The median raphe nucleus in anxiety revisited

The dorsal raphe-to-ventral hippocampal projection modulates reactive aggression through 5HT1B receptors

Maladaptive impulsive aggression is a core symptom of neuropsychiatric disorders such as schizophrenia. While uncontrolled impulsive aggression dampens societal safety, there is a limited understanding of the neural regulation involved in impulsive aggression and its treatment. High levels of impulsivity and aggression have been linked to low serotonin (5-HT) levels. Additionally, post-weaning socially isolated (SI) mice exhibit outbursts of aggression following encountering acute stress, and hyperactivated ventral hippocampus (vHip) involves this stress-provoked escalated aggression. Here, we investigated the potential role of the raphe nucleus projecting to the vHip in modulating aggressive behavior. Chemogenetically activating the dorsal raphe nucleus (DRN) soma projecting the vHip or DRN nerve terminals in the vHip reduced impulsive aggression. The reduction of attack behavior was abolished by the pretreatment of 5HT1BR antagonist SB-224289. However, activating the median raphe nucleus (MRN)-to-vHip pathway ameliorated depression-like behavior but did not affect impulsive aggression. DRN→vHip activation suppressed the vHip downstream area, the ventromedial hypothalamus (VMH), which is a core aggression area. Intra-vHip infusion of 5HT1BR agonists (anpirtoline, CP-93129) suppressed impulsive aggression and decreased c-Fos levels in the vHip neurons projecting to the VMH, suggesting an inhibition mechanism. Our findings indicate that activating the DRN projecting to the vHip is sufficient to inhibit impulsive aggression in a 5HT1BR-dependent manner. Thus, targeting 5HT1BR could serve as a promising therapeutic approach to ameliorate symptoms of impulsive aggression.

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

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

The Dopaminergic Cells in the Median Raphe Region Regulate Social Behavior in Male Mice

According to previous studies, the median raphe region (MRR) is known to contribute significantly to social behavior. Besides serotonin, there have also been reports of a small population of dopaminergic neurons in this region. Dopamine is linked to reward and locomotion, but very little is known about its role in the MRR. To address that, we first confirmed the presence of dopaminergic cells in the MRR of mice (immunohistochemistry, RT-PCR), and then also in humans (RT-PCR) using healthy donor samples to prove translational relevance. Next, we used chemogenetic technology in mice containing the Cre enzyme under the promoter of the dopamine transporter. With the help of an adeno-associated virus, designer receptors exclusively activated by designer drugs (DREADDs) were expressed in the dopaminergic cells of the MRR to manipulate their activity. Four weeks later, we performed an extensive behavioral characterization 30 min after the injection of the artificial ligand (Clozapine-N-Oxide). Stimulation of the dopaminergic cells in the MRR decreased social interest without influencing aggression and with an increase in social discrimination. Additionally, inhibition of the same cells increased the friendly social behavior during social interaction test. No behavioral changes were detected in anxiety, memory or locomotion. All in all, dopaminergic cells were present in both the mouse and human samples from the MRR, and the manipulation of the dopaminergic neurons in the MRR elicited a specific social response.

Altered functional connectivity of brainstem nuclei in new daily persistent headache: Evidence from resting-state functional magnetic resonance imaging

OBJECTIVES

The new daily persistent headache (NDPH) is a rare primary headache disorder. However, the underlying mechanisms of NDPH remain incompletely understood. This study aims to apply seed-based analysis to explore the functional connectivity (FC) of brainstem nuclei in patients with NDPH using resting-state functional magnetic resonance imaging (MRI).

METHODS

The FC analysis from the region of interest (ROI) to whole brain voxels was used to investigate 29 patients with NDPH and 37 well-matched healthy controls (HCs) with 3.0 Tesla MRI. The 76 nuclei in the brainstem atlas were defined as ROIs. Furthermore, we explored the correlations between FC and patients' clinical characteristics and neuropsychological evaluations.

RESULTS

Patients with NDPH exhibited reduced FC in multiple brainstem nuclei compared to HCs (including right inferior medullary reticular formation, right mesencephalic reticular formation, bilateral locus coeruleus, bilateral laterodorsal tegmental nucleus-central gray of the rhombencephalon, median raphe, left medial parabrachial nucleus, periaqueductal gray, and bilateral ventral tegmental area-parabrachial pigmented nucleus complex) and increased FC in periaqueductal gray. No significant correlations were found between the FC of these brain regions and clinical characteristics or neuropsychological evaluations after Bonferroni correction (p > 0.00016).

CONCLUSIONS

Our results demonstrated that patients with NDPH have abnormal FC of brainstem nuclei involved in the perception and regulation of pain and emotions.

Role of α1-GABAA receptors in the serotonergic dorsal raphe nucleus in models of opioid reward, anxiety, and depression

BACKGROUND

The serotonin (5-hydroxytryptamine (5-HT))-mediated system plays an important role in stress-related psychiatric disorders and substance abuse. Our previous studies showed that stress and drug exposure can modulate the dorsal raphe nucleus (DRN)-5-HT system via γ-aminobutyric acid (GABA)A receptors. Moreover, GABAA receptor-mediated inhibition of serotonergic DRN neurons is required for stress-induced reinstatement of opioid seeking.

AIM/METHODS

To further test the role of GABAA receptors in the 5-HT system in stress and opioid-sensitive behaviors, our current study generated mice with conditional genetic deletions of the GABAA α1 subunit to manipulate GABAA receptors in either the DRN or the entire population of 5-HT neurons. The GABAA α1 subunit is a constituent of the most abundant GABAA subtype in the brain and the most highly expressed subunit in 5-HT DRN neurons.

RESULTS

Our results showed that mice with DRN-specific knockout of α1-GABAA receptors exhibited a normal phenotype in tests of anxiety- and depression-like behaviors as well as swim stress-induced reinstatement of morphine-conditioned place preference. By contrast, mice with 5-HT neuron-specific knockout of α1-GABAA receptors exhibited an anxiolytic phenotype at baseline and increased sensitivity to post-morphine withdrawal-induced anxiety.

CONCLUSIONS

Our data suggest that GABAA receptors on 5-HT neurons contribute to anxiety-like behaviors and sensitivity of those behaviors to opioid withdrawal.

Ventral hippocampus is more sensitive to fluoxetine-induced changes in extracellular 5-HT concentration, membrane 5-HT transporter level and immobility times

Hippocampal responses to selective 5-HT reuptake inhibitor (SSRI) have long been studied. However, its sub-regional involvements in mediating SSRI's pharmacological effects have not been fully addressed. The current study sought to investigate neurochemical, neurobiological and neurobehavioral changes in response to direct fluoxetine perfusion into the ventral and dorsal sub-regions of the hippocampus in C57BL/6 mice. Following fluoxetine perfusion, time courses of dialysate 5-HT, 5-HT transporter (5-HTT) protein (total, membrane and cytoplasmic fractions), locomotion, and immobility times in the forced swim test (FST) and tail suspension test (TST) were determined. At baseline, 5-HT uptake efficiency assessed by the no-net-flux microdialysis, and 5-HTT protein were measured as well. Results show that fluoxetine dose-dependently increased dialysate 5-HT, lowered membrane 5-HTT protein and increased cytoplasmic fraction without changing the total level, decreased immobility times in both the FST and TST, with greater responses all detected in the ventral sub-region compared to the dorsal sub-region. Fluoxetine didn't affect locomotor activity, ruling out the possibility that fluoxetine's effects on immobility maybe due to alteration in locomotion. Besides, lower 5-HT uptake efficiency and lower membrane 5-HTT protein level were found in the ventral sub-region at baseline. Together, the sub-regional differences at baseline and in responses to fluoxetine added powerful evidence to support the existence of two distinct 5-HT sub-systems in the hippocampus, with greater changes to fluoxetine detected in the ventral sub-system.

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons

A substantial proportion of raphe neurons are glutamatergic. However, little is known about how these glutamatergic neurons modulate the forebrain. We investigated how glutamatergic median raphe nucleus (MRN) input modulates the medial prefrontal cortex (mPFC), a critical component of fear circuitry. We show that vesicular glutamate transporter 3 (VGLUT3)-expressing MRN neurons activate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons enhances GABAergic transmission in mPFC pyramidal neurons and attenuates fear memory in female but not male mice. Serotonin plays a key role in MRN (VGLUT3) neuron-mediated GABAergic plasticity in the mPFC. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and in mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity.

Sex-specific modulation of the medial prefrontal cortex by glutamatergic median raphe neurons

The current understanding of the neuromodulatory role of the median raphe nucleus (MRN) is primarily based on its putative serotonergic output. However, a significant proportion of raphe neurons are glutamatergic. The present study investigated how glutamatergic MRN input modulates the medial prefrontal cortex (mPFC), a critical component of the fear circuitry. Our studies show that VGLUT3-expressing MRN neurons modulate VGLUT3- and somatostatin-expressing neurons in the mPFC. Consistent with this modulation of mPFC GABAergic neurons, activation of MRN (VGLUT3) neurons suppresses mPFC pyramidal neuron activity and attenuates fear memory in female but not male mice. In agreement with these female-specific effects, we observed sex differences in glutamatergic transmission onto MRN (VGLUT3) neurons and mPFC (VGLUT3) neuron-mediated dual release of glutamate and GABA. Thus, our results demonstrate a cell type-specific modulation of the mPFC by MRN (VGLUT3) neurons and reveal a sex-specific role of this neuromodulation in mPFC synaptic plasticity and fear memory.

Higher order diffusion imaging as a putative index of human sleep-related microstructural changes and glymphatic clearance

The brain has a unique macroscopic waste clearance system, termed the glymphatic system which utilises perivascular tunnels surrounded by astroglia to promote cerebrospinal-interstitial fluid exchange. Rodent studies have demonstrated a marked increase in glymphatic clearance during sleep which has been linked to a sleep-induced expansion of the extracellular space and concomitant reduction in intracellular volume. However, despite being implicated in the pathophysiology of multiple human neurodegenerative disorders, non-invasive techniques for imaging glymphatic clearance in humans are currently limited. Here we acquired multi-shell diffusion weighted MRI (dwMRI) in twenty-one healthy young participants (6 female, 22.3 ± 3.2 years) each scanned twice, once during wakefulness and once during sleep induced by a combination of one night of sleep deprivation and 10 mg of the hypnotic zolpidem 30 min before scanning. To capture hypothesised sleep-associated changes in intra/extracellular space, dwMRI were analysed using higher order diffusion modelling with the prediction that sleep-associated increases in interstitial (extracellular) fluid volume would result in a decrease in diffusion kurtosis, particularly in areas associated with slow wave generation at the onset of sleep. In line with our hypothesis, we observed a global reduction in diffusion kurtosis (t15=2.82, p = 0.006) during sleep as well as regional reductions in brain areas associated with slow wave generation during early sleep and default mode network areas that are highly metabolically active during wakefulness. Analysis with a higher-order representation of diffusion (MAP-MRI) further indicated that changes within the intra/extracellular domain rather than membrane permeability likely underpin the observed sleep-associated decrease in kurtosis. These findings identify higher-order modelling of dwMRI as a potential new non-invasive method for imaging glymphatic clearance and extend rodent findings to suggest that sleep is also associated with an increase in interstitial fluid volume in humans.

Glutamatergic and GABAergic neurons in pontine central gray mediate opposing valence-specific behaviors through a global network

Extracting the valence of environmental cues is critical for animals' survival. How valence in sensory signals is encoded and transformed to produce distinct behavioral responses remains not well understood. Here, we report that the mouse pontine central gray (PCG) contributes to encoding both negative and positive valences. PCG glutamatergic neurons were activated selectively by aversive, but not reward, stimuli, whereas its GABAergic neurons were preferentially activated by reward signals. The optogenetic activation of these two populations resulted in avoidance and preference behavior, respectively, and was sufficient to induce conditioned place aversion/preference. Suppression of them reduced sensory-induced aversive and appetitive behaviors, respectively. These two functionally opponent populations, receiving a broad range of inputs from overlapping yet distinct sources, broadcast valence-specific information to a distributed brain network with distinguishable downstream effectors. Thus, PCG serves as a critical hub to process positive and negative valences of incoming sensory signals and drive valence-specific behaviors with distinct circuits.

Mapping of STB/HAP1 Immunoreactivity in the Mouse Brainstem and its Relationships with Choline Acetyltransferase, with Special Emphasis on Cranial Nerve Motor and Preganglionic Autonomic Nuclei

Huntingtin-associated protein 1 (HAP1) is a core component of stigmoid body (STB) and is known as a neuroprotective interactor with causal agents for various neurodegenerative diseases. Brain regions rich in STB/HAP1 immunoreactivity are usually spared from cell death, whereas brain regions with negligible STB/HAP1 immunoreactivity are the major neurodegenerative targets. Recently, we have shown that STB/HAP1 is abundantly expressed in the spinal preganglionic sympathetic/parasympathetic neurons but absent in the motoneurons of spinal cord, indicating that spinal motoneurons are more vulnerable to neurodegenerative diseases. In light of STB/HAP1 neuroprotective effects, it is also essential to clarify the distribution of STB/HAP1 in another major neurodegenerative target, the brainstem. Here, we examined the expression and detailed immunohistochemical distribution of STB/HAP1 and its relationships with choline acetyltransferase (ChAT) in the midbrain, pons, and medulla oblongata of adult mice. Abundant STB/HAP1 immunoreactive neurons were disseminated in the periaqueductal gray, Edinger-Westphal nucleus, raphe nuclei, locus coeruleus, pedunculopontine tegmental nucleus, superior/inferior salivatory nucleus, and dorsal motor nucleus of vagus. Double-label immunohistochemistry of HAP1 with ChAT (or with urocortin-1 for Edinger-Westphal nucleus centrally projecting population) confirmed that STB/HAP1 was highly present in parasympathetic preganglionic neurons but utterly absent in cranial nerve motor nuclei throughout the brainstem. These results suggest that due to deficient putative STB/HAP1-protectivity, cranial nerve motor nuclei might be more vulnerable to certain neurodegenerative stresses than STB/HAP1-expressing brainstem nuclei, including preganglionic parasympathetic nuclei. Our current results also lay a basic foundation for future studies that seek to clarify the physiological/pathological roles of STB/HAP1 in the brainstem.

Response Flexibility: The Role of the Lateral Habenula

The ability to make appropriate decisions that result in an optimal outcome is critical for survival. This process involves assessing the environment as well as integrating prior knowledge about the environment with information about one’s current internal state. There are many neural structures that play critical roles in mediating these processes, but it is not yet known how such information coalesces to influence behavioral output. The lateral habenula (LHb) has often been cited as a structure critical for adaptive and flexible responding when environmental contexts and internal state changes. A challenge, however, has been understanding how LHb promotes response flexibility. In this review, we hypothesize that the LHb enables flexible responding following the integration of context memory and internal state information by signaling downstream brainstem structures known to drive hippocampal theta. In this way, animals respond more flexibly in a task situation not because the LHb selects a particular action, but rather because LHb enhances a hippocampal neural state that is often associated with greater attention, arousal, and exploration. In freely navigating animals, these are essential conditions that are needed to discover and implement appropriate alternative choices and behaviors. As a corollary to our hypothesis, we describe short- and intermediate-term functions of the LHb. Finally, we discuss the effects on the behavior of LHb dysfunction in short- and intermediate-timescales, and then suggest that new therapies may act on the LHb to alleviate the behavioral impairments following long-term LHb disruption.

Correlation of Expression Changes between Genes Controlling 5-HT Synthesis and Genes Crh and Trh in the Midbrain Raphe Nuclei of Chronically Aggressive and Defeated Male Mice

Midbrain raphe nuclei (MRNs) contain a large number of serotonergic neurons associated with the regulation of numerous types of psychoemotional states and physiological processes. The aim of this work was to study alterations of the MRN transcriptome in mice with prolonged positive or negative fighting experience and to identify key gene networks associated with the regulation of serotonergic system functioning. Numerous genes underwent alterations of transcription in the MRNs of male mice that either manifested aggression or experienced social defeat in daily agonistic interactions. The expression of the Tph2 gene encoding the rate-limiting enzyme of the serotonin synthesis pathway correlated with the expression of many genes, 31 of which were common between aggressive and defeated mice and were downregulated in the MRNs of mice of both experimental groups. Among these common differentially expressed genes (DEGs), there were genes associated with behavior, learning, memory, and synaptic signaling. These results suggested that, in the MRNs of the mice, the transcriptome changes associated with serotonergic regulation of various processes are similar between the two groups (aggressive and defeated). In the MRNs, more DEGs correlating with Tph2 expression were found in defeated mice than in the winners, which is probably a consequence of deeper Tph2 downregulation in the losers. It was shown for the first time that, in both groups of experimental mice, the changes in the transcription of genes controlling the synthesis and transport of serotonin directly correlate with the expression of genes Crh and Trh, which control the synthesis of corticotrophin- and thyrotropin-releasing hormones. Our findings indicate that CRH and TRH locally produced in MRNs are related to serotonergic regulation of brain processes during a chronic social conflict.

16p11 Duplication Disrupts Hippocampal-Orbitofrontal-Amygdala Connectivity, Revealing a Neural Circuit Endophenotype for Schizophrenia

Chromosome 16p11.2 duplications dramatically increase risk for schizophrenia, but the mechanisms remain largely unknown. Here, we show that mice with an equivalent genetic mutation (16p11.2 duplication mice) exhibit impaired hippocampal-orbitofrontal and hippocampal-amygdala functional connectivity. Expression of schizophrenia-relevant GABAergic cell markers (parvalbumin and calbindin) is selectively decreased in orbitofrontal cortex, while somatostatin expression is decreased in lateral amygdala. When 16p11.2 duplication mice are tested in cognitive tasks dependent on hippocampal-orbitofrontal connectivity, performance is impaired in an 8-arm maze "N-back" working memory task and in a touchscreen continuous performance task. Consistent with hippocampal-amygdala dysconnectivity, deficits in ethologically relevant social behaviors are also observed. Overall, the cellular/molecular, brain network, and behavioral alterations markedly mirror those observed in schizophrenia patients. Moreover, the data suggest that 16p11.2 duplications selectively impact hippocampal-amygdaloid-orbitofrontal circuitry, supporting emerging ideas that dysfunction in this network is a core element of schizophrenia and defining a neural circuit endophenotype for the disease.

Continuous psychosocial stress stimulates BMP signaling in dorsal hippocampus concomitant with anxiety-like behavior associated with differential modulation of cell proliferation and neurogenesis

Bone morphogenetic protein (BMP) signaling in the hippocampus regulates psychiatric behaviors and hippocampal neurogenesis in non-stress conditions; however, stress-induced changes in hippocampal BMP signaling have not yet been reported. Therefore, we sought to examine whether psychosocial stress, which induces psychiatric symptoms, affects hippocampal BMP signaling. A total of 32 male Sprague-Dawley rats were exposed to a psychosocial stress using a Resident/Intruder paradigm for ten consecutive days. Subsequently, rats were subjected to a battery of behavioral tests (novelty-suppressed feeding test, sucrose preference test, and forced swimming test) for the evaluation of adult neurogenesis and activity of BMP signaling in the dorsal and ventral hippocampus. Repeated social defeat promoted anxiety-like behaviors, but neither anhedonia nor behavioral despair. Socially defeated rats exhibited an increase in the number of Ki-67-positive cells, decrease in the number of doublecortin (DCX)-positive cells, and decrease only in the dorsal hippocampus of the ratio of DCX-positive to Ki-67-positive cells, a proxy for newly-born cell maturation speed and survival. In contrast, no differences were observed in the number of 5-Bromo-2'-deoxyuridine (BrdU)-positive cells, indicating survival of newly-born cells both in the dorsal and ventral hippocampus. Furthermore, psychosocial stress significantly increased the BMP-4 and phosphorylated Smad1/5/9 expression levels specifically in the dorsal hippocampus. Our findings suggest that repeated psychosocial stress activates BMP signaling and differently affects cell proliferation and neurogenesis exclusively in the dorsal hippocampus, potentially exacerbating anxiety-related symptoms. Targeting BMP signaling is a potential therapeutic strategy for psychiatric disorders.

Median raphe controls acquisition of negative experience in the mouse

Adverse events need to be quickly evaluated and memorized, yet how these processes are coordinated is poorly understood. We discovered a large population of excitatory neurons in mouse median raphe region (MRR) expressing vesicular glutamate transporter 2 (vGluT2) that received inputs from several negative experience-related brain centers, projected to the main aversion centers, and activated the septohippocampal system pivotal for learning of adverse events. These neurons were selectively activated by aversive but not rewarding stimuli. Their stimulation induced place aversion, aggression, depression-related anhedonia, and suppression of reward-seeking behavior and memory acquisition-promoting hippocampal theta oscillations. By contrast, their suppression impaired both contextual and cued fear memory formation. These results suggest that MRR vGluT2 neurons are crucial for the acquisition of negative experiences and may play a central role in depression-related mood disorders.

Role of 5-HT1A Receptor in the Anxiolytic-Relaxant Effects of Bergamot Essential Oil in Rodent

The essential oil obtained by the fresh fruit of Citrus bergamia Risso et Poiteau is used worldwide in aromatherapy to reduce pain, facilitate sleep induction, and/or minimize the effects of stress-induced anxiety. Preclinical pharmacological data demonstrate that bergamot essential oil (BEO) modulates specific neurotransmissions and shows an anxiolytic-relaxant effect not superimposable to that of the benzodiazepine diazepam, suggesting that neurotransmissions, other than GABAergic, could be involved. Several studies on essential oils indicate a role for serotonergic (5-HT) neurotransmission in anxiety. Interestingly, among serotonergic receptors, the 5-HT1A subtype seems to play a key role in the control of anxiety. Here, we report that modulation of the 5-HT1A receptor by selective agonist ((±)8-OH-DPAT) or antagonist (WAY-100635) may influence some of the anxiolytic-relaxant effects of BEO in Open Field and Elevated Plus Maze tests.

Embracing diversity in the 5-HT neuronal system

Neurons that synthesize and release 5-hydroxytryptamine (5-HT; serotonin) express a core set of genes that establish and maintain this neurotransmitter phenotype and distinguish these neurons from other brain cells. Beyond a shared 5-HTergic phenotype, these neurons display divergent cellular properties in relation to anatomy, morphology, hodology, electrophysiology and gene expression, including differential expression of molecules supporting co-transmission of additional neurotransmitters. This diversity suggests that functionally heterogeneous subtypes of 5-HT neurons exist, but linking subsets of these neurons to particular functions has been technically challenging. We discuss recent data from molecular genetic, genomic and functional methods that, when coupled with classical findings, yield a reframing of the 5-HT neuronal system as a conglomeration of diverse subsystems with potential to inspire novel, more targeted therapies for clinically distinct 5-HT-related disorders.

Median raphe serotonin neurons promote anxiety-like behavior via inputs to the dorsal hippocampus

Anxiety disorders may be mediated in part by disruptions in serotonin (5-hydroxytryptamine, 5-HT) system function. Behavioral measures of approach-avoidance conflict suggest that serotonin neurons within the median raphe nucleus (MRN) promote an anxiogenic state, and some evidence indicates this may be mediated by serotonergic signaling within the dorsal hippocampus. Here, we test this hypothesis using an optogenetic approach to examine the contribution of MRN 5-HT neurons and 5-HT innervation of the dorsal hippocampus (dHC) to anxiety-like behaviours in female mice. Mice expressing the excitatory opsin ChR2 were generated by crossing the ePet-cre serotonergic cre-driver line with the conditional Ai32 ChR2 reporter line, resulting in selective expression of ChR2 in 5-HT neurons. Electrophysiological recordings confirmed that this approach enabled reliable optogenetic stimulation of MRN 5-HT neurons, and this stimulation produced downstream 5-HT release in the dHC as measured by in vivo microdialysis. Optogenetic stimulation of the MRN elicited behavioral responses indicative of an anxiogenic effect in three behavioural tests: novelty-suppressed feeding, marble burying and exploration on the elevated-plus maze. These effects were shown to be behaviourally-specific. Stimulation of 5-HT terminals in the dHC recapitulated the anxiety-like behaviour in the novelty-suppressed feeding and marble burying tests. These results show that activation of 5-HT efferents from the MRN rapidly induces expression of anxiety-like behaviour, in part via projections to the dHC. These findings reveal an important neural circuit implicated in the expression of anxiety in female mice.

Habenular connections with the dopaminergic and serotonergic system and their role in stress-related psychiatric disorders

The habenula (Hb) is a phylogenetically old epithalamic structure differentiated into two nuclear complexes, the medial (MHb) and lateral habenula (LHb). After decades of search for a great unifying function, interest in the Hb resurged when it was demonstrated that LHb plays a major role in the encoding of aversive stimuli ranging from noxious stimuli to the loss of predicted rewards. Consistent with a role as an anti-reward center, aberrant LHb activity has now been identified as a key factor in the pathogenesis of major depressive disorder. Moreover, both MHb and LHb emerged as new players in the reward circuitry by primarily mediating the aversive properties of distinct drugs of abuse. Anatomically, the Hb serves as a bridge that links basal forebrain structures with monoaminergic nuclei in the mid- and hindbrain. So far, research on Hb has focused on the role of the LHb in regulating midbrain dopamine release. However, LHb/MHb are also interconnected with the dorsal (DR) and median (MnR) raphe nucleus. Hence, it is conceivable that some of the habenular functions are at least partly mediated by the complex network that links MHb/LHb with pontomesencephalic monoaminergic nuclei. Here, we summarize research about the topography and transmitter phenotype of the reciprocal connections between the LHb and ventral tegmental area-nigra complex, as well as those between the LHb and DR/MnR. Indirect MHb outputs via interpeduncular nucleus to state-setting neuromodulatory networks will also be commented. Finally, we discuss the role of specific LHb-VTA and LHb/MHb-raphe circuits in anxiety and depression.

Effects of maternal separation on serotonergic systems in the dorsal and median raphe nuclei of adult male Tph2-deficient mice

Previous studies have highlighted interactions between serotonergic systems and adverse early life experience as important gene x environment determinants of risk of stress-related psychiatric disorders. Evidence suggests that mice deficient in Tph2, the rate-limiting enzyme for brain serotonin synthesis, display disruptions in behavioral phenotypes relevant to stress-related psychiatric disorders. The aim of this study was to determine how maternal separation in wild-type, heterozygous, and Tph2 knockout mice affects mRNA expression of serotonin-related genes. Serotonergic genes studied included Tph2, the high-affinity, low-capacity, sodium-dependent serotonin transporter (Slc6a4), the serotonin type 1a receptor (Htr1a), and the corticosterone-sensitive, low-affinity, high-capacity sodium-independent serotonin transporter, organic cation transporter 3 (Slc22a3). Furthermore, we studied corticotropin-releasing hormone receptors 1 (Crhr1) and 2 (Crhr2), which play important roles in controlling serotonergic neuronal activity. For this study, offspring of Tph2 heterozygous dams were exposed to daily maternal separation for the first two weeks of life. Adult, male wild-type, heterozygous, and homozygous offspring were subsequently used for molecular analysis. Maternal separation differentially altered serotonergic gene expression in a genotype- and topographically-specific manner. For example, maternal separation increased Slc6a4 mRNA expression in the dorsal part of the dorsal raphe nucleus in Tph2 heterozygous mice, but not in wild-type or knockout mice. Overall, these data are consistent with the hypothesis that gene x environment interactions, including serotonergic genes and adverse early life experience, play an important role in vulnerability to stress-related psychiatric disorders.

Different roles of distinct serotonergic pathways in anxiety-like behavior, antidepressant-like, and anti-impulsive effects

Serotonergic agents have been widely used for treatment of psychiatric disorders, but the therapeutic effects are insufficient and these drugs often induce severe side effects. We need to specify the distinct serotonergic pathways underlying each mental function to overcome these problems. Preclinical studies have demonstrated that the central serotonergic system is involved in several emotional/cognitive functions including anxiety, depression, and impulse control, but it remains unclear whether each function is regulated by a different serotonergic system. We used optogenetic strategy to increase central serotonergic activity in mice and evaluated the behavioral consequences. Pharmacological and genetic tools were used to determine the subtype of 5-HT receptors responsible for the observed effects. We demonstrated that the serotonergic activation in the median raphe nucleus enhanced anxiety-like behavior, the serotonergic activation in the dorsal raphe nucleus exerted antidepressant-like effects, and the serotonergic activation in the median or dorsal raphe nucleus suppressed impulsive action. We also found that different serotonergic terminals, ventral hippocampus, ventral tegmental area/substantia nigra, and subthalamic/parasubthalamic nucleus, are involved in regulating anxiety-like behavior, antidepressant-like, and anti-impulsive effects, respectively. Furthermore, we found, using triple-transgenic mice, that the stimulation of the 5-HT_2C receptor is required to evoke anxiety-like behavior, but not to exert anti-impulsive effects. These results suggest the need for pathway-specific treatments and provide important insights that will help the development of more effective and safer therapeutics. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Unpredictable chronic prenatal stress and manifestation of generalized anxiety and panic in rat's offspring

Often the manifestation of anxiety cannot be explained by known environmental or hereditary factors. With this perspective, it has been reported that prenatal stress may lead to emotional disturbances in the offspring. However, studies relating prenatal stress to anxiety are controversial and generally the stressors used do not mimicks the reality experienced by mothers. Thus, this investigation evaluated the effects of an unpredictable chronic stress scheme applied in one of the three gestational weeks of rats on the manifestation of generalized anxiety and panic disorder in the progeny (males), analyzing, respectively, the avoidances and escapes in the elevated T-maze, at the 1st, 3rd or 6th month of progeny life. Control offspring showed increased generalized anxiety disorder and reduced panic at 6 months. The effects of prenatal stress depended on the gestational week where it occurred and on the progeny age: during the 1st gestational week the generalized anxiety decreased in 6 month old rats. Animals in the 3rd month, prenatally stressed during the last gestational week, showed anxiogenesis and panicogenesis, but effects reverted at the 6th month, when they presented anxiolysis and no changes related to panic. Together the results show that not only the gestational period in which the aversive experience occurred was important, but the age of the evaluated progeny, since the type and the intensity of behaviors related to anxiety may vary with the developmental stage. For the model of stress used in the present study, the effects of prenatal stress were more prominent when the exposure occurred during the 3rd gestational week in rats.

A multiscale cerebral neurochemical connectome of the rat brain

Understanding the rat neurochemical connectome is fundamental for exploring neuronal information processing. By using advanced data mining, supervised machine learning, and network analysis, this study integrates over 5 decades of neuroanatomical investigations into a multiscale, multilayer neurochemical connectome of the rat brain. This neurochemical connectivity database (ChemNetDB) is supported by comprehensive systematically-determined receptor distribution maps. The rat connectome has an onion-type structural organization and shares a number of structural features with mesoscale connectomes of mouse and macaque. Furthermore, we demonstrate that extremal values of graph theoretical measures (e.g., degree and betweenness) are associated with evolutionary-conserved deep brain structures such as amygdala, bed nucleus of the stria terminalis, dorsal raphe, and lateral hypothalamus, which regulate primitive, yet fundamental functions, such as circadian rhythms, reward, aggression, anxiety, and fear. The ChemNetDB is a freely available resource for systems analysis of motor, sensory, emotional, and cognitive information processing.

Interaction between estradiol and 5-HT1A receptors in the median raphe nucleus on acquisition of aversive information and association to the context in ovariectomized rats

The median raphe nucleus (MRN) is related to stress resistance and defensive responses, a crucial source of serotonergic neurons that project to prosencephalic structures related to stress and anxiety. Estrogen receptors were identified in this mesencephalic structure. It is possible that the estrogen action is related to serotonin effect on somatodendritic 5-HT_1A receptors, inhibiting the function of serotonergic neurons and thus preventing of the stress effect and inducing anxiolysis. So, in order to evaluate these aspects, female Wistar rats were ovariectomized and 21 days later were given a direct microinjection of estradiol benzoate (EB) (1200 ng) into the MRN, preceded by microinjections of saline or WAY100.635 (100 ng), a 5-HT_1A receptor antagonist. Immediately after the two microinjections, the ovariectomized rats were conditioned with an aversive event (foot shock) session in a Skinner box. Twenty-four hours later, they were exposed to the same context in a test session for 5 min for behavioral assessment: freezing, rearing, locomotion, grooming, and autonomic responses (fecal boluses and micturition). EB microinjection in the MRN prior to the exposure of animals to the foot shocks in the conditioning session did not alter their behavior in this session, but neutralized the association of the aversive experience to the context: there was a decrease in the expression of freezing and an increased rearing activity in the test session. This effect was reversed by prior microinjection of WAY100.635. In conclusion, EB acted on serotonergic neurons in the MRN of the ovariectomized rats, impairing the association of the aversive experience to the context, by co-modulating the functionality of somatodendritic 5-HT_1A.

Functional neuroanatomy in panic disorder: Status quo of the research

AIM

To provide an overview of the current research in the functional neuroanatomy of panic disorder.

METHODS

Panic disorder (PD) is a frequent psychiatric disease. Gorman et al (1989; 2000) proposed a comprehensive neuroanatomical model of PD, which suggested that fear- and anxiety-related responses are mediated by a so-called “fear network” which is centered in the amygdala and includes the hippocampus, thalamus, hypothalamus, periaqueductal gray region, locus coeruleus and other brainstem sites. We performed a systematic search by the electronic database PubMed. Thereby, the main focus was laid on recent neurofunctional, neurostructural, and neurochemical studies (from the period between January 2012 and April 2016). Within this frame, special attention was given to the emerging field of imaging genetics.

RESULTS

We noted that many neuroimaging studies have reinforced the role of the “fear network” regions in the pathophysiology of panic disorder. However, recent functional studies suggest abnormal activation mainly in an extended fear network comprising brainstem, anterior and midcingulate cortex (ACC and MCC), insula, and lateral as well as medial parts of the prefrontal cortex. Interestingly, differences in the amygdala activation were not as consistently reported as one would predict from the hypothesis of Gorman et al (2000). Indeed, amygdala hyperactivation seems to strongly depend on stimuli and experimental paradigms, sample heterogeneity and size, as well as on limitations of neuroimaging techniques. Advanced neurochemical studies have substantiated the major role of serotonergic, noradrenergic and glutamatergic neurotransmission in the pathophysiology of PD. However, alterations of GABAergic function in PD are still a matter of debate and also their specificity remains questionable. A promising new research approach is “imaging genetics”. Imaging genetic studies are designed to evaluate the impact of genetic variations (polymorphisms) on cerebral function in regions critical for PD. Most recently, imaging genetic studies have not only confirmed the importance of serotonergic and noradrenergic transmission in the etiology of PD but also indicated the significance of neuropeptide S receptor, CRH receptor, human TransMEMbrane protein (TMEM123D), and amiloride-sensitive cation channel 2 (ACCN2) genes.

CONCLUSION

In light of these findings it is conceivable that in the near future this research will lead to the development of clinically useful tools like predictive biomarkers or novel treatment options.

Mirtazapine exerts an anxiolytic-like effect through activation of the median raphe nucleus-dorsal hippocampal 5-HT pathway in contextual fear conditioning in rats

The functional role of serotonergic projections from the median raphe nucleus (MRN) to the dorsal hippocampus (DH) in anxiety remains understood poorly. The purpose of the present research was to examine the functional role of this pathway, using the contextual fear conditioning (CFC) model of anxiety. We show that intra-MRN microinjection of mirtazapine, a noradrenergic and specific serotonergic antidepressant, reduced freezing in CFC without affecting general motor activity dose-dependently, suggesting an anxiolytic-like effect. In addition, intra-MRN microinjection of mirtazapine dose-dependently increased extracellular concentrations of serotonin (5-HT) but not dopamine in the DH. Importantly, intra-DH pre-microinjection of WAY-100635, a 5-HT1A antagonist, significantly attenuated the effect of mirtazapine on freezing. These results, for the first time, suggest that activation of the MRN-DH 5-HT1A pathway exerts an anxiolytic-like effect in CFC. This is consistent with the literature that the hippocampus is essential for retrieval of contextual memory and that 5-HT1A receptor activation in the hippocampus primarily exerts an inhibitory effect on the neuronal activity.

Combined treatment with subchronic lithium and acute intracerebral mirtazapine microinjection into the median raphe nucleus exerted an anxiolytic-like effect synergistically

Although preclinical and clinical studies have established the efficacy of lithium augmentation of antidepressant drugs, the mechanism of action of lithium augmentation is not fully understood. Our previous study reported that subchronic lithium treatment enhanced the anxiolytic-like effect of systemic mirtazapine. In the present study, we examined the effect of subchronic lithium in combination with acute local intracerebral injection of mirtazapine on fear-related behaviors in a contextual fear conditioning test in rats to clarify the target brain region of lithium augmentation of mirtazapine. After conditioning by footshock, diet (food pellets) containing Li2CO3 at a concentration of 0.2% was administered for 7 days. Ten min before testing and 7 days after conditioning, mirtazapine (3μg/site) in a volume of 0.5µl was acutely injected into the median raphe nucleus (MRN), hippocampus or amygdala. The combination of subchronic lithium and acute mirtazapine microinjection into the MRN but not the hippocampus or the amygdala reduced fear expression synergistically. These results suggest that intra-MRN mirtazapine treatment with subchronic lithium exerts the anxiolytic-like effect through the facilitation of the MRN-5HT pathway.

Ongoing behavioral state information signaled in the lateral habenula guides choice flexibility in freely moving rats

The lateral habenula (LHb) plays a role in a wide variety of behaviors ranging from maternal care, to sleep, to various forms of cognition. One prominent theory with ample supporting evidence is that the LHb serves to relay basal ganglia and limbic signals about negative outcomes to midbrain monoaminergic systems. This makes it likely that the LHb is critically involved in behavioral flexibility as all of these systems have been shown to contribute when flexible behavior is required. Behavioral flexibility is commonly examined across species and is impaired in various neuropsychiatric conditions including autism, depression, addiction, and schizophrenia; conditions in which the LHb is thought to play a role. Therefore, a thorough examination of the role of the LHb in behavioral flexibility serves multiple functions including understanding possible connections with neuropsychiatric illnesses and additional insight into its role in cognition in general. Here, we assess the LHb’s role in behavioral flexibility through comparisons of the roles its afferent and efferent pathways are known to play. Additionally, we provide new evidence supporting the LHb contributions to behavioral flexibility through organization of specific goal directed actions under cognitively demanding conditions. Specifically, in the first experiment, a majority of neurons recorded from the LHb were found to correlate with velocity on a spatial navigation task and did not change significantly when reward outcomes were manipulated. Additionally, measurements of local field potential (LFP) in the theta band revealed significant changes in power relative to velocity and reward location. In a second set of experiments, inactivation of the LHb with the gamma-aminobutyric acid (GABA) agonists baclofen and muscimol led to an impairment in a spatial/response based repeated probabilistic reversal learning task. Control experiments revealed that this impairment was likely due to the demands of repeated switching behaviors as rats were unimpaired on initial discrimination acquisition or retention of probabilistic learning. Taken together, these novel findings compliment other work discussed supporting a role for the LHb in action selection when cognitive or emotional demands are increased. Finally, we discuss future mechanisms by which a superior understanding of the LHb can be obtained through additional examination of behavioral flexibility tasks.

Differences in Anxiety-Like Behavior within a Batch of Wistar Rats Are Associated with Differences in Serotonergic Transmission, Enhanced by Acute SRI Administration, and Abolished By Serotonin Depletion

BACKGROUND

The anxiety-reducing effect of long-term administration of serotonin reuptake inhibitors is usually seen only in subjects with anxiety disorders, and such patients are also abnormally inclined to experience a paradoxical anxiety-enhancing effect of acute serotonin reuptake inhibition. These unique responses to serotonin reuptake inhibitors in anxiety-prone subjects suggest, as do genetic association studies, that inter-individual differences in anxiety may be associated with differences in serotonergic transmission.

METHODS

The one-third of the animals within a batch of Wistar rats most inclined to spend time on open arms in the elevated plus maze were compared with the one-third most inclined to avoid them with respect to indices of brain serotonergic transmission and how their behavior was influenced by serotonin-modulating drugs.

RESULTS

"Anxious" rats displayed higher expression of the tryptophan hydroxylase-2 gene and higher levels of the tryptophan hydroxylase-2 protein in raphe and also higher levels of serotonin in amygdala. Supporting these differences to be important for the behavioral differences, serotonin depletion obtained by the tryptophan hydroxylase-2 inhibitor p-chlorophenylalanine eliminated them by reducing anxiety in "anxious" but not "non-anxious" rats. Acute administration of a serotonin reuptake inhibitor, paroxetine, exerted an anxiety-enhancing effect in "anxious" but not "non-anxious" rats, which was eliminated by long-term pretreatment with another serotonin reuptake inhibitor, escitalopram.

CONCLUSIONS

Differences in an anxiogenic impact of serotonin, which is enhanced by acute serotonin reuptake inhibitor administration, may contribute to differences in anxiety-like behavior amongst Wistar rats.

Effect of lithium on behavioral disinhibition induced by electrolytic lesion of the median raphe nucleus

RATIONALE

Alterations in brainstem circuits have been proposed as a possible mechanism underlying the etiology of mood disorders. Projections from the median raphe nucleus (MnR) modulate dopaminergic activity in the forebrain and are also part of a behavioral disinhibition/inhibition system that produces phenotypes resembling behavioral variations manifested during manic and depressive phases of bipolar disorder.

OBJECTIVE

The aim of this study is to assess the effect of chronic lithium treatment on behavioral disinhibition induced by MnR lesions.

METHODS

MnR electrolytic lesions were performed in C57BL/6J mice, with sham-operated and intact animals as control groups. Following recovery, mice were chronically treated with lithium (LiCl, added in chow) followed by behavioral testing.

RESULTS

MnR lesion induced manic-like behavioral alterations including hyperactivity in the open field (OF), stereotyped circling, anxiolytic/risk taking in the elevated plus maze (EPM) and light/dark box (LDB) tests, and increased basal body temperature. Lithium was specifically effective in reducing OF hyperactivity and stereotypy but did not reverse (EPM) or had a nonspecific effect (LDB) on anxiety/risk-taking measures. Additionally, lithium decreased saccharin preference and prevented weight loss during single housing.

CONCLUSIONS

Our data support electrolytic lesions of the MnR as an experimental model of a hyper-excitable/disinhibited phenotype consistent with some aspects of mania that are attenuated by the mood stabilizer lithium. Given lithium's relatively specific efficacy in treating mania, these data support the hypothesis that manic symptoms derive not only from the stimulation of excitatory systems but also from inactivation or decreased activity of inhibitory mechanisms.

Impact of chronic stressors on the anxiety profile of pregnant rats

The manifestation of anxiety during pregnancy can be caused by multiple factors and may have emotional and physical consequences for both the mother and the fetus. The prevalence of gestational anxiety has grown in recent years, making the development of studies for its comprehension essential. Thus, the aim of this investigation was to evaluate the effects of predictable and unpredictable chronic stressors on the anxiety profile of rats in three distinct stages of pregnancy (1st, 2nd and 3rd weeks). Wistar dams were divided into three groups: control, social separation and unpredictable chronic stress. Behavioral assessments were conducted in the Elevated Plus-Maze at the end of the 1st, 2nd and 3rd weeks of gestation. The results showed that there was increased anxiety in the proximity of parturition in control dams. Chronic stressors differentially affected the behavior of pregnant rats according to the gestational period where they were applied: social separation decreased anxiety at the end of the 3rd week, while unpredictable chronic stress caused increased anxiety, especially at the end of the 2nd gestational week. These results show that there is a critical time during pregnancy for the onset of anxiety in control rats, depending on the gestational stage. The exposure to different types of chronic stressors may result in distinct behaviors related to this disorder.

Ingestive and locomotor behaviours induced by pharmacological manipulation of <Alpha>-adrenoceptors into the median raphe nucleus

The present study evaluated the involvement of α-adrenoceptors of the median raphe nucleus (MRN) in satiated rats, in food and water intake and motor behaviour. Control groups were treated with saline (SAL) or adrenaline (ADR), injected into the MRN seven minutes after injection of the vehicle used to solubilize the antagonists, propylene glycol (PLG) or SAL. Experimental groups were treated with an α-adrenoceptor antagonist, prazosin (α1, 20 or 40 nmol) or yohimbine (α2, 20 or 40 nmol) or phentolamine (non-selective α, 20 or 40 nmol), followed (later) by injection of ADR or SAL. Behaviour was recorded for 30 min. The injection of ADR and the blockade of α1 receptors resulted in hyperphagia whereas blocking α2 or α1 and α2 simultaneously did not change feeding behaviour. Pre-treatment with prazosin, followed by injection of ADR was not able to cause an increase in the amount of food ingested, while the higher dose of the α1 antagonist reduced the latency to start feeding. Pre-treatment with prazosin also caused hyperactivity. However, pre-treatment with phentolamine or yohimbine was able to block ADR-induced feeding. The present study supports the hypothesis that there is a tonic activation of α1-adrenoceptors in the MRN in satiated rats, which activates an inhibitory influence in areas that control food intake. Injection of ADR seems to activate α2 receptors, resulting in a decrease in the availability of endogenous catecholamines, which reduces the release of the signal that inhibits food intake, leading to hyperphagia.

Executive and modulatory neural circuits of defensive reactions: implications for panic disorder

The present review covers two independent approaches, a neuroanatomical and a pharmacological (focused on serotonergic transmission), which converge in highlighting the critical role of the hypothalamus and midbrain periaqueductal gray matter in the generation of panic attacks and in the mechanism of action of current antipanic medication. Accordingly, innate and learned fear responses to different threats (i.e., predator, aggressive members of the same species, interoceptive threats and painful stimuli) are processed by independent circuits involving corticolimbic regions (the amygdala, the hippocampus and the prefrontal and insular cortices) and downstream hypothalamic and brainstem circuits. As for the drug treatment, animal models of panic indicate that the drugs currently used for treating panic disorder should work by enhancing 5-HT inhibition of neural systems that command proximal defense in both the dorsal periaqueductal gray and in the medial hypothalamus. For the anticipatory anxiety, the reviewed evidence points to corticolimbic structures, such as the amygdala, the septo-hippocampus and the prefrontal cortex, as its main neural substrate, modulated by stimulation of 5-HT2C and 5-HT1A receptors.