The ventral hippocampus and the lateral septum work in tandem to regulate rats' open-arm exploration in the elevated plus-maze

Prenatal tetrahydrocannabinol and cannabidiol exposure produce sex-specific pathophysiological phenotypes in the adolescent prefrontal cortex and hippocampus

Clinical and preclinical evidence has demonstrated an increased risk for neuropsychiatric disorders following prenatal cannabinoid exposure. However, given the phytochemical complexity of cannabis, there is a need to understand how specific components of cannabis may contribute to these neurodevelopmental risks later in life. To investigate this, a rat model of prenatal cannabinoid exposure was utilized to examine the impacts of specific cannabis constituents (Δ9-tetrahydrocannabinol [THC]; cannabidiol [CBD]) alone and in combination on future neuropsychiatric liability in male and female offspring. Prenatal THC and CBD exposure were associated with low birth weight. At adolescence, offspring displayed sex-specific behavioural changes in anxiety, temporal order and social cognition, and sensorimotor gating. These phenotypes were associated with sex and treatment-specific neuronal and gene transcriptional alterations in the prefrontal cortex, and ventral hippocampus, regions where the endocannabinoid system is implicated in affective and cognitive development. Electrophysiology and RT-qPCR analysis in these regions implicated dysregulation of the endocannabinoid system and balance of excitatory and inhibitory signalling in the developmental consequences of prenatal cannabinoids. These findings reveal critical insights into how specific cannabinoids can differentially impact the developing fetal brains of males and females to enhance subsequent neuropsychiatric risk.

Ventral hippocampal projections to infralimbic cortex and basolateral amygdala are differentially activated by contextual fear and extinction recall

Fear and extinction learning are thought to generate distinct and competing memory representations in the hippocampus. How these memory representations modulate the expression of appropriate behavioral responses remains unclear. To investigate this question, we used cholera toxin B subunit to retrolabel ventral hippocampal (vHPC) neurons projecting to the infralimbic cortex (IL) and basolateral amygdala (BLA) and then quantified c-Fos immediate early gene activity within these populations following expression of either contextual fear recall or contextual fear extinction recall. Fear recall was associated with increased c-Fos expression in vHPC projections to the BLA, whereas extinction recall was associated with increased activity in vHPC projections to IL. A control experiment was performed to confirm that the apparent shift in projection neuron activity was associated with extinction learning rather than mere context exposure. Overall, results indicate that hippocampal contextual fear and extinction memory representations differentially activate vHPC projections to IL and BLA. These findings suggest that hippocampal memory representations orchestrate appropriate behavioral responses through selective activation of projection pathways.

Developmental origin and local signals cooperate to determine septal astrocyte identity

Astrocyte specification during development is influenced by both intrinsic and extrinsic factors, but the precise contribution of each remains poorly understood. Here we show that septal astrocytes from Nkx2.1 and Zic4 expressing progenitor zones are allocated into non-overlapping domains of the medial (MS) and lateral septal nuclei (LS) respectively. Astrocytes in these areas exhibit distinctive molecular and morphological features tailored to the unique cellular and synaptic circuit environment of each nucleus. Using single-nucleus (sn) RNA sequencing, we trace the developmental trajectories of cells in the septum and find that neurons and astrocytes undergo region and developmental stage-specific local cell-cell interactions. We show that expression of the classic morphogens Sonic hedgehog (Shh) and Fibroblast growth factors (Fgfs) by MS and LS neurons respectively, functions to promote the molecular specification of local astrocytes in each region. Finally, using heterotopic cell transplantation, we show that both morphological and molecular specifications of septal astrocytes are highly dependent on the local microenvironment, regardless of developmental origins. Our data highlights the complex interplay between intrinsic and extrinsic factors shaping astrocyte identities and illustrates the importance of the local environment in determining astrocyte functional specialization.

Hippocampus: Molecular, Cellular, and Circuit Features in Anxiety

Anxiety disorders are currently a major psychiatric and social problem, the mechanisms of which have been only partially elucidated. The hippocampus serves as a major target of stress mediators and is closely related to anxiety modulation. Yet so far, its complex anatomy has been a challenge for research on the mechanisms of anxiety regulation. Recent advances in imaging, virus tracking, and optogenetics/chemogenetics have permitted elucidation of the activity, connectivity, and function of specific cell types within the hippocampus and its connected brain regions, providing mechanistic insights into the elaborate organization of the hippocampal circuitry underlying anxiety. Studies of hippocampal neurotransmitter systems, including glutamatergic, GABAergic, cholinergic, dopaminergic, and serotonergic systems, have contributed to the interpretation of the underlying neural mechanisms of anxiety. Neuropeptides and neuroinflammatory factors are also involved in anxiety modulation. This review comprehensively summarizes the hippocampal mechanisms associated with anxiety modulation, based on molecular, cellular, and circuit properties, to provide tailored targets for future anxiety treatment.

Behavioral phenotype, intestinal microbiome, and brain neuronal activity of male serotonin transporter knockout mice

The serotonin transporter (5-HTT) plays a critical role in the regulation of serotonin neurotransmission. Mice genetically deficient in 5-HTT expression have been used to study the physiological functions of 5-HTT in the brain and have been proposed as a potential animal model for neuropsychiatric and neurodevelopmental disorders. Recent studies have provided evidence for a link between the gut-brain axis and mood disorders. However, the effects of 5-HTT deficiency on gut microbiota, brain function, and behavior remain to be fully characterized. Here we investigated the effects of 5-HTT deficiency on different types of behavior, the gut microbiome, and brain c-Fos expression as a marker of neuronal activation in response to the forced swim test for assessing depression-related behavior in male 5-HTT knockout mice. Behavioral analysis using a battery of 16 different tests showed that 5-HTT-/- mice exhibited markedly reduced locomotor activity, decreased pain sensitivity, reduced motor function, increased anxiety-like and depression-related behavior, altered social behavior in novel and familiar environments, normal working memory, enhanced spatial reference memory, and impaired fear memory compared to 5-HTT+/+ mice. 5-HTT+/- mice showed slightly reduced locomotor activity and impaired social behavior compared to 5-HTT+/+ mice. Analysis of 16S rRNA gene amplicons showed that 5-HTT-/- mice had altered gut microbiota abundances, such as a decrease in Allobaculum, Bifidobacterium, Clostridium sensu stricto, and Turicibacter, compared to 5-HTT+/+ mice. This study also showed that after exposure to the forced swim test, the number of c-Fos-positive cells was higher in the paraventricular thalamus and lateral hypothalamus and was lower in the prefrontal cortical regions, nucleus accumbens shell, dorsolateral septal nucleus, hippocampal regions, and ventromedial hypothalamus in 5-HTT-/- mice than in 5-HTT+/+ mice. These phenotypes of 5-HTT-/- mice partially recapitulate clinical observations in humans with major depressive disorder. The present findings indicate that 5-HTT-deficient mice serve as a good and valid animal model to study anxiety and depression with altered gut microbial composition and abnormal neuronal activity in the brain, highlighting the importance of 5-HTT in brain function and the mechanisms underlying the regulation of anxiety and depression.

Lateral septum-lateral hypothalamus circuit dysfunction in comorbid pain and anxiety

Pain and anxiety comorbidities are a common health problem, but the neural mechanisms underlying comorbidity remain unclear. We propose that comorbidity implies that similar brain regions and neural circuits, with the lateral septum (LS) as a major candidate, process pain and anxiety. From results of behavioral and neurophysiological experiments combined with selective LS manipulation in mice, we find that LS GABAergic neurons were critical for both pain and anxiety. Selective activation of LS GABAergic neurons induced hyperalgesia and anxiety-like behaviors. In contrast, selective inhibition of LS GABAergic neurons reduced nocifensive withdrawal responses and anxiety-like behaviors. This was found in two mouse models, one for chronic inflammatory pain (induced by complete Freund's adjuvant) and one for anxiety (induced by chronic restraint stress). Additionally, using TetTag chemogenetics to functionally mark LS neurons, we found that activation of LS neurons by acute pain stimulation could induce anxiety-like behaviors and vice versa. Furthermore, we show that LS GABAergic projection to the lateral hypothalamus (LH) plays an important role in the regulation of pain and anxiety comorbidities. Our study revealed that LS GABAergic neurons, and especially the LS^GABAergic-LH circuit, are a critical to the modulation of pain and anxiety comorbidities.

Sleep fragmentation affects glymphatic system through the different expression of AQP4 in wild type and 5xFAD mouse models

Alzheimer's disease (AD) is characterized by genetic and multifactorial risk factors. Many studies correlate AD to sleep disorders. In this study, we performed and validated a mouse model of AD and sleep fragmentation, which properly mimics a real condition of intermittent awakening. We noticed that sleep fragmentation induces a general acceleration of AD progression in 5xFAD mice, while in wild type mice it affects cognitive behaviors in particular learning and memory. Both these events may be correlated to aquaporin-4 (AQP4) modulation, a crucial player of the glymphatic system activity. In particular, sleep fragmentation differentially affects aquaporin-4 channel (AQP4) expression according to the stage of the disease, with an up-regulation in younger animals, while such change cannot be detected in older ones. Moreover, in wild type mice sleep fragmentation affects cognitive behaviors, in particular learning and memory, by compromising the glymphatic system through the decrease of AQP4. Nevertheless, an in-depth study is needed to better understand the mechanism by which AQP4 is modulated and whether it could be considered a risk factor for the disease development in wild type mice. If our hypotheses are going to be confirmed, AQP4 modulation may represent the convergence point between AD and sleep disorder pathogenic mechanisms.

Sex differences in amygdalohippocampal oscillations and neuronal activation in a rodent anxiety model and in response to infralimbic deep brain stimulation

Introduction

Depression and anxiety are highly comorbid mental disorders with marked sex differences. Both disorders show altered activity in the amygdala, hippocampus, and prefrontal cortex. Infralimbic deep brain stimulation (DBS-IL) has anxiolytic and antidepressant effects, but the underlying mechanisms remain unclear. We aimed to contribute to understanding sex differences in the neurobiology of these disorders.

Methods

In male and female rats, we recorded neural oscillations along the dorsoventral axis of the hippocampus and the amygdala in response to an anxiogenic drug, FG-7142. Following this, we applied DBS-IL.

Results

Surprisingly, in females, the anxiogenic drug failed to induce most of the changes observed in males. We found sex differences in slow, delta, theta, and beta oscillations, and the amygdalo-hippocampal communication in response to FG-7142, with modest changes in females. Females had a more prominent basal gamma, and the drug altered this band only in males. We also analyzed c-Fos expression in both sexes in stress-related structures in response to FG-7142, DBS-IL, and combined interventions. With the anxiogenic drug, females showed reduced expression in the nucleus incertus, amygdala, septohippocampal network, and neocortical levels. In both experiments, the DBS-IL reversed FG-7142-induced effects, with a more substantial effect in males than females.

Discussion

Here, we show a reduced response in female rats which contrasts with the higher prevalence of anxiety in women but is consistent with other studies in rodents. Our results open compelling questions about sex differences in the neurobiology of anxiety and depression and their study in animal models.

Ventral hippocampus-lateral septum circuitry promotes foraging-related memory

Remembering the location of a food or water source is essential for survival. Here, we reveal that spatial memory for food location is reflected in ventral hippocampus (HPCv) neuron activity and is impaired by HPCv lesion. HPCv mediation of foraging-related memory involves communication to the lateral septum (LS), as either reversible or chronic disconnection of HPCv-to-LS signaling impairs spatial memory retention for food or water location. This neural pathway selectively encodes appetitive spatial memory, as HPCv-LS disconnection does not affect spatial memory for escape location in a negative reinforcement procedure, food intake, or social and olfactory-based appetitive learning. Neural pathway tracing and functional mapping analyses reveal that LS neurons recruited during the appetitive spatial memory procedure are primarily GABAergic neurons that project to the lateral hypothalamus. Collective results emphasize that the neural substrates controlling spatial memory are outcome specific based on reinforcer modality.

Aversive Contexts Reduce Activity in the Ventral Subiculum- BNST Pathway

Many anxiety disorders can be characterized by abnormalities in detecting and learning about threats, and the inability to reduce fear responses in non-threatening environments. PTSD may be the most representative of context processing pathology, as intrusive memories are experienced in "safe" contexts. The ventral subiculum (vSUB), the main output of the ventral hippocampus, encodes environmental cues and is critical for context processing. The bed nucleus of the stria terminalis (BNST) contributes to anxiety-like behaviors as well as context fear conditioning. Given the important roles of the BNST and the vSUB in these anxiety and fear-related behaviors, and the anatomical connections between the two brain regions, the major aims of this study were to characterize the anatomy and function of the vSUB-BNST pathway. First, using the retrograde tracer cholera toxin, we mapped the topographical arrangement of the vSUB-BNST pathway. Dual retrograde tracing experiments revealed neurons projecting to the BNST and those projecting to the basolateral amygdala are distinct populations. Second, we assessed whether activity in this pathway, as indexed by FOS immunohistochemistry, was modulated by context fear conditioning. Our data reveal less activation of the vSUB-BNST pathway in both males and females in aversive contexts and the greatest activation when animals explored a neutral familiar context. In addition, the vSUB of females contained fewer GABAergic neurons compared to males. These findings suggest that the vSUB-BNST pathway is involved in eliciting appropriate responses to contexts.

Parallel ventral hippocampus-lateral septum pathways differentially regulate approach-avoidance conflict

The ability to resolve an approach-avoidance conflict is critical to adaptive behavior. The ventral CA3 (vCA3) and CA1 (vCA1) subfields of the ventral hippocampus (vHPC) have been shown to facilitate avoidance and approach behavior, respectively, in the face of motivational conflict, but the neural circuits by which this subfield-specific regulation is implemented is unknown. We demonstrate that two distinct pathways from these subfields to lateral septum (LS) contribute to this divergent control. In Long-Evans rats, chemogenetic inhibition of the vCA3- LS caudodorsal (cd) pathway potentiated approach towards a learned conflict-eliciting stimulus, while inhibition of the vCA1-LS rostroventral (rv) pathway potentiated approach non-specifically. Additionally, vCA3-LScd inhibited animals were less hesitant to explore food during environmental uncertainty, while the vCA1- LSrv inhibited animals took longer to initiate food exploration. These findings suggest that the vHPC influences multiple behavioral systems via differential projections to the LS, which in turn send inhibitory projections to motivational centres of the brain.

Current Understanding of the Neural Circuitry in the Comorbidity of Chronic Pain and Anxiety

Chronic pain patients often develop mental disorders, and anxiety disorders are common. We hypothesize that the comorbid anxiety results from an imbalance between the reward and antireward system due to persistent pain, which leads to the dysfunction of the pain and anxiety regulatory system. In this review, we will focus on changes in neuroplasticity, especially in neural circuits, during chronic pain and anxiety as observed in animal studies. Several neural circuits within specific regions of the brain, including the nucleus accumbens, lateral habenular, parabrachial nucleus, medial septum, anterior cingulate cortex, amygdala, hippocampus, medial prefrontal cortex, and bed nucleus of the stria terminalis, will be discussed based on novel findings after chemogenetic or optogenetic manipulation. We believe that these animal studies provide novel insights into human conditions and can guide clinical practice.

Conjunctive spatial and self-motion codes are topographically organized in the GABAergic cells of the lateral septum

The hippocampal spatial code’s relevance for downstream neuronal populations—particularly its major subcortical output the lateral septum (LS)—is still poorly understood. Here, using calcium imaging combined with unbiased analytical methods, we functionally characterized and compared the spatial tuning of LS GABAergic cells to those of dorsal CA3 and CA1 cells. We identified a significant number of LS cells that are modulated by place, speed, acceleration, and direction, as well as conjunctions of these properties, directly comparable to hippocampal CA1 and CA3 spatially modulated cells. Interestingly, Bayesian decoding of position based on LS spatial cells reflected the animal’s location as accurately as decoding using the activity of hippocampal pyramidal cells. A portion of LS cells showed stable spatial codes over the course of multiple days, potentially reflecting long-term episodic memory. The distributions of cells exhibiting these properties formed gradients along the anterior–posterior and dorsal–ventral axes of the LS, directly reflecting the topographical organization of hippocampal inputs to the LS. Finally, we show using transsynaptic tracing that LS neurons receiving CA3 and CA1 excitatory input send projections to the hypothalamus and medial septum, regions that are not targeted directly by principal cells of the dorsal hippocampus. Together, our findings demonstrate that the LS accurately and robustly represents spatial, directional as well as self-motion information and is uniquely positioned to relay this information from the hippocampus to its downstream regions, thus occupying a key position within a distributed spatial memory network.

Calcium imaging of neurons in freely behaving mice reveals how the lateral septum, the main output of the hippocampal place cells, effectively represents information about not only location, but also head direction and self-movement, and may be pivotal in sending this information to downstream brain regions.

GABAergic Neurons in the Dorsal-Intermediate Lateral Septum Regulate Sleep-Wakefulness and Anesthesia in Mice

BACKGROUND

The γ-aminobutyric acid-mediated (GABAergic) inhibitory system in the brain is critical for regulation of sleep-wake and general anesthesia. The lateral septum contains mainly GABAergic neurons, being cytoarchitectonically divided into the dorsal, intermediate, and ventral parts. This study hypothesized that GABAergic neurons of the lateral septum participate in the control of wakefulness and promote recovery from anesthesia.

METHODS

By employing fiber photometry, chemogenetic and optogenetic neuronal manipulations, anterograde tracing, in vivo electrophysiology, and electroencephalogram/electromyography recordings in adult male mice, the authors measured the role of lateral septum GABAergic neurons to the control of sleep-wake transition and anesthesia emergence and the corresponding neuron circuits in arousal and emergence control.

RESULTS

The GABAergic neurons of the lateral septum exhibited high activities during the awake state by in vivo fiber photometry recordings (awake vs. non-rapid eye movement sleep: 3.3 ± 1.4% vs. -1.3 ± 1.2%, P < 0.001, n = 7 mice/group; awake vs. anesthesia: 2.6 ± 1.2% vs. -1.3 ± 0.8%, P < 0.001, n = 7 mice/group). Using chemogenetic stimulation of lateral septum GABAergic neurons resulted in a 100.5% increase in wakefulness and a 51.2% reduction in non-rapid eye movement sleep. Optogenetic activation of these GABAergic neurons promoted wakefulness from sleep (median [25th, 75th percentiles]: 153.0 [115.9, 179.7] s to 4.0 [3.4, 4.6] s, P = 0.009, n = 5 mice/group) and accelerated emergence from isoflurane anesthesia (514.4 ± 122.2 s vs. 226.5 ± 53.3 s, P < 0.001, n = 8 mice/group). Furthermore, the authors demonstrated that the lateral septum GABAergic neurons send 70.7% (228 of 323 cells) of monosynaptic projections to the ventral tegmental area GABAergic neurons, preferentially inhibiting their activities and thus regulating wakefulness and isoflurane anesthesia depth.

CONCLUSIONS

The results uncover a fundamental role of the lateral septum GABAergic neurons and their circuit in maintaining awake state and promoting general anesthesia emergence time.

EDITOR’S PERSPECTIVE

Anxiety and Alzheimer's disease: Behavioral analysis and neural basis in rodent models of Alzheimer's-related neuropathology

Alzheimer's disease (AD) pathology is commonly associated with cognitive decline but is also composed of neuropsychiatric symptoms including psychological distress and alterations in mood, including anxiety and depression. Emotional dysfunction in AD is frequently modeled using tests of anxiety-like behavior in transgenic rodents. These tests often include the elevated plus-maze, light/dark test and open field test. In this review, we describe prototypical behavioral paradigms used to examine emotional dysfunction in transgenic models of AD, specifically anxiety-like behavior. Next, we summarize the results of studies examining anxiety-like behavior in transgenic rodents, noting that the behavioral outcomes using these paradigms have produced inconsistent results. We suggest that future research will benefit from using a battery of tests to examine emotional behavior in transgenic AD models. We conclude by discussing putative, overlapping neurobiological mechanisms underlying AD-related neuropathology, stress and anxiety-like behavior reported in AD models.

Regulation of depression-related behaviors by GABAergic neurons in the lateral septum through periaqueductal gray neuronal projections

Major depressive disorder (MDD) is a serious and widespread mental illness worldwide. The abnormality of neuronal networks may contribute to the etiology of MDD. However, the neural connections underlying the main symptoms of MDD need further elucidation. Here, we found that GABAergic neurons in the lateral septum (LS) were activated by chronic unpredictable stress (CUS), with increased numbers of ΔFosB-labeled neurons. LS neuronal activity was modulated using a chemogenetic approach. Activation of LS neurons caused a depressive phenotype, as shown by increased immobility in the forced swim test, and induced increased susceptibility to subthreshold chronic stress, as indicated by decreased female urine sniffing time and preference for sucrose in depression-related behavior detection, whereas suppression of LS neuronal activity induced an antidepressant effect under basal and stressed conditions. Moreover, we found that the LS showed strong neuronal projections to the dorsal periaqueductal gray (dPAG); activation of dPAG-projecting GABAergic neurons in the LS produced the same depressive behaviors and stress susceptibility as induced by the activation of the majority of LS GABAergic neurons. Finally, we found that activation of neuronal fibers in the dPAG derived from the LS showed depression-related behaviors, as suggested by the decreased female urine sniffing time and sucrose preference in female urine sniffing and sucrose preference tests respectively. Our findings indicate that LS is a key depression-controlling nucleus, and that the LS-PAG projection is an essential effector circuit for morbidity and treatment in depression.

Lateral septum as a nexus for mood, motivation, and movement

The lateral septum (LS) has been implicated in a wide variety of functions, including emotional, motivational, and spatial behavior, and the LS may regulate interactions between the hippocampus and other regions that mediate goal directed behavior. In this review, we suggest that the lateral septum incorporates movement into the evaluation of environmental context with respect to motivation, anxiety, and reward to output an 'integrated movement value signal'. Specifically, hippocampally-derived contextual information may be combined with reinforcement or motivational information in the LS to inform task-relevant decisions. We will discuss how movement is represented in the LS and the literature on the LS's involvement in mood and motivation. We will then connect these results to LS movement-related literature and hypotheses about the role of the lateral septum. We suggest that the LS may communicate a movement-scaled reward signal via changes in place-, movement-, and reward-related firing, and that the LS should be considered a fundamental node of affect and locomotor pathways in the brain.

Neuronal diversity and reciprocal connectivity between the vertebrate hippocampus and septum

A hallmark of the nervous system is the precision with which myriad cell types are integrated into functional networks that control complex behaviors. The limbic system governs evolutionarily conserved processes essential for survival. The septum and the hippocampus are central to the limbic system, and control not only emotion-related behaviors but also learning and memory. Here, we provide a developmental and evolutionary perspective of the hippocampus and septum and highlight the neuronal diversity and circuitry that connects these two central components of the limbic system. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles Comparative Development and Evolution > Regulation of Organ Diversity.

Interdependence between dorsal and ventral hippocampus during spatial navigation

INTRODUCTION

The hippocampus is linked to the formation and retrieval of episodic memories and spatial navigation. In rats, it is an elongated structure divided into dorsal (septal) and ventral (temporal) regions paralleling the respective division in the posterior and anterior hippocampus in humans. The dorsal hippocampus has been suggested to be more important for spatial processing and the ventral to processing anxiety-based behaviors. Far less is known regarding the degree to which these different regions interact during information processing. The anatomical connectivity suggests a flow of information between the dorsal and ventral regions; conversely, there are also commissural connections to the contralateral hippocampus. The current study examined the extent to which information from the dorsal hippocampus interacts with processing in the ipsilateral and contralateral ventral hippocampus following the acquisition of a spatial task.

METHODS

Rats were well-trained on a spatial reference version of the water maze, followed by muscimol inactivation of different hippocampal subregions in a within-animal repeated design. Various combinations of bilateral, ipsilateral, and contralateral infusions were used.

RESULTS

Combined dorsal and ventral inactivation produced a severe impairment in spatial performance. Inactivation of only the dorsal or ventral regions resulted in intermediate impairment with performance levels falling between controls and combined inactivation. Performance was impaired during contralateral inactivation and was almost equivalent to bilateral dorsal and ventral hippocampus inactivation, while ipsilateral inactivation resulted in little impairment.

CONCLUSIONS

Taken together, results indicate that for spatial processing, the hippocampus functions as a single integrated structure along the longitudinal axis.

Neuroligin-2 Determines Inhibitory Synaptic Transmission in the Lateral Septum to Optimize Stress-Induced Neuronal Activation and Avoidance Behavior

BACKGROUND

Investigations in the neocortex have revealed that the balance of excitatory and inhibitory synaptic transmission (E/I ratio) is important for proper information processing. The disturbance of this balance underlies many neuropsychiatric illnesses, including autism spectrum disorder and schizophrenia. However, little is known about the contribution of E/I balance to the functioning of subcortical brain regions, such as the lateral septum (LS), a structure that plays important roles in regulating anxiety-related behavior.

METHODS

We manipulated E/I balance in the mouse LS by localized conditional deletion of neuroligin-2, a postsynaptic cell adhesion protein located at gamma-aminobutyric acidergic synapses and important for inhibitory synaptic transmission. We then performed analyses of synaptic transmission in the LS, stress-induced expression of immediate early gene c-fos, and anxiety-related and depression-related behavior.

RESULTS

The absence of neuroligin-2 in the LS in the mature mouse brain resulted in postsynaptic impairment of inhibitory synaptic transmission. Importantly, the reduced inhibition and resulting E/I imbalance decreased the responsiveness of LS neurons to stress. Furthermore, this E/I imbalance in the LS was associated with impaired stress-induced activation of downstream hypothalamic nuclei and reduced avoidance behavior of the animals in the elevated plus maze.

CONCLUSIONS

Our results described the synaptic function of neuroligin-2 in the LS, uncovered a positive association between c-Fos-expressing neurons in the LS and downstream hypothalamic areas and avoidance behavior, and demonstrated that intact inhibitory synaptic transmission and proper E/I balance are required for the optimal functioning of this subcortical circuit.

Dopamine D1-like receptors in the dorsomedial prefrontal cortex regulate contextual fear conditioning

RATIONALE

Dopamine D1 receptor (D1R) signalling is involved in contextual fear conditioning. The D1R antagonist SCH23390 impairs the acquisition of contextual fear when administered systemically or infused locally into the dorsal hippocampus or basolateral amygdala.

OBJECTIVES

We determined if state dependency may account for the impairment in contextual fear conditioning caused by systemic SCH23390 administration. We also examined if the dorsomedial prefrontal cortex (dmPFC), nucleus accumbens (NAc), and ventral hippocampus (VH) are involved in mediating the effect of systemic SCH23390 treatment on contextual fear conditioning.

METHODS

In experiment 1, SCH23390 (0.1 mg/kg) or vehicle was given before contextual fear conditioning and/or retrieval. In experiment 2, SCH23390 (2.5 μg/0.5 uL) or vehicle was infused locally into dmPFC, NAc, or VH before contextual fear conditioning, and retrieval was tested drug-free. Freezing was quantified as a measure of contextual fear.

RESULTS

In experiment 1, SCH23390 given before conditioning or before both conditioning and retrieval decreased freezing at retrieval, whereas SCH23390 given only before retrieval had no effect. In experiment 2, SCH23390 infused into dmPFC before conditioning decreased freezing at retrieval, while infusion of SCH23390 into NAc or VH had no effect.

CONCLUSIONS

The results of experiment 1 confirm those of previous studies indicating that D1Rs are required for the acquisition but not retrieval of contextual fear and rule out state dependency as an explanation for these findings. Moreover, the results of experiment 2 provide evidence that dmPFC is also part of the neural circuitry through which D1R signalling regulates contextual fear conditioning.

The microbiome regulates amygdala-dependent fear recall

The amygdala is a key brain region that is critically involved in the processing and expression of anxiety and fear-related signals. In parallel, a growing number of preclinical and human studies have implicated the microbiome-gut-brain in regulating anxiety and stress-related responses. However, the role of the microbiome in fear-related behaviours is unclear. To this end we investigated the importance of the host microbiome on amygdala-dependent behavioural readouts using the cued fear conditioning paradigm. We also assessed changes in neuronal transcription and post-transcriptional regulation in the amygdala of naive and stimulated germ-free (GF) mice, using a genome-wide transcriptome profiling approach. Our results reveal that GF mice display reduced freezing during the cued memory retention test. Moreover, we demonstrate that under baseline conditions, GF mice display altered transcriptional profile with a marked increase in immediate-early genes (for example, Fos, Egr2, Fosb, Arc) as well as genes implicated in neural activity, synaptic transmission and nervous system development. We also found a predicted interaction between mRNA and specific microRNAs that are differentially regulated in GF mice. Interestingly, colonized GF mice (ex-GF) were behaviourally comparable to conventionally raised (CON) mice. Together, our data demonstrates a unique transcriptional response in GF animals, likely because of already elevated levels of immediate-early gene expression and the potentially underlying neuronal hyperactivity that in turn primes the amygdala for a different transcriptional response. Thus, we demonstrate for what is to our knowledge the first time that the presence of the host microbiome is crucial for the appropriate behavioural response during amygdala-dependent memory retention.

The role of CA3-LS-VTA loop in the formation of conditioned place preference induced by context-associated reward memory for morphine

Addiction-related behaviors, such as conditioned place preference (CPP), require animals to remember an association between environmental cue and drug treatment, and exposure to environmental cue is one of the key contributing factors to relapse. However, how central neural circuit participates in the formation of CPP induced by stimulus of morphine-paired environment remains unknown. In the present study, we found that reexposure to morphine-paired environment significantly increased the activity of hippocampal CA3 neurons, increased the excitability of GABAergic neurons and expression of glutamic acid decarboxylase 65/67 in the caudal lateral septum (LSc) and decreased the activity of GABAergic neurons and GAD65/67 expression in ventral tegmental area (VTA), leading to activation (disinhibition) of dopaminergic neurons. Inactivation of CA3 neurons attenuated GABAergic neurons activity and decreased the upregulation of GAD65/67 in LSc, prevented the dopaminergic neurons activation,and GAD65/67 downregulation in VTA and ameliorated the CPP behavior following exposure to morphine-paired context. Blockade of NMDA receptor in LSc also prevented the upregulation of GAD65/67 in LSc and formation of CPP induced by stimulus of morphine-paired environment. Suppression of GAD activity in LSc also remarkably attenuated the dopaminergic neurons activation and the GAD65/67 downregulation in VTA and prevented the formation of CPP induced by reexposure to morphine-associated context. Collectively, these results, for the first time, illustrated the involvement of neural circuitry of CA3-LSc-VTA, through integration of the contexts and reward information, participated in the reinstatement of CPP induced by exposure to morphine-associated context, which advanced our understanding on neurobiological basis for the context-associated memory and rewarding behavior.

Chronic Fluoxetine Induces Activity Changes in Recovery From Poststroke Anxiety, Depression, and Cognitive Impairment

Poststroke depression (PSD) is a common outcome of stroke that limits recovery and is only partially responsive to chronic antidepressant treatment. In order to elucidate changes in the cortical-limbic circuitry associated with PSD and its treatment, we examined a novel mouse model of persistent PSD. Focal endothelin-1-induced ischemia of the left medial prefrontal cortex (mPFC) in male C57BL6 mice resulted in a chronic anxiety and depression phenotype. Here, we show severe cognitive impairment in spatial learning and memory in the stroke mice. The behavioral and cognitive phenotypes were reversed by chronic (4-week) treatment with fluoxetine, alone or with voluntary exercise (free-running wheel), but not by exercise alone. To assess chronic cellular activation, FosB+ cells were co-labeled for markers of glutamate/pyramidal (VGluT1-3/CaMKIIα), γ-aminobutyric acid (GAD67), and serotonin (TPH). At 6 weeks poststroke versus sham (or 4 days poststroke), left mPFC stroke induced widespread FosB activation, more on the right (contralesional) than on the left side. Stroke activated glutamate cells of the mPFC, nucleus accumbens, amygdala, hippocampus, and raphe serotonin neurons. Chronic fluoxetine balanced bilateral neuronal activity, reducing total FosB and FosB/CamKII+ cells (mPFC, nucleus accumbens), and unlike exercise, increasing FosB/GAD67+ cells (septum, amygdala) or both (hippocampus, raphe). In summary, chronic antidepressant but not exercise mediates recovery in this unilateral ischemic PSD model that is associated with region-specific reversal of stroke-induced pyramidal cell hyperactivity and increase in γ-aminobutyric acidergic activity. Targeted brain stimulation to restore brain activity could provide a rational approach for treating clinical PSD.

The role of CA3 GABAB receptors on anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 with respect to Ca2+ ions

Glutamatergic and GABAergic systems play key roles in the hippocampus and affect the pathogenesis of anxiety- and memory-related processes. Some investigations have assessed the role of balancing the function of these two systems in different areas of the central nervous system (CNS) as an approach to manage the related disorders. We investigated the anxiety and avoidance memory states using the test-retest protocol in the elevated plus maze to understand the role of GABA_B receptors (GABA_BRs) in relation to the NMDA receptor blockade by D-AP5 (an NMDA receptor antagonist). Also, we examined the function of Ca²⁺ ions by blocking its entrance to the cell using SKF96365 (a Ca²⁺ channel blocker). The drugs were injected into the CA3 region before the test. Our data showed that D-AP5 induced anxiolytic-like behaviors and impaired the avoidance memory. Injection of baclofen (a GABA_BR agonist), but not phaclofen (a GABA_BR antagonist) induced anxiolytic-like behaviors. Neither baclofen nor phaclofen altered avoidance memory-related behaviors. When baclofen was injected before D-AP5, it potentiated the anxiolytic-like behaviors induced by D-AP5, but counteracted its effect on avoidance memory. Phaclofen pretreatment attenuated D-AP5-induced anxiolytic-like behaviors, but potentiated its effect on avoidance memory. The effect of baclofen application before D-AP5 on anxiety and phaclofen application before D-AP5 on avoidance memory at the heist doses were accompanied by a decrease in locomotion. The application of SKF96365 did not alter anxiety-like behaviors but induced avoidance memory impairment. SKF96365 application before the combination of baclofen and D-AP5 counteracted the effects produced by the combination of baclofen and D-AP5 on anxiety and memory states. Our findings showed that the CA3 GABA_BRs had a critical role in anxiolytic-like behaviors and avoidance memory deficit induced by D-AP5 and confirmed the role of Ca²⁺ ions in the observed results.

Bidirectional Control of Anxiety-Related Behaviors in Mice: Role of Inputs Arising from the Ventral Hippocampus to the Lateral Septum and Medial Prefrontal Cortex

Anxiety is an adaptive response to potentially threatening situations. Exaggerated and uncontrolled anxiety responses become maladaptive and lead to anxiety disorders. Anxiety is shaped by a network of forebrain structures, including the hippocampus, septum, and prefrontal cortex. In particular, neural inputs arising from the ventral hippocampus (vHPC) to the lateral septum (LS) and medial prefrontal cortex (mPFC) are thought to serve as principal components of the anxiety circuit. However, the role of vHPC-to-LS and vHPC-to-mPFC signals in anxiety is unclear, as no study has directly compared their behavioral contribution at circuit level. We targeted LS-projecting vHPC cells and mPFC-projecting vHPC cells by injecting the retrogradely propagating canine adenovirus encoding Cre recombinase into the LS or mPFC, and injecting a Cre-responsive AAV (AAV8-hSyn-FLEX-hM3D or hM4D) into the vHPC. Consequences of manipulating these neurons were examined in well-established tests of anxiety. Chemogenetic manipulation of LS-projecting vHPC cells led to bidirectional changes in anxiety: activation of LS-projecting vHPC cells decreased anxiety whereas inhibition of these cells produced opposite anxiety-promoting effects. The observed anxiety-reducing function of LS-projecting cells was in contrast with the function of mPFC-projecting cells, which promoted anxiety. In addition, double retrograde tracing demonstrated that LS- and mPFC-projecting cells represent two largely anatomically distinct cell groups. Altogether, our findings suggest that the vHPC houses discrete populations of cells that either promote or suppress anxiety through differences in their projection targets. Disruption of the intricate balance in the activity of these two neuron populations may drive inappropriate behavioral responses seen in anxiety disorders.

Dissecting the Function of Hippocampal Oscillations in a Human Anxiety Model

Neural oscillations in hippocampus and medial prefrontal cortex (mPFC) are a hallmark of rodent anxiety models that build on conflict between approach and avoidance. Yet, the function of these oscillations, and their expression in humans, remain elusive. Here, we used magnetoencephalography (MEG) to investigate neural oscillations in a task that simulated approach–avoidance conflict, wherein 23 male and female human participants collected monetary tokens under a threat of virtual predation. Probability of threat was signaled by color and learned beforehand by direct experience. Magnitude of threat corresponded to a possible monetary loss, signaled as a quantity. We focused our analyses on an a priori defined region-of-interest, the bilateral hippocampus. Oscillatory power under conflict was linearly predicted by threat probability in a location consistent with right mid-hippocampus. This pattern was specific to the hippocampus, most pronounced in the gamma band, and not explained by spatial movement or anxiety-like behavior. Gamma power was modulated by slower theta rhythms, and this theta modulation increased with threat probability. Furthermore, theta oscillations in the same location showed greater synchrony with mPFC theta with increased threat probability. Strikingly, these findings were not seen in relation to an increase in threat magnitude, which was explicitly signaled as a quantity and induced similar behavioral responses as learned threat probability. Thus, our findings suggest that the expression of hippocampal and mPFC oscillatory activity in the context of anxiety is specifically linked to threat memory. These findings resonate with neurocomputational accounts of the role played by hippocampal oscillations in memory.

SIGNIFICANCE STATEMENT We use a biologically relevant approach–avoidance conflict test in humans while recording neural oscillations with magnetoencephalography to investigate the expression and function of hippocampal oscillations in human anxiety. Extending nonhuman studies, we can assign a possible function to hippocampal oscillations in this task, namely threat memory communication. This blends into recent attempts to elucidate the role of brain synchronization in defensive responses to threat.

Anxiety and Nicotine Dependence: Emerging Role of the Habenulo-Interpeduncular Axis

While innovative modern neuroscience approaches have aided in discerning brain circuitry underlying negative emotional behaviors including fear and anxiety responses, how these circuits are recruited in normal and pathological conditions remains poorly understood. Recently, genetic tools that selectively manipulate single neuronal populations have uncovered an understudied circuit, the medial habenula (mHb)-interpeduncular (IPN) axis, that modulates basal negative emotional responses. Interestingly, the mHb-IPN pathway also represents an essential circuit that signals heightened anxiety induced by nicotine withdrawal. Insights into how this circuit interconnects with regions more classically associated with anxiety, and how chronic nicotine exposure induces neuroadaptations resulting in an anxiogenic state, may thereby provide novel strategies and molecular targets for therapies that facilitate smoking cessation, as well as for anxiety relief.

Resolving the neural circuits of anxiety

Although anxiety disorders represent a major societal problem demanding new therapeutic targets, these efforts have languished in the absence of a mechanistic understanding of this subjective emotional state. While it is impossible to know with certainty the subjective experience of a rodent, rodent models hold promise in dissecting well-conserved limbic circuits. The application of modern approaches in neuroscience has already begun to unmask the neural circuit intricacies underlying anxiety by allowing direct examination of hypotheses drawn from existing psychological concepts. This information points toward an updated conceptual model for what neural circuit perturbations could give rise to pathological anxiety and thereby provides a roadmap for future therapeutic development.

Functional differentiation of adult-born neurons along the septotemporal axis of the dentate gyrus

Over the past several decades, the proliferation and integration of adult-born neurons into existing hippocampal circuitry has been implicated in a wide range of behaviors, including novelty recognition, pattern separation, spatial learning, anxiety behaviors, and antidepressant response. In this review, we suggest that the diversity in behavioral requirements for new neurons may be partly caused by separate functional roles of individual neurogenic niches. Growing evidence shows that the hippocampal formation can be compartmentalized not only along the classic trisynaptic circuit, but also along a longitudinal septotemporal axis. We suggest that subpopulations of hippocampal adult-born neurons may be specialized for distinct mnemonic- or mood-related behavioral tasks. We will examine the literature supporting a functional and anatomical dissociation of the hippocampus along the longitudinal axis and discuss techniques to functionally dissect the roles of adult-born hippocampal neurons in these distinct subregions.

The lateral septum as a regulator of hippocampal theta oscillations and defensive behavior in rats

Hippocampal theta oscillations are linked to various processes, including locomotion, learning and memory, and defense and affect. The lateral septum (LS) has been implicated in the generation of the hippocampal theta rhythm, but its precise role in this process is not well understood. Here, we investigated the effects of direct pharmacological inhibition or disinhibition of the dorsal LS (dLS) on the frequency of hippocampal theta activity elicited by stimulation of the reticular formation in urethane-anesthetized rats. We found that bilateral infusions of the GABAA receptor agonist muscimol into the dLS significantly increased theta frequency. Strikingly, intra-dLS infusions of the GABAA receptor antagonist GABAzine largely abolished reticularly elicited theta activity. We also locally injected these same compounds into the medial septum (MS) to test for neuroanatomical specificity. In contrast to the effects seen in the dLS, intra-MS infusions of muscimol had no effect on theta frequency, whereas intra-MS infusions of GABAzine increased theta frequency. Given the hypothesized role of hippocampal theta in behavioral defense, we also examined the effects of intra-dLS application of muscimol in two models of anxiety, the elevated plus maze and the novelty-induced suppression of feeding paradigm; both tests revealed clear, anxiolytic-like effects following muscimol infusions. The fact that dLS-muscimol increased theta frequency while also reducing anxiety-like behaviors challenges the influential theta suppression model of anxiolysis, which predicts a slowing of theta with anxiolytic compounds. More importantly, the experiments reveal a novel role of the LS, especially its dorsal aspects, as an important gating mechanism for the expression of theta oscillations in the rodent hippocampus.

Prenatal stress-induced programming of genome-wide promoter DNA methylation in 5-HTT-deficient mice

The serotonin transporter gene (5-HTT/SLC6A4)-linked polymorphic region has been suggested to have a modulatory role in mediating effects of early-life stress exposure on psychopathology rendering carriers of the low-expression short (s)-variant more vulnerable to environmental adversity in later life. The underlying molecular mechanisms of this gene-by-environment interaction are not well understood, but epigenetic regulation including differential DNA methylation has been postulated to have a critical role. Recently, we used a maternal restraint stress paradigm of prenatal stress (PS) in 5-HTT-deficient mice and showed that the effects on behavior and gene expression were particularly marked in the hippocampus of female 5-Htt+/- offspring. Here, we examined to which extent these effects are mediated by differential methylation of DNA. For this purpose, we performed a genome-wide hippocampal DNA methylation screening using methylated-DNA immunoprecipitation (MeDIP) on Affymetrix GeneChip Mouse Promoter 1.0 R arrays. Using hippocampal DNA from the same mice as assessed before enabled us to correlate gene-specific DNA methylation, mRNA expression and behavior. We found that 5-Htt genotype, PS and their interaction differentially affected the DNA methylation signature of numerous genes, a subset of which showed overlap with the expression profiles of the corresponding transcripts. For example, a differentially methylated region in the gene encoding myelin basic protein (Mbp) was associated with its expression in a 5-Htt-, PS- and 5-Htt × PS-dependent manner. Subsequent fine-mapping of this Mbp locus linked the methylation status of two specific CpG sites to Mbp expression and anxiety-related behavior. In conclusion, hippocampal DNA methylation patterns and expression profiles of female prenatally stressed 5-Htt+/- mice suggest that distinct molecular mechanisms, some of which are promoter methylation-dependent, contribute to the behavioral effects of the 5-Htt genotype, PS exposure and their interaction.

Tests of unconditioned anxiety - pitfalls and disappointments

The plus-maze, the light-dark box and the open-field are the main current tests of unconditioned anxiety for mice and rats. Despite their disappointing achievements, they remain as popular as ever and seem to play an important role in an ever-growing demand for behavioral phenotyping and drug screening. Numerous reviews have repeatedly reported their lack of consistency and reliability but they failed to address the core question of whether these tests do provide unequivocal measures of fear-induced anxiety, that these measurements are not confused with measures of fear-induced avoidance or natural preference responses - i.e. discriminant validity. In the present report, I examined numerous issues that undermine the validity of the current tests, and I highlighted various flaws in the aspects of these tests and the methodologies pursued. This report concludes that the evidence in support of the validity of the plus-maze, the light/dark box and the open-field as anxiety tests is poor and methodologically questionable.

VPAC receptor signaling modulates grouping behavior and social responses to contextual novelty in a gregarious finch: a role for a putative prefrontal cortex homologue

In both mammals and birds, vasoactive intestinal polypeptide (VIP) neurons and fibers are present in virtually every brain area that is important for social behavior. VIP influences aggression in birds, social recognition in rodents, and prolactin secretion in both taxa, but other possible functions in social modulation remain little explored. VIP effects are mediated by VPAC receptors, which bind both VIP and pituitary adenylate cyclase activating peptide. Within the lateral septum and medial bed nucleus of the stria terminalis, VPAC receptors are found at higher densities in gregarious finch species relative to territorial species, suggesting that VPAC receptor activation promotes social contact and/or preference for larger groups. Here we here test this hypothesis in zebra finches (Taeniopygia guttata), and also examine the relevance of VPAC receptors to anxiety-like processes. Intraventricular infusions of the VPAC receptor antagonist, neurotensin6-11 mouseVIP7-28, strongly reduce social contact when animals are tested in a novel environment, and exert sex-specific effects on grouping behavior. Specifically, VPAC receptor antagonism reduces gregariousness in females but increases gregariousness in males. Interestingly, VPAC antagonism in the medial pallium (putative prefrontal cortex homologue) significantly reduces gregariousness in both sexes, suggesting site-specific effects of VIP signaling. However, VPAC antagonism does not modulate novel-familiar social preferences in a familiar environment or general anxiety-like behaviors. The current results suggest that endogenous activation of VPAC receptors promotes social contact under novel environmental conditions, a function that may be accentuated in gregarious species. Moreover, endogenous VIP modulates gregariousness in both males and females.

Lateral septal infusions of the neuropeptide Y Y2 receptor agonist, NPY(13-36) differentially affect different defensive behaviors in male, Long Evans rats

The lateral septum has been extensively implicated in regulating anxiety-related defensive behaviors in the rat. Neuropeptide Y (NPY) contributes to anxiety, likely through activity at the NPY Y1 and/or Y2 receptor binding sites. Although the lateral septum contains the highest density of Y2 receptors in brain, the involvement of this receptor in anxiety-related defensive behaviors is not clear. Thus, the purpose of the current study was to characterize lateral septal Y2 receptor contributions to rats' defensive responses to threat and/or potentially threatening environments. We investigated this by infusing the NPY Y2 agonist NPY13-36 into the lateral septum and testing rats across a battery of animal models of anxiety (Experiment 1). To verify the role of Y2 in mediating the observed effects, rats were pre-infused with the potent and highly selective Y2 antagonist BIIE 0246 prior to infusion with NPY13-36 (Experiment 2). Infusions of NPY13-36 into the lateral septum increased rats' open-arm exploration in the elevated plus-maze test (p<0.01) and decreased the proportion of rats' that buried (p<0.05) as well as their latency to initiate burying in the shock-probe burying test (p<0.01). By contrast, NPY13-36 did not affect either anxiety- or appetite-related responses in the novelty-induced suppression of feeding test (all ps>0.3; Experiment 1). Pre-treatment with the Y2 antagonist BIIE 0246 prevented the anxiolytic-like actions of NPY13-36 in the plus-maze but not in the shock-probe test (Experiment 2). Thus, it appears that the anxiolytic-like actions of lateral septal NPY13-36 are mediated by the Y2 receptor in a test-specific manner.

Neuromodulation by oxytocin and vasopressin

Oxytocin (OT) and vasopressin (VP) are two closely related neuropeptides, widely known for their peripheral hormonal effects. Specific receptors have also been found in the brain, where their neuromodulatory actions have meanwhile been described in a large number of regions. Recently, it has become possible to study their endogenous neuropeptide release with the help of OT/VP promoter-driven expression of fluorescent proteins and light-activated ion channels. In this review, I summarize the neuromodulatory effects of OT and VP in different brain regions by grouping these into different behavioral systems, highlighting their concerted, and at times opposite, effects on different aspects of behavior.

Systems genetics of the lateral septal nucleus in mouse: heritability, genetic control, and covariation with behavioral and morphological traits

The lateral septum has strong efferent projections to hypothalamic and midbrain regions, and has been associated with modulation of social behavior, anxiety, fear conditioning, memory-related behaviors, and the mesolimbic reward pathways. Understanding natural variation of lateral septal anatomy and function, as well as its genetic modulation, may provide important insights into individual differences in these evolutionarily important functions. Here we address these issues by using efficient and unbiased stereological probes to estimate the volume of the lateral septum in the BXD line of recombinant inbred mice. Lateral septum volume is a highly variable trait, with a 2.5-fold difference among animals. We find that this trait covaries with a number of behavioral and physiological phenotypes, many of which have already been associated with behaviors modulated by the lateral septum, such as spatial learning, anxiety, and reward-seeking. Heritability of lateral septal volume is moderate (h(2) = 0.52), and much of the heritable variation is caused by a locus on the distal portion of chromosome (Chr) 1. Composite interval analysis identified a secondary interval on Chr 2 that works additively with the Chr 1 locus to increase lateral septum volume. Using bioinformatic resources, we identified plausible candidate genes in both intervals that may influence the volume of this key nucleus, as well as associated behaviors.

Social status alters defeat-induced neural activation in Syrian hamsters

Although exposure to social stress leads to increased depression-like and anxiety-like behavior, some individuals are more vulnerable than others to these stress-induced changes in behavior. Prior social experience is one factor that can modulate how individuals respond to stressful events. In this study, we investigated whether experience-dependent resistance to the behavioral consequences of social defeat was associated with a specific pattern of neural activation. We paired weight-matched male Syrian hamsters in daily aggressive encounters for 2 weeks, during which they formed a stable dominance relationship. We also included control animals that were exposed to an empty cage each day for 2 weeks. Twenty-four hours after the final pairing or empty cage exposure, half of the subjects were socially defeated in 3, 5-min encounters, whereas the others were not socially defeated. Twenty-four hours after social defeat, animals were tested for conditioned defeat in a 5-min social interaction test with a non-aggressive intruder. We collected brains after social defeat and processed the tissue for c-Fos immunoreactivity. We found that dominants were more likely than subordinates to counter-attack the resident aggressor during social defeat, and they showed less submissive and defensive behavior at conditioned defeat testing compared with subordinates. Also, social status was associated with distinct patterns of defeat-induced neural activation in select brain regions, including the amygdala, prefrontal cortex, hypothalamus, and lateral septum. Our results indicate that social status is an important form of prior experience that predicts both initial coping style and the degree of resistance to social defeat. Further, the differences in defeat-induced neural activation suggest possible brain regions that may control resistance to conditioned defeat in dominant individuals.

Central metabolite changes and activation of microglia after peripheral interleukin-2 challenge

Interleukin (IL)-2 regulates the immune response through the proliferation of activated T-cells and also exerts effects on the central nervous system (CNS). Alongside having marked neurobehavioral effects, IL-2 has been suggested to impact on various psychiatric disorders. The immune-CNS communication of IL-2 remains unclear, although, it is suggested that microglia are the source and target of IL-2. Here, we analyzed changes in brain metabolites following a peripheral IL-2 challenge and examined the contribution of microglia in mediating these effects. Rats were assessed by magnetic resonance spectroscopy (MRS) in a 9.4 T scanner for baseline metabolite levels in the prefrontal cortex (PFC) and the hippocampus. After 7 days animals were scanned again following a single injection of IL-2 (2.5 μg/kg) and then tested on the elevated plus-maze for the correlation of IL-2-induced brain metabolites and measures of anxiety. In another experiment CD25(+) microglia cells were determined. A separate group of rats was injected either with IL-2 or vehicle, and afterward the PFC and hippocampus were dissected and fluorescence activated cell sorting (FACS) analysis was performed. The MRS scans in the intra-individual study design showed a significant increase in myo-inositol in the analyzed regions. A significant correlation of anxiety-like measures and myo-inositol, a marker for microglia activity, was found in the hippocampus. The FACS analysis showed a significant increase in CD25(+) microglia in the hippocampus compared to controls. The results support the role of microglia as a mediator in the immune-CNS communication and the effects of peripheral IL-2.

Pattern of forebrain activation in high novelty-seeking rats following aggressive encounter

We have previously demonstrated that selectively-bred High (bHR) and Low (bLR) novelty-seeking rats exhibit agonistic differences, with bHRs acting in a highly aggressive manner when facing homecage intrusion. In order to discover the specific neuronal pathways responsible for bHRs' high levels of aggression, the present study compared c-fos mRNA expression in several forebrain regions of bHR/bLR males following this experience. bHR/bLR males were housed with female rats for 2 weeks, and then the females were replaced with a male intruder for 10 min. bHR/bLR residents were subsequently sacrificed by rapid decapitation, and their brains were removed and processed for c-fos in situ hybridization. Intrusion elicited robust c-fos mRNA expression in both phenotypes throughout the forebrain, including the septum, amygdala, hippocampus, cingulate cortex, and the hypothalamus. However, bHRs and bLRs exhibited distinct activation patterns in select areas. Compared to bHR rats, bLRs expressed greater c-fos in the lateral septum and within multiple hypothalamic nuclei, while bHRs showed greater activation in the arcuate hypothalamic nucleus and in the hippocampus. No bHR/bLR differences in c-fos expression were detected in the amygdala, cortical regions, and striatum. We also found divergent 5-HT1A receptor mRNA expression within some of these same areas, with bLRs having greater 5-HT1A, but not 5-HT1B, receptor mRNA levels in the septum, hippocampus and cingulate cortex. These findings, together with our earlier work, suggest that bHRs exhibit altered serotonergic functioning within select circuits during an aggressive encounter.

Infusions of neuropeptide Y into the lateral septum reduce anxiety-related behaviors in the rat

Neuropeptide Y (NPY) is one of the most abundant peptides in mammalian brain and NPY-like-immunoreactivity is highly expressed in the lateral septum, an area extensively involved in anxiety regulation. NPY counteracts the neurochemical and behavioral responses to acute threat in animal models, and intracerebroventricular (i.c.v.) administration of NPY at low doses is anxiolytic. Less is known about the specific contributions of the lateral septum to NPY-mediated anxiety regulation. In Experiment 1, the effects of infusions of NPY (1.5 μg) into the lateral septum were investigated in three animal models of anxiety: the elevated plus-maze, novelty-induced suppression of feeding, and shock-probe burying tests. Experiment 2 examined the role of the NPY Y1 receptor in these models by co-infusing the Y1 antagonist BIBO 3304 (0.15 μg, 0.30 μg) with NPY into the lateral septum. In the elevated plus-maze, there were no changes in rats' open arm exploration, the index of anxiety reduction in this test. In the novelty-induced suppression of feeding test, rats infused with NPY showed decreases in the latency to consume a palatable snack in a novel (but not familiar) environment, suggesting a reduction in anxiety independent of increases in appetite. This anxiolysis was attenuated by co-infusion with BIBO 3304 (0.30 μg) in Experiment 2. Lastly, rats infused with NPY showed decreases in the duration of burying behavior in the shock-probe burying test, also indicative of anxiety reduction. However, unlike in the feeding test, BIBO 3304 did not attenuate the NPY-induced anxiolysis in the shock-probe test. It is concluded that NPY produces anxiolytic-like actions in the lateral septum in two animal models of anxiety: the novelty-induced suppression of feeding, and shock-probe burying tests, and that this anxiolysis is dependent on Y1 receptor activation in the feeding test.