Serotonin transporter knockout and repeated social defeat stress: impact on neuronal morphology and plasticity in limbic brain areas

Serotonergic-dependent effects of exercise and elevated stress hormone on small non-coding RNA transcriptomics and proteomics in a mouse model of affective disorders

Environmental changes may alter gene expression in depression and anxiety disorders through epigenetic regulation, including via small non-coding RNAs (sncRNAs) and their major subclass, microRNAs (miRNAs). However, underlying mechanisms mediating miRNA regulation in response to changing environmental stimuli are unclear. Using the serotonin transporter (5-HTT) knockout (KO) mouse model of depression/anxiety, this study aimed to compare the effects of voluntary exercise (EX) versus chronic treatment with the stress hormone corticosterone (CT), on hippocampal miRNA transcriptome and proteome in five comparison groups: WT-SH vs. KO-SH; WT-SH vs. WT-EX; KO-SH vs. KO-EX; WT-SH vs. WT-CT; KO-SH vs. KO-CT. We hypothesized that treatment with stress hormone will result in miRNA and proteomics changes observed in genetic model of depression, while exercise will have beneficial effects similar to antidepressant treatment. Using high-throughput sequencing of miRNAs and mass spectrometry (MS)-based approaches for protein expression, we revealed 337 differentially expressed (DE) miRNAs and 67 proteins in 5-HTT KO mice compared to wild-type (WT) control mice in standard-housing conditions. After exercise, there were 200 DE miRNAs and 3 DE proteins in WT mice, and 20 DE miRNAs and 95 DE proteins in 5-HTT KO mice, while corticosterone treatment led to 168 DE miRNAs and 1 DE protein in WT, and 21 DE miRNAs and 21 DE proteins in 5-HTT KO mice. Serotonergic dysfunction (due to the 5-HTT KO gene mutation) induced altered expression of miRNAs and proteins involved in regulation of neurodevelopment, neurogenesis and neuroinflammatory responses. Treatment with the stress hormone corticosterone in WT mice activated pathways which were also found altered in 5-HTT KO mice, while exercise caused antidepressant-like effects. These findings suggest that functional 5-HTT might be required for the beneficial effects of exercise on miRNA expression. Our study is the first to explore how gene-environment interactions affect miRNA/proteomic composition in a mouse model of depression/anxiety, and extends our understanding of gene-environmental interactions underlying these affective disorders.

Quantifying multilabeled brain cells in the whole prefrontal cortex reveals reduced inhibitory and a subtype of excitatory neuronal marker expression in serotonin transporter knockout rats

The prefrontal cortex regulates emotions and is influenced by serotonin. Rodents lacking the serotonin transporter (5-HTT) show increased anxiety and changes in excitatory and inhibitory cell markers in the prefrontal cortex. However, these observations are constrained by limitations in brain representation and cell segmentation, as standard immunohistochemistry is inadequate to consider volume variations in regions of interest. We utilized the deep learning network of the StarDist method in combination with novel open-source methods for automated cell counts in a wide range of prefrontal cortex subregions. We found that 5-HTT knockout rats displayed increased anxiety and diminished relative numbers of subclass excitatory VGluT2+ and activated ΔFosB+ cells in the infralimbic and prelimbic cortices and of inhibitory GAD67+ cells in the prelimbic cortex. Anxiety levels and ΔFosB cell counts were positively correlated in wild-type, but not in knockout, rats. In conclusion, we present a novel method to quantify whole brain subregions of multilabeled cells in animal models and demonstrate reduced excitatory and inhibitory neuronal marker expression in prefrontal cortex subregions of 5-HTT knockout rats.

More and Less Fear in Serotonin Transporter Knockout Mice

Recent theories suggest that reduced serotonin transporter (5-HTT) function, which increases serotonin (5-HT) levels at the synapse, enhances neural plasticity and affects sensitivity to environmental cues. This may promote learning about emotionally relevant events. However, the boundaries that define such emotional learning remain to be established. This was investigated using 5-HTT knockout (5-HTTKO) mice which provide a model of long-term elevated 5-HT transmission and are associated with increased anxiety. Compared to wild-type controls, 5-HTTKO mice were faster to discriminate between an auditory cue that predicted footshock (CS+) and a cue predicting no footshock (CS-). Notably, this enhanced discrimination performance was driven not by faster learning that the CS+ predicted footshock, but rather by faster learning that the CS- cue signals the absence of footshock and thus provides temporary relief from fear/anxiety. Similarly, 5-HTTKO mice were also faster to reduce their fear of the CS+ cue during subsequent extinction. These findings are consistent with facilitated inhibitory learning that predicts the absence of potential threats in 5-HTTKO mice. However, 5-HTTKO mice also exhibited increased generalisation of fear learning about ambiguous aversive cues in a novel context, different from the training context. Thus, 5-HTTKO mice can exhibit both more and less fear compared to wild-type controls. Taken together, our results support the idea that loss of 5-HTT function, and corresponding increases in synaptic 5-HT availability, may facilitate learning by priming of aversive memories. This both facilitates inhibitory learning for fear memories but also enhances generalisation of fear.

Serotonin transporter knockout in rats reduces beta- and gamma-band functional connectivity between the orbitofrontal cortex and amygdala during auditory discrimination

The orbitofrontal cortex and amygdala collaborate in outcome-guided decision-making through reciprocal projections. While serotonin transporter knockout (SERT-/-) rodents show changes in outcome-guided decision-making, and in orbitofrontal cortex and amygdala neuronal activity, it remains unclear whether SERT genotype modulates orbitofrontal cortex-amygdala synchronization. We trained SERT-/- and SERT+/+ male rats to execute a task requiring to discriminate between two auditory stimuli, one predictive of a reward (CS+) and the other not (CS-), by responding through nose pokes in opposite-side ports. Overall, task acquisition was not influenced by genotype. Next, we simultaneously recorded local field potentials in the orbitofrontal cortex and amygdala of both hemispheres while the rats performed the task. Behaviorally, SERT-/- rats showed a nonsignificant trend for more accurate responses to the CS-. Electrophysiologically, orbitofrontal cortex-amygdala synchronization in the beta and gamma frequency bands during response selection was significantly reduced and associated with decreased hubness and clustering coefficient in both regions in SERT-/- rats compared to SERT+/+ rats. Conversely, theta synchronization at the time of behavioral response in the port associated with reward was similar in both genotypes. Together, our findings reveal the modulation by SERT genotype of the orbitofrontal cortex-amygdala functional connectivity during an auditory discrimination task.

Time moderates the interplay between 5-HTTLPR and stress on depression risk: gene x environment interaction as a dynamic process

The serotonin-transporter-linked promoter region (5-HTTLPR) has been widely investigated as contributing to depression vulnerability. Nevertheless, empirical research provides wide contrasting findings regarding its involvement in the etiopathogenesis of the disorder. Our hypothesis was that such discrepancy can be explained considering time as moderating factor. We explored this hypothesis, exploiting a meta analytic approach. We searched PubMed, PsychoINFO, Scopus and EMBASE databases and 1096 studies were identified and screened, resulting in 22 studies to be included in the meta-analyses. The effect of the 5-HTTLPR x stress interaction on depression risk was found to be moderated by the following temporal factors: the duration of stress (i.e. chronic vs. acute) and the time interval between end of stress and assessment of depression (i.e. within 1 year vs. more than 1 year). When stratifying for the duration of stress, the effect of the 5-HTTLPR x stress interaction emerged only in the case of chronic stress, with a significant subgroup difference (p = 0.004). The stratification according to time interval revealed a significant interaction only for intervals within 1 year, though no difference between subgroups was found. The critical role of time interval clearly emerged when considering only chronic stress: a significant effect of the 5-HTTLPR and stress interaction was confirmed exclusively within 1 year and a significant subgroup difference was found (p = 0.01). These results show that the 5-HTTLPR x stress interaction is a dynamic process, producing different effects at different time points, and indirectly confirm that s-allele carriers are both at higher risk and more capable to recover from depression. Overall, these findings expand the current view of the interplay between 5-HTTLPR and stress adding the temporal dimension, that results in a three-way interaction: gene x environment x time.

Parsing neural circuits of fear learning and extinction across basic and clinical neuroscience: Towards better translation

Over the past decades, studies of fear learning and extinction have advanced our understanding of the neurobiology of threat and safety learning. Animal studies can provide mechanistic/causal insights into human brain regions and their functional connectivity involved in fear learning and extinction. Findings in humans, conversely, may further enrich our understanding of neural circuits in animals by providing macroscopic insights at the level of brain-wide networks. Nevertheless, there is still much room for improvement in translation between basic and clinical research on fear learning and extinction. Through the lens of neural circuits, in this article, we aim to review the current knowledge of fear learning and extinction in both animals and humans, and to propose strategies to fill in the current knowledge gap for the purpose of enhancing clinical benefits.

Early-life exposure to selective serotonin reuptake inhibitors: Long-term effects on pain and affective comorbidities

A growing body of evidence indicates that early‐life exposure to selective serotonin reuptake inhibitor has long‐term consequences on the offspring's pain in addition to affective disorders like anxiety disorder and major depression. Serotonin, besides its role in regulating pain and emotions, promotes neuronal network formation. The prefrontal cortex and the amygdala are two key brain regions involved in the modulation of pain and its affective comorbidities. Thus, the aim of this review is to understand how early‐life selective serotonin reuptake inhibitor exposure alters the developing prefrontal cortex and amygdala and thereby underlies the long‐term changes in pain and its affective comorbidities in later life. While there is still limited data on the effects of early‐life selective serotonin reuptake inhibitor exposure on pain, there is a substantial body of evidence on its affective comorbidities. From this perspective paper, four conclusions emerged. First, early‐life selective serotonin reuptake inhibitor exposure results in long‐term nociceptive effects, which needs to be consistently studied to clarify. Second, it results in enhanced depressive‐like behaviour and diminished exploratory behaviour in adult rodents. Third, early‐life selective serotonin reuptake inhibitor exposure alters serotonergic levels, transcription factors expression, and brain‐derived neurotrophic factor levels, resulting in hyperconnectivity within the amygdala and the prefrontal cortex. Finally, it affects antinociceptive inputs of the prefrontal cortex and the amygdala in the spinal cord. We conclude that early‐life selective serotonin reuptake inhibitor exposure affects the maturation of prefrontal cortex and amygdala circuits and thereby enhances their antinociceptive inputs in the spinal cord.

Serotonin transporter genotype modulates resting state and predator stress-induced amygdala perfusion in mice in a sex-dependent manner

The serotonin transporter (5-HTT) is a key molecule of serotoninergic neurotransmission and target of many anxiolytics and antidepressants. In humans, 5-HTT gene variants resulting in lower expression levels are associated with behavioral traits of anxiety. Furthermore, functional magnetic resonance imaging (fMRI) studies reported increased cerebral blood flow (CBF) during resting state (RS) and amygdala hyperreactivity. 5-HTT deficient mice as an established animal model for anxiety disorders seem to be well suited for investigating amygdala (re-)activity in an fMRI study. We investigated wildtype (5-HTT+/+), heterozygous (5-HTT+/-), and homozygous 5-HTT-knockout mice (5-HTT-/-) of both sexes in an ultra-high-field 17.6 Tesla magnetic resonance scanner. CBF was measured with continuous arterial spin labeling during RS, stimulation state (SS; with odor of rats as aversive stimulus), and post-stimulation state (PS). Subsequently, post mortem c-Fos immunohistochemistry elucidated neural activation on cellular level. The results showed that in reaction to the aversive odor CBF in total brain and amygdala of all mice significantly increased. In male 5-HTT+/+ mice amygdala RS CBF levels were found to be significantly lower than in 5-HTT+/- mice. From RS to SS 5-HTT+/+ amygdala perfusion significantly increased compared to both 5-HTT+/- and 5-HTT-/- mice. Perfusion level changes of male mice correlated with the density of c-Fos-immunoreactive cells in the amygdaloid nuclei. In female mice the perfusion was not modulated by the 5-Htt-genotype, but by estrous cycle stages. We conclude that amygdala reactivity is modulated by the 5-Htt genotype in males. In females, gonadal hormones have an impact which might have obscured genotype effects. Furthermore, our results demonstrate experimental support for the tonic model of 5-HTTLPR function.

Mechanisms underlying remediation of depression-associated anxiety by chronic N-acetyl cysteine treatment

RATIONALE

Anxiety is one of the most comorbid conditions with major depressive disorder (MDD). Depression-associated anxiety often stems from the dysfunctional hypothalamic-pituitary-adrenal (HPA) axis and its altered regulation by the amygdala. Furthermore, MDD is associated with altered glutamatergic processing leading to anxiety and impaired regulation of the HPA axis. Recent studies have demonstrated that N-acetyl cysteine (NAC), a pleiotropic drug, exerts antidepressant-like effect by modulation of hippocampal functions, periterminal release of glutamate, and/or redox systems. However, the effects of NAC on depression-associated anxiety, HPA axis hyperactivity, and amygdalar dysfunctions are relatively unknown.

OBJECTIVES

Accordingly, we evaluated the effect of NAC on neonatal clomipramine (CLI)-induced adulthood anxiety and accompanying changes in plasma corticosterone levels, amygdalar volumes, neuronal/glial densities, levels of monoamines, and their metabolites in the amygdalar complex.

RESULTS

We found that chronic treatment with NAC reverses CLI-induced anhedonia and enhanced anxiety. Interestingly, attenuation of CLI-associated anxiety in NAC-treated rats were accompanied by a reversal of adrenal and spleen hypertrophy, and normalization of enhanced plasma corticosterone levels, indicating improved HPA axis functioning. Furthermore, NAC treatment was sufficient to reverse volumetric hypertrophy of basolateral amygdala (BLA), and altered noradrenaline (NA) metabolism in the amygdalar complex. The effects of NAC in the reversal of CLI-induced impairments were similar to that of fluoxetine (FLX).

CONCLUSIONS

We suggest that beneficial effects of NAC on antidepressive- and antianxiety-like behaviors are at least in part mediated via restoration of amygdalar and HPA axis functioning. Our results support the hypothesis that NAC might be evolved as a therapeutic strategy for reversal of amygdalar dysfunction in depression.

Neonatal Tactile Stimulation Alters Behaviors in Heterozygous Serotonin Transporter Male Rats: Role of the Amygdala

The serotonin transporter (SERT) gene, especially the short allele of the human serotonin transporter linked polymorphic region (5-HTTLPR), has been associated with the development of stress-related neuropsychiatric disorders. In line, exposure to early life stress in SERT knockout animals contributes to anxiety- and depression-like behavior. However, there is a lack of investigation of how early-life exposure to beneficial stimuli, such as tactile stimulation (TS), affects later life behavior in these animals. In this study, we investigated the effect of TS on social, anxiety, and anhedonic behavior in heterozygous SERT knockouts rats and wild-type controls and its impact on gene expression in the basolateral amygdala. Heterozygous SERT^+/– rats were submitted to TS during postnatal days 8–14, for 10 min per day. In adulthood, rats were assessed for social and affective behavior. Besides, brain-derived neurotrophic factor (Bdnf) gene expression and its isoforms, components of glutamatergic and GABAergic systems as well as glucocorticoid-responsive genes were measured in the basolateral amygdala. We found that exposure to neonatal TS improved social and affective behavior in SERT^+/– animals compared to naïve SERT^+/– animals and was normalized to the level of naïve SERT^+/+ animals. At the molecular level, we observed that TS per se affected Bdnf, the glucocorticoid-responsive genes Nr4a1, Gadd45β, the co-chaperone Fkbp5 as well as glutamatergic and GABAergic gene expression markers including the enzyme Gad67, the vesicular GABA transporter, and the vesicular glutamate transporter genes. Our results suggest that exposure of SERT^+/– rats to neonatal TS can normalize their phenotype in adulthood and that TS per se alters the expression of plasticity and stress-related genes in the basolateral amygdala. These findings demonstrate the potential effect of a supportive stimulus in SERT rodents, which are more susceptible to develop psychiatric disorders.

The involvement of oxytocin in the effects of chronic social defeat stress on emotional behaviours in adult female mandarin voles

Chronic social defeat stress (CSDS) can induce anxiety and depression in male rodents, but the prevalence of anxiety and depression is much higher in females, and effects of CSDS on adult females and its underlying mechanism remain unclear. Oxytocin is a stress-buffering hormone in the brain that modulates the physiological effects of stress. Strikingly, research regarding the effect of oxytocin on emotional changes caused by CSDS is still lacking in females. Thus, we focused on the involvement of the oxytocin system in changes in emotional regulation induced by CSDS in female voles. Seventy-day-old female mandarin voles (Microtus mandarinus) were exposed to aggressive adult females for 14 days, and the effects of CSDS on emotion and regulation of oxytocin system were characterized. In addition, we injected vehicle, oxytocin and oxytocin receptor antagonist into the nucleus accumbens (Nacc) of female voles to investigate the involvement of Nacc oxytocin in the effect of CSDS on emotion. Herein, we reported that CSDS increased anxiety and depression-like behaviour and the circulating level of corticosterone, but decreased the number of oxytocin projections and the protein and mRNA expression levels of oxytocin receptor in the Nacc. Injection of oxytocin into the Nacc reversed the effects of CSDS on anxiety-like and depressive-like behaviour, whereas combined injections of oxytocin and oxytocin receptor antagonist eliminated these effects. In conclusion, CSDS increases the levels of anxiety and depression possibly via a reduction in oxytocin projections and the oxytocin receptor level in the Nacc. Nacc oxytocin may be involved in the effects of CSDS on emotional behaviours.

Alterations of Glutamatergic Markers in the Prefrontal Cortex of Serotonin Transporter Knockout Rats: A Developmental Timeline

The serotoninergic system plays a key role in environmental sensitivity, potentially through down-stream effects on the GABAergic and glutamatergic systems. We previously demonstrated that juvenile serotonin transporter knockout (SERT-/-) rats, showing increased environmental sensitivity, exhibit a decreased GABA-mediated inhibitory tone in the cortex. Since the GABAergic and glutamatergic systems are tightly interconnected, we here analyzed glutamatergic markers in the prefrontal cortex of SERT-/- rats, from the early stages of life until adulthood. We found that SERT inactivation in pre-weaning, juvenile, and adult rats was associated with reduced expression of proteins essential for the glutamatergic synapses such as GluN1, PSD95, CDC42, and SEPT7. These lifelong molecular changes may destabilize glutamatergic signaling and possibly contribute to stress sensitivity and vulnerability to stress-related disorders associated with SERT alteration.

Associations Between Brain Structure and Connectivity in Infants and Exposure to Selective Serotonin Reuptake Inhibitors During Pregnancy

IMPORTANCE

Selective serotonin reuptake inhibitor (SSRI) use among pregnant women is increasing, yet the association between prenatal SSRI exposure and fetal neurodevelopment is poorly understood. Animal studies show that perinatal SSRI exposure alters limbic circuitry and produces anxiety and depressive-like behaviors after adolescence, but literature on prenatal SSRI exposure in humans is limited and mixed.

OBJECTIVE

To examine associations between prenatal SSRI exposure and brain development using structural and diffusion magnetic resonance imaging (MRI).

DESIGN, SETTING, AND PARTICIPANTS

A cohort study conducted at Columbia University Medical Center and New York State Psychiatric Institute included 98 infants: 16 with in utero SSRI exposure, 21 with in utero untreated maternal depression exposure, and 61 healthy controls. Data were collected between January 6, 2011, and October 25, 2016.

EXPOSURES

Selective serotonin reuptake inhibitors and untreated maternal depression.

MAIN OUTCOMES AND MEASURES

Gray matter volume estimates using structural MRI with voxel-based morphometry and white matter structural connectivity (connectome) using diffusion MRI with probabilistic tractography.

RESULTS

The sample included 98 mother (31 [32%] white, 26 [27%] Hispanic/Latina, 26 [27%] black/African American, 15 [15%] other) and infant (46 [47%] boys, 52 [53%] girls) dyads. Mean (SD) age of the infants at the time of the scan was 3.43 (1.50) weeks. Voxel-based morphometry showed significant gray matter volume expansion in the right amygdala (Cohen d = 0.65; 95% CI, 0.06-1.23) and right insula (Cohen d = 0.86; 95% CI, 0.26-1.14) in SSRI-exposed infants compared with both healthy controls and infants exposed to untreated maternal depression (P < .05; whole-brain correction). In connectome-level analysis of white matter structural connectivity, the SSRI group showed a significant increase in connectivity between the right amygdala and the right insula with a large effect size (Cohen d = 0.99; 95% CI, 0.40-1.57) compared with healthy controls and untreated depression (P < .05; whole connectome correction).

CONCLUSIONS AND RELEVANCE

Our findings suggest that prenatal SSRI exposure has an association with fetal brain development, particularly in brain regions critical to emotional processing. The study highlights the need for further research on the potential long-term behavioral and psychological outcomes of these neurodevelopmental changes.

OCD-like behavior is caused by dysfunction of thalamo-amygdala circuits and upregulated TrkB/ERK-MAPK signaling as a result of SPRED2 deficiency

Obsessive-compulsive disorder (OCD) is a common neuropsychiatric disease affecting about 2% of the general population. It is characterized by persistent intrusive thoughts and repetitive ritualized behaviors. While gene variations, malfunction of cortico-striato-thalamo-cortical (CSTC) circuits, and dysregulated synaptic transmission have been implicated in the pathogenesis of OCD, the underlying mechanisms remain largely unknown. Here we show that OCD-like behavior in mice is caused by deficiency of SPRED2, a protein expressed in various brain regions and a potent inhibitor of Ras/ERK-MAPK signaling. Excessive self-grooming, reflecting OCD-like behavior in rodents, resulted in facial skin lesions in SPRED2 knockout (KO) mice. This was alleviated by treatment with the selective serotonin reuptake inhibitor fluoxetine. In addition to the previously suggested involvement of cortico-striatal circuits, electrophysiological measurements revealed altered transmission at thalamo-amygdala synapses and morphological differences in lateral amygdala neurons of SPRED2 KO mice. Changes in synaptic function were accompanied by dysregulated expression of various pre- and postsynaptic proteins in the amygdala. This was a result of altered gene transcription and triggered upstream by upregulated tropomyosin receptor kinase B (TrkB)/ERK-MAPK signaling in the amygdala of SPRED2 KO mice. Pathway overactivation was mediated by increased activity of TrkB, Ras, and ERK as a specific result of SPRED2 deficiency and not elicited by elevated brain-derived neurotrophic factor levels. Using the MEK inhibitor selumetinib, we suppressed TrkB/ERK-MAPK pathway activity in vivo and reduced OCD-like grooming in SPRED2 KO mice. Altogether, this study identifies SPRED2 as a promising new regulator, TrkB/ERK-MAPK signaling as a novel mediating mechanism, and thalamo-amygdala synapses as critical circuitry involved in the pathogenesis of OCD.

Genetic and biochemical changes of the serotonergic system in migraine pathobiology

Migraine is a brain disorder characterized by a piercing headache which affects one side of the head, located mainly at the temples and in the area around the eye. Migraine imparts substantial suffering to the family in addition to the sufferer, particularly as it affects three times more women than men and is most prevalent between the ages of 25 and 45, the years of child rearing. Migraine typically occurs in individuals with a genetic predisposition and is aggravated by specific environmental triggers. Attempts to study the biochemistry of migraine began as early as the 1960s and were primarily directed at serotonin metabolism after an increase of 5-hydroxyindoleacetic acid (5-HIAA), the main metabolite of serotonin was observed in urine of migraineurs. Genetic and biochemical studies have primarily focused on the neurotransmitter serotonin, considering receptor binding, transport and synthesis of serotonin and have investigated serotonergic mediators including enzymes, receptors as well as intermediary metabolites. These studies have been mainly assayed in blood, CSF and urine as the most accessible fluids. More recently PET imaging technology integrated with a metabolomics and a systems biology platform are being applied to study serotonergic biology. The general trend observed is that migraine patients have alterations of neurotransmitter metabolism detected in biological fluids with different biochemistry from controls, however the interpretation of the biological significance of these peripheral changes is unresolved. In this review we present the biology of the serotonergic system and metabolic routes for serotonin and discuss results of biochemical studies with regard to alterations in serotonin in brain, cerebrospinal fluid, saliva, platelets, plasma and urine of migraine patients.

Impact of varying social experiences during life history on behaviour, gene expression, and vasopressin receptor gene methylation in mice

Both negative and positive social experiences during sensitive life phases profoundly shape brain and behaviour. Current research is therefore increasingly focusing on mechanisms mediating the interaction between varying life experiences and the epigenome. Here, male mice grew up under either adverse or beneficial conditions until adulthood, when they were subdivided into groups exposed to situations that either matched or mismatched previous conditions. It was investigated whether the resulting four life histories were associated with changes in anxiety-like behaviour, gene expression of selected genes involved in anxiety and stress circuits, and arginine vasopressin receptor 1a (Avpr1a) gene methylation. Varying experiences during life significantly modulated (1) anxiety-like behaviour; (2) hippocampal gene expression of Avpr1a, serotonin receptor 1a (Htr1a), monoamine oxidase A (Maoa), myelin basic protein (Mbp), glucocorticoid receptor (Nr3c1), growth hormone (Gh); and (3) hippocampal DNA methylation within the Avpr1a gene. Notably, mice experiencing early beneficial and later adverse conditions showed a most pronounced downregulation of Avpr1a expression, accompanied by low anxiety-like behaviour. This decrease in Avpr1a expression may have been, in part, a consequence of increased methylation in the Avpr1a gene. In summary, this study highlights the impact of interactive social experiences throughout life on the hippocampal epigenotype and associated behaviour.

Serotonin and neuroplasticity - Links between molecular, functional and structural pathophysiology in depression

Serotonin modulates neuroplasticity, especially during early life, and dysfunctions in both systems likewise contribute to pathophysiology of depression. Recent findings demonstrate that serotonin reuptake inhibitors trigger reactivation of juvenile-like neuroplasticity. How these findings translate to clinical antidepressant treatment in major depressive disorder remains unclear. With this review, we link preclinical with clinical work on serotonin and neuroplasticity to bring two pathophysiologic models in clinical depression closer together. Dysfunctional developmental plasticity impacts on later-life cognitive and emotional functions, changes of synaptic serotonin levels and receptor levels are coupled with altered synaptic plasticity and neurogenesis. Structural magnetic resonance imaging in patients reveals disease-state-specific reductions of gray matter, a marker of neuroplasticity, and reversibility upon selective serotonin reuptake inhibitor treatment. Translational evidence from magnetic resonance imaging in animals support that reduced densities and sizes of neurons and reduced hippocampal volumes in depressive patients could be attributable to changes of serotonergic neuroplasticity. Since ketamine, physical exercise or learning enhance neuroplasticity, combinatory paradigms with selective serotonin reuptake inhibitors could enhance clinical treatment of depression.

Chronic Social Defeat Stress Modulates Dendritic Spines Structural Plasticity in Adult Mouse Frontal Association Cortex

Chronic stress is associated with occurrence of many mental disorders. Previous studies have shown that dendrites and spines of pyramidal neurons of the prefrontal cortex undergo drastic reorganization following chronic stress experience. So the prefrontal cortex is believed to play a key role in response of neural system to chronic stress. However, how stress induces dynamic structural changes in neural circuit of prefrontal cortex remains unknown. In the present study, we examined the effects of chronic social defeat stress on dendritic spine structural plasticity in the mouse frontal association (FrA) cortex in vivo using two-photon microscopy. We found that chronic stress altered spine dynamics in FrA and increased the connectivity in FrA neural circuits. We also found that the changes in spine dynamics in FrA are correlated with the deficit of sucrose preference in defeated mice. Our findings suggest that chronic stress experience leads to adaptive change in neural circuits that may be important for encoding stress experience related memory and anhedonia.

Sensory processing sensitivity and serotonin gene variance: Insights into mechanisms shaping environmental sensitivity

Current research supports the notion that the apparently innate trait Sensory Processing Sensitivity (SPS) may act as a modulator of development as function of the environment. Interestingly, the common serotonin transporter linked polymorphic region (5-HTTLPR) does the same. While neural mechanisms underlying SPS are largely unknown, those associated with the 5-HTTLPR have been extensively investigated. We perform a comparative analysis of research findings on sensory processing facets associated with the trait and polymorphism to: 1. detect shared phenotypes and frame a hypothesis towards neural mechanisms underlying SPS; 2. increase the understanding of 5-HTTLPR-associated behavioral patterns. Trait and polymorphism are both associated with differential susceptibility to environmental stimuli; additionally, both involve 1. having stronger emotional reactions, 2. processing of sensory information more deeply, 3. being more aware of environmental subtleties, and 4. being easily overstimulated. We discuss neural mechanisms and environmental conditions that may underlie these four facets. Besides urging the actual assessment of the link between the two, the conclusions of our analyses may guide and focus future research strategies.

A dendritic organization of lateral amygdala neurons in fear susceptible and resistant mice

Subtle differences in neuronal microanatomy may be coded in individuals with genetic susceptibility for neuropsychiatric disorders. Genetic susceptibility is a significant risk factor in the development of anxiety disorders, including post-traumatic stress disorder (PTSD). Pavlovian fear conditioning has been proposed to model key aspects of PTSD. According to this theory, PTSD begins with the formation of a traumatic memory which connects relevant environmental stimuli to significant threats to life. The lateral amygdala (LA) is considered to be a key network hub for the establishment of Pavlovian fear conditioning. Substantial research has also linked the LA to PTSD. Here we used a genetic mouse model of fear susceptibility (F-S) and resistance (F-R) to investigate the dendritic and spine structure of principal neurons located in the LA. F-S and F-R lines were bi-directionally selected based on divergent levels of contextual and cued conditioned freezing in response to fear-evoking footshocks. We examined LA principal neuron dendritic and spine morphology in the offspring of experimentally naive F-S and F-R mice. We found differences in the spatial distribution of dendritic branch points across the length of the dendrite tree, with a significant increase in branch points at more distal locations in the F-S compared with F-R line. These results suggest a genetic predisposition toward differences in fear memory strength associated with a dendritic branch point organization of principal neurons in the LA. These micro-anatomical differences in neuron structure in a genetic mouse model of fear susceptibility and resistance provide important insights into the cellular mechanisms of pathophysiology underlying genetic predispositions to anxiety and PTSD.

Qualitatively different effect of repeated stress during adolescence on principal neuron morphology across lateral and basal nuclei of the rat amygdala

Repeated stress can elicit symptoms of depression and anxiety. The amygdala is a significant contributor to the expression of emotion and the basolateral amygdala (BLA) is a major target for the effects of stress on emotion. The adolescent time period may be particularly susceptible to the effects of stress on emotion. While repeated stress has been demonstrated to modify the morphology of BLA neurons in adult rats, little is known about its effects on BLA neurons during adolescence. This study tests the effects of repeated stress during adolescence on BLA neuronal morphology, and whether these are similar to the effects of stress during adulthood. The BLA includes the basal (BA) and lateral (LAT) nuclei, which are differentially responsive to stress in adults. Therefore, effects of stress during adolescence were compared between the BA and LAT nuclei. Morphological features of reconstructed BLA neurons were examined using Golgi-Cox-stained tissue from control or repeated restraint stress-exposed rats. We found subtle dendritic growth coupled with loss of spines after repeated stress during adolescence. The magnitude and dendritic location of these differences varied between the BA and LAT nuclei in strong contrast to the stress-induced increases in spine number seen in adults. These results demonstrate that repeated stress during adolescence has markedly different effects on BLA neuronal morphology, and the extent of these changes is BLA nucleus-dependent. Moreover, altered neuroanatomy was associated with age-dependent effects of repeated stress on generalization of fear, and may point to the necessity for different approaches to target stress-induced changes in adolescents.

Serotonin-related pathways and developmental plasticity: relevance for psychiatric disorders

Risk for adult psychiatric disorders is partially determined by early-life alterations occurring during neural circuit formation and maturation. In this perspective, recent data show that the serotonin system regulates key cellular processes involved in the construction of cortical circuits. Translational data for rodents indicate that early-life serotonin dysregulation leads to a wide range of behavioral alterations, ranging from stress-related phenotypes to social deficits. Studies in humans have revealed that serotonin-related genetic variants interact with early-life stress to regulate stress-induced cortisol responsiveness and activate the neural circuits involved in mood and anxiety disorders. Emerging data demonstrate that early-life adversity induces epigenetic modifications in serotonin-related genes. Finally, recent findings reveal that selective serotonin reuptake inhibitors can reinstate juvenile-like forms of neural plasticity, thus allowing the erasure of long-lasting fear memories. These approaches are providing new insights on the biological mechanisms and clinical application of antidepressants.

Developmental alterations in anxiety and cognitive behavior in serotonin transporter mutant mice

RATIONALE

A promoter variant of the serotonin transporter (SERT) gene is known to affect emotional and cognitive regulation. In particular, the "short" allelic variant is implicated in the etiology of multiple neuropsychiatric disorders. Heterozygous (SERT(+/-)) and homozygous (SERT(-/-)) SERT mutant mice are valuable tools for understanding the mechanisms of altered SERT levels. Although these genetic effects are well investigated in adulthood, the developmental trajectory of altered SERT levels for behavior has not been investigated.

OBJECTIVES

We assessed anxiety-like and cognitive behaviors in SERT mutant mice in early adolescence and adulthood to examine the developmental consequences of reduced SERT levels. Spine density of pyramidal neurons was also measured in corticolimbic brain regions.

RESULTS

Adult SERT(-/-) mice exhibited increased anxiety-like behavior, but these differences were not observed in early adolescent SERT(-/-) mice. Conversely, SERT(+/-) and SERT(-/-) mice did display higher spontaneous alternation during early adolescence and adulthood. SERT(+/-) and SERT(-/-) also exhibited greater neuronal spine densities in the orbitofrontal but not the medial prefrontal cortices. Adult SERT(-/-) mice also showed an increased spine density in the basolateral amygdala.

CONCLUSIONS

Developmental alterations of the serotonergic system caused by genetic inactivation of SERT can have different influences on anxiety-like and cognitive behaviors through early adolescence into adulthood, which may be associated with changes of spine density in the prefrontal cortex and amygdala. The altered maturation of serotonergic systems may lead to specific age-related vulnerabilities to psychopathologies that develop during adolescence.

Effects of negative stressors on DNA methylation in the brain: implications for mood and anxiety disorders

Stress is a major contributor to anxiety and mood disorders. The recent discovery of epigenetic changes in the brain resulting from stress has enhanced our understanding of the mechanism by which stress is able to promote these disorders. Although epigenetics encompasses chemical modifications that occur at both DNA and histones, much attention has been focused on stress-induced DNA methylation changes on behavior. Here, we review the effect of stress-induced DNA methylation changes on physiological mechanisms that govern behavior and cognition, dysregulation of which can be harmful to mental health. A literature review was performed in the areas of DNA methylation, stress, and their impact on the brain and psychiatric illness. Key findings center on genes involved in the hypothalamic-pituitary-adrenal axis, neurotransmission and neuroplasticity. Using animal models of different stress paradigms and clinical studies, we detail how DNA methylation changes to these genes can alter physiological mechanisms that influence behavior. Appropriate levels of gene expression in the brain play an important role in mental health. This dynamic control can be disrupted by stress-induced changes to DNA methylation patterns. Advancement in other areas of epigenetics, such as histone modifications and the discovery of the novel DNA epigenetic mark, 5-hydroxymethylcytosine, could provide additional avenues to consider when determining the epigenetic effects of stress on the brain.

Mechanisms to medicines: elucidating neural and molecular substrates of fear extinction to identify novel treatments for anxiety disorders

The burden of anxiety disorders is growing, but the efficacy of available anxiolytic treatments remains inadequate. Cognitive behavioural therapy for anxiety disorders focuses on identifying and modifying maladaptive patterns of thinking and behaving, and has a testable analogue in rodents in the form of fear extinction. A large preclinical literature has amassed in recent years describing the neural and molecular basis of fear extinction in rodents. In this review, we discuss how this work is being harnessed to foster translational research on anxiety disorders and facilitate the search for new anxiolytic treatments. We begin by summarizing the anatomical and functional connectivity of a medial prefrontal cortex (mPFC)-amygdala circuit that subserves fear extinction, including new insights from optogenetics. We then cover some of the approaches that have been taken to model impaired fear extinction and associated impairments with mPFC-amygdala dysfunction. The principal goal of the review is to evaluate evidence that various neurotransmitter and neuromodulator systems mediate fear extinction by modulating the mPFC-amygdala circuitry. To that end, we describe studies that have tested how fear extinction is impaired or facilitated by pharmacological manipulations of dopamine, noradrenaline, 5-HT, GABA, glutamate, neuropeptides, endocannabinoids and various other systems, which either directly target the mPFC-amygdala circuit, or produce behavioural effects that are coincident with functional changes in the circuit. We conclude that there are good grounds to be optimistic that the progress in defining the molecular substrates of mPFC-amygdala circuit function can be effectively leveraged to identify plausible candidates for extinction-promoting therapies for anxiety disorders.

Mice with compromised 5-HTT function lack phosphotyrosine-mediated inhibitory control over prefrontal 5-HT responses

The activity of the prefrontal cortex is essential for normal emotional processing and is strongly modulated by serotonin (5-HT). Yet, little is known about the regulatory mechanisms that control the activity of the prefrontal 5-HT receptors. Here, we found and characterized a deregulation of prefrontal 5-HT receptor electrophysiological signaling in mouse models of disrupted serotonin transporter (5-HTT) function, a risk factor for emotional and cognitive disturbances. We identified a novel tyrosine kinase-dependent mechanism that regulates 5-HT-mediated inhibition of prefrontal pyramidal neurons. We report that mice with compromised 5-HTT, resulting from either genetic deletion or brief treatment with selective serotonin reuptake inhibitors during development, have amplified 5-HT1A receptor-mediated currents in adulthood. These greater inhibitory effects of 5-HT are accompanied by enhanced downstream coupling to Kir3 channels. Notably, in normal wild-type mice, we found that these larger 5-HT1A responses can be mimicked through inhibition of Src family tyrosine kinases. By comparison, in our 5-HTT mouse models, the larger 5-HT1A responses were rapidly reduced through inhibition of tyrosine phosphatases. Our findings implicate tyrosine phosphorylation in regulating the electrophysiological effects of prefrontal 5-HT1A receptors with implications for neuropsychiatric diseases associated with emotional dysfunction, such as anxiety and depressive disorders.

Effects of repeated adolescent stress and serotonin transporter gene partial knockout in mice on behaviors and brain structures relevant to major depression

In humans, exposure to stress during development is associated with structural and functional alterations of the prefrontal cortex (PFC), amygdala (AMY), and hippocampus (HC) and their circuits of connectivity, and with an increased risk for developing major depressive disorder particularly in carriers of the short (s) variant of the serotonin transporter (5-HTT) gene-linked polymorphic region (5-HTTLPR). Although changes in these regions are found in carriers of the s allele and/or in depressed patients, evidence for a specific genotype × developmental stress effect on brain structure and function is limited. Here, we investigated the effect of repeated stress exposure during adolescence in mice with partial knockout of the 5-HTT gene (HET) vs. wildtype (WT) on early-adulthood behavioral measures and brain structure [using magnetic resonance imaging (MRI)] relevant to human major depression. Behaviorally, adolescent stress (AS) increased anxiety and decreased activity and did so to a similar degree in HET and WT. In a probabilistic reversal learning task, HET-AS mice achieved fewer reversals than did HET-No-AS mice. 5-HTT genotype and AS were without effect on corticosterone stress response. In terms of structural brain differences, AS reduced the volume of two long-range white matter tracts, the optic tract (OT) and the cerebral peduncle (CP), in WT mice specifically. In a region-of-interest analysis, AS was associated with increased HC volume and HET genotype with a decreased frontal lobe volume. In conclusion, we found that 5-HTT and AS genotype exerted long-term effects on behavior and development of brain regions relevant to human depression.

5-HTT deficiency affects neuroplasticity and increases stress sensitivity resulting in altered spatial learning performance in the Morris water maze but not in the Barnes maze

The purpose of this study was to evaluate whether spatial hippocampus-dependent learning is affected by the serotonergic system and stress. Therefore, 5-HTT knockout (-/-), heterozygous (+/-) and wildtype (+/+) mice were subjected to the Barnes maze (BM) and the Morris water maze (WM), the latter being discussed as more aversive. Additionally, immediate early gene (IEG) expression, hippocampal adult neurogenesis (aN), and blood plasma corticosterone were analyzed.

While the performance of 5-HTT-/- mice in the BM was undistinguishable from both other genotypes, they performed worse in the WM. However, in the course of the repeated WM trials 5-HTT-/- mice advanced to wildtype level. The experience of a single trial of either the WM or the BM resulted in increased plasma corticosterone levels in all genotypes. After several trials 5-HTT-/- mice exhibited higher corticosterone concentrations compared with both other genotypes in both tests. Corticosterone levels were highest in 5-HTT-/- mice tested in the WM indicating greater aversiveness of the WM and a greater stress sensitivity of 5-HTT deficient mice.

Quantitative immunohistochemistry in the hippocampus revealed increased cell counts positive for the IEG products cFos and Arc as well as for proliferation marker Ki67 and immature neuron marker NeuroD in 5-HTT-/- mice compared to 5-HTT+/+ mice, irrespective of the test. Most differences were found in the suprapyramidal blade of the dentate gyrus of the septal hippocampus. Ki67-immunohistochemistry revealed a genotype x environment interaction with 5-HTT genotype differences in naïve controls and WM experience exclusively yielding more Ki67-positive cells in 5-HTT+/+ mice. Moreover, in 5-HTT-/- mice we demonstrate that learning performance correlates with the extent of aN.

Overall, higher baseline IEG expression and increased an in the hippocampus of 5-HTT-/- mice together with increased stress sensitivity may constitute the neurobiological correlate of raised alertness, possibly impeding optimal learning performance in the more stressful WM.

Convergent effects of mouse Pet-1 deletion and human PET-1 variation on amygdala fear and threat processing

Serotonin is critical for shaping the development of neural circuits regulating emotion. Pet-1 (FEV-1) is an ETS-domain transcription factor essential for differentiation and forebrain targeting of serotonin neurons. Constitutive Pet-1 knockout (KO) causes major loss of serotonin neurons and forebrain serotonin availability, and behavioral abnormalities. We phenotyped Pet-1 KO mice for fear conditioning and extinction, and on a battery of assays for anxiety- and depression-related behaviors. Morphology of Golgi-stained neurons in basolateral amygdala (BLA) and prelimbic cortex was examined. Using human imaging genetics, a common variant (rs860573) in the PET-1 (FEV) gene was tested for effects on threat-related amygdala reactivity and psychopathology in 88 Asian-ancestry subjects. Pet-1 KO mice exhibited increased acquisition and expression of fear, and elevated fear recovery following extinction, relative to wild-type (WT). BLA dendrites of Pet-1 KO mice were significantly longer than in WT. Human PET-1 variation associated with differences in amygdala threat processing and psychopathology. This novel evidence for the role of Pet-1 in fear processing and dendritic organization of amygdala neurons and in human amygdala threat processing extends a growing literature demonstrating the influence of genetic variation in the serotonin system on emotional regulation via effects on structure and function of underlying corticolimbic circuitry.

The role of the serotonergic and GABA system in translational approaches in drug discovery for anxiety disorders

There is ample evidence that genetic factors play an important role in anxiety disorders. In support, human genome-wide association studies have implicated several novel candidate genes. However, illumination of such genetic factors involved in anxiety disorders has not resulted in novel drugs over the past decades. A complicating factor is the heterogeneous classification of anxiety disorders in the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV-TR) and diverging operationalization of anxiety used in preclinical and clinical studies. Currently, there is an increasing focus on the gene × environment (G × E) interaction in anxiety as genes do not operate in isolation and environmental factors have been found to significantly contribute to the development of anxiety disorders in at-risk individuals. Nevertheless, extensive research on G × E mechanisms in anxiety has not resulted in major breakthroughs in drug discovery. Modification of individual genes in rodent models has enabled the specific study of anxiety in preclinical studies. In this context, two extensively studied neurotransmitters involved in anxiety are the gamma-aminobutyric acid (GABA) and 5-HT (5-hydroxytryptamine) system. In this review, we illustrate the complex interplay between genes and environment in anxiety processes by reviewing preclinical and clinical studies on the serotonin transporter (5-HTT), 5-HT1A receptor, 5-HT2 receptor, and GABAA receptor. Even though targets from the serotonin and GABA system have yielded drugs with known anxiolytic efficacy, the relation between the genetic background of these targets and anxiety symptoms and development of anxiety disorders is largely unknown. The aim of this review is to show the vast complexity of genetic and environmental factors in anxiety disorders. In light of the difficulty with which common genetic variants are identified in anxiety disorders, animal models with translational validity may aid in elucidating the neurobiological background of these genes and their possible role in anxiety. We argue that, in addition to human genetic studies, translational models are essential to map anxiety-related genes and to enhance our understanding of anxiety disorders in order to develop potentially novel treatment strategies.

Effect of acute stressor and serotonin transporter genotype on amygdala first wave transcriptome in mice

The most prominent brain region evaluating the significance of external stimuli immediately after their onset is the amygdala. Stimuli evaluated as being stressful actuate a number of physiological processes as an immediate stress response. Variation in the serotonin transporter gene has been associated with increased anxiety- and depression-like behavior, altered stress reactivity and adaptation, and pathophysiology of stress-related disorders. In this study the instant reactions to an acute stressor were measured in a serotonin transporter knockout mouse model. Mice lacking the serotonin transporter were verified to be more anxious than their wild-type conspecifics. Genome-wide gene expression changes in the amygdala were measured after the mice were subjected to control condition or to an acute stressor of one minute exposure to water. The dissection of amygdalae and stabilization of RNA was conducted within nine minutes after the onset of the stressor. This extremely short protocol allowed for analysis of first wave primary response genes, typically induced within five to ten minutes of stimulation, and was performed using Affymetrix GeneChip Mouse Gene 1.0 ST Arrays. RNA profiling revealed a largely new set of differentially expressed primary response genes between the conditions acute stress and control that differed distinctly between wild-type and knockout mice. Consequently, functional categorization and pathway analysis indicated genes related to neuroplasticity and adaptation in wild-types whereas knockouts were characterized by impaired plasticity and genes more related to chronic stress and pathophysiology. Our study therefore disclosed different coping styles dependent on serotonin transporter genotype even directly after the onset of stress and accentuates the role of the serotonergic system in processing stressors and threat in the amygdala. Moreover, several of the first wave primary response genes that we found might provide promising targets for future therapeutic interventions of stress-related disorders also in humans.

Functional and structural neural network characterization of serotonin transporter knockout rats

Brain serotonin homeostasis is crucially maintained by the serotonin transporter (5-HTT), and its down-regulation has been linked to increased vulnerability for anxiety- and depression-related behavior. Studies in 5-HTT knockout (5-HTT^-/-) rodents have associated inherited reduced functional expression of 5-HTT with increased sensitivity to adverse as well as rewarding environmental stimuli, and in particular cocaine hyperresponsivity. 5-HTT down-regulation may affect normal neuronal wiring of implicated corticolimbic cerebral structures. To further our understanding of its contribution to potential alterations in basal functional and structural properties of neural network configurations, we applied resting-state functional MRI (fMRI), pharmacological MRI of cocaine-induced activation, and diffusion tensor imaging (DTI) in 5-HTT^-/- rats and wild-type controls (5-HTT^+/+). We found that baseline functional connectivity values and cocaine-induced neural activity within the corticolimbic network was not significantly altered in 5-HTT^-/- versus 5-HTT^+/+ rats. Similarly, DTI revealed mostly intact white matter structural integrity, except for a reduced fractional anisotropy in the genu of the corpus callosum of 5-HTT^-/- rats. At the macroscopic level, analyses of complex graphs constructed from either functional connectivity values or structural DTI-based tractography results revealed that key properties of brain network organization were essentially similar between 5-HTT^+/+ and 5-HTT^-/- rats. The individual tests for differences between 5-HTT^+/+ and 5-HTT^-/- rats were capable of detecting significant effects ranging from 5.8% (fractional anisotropy) to 26.1% (pharmacological MRI) and 29.3% (functional connectivity). Tentatively, lower fractional anisotropy in the genu of the corpus callosum could indicate a reduced capacity for information integration across hemispheres in 5-HTT^-/- rats. Overall, the comparison of 5-HTT^-/- and wild-type rats suggests mostly limited effects of 5-HTT genotype on MRI-based measures of brain morphology and function.

Methods of dendritic spine detection: from Golgi to high-resolution optical imaging

Dendritic spines, the bulbous protrusions that form the postsynaptic half of excitatory synapses, are one of the most prominent features of neurons and have been imaged and studied for over a century. In that time, changes in the number and morphology of dendritic spines have been correlated to the developmental process as well as the pathophysiology of a number of neurodegenerative diseases. Due to the sheer scale of synaptic connectivity in the brain, work to date has merely scratched the surface in the study of normal spine function and pathology. This review will highlight traditional approaches to the imaging of dendritic spines and newer approaches made possible by advances in microscopy, protein engineering, and image analysis. The review will also describe recent work that is leading researchers toward the possibility of a systematic and comprehensive study of spine anatomy throughout the brain.

Serotonergic innervation and serotonin receptor expression of NPY-producing neurons in the rat lateral and basolateral amygdaloid nuclei

Pharmacobehavioral studies in experimental animals, and imaging studies in humans, indicate that serotonergic transmission in the amygdala plays a key role in emotional processing, especially for anxiety-related stimuli. The lateral and basolateral amygdaloid nuclei receive a dense serotonergic innervation in all species studied to date. We investigated interrelations between serotonergic afferents and neuropeptide Y (NPY)-producing neurons, which are a subpopulation of inhibitory interneurons in the rat lateral and basolateral nuclei with particularly strong anxiolytic properties. Dual light microscopic immunolabeling showed numerous appositions of serotonergic afferents on NPY-immunoreactive somata. Using electron microscopy, direct membrane appositions and synaptic contacts between serotonin-containing axon terminals and NPY-immunoreactive cellular profiles were unequivocally established. Double in situ hybridization documented that more than 50 %, and about 30–40 % of NPY mRNA-producing neurons, co-expressed inhibitory 5-HT1A and excitatory 5-HT2C mRNA receptor subtype mRNA, respectively, in both nuclei with no gender differences. Triple in situ hybridization showed that individual NPY mRNA-producing interneurons co-express both 5-HT1A and 5-HT2C mRNAs. Co-expression of NPY and 5-HT3 mRNA was not observed. The results demonstrate that serotonergic afferents provide substantial innervation of NPY-producing neurons in the rat lateral and basolateral amygdaloid nuclei. Studies of serotonin receptor subtype co-expression indicate a differential impact of the serotonergic innervation on this small, but important, population of anxiolytic interneurons, and provide the basis for future studies of the circuitry underlying serotonergic modulation of emotional stimulus processing in the amygdala.

Social defeat: impact on fear extinction and amygdala-prefrontal cortical theta synchrony in 5-HTT deficient mice

Emotions, such as fear and anxiety, can be modulated by both environmental and genetic factors. One genetic factor is for example the genetically encoded variation of the serotonin transporter (5-HTT) expression. In this context, the 5-HTT plays a key role in the regulation of central 5-HT neurotransmission, which is critically involved in the physiological regulation of emotions including fear and anxiety. However, a systematic study which examines the combined influence of environmental and genetic factors on fear-related behavior and the underlying neurophysiological basis is missing. Therefore, in this study we used the 5-HTT-deficient mouse model for studying emotional dysregulation to evaluate consequences of genotype specific disruption of 5-HTT function and repeated social defeat for fear-related behaviors and corresponding neurophysiological activities in the lateral amygdala (LA) and infralimbic region of the medial prefrontal cortex (mPFC) in male 5-HTT wild-type (+/+), homo- (−/−) and heterozygous (+/−) mice. Naive males and experienced losers (generated in a resident-intruder paradigm) of all three genotypes, unilaterally equipped with recording electrodes in LA and mPFC, underwent a Pavlovian fear conditioning. Fear memory and extinction of conditioned fear was examined while recording neuronal activity simultaneously with fear-related behavior. Compared to naive 5-HTT+/+ and +/− mice, 5-HTT−/− mice showed impaired recall of extinction. In addition, 5-HTT−/− and +/− experienced losers showed delayed extinction learning and impaired recall of extinction. Impaired behavioral responses were accompanied by increased theta synchronization between the LA and mPFC during extinction learning in 5-HTT-/− and +/− losers. Furthermore, impaired extinction recall was accompanied with increased theta synchronization in 5-HTT−/− naive and in 5-HTT−/− and +/− loser mice. In conclusion, extinction learning and memory of conditioned fear can be modulated by both the 5-HTT gene activity and social experiences in adulthood, accompanied by corresponding alterations of the theta activity in the amygdala-prefrontal cortex network.

Serotonin transporter genotype x construction stress interaction in rats

A well-known example for gene x environment interactions in psychiatry is the one involving the low activity (s) allelic variant of the serotonin transporter (5-HTT) promoter polymorphism (5-HTTLPR) that in the context of stress increases risk for depression. In analogy, 5-HTT knockout rodents are highly responsive to early life, but also adult external stressors, albeit conflicting data have been obtained. In our study on emotion and cognition using homozygous 5-HTT knockout (5-HTT(-/-)) and wild-type (5-HTT(+/+)) rats we have been confronted with animal facility construction, which were associated with severe lifetime stress (noise and vibrations). To assess the impact of construction stress on well-established 5-HTT(-/-) rat phenotypes we conducted ad hoc analyses of 5-HTT(-/-) and 5-HTT(+/+) rats that grew up before and during the construction. The reproductive capacity of the parents of the experimental 5-HTT(+/-) rats was significantly decreased. Further, 5-HTT(-/-) anxiety-related phenotypes in the elevated plus maze and social interaction tests were abolished after construction noise exposure, due to increased anxiety in 5-HTT(+/+) rats and decreased anxiety in 5-HTT(-/-) rats (social interaction test only). In addition, reversal learning was improved in 5-HTT(+/+) and, to a milder extent, decreased in 5-HTT(-/-) rats. Finally, construction stress genotype-independently increased behavioural despair in the forced swim test. In conclusion, severe construction stress induces 5-HTT genotype-dependent 'for-better-and-for-worse' effects. These data importantly contribute to the understanding of 5-HTT gene x environment interactions and show the risk of losing genotype effects by construction stress.

When the serotonin transporter gene meets adversity: the contribution of animal models to understanding epigenetic mechanisms in affective disorders and resilience

Although converging epidemiological evidence links exposure to stressful life events with increased risk for affective spectrum disorders, there is extraordinary interindividual variability in vulnerability to adversity. The environmentally moderated penetrance of genetic variation is thought to play a major role in determining who will either develop disease or remain resilient. Research on genetic factors in the aetiology of disorders of emotion regulation has, nevertheless, been complicated by a mysterious discrepancy between high heritability estimates and a scarcity of replicable gene-disorder associations. One explanation for this incongruity is that at least some specific gene effects are conditional on environmental cues, i.e. gene-by-environment interaction (G × E) is present. For example, a remarkable number of studies reported an association of variation in the human serotonin (5-HT) transporter gene (SLC6A4, 5-HTT, SERT) with emotional and cognitive traits as well as increased risk for depression in interaction with psychosocial adversity. The results from investigations in non-human primate and mouse support the occurrence of G × E interaction by showing that variation of 5-HTT function is associated with a vulnerability to adversity across the lifespan leading to unfavourable outcomes resembling various neuropsychiatric disorders. The neural and molecular mechanisms by which environmental adversity in early life increases disease risk in adulthood are not known but may include epigenetic programming of gene expression during development. Epigenetic mechanisms, such as DNA methylation and chromatin modification, are dynamic and reversible and may also provide targets for intervention strategies (see Bountra et al., Curr Top Behav Neurosci, 2011). Animal models amenable to genetic manipulation are useful in the identification of molecular mechanisms underlying epigenetic programming by adverse environments and individual differences in resilience to stress. Therefore, deeper insight into the role of epigenetic regulation in the process of neurodevelopmental programmes is likely to result in early diagnosis of affective spectrum disorders and will contribute to the design of innovative treatments targeting neural pathways that foster resilience.