Specifically, the author has shown that the direct light perception of premammillary nucleus dopamine-melatonin (PMM DA-Mel) neurons is associated with the reproductive activation in birds. Although further research is required to establish the functional role of Opn4 in the ventral tegmental area (VTA), dorsal raphe nucleus, and caudal raphe nucleus in the light perception and physiological responses of birds, it is an exciting prospect because the previous results in birds support this hypothesis that Opn4 in the midbrain DA and serotonin neurons may play significant roles on the light-induced welfare of birds.

Binge-like Alcohol Exposure in Adolescence: Behavioural, Neuroendocrine and Molecular Evidence of Abnormal Neuroplasticity… and Return

Binge alcohol consumption among adolescents affects the developing neural networks underpinning reward and stress processing in the nucleus accumbens (NAc). This study explores in rats the long-lasting effects of early intermittent exposure to intoxicating alcohol levels at adolescence, on: (1) the response to natural positive stimuli and inescapable stress; (2) stress-axis functionality; and (3) dopaminergic and glutamatergic neuroadaptation in the NAc. We also assess the potential effects of the non-intoxicating phytocannabinoid cannabidiol, to counteract (or reverse) the development of detrimental consequences of binge-like alcohol exposure. Our results show that adolescent binge-like alcohol exposure alters the sensitivity to positive stimuli, exerts social and novelty-triggered anxiety-like behaviour, and passive stress-coping during early and prolonged withdrawal. In addition, serum corticosterone and hypothalamic and NAc corticotropin-releasing hormone levels progressively increase during withdrawal. Besides, NAc tyrosine hydroxylase levels increase at late withdrawal, while the expression of dopamine transporter, D1 and D2 receptors is dynamically altered during binge and withdrawal. Furthermore, the expression of markers of excitatory postsynaptic signaling—PSD95; Homer-1 and -2 and the activity-regulated spine-morphing proteins Arc, LIM Kinase 1 and FOXP1—increase at late withdrawal. Notably, subchronic cannabidiol, during withdrawal, attenuates social- and novelty-induced aversion and passive stress-coping and rectifies the hyper-responsive stress axis and NAc dopamine and glutamate-related neuroplasticity. Overall, the exposure to binge-like alcohol levels in adolescent rats makes the NAc, during withdrawal, a locus minoris resistentiae as a result of perturbations in neuroplasticity and in stress-axis homeostasis. Cannabidiol holds a promising potential for increasing behavioural, neuroendocrine and molecular resilience against binge-like alcohol harmful effects.

Striatal Shati/Nat8l-BDNF pathways determine the sensitivity to social defeat stress in mice through epigenetic regulation

The global number of patients with depression increases in correlation to exposure to social stress. Chronic stress does not trigger depression in all individuals, as some remain resilient. The underlying molecular mechanisms that contribute to stress sensitivity have been poorly understood, although revealing the regulation of stress sensitivity could help develop treatments for depression. We previously found that striatal Shati/Nat8l, an N-acetyltransferase, was increased in a depression mouse model. We investigated the roles of Shati/Nat8l in stress sensitivity in mice and found that Shati/Nat8l and brain-derived neurotrophic factor (BDNF) levels in the dorsal striatum were increased in stress-susceptible mice but not in resilient mice exposed to repeated social defeat stress (RSDS). Knockdown of Shati/Nat8l in the dorsal striatum induced resilience to RSDS. In addition, blockade of BDNF signaling in the dorsal striatum by ANA-12, a BDNF-specific receptor tropomyosin-receptor-kinase B (TrkB) inhibitor, also induced resilience to stress. Shati/Nat8l is correlated with BDNF expression after RSDS, and BDNF is downstream of Shati/Nat8l pathways in the dorsal striatum; Shati/Nat8l is epigenetically regulated by BDNF via histone acetylation. Our results demonstrate that striatal Shati/Nat8l-BDNF pathways determine stress sensitivity through epigenetic regulation. The striatal Shati/Nat8l-BDNF pathway could be a novel target for treatments of depression and could establish a novel therapeutic strategy for depression patients.

The Neurocircuitry of Posttraumatic Stress Disorder and Major Depression: Insights Into Overlapping and Distinct Circuit Dysfunction-A Tribute to Ron Duman

The neurocircuitry that contributes to the pathophysiology of posttraumatic stress disorder and major depressive disorder, psychiatric conditions that exhibit a high degree of comorbidity, likely involves both overlapping and unique structural and functional changes within multiple limbic brain regions. In this review, we discuss neurobiological alterations that are associated with posttraumatic stress disorder and major depressive disorder and highlight both similarities and differences that may exist between these disorders to argue for the existence of a shared neurobiology. We highlight the key contributions based on preclinical studies, emerging from the late Professor Ronald Duman's research, that have shaped our understanding of the neurocircuitry that contributes to both the etiopathology and treatment of major depressive disorder and posttraumatic stress disorder.

Light Modulation of Brain and Development of Relevant Equipment

Light modulation plays an important role in understanding the pathology of brain disorders and improving brain function. Optogenetic techniques can activate or silence targeted neurons with high temporal and spatial accuracy and provide precise control, and have recently become a method for quick manipulation of genetically identified types of neurons. Photobiomodulation (PBM) is light therapy that utilizes non-ionizing light sources, including lasers, light emitting diodes, or broadband light. It provides a safe means of modulating brain activity without any irreversible damage and has established optimal treatment parameters in clinical practice. This manuscript reviews 1) how optogenetic approaches have been used to dissect neural circuits in animal models of Alzheimer's disease, Parkinson's disease, and depression, and 2) how low level transcranial lasers and LED stimulation in humans improves brain activity patterns in these diseases. State-of-the-art brain machine interfaces that can record neural activity and stimulate neurons with light have good prospects in the future.

Nucleus Accumbens Tac1-Expressing Neurons Mediate Stress-Induced Anhedonia-like Behavior in Mice

Major depressive disorder (MDD) presents with two primary symptoms: depressed mood and anhedonia, which suggests that distinct neuronal circuits may regulate MDD. However, the underlying circuits of these individual symptoms linked to depression remain elusive. Herein, we identify a discrete circuit of tachykinin precursor 1 (Tac1)-expressing neurons in the nucleus accumbens (NAc) lateral shell, which project to ventral pallidum and contribute to stress-induced anhedonia-like behavior. Selective inhibition and activation of Tac1^NAc neurons bidirectionally modulate stress susceptibility, revealing that Tac1 neurons in the NAc are critical for regulating anhedonia-like behaviors. We find that a subpopulation of VP neurons receives inhibitory inputs from Tac1^NAc neurons and exhibits decreased excitability in susceptible mice. Furthermore, the inhibition of the neurokinin 1 receptor promotes susceptibility to social stress. Overall, our study reveals a discrete circuit regulating anhedonia-like behavior in mice.

Long-term behavioral and cell-type-specific molecular effects of early life stress are mediated by H3K79me2 dynamics in medium spiny neurons

Animals susceptible to chronic social defeat stress (CSDS) exhibit depression-related behaviors, with aberrant transcription across several limbic brain regions, most notably in the nucleus accumbens (NAc). Early life stress (ELS) promotes susceptibility to CSDS in adulthood, but associated enduring changes in transcriptional control mechanisms in the NAc have not yet been investigated. In this study, we examined long-lasting changes to histone modifications in the NAc of male and female mice exposed to ELS. Dimethylation of lysine 79 of histone H3 (H3K79me2) and the enzymes (DOT1L and KDM2B) that control this modification are enriched in D2-type medium spiny neurons and are shown to be crucial for the expression of ELS-induced stress susceptibility. We mapped the site-specific regulation of this histone mark genome wide to reveal the transcriptional networks it modulates. Finally, systemic delivery of a small molecule inhibitor of DOT1L reversed ELS-induced behavioral deficits, indicating the clinical relevance of this epigenetic mechanism.

Ketamine for post-traumatic stress disorders and it's possible therapeutic mechanism

Posttraumatic stress disorder (PTSD) is a devastating medical illness, for which currently available pharmacotherapies have poor efficacy. Accumulating evidence from clinical and preclinical animal investigations supports that ketamine exhibits a rapid and persistent effect against PTSD, though the underlying molecular mechanism remains to be clarified. In this literature review, we recapitulate the achievements from early ketamine studies to the most up-to-date discoveries, with an effort to discuss an inclusive therapeutic role of ketamine for PTSD treatment and its possible therapeutic mechanism. Ketamine seems to have an inimitable mechanism of action entailing glutamate modulation via actions at the N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors, as well as downstream activation of brain-derived neurotrophic factor (BDNF) and mechanistic target of rapamycin (mTOR) signaling pathways to potentiate synaptic plasticity.

Methylmercury induces hyperalgesia/allodynia through spinal cord dorsal horn neuronal activation and subsequent somatosensory cortical circuit formation in rats

Methylmercury (MeHg) is known to cause serious neurological deficits in humans. In this study, we investigated the occurrence of MeHg-mediated neuropathic pain and identified the underlying pathophysiological mechanism in a rat model of MeHg exposure. Rats were exposed to MeHg (20 ppm in drinking water) for 3 weeks. Neurological damage was observed in the primary afferent neuronal system, including the dorsal root nerve and the dorsal column of the spinal cord. The MeHg-exposed rats showed hyperalgesia/allodynia, compared to controls, as evidenced by a significant decrease in the threshold of mechanical pain evaluated using an algometer with calibrated forceps. Immunohistochemistry revealed the accumulation of activated microglia in the dorsal root nerve, dorsal column, and dorsal horn of the spinal cord. Western blot analyses of the dorsal part of the spinal cord demonstrated an increase in inflammotoxic and inflammatory cytokines and a neuronal activation related protein, phospho-CRE bunding protein (CREB). The results suggest that dorsal horn neuronal activation was mediated by inflammatory factors excreted by accumulated microglia. Furthermore, analyses of the cerebral cortex demonstrated increased expression of phospho-CREB and thrombospondin-1, which is known to be an important factor for excitatory synapse formation, specifically in the somatosensory cortical area. In addition, the expression of pre- and post-synaptic markers was increased in this cortex area. These results suggested that the new cortical circuit was wired specifically in the somatosensory cortex. In conclusion, MeHg-mediated dorsal horn neuronal activation with inflammatory microglia might induce somatosensory cortical rewiring, leading to hyperalgesia/allodynia.

Behaviors Related to Psychiatric Disorders and Pain Perception in C57BL/6J Mice During Different Phases of Estrous Cycle

Robust sex difference among humans regarding psychiatry- and pain-related behaviors is being researched; however, the use of female mice in preclinical research is relatively rare due to an unchecked potential behavioral variation over the estrous cycle. In the present study, a battery of psychiatry- and pain-related behaviors are examined under physiological condition in female C57BL/6J mice over different estrous cycle phases: proestrus, estrous, metestrous, diestrous. Our behavioral results reveal that there is no significant difference over different phases of the estrous cycle in social interaction test, sucrose preference test, tail suspension test, open field test, marble burying test, novelty-suppressed feeding test, Hargreaves thermal pain test, and Von Frey mechanical pain test. These findings implicate those psychiatry- and pain-related behaviors in normal female C57BL/6J mice appear to be relatively consistent throughout the estrous cycle; the estrous cycle might not be a main contributor to female C57BL/6J mice’s variability of behaviors.

Blunted diurnal firing in lateral habenula projections to dorsal raphe nucleus and delayed photoentrainment in stress-susceptible mice

Daily rhythms are disrupted in patients with mood disorders. The lateral habenula (LHb) and dorsal raphe nucleus (DRN) contribute to circadian timekeeping and regulate mood. Thus, pathophysiology in these nuclei may be responsible for aberrations in daily rhythms during mood disorders. Using the 15-day chronic social defeat stress (CSDS) paradigm and in vitro slice electrophysiology, we measured the effects of stress on diurnal rhythms in firing of LHb cells projecting to the DRN (cells^LHb→DRN) and unlabeled DRN cells. We also performed optogenetic experiments to investigate if increased firing in cells^LHb→DRN during exposure to a weak 7-day social defeat stress (SDS) paradigm induces stress-susceptibility. Last, we investigated whether exposure to CSDS affected the ability of mice to photoentrain to a new light–dark (LD) cycle. The cells^LHb→DRN and unlabeled DRN cells of stress-susceptible mice express greater blunted diurnal firing compared to stress-näive (control) and stress-resilient mice. Daytime optogenetic activation of cells^LHb→DRN during SDS induces stress-susceptibility which shows the direct correlation between increased activity in this circuit and putative mood disorders. Finally, we found that stress-susceptible mice are slower, while stress-resilient mice are faster, at photoentraining to a new LD cycle. Our findings suggest that exposure to strong stressors induces blunted daily rhythms in firing in cells^LHb→DRN, DRN cells and decreases the initial rate of photoentrainment in susceptible-mice. In contrast, resilient-mice may undergo homeostatic adaptations that maintain daily rhythms in firing in cells^LHb→DRN and also show rapid photoentrainment to a new LD cycle.

Daily rhythms are disrupted in patients suffering from mood disorders, and it is known that the lateral habenula and dorsal raphe nucleus contribute to circadian timekeeping and regulate mood. This study shows that stress-susceptible mice have blunted and inverted diurnal firing rhythms in lateral habenula cells that project to the dorsal raphe nucleus, and have a slow rate of photoentrainment to a new light cycle.

Neural circuits of social behaviors: Innate yet flexible

Social behaviors, such as mating, fighting, and parenting, are fundamental for survival of any vertebrate species. All members of a species express social behaviors in a stereotypical and species-specific way without training because of developmentally hardwired neural circuits dedicated to these behaviors. Despite being innate, social behaviors are flexible. The readiness to interact with a social target or engage in specific social acts can vary widely based on reproductive state, social experience, and many other internal and external factors. Such high flexibility gives vertebrates the ability to release the relevant behavior at the right moment and toward the right target. This maximizes reproductive success while minimizing the cost and risk associated with behavioral expression. Decades of research have revealed the basic neural circuits underlying each innate social behavior. The neural mechanisms that support behavioral plasticity have also started to emerge. Here we provide an overview of these social behaviors and their underlying neural circuits and then discuss in detail recent findings regarding the neural processes that support the flexibility of innate social behaviors.

Optogenetics: A revolutionary approach for the study of depression

Depression is a severe and chronic mental disorder that affects millions of individuals worldwide. Symptoms include depressed mood, loss of interest, reduced motivation and suicidal thoughts. Even though findings from genetic, molecular and imaging studies have helped provide some clues regarding the mechanisms underlying depression-like behaviors, there are still many unanswered questions that need to be addressed. Optogenetics, a technique developed in the early 2000s, has proved effective in the study and treatment of depression and depression-like behaviors and has revolutionized already known experimental techniques. This technique employs light and genetic tools to either inhibit or excite specific neurons or pathways within the brain. In this review paper, an up-to-date understanding of the use of optogenetics in the study of depression-like behaviors is provided, along with suggestions for future research directions.

Animal models for the study of depressive disorder

Depressive disorder is one of the most widespread forms of psychiatric pathology, worldwide. According to a report by the World Health Organization, the number of people with depression, globally, is increasing dramatically with each year. Previous studies have demonstrated that various factors, including genetics and environmental stress, contribute to the risk of depression. As such, it is crucial to develop a detailed understanding of the pathogenesis of depressive disorder and animal studies are essential for identifying the mechanisms and genetic disorders underlying depression. Recently, many researchers have reported on the pathology of depression via various models of depressive disorder. Given that different animal models of depression show differences in terms of patterns of depressive behavior and pathology, the comparison between depressive animal models is necessary for progress in the field of the depression study. However, the various animal models of depression have not been fully compared or evaluated until now. In this paper, we reviewed the pathophysiology of the depressive disorder and its current animal models with the analysis of their transcriptomic profiles. We provide insights for selecting different animal models for the study of depression.

Shisa6 mediates cell-type specific regulation of depression in the nucleus accumbens

Depression is the leading cause of disability and produces enormous health and economic burdens. Current treatment approaches for depression are largely ineffective and leave more than 50% of patients symptomatic, mainly because of non-selective and broad action of antidepressants. Thus, there is an urgent need to design and develop novel therapeutics to treat depression. Given the heterogeneity and complexity of the brain, identification of molecular mechanisms within specific cell-types responsible for producing depression-like behaviors will advance development of therapies. In the reward circuitry, the nucleus accumbens (NAc) is a key brain region of depression pathophysiology, possibly based on differential activity of D1- or D2- medium spiny neurons (MSNs). Here we report a circuit- and cell-type specific molecular target for depression, Shisa6, recently defined as an AMPAR component, which is increased only in D1-MSNs in the NAc of susceptible mice. Using the Ribotag approach, we dissected the transcriptional profile of D1- and D2-MSNs by RNA sequencing following a mouse model of depression, chronic social defeat stress (CSDS). Bioinformatic analyses identified cell-type specific genes that may contribute to the pathogenesis of depression, including Shisa6. We found selective optogenetic activation of the ventral tegmental area (VTA) to NAc circuit increases Shisa6 expression in D1-MSNs. Shisa6 is specifically located in excitatory synapses of D1-MSNs and increases excitability of neurons, which promotes anxiety- and depression-like behaviors in mice. Cell-type and circuit-specific action of Shisa6, which directly modulates excitatory synapses that convey aversive information, identifies the protein as a potential rapid-antidepressant target for aberrant circuit function in depression.

Depression and substance use disorders: Clinical comorbidity and shared neurobiology

Mood disorders, including major depressive disorder (MDD), are the most prevalent psychiatric illnesses, and pose an incredible burden to society, both in terms of disability and in terms of costs associated with medical care and lost work time. MDD has extremely high rates of comorbidity with substance use disorders (SUD) as many of the same neurobiological circuits and molecular mechanisms regulate the reward pathways disrupted in both conditions. MDD may induce SUDs, SUD may contribute to MDD development, or underlying vulnerabilities and common life experience may confer risk to developing both conditions. In this chapter we explore theories of MDD and SUD comorbidity, the neurobiological underpinnings of depression, overlapping cellular and molecular pathways for both conditions, and current treatment approaches for these comorbid conditions.

Early-life stress alters affective behaviors in adult mice through persistent activation of CRH-BDNF signaling in the oval bed nucleus of the stria terminalis

Early-life stress (ELS) leads to stress-related psychopathology in adulthood. Although dysfunction of corticotropin-releasing hormone (CRH) signaling in the bed nucleus of the stria terminalis (BNST) mediates chronic stress-induced maladaptive affective behaviors that are historically associated with mood disorders such as anxiety and depression, it remains unknown whether ELS affects CRH function in the adult BNST. Here we applied a well-established ELS paradigm (24 h maternal separation (MS) at postnatal day 3) and assessed the effects on CRH signaling and electrophysiology in the oval nucleus of BNST (ovBNST) of adult male mouse offspring. ELS increased maladaptive affective behaviors, and amplified mEPSCs and decreased M-currents (a voltage-gated K⁺ current critical for stabilizing membrane potential) in ovBNST CRH neurons, suggesting enhanced cellular excitability. Furthermore, ELS increased the numbers of CRH⁺ and PACAP⁺ (the pituitary adenylate cyclase-activating polypeptide, an upstream CRH regulator) cells and decreased STEP⁺ (striatal-enriched protein tyrosine phosphatase, a CRH inhibitor) cells in BNST. Interestingly, ELS also increased BNST brain-derived neurotrophic factor (BDNF) expression, indicating enhanced neuronal plasticity. These electrophysiological and behavioral effects of ELS were reversed by chronic application of the CRHR1-selective antagonist R121919 into ovBNST, but not when BDNF was co-administered. In addition, the neurophysiological effects of BDNF on M-currents and mEPSCs in BNST CRH neurons mimic effects and were abolished by PKC antagonism. Together, our findings indicate that ELS results in a long-lasting activation of CRH signaling in the mouse ovBNST. These data highlight a regulatory role of CRHR1 in the BNST and for BDNF signaling in mediating ELS-induced long-term behavioral changes.

Methylmercury-Mediated Oxidative Stress and Activation of the Cellular Protective System

Methylmercury (MeHg) is a well-known neurotoxicant that causes severe intoxication in humans. In Japan, it is referred to as Minamata disease, which involves two characteristic clinical forms: fetal type and adult type depending on the exposed age. In addition to MeHg burden level, individual susceptibility to MeHg plays a role in the manifestation of MeHg toxicity. Research progress has pointed out the importance of oxidative stress in the pathogenesis of MeHg toxicity. MeHg has a high affinity for selenohydryl groups, sulfhydryl groups, and selenides. It has been clarified that such affinity characteristics cause the impairment of antioxidant enzymes and proteins, resulting in the disruption of antioxidant systems. Furthermore, MeHg-induced intracellular selenium deficiency due to the greater affinity of MeHg for selenohydryl groups and selenides leads to failure in the recoding of a UGA codon for selenocysteine and results in the degradation of antioxidant selenoenzyme mRNA by nonsense-mediated mRNA decay. The defect of antioxidant selenoenzyme replenishment exacerbates MeHg-mediated oxidative stress. On the other hand, it has also been revealed that MeHg can directly activate the antioxidant Keap1/Nrf2 signaling pathway. This review summarizes the incidence of MeHg-mediated oxidative stress from the viewpoint of the individual intracellular redox system interactions and the MeHg-mediated aforementioned intracellular events. In addition, the mechanisms of cellular stress pathways and neuronal cell death triggered by MeHg-mediated oxidative stress and direct interactions of MeHg with reactive residues of proteins are mentioned.

Molecular and neurocircuitry mechanisms of social avoidance

Humans and animals live in social relationships shaped by actions of approach and avoidance. Both are crucial for normal physical and mental development, survival, and well-being. Active withdrawal from social interaction is often induced by the perception of threat or unpleasant social experience and relies on adaptive mechanisms within neuronal networks associated with social behavior. In case of confrontation with overly strong or persistent stressors and/or dispositions of the affected individual, maladaptive processes in the neuronal circuitries and its associated transmitters and modulators lead to pathological social avoidance. This review focuses on active, fear-driven social avoidance, affected circuits within the mesocorticolimbic system and associated regions and a selection of molecular modulators that promise translational potential. A comprehensive review of human research in this field is followed by a reflection on animal studies that offer a broader and often more detailed range of analytical methodologies. Finally, we take a critical look at challenges that could be addressed in future translational research on fear-driven social avoidance.

Different adaptations of dopamine release in Nucleus Accumbens shell and core of individual alcohol drinking groups of mice

Alcohol use disorder (AUD) places a tremendous burden on society, with approximately two billion alcohol users in the world. While most people drink alcohol recreationally, a subpopulation (3-5%) engages in reckless and compulsive drinking, leading to the development of AUD and alcohol dependence. The Ventral Tegmental Area (VTA)-Nucleus Accumbens (NAc) circuit has been shown to encode rewarding stimuli and drive individual alcohol drinking behavior. Our previous work successfully separated C57BL/6J isogenic mice into high or low alcohol drinking subgroups after a 12-day, two-bottle choice voluntary alcohol access paradigm. Electrophysiological studies revealed that low alcohol drinking mice exhibited elevated spontaneous and burst firing properties of their VTA dopamine (DA) neurons and specifically mimicking this pattern of activity in VTA-NAc neurons in high alcohol drinking mice using optogenetics decreased their alcohol preference. It is also known that VTA DA neurons encode the salience and rewarding properties of external stimuli while also regulating downstream dopamine concentrations. Here, as a follow-up to this study, we utilized Fast Scan Cyclic Voltammetry (FSCV) to examine dopamine release in the NAc shell and core between alcohol drinking groups. We observed dynamic changes of dopamine release in the core of high drinking mice, but failed to see widely significant differences of dopamine release in the shell of both groups, when compared with ethanol-naive controls. Overall, the present data suggest subregion-specific differences of evoked dopamine release in the NAc of low and high alcohol drinking mice, and may provide an anatomical substrate for individual alcohol drinking behavior. This article is part of the special issue on Stress, Addiction and Plasticity.