Exposure to Acute and Chronic Fluoxetine has Differential Effects on Sociability and Activity of Serotonergic Neurons in the Dorsal Raphe Nucleus of Juvenile Male BALB/c Mice

Focal pharmacological manipulation of serotonin signaling in the amygdala does not alter social behavior

Serotonin signaling plays critical roles in social and emotional behaviors. Likewise, decades of research demonstrate that the amygdala is a prime modulator of social behavior. Permanent excitotoxic lesions and transient amygdala inactivation consistently increase social behaviors in non-human primates. In rodents, acute systemic administration of drugs that increase serotonin signaling is associated with decreased social interactions. However, in primates, the direct involvement of serotonin signaling in the amygdala, particularly in affiliative social interaction, remains unexplored. Here, we examined the effects of serotonin manipulations within the amygdala on social behavior in eight pairs of familiar male macaques. We microinfused drugs targeting the serotonin system into either the basolateral (BLA) or central (CeA) amygdala and measured changes in social behavior. Surprisingly, the results demonstrated no significant differences in social behavior following the infusion of a selective serotonin reuptake inhibitor, 5-HT1A agonist or antagonist, 5-HT2A agonist or antagonist, or 5-HT3 agonist or antagonist into either the BLA or CeA. These findings suggest that serotonin signaling in the amygdala does not directly contribute to the regulation of social behavior between familiar conspecifics. Future research should explore alternative mechanisms and potential interactions with other brain regions to gain a comprehensive understanding of the complex neural circuitry governing social behavior.

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.

The Role of Dorsal Raphe Nucleus Serotonergic Systems in Emotional Learning and Memory in Male BALB/c Mice

Selective serotonin reuptake inhibitors (SSRIs) are the first-line pharmacological treatment for a variety of anxiety-, trauma- and stressor-related disorders. Although they are efficacious, therapeutic improvements require several weeks of treatment and are often associated with an initial exacerbation of symptoms. The dorsal raphe nucleus (DR) has been proposed as an important target for the modulation of emotional responses and the therapeutic effects of SSRIs. Using a fear-conditioning paradigm we aimed to understand how SSRIs affect emotional learning and memory, and their effects on serotonergic circuitry. Adult male BALB/c mice were treated with vehicle (n = 16) or the SSRI fluoxetine (18 mg/kg/d) acutely (n = 16), or chronically (21d, n = 16), prior to fear conditioning. Treatment was stopped, and half of the mice (n = 8/treatment group) were exposed to cued fear memory recall 72 h later. Activation of DR serotonergic neurons during fear conditioning (Experiment 1) or fear memory recall (Experiment 2), was measured using dual-label immunohistochemistry for Tph2 and c-Fos. Acute and chronic fluoxetine treatment reduced associative fear learning without affecting memory recall and had opposite effects on anxiety-like behaviour. Acute fluoxetine decreased serotonergic activity in the DR, while chronic treatment led to serotonergic activity that was indistinguishable from that of control levels in DRD and DRV subpopulations. Chronic fluoxetine facilitated fear extinction, which was associated with rostral DRD inhibition. These findings provide further evidence that SSRIs can alter aspects of learning and memory processes and are consistent with a role for discrete populations of DR serotonergic neurons in regulating fear- and anxiety-related behaviours.

Adult hippocampal neurogenesis and social behavioural deficits in the R451C Neuroligin3 mouse model of autism are reverted by the antidepressant fluoxetine

Neuron generation persists throughout life in the hippocampus but is altered in animal models of neurological and neuropsychiatric diseases, suggesting that disease-associated decline in cognitive and emotional hippocampal-dependent behaviours might be functionally linked with dysregulation of postnatal neurogenesis. Depletion of the adult neural stem/progenitor cell (NSPCs) pool and neurogenic decline have been recently described in mice expressing synaptic susceptibility genes associated with autism spectrum disorder (ASDs). To gain further insight into mechanisms regulating neurogenesis in mice carrying mutations in synaptic genes related to monogenic ASDs, we used the R451C Neuroligin3 knock-in (Nlgn3 KI) mouse, which is characterized by structural brain abnormalities, deficits in synaptic functions and reduced sociability. We show that the number of adult-born neurons, but not the size of the NSPC pool, was reduced in the ventral dentate gyrus in knock-in mice. Notably, this neurogenic decline was rescued by daily injecting mice with 10 mg/Kg of the antidepressant fluoxetine for 20 consecutive days. Sustained treatment also improved KI mice's sociability and increased the number of c-Fos active adult-born neurons, compared with vehicle-injected KI mice. Our study uncovers neurogenesis-mediated alterations in the brain of R451C KI mouse, showing that the R451C Nlgn3 mutation leads to lasting, albeit pharmacologically reversible, changes in the brain, affecting neuron formation in the adult hippocampus. Our results suggest that fluoxetine can ameliorate social behaviour in KI mice, at least in part, by rescuing adult hippocampal neurogenesis, which may be relevant for the pharmacological treatment of ASDs.

Novel Antidepressant-Like Properties of the Iron Chelator Deferiprone in a Mouse Model of Depression

Depressed individuals who carry the short allele for the serotonin-transporter-linked promotor region of the gene are more vulnerable to stress and have reduced response to first-line antidepressants such as selective serotonin reuptake inhibitors. Since depression severity has been reported to correlate with brain iron levels, the present study aimed to characterise the potential antidepressant properties of the iron chelator deferiprone. Using the serotonin transporter knock-out (5-HTT KO) mouse model, we assessed the behavioural effects of acute deferiprone on the Porsolt swim test (PST) and novelty-suppressed feeding test (NSFT). Brain and blood iron levels were also measured following acute deferiprone. To determine the relevant brain regions activated by deferiprone, we then measured c-Fos expression and applied network-based analyses. We found that deferiprone reduced immobility time in the PST in 5-HTT KO mice and reduced latency to feed in the NSFT in both genotypes, suggesting potential antidepressant-like effects. There was no effect on brain or blood iron levels following deferiprone treatment, potentially indicating an acute iron-independent mechanism. Deferiprone reversed the increase in c-Fos expression induced by swim stress in 5-HTT KO mice in the lateral amygdala. Functional network analyses suggest that hub regions of activity in mice treated with deferiprone include the caudate putamen and prefrontal cortex. The PST-induced increase in network modularity in wild-type mice was not observed in 5-HTT KO mice. Altogether, our data show that the antidepressant-like effects of deferiprone could be acting via an iron-independent mechanism and that these therapeutic effects are underpinned by changes in neuronal activity in the lateral amygdala.

Acute treatment with 5-hydroxytryptophan increases social approach behaviour but does not activate serotonergic neurons in the dorsal raphe nucleus in juvenile male BALB/c mice: A model of human disorders with deficits of sociability

BACKGROUND

The BALB/c mouse has been proposed as a model of human psychiatric disorders characterised by elevated anxiety and altered sociability. Juvenile BALB/c mice show decreased social exploratory behaviour, increased anxiety, and reduced brain serotonin synthesis compared to other strains including C57BL/6J mice.

AIM

To determine whether supplementation of brain serotonin synthesis alters social behaviour and activation of serotonergic neurons across subregions of the dorsal raphe nucleus (DR) in BALB/c mice.

METHODS

Juvenile male BALB/c mice were assigned to one of four treatment conditions: vehicle/vehicle, carbidopa (25 mg/kg)/vehicle, vehicle/5-HTP (10 mg/kg), carbidopa (25 mg/kg)/5-HTP (10 mg/kg). Social behaviour was measured using the three-chamber social approach test, followed by immunohistochemical staining for TPH2 and c-Fos to measure activation of serotonergic neurons across subregions of the DR.

RESULTS

Mice treated with carbidopa/5-HTP spent more time in the social cage zone and covered more distance in the social approach test compared to other treatment groups. There was no difference between treatment groups in the activation of serotonergic neurons across subregions of the DR. However, the DRD was associated with increased social approach behaviour in carbidopa/5-HTP treated animals.

CONCLUSIONS

Supplementation of serotonin synthesis can increase social approach behaviour in juvenile BALB/c mice. An increase in locomotor behaviour was also observed suggesting that increasing central serotonin synthesis may have led to a reduction in state anxiety, manifesting in increased exploratory behaviour. As no effect on serotonergic activation within the DR was found, alternative mechanisms are likely important for the effects of 5-HTP on social behaviour.

Serotonin Receptors as Therapeutic Targets for Autism Spectrum Disorder Treatment

Autism spectrum disorder (ASD) is a group of neurodevelopmental disorders characterized by repetitive and stereotyped behaviors as well as difficulties with social interaction and communication. According to reports for prevalence rates of ASD, approximately 1~2% of children worldwide have been diagnosed with ASD. Although there are a couple of FDA (Food and Drug Administration)—approved drugs for ASD treatment such as aripiprazole and risperidone, they are efficient for alleviating aggression, hyperactivity, and self-injury but not the core symptoms. Serotonin (5-hydroxytryptamine, 5-HT) as a neurotransmitter plays a crucial role in the early neurodevelopmental stage. In particular, 5-HT has been known to regulate a variety of neurobiological processes including neurite outgrowth, dendritic spine morphology, shaping neuronal circuits, synaptic transmission, and synaptic plasticity. Given the roles of serotonergic systems, the 5-HT receptors (5-HTRs) become emerging as potential therapeutic targets in the ASD. In this review, we will focus on the recent development of small molecule modulators of 5-HTRs as therapeutic targets for the ASD treatment.

Behavioral and cortisol analysis of the anti-stress effect of Polygonum minus (Huds) extracts in chronic unpredictable stress (CUS) zebrafish model

The World Health Organization (WHO) recorded approximately 350 million people worldwide have suffered from mental health disorders, such as depression, anxiety, schizophrenia, and addictive behaviors. The search for new drugs from nature has drawn on many biological resources and human practices. In this study, leaves of Polygonum minus standardized extract (Biokesum®), 1 and 100 mg/L were used to evaluate the anti-stress effect in the chronic unpredictable stress (CUS) zebrafish model. Five groups of zebrafish were manipulated in this study, comprising control, chronic unpredictable stress (CUS), CUS + Biokesum® 1 mg/L (4 days, 20 min/day, immersion) CUS + Biokesum® 100 mg/L (4 days, 20 min/day, immersion) and CUS + fluoxetine 0.6 mg/L (4 days, 20 min/day, immersion). Four different behavioral tests were used, i.e. open-field test, social interaction test, light and dark test, and exploratory test. After four consecutive days of treatment, the zebrafish were sacrificed for whole-body cortisol analysis. The exploratory test showed a significant change upon P. minus treatment (one-way ANOVA; p = 0.0011). Cortisol analysis showed a decrease of cortisol level after treatment with the extract and fluoxetine, without significant difference. These results showed that zebrafish is a reliable model to study the anti-stress effect of compounds or herbal extract.

Acute administration of fluoxetine increases social avoidance and risk assessment behaviors in a sex- and social stress-dependent manner in Syrian hamsters (Mesocricetus auratus)

Most studies investigating the effects of acute administration of selective serotonin reuptake inhibitors (SSRI) on responses to social stress have been conducted with males. This is despite the fact that SSRIs remain the primary pharmacotherapy for social stress-related disorders for both sexes and that the prevalence of these disorders is twofold higher in women than in men. To determine whether acute treatment with the SSRI, fluoxetine, alters behavioral responses to social defeat stress in a sex- or social stress-dependent manner, male and female Syrian hamsters were subjected to one of three social defeat conditions: no defeat (placed into an empty resident aggressor (RA) cage), a single defeat by one RA for 15 min, or three consecutive defeats using different RAs for 5 min each. The day following social defeat, subjects were infused with either vehicle or fluoxetine (20 mg/kg, I.P.) 2 h prior to a 5 min social avoidance test. Overall, we found that fluoxetine increased social vigilance regardless of sex or defeat condition. We also found that fluoxetine affected social avoidance in a sex by stress intensity interaction, such that fluoxetine increased avoidance in no defeat males and in males defeated once but significantly increased avoidance in females only after three defeats. These data suggest that treatment with an SSRI could initially exacerbate the effects of social stress in both sexes. These data also emphasize the importance of including sex as a biological variable when investigating the efficacy of pharmacotherapy for stress-related disorders.

A Systematic Review of Brainstem Contributions to Autism Spectrum Disorder

Autism spectrum disorder (ASD) is a neurodevelopmental disorder that affects one in 66 children in Canada. The contributions of changes in the cortex and cerebellum to autism have been studied for decades. However, our understanding of brainstem contributions has only started to emerge more recently. Disruptions of sensory processing, startle response, sensory filtering, sensorimotor gating, multisensory integration and sleep are all features of ASD and are processes in which the brainstem is involved. In addition, preliminary research into brainstem contribution emphasizes the importance of the developmental timeline rather than just the mature brainstem. Therefore, the purpose of this systematic review is to compile histological, behavioral, neuroimaging, and electrophysiological evidence from human and animal studies about brainstem contributions and their functional implications in autism. Moreover, due to the developmental nature of autism, the review pays attention to the atypical brainstem development and compares findings based on age. Overall, there is evidence of an important role of brainstem disruptions in ASD, but there is still the need to examine the brainstem across the life span, from infancy to adulthood which could lead the way for early diagnosis and possibly treatment of ASD.

Involvement of dorsal raphe nucleus serotonergic systems in social approach-avoidance behaviour and in the response to fluoxetine treatment in peri-adolescent female BALB/c mice

Serotonergic systems are involved in the development and regulation of social behaviour, and drugs that target serotonin neurotransmission, such as selective serotonin reuptake inhibitors (SSRIs), also alter aspects of social approach-avoidance. The midbrain dorsal raphe nucleus (DR), which is a major serotonergic nucleus and main source of serotonergic innervation of the forebrain, has been proposed as an important target for SSRIs, although evidence in females is lacking. In this study, we examined the involvement of the DR serotonergic systems in social behaviour and in response to SSRI treatment, using peri-adolescent female BALB/c mice. Mice were exposed to the SSRI fluoxetine either chronically (18 mg/kg/day, in drinking water, for 12 days) or acutely (18 mg/kg, i.p.), or to vehicle control condition (0.9 % saline, i.p.), prior to being exposed to the three-chambered sociability test. Activation of serotonergic neurons across subregions of the DR were subsequently measured, using dual-label immunohistochemistry for TPH2 and c-Fos. Acute fluoxetine administration increased generalised and social avoidance, while mice exposed to chronic fluoxetine treatment showed levels of social approach behaviour that were comparable to controls. Serotonergic populations across the DR showed reduced activity following acute fluoxetine treatment. Further, activation of serotonergic neurons in the ventral DR correlated with social approach behaviour in vehicle-treated control mice. These data provide some support for the involvement of discrete populations of DR serotonergic neurons in the regulation of social approach-avoidance, although more research is needed to understand the effects and mechanisms of chronic SSRI treatment in females.

Long-term effects of young-adult methamphetamine on dorsal raphe serotonin systems in mice: Role of brain-derived neurotrophic factor

To assess the long-term effects of chronic adolescent methamphetamine (METH) treatment on the serotonin system in the brain, we used serotonin-1A receptor (5-HT_1A) and serotonin transporter (SERT) autoradiography, and quantitative tryptophan-hydroxylase 2 (TPH2) immunohistochemistry in the raphe nuclei of mice. Because of the modulatory role of brain-derived neurotrophic factor (BDNF) on the serotonin system and the effects of METH, we included both BDNF heterozygous (HET) mice and wildtype (WT) controls. Male and female mice of both genotypes were treated with an escalating METH dose regimen from the age of 6-9 weeks. At least two weeks later, acute locomotor hyperactivity induced by a 5 mg/kg D-amphetamine challenge was significantly enhanced in METH-pretreated mice, showing long-term sensitisation. METH pretreatment caused a small, but significant decrease of 5-HT_1A receptor binding in the dorsal raphe nucleus (DRN) of males independent of genotype, but there were no changes in the median raphe nucleus (MRN) or in SERT binding density. METH treatment reduced the number of TPH2 positive cells in ventral subregions of the rostral and medial DRN independent of genotype. METH treatment selectively reduced DRN cell counts in BDNF HET mice compared to wildtype mice in medial and caudal ventrolateral subregions previously associated with panic-like behaviour. The data increase our understanding of the long-term and selective effects of METH on brain serotonin systems. These findings could be relevant for some of the psychosis-like symptoms associated with long-term METH use.

Somatosensorimotor and Odor Modification, Along with Serotonergic Processes Underlying the Social Deficits in BTBR T+ Itpr3tf/J and BALB/cJ Mouse Models of Autism

Autism is a complex spectrum of disorders characterized by core behavioral deficits in social communicative behavior, which are also required for comprehensive analysis of preclinical mouse models. As animal models of the core behavioral deficits in autism, two inbred mouse strains, BTBR T+ Itpr3tf/J (BTBR) and BALB/cJ (BALB), were compared with the standard social strain, C57BL/6J (B6), regarding a variety of behavioral factors underlying social communicative interactions, including olfactory and tactile sensory processes, social recognition abilities and behavioral expression strategies. Although both female BTBR and BALB mice can express social recognition and approach behavior depending on the stimuli they encounter, the available sensory modalities, along with modulation of the serotonergic system, differ between the two strains. BALB mice have deficits in using volatile olfactory cues and tactile information in a social context; they fail to exhibit a social approach to volatile cues and seek nonvolatile cues by exhibiting substantial sniff/contact behavior when allowed direct contact with social opponents. Systemic injection of the serotonin (5-HT1A) agonist buspirone has little effect on these social deficits, suggesting a congenitally degraded serotonergic system in BALB mice. In contrast, BTBR mice exhibit impaired body coordination and social motivation-modified olfactory signals, which are relevant to a reduced social approach. A systemic injection of the 5-HT1A agonist restored these social deficits in BTBR mice, indicating that a downregulated serotonergic system is involved in the social deficits exhibited by BTBR mice.

Effects of repeated voluntary or forced exercise on brainstem serotonergic systems in rats

Voluntary exercise increases stress resistance by modulating stress-responsive neurocircuitry, including brainstem serotonergic systems. However, it remains unknown how exercise produces adaptations to serotonergic systems. Recruitment of serotonergic systems during repeated, daily exercise could contribute to the adaptations in serotonergic systems following exercise, but whether repeated voluntary exercise recruits serotonergic systems is unknown. In this study, we investigated the effects of six weeks of voluntary or forced exercise on rat brain serotonergic systems. Specifically, we analyzed c-Fos and FosB/ΔFosB as markers of acute and chronic cellular activation, respectively, in combination with tryptophan hydroxylase, a marker of serotonergic neurons, within subregions of the dorsal raphe nucleus using immunohistochemical staining. Compared to sedentary controls, rats exposed to repeated forced exercise, but not repeated voluntary exercise, displayed decreased c-Fos expression in serotonergic neurons in the rostral dorsal portion of the dorsal raphe nucleus (DRD) and increased c-Fos expression in serotonergic neurons in the caudal DR (DRC), and interfascicular part of the dorsal raphe nucleus (DRI) during the active phase of the diurnal activity rhythm. Similarly, increases in c-Fos expression in serotonergic neurons in the DRC, DRI, and ventral portion of the dorsal raphe nucleus (DRV) were observed in rats exposed to repeated forced exercise, compared to rats exposed to repeated voluntary exercise. Six weeks of forced exercise, relative to the sedentary control condition, also increased FosB/ΔFosB expression in DRD, DRI, and DRV serotonergic neurons. While both voluntary and forced exercise increase stress resistance, these results suggest that repeated forced exercise, but not repeated voluntary exercise, increases activation of DRI serotonergic neurons, an effect that may contribute to the stress resistance effects of forced exercise. These results also suggest that mechanisms of exercise-induced stress resistance may differ depending on the controllability of the exercise.

Antidepressants in Surface Waters: Fluoxetine Influences Mosquitofish Anxiety-Related Behavior at Environmentally Relevant Levels

Pharmaceutical contamination is an increasing problem globally. In this regard, the selective serotonin reuptake inhibitors (SSRIs)-a group of antidepressants-are particularly concerning. By disrupting the serotonergic system, SSRIs have the potential to affect ecologically important behaviors in exposed wildlife. Despite this, the nature and magnitude of behavioral perturbations resulting from environmentally relevant SSRI exposure among species is poorly understood. Accordingly, we investigated the effects of two field-realistic levels of the SSRI fluoxetine (61 and 352 ng/L) on sociability and anxiety-related behaviors in eastern mosquitofish ( Gambusia holbrooki) for 28 days. Additionally, we measured whole-body tissue concentrations of fluoxetine and norfluoxetine. We found that fluoxetine altered anxiety-related behavior but not sociability. Specifically, female fish showed reduced anxiety-related behavior at the lower treatment level, while males showed an increase at the higher treatment level. In addition, we report a biomass-dependent and sex-specific accumulation of fluoxetine and norfluoxetine, with smaller fish showing higher relative tissue concentrations, with this relationship being more pronounced in males. Our study provides evidence for nonmonotonic and sex-specific effects of fluoxetine exposure at field-realistic concentrations. More broadly, our study demonstrated that neuroactive pharmaceuticals, such as fluoxetine, can affect aquatic life by causing subtle but important shifts in ecologically relevant behaviors.