Social stress alters inhibitory synaptic input to distinct subpopulations of raphe serotonin neurons

Gut Barrier Dysfunction and Type 2 Immunity: Implications for Compulsive Behavior

To date, much of the focus of gut-brain axis research has been on gut microbiota regulation of anxiety and stress-related behaviors. Much less attention has been directed to potential connections between gut microbiota and compulsive behavior. Here, we discuss a potential link between gut barrier dysfunction and compulsive behavior that is mediated through "type 2" rather than "type 1" inflammation. We examine connections between compulsive behavior and type 2 inflammation in Tourette syndrome, obsessive-compulsive disorder, autism, addiction, and post-traumatic stress disorder. Next, we discuss potential connections between gut barrier dysfunction, type 2 inflammation, and compulsive behavior. We posit a potential mechanism whereby gut barrier dysfunction-associated type 2 inflammation may drive compulsive behavior through histamine regulation of dopamine neurotransmission. Finally, we discuss the possibility of exploiting the greater accessibility of the gut relative to the brain in identifying targets to treat compulsive behavior disorders.

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.

Changes in striatal dopamine transporters in bipolar disorder and valproate treatment

Background

Previous studies suggested that a disturbance of the dopamine system underlies the pathophysiology of bipolar disorder (BD). In addition, the therapeutic action of medications for treating BD, such as valproate (VPA), might modulate dopamine system activity, but it remains unclear. Here, we aimed to investigate the role of the striatal dopamine transporter (DAT) in BD patients and in social defeat (SD) mice treated with VPA.

Methods

We enrolled community-dwelling controls (N = 18) and BD patients (N = 23) who were treated with VPA in a euthymic stage. The striatal DAT availabilities were approached by TRODAT-1 single photon emission computed tomography. We also established a chronic SD mouse model and treated mice with 350 mg/kg VPA for 3 weeks. Behavioral tests were administered, and striatal DAT expression levels were determined.

Results

In humans, the level of striatal DAT availability was significantly higher in euthymic BD patients (1.52 ± 0.17 and 1.37 ± 0.23, p = 0.015). Moreover, the level of striatal DAT availability was also negatively correlated with the VPA concentration in BD patients (r = −0.653, p = 0.003). In SD mice, the expression of striatal DAT significantly increased (p < 0.001), and the SD effect on DAT expression was rescued by VPA treatment.

Conclusions

The striatal DAT might play a role in the pathophysiology of BD and in the therapeutic mechanism of VPA. The homeostasis of DAT might represent a new therapeutic strategy for BD patients.

Inhibition of monoamine oxidase attenuates social defeat-induced memory impairment in goldfish, (Carassius auratus): A possible involvement of synaptic proteins and BDNF

Social defeat (SD) has been implicated in different modulatory effects of physiology and behaviour including learning and memory. We designed an experiment to test the functional role of monoamine oxidase (MAO) in regulation of synaptic transmission, synaptic plasticity and memory in goldfish Carassius auratus. To test this, individuals were divided into three groups: (i) control; (ii) social defeat (SD) group (individuals were subjected to social defeat for 10 min by Pseudotropheus demasoni) and (iii) SD + MAO inhibitor pre-treated group. All experimental groups were subjected to spatial learning and then memory. Our results suggest that SD affects a spatial learning and memory, whereas SD exerts no influence on MAOI pre-treated group. In addition, we noted that the expression of monoamine oxidase-A (MAO-A) was up-regulated and level of serotonin (5-hydroxytryptamine; 5-HT), expression of serotonin transporter (SERT), synaptophysin (SYP), synaptotagmin -1 (SYT-1), N-methyl-D-asparate (NMDA) receptors subunits (NR2A and NR2B), postsynaptic density-95 (PSD-95) and brain-derived neurotrophic factor (BDNF) were reduced by SD, while MAOIs pretreatment protects the effect of SD. Taken together, our results suggest that MAO is an essential component in the serotonergic system that finely tunes the level of 5-HT, which further regulates the molecules involving in synaptic transmission, synaptic plasticity and memory.

Sex-Specific Role for Dopamine Receptor D2 in Dorsal Raphe Serotonergic Neuron Modulation of Defensive Acoustic Startle and Dominance Behavior

Brain networks underlying states of social and sensory alertness are normally adaptive, influenced by serotonin and dopamine (DA), and abnormal in neuropsychiatric disorders, often with sex-specific manifestations. Underlying circuits, cells, and molecules are just beginning to be delineated. Implicated is a subtype of serotonergic neuron denoted Drd2-Pet1, distinguished by expression of the type-2 DA receptor (Drd2) gene, inhibited cell-autonomously by DRD2 agonism in slice, and, when constitutively silenced in male mice, affects levels of defensive and exploratory behaviors (Niederkofler et al., 2016). Unknown has been whether DRD2 signaling in these Pet1 neurons contributes to their capacity for shaping defensive behaviors. To address this, we generated mice in which Drd2 gene sequences were deleted selectively in Pet1 neurons. We found that Drd2^Pet1-CKO males, but not females, demonstrated increased winning against sex-matched controls in a social dominance assay. Drd2^Pet1-CKO females, but not males, exhibited blunting of the acoustic startle response, a protective, defensive reflex. Indistinguishable from controls were auditory brainstem responses (ABRs), locomotion, cognition, and anxiety-like and depression-like behaviors. Analyzing wild-type Drd2-Pet1 neurons, we found sex-specific differences in the proportional distribution of axonal collaterals, in action potential (AP) duration, and in transcript levels of Gad2, important for GABA synthesis. Drd2^Pet1-CKO cells displayed sex-specific differences in the percentage of cells harboring Gad2 transcripts. Our results suggest that DRD2 function in Drd2-Pet1 neurons is required for normal defensive/protective behaviors in a sex-specific manner, which may be influenced by the identified sex-specific molecular and cellular features. Related behaviors in humans too show sex differences, suggesting translational relevance.

Ketamine Administration Reverses Corticosterone-Induced Alterations in Excitatory and Inhibitory Transmission in the Rat Dorsal Raphe Nucleus

Ketamine, a N-methyl-D-aspartate (NMDA) receptor antagonist, exerts rapid antidepressant effects in human patients and ameliorates depressive-like behavioral effects of chronic stress in animal models. Chronic stress and elevated corticosterone levels have been shown to modify serotonin (5-HT) neurotransmission, and ketamine's antidepressant-like activity involves a 5-HT-dependent mechanism. However, it is not known if and how ketamine affects the electrophysiological characteristics of neurons and synaptic transmission within the dorsal raphe nucleus (DRN), the main source of 5-HT forebrain projections. Our study was aimed at investigating the effects of a single ketamine administration on excitatory and inhibitory transmission in the DRN of rats which had previously been administered corticosterone twice daily for 7 days. Spontaneous excitatory and inhibitory postsynaptic currents (sEPSCs and sIPSCs) were then recorded from DRN projection cells in ex vivo slice preparations obtained 24 h after ketamine injection. Repeated corticosterone administration increased sEPSC frequency and decreased sIPSC frequency in DRN projection cells. There were no changes either in the amplitude of postsynaptic currents or in the excitability of these cells. In slices prepared from rats with ketamine administered after the end of corticosterone treatment, the frequencies of sEPSCs and sIPSCs were similar to those in control preparations. These data indicate that a single administration of ketamine reversed the effects of corticosterone on excitatory and inhibitory transmission in the DRN.

Psychoanalysis and Affective Neuroscience. The Motivational/Emotional System of Aggression in Human Relations

This article highlights the evolutionary biological epistemology in Freud psychoanalytic theory. The concepts of aggressive and sexual drives are cornerstones of the psychoanalytic epistemological system, concerning the motivational/emotional roots of mental functioning. These biological roots of mental functioning, especially with regard to aggressive drive, have gradually faded away from psychoanalytic epistemology, as we show in this article. Currently, however, Neurosciences, and in particular Affective Neuroscience (Panksepp, 1998), can help us to have a better understanding of the biological roots of human mental functioning. The motivational/emotional systems studied by Affective Neuroscience can give a new epistemological foundation to the aggressive drive concept in psychoanalytic theory. Over the course of human evolution, motivational/emotional systems have played a role in social relationships and also in mental functioning. In this regard, among the various types of aggression (ANGER in Panksepp taxonomy 1998) that we consider in our article, inter-male aggression, also named Dominance motivational/emotional system, is that which regulates social interactions between sexually matured adults. This type of aggression acts in complementary connection with FEAR motivational/emotional system that regulate submissive behavior and social defeat, and the latter one is of the more important stressors. The interaction between aggression and FEAR motivational/emotional systems gives rise to agonistic behavior or dominance/submission motivational/emotional system, as we propose in our article. There is now a large literature that identifies in the dynamic of Competitive behavior, which is one of the main factors of mental illness. When social interactions activate the competitive behavior, the subject can perceive himself as "destined to victory" or "destined to defeat," activating either behaviors or emotions connected to the Involuntary Defeat Strategy or Involuntary Dominant Strategy (Sloman, 2002), which we can find in many types of mental disorders, for example, mood disorders or anxiety disorders.

Molecular Characterization of GABA-A Receptor Subunit Diversity within Major Peripheral Organs and Their Plasticity in Response to Early Life Psychosocial Stress

Gamma aminobutyric acid (GABA) subtype A receptors (GABA_ARs) are integral membrane ion channels composed of five individual proteins or subunits. Up to 19 different GABA_AR subunits (α1–6, β1–3, γ1–3, δ, ε, θ, π, and ρ1–3) have been identified, resulting in anatomically, physiologically, and pharmacologically distinct multiple receptor subtypes, and therefore GABA-mediated inhibition, across the central nervous system (CNS). Additionally, GABA_AR-modulating drugs are important tools in clinical medicine, although their use is limited by adverse effects. While significant advances have been made in terms of characterizing the GABA_AR system within the brain, relatively less is known about the molecular phenotypes within the peripheral nervous system of major organ systems. This represents a potentially missed therapeutic opportunity in terms of utilizing or repurposing clinically available GABA_AR drugs, as well as promising research compounds discarded due to their poor CNS penetrance, for the treatment of peripheral disorders. In addition, a broader understanding of the peripheral GABA_AR subtype repertoires will contribute to the design of therapies which minimize peripheral side-effects when treating CNS disorders. We have recently provided a high resolution molecular and function characterization of the GABA_ARs within the enteric nervous system of the mouse colon. In this study, the aim was to determine the constituent GABA_AR subunit expression profiles of the mouse bladder, heart, liver, kidney, lung, and stomach, using reverse transcription polymerase chain reaction and western blotting with brain as control. The data indicate that while some subunits are expressed widely across various organs (α3–5), others are restricted to individual organs (γ2, only stomach). Furthermore, we demonstrate complex organ-specific developmental expression plasticity of the transporters which determine the chloride gradient within cells, and therefore whether GABA_AR activation has a depolarizing or hyperpolarizing effect. Finally, we demonstrate that prior exposure to early life psychosocial stress induces significant changes in peripheral GABA_AR subunit expression and chloride transporters, in an organ- and subunit-specific manner. Collectively, the data demonstrate the molecular diversity of the peripheral GABA_AR system and how this changes dynamically in response to life experience. This provides a molecular platform for functional analyses of the GABA–GABA_AR system in health, and in diseases affecting various peripheral organs.

The 5-HT7 receptor antagonist SB 269970 ameliorates corticosterone-induced alterations in 5-HT7 receptor-mediated modulation of GABAergic transmission in the rat dorsal raphe nucleus

RATIONALE

Chronic stress and corticosterone have been shown to affect serotonin (5-HT) neurotransmission; however, the influence of stress on the activity of the dorsal raphe nucleus (DRN), the main source of 5-HT in the forebrain, is not well understood. In particular, it is unknown if and how stress modifies DRN 5-HT₇ receptors, which are involved in the modulation of the firing of local inhibitory interneurons responsible for regulating the activity of DRN projection cells.

OBJECTIVES

Our study aimed to investigate the effect of repeated corticosterone injections on the modulation of the inhibitory transmission within the DRN by 5-HT₇ receptors and whether it could be reversed by treatment with a 5-HT₇ receptor antagonist.

METHODS

Male Wistar rats received corticosterone injections repeated twice daily for 14 days. Spontaneous inhibitory postsynaptic currents (sIPSCs) were then recorded from DRN projection cells in ex vivo slice preparations obtained 24 h after the last injection.

RESULTS

Repeated corticosterone administration resulted in decreased frequency, but not amplitude, of sIPSCs in DRN projection cells. There were no changes in the excitability of these cells; however, corticosterone treatment suppressed the 5-HT₇ receptor-mediated increase in sIPSC frequency. Administration of the 5-HT₇ receptor antagonist SB 269970 for 7 days beginning on the eighth day of corticosterone treatment reversed the detrimental effects of corticosterone on 5-HT₇ receptor reactivity and GABAergic transmission in the DRN.

CONCLUSIONS

Elevated corticosterone level reduces DRN 5HT₇ receptor reactivity and decreases GABAergic transmission within the DRN, which can be reversed by the 5-HT₇ receptor antagonist SB 269970.

Social experience alters socially induced serotonergic fluctuations in the inferior colliculus

Past social experience and current social context shape the responses of animals to social signals. The serotonergic system is one potential mechanism by which both experiential and contextual factors could be conveyed to sensory systems, such as the auditory system, for multiple reasons. 1) Many features of the serotonergic system are sensitive to social experience. 2) Elevations in serotonergic activity are triggered by social partners, and variations in socially triggered serotonergic responses reflect behavioral differences among social encounters. 3) Serotonin is an auditory neuromodulator, altering how auditory neurons respond to sounds including conspecific vocalizations. In this study, we tested how social experience influences the socially triggered serotonergic response in the inferior colliculus, an auditory midbrain region with an important role in vocalization processing. We used carbon fiber voltammetry to measure serotonin during social interactions of male mice ( Mus musculus) from different social backgrounds: 4 weeks of grouped or individual housing. When paired with an unfamiliar male, both group-housed and individually housed males demonstrated elevations in serotonin; however, individually housed males exhibited socially triggered serotonergic responses with delayed time courses compared with the group-housed males. Furthermore, group-housed males displayed previously described correlations between the socially triggered serotonergic response and behaviors such as social investigation. In contrast, individually housed males did not show these serotonin-behavior relationships. These results suggest that social experience gained via social housing may shape the ability of the central serotonergic system to encode social context in sensory regions.

NEW & NOTEWORTHY We demonstrate that past social experience influences the fidelity with which the serotonergic system represents social context in an auditory region. Social experience altered the time course of socially triggered serotonergic responses and changed how the serotonergic system reflects behavioral variations among social encounters of the same context. These findings are significant to the study of communication, suggesting that centralized neuromodulatory systems potentially convey integrated information regarding past experience and current context to primary sensory regions.

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

Association of social defeat stress-induced anhedonia-like symptoms with mGluR1-dependent decrease in membrane-bound AMPA-GluR1 in the mouse ventral midbrain

Anhedonia is a core symptom of social defeat stress (SDS)-induced depression associated with the reward system. We previously reported that decreased membrane-bound AMPA-GluR1 in the reward system is associated with lipopolysaccharide-induced anhedonia-like symptoms. Since group I metabotropic glutamate receptor (mGluR) activation reduces the surface density of GluR1, we examined whether group I mGluR-dependent decrease in membrane-bound GluR1 in the reward system is involved in SDS-induced anhedonia-like symptoms. Mice exposed to SDS for 4 consecutive days had markedly decreased membrane-bound GluR1 and GluR2 in the prefrontal cortex (PFC) and membrane-bound GluR1 in the ventral midbrain (VM) along with lower sucrose preference (SP). Intra-PFC injection of the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG; 100 μmol) demonstrated decrease in membrane-bound GluR1 and GluR2 in the PFC 2 and 24 h and membrane-bound GluR1 in the VM 24 h after injection. Moreover, intra-PFC injection of DHPG decreased SP only in the second 24-h (24-48 h) period. Conversely, intra-VM injection of DHPG decreased SP in both the first and second 24-h period and decreased membrane-bound GluR1 in the VM 2 and 24 h after injection. Pre-treatment with the mGluR1 antagonist JNJ16259685 (30 mg/kg, subcutaneous) prevented SDS-decreased SP and membrane-bound GluR1 in the VM. The mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine (MPEP; 10 mg/kg, subcutaneous) prevented SDS-induced decrease in membrane-bound GluR1 and GluR2 in the PFC, whereas MPEP did not affect SDS-induced decrease in SP and membrane-bound GluR1 in the VM. These results suggest that mGluR1-mediated decrease in membrane-bound GluR1 in VM is involved in SDS-induced anhedonia-like symptoms.

Refining the Role of 5-HT in Postnatal Development of Brain Circuits

Changing serotonin (5-hydroxytryptamine, 5-HT) brain levels during critical periods in development has long-lasting effects on brain function, particularly on later anxiety/depression-related behaviors in adulthood. A large part of the known developmental effects of 5-HT occur during critical periods of postnatal life, when activity-dependent mechanisms remodel neural circuits. This was first demonstrated for the maturation of sensory brain maps in the barrel cortex and the visual system. More recently this has been extended to the 5-HT raphe circuits themselves and to limbic circuits. Recent studies overviewed here used new genetic models in mice and rats and combined physiological and structural approaches to provide new insights on the cellular and molecular mechanisms controlled by 5-HT during late stages of neural circuit maturation in the raphe projections, the somatosensory cortex and the visual system. Similar mechanisms appear to be also involved in the maturation of limbic circuits such as prefrontal circuits. The latter are of particular relevance to understand the impact of transient 5-HT dysfunction during postnatal life on psychiatric illnesses and emotional disorders in adult life.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Social defeat as an animal model for depression

Depression is one of the most disabling medical conditions in the world today, yet its etiologies remain unclear and current treatments are not wholly effective. Animal models are a powerful tool to investigate possible causes and treatments for human diseases. We describe an animal model of social defeat as a possible model for human depression. We discuss the paradigm, behavioral correlates to depression, and potential underlying neurobiological mechanisms with an eye toward possible future therapies.

Lateral habenula and the rostromedial tegmental nucleus innervate neurochemically distinct subdivisions of the dorsal raphe nucleus in the rat

The lateral habenula (LHb) is an epithalamic structure differentiated in a medial (LHbM) and a lateral division (LHbL). Together with the rostromedial tegmental nucleus (RMTg), the LHb has been implicated in the processing of aversive stimuli and inhibitory control of monoamine nuclei. The inhibitory LHb influence on midbrain dopamine neurons has been shown to be mainly mediated by the RMTg, a mostly GABAergic nucleus that receives a dominant input from the LHbL. Interestingly, the RMTg also projects to the dorsal raphe nucleus (DR), which also receives direct LHb projections. To compare the organization and transmitter phenotype of LHb projections to the DR, direct and indirect via the RMTg, we first placed injections of the anterograde tracer Phaseolus vulgaris leucoagglutinin into the LHb or the RMTg. We then confirmed our findings by retrograde tracing and investigated a possible GABAergic phenotype of DR-projecting RMTg neurons by combining retrograde tracing with in situ hybridization for GAD67. We found only moderate direct LHb projections to the DR, which mainly emerged from the LHbM and were predominantly directed to the serotonin-rich caudal DR. In contrast, RMTg projections to the DR were more robust, emerged from RMTg neurons enriched in GAD67 mRNA, and were focally directed to a distinctive DR subdivision immunohistochemically characterized as poor in serotonin and enriched in presumptive glutamatergic neurons. Thus, besides its well-acknowledged role as a GABAergic control center for the ventral tegmental area (VTA)-nigra complex, our findings indicate that the RMTg is also a major GABAergic relay between the LHb and the DR.

Antidepressant-like effects of cortical deep brain stimulation coincide with pro-neuroplastic adaptations of serotonin systems

BACKGROUND

Cortical deep brain stimulation (DBS) is a promising therapeutic option for treatment-refractory depression, but its mode of action remains enigmatic. Serotonin (5-HT) systems are engaged indirectly by ventromedial prefrontal cortex (vmPFC) DBS. Resulting neuroplastic changes in 5-HT systems could thus coincide with the long-term therapeutic activity of vmPFC DBS.

METHODS

We tested this hypothesis by evaluating the antidepressant-like activity of vmPFC DBS in the chronic social defeat stress (CSDS) model of depression (n = 8-13 mice/group). Circuit-wide activation induced by vmPFC DBS was mapped with c-Fos immunolabeling. The effects of chronic vmPFC DBS on the physiology and morphology of genetically identified 5-HT cells from the dorsal raphe nucleus (DRN) were examined with whole-cell recording, somatodendritic three-dimensional reconstructions and morphometric analyses of presynaptic boutons along 5-HT axons.

RESULTS

Acute DBS drove c-Fos expression locally in the vmPFC and in several distal monosynaptically connected regions, including the DRN. Chronic DBS reversed CSDS-induced social avoidance, restored the disrupted balance of excitatory/inhibitory inputs onto 5-HT neurons, and reversed 5-HT hypoexcitability observed after CSDS. Furthermore, vmPFC DBS reversed CSDS-induced arborization of 5-HT dendrites in the DRN and increased the size and density of 5-HT presynaptic terminals in the dentate gyrus and vmPFC.

CONCLUSIONS

We validate a new preclinical paradigm to examine cellular mechanisms underlying the antidepressant-like activity of vmPFC DBS and identify dramatic circuit-mediated cellular adaptations that coincide with this treatment. These neuroplastic changes of 5-HT neurons might contribute to the progressive mood improvements reported in patients treated with chronic courses of cortical DBS.

Localisation and stress-induced plasticity of GABAA receptor subunits within the cellular networks of the mouse dorsal raphe nucleus

The dorsal raphe nucleus (DRN) provides the major source of serotonin to the central nervous system (CNS) and modulates diverse neural functions including mood. Furthermore, DRN cellular networks are engaged in the stress-response at the CNS level allowing for adaptive behavioural responses, whilst stress-induced dysregulation of DRN and serotonin release is implicated in psychiatric disorders. Therefore, identifying the molecules regulating DRN activity is fundamental to understand DRN function in health and disease. GABAA receptors (GABAARs) allow for brain region, cell type and subcellular domain-specific GABA-mediated inhibitory currents and are thus key regulators of neuronal activity. Yet, the GABAAR subtypes expressed within the neurochemically diverse cell types of the mouse DRN are poorly described. In this study, immunohistochemistry and confocal microscopy revealed that all serotonergic neurons expressed immunoreactivity for the GABAAR alpha2 and 3 subunits, although the respective signals were co-localised to varying degrees with inhibitory synaptic marker proteins. Only a topographically located sub-population of serotonergic neurons exhibited GABAAR alpha1 subunit immunoreactivity. However, all GABAergic as well as non-GABAergic, non-serotonergic neurons within the DRN expressed GABAAR alpha1 subunit immunoreactivity. Intriguingly, immunoreactivity for the GABAAR gamma2 subunit was enriched on GABAergic rather than serotonergic neurons. Finally, repeated restraint stress increased the expression of the GABAAR alpha3 subunit at the mRNA and protein level. The study demonstrates the identity and location of distinct GABAAR subunits within the cellular networks of the mouse DRN and that stress impacts on the expression levels of particular subunits at the gene and protein level.

Optogenetic modulation of descending prefrontocortical inputs to the dorsal raphe bidirectionally bias socioaffective choices after social defeat

It has been well established that modulating serotonin (5-HT) levels in humans and animals affects perception and response to social threats, however the circuit mechanisms that control 5-HT output during social interaction are not well understood. A better understanding of these systems could provide groundwork for more precise and efficient therapeutic interventions. Here we examined the organization and plasticity of microcircuits implicated in top-down control of 5-HT neurons in the dorsal raphe nucleus (DRN) by excitatory inputs from the ventromedial prefrontal cortex (vmPFC) and their role in social approach-avoidance decisions. We did this in the context of a social defeat model that induces a long lasting form of social aversion that is reversible by antidepressants. We first used viral tracing and Cre-dependent genetic identification of vmPFC glutamatergic synapses in the DRN to determine their topographic distribution in relation to 5-HT and GABAergic subregions and found that excitatory vmPFC projections primarily localized to GABA-rich areas of the DRN. We then used optogenetics in combination with cFos mapping and slice electrophysiology to establish the functional effects of repeatedly driving vmPFC inputs in DRN. We provide the first direct evidence that vmPFC axons drive synaptic activity and immediate early gene expression in genetically identified DRN GABA neurons through an AMPA receptor-dependent mechanism. In contrast, we did not detect vmPFC-driven synaptic activity in 5-HT neurons and cFos induction in 5-HT neurons was limited. Finally we show that optogenetically increasing or decreasing excitatory vmPFC input to the DRN during sensory exposure to an aggressor's cues enhances or diminishes avoidance bias, respectively. These results clarify the functional organization of vmPFC-DRN pathways and identify GABAergic neurons as a key cellular element filtering top-down vmPFC influences on affect-regulating 5-HT output.

Serotonergic neuron regulation informed by in vivo single-cell transcriptomics

Despite the recognized importance of the dorsal raphe (DR) serotonergic (5-HT) nuclei in the pathophysiology of depression and anxiety, the molecular components/putative drug targets expressed by these neurons are poorly characterized. Utilizing the promoter of an ETS domain transcription factor that is a stable marker of 5-HT neurons (Pet-1) to drive 5-HT neuronal expression of YFP, we identified 5-HT neurons in live acute slices. We isolated RNA from single 5-HT neurons in the ventromedial and lateral wings of the DR and performed single-cell RNA-Seq analysis identifying >500 G-protein coupled receptors (GPCRs) including receptors for classical transmitters, lipid signals, and peptides as well as dozens of orphan-GPCRs. Using these data to inform our selection of receptors to assess, we found that oxytocin and lysophosphatidic acid 1 receptors are translated and active in costimulating, with the α1-adrenergic receptor, the firing of DR 5-HT neurons, while the effects of histamine are inhibitory and exerted at H3 histamine receptors. The inhibitory histamine response provides evidence for tonic in vivo histamine inhibition of 5-HT neurons. This study illustrates that unbiased single-cell transcriptomics coupled with functional analyses provides novel insights into how neurons and neuronal systems are regulated.

Raphe GABAergic neurons mediate the acquisition of avoidance after social defeat

Serotonin (5-HT) modulates neural responses to socioaffective cues and can bias approach or avoidance behavioral decisions, yet the cellular mechanisms underlying its contribution to the regulation of social experiences remain poorly understood. We hypothesized that GABAergic neurons in the dorsal raphe nucleus (DRN) may participate in socioaffective regulation by controlling serotonergic tone during social interaction. We tested this hypothesis using whole-cell recording techniques in genetically identified DRN GABA and 5-HT neurons in mice exposed to social defeat, a model that induces long-lasting avoidance behaviors in a subset of mice responsive to serotonergic antidepressants. Our results revealed that social defeat engaged DRN GABA neurons and drove GABAergic sensitization that strengthened inhibition of 5-HT neurons in mice that were susceptible, but not resilient to social defeat. Furthermore, optogenetic silencing of DRN GABA neurons disinhibited neighboring 5-HT neurons and prevented the acquisition of social avoidance in mice exposed to a social threat, but did not affect a previously acquired avoidance phenotype. We provide the first characterization of GABA neurons in the DRN that monosynaptically inhibit 5-HT neurons and reveal their key role in neuroplastic processes underlying the development of social avoidance.

Relationships between radial glial progenitors and 5-HT neurons in the paraventricular organ of adult zebrafish - potential effects of serotonin on adult neurogenesis

In non-mammalian vertebrates, serotonin (5-HT)-producing neurons exist in the paraventricular organ (PVO), a diencephalic structure containing cerebrospinal fluid (CSF)-contacting neurons exhibiting 5-HT or dopamine (DA) immunoreactivity. Because the brain of the adult teleost is known for its neurogenic activity supported, for a large part, by radial glial progenitors, this study addresses the origin of newborn 5-HT neurons in the hypothalamus of adult zebrafish. In this species, the PVO exhibits numerous radial glial cells (RGCs) whose somata are located at a certain distance from the ventricle. To study relationships between RGCs and 5-HT CSF-contacting neurons, we performed 5-HT immunohistochemistry in transgenic tg(cyp19a1b-GFP) zebrafish in which RGCs are labelled with GFP under the control of the cyp19a1b promoter. We show that the somata of the 5-HT neurons are located closer to the ventricle than those of RGCs. RGCs extend towards the ventricle cytoplasmic processes that form a continuous barrier along the ventricular surface. In turn, 5-HT neurons contact the CSF via processes that cross this barrier through small pores. Further experiments using proliferating cell nuclear antigen or 5-bromo-2'-deoxyuridine indicate that RGCs proliferate and give birth to 5-HT neurons migrating centripetally instead of centrifugally as in other brain regions. Furthermore, treatment of adult zebrafish with tryptophan hydroxylase inhibitor causes a significant decrease in the number of proliferating cells in the PVO, but not in the mediobasal hypothalamus. These data point to the PVO as an intriguing region in which 5-HT appears to promote genesis of 5-HT neurons that accumulate along the brain ventricles and contact the CSF.