Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat

Regulation of stress-induced sleep perturbations by dorsal raphe VGLUT3 neurons in male mice

Exposure to stressors has profound effects on sleep that have been linked to serotonin (5-HT) neurons of the dorsal raphe nucleus (DR). However, the DR also comprises glutamatergic neurons expressing vesicular glutamate transporter type 3 (DRVGLUT3), leading us to examine their role. Cell-type-specific tracing revealed that DRVGLUT3 neurons project to brain areas regulating arousal and stress. We found that chemogenetic activation of DRVGLUT3 neurons mimics stress-induced sleep perturbations. Furthermore, deleting VGLUT3 in the DR attenuated stress-induced sleep perturbations, especially after social defeat stress. In the DR, VGLUT3 is found in subsets of 5-HT and non-5-HT neurons. We observed that both populations are activated by acute stress, including those projecting to the ventral tegmental area. However, deleting VGLUT3 in 5-HT neurons minimally affected sleep regulation. These findings suggest that VGLUT3 expression in the DR drives stress-induced sleep perturbations, possibly involving non-5-HT DRVGLUT3 neurons.

Dorsal raphe neurons integrate the values of reward amount, delay, and uncertainty in multi-attribute decision-making

The dorsal raphe nucleus (DRN) is implicated in psychiatric disorders that feature impaired sensitivity to reward amount, impulsivity when facing reward delays, and risk-seeking when confronting reward uncertainty. However, it has been unclear whether and how DRN neurons signal reward amount, reward delay, and reward uncertainty during multi-attribute value-based decision-making, where subjects consider these attributes to make a choice. We recorded DRN neurons as monkeys chose between offers whose attributes, namely expected reward amount, reward delay, and reward uncertainty, varied independently. Many DRN neurons signaled offer attributes, and this population tended to integrate the attributes in a manner that reflected monkeys' preferences for amount, delay, and uncertainty. After decision-making, in response to post-decision feedback, these same neurons signaled signed reward prediction errors, suggesting a broader role in tracking value across task epochs and behavioral contexts. Our data illustrate how the DRN participates in value computations, guiding theories about the role of the DRN in decision-making and psychiatric disease.

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.

5-HT Neurons Integrate GABA and Dopamine Inputs to Regulate Meal Initiation

Obesity is a growing global health epidemic with limited effective therapeutics. Serotonin (5-HT) is one major neurotransmitter which remains an excellent target for new weight-loss therapies, but there remains a gap in knowledge on the mechanisms involved in 5-HT produced in the dorsal Raphe nucleus (DRN) and its involvement in meal initiation. Using a closed-loop optogenetic feeding paradigm, we showed that the 5-HTDRN→arcuate nucleus (ARH) circuit plays an important role in regulating meal initiation. Incorporating electrophysiology and ChannelRhodopsin-2-Assisted Circuit Mapping, we demonstrated that 5-HTDRN neurons receive inhibitory input partially from GABAergic neurons in the DRN, and the 5-HT response to GABAergic inputs can be enhanced by hunger. Additionally, deletion of the GABAA receptor subunit in 5-HT neurons inhibits meal initiation with no effect on the satiation process. Finally, we identified the instrumental role of dopaminergic inputs via dopamine receptor D2 in 5-HTDRN neurons in enhancing the response to GABA-induced feeding. Thus, our results indicate that 5-HTDRN neurons are inhibited by synergistic inhibitory actions of GABA and dopamine, which allows for the initiation of a meal.

A midbrain GABAergic circuit constrains wakefulness in a mouse model of stress

Enhancement of wakefulness is a prerequisite for adaptive behaviors to cope with acute stress, but hyperarousal is associated with impaired behavioral performance. Although the neural circuitries promoting wakefulness in acute stress conditions have been extensively identified, less is known about the circuit mechanisms constraining wakefulness to prevent hyperarousal. Here, we found that chemogenetic or optogenetic activation of GAD2-positive GABAergic neurons in the midbrain dorsal raphe nucleus (DRNGAD2) decreased wakefulness, while inhibition or ablation of these neurons produced an increase in wakefulness along with hyperactivity. Surprisingly, DRNGAD2 neurons were paradoxically wakefulness-active and were further activated by acute stress. Bidirectional manipulations revealed that DRNGAD2 neurons constrained the increase of wakefulness and arousal level in a mouse model of stress. Circuit-specific investigations demonstrated that DRNGAD2 neurons constrained wakefulness via inhibition of the wakefulness-promoting paraventricular thalamus. Therefore, the present study identified a wakefulness-constraining role DRNGAD2 neurons in acute stress conditions.

The role of serotonin in depression-A historical roundup and future directions

Depression is one of the most common psychiatric disorders worldwide, affecting approximately 280 million people, with probably much higher unrecorded cases. Depression is associated with symptoms such as anhedonia, feelings of hopelessness, sleep disturbances, and even suicidal thoughts. Tragically, more than 700 000 people commit suicide each year. Although depression has been studied for many decades, the exact mechanisms that lead to depression are still unknown, and available treatments only help a fraction of patients. In the late 1960s, the serotonin hypothesis was published, suggesting that serotonin is the key player in depressive disorders. However, this hypothesis is being increasingly doubted as there is evidence for the influence of other neurotransmitters, such as noradrenaline, glutamate, and dopamine, as well as larger systemic causes such as altered activity in the limbic network or inflammatory processes. In this narrative review, we aim to contribute to the ongoing debate on the involvement of serotonin in depression. We will review the evolution of antidepressant treatments, systemic research on depression over the years, and future research applications that will help to bridge the gap between systemic research and neurotransmitter dynamics using biosensors. These new tools in combination with systemic applications, will in the future provide a deeper understanding of the serotonergic dynamics in depression.

Integrative Research of Neuropharmacology and Informatics Pharmacology for Mental Disorder

Mental illness poses a huge social burden, accounting for approximately 14% of all deaths. Depression, a major component of mental illness, affects approximately 300 million people worldwide, mainly in developed countries, and is not only a major social burden but also a cause of suicide. The social burden of depression is estimated to increase further in developing countries, and overcoming it is a pressing issue for all countries, including Japan. Although clinical evidence has demonstrated the efficacy of serotonergic neurotransmission enhancers in the treatment of depression, the full picture of their therapeutic effects has not yet been fully elucidated. In this review, we show that the hyperactivity of serotonin neurons, especially those in the dorsal raphe nucleus, is commonly induced by various antidepressants within a period corresponding to the onset of their clinical efficacy. We established quantitative prediction methods for pharmacological activity using only chemical structures to translate the biological understanding of mental disorders, including major depressive disorders, into clinically effective therapeutics. Our method exhibited better performance than the previously reported methods of quantitative prediction, while targeting a larger number of proteins. Our article suggests the importance of integrative neuropharmacology and informatics-based pharmacology studies to understand the biological basis of mental disorders and facilitate drug development for these disorders.

Reduction of p11 in dorsal raphe nucleus serotonergic neurons mediates depression-like behaviors

The pathology of depression is related to the imbalance of various neurotransmitters. The dorsal raphe nucleus (DRN), the main brain region producing 5-HT, is crucially involved in the pathophysiology of depression. It contains several neuron types, in which GABAergic neurons are activated by stimuli associated with negative experiences and 5-HT neurons are activated by reward signals. However, little is known about its underlying molecular mechanisms. Here, we found that p11, a multifunctional protein associated with depression, was down-regulated by chronic social defeat stress in 5-HTDRN neurons. Knockdown of p11 in DRN induced depression-like behaviors, while its overexpression in 5-HTDRN neurons alleviated depression-like behavior caused by chronic social defeat stress. Further, p11 regulates membrane trafficking of glutamate receptors in 5-HTDRN neurons, suggesting a possible molecular mechanism underlying the participation of p11 in the pathological process of depression. This may facilitate the understanding of the molecular and cellular basis of depression.

Resting-state fMRI reveals changes within the anxiety and social avoidance circuitry of the brain in mice with psoriasis-like skin lesions

Psoriasis is an autoimmune skin disease that often co-occurs with psychological morbidities such as anxiety and depression, and psychosocial issues also lead psoriasis patients to avoid other people. However, the precise mechanism underlying the comorbidity of psoriasis and anxiety is unknown. Also, whether the social avoidance phenomenon seen in human patients also exists in psoriasis-like animal models remains unknown. In the present study, anxiety-like behaviours and social avoidance-like behaviours were observed in an imiquimod-induced psoriasis-like C57-BL6 mouse model along with typical psoriasis-like dermatitis and itch-like behaviours. The 11.7T resting-state functional magnetic resonance imaging showed differences in brain regions between the model and control group, and voxel-based morphometry showed that the grey matter volume changed in the basal forebrain region, anterior commissure intrabulbar and striatum in the psoriasis-like mice. Seed-based resting state functional connectivity analysis revealed connectivity changes in the amygdala, periaqueductal gray, raphe nuclei and lateral septum. We conclude that the imiquimod-induced psoriasis-like C57-BL6 mouse model is well suited for mechanistic studies and for performing preclinical therapeutic trials for treating anxiety and pathological social avoidance in psoriasis patients.

Stress, associative learning, and decision-making

Exposure to acute and chronic stress has significant effects on the basic mechanisms of associative learning and memory. Stress can both impair and enhance associative learning depending on type, intensity, and persistence of the stressor, the subject's sex, the context that the stress and behavior is experienced in, and the type of associative learning taking place. In some cases, stress can cause or exacerbate the maladaptive behavior that underlies numerous psychiatric conditions including anxiety disorders, obsessive-compulsive disorder, post-traumatic stress disorder, substance use disorder, and others. Therefore, it is critical to understand how the varied effects of stress, which may normally facilitate adaptive behavior, can also become maladaptive and even harmful. In this review, we highlight several findings of associative learning and decision-making processes that are affected by stress in both human and non-human subjects and how they are related to one another. An emerging theme from this work is that stress biases behavior towards less flexible strategies that may reflect a cautious insensitivity to changing contingencies. We consider how this inflexibility has been observed in different associative learning procedures and suggest that a goal for the field should be to clarify how factors such as sex and previous experience influence this inflexibility.

Dorsal raphe neurons signal integrated value during multi-attribute decision-making

The dorsal raphe nucleus (DRN) is implicated in psychiatric disorders that feature impaired sensitivity to reward amount, impulsivity when facing reward delays, and risk-seeking when grappling with reward uncertainty. However, whether and how DRN neurons signal reward amount, reward delay, and reward uncertainty during multi-attribute value-based decision-making, where subjects consider all these attributes to make a choice, is unclear. We recorded DRN neurons as monkeys chose between offers whose attributes, namely expected reward amount, reward delay, and reward uncertainty, varied independently. Many DRN neurons signaled offer attributes. Remarkably, these neurons commonly integrated offer attributes in a manner that reflected monkeys' overall preferences for amount, delay, and uncertainty. After decision-making, in response to post-decision feedback, these same neurons signaled signed reward prediction errors, suggesting a broader role in tracking value across task epochs and behavioral contexts. Our data illustrate how DRN participates in integrated value computations, guiding theories of DRN in decision-making and psychiatric disease.

Inhibition of dorsal raphe GABAergic neurons blocks hyperalgesia during heroin withdrawal

Opioid withdrawal signs, such as hyperalgesia, are manifestations of opioid use disorder that may contribute to opioid seeking and taking. We have previously identified an association between dorsal raphe (DR) neurons and the expression of hyperalgesia during spontaneous heroin withdrawal. Here, we found that chemogenetic inhibition of DR neurons decreased hyperalgesia during spontaneous heroin withdrawal in male and female C57/B6 mice. By neuroanatomy, we identified three major subtypes of DR neurons expressing μ-opioid receptors (MOR) that were activated in hyperalgesia during spontaneous withdrawal, those expressing vesicular GABA transporter (VGaT), glutamate transporter 3 (VGluT3), or co-expressing VGluT3 and tryptophan hydroxylase (TPH). In contrast, we identified a small population of DR-MOR neurons expressing solely TPH, which were not activated in hyperalgesia during spontaneous withdrawal. Collectively, these findings indicate a role of the DR in hyperalgesia during spontaneous heroin withdrawal mediated, in part, by the activation of local MOR-GABAergic, MOR-glutamatergic and MOR-co-releasing glutamatergic-serotonergic neurons. We found that  specific chemogenetic inhibition of DR-VGaT neurons blocked hyperalgesia during spontaneous heroin withdrawal in male and female mice. Collectively, these findings indicate that DR-GABAergic neurons play a role in the expression of hyperalgesia during spontaneous heroin withdrawal.

Deep Brain Stimulation of the dorsal raphe induces anxiolytic and panicolytic-like effects and alters serotonin immunoreactivity

Previously we showed that Deep Brain Stimulation (DBS) of the dorsal region (DRD) and of the lateral wings of the dorsal raphe (lwDR) respectively decreases anxiety and panic-like responses in the elevated T-maze (ETM). This study investigates neurobiological alterations which might respond for these behavioral effects. Male Wistar rats were submitted to high-frequency stimulation (100µA, 100Hz) of the DRD or of the lwDR for 1h, and subsequently tested in the avoidance or escape tasks of the ETM. Since serotonin (5-HT) reuptake inhibitors are first line pharmacological treatment for anxiety disorders, we also tested the effects of chronic fluoxetine administration (10mg/kg, IP, 21 days) on a separate group of rats. An open field was used for locomotor activity assessment. Additionally, we evaluated c-Fos immunoreactivity (Fos-ir) in serotonergic cells of the dorsal raphe (DR). Results showed that DBS of the DRD decreases avoidance reactions, an anxiolytic-like effect, without altering escape or locomotor activity. Both fluoxetine and DBS of the lwDR decreased escape responses in the ETM, a panicolytic-like effect, without altering avoidance measurements or locomotor activity. While DBS of the DRD decreased double immunostaining in the DRD, DBS of the lwDR increased Fos-ir and double immunostaining in the DRD and lwDR. Fluoxetine also increased double immunostaining in the lwDR and in the DRV but decreased it in the DRD. These results suggest that both the anxiolytic and panicolytic-like effects of DBS and fluoxetine are related to 5-HT modulation in different subnuclei of the DR.

Neurochemical mechanisms of deep brain stimulation for depression in animal models

Deep brain stimulation (DBS) has emerged as a neuromodulation therapy for treatment-resistant depression, but its actual efficacy and mechanisms of action are still unclear. Changes in neurochemical transmission are important mechanisms of antidepressant therapies. Here, we review the preclinical DBS literature reporting behavioural and neurochemical data associated with its antidepressant-like effects. The most commonly studied target in preclinical models was the ventromedial prefrontal cortex (vmPFC). In rodents, DBS delivered to this target induced serotonin (5-HT) release and increased 5-HT1_B receptor expression. The antidepressant-like effects of vmPFC DBS seemed to be independent of the serotonin transporter and potentially mediated by the direct modulation of prefrontal projections to the raphe. Adenosinergic and glutamatergic transmission might have also play a role. Medial forebrain bundle (MFB) DBS increased dopamine levels and reduced D2 receptor expression, whereas nucleus accumbens (NAcc), and lateral habenula (LHb) stimulation increased catecholamine levels in different brain regions. In rodents, subthalamic nucleus (STN) DBS induced robust depression-like responses associated with a reduction in serotonergic transmission, as revealed by a decrease in serotonin release. Some of these effects seemed to be mediated by 5HT1_A receptors. In conclusion, the antidepressant-like effects of DBS in preclinical models have been well documented in multiple targets. Though variable mechanisms have been proposed, DBS-induced acute and long-term changes in neurochemical substrates seem to play an important role in the antidepressant-like effects of this therapy.

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.

Dorsal raphe serotonergic neurons preferentially reactivate dorsal dentate gyrus cell ensembles associated with positive experience

Major depressive disorder (MDD) is among the most common mental illnesses. Serotonergic (5-HT) neurons are central to the pathophysiology and treatment of MDD. Repeatedly recalling positive episodes is effective for MDD. Stimulating 5-HT neurons of the dorsal raphe nucleus (DRN) or neuronal ensembles in the dorsal dentate gyrus (dDG) associated with positive memories reverses the stress-induced behavioral abnormalities. Despite this phenotypic similarity, their causal relationship is unclear. This study revealed that the DRN 5-HT neurons activate dDG neurons; surprisingly, this activation was specifically observed in positive memory ensembles rather than neutral or negative ensembles. Furthermore, we revealed that dopaminergic signaling induced by activation of DRN 5-HT neurons projecting to the ventral tegmental area mediates an increase in active coping behavior and positive dDG ensemble reactivation. Our study identifies a role of DRN 5-HT neurons as specific reactivators of positive memories and provides insights into how serotonin elicits antidepressive effects.

Degeneracy and stability in neural circuits of dopamine and serotonin neuromodulators: A theoretical consideration

Degenerate neural circuits perform the same function despite being structurally different. However, it is unclear whether neural circuits with interacting neuromodulator sources can themselves degenerate while maintaining the same neuromodulatory function. Here, we address this by computationally modeling the neural circuits of neuromodulators serotonin and dopamine, local glutamatergic and GABAergic interneurons, and their possible interactions, under reward/punishment-based conditioning tasks. The neural modeling is constrained by relevant experimental studies of the VTA or DRN system using, e.g., electrophysiology, optogenetics, and voltammetry. We first show that a single parsimonious, sparsely connected neural circuit model can recapitulate several separate experimental findings that indicated diverse, heterogeneous, distributed, and mixed DRNVTA neuronal signaling in reward and punishment tasks. The inability of this model to recapitulate all observed neuronal signaling suggests potentially multiple circuits acting in parallel. Then using computational simulations and dynamical systems analysis, we demonstrate that several different stable circuit architectures can produce the same observed network activity profile, hence demonstrating degeneracy. Due to the extensive D2-mediated connections in the investigated circuits, we simulate the D2 receptor agonist by increasing the connection strengths emanating from the VTA DA neurons. We found that the simulated D2 agonist can distinguish among sub-groups of the degenerate neural circuits based on substantial deviations in specific neural populations' activities in reward and punishment conditions. This forms a testable model prediction using pharmacological means. Overall, this theoretical work suggests the plausibility of degeneracy within neuromodulator circuitry and has important implications for the stable and robust maintenance of neuromodulatory functions.

Serotonin modulates an inhibitory input to the central amygdala from the ventral periaqueductal gray

Fear is an adaptive state that drives defensive behavioral responses to specific and imminent threats. The central nucleus of the amygdala (CeA) is a critical site of adaptations that are required for the acquisition and expression of fear, in part due to alterations in the activity of inputs to the CeA. Here, we characterize a novel GABAergic input to the CeA from the ventral periaqueductal gray (vPAG) using fiber photometry and ex vivo whole-cell slice electrophysiology combined with optogenetics and pharmacology. GABA transmission from this ascending vPAG-CeA input was enhanced by serotonin via activation of serotonin type 2 C (5HT2C) receptors. Results suggest that these receptors are presynaptic. Interestingly, we found that GABA release from the vPAG-CeA input is enhanced following fear learning via activation of 5HT2C receptors and that this pathway is dynamically engaged in response to aversive stimuli. Additionally, we characterized serotonin release in the CeA during fear learning and recall for the first time using fiber photometry coupled to a serotonin biosensor. Together, these findings describe a mechanism by which serotonin modulates GABA release from ascending vPAG GABA inputs to the CeA and characterize a role for this pathway in fear.

Functional characterization of cannabidiol effect on the serotonergic neurons of the dorsal raphe nucleus in rat brain slices

Cannabidiol (CBD), the main non-psychoactive cannabinoid found in the cannabis plant, elicits several pharmacological effects via the 5-HT1A receptor. The dorsal raphe nucleus (DRN) is the main serotonergic cluster in the brain that expresses the 5-HT1A receptor. To date, the effect of CBD on the neuronal activity of DRN 5-HT cells and its interaction with somatodendritic 5-HT1A autoreceptors have not been characterized. Our aim was to study the effect of CBD on the firing activity of DRN 5-HT cells and the 5-HT1A autoreceptor activation by electrophysiological and calcium imaging techniques in male Sprague–Dawley rat brain slices. Perfusion with CBD (30 μM, 10 min) did not significantly change the firing rate of DRN 5-HT cells or the inhibitory effect of 5-HT (50–100 μM, 1 min). However, in the presence of CBD (30 μM, 10 min), the inhibitory effects of 8-OH-DPAT (10 nM) and ipsapirone (100 nM) were reduced by 66% and 53%, respectively. CBD failed to reverse ipsapirone-induced inhibition, whereas perfusion with the 5-HT1A receptor antagonist WAY100635 (30 nM) completely restored by 97.05 ± 14.63% the firing activity of 5-HT cells. Administration of AM251 (1 µM), MDL100907 (30 nM), or picrotoxin (20 μM) did not change the blockade produced by CBD (30 μM) on ipsapirone-induced inhibition. Our study also shows that CBD failed to modify the KCl (15 mM, 4 min)-evoked increase in [Ca2+]i or the inhibitory effect of ipsapirone (1 μM, 4 min) on KCl-evoked [Ca2+]i. In conclusion, CBD does not activate 5-HT1A autoreceptors, but it hindered the inhibitory effect produced by selective 5-HT1A receptor agonists on the firing activity of DRN 5-HT cells through a mechanism that does not involve CB1, 5-HT2A, or GABAA receptors. Our data support a negative allosteric modulation of DRN somatodendritic 5-HT1A receptor by CBD.

Deletion of the P/Q-Type Calcium Channel from Serotonergic Neurons Drives Male Aggression in Mice

Aggressive behavior is one of the most conserved social interactions in nature and serves as a crucial evolutionary trait. Serotonin (5-HT) plays a key role in the regulation of our emotions, such as anxiety and aggression, but which molecules and mechanisms in the serotonergic system are involved in violent behavior are still unknown. In this study, we show that deletion of the P/Q-type calcium channel selectively from serotonergic neurons in the dorsal raphe nuclei (DRN) augments aggressive behavior in male mice, while anxiety is not affected. These mice demonstrated increased induction of the immediate early gene c-fos and in vivo serotonergic firing activity in the DRN. The ventrolateral part of the ventromedial hypothalamus is also a prominent region of the brain mediating aggression. We confirmed a monosynaptic projection from the DRN to the ventrolateral part of the ventromedial hypothalamus, and silencing these projections with an inhibitory designer receptor exclusively activated by a designer drug effectively reduced aggressive behavior. Overall, our findings show that deletion of the P/Q-type calcium channel from DRN neurons is sufficient to induce male aggression in mice and regulating its activity may serve as a therapeutic approach to treat violent behavior.SIGNIFICANCE STATEMENT In this study, we show that P/Q-type calcium channel is mediating aggression in serotonergic neurons from the dorsal raphe nucleus via monosynaptic projections to the ventrolateral part of the ventromedial hypothalamus. More importantly, silencing these projections reduced aggressive behavior in mice and may serve as a therapeutic approach for treating aggression in humans.

Encore: Behavioural animal models of stress, depression and mood disorders

Animal studies over the past two decades have led to extensive advances in our understanding of pathogenesis of depressive and mood disorders. Among these, rodent behavioural models proved to be of highest informative value. Here, we present a comprehensive overview of the most popular behavioural models with respect to physiological, circuit, and molecular biological correlates. Behavioural stress paradigms and behavioural tests are assessed in terms of outcomes, strengths, weaknesses, and translational value, especially in the domain of pharmacological studies.

Optogenetic Methods to Investigate Brain Alterations in Preclinical Models

Investigating the neuronal dynamics supporting brain functions and understanding how the alterations in these mechanisms result in pathological conditions represents a fundamental challenge. Preclinical research on model organisms allows for a multiscale and multiparametric analysis in vivo of the neuronal mechanisms and holds the potential for better linking the symptoms of a neurological disorder to the underlying cellular and circuit alterations, eventually leading to the identification of therapeutic/rescue strategies. In recent years, brain research in model organisms has taken advantage, along with other techniques, of the development and continuous refinement of methods that use light and optical approaches to reconstruct the activity of brain circuits at the cellular and system levels, and to probe the impact of the different neuronal components in the observed dynamics. These tools, combining low-invasiveness of optical approaches with the power of genetic engineering, are currently revolutionizing the way, the scale and the perspective of investigating brain diseases. The aim of this review is to describe how brain functions can be investigated with optical approaches currently available and to illustrate how these techniques have been adopted to study pathological alterations of brain physiology.

Electrophysiological Characteristics of Dorsal Raphe Nucleus in Tail Suspension Test

The dorsal raphe nucleus (DRN) is a major source of serotonin in the central nervous system, which is closely related to depression-like behaviors and is modulated by local GABAergic interneurons. Although serotonin neurons are known to be activated by struggling behavior in tail suspension test (TST), the exact electrophysiological characteristics are still unclear. Here, we combined in vivo electrode recording and behavioral test to explore the mice neuron electrophysiology in DRN during TST and observed that gamma oscillation was related to despair-like behaviors whereas burst fraction was crucial for survival-like behaviors. We reported the identification of a subpopulation of DRN neurons which change their firing rates when mice get into and during TST immobile states. Both increase (putative despair units, D units for short) and decrease (putative survival units, S units for short) in firing rate were observed. Furthermore, using optogenetics to identify parvalbumin-positive (PV+) and serotonin transporter-positive (SERT+) neurons, we found that SERT+ neurons were almost S units. Interestingly, those that have been identified PV+ neurons include ~20% of D units and ~50% of S units. These results suggest that electrophysiological characteristics incorporated in despair-like behavior studies can provide new insight into the study of anti-depression targets, and GABAergic interneuron is a complex key hub to the coding and regulation of local neural network.

Functional Interaction Between GABAergic Neurons in the Ventral Tegmental Area and Serotonergic Neurons in the Dorsal Raphe Nucleus

GABAergic neurons in the ventral tegmental area (VTA) have brain-wide projections and are involved in multiple behavioral and physiological functions. Here, we revealed the responsiveness of Gad67+ neurons in VTA (VTA_Gad67+) to various neurotransmitters involved in the regulation of sleep/wakefulness by slice patch clamp recording. Among the substances tested, a cholinergic agonist activated, but serotonin, dopamine and histamine inhibited these neurons. Dense VTA_Gad67+ neuronal projections were observed in brain areas regulating sleep/wakefulness, including the central amygdala (CeA), dorsal raphe nucleus (DRN), and locus coeruleus (LC). Using a combination of electrophysiology and optogenetic studies, we showed that VTA_Gad67+ neurons inhibited all neurons recorded in the DRN, but did not inhibit randomly recorded neurons in the CeA and LC. Further examination revealed that the serotonergic neurons in the DRN (DRN_5–HT) were monosynaptically innervated and inhibited by VTA_Gad67+ neurons. All recorded DRN_5–HT neurons received inhibitory input from VTA_Gad67+ neurons, while only one quarter of them received inhibitory input from local GABAergic neurons. Gad67+ neurons in the DRN (DRN_Gad67+) also received monosynaptic inhibitory input from VTA_Gad67+ neurons. Taken together, we found that VTA_Gad67+ neurons were integrated in many inputs, and their output inhibits DRN_5–HT neurons, which may regulate physiological functions including sleep/wakefulness.

Selective targeting of chronic social stress-induced activated neurons identifies neurogenesis-related genes to be associated with resilience in female mice

Prolonged social stress is a major cause for depression in humans and is associated with a wide range of subsequent pathophysiological changes such as elevated blood pressure. A routinely used model for investigating this kind of stress in mice is the chronic social defeat paradigm where a smaller intruder is exposed to an aggressive inhabitant of a home cage. This model is restricted to males and includes a high proportion of physical stress that might e.g., interfere with immunological aspects of the stress. The prevalence of depression in humans is even higher in women than in men. Therefore, expanding models to female individuals is desirable. We here tested the social instability model as a tool for administering chronic social stress to female C57BL/6J mice and analyzed short-term as well as long-lasting effects. Animals were housed in groups of four and were shuffled two times a week, resulting in a permanent re-structuration of their social hierarchy. While directly after the stress exposure, serum corticosterone was elevated, increased body weight and fat deposits were observed in stressed mice even one year after discontinuation of the stress. At the behavioral level, animals could be stratified into resilient and susceptible animals directly post-stress, but those subgroups were not distinguishable any more in the long-term analysis. To identify molecular contributors to resilience in the here presented social instability induced stress model, Arc-activity dependent trapping of neurons was conducted in Arc-creERT2/sun1sfGFP mice. RNA samples derived from activated nuclei from the ventral hippocampus, a brain region involved in stress-regulation during attacks or explorative behavior of mice, were subjected to a neurogenesis pathway array. While several genes were differentially regulated by stress, in particular, artemin, a neurotrophic factor was upregulated in resilient versus susceptible individuals.

Stress-sensitive inference of task controllability

Estimating the controllability of the environment enables agents to better predict upcoming events and decide when to engage controlled action selection. How does the human brain estimate controllability? Trial-by-trial analysis of choices, decision times and neural activity in an explore-and-predict task demonstrate that humans solve this problem by comparing the predictions of an 'actor' model with those of a reduced 'spectator' model of their environment. Neural blood oxygen level-dependent responses within striatal and medial prefrontal areas tracked the instantaneous difference in the prediction errors generated by these two statistical learning models. Blood oxygen level-dependent activity in the posterior cingulate, temporoparietal and prefrontal cortices covaried with changes in estimated controllability. Exposure to inescapable stressors biased controllability estimates downward and increased reliance on the spectator model in an anxiety-dependent fashion. Taken together, these findings provide a mechanistic account of controllability inference and its distortion by stress exposure.

Resilience to the effects of social stress on vulnerability to developing drug addiction

We review the still scarce but growing literature on resilience to the effects of social stress on the rewarding properties of drugs of abuse. We define the concept of resilience and how it is applied to the field of drug addiction research. We also describe the internal and external protective factors associated with resilience, such as individual behavioral traits and social support. We then explain the physiological response to stress and how it is modulated by resilience factors. In the subsequent section, we describe the animal models commonly used in the study of resilience to social stress, and we focus on the effects of chronic social defeat (SD), a kind of stress induced by repeated experience of defeat in an agonistic encounter, on different animal behaviors (depression- and anxiety-like behavior, cognitive impairment and addiction-like symptoms). We then summarize the current knowledge on the neurobiological substrates of resilience derived from studies of resilience to the effects of chronic SD stress on depression- and anxiety-related behaviors in rodents. Finally, we focus on the limited studies carried out to explore resilience to the effects of SD stress on the rewarding properties of drugs of abuse, describing the current state of knowledge and suggesting future research directions.

Spike firing attenuation of serotonin neurons in learned helplessness rats is reversed by ketamine

Animals suffering from uncontrollable stress sometimes show low effort to escape stress (learned helplessness). Changes in serotonin (5-hydroxytryptamine) signalling are thought to underlie this behaviour. Although the release of 5-hydroxytryptamine is triggered by the action potential firing of dorsal raphe nuclei 5-hydroxytryptamine neurons, the electrophysiological changes induced by uncontrollable stress are largely unclear. Herein, we examined electrophysiological differences among 5-hydroxytryptamine neurons in naïve rats, learned helplessness rats and rats resistant to inescapable stress (non-learned helplessness). Five-week-old male Sprague Dawley rats were exposed to inescapable foot shocks. After an avoidance test session, rats were classified as learned helplessness or non-learned helplessness. Activity-dependent 5-hydroxytryptamine release induced by the administration of high-potassium solution was slower in free-moving learned helplessness rats. Subthreshold electrophysiological properties of 5-hydroxytryptamine neurons were identical among the three rat groups, but the depolarization-induced spike firing was significantly attenuated in learned helplessness rats. To clarify the underlying mechanisms, potassium (K+) channels regulating the spike firing were initially examined using naïve rats. K+ channels sensitive to 500 μM tetraethylammonium caused rapid repolarization of the action potential and the small conductance calcium-activated K+ channels produced afterhyperpolarization. Additionally, dendrotoxin-I, a blocker of Kv1.1 (encoded by Kcna1), Kv1.2 (encoded by Kcna2) and Kv1.6 (encoded by Kcna6) voltage-dependent K+ channels, weakly enhanced the spike firing frequency during depolarizing current injections without changes in individual spike waveforms in naïve rats. We found that dendrotoxin-I significantly enhanced the spike firing of 5-hydroxytryptamine neurons in learned helplessness rats. Consequently, the difference in spike firing among the three rat groups was abolished in the presence of dendrotoxin-I. These results suggest that the upregulation of dendrotoxin-I-sensitive Kv1 channels underlies the firing attenuation of 5-hydroxytryptamine neurons in learned helplessness rats. We also found that the antidepressant ketamine facilitated the spike firing of 5-hydroxytryptamine neurons and abolished the firing difference between learned helplessness and non-learned helplessness by suppressing dendrotoxin-I-sensitive Kv1 channels. The dendrotoxin-I-sensitive Kv1 channel may be a potential target for developing drugs to control activity of 5-hydroxytryptamine neurons.

Whole-brain connectivity atlas of glutamatergic and GABAergic neurons in the mouse dorsal and median raphe nuclei

The dorsal raphe nucleus (DR) and median raphe nucleus (MR) contain populations of glutamatergic and GABAergic neurons that regulate diverse behavioral functions. However, their whole-brain input-output circuits remain incompletely elucidated. We used viral tracing combined with fluorescence micro-optical sectioning tomography to generate a comprehensive whole-brain atlas of inputs and outputs of glutamatergic and GABAergic neurons in the DR and MR. We found that these neurons received inputs from similar upstream brain regions. The glutamatergic and GABAergic neurons in the same raphe nucleus had divergent projection patterns with differences in critical brain regions. Specifically, MR glutamatergic neurons projected to the lateral habenula through multiple pathways. Correlation and cluster analysis revealed that glutamatergic and GABAergic neurons in the same raphe nucleus received heterogeneous inputs and sent different collateral projections. This connectivity atlas further elucidates the anatomical architecture of the raphe nuclei, which could facilitate better understanding of their behavioral functions.

Neurophysiological dynamics for psychological resilience: A view from the temporal axis

When an individual is faced with adversity, the brain and body work cooperatively to adapt to it. This adaptive process is termed psychological resilience, and recent studies have identified several neurophysiological factors ("neurophysiological resilience"), such as monoamines, oscillatory brain activity, hemodynamics, autonomic activity, stress hormones, and immune systems. Each factor is activated in an interactive manner during specific time windows after exposure to stress. Thus, the differences in psychological resilience levels among individuals can be characterized by differences in the temporal dynamics of neurophysiological resilience. In this review, after briefly introducing the frequently used approaches in this research field and the well-known factors of neurophysiological resilience, we summarize the temporal dynamics of neurophysiological resilience. This viewpoint clarifies an important time window, the more-than-one-hour scale, but the neurophysiological dynamics during this window remain elusive. To address this issue, we propose exploring brain-wide oscillatory activities using concurrent functional magnetic resonance imaging (fMRI) and electroencephalogram (EEG) techniques.

The dorsal raphe nucleus in the control of energy balance

Energy balance is orchestrated by an extended network of highly interconnected nuclei across the central nervous system. While much is known about the hypothalamic circuits regulating energy homeostasis, the 'extra-hypothalamic' circuits involved are relatively poorly understood. In this review, we focus on the brainstem's dorsal raphe nucleus (DRN), integrating decades of research linking this structure to the physiologic and behavioral responses that maintain proper energy stores. DRN neurons sense and respond to interoceptive and exteroceptive cues related to energy imbalance and in turn induce appropriate alterations in energy intake and expenditure. The DRN is also molecularly differentiable, with different populations playing distinct and often opposing roles in controlling energy balance. These populations are integrated into the extended circuit known to regulate energy balance. Overall, this review summarizes the key evidence demonstrating an important role for the DRN in regulating energy balance.

Cannabidiol and the corticoraphe circuit in post-traumatic stress disorder

Post-Traumatic Stress Disorder (PTSD), characterized by re-experiencing, avoidance, negative affect, and impaired memory processing, may develop after traumatic events. PTSD is complicated by impaired plasticity and medial prefrontal cortex (mPFC) activity, hyperactivity of the amygdala, and impaired fear extinction. Cannabidiol (CBD) is a promising candidate for treatment due to its multimodal action that enhances plasticity and calms hyperexcitability. CBD’s mechanism in the mPFC of PTSD patients has been explored extensively, but literature on the mechanism in the dorsal raphe nucleus (DRN) is lacking. Following the PRISMA guidelines, we examined current literature regarding CBD in PTSD and overlapping symptomologies to propose a mechanism by which CBD treats PTSD via corticoraphe circuit. Acute CBD inhibits excess 5-HT release from DRN to amygdala and releases anandamide (AEA) onto amygdala inputs. By first reducing amygdala and DRN hyperactivity, CBD begins to ameliorate activity disparity between mPFC and amygdala. Chronic CBD recruits the mPFC, creating harmonious corticoraphe signaling. DRN releases enough 5-HT to ameliorate mPFC hypoactivity, while the mPFC continuously excites DRN 5-HT neurons via glutamate. Meanwhile, AEA regulates corticoraphe activity to stabilize signaling. AEA prevents DRN GABAergic interneurons from inhibiting 5-HT release so the DRN can assist the mPFC in overcoming its hypoactivity. DRN-mediated restoration of mPFC activity underlies CBD’s mechanism on fear extinction and learning of stress coping.

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CBD reduces PTSD symptoms via the DRN and corticoraphe circuit.

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Acute effects of CBD reduce DRN-amygdala excitatory signaling to lessen the activity disparity between amygdala and mPFC.

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Chronic CBD officially resolves mPFC hypoactivity by facilitating 5-HT release from DRN to mPFC.

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CBD-facilitated endocannabinoid signaling stabilizes DRN activity and restores mPFC inhibitory control.

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Chronically administered CBD acts via the corticoraphe circuit to favor fear extinction over fear memory reconsolidation.

mTOR Knockdown in the Infralimbic Cortex Evokes A Depressive-like State in Mouse

Fast and sustained antidepressant effects of ketamine identified the mammalian target of rapamycin (mTOR) signaling pathway as the main modulator of its antidepressive effects. Thus, mTOR signaling has become integral for the preclinical evaluation of novel compounds to treat depression. However, causality between mTOR and depression has yet to be determined. To address this, we knocked down mTOR expression in mice using an acute intracerebral infusion of small interfering RNAs (siRNA) in the infralimbic (IL) or prelimbic (PrL) cortices of the medial prefrontal cortex (mPFC), and evaluated depressive- and anxious-like behaviors. mTOR knockdown in IL, but not PrL, cortex produced a robust depressive-like phenotype in mice, as assessed in the forced swimming test (FST) and the tail suspension test (TST). This phenotype was associated with significant reductions of mTOR mRNA and protein levels 48 h post-infusion. In parallel, decreased brain-derived neurotrophic factor (BDNF) expression was found bilaterally in both IL and PrL cortices along with a dysregulation of serotonin (5-HT) and glutamate (Glu) release in the dorsal raphe nucleus (DRN). Overall, our results demonstrate causality between mTOR expression in the IL cortex and depressive-like behaviors, but not in anxiety.

Angiotensin II induces cognitive decline and anxiety-like behavior via disturbing pattern of theta-gamma oscillations

Hypertension is the most common chronic disease accompanied by cognitive decline and anxiety-like behavior. Angiotensin II (Ang II) induces hypertension by activating angiotensin II receptor subtype 1 (AT1R). The purpose of the study was to examine the potential underlying mechanism of alterations in cognition and anxiety-like behavior induced by Ang II. Adult C57 mice were intraperitoneal injected with either 1 mg/kg/d Ang II or saline individually for 14 consecutive days. Ang II resulted in cognitive decline and anxious like behavior in C57 mice. Moreover, Ang II disturbed bidirectional synaptic plasticity and neural oscillation coupling between high theta and gamma on PP (perforant pathway)-DG (dentate gyrus) pathway. In addition, Ang II decreased the expression of N-methyl-d-aspartate receptor (NR) 2A and NR 2B and increased the expression of GABA_AR α1. The data suggest that Ang II disturb neural oscillations via altering excitatory and inhibitory (E/I) balance and eventually damage cognition and anxiety-like behavior in mice.

Using social rank as the lens to focus on the neural circuitry driving stress coping styles

Social hierarchy position in humans is negatively correlated with stress-related psychiatric disease risk. Animal models have largely corroborated human studies, showing that social rank can impact stress susceptibility and is considered to be a major risk factor in the development of psychiatric illness. Differences in stress coping style is one of several factors that mediate this relationship between social rank and stress susceptibility. Coping styles encompass correlated groupings of behaviors associated with differential physiological stress responses. Here, we discuss recent insights from animal models that highlight several neural circuits that can contribute to social rank-associated differences in coping style.

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.

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.

Down-regulation of habenular calcium-dependent secretion activator 2 induces despair-like behavior

Calcium-dependent secretion activator 2 (CAPS2) regulates the trafficking and exocytosis of neuropeptide-containing dense-core vesicles (DCVs). CAPS2 is prominently expressed in the medial habenula (MHb), which is related to depressive behavior; however, how MHb neurons cause depressive symptoms and the role of CAPS2 remains unclear. We hypothesized that dysfunction of MHb CAPS neurons might cause defects in neuropeptide secretion and the activity of monoaminergic centers, resulting in depressive-like behaviors. In this study, we examined (1) CAPS2 expression in the habenula of depression animal models and major depressive disorder patients and (2) the effects of down-regulation of MHb CAPS2 on the animal behaviors, synaptic transmission in the interpeduncular nucleus (IPN), and neuronal activity of monoamine centers. Habenular CAPS2 expression was decreased in the rat chronic restraint stress model, mouse learned helplessness model, and showed tendency to decrease in depression patients who died by suicide. Knockdown of CAPS2 in the mouse habenula evoked despair-like behavior and a reduction of the release of DCVs in the IPN. Neuronal activity of IPN and monoaminergic centers was also reduced. These results implicate MHb CAPS2 as playing a pivotal role in depressive behavior through the regulation of neuropeptide secretion of the MHb-IPN pathway and the activity of monoaminergic centers.

Attachment Insecurity in Rats Subjected to Maternal Separation and Early Weaning: Sex Differences

Attachment insecurity in the forms of attachment anxiety and avoidance is associated with mental disorders in humans. In this research field, rodents, especially mice and rats, are commonly used to study social behaviors and underlying biological mechanisms due to their pronounced sociability. However, quantitative assessment of attachment security/insecurity in rodents has been a major challenge. The present study identified attachment insecurity behaviors in rats subjected to maternal separation (MS) during postnatal days (PD) 2-16 and early weaning (EW) during PD 17-21. This MSEW procedure has been used to mimic early life neglect in humans. After MSEW, rats continued to survive until early adulthood when they were subjected to open-field, social interaction, and elevated-plus maze tests. Compared to CNT rats in either gender, MSEW rats moved longer distances at higher velocities in the open-field. The MSEW rats also showed lower ratios of travel distance at central zone over that on whole arena of the open-field compared to CNT rats. In social interaction test, male CNT rats preferred to investigate an empty cage than females; whereas female CNT rats spent more time with a partner-containing cage as compared to males. This gender-specific difference was reversed in MSEW rats. On elevated-plus maze female CNT rats exhibited more risk-taking behaviors as compared to male counterparts. Moreover, female MSEW rats experienced a greater difficulty in making a decision on whether approaching to or averting from which arms of elevated-plus maze. Taken together, male MSEW rats behaved like attachment anxiety while females' phenotype is alike to attachment avoidance described in humans. These results shall prompt further application of MSEW rat in abnormal psychology and biological psychiatry research.

Resilience to fear: The role of individual factors in amygdala response to stressors

Resilience to stress is an adaptive process that varies individually. Resilience refers to the adaptation, or the ability to maintain or regain mental health, despite being subject to adverse situation. Resilience is a dynamic concept that reflects a combination of internal individual factors, including age and gender interacting with external factors such as social, cultural and environmental factors. In the last decade, we have witnessed an increase in the prevalence of anxiety disorders, including post-traumatic stress disorder. Given that stress in unavoidable, it is of great interest to understand the neurophysiological mechanisms of resilience, the individual factors that may contribute to susceptibility and promote efficacious approaches to improve resilience. Here, we address this complex question, attempting at defining clear and operational definitions that may allow us to improve our analysis of behavior incorporating individuality. We examine how individual perception of the stressor can alter the outcome of an adverse situation using as an example, the fear-conditioning paradigm and discuss how individual differences in the reward system can contribute to resilience. Given the central role of the endocannabinoid system in regulating fear responses and anxiety, we discuss the evidence that polymorphisms in several molecules of this signaling system contribute to different anxiety phenotypes. The endocannabinoid system is highly interconnected with the serotoninergic and dopaminergic modulatory systems, contributing to individual differences in stress perception and coping mechanisms. We review how the individual variability in these modulatory systems can be used towards a multivariable assessment of stress risk. Incorporating individuality in our research will allow us to define biomarkers of anxiety disorders as well as assess prognosis, towards a personalized clinical approach to mental health.

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.

Prefrontal cortex circuits in depression and anxiety: contribution of discrete neuronal populations and target regions

Our understanding of depression and its treatment has advanced with the advent of ketamine as a rapid-acting antidepressant and the development and refinement of tools capable of selectively altering the activity of populations of neuronal subtypes. This work has resulted in a paradigm shift away from dysregulation of single neurotransmitter systems in depression towards circuit level abnormalities impacting function across multiple brain regions and neurotransmitter systems. Studies on the features of circuit level abnormalities demonstrate structural changes within the prefrontal cortex (PFC) and functional changes in its communication with distal brain structures. Treatments that impact the activity of brain regions, such as transcranial magnetic stimulation or rapid-acting antidepressants like ketamine, appear to reverse depression associated circuit abnormalities though the mechanisms underlying the reversal, as well as development of these abnormalities remains unclear. Recently developed optogenetic and chemogenetic tools that allow high-fidelity control of neuronal activity in preclinical models have begun to elucidate the contributions of the PFC and its circuitry to depression- and anxiety-like behavior. These tools offer unprecedented access to specific circuits and neuronal subpopulations that promise to offer a refined view of the circuit mechanisms surrounding depression and potential mechanistic targets for development and reversal of depression associated circuit abnormalities.

The Dorsal Raphe Regulates the Duration of Attack through the Medial Orbitofrontal Cortex and Medial Amygdala

The dorsal raphe (DR) is an evolutionarily conserved brain structure that is involved in aggressive behavior. It projects onto numerous cortical and limbic areas underlying attack behavior. The specific neurocircuit through which the DR regulates aggression, however, is largely unclear. In this study we show that DR neurons expressing CaMKIIα are activated by attack behavior in mice. These neurons project to the medial aspect of the orbitofrontal cortex (OFC; MeOC) and the medial amygdala (MeA), two key regions within the neural circuit known to control aggressive behavior. Using an in vivo optogenetic approach, we show that attack bouts are shortened by inhibiting CaMKIIα+ neurons in the DR and their axons at the MeOC and prolonged by stimulating the DR-MeOC axons during an attack. By contrast, stimulating the axons of CaMKIIα+ DR neurons at the MeA shortens attack. Notably, neither the DR-MeOC or DR-MeA pathway initiates attack when stimulated. These results indicate that the DR-MeOC and DR-MeA pathways regulate the duration of attack behavior in opposite directions, revealing a circuit mechanism for the control of attack by the DR.

Molecular Fingerprint and Developmental Regulation of the Tegmental GABAergic and Glutamatergic Neurons Derived from the Anterior Hindbrain

Tegmental nuclei in the ventral midbrain and anterior hindbrain control motivated behavior, mood, memory, and movement. These nuclei contain inhibitory GABAergic and excitatory glutamatergic neurons, whose molecular diversity and development remain largely unraveled. Many tegmental neurons originate in the embryonic ventral rhombomere 1 (r1), where GABAergic fate is regulated by the transcription factor (TF) Tal1. We used single-cell mRNA sequencing of the mouse ventral r1 to characterize the Tal1-dependent and independent neuronal precursors. We describe gene expression dynamics during bifurcation of the GABAergic and glutamatergic lineages and show how active Notch signaling promotes GABAergic fate selection in post-mitotic precursors. We identify GABAergic precursor subtypes that give rise to distinct tegmental nuclei and demonstrate that Sox14 and Zfpm2, two TFs downstream of Tal1, are necessary for the differentiation of specific tegmental GABAergic neurons. Our results provide a framework for understanding the development of cellular diversity in the tegmental nuclei.

A discrete serotonergic circuit regulates vulnerability to social stress

Exposure to social stress and dysregulated serotonergic neurotransmission have both been implicated in the etiology of psychiatric disorders. However, the serotonergic circuit involved in stress vulnerability is still unknown. Here, we explored whether a serotonergic input from the dorsal raphe (DR) to ventral tegmental area (VTA) influences vulnerability to social stress. We identified a distinct, anatomically and functionally defined serotonergic subpopulation in the DR that projects to the VTA (5-HTDR→VTA neurons). Moreover, we found that susceptibility to social stress decreased the firing activity of 5-HTDR→VTA neurons. Importantly, the bidirectional manipulation of 5-HTDR→VTA neurons could modulate susceptibility to social stress. Our findings reveal that the activity of 5-HTDR→VTA neurons may be an essential factor in determining individual levels of susceptibility to social stress and suggest that targeting specific serotonergic circuits may aid the development of therapies for the treatment of stress-related disorders.

Enriched Environment and Social Isolation Affect Cognition Ability via Altering Excitatory and Inhibitory Synaptic Density in Mice Hippocampus

The purpose of the study was to examine whether the underlying mechanism of the alteration of cognitive ability and synaptic plasticity induced by the housing environment is associated with the balance of excitatory/inhibitory synaptic density. Enriched environment (EE) and social isolation (SI) are two different housing environment, and one is to give multiple sensory environments, the other is to give monotonous and lonely environment. Male 4-week-old C57 mice were divided into three groups: CON, EE and SI. They were housed in the different cage until 3 months of age. Morris water maze and novel object recognition were performed. Long term potentiation (LTP), depotentiation (DEP) and local field potentials were recorded in the hippocampal perforant pathway and dentate gyrus (DG) region. The data showed that EE enhanced the ability of spatial learning, reversal learning and memory as well as LTP/DEP in the hippocampal DG region. Meanwhile, SI reduced those abilities and the level of LTP/DEP. Moreover, there were higher couplings of both phase-amplitude and phase-phase in the EE group, and lower couplings of them in the SI group compared to that in the CON group. Western blot and immunofluorescence analysis showed that EE significantly enhanced the level of PSD-95, NR2B and DCX; however, SI reduced them but increased GABA_AR_α1 and decreased DCX levels. The data suggests that the cognitive functions, synaptic plasticity, neurogenesis and neuronal oscillatory patterns were significantly impacted by housing environment via possibly changing the balance of excitatory and inhibitory synaptic density.

Corticotropin releasing factor modulates excitatory synaptic transmission

The mammalian brain contains many regions which synthesize and release the hormone and transmitter corticotropin releasing factor. This peptide is a key player in the function of the hypothalamic-pituitary-adrenal axis and has major role in mediating the endocrine limb of the stress response. However, there are several regions outside of the paraventricular nucleus of the hypothalamus which synthesize this peptide in which it has a role more akin to a classical neurotransmitter. A significant body of literature exists in which its role as a transmitter and its cellular effects in many brain regions, as well as how it affects various forms of behavior, is described. However, the receptors which corticotropin releasing factor interacts with in the brain are G-protein coupled receptors, and therefore their activation promotes a multitude of cellular effects. Despite this, comparatively little research has been done to investigate how this peptide affects excitatory synaptic transmission in the brain. This is important because both excitatory and inhibitory regulation of physiology are important extrinsic factors in the operation of neurons which occur in conjunction with their intrinsic properties. By not taking into account how corticotropin releasing factor affects these processes, a complete picture of this peptide's role in brain function is not available. In this chapter, the limited body of research which has explicitly investigated how corticotropin releasing factor affects excitatory synaptic transmission in various brain regions will be explored.

A single-cell transcriptomic and anatomic atlas of mouse dorsal raphe Pet1 neurons

Among the brainstem raphe nuclei, the dorsal raphe nucleus (DR) contains the greatest number of Pet1-lineage neurons, a predominantly serotonergic group distributed throughout DR subdomains. These neurons collectively regulate diverse physiology and behavior and are often therapeutically targeted to treat affective disorders. Characterizing Pet1 neuron molecular heterogeneity and relating it to anatomy is vital for understanding DR functional organization, with potential to inform therapeutic separability. Here we use high-throughput and DR subdomain-targeted single-cell transcriptomics and intersectional genetic tools to map molecular and anatomical diversity of DR-Pet1 neurons. We describe up to fourteen neuron subtypes, many showing biased cell body distributions across the DR. We further show that P2ry1-Pet1 DR neurons - the most molecularly distinct subtype - possess unique efferent projections and electrophysiological properties. These data complement and extend previous DR characterizations, combining intersectional genetics with multiple transcriptomic modalities to achieve fine-scale molecular and anatomic identification of Pet1 neuron subtypes.