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

The mast cells - Cytokines axis in Autism Spectrum Disorder

Autism Spectrum Disorder (ASD) is a neurodevelopmental disturbance, diagnosed in early childhood. It is associated with varying degrees of dysfunctional communication and social skills, repetitive and stereotypic behaviors. Regardless of the constant increase in the number of diagnosed patients, there are still no established treatment schemes in global practice. Many children with ASD have allergic symptoms, often in the absence of mast cell (MC) positive tests. Activation of MCs may release molecules related to inflammation and neurotoxicity, which contribute to the pathogenesis of ASD. The aim of the present paper is to enrich the current knowledge regarding the relationship between MCs and ASD by providing PPI network analysis-based data that reveal key molecules and immune pathways associated with MCs in the pathogenesis of autism. Network and enrichment analyzes were performed using receptor information and secreted molecules from activated MCs identified in ASD patients. Our analyses revealed cytokines and key marker molecules for MCs degranulation, molecular pathways of key mediators released during cell degranulation, as well as various receptors. Understanding the relationship between ASD and the activation of MCs, as well as the involved molecules and interactions, is important for elucidating the pathogenesis of ASD and developing effective future treatments for autistic patients by discovering new therapeutic target molecules.

Reviewing explore/exploit decision-making as a transdiagnostic target for psychosis, depression, and anxiety

In many everyday decisions, individuals choose between trialling something novel or something they know well. Deciding when to try a new option or stick with an option that is already known to you, known as the "explore/exploit" dilemma, is an important feature of cognition that characterises a range of decision-making contexts encountered by humans. Recent evidence has suggested preferences in explore/exploit biases are associated with psychopathology, although this has typically been examined within individual disorders. The current review examined whether explore/exploit decision-making represents a promising transdiagnostic target for psychosis, depression, and anxiety. A systematic search of academic databases was conducted, yielding a total of 29 studies. Studies examining psychosis were mostly consistent in showing that individuals with psychosis explored more compared with individuals without psychosis. The literature on anxiety and depression was more heterogenous; some studies found that anxiety and depression were associated with more exploration, whereas other studies demonstrated reduced exploration in anxiety and depression. However, examining a subset of studies that employed case-control methods, there was some evidence that both anxiety and depression also were associated with increased exploration. Due to the heterogeneity across the literature, we suggest that there is insufficient evidence to conclude whether explore/exploit decision-making is a transdiagnostic target for psychosis, depression, and anxiety. However, alongside our advisory groups of lived experience advisors, we suggest that this context of decision-making is a promising candidate that merits further investigation using well-powered, longitudinal designs. Such work also should examine whether biases in explore/exploit choices are amenable to intervention.

Prolame produces anxiolytic- and antidepressant-like effects in middle-aged female rats with less uterotrophic effects than 17β-estradiol

Estrogen hormone replacement therapy (EHRT), improving women's life quality at menopause, reduces anxiety and depression symptoms associated with ovarian hormonal decline. However, its potential adverse effects, like thromboembolism and cancer risk, limit its use. Prolame is a synthetic 17β-amino estrogen with antithrombotic actions that exerts anxiolytic- and antidepressant-like effects on young adult ovariectomized female rats. It is unknown if prolame's effects may be observed in age and endocrine conditions emulating menopause. This study aimed to identify the antidepressant- and anxiolytic-like effects of prolame and E2 (used as a reference estrogen treatment) in middle-aged female rats coursing with irregular cycles, in two different conditions: ovariectomized or gonadally intact. Results were compared with those from young adult ovariectomized rats. Prolame (60 or 120 μg/kg), 17β-estradiol (E2, 40 or 80 μg/kg), or vehicle were chronically administered, and their effects were evaluated in the elevated plus-maze, defensive burying behavior test, open field test, and forced swimming test. Uterotrophic actions were estimated by uterine weight related to body weight. Prolame and E2 produced robust anxiolytic- and antidepressant-like effects in young adult ovariectomized rats, but these effects were absent in gonadally intact middle-aged rats. Interestingly, only prolame induced anxiolytic- and antidepressant-like effects in middle-aged ovariectomized rats. Uterotrophic effects of prolame were weaker than E2 effects, notably in middle-aged females. Altogether, present data support the notion that prolame has the potential to be considered an EHRT with relevant psychoactive actions and with apparently lower adverse-side effects, especially in middle-aged populations.

Disfunction of dorsal raphe nucleus-hippocampus serotonergic-HTR3 transmission results in anxiety phenotype of Neuroplastin 65-deficient mice

Anxiety disorders are the most common psychiatric condition, but the etiology of anxiety disorders remains largely unclear. Our previous studies have shown that neuroplastin 65 deficiency (NP65-/-) mice exhibit abnormal social and mental behaviors and decreased expression of tryptophan hydroxylase 2 (TPH2) protein. However, whether a causal relationship between TPH2 reduction and anxiety disorders exists needs to be determined. In present study, we found that replenishment of TPH2 in dorsal raphe nucleus (DRN) enhanced 5-HT level in the hippocampus and alleviated anxiety-like behaviors. In addition, injection of AAV-NP65 in DRN significantly increased TPH2 expression in DRN and hippocampus, and reduced anxiety-like behaviors. Acute administration of exogenous 5-HT or HTR3 agonist SR57227A in hippocampus mitigated anxiety-like behaviors in NP65-/- mice. Moreover, replenishment of TPH2 in DRN partly repaired the impairment of long-term potentiation (LTP) maintenance in hippocampus of NP65-/- mice. Finally, we found that loss of NP65 lowered transcription factors Lmx1b expression in postnatal stage and replenishment of NP65 in DRN reversed the decrease in Lmx1b expression of NP65-/- mice. Together, our findings reveal that NP65 deficiency induces anxiety phenotype by downregulating DRN-hippocampus serotonergic-HTR3 transmission. These studies provide a novel and insightful view about NP65 function, suggesting an attractive potential target for treatment of anxiety disorders.

CX3CR1 mediates motor dysfunction in mice through 5-HTR2a

CX3CR1 knockout could induce motor dysfunction in several neurological disease models mainly through regulating microglia's function. While CX3CR1 was expressed on neurons in a few reports, whether neuronal CX3CR1 could affect the function of neurons and mediate motor dysfunction under physiological conditions is unknown. To elucidate the roles of neuronal CX3CR1 on motor dysfunction, CX3CR1 knockout mice were created. Rotarod test and Open field test found that the CX3CR1-/- mice's motor capacity was reduced. Immunofluorescence staining detected the expression of CX3CR1 in neurons both in vivo and in vitro. Immunohistochemistry and West blot found that knockout of CX3CR1 did not affect the neurons' number in both spinal cord and brain of mice. While inhibiting the function of CX3CR1 by AZD8797 could decrease the expression of 5-Hydroxytryptamine receptor(5-HTR2a), which involved in the regulation of motor function. Further investigation revealed that CX3CR1 regulated the expression of HTR2a through the NF-κB pathway. For the first time, we reported that neuronal CXCR1 mediates motor dysfunction. Our results suggest that modulating CXCR1 activity offers a novel therapeutic strategy for motor dysfunction.

HCN1 in the lateral habenula contributes to morphine abstinence-induced anxiety-like behaviors in male mice

Anxiety disorders, common symptoms during morphine withdrawal, are important negative reinforcement factors leading to relapse. Lateral habenula serves as a negative reinforcement center, however its role in morphine withdrawal-induced anxiety remains uncovered. The hyperpolarization activated cyclic nucleotide-gated (HCN) channels have been reported to be important in emotion processing and addiction, but the role of HCN in anxiety from drug protracted abstinence remains elusive. In this study, by using behavioral test, Western blot, immunofluorescence, electrophysiology and virus-mediated regulation of HCN, we found that: (1) Intra-LHb injection of selective HCN blocker ZD7288 alleviated anxiety-like behaviors in morphine protracted abstinent male mice. (2) The LHb neuronal activity was increased by morphine protracted abstinence. (3) LHb neurons were inhibited by ZD7288 and activated by 8-Br-cAMP respectively, which were enhanced by morphine withdrawal. (4) HCN1 in the LHb was upregulated by morphine withdrawal. (5) Virus-mediated overexpression of HCN1 in the LHb was sufficient to produce anxiety-like behaviors in male mice and virus-mediated knockdown of HCN1 in the LHb prevented the anxiety-like behaviors in male mice. The findings reveal that selective blockade of HCN1 channels in the LHb may represent a therapeutic approach to morphine withdrawal-induced anxiety.

Gut-Brain Axis Impact on Canine Anxiety Disorders: New Challenges for Behavioral Veterinary Medicine

Anxiety disorders in dogs are ever-growing and represent an important concern in the veterinary behavior field. These disorders are often disregarded in veterinary clinical practice, negatively impacting the animal's and owner's quality of life. Moreover, these anxiety disorders can potentially result in the abandonment or euthanasia of dogs. Growing evidence shows that the gut microbiota is a central player in the gut-brain axis. A variety of microorganisms inhabit the intestines of dogs, which are essential in maintaining intestinal homeostasis. These microbes can impact mental health through several mechanisms, including metabolic, neural, endocrine, and immune-mediated pathways. The disruption of a balanced composition of resident commensal communities, or dysbiosis, is implicated in several pathological conditions, including mental disorders such as anxiety. Studies carried out in rodent models and humans demonstrate that the intestinal microbiota can influence mental health through these mechanisms, including anxiety disorders. Furthermore, novel therapeutic strategies using prebiotics and probiotics have been shown to ameliorate anxiety-related symptoms. However, regarding the canine veterinary behavior field, there is still a lack of insightful research on this topic. In this review, we explore the few but relevant studies performed on canine anxiety disorders. We agree that innovative bacterial therapeutical approaches for canine anxiety disorders will become a promising field of investigation and certainly pave the way for new approaches to these behavioral conditions.

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

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

The Role of Serotonin in Fear Learning and Memory: A Systematic Review of Human Studies

Fear is characterized by distinct behavioral and physiological responses that are essential for the survival of the human species. Fear conditioning (FC) serves as a valuable model for studying the acquisition, extinction, and expression of fear. The serotonin (5-hydroxytryptamine, 5-HT) system is known to play a significant role in emotional and motivational aspects of human behavior, including fear learning and expression. Accumulating evidence from both animal and human studies suggests that brain regions involved in FC, such as the amygdala, hippocampus, and prefrontal cortex, possess a high density of 5-HT receptors, implicating the crucial involvement of serotonin in aversive learning. Additionally, studies exploring serotonin gene polymorphisms have indicated their potential influence on FC. Therefore, the objective of this work was to review the existing evidence linking 5-HT with fear learning and memory in humans. Through a comprehensive screening of the PubMed and Web of Science databases, 29 relevant studies were included in the final review. These studies investigated the relationship between serotonin and fear learning using drug manipulations or by studying 5-HT-related gene polymorphisms. The results suggest that elevated levels of 5-HT enhance aversive learning, indicating that the modulation of serotonin 5-HT2A receptors regulates the expression of fear responses in humans. Understanding the role of this neurochemical messenger in associative aversive learning can provide insights into psychiatric disorders such as anxiety and post-traumatic stress disorder (PTSD), among others.

Serotonergic circuit dysregulation underlying autism-related phenotypes in BTBR mouse model of autism

The inbred mouse strain, BTBR T⁺Itpr3^tf/J (BTBR), possesses neuronal and circuit abnormalities that underlie atypical behavioral profiles resembling the major symptoms of human autism spectrum disorder (ASD). Forebrain serotonin (5-HT) transmission has been implicated in ASD-related behavioral alterations. In this study, we assessed 5-HT signals and the functional responsiveness in BTBR mice compared to standard C57BL/6J (B6) control mice to elucidate how 5-HT alterations contribute to behavioral abnormalities in BTBR mice. A lower number of 5-HT neurons in the median raphe, but not in the dorsal raphe, was observed in male and female BTBR mice. Acute systemic injection of buspirone, a 5-HT1A receptor agonist, induced c-Fos in several brain regions in both B6 and BTBR mice; however, blunted c-Fos induction in BTBR mice was documented in the cingulate cortex, basolateral amygdala (BLA), and ventral hippocampus (Hipp). Decreased c-Fos responses in these regions are associated with a lack of buspirone effects on anxiety-like behavior in BTBR mice. Analysis of mRNA expression following acute buspirone injection indicated that 5HTR1a gene downregulation (or upregulation) occurred in the BLA and Hipp of B6 mice, respectively, but not BTBR mice. The mRNA expression of factors associated with neurogenesis or the pro-inflammatory state was not consistently altered by acute buspirone injection. Therefore, 5-HT responsivity via 5-HT1A receptors in the BLA and Hipp are linked to anxiety-like behavior, in which circuits are disrupted in BTBR mice. Other distinct 5-HT circuits from the BLA and Hipp that regulate social behavior are restricted but preserved in BTBR mice.

Characteristics of tissue distribution, metabolism, effects on brain catecholamines, and environmental exposure of frogs to neonicotinoid insecticides

Pesticide exposure is considered to be one important factor responsible for declining amphibian populations worldwide. The usage of neonicotinoid insecticides (NNIs) has markedly increased in recent years, and there are concerns regarding the effects of NNI-induced toxicity on the development and behavior of amphibians. However, there have been few reports on the metabolism, distribution, and neurotoxicity of NNIs in amphibians. In this study, we exposed the Western clawed frog (Silurana tropicalis) to clothianidin (CLT) in water. After 24 h of exposure, the highest concentrations were detected in the skin, indicating that frogs are at a high risk of absorbing CLT through their skin along with water. Excretion of CLT was estimated based on the concentrations of CLT metabolites in the water until 48 h of exposure. The findings showed that frogs had higher CLT metabolic ability than zebrafish. Serotonin levels in the brain were lower in the high-concentration CLT exposure group than in the control group, although the difference was not statistically significant. This suggested that catecholamine-related effects of CLT on the brain cannot be disregarded. In addition, quantitative analyses of NNI residue in wild frogs, soil, and water in agricultural areas in Hokkaido, Japan, were performed and four NNIs were detected. These results indicated the possible risk of NNI-induced toxicity in frogs. This is the first report of the characteristics of tissue distribution and metabolism of NNIs in frogs, which may facilitate the design of appropriate conservation programs for amphibians.

Increased subcortical brain activity in anxious but not depressed individuals

BACKGROUND

Anxiety and depressive symptoms usually co-occur. Neuroimaging abnormalities in patients with depression and anxiety disorders are therefore related to a combination of symptoms. Here, we used a large population study to select individuals with anxiety, depressive, or both anxiety and depressive symptoms to identify whether neuroimaging differences are unique or shared between anxiety and depressive symptoms.

METHODS

We selected four groups of 200 individuals (anxiety, depression, anxiety and depression, controls) from the UK Biobank, matched for age, sex, intelligence, and educational attainment (total N = 800). We extracted the amplitude of low frequency fluctuations (ALFF) from resting-state functional magnetic resonance imaging data, which indexes spontaneous neuronal activity. Group differences were assessed using permutation testing to correct for multiple comparisons, with age, sex, IQ, and head motion as covariates.

RESULTS

Compared to controls, anxious individuals had higher ALFF values in many subcortical brain regions including the striatum, thalamus, medial temporal lobe, midbrain, pons, as well as the cerebellum. Anxious individuals also showed higher ALFF in the hippocampus, parahippocampal gyrus, cerebellum, and pons compared to individuals with depressive symptoms. No significant differences were found for the depression and combined anxiety/depression groups. Post-hoc tests with largest possible samples showed comparable results in the anxiety group and in the combined group, but still no significant differences for the depression group.

CONCLUSIONS

Anxiety but not depressive symptoms were associated with increased subcortical activity during rest. This suggest that anxiety symptoms may have the largest contribution to the neuroimaging differences in individuals with depression and anxiety disorders.

Medial Prefrontal Cortex Serotonin Input Regulates Cognitive Flexibility in Mice

The medial prefrontal cortex (mPFC) regulates cognitive flexibility and emotional behavior. Neurons that release serotonin project to the mPFC, and serotonergic drugs influence emotion and cognition. Yet, the specific roles of endogenous serotonin release in the mPFC on neurophysiology and behavior are unknown. We show that axonal serotonin release in the mPFC directly inhibits the major mPFC output neurons. In serotonergic neurons projecting from the dorsal raphe to the mPFC, we find endogenous activity signatures pre-reward retrieval and at reward retrieval during a cognitive flexibility task. In vivo optogenetic activation of this pathway during pre-reward retrieval selectively improved extradimensional rule shift performance while inhibition impaired it, demonstrating sufficiency and necessity for mPFC serotonin release in cognitive flexibility. Locomotor activity and anxiety-like behavior were not affected by either optogenetic manipulation. Collectively, our data reveal a powerful and specific modulatory role of endogenous serotonin release from dorsal raphe-to-mPFC projecting neurons in cognitive flexibility.

mGluR5 in hippocampal CA1 pyramidal neurons mediates stress-induced anxiety-like behavior

Pharmacological manipulation of mGluR5 has showed that mGluR5 is implicated in the pathophysiology of anxiety and mGluR5 has been proposed as a potential drug target for anxiety disorders. Nevertheless, the mechanism underlying the mGluR5 involvement in stress-induced anxiety-like behavior remains largely unknown. Here, we found that chronic restraint stress induced anxiety-like behavior and decreased the expression of mGluR5 in hippocampal CA1. Specific knockdown of mGluR5 in hippocampal CA1 pyramidal neurons produced anxiety-like behavior. Furthermore, both chronic restraint stress and mGluR5 knockdown impaired inhibitory synaptic inputs in hippocampal CA1 pyramidal neurons. Notably, positive allosteric modulator of mGluR5 rescued stress-induced anxiety-like behavior and restored the inhibitory synaptic inputs. These findings point to an essential role for mGluR5 in hippocampal CA1 pyramidal neurons in mediating stress-induced anxiety-like behavior.

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.

The cerebellum and anxiety

Although the cerebellum is traditionally known for its role in motor functions, recent evidence points toward the additional involvement of the cerebellum in an array of non-motor functions. One such non-motor function is anxiety behavior: a series of recent studies now implicate the cerebellum in anxiety. Here, we review evidence regarding the possible role of the cerebellum in anxiety-ranging from clinical studies to experimental manipulation of neural activity-that collectively points toward a role for the cerebellum, and possibly a specific topographical locus within the cerebellum, as one of the orchestrators of anxiety responses.

Serotonin regulation of behavior via large-scale neuromodulation of serotonin receptor networks

Although we understand how serotonin receptors function at the single-cell level, what role different serotonin receptors play in regulating brain-wide activity and, in turn, human behavior, remains unknown. Here, we developed transcriptomic-neuroimaging mapping to characterize brain-wide functional signatures associated with specific serotonin receptors: serotonin receptor networks (SRNs). Probing SRNs with optogenetics-functional magnetic resonance imaging (MRI) and pharmacology in mice, we show that activation of dorsal raphe serotonin neurons differentially modulates the amplitude and functional connectivity of different SRNs, showing that receptors' spatial distributions can confer specificity not only at the local, but also at the brain-wide, network level. In humans, using resting-state functional MRI, SRNs replicate established divisions of serotonin effects on impulsivity and negative biases. These results provide compelling evidence that heterogeneous brain-wide distributions of different serotonin receptor types may underpin behaviorally distinct modes of serotonin regulation. This suggests that serotonin neurons may regulate multiple aspects of human behavior via modulation of large-scale receptor networks.

Raphe serotonin projections dynamically regulate feeding behavior through targeting inhibitory circuits from rostral zona incerta to paraventricular thalamus

OBJECTIVE

Rostral zona incerta (ZIR) evokes feeding by sending GABA transmission to paraventricular thalamus (PVT). Although central serotonin (5-HT) signaling is known to play critical roles in the regulation of food intake and eating disorders, it remains unknown whether raphe 5-HT neurons functionally innervate ZIR-PVT neural pathway for feeding control. Here, we sought to reveal how raphe 5-HT signaling regulates both ZIR and PVT for feeding control.

METHODS

We used retrograde neural tracers to map 5-HT projections in Sert-Cre mice and slice electrophysiology to examine the mechanism by which 5-HT modulates ZIR GABA neurons. We also used optogenetics to test the effects of raphe-ZIR and raphe-PVT 5-HT projections on feeding motivation and food intake in mice regularly fed, 24 h fasted, and with intermittent high-fat high-sugar (HFHS) diet. In addition, we applied RNAscope in situ hybridization to identify 5-HT receptor subtype mRNA in ZIR.

RESULTS

We show raphe 5-HT neurons sent projections to both ZIR and PVT with partial collateral axons. Photostimulation of 5-HT projections inhibited ZIR but excited PVT neurons to decrease motivated food consumption. However, both acute food deprivation and intermittent HFHS diet downregulated 5-HT inhibition on ZIR GABA neurons, abolishing the inhibitory regulation of raphe-ZIR 5-HT projections on feeding motivation and food intake. Furthermore, we found high-level 5-HT1a and 5-HT2c as well as low-level 5-HT7 mRNA expression in ZIR. Intermittent HFHS diet increased 5-HT7 but not 5-HT1a or 5-HT2c mRNA levels in the ZIR.

CONCLUSIONS

Our results reveal that raphe-ZIR 5-HT projections dynamically regulate ZIR GABA neurons for feeding control, supporting that a dynamic fluctuation of ZIR 5-HT inhibition authorizes daily food intake but a sustained change of ZIR 5-HT signaling leads to overeating induced by HFHS diet.

Oxytocin interactions with central dopamine and serotonin systems regulate different components of motherhood

The role of oxytocin in maternal caregiving and other postpartum behaviours has been studied for more than five decades. How oxytocin interacts with other neurochemical systems to enact these behavioural changes, however, is only slowly being elucidated. The best-studied oxytocin-neurotransmitter interaction is with the mesolimbic dopamine system, and this interaction is essential for maternal motivation and active caregiving behaviours such as retrieval of pups. Considerably less attention has been dedicated to investigating how oxytocin interacts with central serotonin to influence postpartum behaviour. Recently, it has become clear that while oxytocin-dopamine interactions regulate the motivational and pup-approach aspects of maternal caregiving behaviours, oxytocin-serotonin interactions appear to regulate nearly all other aspects including postpartum nursing, aggression, anxiety-like behaviour and stress coping strategy. Collectively, oxytocin's interactions with central dopamine and serotonin systems are thus critical for the entire suite of behavioural adaptations exhibited in the postpartum period, and these sites of interaction are potential pharmacological targets for where oxytocin could help to ameliorate deficits in maternal caregiving and poor postpartum mental health. This article is part of the theme issue 'Interplays between oxytocin and other neuromodulators in shaping complex social behaviours'.

Structural and metabolic activity differences in serotonergic cell groups in a rat model of individual differences of emotionality and stress reactivity

Serotonin regulates a diverse set of functions, including emotional behavior, cognition, sociability, appetite, and sleep. Serotonin is also a key trophic factor that shapes neurodevelopmental processes. Genetic and environmental factors that drive individual differences in the serotonergic system have the capacity to impact brain structure and behavior, and likely contribute to pathophysiological processes involved in neuropsychiatric disorders. Using adult rats selectively bred for low novelty exploration (Low Responders, LR), we previously demonstrated pronounced increases in the levels of their anxiety- and depression- relevant behaviors as compared to the selectively bred High Novelty Responder (HR) rats. These behavioral differences were accompanied by alterations in the expression of genes that regulate serotonin synthesis in the brainstem, and its signaling in the forebrain. The present study extends these observations with a focus on the organization and the metabolism of brainstem serotonin cell groups that provide serotonergic innervation of the hippocampus and other limbic regions of male HR/LR rats. Using design-based stereology, we found the median raphe (MnR) in adult male LR rats contains increased number of serotonergic neurons as compared to the HRs. This is preceded by an increase in the metabolic activity of the caudal dorsal raphe (DRC) and the intrafascicular DR (DRI) during early postnatal development. These findings suggest that structural and functional differences in the raphe-limbic projections shape behavioral inhibition throughout the lifespan.

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.

A New Player in the Hippocampus: A Review on VGLUT3+ Neurons and Their Role in the Regulation of Hippocampal Activity and Behaviour

Glutamate is the most abundant excitatory amino acid in the central nervous system. Neurons using glutamate as a neurotransmitter can be characterised by vesicular glutamate transporters (VGLUTs). Among the three subtypes, VGLUT3 is unique, co-localising with other "classical" neurotransmitters, such as the inhibitory GABA. Glutamate, manipulated by VGLUT3, can modulate the packaging as well as the release of other neurotransmitters and serve as a retrograde signal through its release from the somata and dendrites. Its contribution to sensory processes (including seeing, hearing, and mechanosensation) is well characterised. However, its involvement in learning and memory can only be assumed based on its prominent hippocampal presence. Although VGLUT3-expressing neurons are detectable in the hippocampus, most of the hippocampal VGLUT3 positivity can be found on nerve terminals, presumably coming from the median raphe. This hippocampal glutamatergic network plays a pivotal role in several important processes (e.g., learning and memory, emotions, epilepsy, cardiovascular regulation). Indirect information from anatomical studies and KO mice strains suggests the contribution of local VGLUT3-positive hippocampal neurons as well as afferentations in these events. However, further studies making use of more specific tools (e.g., Cre-mice, opto- and chemogenetics) are needed to confirm these assumptions.

Different Exercise Time on 5-HT and Anxiety-like Behavior in the Rat With Vascular Dementia

BACKGROUND

Previous studies have demonstrated that pre-exercise suppresses anxiety-like behavior, but the effects of different exercise times on vascular dementia induced anxiety-like behavior have not been well investigated.

OBJECTIVE

The present study aims to investigate the underlying neurochemical mechanism of different pre-vascular-dementia exercise times on 5-HT and anxiety-like behavior in rats with vascular dementia.

METHODS

32 Sprague-Dawley (SD) rats were randomly divided into 4 groups: sham group (S group, n = 8), vascular dementia group (VD group, n = 8), 1-week physical exercise and vascular dementia group (1WVD group, n = 8), and 4 weeks physical exercise and vascular dementia group (4WVD group, n = 8). 1 week and 4 weeks of voluntary wheel running were used as pre-exercise training. The vascular dementia model was established by bilateral common carotid arteries occlusion (BCCAo) for 1 week. But bilateral common carotid arteries were not ligated in the sham group. The level of hippocampal 5-HT was detected with in vivo microdialysis coupled with high-performance liquid chromatography (MD-HPLC). Elevated plus maze (EPM), open field (OF), and light/dark box test were used to test anxiety-like behavior.

RESULTS

Compared with the C group, the hippocampal 5-HT was significantly decreased in the VD group after 1 week of ligated operation. The hippocampal 5-HT levels in 1WVD and 4WVD groups were substantially higher than the level in the VD group. The hippocampal 5-HT level has no significant difference among C, 1WVD, and 4WVD. Behavioral data suggested that the rats in the VD group developed obvious anxiety-like behavior after 1 week of ligation surgery. Still, the rats in 1WVD and 4WVD groups did not show significant anxiety-like behavior.

CONCLUSION

Both 1 week and 4 weeks of voluntary running wheel exercise can inhibit the anxiety-like behavior in rats with vascular dementia by upregulating 5-HT levels in the hippocampus in the VD model.

Pharmacogenetic excitation of the median raphe region affects social and depressive-like behavior and core body temperature in male mice

AIMS

Median raphe region (MRR) is an important bottom-up regulatory center for various behaviors as well as vegetative functions, but detailed descriptions and links between the two are still largely unexplored.

METHODS

Pharmacogenetics was used to study the role of MRR in social (sociability, social interaction, resident intruder test) and emotional behavior (forced swim test) parallel with some vegetative changes (biotelemetry: core body temperature). Additionally, to validate pharmacogenetics, the effect of clozapine-N-oxide (CNO), the ligand of the artificial receptor, was studied by measuring (i) serum and brainstem concentrations of CNO and clozapine; (ii) MRR stimulation induced neurotransmitter release in hippocampus; (iii) CNO induced changes in body temperature and locomotor activity.

KEY FINDINGS

MRR stimulation decreased locomotion, increased friendly social behavior in the resident intruder test and enhanced depressive-like behavior. The latter was accompanied by diminished decrease in core body temperature. Thirty minutes after CNO injection clozapine was predominant in the brainstem. Nonetheless, peripheral CNO injection was able to induce glutamate release in the hippocampus. CNO had no immediate (<30 min) or chronic (repeated injections) effect on the body temperature or locomotion.

SIGNIFICANCE

We confirmed the role of MRR in locomotion, social and depressive-like behavior. Most interestingly, only depressive-like behavior was accompanied by changed body temperature regulation, which was also observed in human depressive disorders previously. This indicates clinical relevance of our findings. Despite low penetration, CNO acts centrally, but does not influence the examined basic parameters, being suitable for repeated behavioral testing.

Insufficiency of ventral hippocampus to medial prefrontal cortex transmission explains antidepressant non-response

BACKGROUND

There is extensive evidence that antidepressant drugs restore normal brain function by repairing damage to ventral hippocampus (vHPC) and medial prefrontal cortex (mPFC). While the damage is more extensive in hippocampus, the evidence of treatments, such as deep brain stimulation, suggests that functional changes in prefrontal cortex may be more critical. We hypothesized that antidepressant non-response may result from an insufficiency of transmission from vHPC to mPFC.

METHOD

Antidepressant non-responsive Wistar Kyoto (WKY) rats were subjected to chronic mild stress (CMS), then treated with chronic daily administration of the antidepressant drug venlafaxine (VEN) and/or repeated weekly optogenetic stimulation (OGS) of afferents to mPFC originating from vHPC or dorsal HPC (dHPC).

RESULTS

As in many previous studies, CMS decreased sucrose intake, open-arm entries on the elevated plus maze (EPM), and novel object recognition (NOR). Neither VEN nor vHPC-mPFC OGS alone was effective in reversing the effects of CMS, but the combination of chronic VEN and repeated OGS restored normal behaviour on all three measures. dHPC-mPFC OGS restored normal behaviour in the EPM and NOR test irrespective of concomitant VEN treatment, and had no effect on sucrose intake.

CONCLUSIONS

The synergism between VEN and vHPC-mPFC OGS supports the hypothesis that the antidepressant non-responsiveness of WKY rats results from a failure of antidepressant treatment fully to restore transmission in the vHPC-mPFC pathway.

Acute Stress Exposure Alters Food-Related Brain Monoaminergic Profiles in a Rat Model of Anorexia

BACKGROUND

Adverse life experiences are a major risk factor for anorexia nervosa (AN). Eating-provoked anxiousness associated with AN is postulated to be due to food-related exaggerated serotonin activity in the brain and imbalances of monoamine neurotransmitters.

OBJECTIVES

Using a rodent model of stress-induced hypophagia, we investigated if stress exposure augments food-related serotonin turnover and imbalances in measures of brain serotonin and dopamine activity in manners consistent with anxiousness toward food and restricted eating.

METHODS

Adult male F344 rats were conditioned to associate an audio cue with daily food over 2 weeks, after which half of the rats were exposed to a single episode of tail shocks (stress) or left undisturbed (nonstressed). All rats were killed 48 h later, during a control period, the food-associated cue, or a period of food access. Serotonin, dopamine, and norepinephrine, as well as metabolite concentrations, were assessed across brain regions comprising reward, emotion, and feeding circuits relevant to AN in acutely stressed and nonstressed rats using HPLC. Statistical significance level was 5%.

RESULTS

Stress-induced rat hypophagia paralleled an augmented serotonin turnover in response to the food-associated cue in the hypothalamus and hippocampus, as well as food access in the hypothalamus and cortical areas (all P < 0.05). Stress exposure increased the ratio of serotonin to dopamine metabolites across several brain areas, but the magnitude of this imbalance was further augmented during the food-associated cue and food access in the brainstem, hippocampus, and cortical areas (all P < 0.05). Finally, stress lowered norepinephrine concentrations by 18% in the hypothalamus (P < 0.05).

CONCLUSIONS

The observed stress-induced changes to monoamine profiles in rats could have key implications for physiological states that contribute to restricted eating and may hold relevance for the development of AN precipitated by adverse life experiences.

Female but not male rats show biphasic effects of low doses of Δ9-tetrahydrocannabinol on anxiety: can cannabidiol interfere with these effects?

Δ⁹-tetrahydrocannabinol (THC) is the main phytocannabinoid present in the Cannabis sativa. It can produce dose-dependent anxiolytic or anxiogenic effects in males. THC effects on anxiety have scarcely been studied in females, despite their higher prevalence of anxiety disorders. Cannabidiol, another phytocannabinoid, has been reported to attenuate anxiety and some THC-induced effects. The present study aimed to investigate the behavioral and neurochemical effects of THC administered alone or combined with CBD in naturally cycling female rats tested in the elevated plus-maze. Systemically administered THC produced biphasic effects in females, anxiolytic at low doses (0.075 or 0.1 mg/kg) and anxiogenic at a higher dose (1.0 mg/kg). No anxiety changes were observed in males treated with the same THC dose range. The anxiogenic effect of THC was prevented by co-administration of CBD (1.0 or 3.0 mg/kg). CBD (3.0 mg/kg) caused an anxiolytic effect. At a lower dose (1.0 mg/kg), it facilitated the anxiolytic effect of the low THC dose. The anxiogenic effect of THC was accompanied by increased dopamine levels in the medial prefrontal cortex (mPFC) and nucleus accumbens (NAc). In contrast, its anxiolytic effect was associated with increased mPFC serotonin concentrations. The anxiolytic effect of CBD was accompanied by increased mPFC serotonin turnover. Together, these results indicate that female rats are susceptible to the biphasic effects of low THC doses on anxiety. These effects could depend on mPFC and NAc dopaminergic and serotoninergic neurotransmissions. CBD could minimize potential THC high-dose side effects whereas enhancing the anxiolytic action of its low doses in females.

Altered behaviour, dopamine and norepinephrine regulation in stressed mice heterozygous in TPH2 gene

Gene-environment interaction (GxE) determines the vulnerability of an individual to a spectrum of stress-related neuropsychiatric disorders. Increased impulsivity, excessive aggression, and other behavioural characteristics are associated with variants within the tryptophan hydroxylase-2 (Tph2) gene, a key enzyme in brain serotonin synthesis. This phenotype is recapitulated in naïve mice with complete, but not with partial Tph2 inactivation. Tph2 haploinsufficiency in animals reflects allelic variation of Tph2 facilitating the elucidation of respective GxE mechanisms. Recently, we showed excessive aggression and altered serotonin brain metabolism in heterozygous Tph2-deficient male mice (Tph2^+/-) after predator stress exposure. Here, we sought to extend these studies by investigating aggressive and anxiety-like behaviours, sociability, and the brain metabolism of dopamine and noradrenaline. Separately, Tph2^+/- mice were examined for exploration activity in a novel environment and for the potentiation of helplessness in the modified swim test (ModFST). Predation stress procedure increased measures of aggression, dominancy, and suppressed sociability in Tph2^+/- mice, which was the opposite of that observed in control mice. Anxiety-like behaviour was unaltered in the mutants and elevated in controls. Tph2^+/- mice exposed to environmental novelty or to the ModFST exhibited increased novelty exploration and no increase in floating behaviour compared to controls, which is suggestive of resilience to stress and despair. High-performance liquid chromatography (HPLC) revealed significant genotype-dependent differences in the metabolism of dopamine, and norepinephrine within the brain tissue. In conclusion, environmentally challenged Tph2^+/- mice exhibit behaviours that resemble the behaviour of non-stressed null mutants, which reveals how GxE interaction studies can unmask latent genetically determined predispositions.

Illuminating Neural Circuits in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and there is currently no cure. Neural circuit dysfunction is the fundamental mechanism underlying the learning and memory deficits in patients with AD. Therefore, it is important to understand the structural features and mechanisms underlying the deregulated circuits during AD progression, by which new tools for intervention can be developed. Here, we briefly summarize the most recently established cutting-edge experimental approaches and key techniques that enable neural circuit tracing and manipulation of their activity. We also discuss the advantages and limitations of these approaches. Finally, we review the applications of these techniques in the discovery of circuit mechanisms underlying β-amyloid and tau pathologies during AD progression, and as well as the strategies for targeted AD treatments.

The Effects of Statins on Neurotransmission and Their Neuroprotective Role in Neurological and Psychiatric Disorders

Statins are among the most widely used drug classes in the world. Apart from their basic mechanism of action, which is lowering cholesterol levels, many pleiotropic effects have been described so far, such as anti-inflammatory and antiatherosclerotic effects. A growing number of scientific reports have proven that these drugs have a beneficial effect on the functioning of the nervous system. The first reports proving that lipid-lowering therapy can influence the development of neurological and psychiatric diseases appeared in the 1990s. Despite numerous studies about the mechanisms by which statins may affect the functioning of the central nervous system (CNS), there are still no clear data explaining this effect. Most studies have focused on the metabolic effects of this group of drugs, however authors have also described the pleiotropic effects of statins, pointing to their probable impact on the neurotransmitter system and neuroprotective effects. The aim of this paper was to review the literature describing the impacts of statins on dopamine, serotonin, acetylcholine, and glutamate neurotransmission, as well as their neuroprotective role. This paper focuses on the mechanisms by which statins affect neurotransmission, as well as on their impacts on neurological and psychiatric diseases such as Parkinson's disease (PD), Alzheimer's disease (AD), vascular dementia (VD), stroke, and depression. The pleiotropic effects of statin usage could potentially open floodgates for research in these treatment domains, catching the attention of researchers and clinicians across the globe.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

Serotonin modulation of hippocampal functions: From anatomy to neurotherapeutics

The hippocampal region receives a dense serotoninergic innervation originating from both medial and dorsal raphe nuclei. This innervation regulates hippocampal activity through the activation of distinct receptor families that are expressed in excitatory and inhibitory neurons, terminals of several afferent neurotransmitter systems, and glial cells. Preclinical and clinical studies indicate that hippocampal dysfunctions are involved in learning and memory deficits, dementia, Alzheimer's disease, epilepsy and mood disorders such as anxiety, depression and post-traumatic syndrome disorder, whereas the hippocampus participates also in the therapeutic mechanisms of numerous medicines. Not surprisingly, several drugs acting via 5-HT mechanisms are efficacious to some extent in some diseases and the link between 5-HT and the hippocampus although clear remains difficult to untangle. For this reason, we review reported data concerning the distribution and the functional roles of the 5-HT receptors in the hippocampal region in health and disease. The impact of the 5-HT systems on the hippocampal function is such that the research of new 5-HT mechanisms and drugs is still very active. It concerns notably drugs acting at the 5-HT_1A,_2A,_2C,_4,6 receptor subtypes, in addition to the already existing drugs including the selective serotonin reuptake inhibitors.

The Roles of Serotonin in Neuropsychiatric Disorders

The serotonergic system extends throughout the central nervous system (CNS) and the gastrointestinal (GI) tract. In the CNS, serotonin (5-HT, 5-hydroxytryptamine) modulates a broad spectrum of functions, including mood, cognition, anxiety, learning, memory, reward processing, and sleep. These processes are mediated through 5-HT binding to 5-HT receptors (5-HTRs), are classified into seven distinct groups. Deficits in the serotonergic system can result in various pathological conditions, particularly depression, schizophrenia, mood disorders, and autism. In this review, we outlined the complexity of serotonergic modulation of physiologic and pathologic processes. Moreover, we provided experimental and clinical evidence of 5-HT's involvement in neuropsychiatric disorders and discussed the molecular mechanisms that underlie these illnesses and contribute to the new therapies.

Neurochemically and Hodologically Distinct Ascending VGLUT3 versus Serotonin Subsystems Comprise the r2-Pet1 Median Raphe

Brainstem median raphe (MR) neurons expressing the serotonergic regulator gene Pet1 send collateralized projections to forebrain regions to modulate affective, memory-related, and circadian behaviors. Some Pet1 neurons express a surprisingly incomplete battery of serotonin pathway genes, with somata lacking transcripts for tryptophan hydroxylase 2 (Tph2) encoding the rate-limiting enzyme for serotonin [5-hydroxytryptamine (5-HT)] synthesis, but abundant for vesicular glutamate transporter type 3 (Vglut3) encoding a synaptic vesicle-associated glutamate transporter. Genetic fate maps show these nonclassical, putatively glutamatergic Pet1 neurons in the MR arise embryonically from the same progenitor cell compartment-hindbrain rhombomere 2 (r2)-as serotonergic TPH2⁺ MR Pet1 neurons. Well established is the distribution of efferents en masse from r2-derived, Pet1-neurons; unknown is the relationship between these efferent targets and the specific constituent source-neuron subgroups identified as r2-Pet1^{Tph2 -high} versus r2-Pet1^{Vglut3 -high} Using male and female mice, we found r2-Pet1 axonal boutons segregated anatomically largely by serotonin⁺ versus VGLUT3⁺ identity. The former present in the suprachiasmatic nucleus, paraventricular nucleus of the thalamus, and olfactory bulb; the latter are found in the hippocampus, cortex, and septum. Thus r2-Pet1^{Tph2- high} and r2-Pet1^{Vglut3- high} neurons likely regulate distinct brain regions and behaviors. Some r2-Pet1 boutons encased interneuron somata, forming specialized presynaptic "baskets" of VGLUT3⁺ or VGLUT3⁺/5-HT⁺ identity; this suggests that some r2-Pet1^{Vglut3- high} neurons may regulate local networks, perhaps with differential kinetics via glutamate versus serotonin signaling. Fibers from other Pet1 neurons (non-r2-derived) were observed in many of these same baskets, suggesting multifaceted regulation. Collectively, these findings inform brain organization and new circuit nodes for therapeutic considerations.SIGNIFICANCE STATEMENT Our findings match axonal bouton neurochemical identity with distant cell bodies in the brainstem raphe. The results are significant because they suggest that disparate neuronal subsystems derive from Pet1 ⁺ precursor cells of the embryonic progenitor compartment rhombomere 2 (r2). Of these r2-Pet1 neuronal subsystems, one appears largely serotonergic, as expected given expression of the serotonergic regulator PET1, and projects to the olfactory bulb, thalamus, and suprachiasmatic nucleus. Another expresses VGLUT3, suggesting principally glutamate transmission, and projects to the hippocampus, septum, and cortex. Some r2-Pet1 boutons-those that are VGLUT3⁺ or VGLUT3⁺/5-HT⁺ co-positive-comprise "baskets" encasing interneurons, suggesting that they control local networks perhaps with differential kinetics via glutamate versus serotonin signaling. Results inform brain organization and circuit nodes for therapeutic consideration.

The impact of insulin on the serotonergic system and consequences on diabetes-associated mood disorders

The idea that insulin could influence emotional behaviours has long been suggested. However, the underlying mechanisms have yet to be solved and there is no direct and clear-cut evidence demonstrating that such action involves brain serotonergic neurones. Indeed, initial arguments in favour of the association between insulin, serotonin and mood arise from clinical or animal studies showing that impaired insulin action in type 1 or type 2 diabetes causes anxiety- and depressive symptoms along with blunted plasma and brain serotonin levels. The present review synthesises the main mechanistic hypotheses that might explain the comorbidity between diabetes and depression. It also provides a state of knowledge of the direct and indirect experimental evidence that insulin modulates brain serotonergic neurones. Finally, it highlights the literature suggesting that antidiabetic drugs present antidepressant-like effects and, conversely, that serotonergic antidepressants impact glucose homeostasis. Overall, this review provides mechanistic insights into how insulin signalling alters serotonergic neurotransmission and related behaviours bringing new targets for therapeutic options.

Serotonergic neurons in the treatment of mood disorders: The dialogue with astrocytes

Astrocytes were traditionally regarded as cells important to neuronal activity, providing both metabolic and structural supports. Recent evidence suggests that they may also play a crucial role in the control of higher brain functions. In keeping with this hypothesis, it is now well accepted that astrocytes contribute to stress but also react to antidepressant drugs as they express serotonergic transporters and receptors. However, the downstream mechanisms leading to the fine-tuned regulation of mood are still unknown. This chapter pays attention to the role of astrocytes in the regulation of emotional behavior and related serotonergic neurotransmission. In particular, it gives a current state of the clinical and preclinical evidence showing that astrocytes respond to environmental conditions and antidepressant drugs through the release of gliotransmitters and neurotrophic factors which in turn, influence serotonergic tone in discrete brain areas. This state-of-the-art review aims at demonstrating the remarkable potential for novel therapeutic antidepressant strategies targeting these glial cells.

The Interpeduncular-Ventral Hippocampus Pathway Mediates Active Stress Coping and Natural Reward

Maladaptive stress-related behaviors are integral to multiple complex psychiatric disorders, and it has been well established that serotonergic signaling mediates various aspects of these maladaptive states. In these studies, we sought to uncover the function of a previously undefined serotonergic pathway, which projects from the interpeduncular nucleus (IPN) to the ventral hippocampus (vHipp). Intersectional retrograde and chemogenetic viral manipulation strategies were employed to manipulate the function of the IPN-vHipp pathway during a variety of behavioral measures in male mice. We found a significant effect of circuit inhibition on behaviors associated with coping strategies and natural reward. Specifically, inhibition of the IPN-vHipp pathway dramatically increased active stress-induced escape behaviors, in addition to moderately affecting sucrose consumption and food self-administration. During inhibition of this pathway, agonist activation of serotonergic 5-HT_2A/2C receptors in the vHipp reversed the effects of IPN-vHipp circuit inhibition on active escape behaviors, thereby supporting the synaptic mechanism underlying the behavioral effects evidenced. IPN-vHipp inhibition did not induce differences in generalized locomotion, anxiety-associated behavior, and intravenous nicotine self-administration. Importantly, these findings are in opposition to the canonical understanding of serotonin in such escape behaviors, indicating that serotonin exerts opposing effects on behavior in a pathway-specific manner in the brain. Taken together, these findings thereby have important implications for our understanding of serotonergic signaling and associated therapeutic approaches for the treatment of disease symptomology.

Serotonin research: Crossing scales and boundaries

Nearly 100 years after the discovery of serotonin, its role remains elusive. Modulation of serotonin transmission is considered in numerous central nervous system (CNS) diseases including depression, anxiety, schizophrenia, obsessive-compulsive disorders, addiction, Parkinson's disease, and Alzheimer's disease. The therapeutic strategies based on serotonin systems have evolved thanks to better identification of the involvement of serotonin in various diseases, the better use of animal models, a better understanding of the molecular environment of serotonin receptors, and ultimately the better understanding of the interaction of serotonin neurotransmission with other biological systems. Some 5-HT receptors are still the object of numerous investigations including 5-HT_1A, 5-HT_2A, and 5-HT₆ receptor subtypes. It is noteworthy that the direction of research is moving towards a simultaneous action at multiple targets either through different 5-HT targets or the consideration of both 5-HT and other targets to achieve better therapeutic responses.