5-HT1B mRNA expression after chronic social stress

Divergent effects of chronic continuous and intermittent social defeat stress on emotional behaviors: Impact on phosphorylated CREB and BDNF protein levels in the rat hippocampus

Chronic psychosocial stress stands as a significant heterogeneous risk factor for psychiatric disorders. The brain's physiological response to such stress varies based on the frequency and intensity of stress episodes. However, whether stress episodes divergently could affect hippocampal cyclic AMP response element-binding protein (CREB)-brain-derived neurotrophic factor (BDNF) signaling remains unclear, a key regulator of psychiatric symptoms. We aimed to assess how two distinct patterns of social defeat stress exposure impact anxiety- and depression-like behaviors, fear, and hippocampal CREB-BDNF signaling in adult male rats. To explore this, adult male Sprague-Dawley rats were subjected to psychosocial stress using a Resident/Intruder paradigm for ten consecutive days (continuous social defeat stress: [CS]) or ten social defeat stress over the course of 21 days (intermittent social defeat stress [IS]). Behavioral tests (including novelty-suppressed feeding test, forced swimming test, and contextually conditioned fear) were conducted. Protein expression levels of phosphorylated CREB and BDNF in the dorsal and ventral hippocampi were examined. CS led to heightened anxiety-like behavior, fear, and increased levels of phosphorylated CREB in both the dorsal and ventral hippocampi. Conversely, IS resulted in increased anxiety-like behavior and behavioral despair alongside decreased levels of phosphorylated CREB and BDNF, particularly in the dorsal hippocampus. These findings indicate that chronic psychosocial stress divergently affects hippocampal CREB-BDNF signaling and emotional regulation depending on the stress episode. Such insights could enhance our understanding of the molecular basis of the heterogeneity of psychiatric disorders and facilitate the development of innovative treatment approaches to patients with psychiatric disorders.

Understanding the Molecular Regulation of Serotonin Receptor 5-HTR1B-β-Arrestin1 Complex in Stress and Anxiety Disorders

The serotonin receptor subtype 5-HTR1B is widely distributed in the brain with an important role in various behavioral implications including neurological conditions and psychiatric disorders. The neuromodulatory action of 5-HTR1B largely depends upon its arrestin mediated signaling pathway. In this study, we tried to investigate the role of unusually long intracellular loop 3 (ICL3) region of the serotonin receptor 5-HTR1B in interaction with β-arrestin1 (Arr2) to compensate for the absence of the long cytoplasmic tail. Molecular modeling and docking tools were employed to obtain a suitable molecular conformation of the ICL3 region in complex with Arr2 which dictates the specific complex formation of 5-HTR1B with Arr2. This reveals the novel molecular mechanism of phosphorylated ICL3 mediated GPCR-arrestin interaction in the absence of the long cytoplasmic tail. The in-cell disulfide cross-linking experiments and molecular dynamics simulations of the complex further validate the model of 5-HTR1B-ICL3-Arr2 complex. Two serine residues (Ser281 and Ser295) within the 5-HTR1B-ICL3 region were found to be occupying the electropositive pocket of Arr2 in our model and might be crucial for phosphorylation and specific Arr2 binding. The alignment studies of these residues showed them to be conserved only across 5-HTR1B mammalian species. Thus, our studies were able to predict a molecular conformation of 5-HTR1B-Arr2 and identify the role of long ICL3 in the signaling process which might be crucial in designing targeted drugs (biased agonists) that promote GPCR-Arr2 signaling to deter the effects of stress and anxiety-like disorders.

Stress decreases serotonin tone in the nucleus accumbens in male mice to promote aversion and potentiate cocaine preference via decreased stimulation of 5-HT1B receptors

Stress-induced release of dynorphins (Dyn) activates kappa opioid receptors (KOR) in serotonergic neurons to produce dysphoria and potentiate drug reward; however, the circuit mechanisms responsible for this effect are not known. In male mice, we found that conditional deletion of KOR from Slc6a4 (SERT)-expressing neurons blocked stress-induced potentiation of cocaine conditioned place preference (CPP). Within the dorsal raphe nucleus (DRN), two overlapping populations of KOR-expressing neurons: Slc17a8 (VGluT3) and SERT, were distinguished functionally and anatomically. Optogenetic inhibition of these SERT⁺ neurons potentiated subsequent cocaine CPP, whereas optical inhibition of the VGluT3⁺ neurons blocked subsequent cocaine CPP. SERT⁺/VGluT3^- expressing neurons were concentrated in the lateral aspect of the DRN. SERT projections from the DRN were observed in the medial nucleus accumbens (mNAc), but VGluT3 projections were not. Optical inhibition of SERT⁺ neurons produced place aversion, whereas optical stimulation of SERT⁺ terminals in the mNAc attenuated stress-induced increases in forced swim immobility and subsequent cocaine CPP. KOR neurons projecting to mNAc were confined to the lateral aspect of the DRN, and the principal source of dynorphinergic (Pdyn) afferents in the mNAc was from local neurons. Excision of Pdyn from the mNAc blocked stress-potentiation of cocaine CPP. Prior studies suggested that stress-induced dynorphin release within the mNAc activates KOR to potentiate cocaine preference by a reduction in 5-HT tone. Consistent with this hypothesis, a transient pharmacological blockade of mNAc 5-HT_1B receptors potentiated subsequent cocaine CPP. 5-HT_1B is known to be expressed on 5-HT terminals in NAc, and 5-HT_1B transcript was also detected in Pdyn⁺, Adora2a⁺ and ChAT⁺ (markers for direct pathway, indirect pathway, and cholinergic interneurons, respectively). Following stress exposure, 5-HT_1B transcript was selectively elevated in Pdyn⁺ cells of the mNAc. These findings suggest that Dyn/KOR regulates serotonin activation of 5HT_1B receptors within the mNAc and dynamically controls stress response, affect, and drug reward.

Development of Mixed Anxiety/Depression-Like State as a Consequence of Chronic Anxiety: Review of Experimental Data

The review presents experimental data considered from the point of view of dynamic changes in the brain neurochemistry, physiology, and behavior of animals during the development of mixed anxiety/depression-like disorder caused by chronic social stress from norm to severe psychopathology. Evidences are presented to support the hypothesis that chronic anxiety rather than social defeat stress is an etiological factor in depression. The consequences of chronic anxiety for human health and social life are discussed.

Intracranial mast cells contribute to the control of social behavior in male mice

Mast cells (MCs) exist intracranially and have been reported to affect higher brain functions in rodents. However, the role of MCs in the regulation of emotionality and social behavior is unclear. In the present study, using male mice, we examined the relationship between MCs and social behavior and investigated the underlying mechanisms. Wild-type male mice intraventricularly injected with a degranulator of MCs exhibited a marked increase in a three-chamber sociability test. In addition, removal of MCs in Mast cell-specific Toxin Receptor-mediated Conditional cell Knock out (Mas-TRECK) male mice showed reduced social preference levels in a three-chamber sociability test without other behavioral changes, such as anxiety-like and depression-like behavior. Mas-TRECK male mice also had reduced serotonin content and serotonin receptor expression and increased oxytocin receptor expression in the brain. These results suggested that MCs may contribute to the regulation of social behavior in male mice. This effect may be partially mediated by serotonin derived from MCs in the brain.

Selective Regulation of 5-HT1B Serotonin Receptor Expression in the Striatum by Dopamine Depletion and Repeated L-DOPA Treatment: Relationship to L-DOPA-Induced Dyskinesias

Dopamine and serotonin in the basal ganglia interact in a bidirectional manner. On the one hand, serotonin (5-HT) receptors regulate the effects of dopamine agonists on several levels, ranging from molecular signaling to behavior. These interactions include 5-HT receptor-mediated facilitation of dopamine receptor-induced gene regulation in striatal output pathways, which involves the 5-HT1B receptor and others. Conversely, there is evidence that dopamine action by psychostimulants regulates 5-HT1B receptor expression in the striatum. To further investigate the effects of dopamine and agonists on 5-HT receptors, we assessed the expression of 5-HT1B and other serotonin receptor subtypes in the striatum after unilateral dopamine depletion by 6-OHDA and subsequent treatment with L-DOPA (5 mg/kg; 4 weeks). Neither dopamine depletion nor L-DOPA treatment produced significant changes in 5-HT2C, 5-HT4, or 5-HT6 receptor expression in the striatum. In contrast, the 6-OHDA lesion caused a (modest) increase in 5-HT1B mRNA levels throughout the striatum. Moreover, repeated L-DOPA treatment markedly further elevated 5-HT1B expression in the dopamine-depleted striatum, an effect that was most robust in the sensorimotor striatum. A minor L-DOPA-induced increase in 5-HT1B expression was also seen in the intact striatum. These changes in 5-HT1B expression mimicked changes in the expression of neuropeptide markers (dynorphin, enkephalin mRNA) in striatal projection neurons. After repeated L-DOPA treatment, the severity of L-DOPA-induced dyskinesias and turning behavior was positively correlated with the increase in 5-HT1B expression in the associative, but not sensorimotor, striatum ipsilateral to the lesion, suggesting that associative striatal 5-HT1B receptors may play a role in L-DOPA-induced behavioral abnormalities.

5-HT release in nucleus accumbens rescues social deficits in mouse autism model

Dysfunction in prosocial interactions is a core symptom of autism spectrum disorder. However, the neural mechanisms that underlie sociability are poorly understood, limiting the rational development of therapies to treat social deficits. Here we show in mice that bidirectional modulation of the release of serotonin (5-HT) from dorsal raphe neurons in the nucleus accumbens bidirectionally modifies sociability. In a mouse model of a common genetic cause of autism spectrum disorder-a copy number variation on chromosome 16p11.2-genetic deletion of the syntenic region from 5-HT neurons induces deficits in social behaviour and decreases dorsal raphe 5-HT neuronal activity. These sociability deficits can be rescued by optogenetic activation of dorsal raphe 5-HT neurons, an effect requiring and mimicked by activation of 5-HT1b receptors in the nucleus accumbens. These results demonstrate an unexpected role for 5-HT action in the nucleus accumbens in social behaviours, and suggest that targeting this mechanism may prove therapeutically beneficial.

Recovery of stress-impaired social behavior by an antagonist of the CRF binding protein, CRF6-33, in the bed nucleus of the stria terminalis of male rats

Social stress is recognized to promote the development of neuropsychiatric and mood disorders. Corticotropin releasing factor (CRF) is an important neuropeptide activated by social stress, and it contributes to neural and behavioral adaptations, as indicated by impaired social interactions and anhedonic effects. Few studies have focused on the role of the CRF binding protein (CRFBP), a component of the CRF system, and its activity in the bed nucleus of stria terminalis (BNST), a limbic structure connecting amygdala and hypothalamus. In this study, animals' preference for sweet solutions was examined as an index of stress-induced anhedonic responses in Wistar rats subjected to four brief intermittent episodes of social defeat. Next, social approach was assessed after local infusions of the CRFBP antagonist, CRF fragment 6-33 (CRF_6-33) into the BNST. The experience of brief episodes of social defeat impaired social approach behaviors in male rats. However, intra-BNST CRF_6-33 infusions restored social approach in stressed animals to the levels of non-stressed rats. CRF_6-33 acted selectively on social interaction and did not alter general exploration in nether stressed nor non-stressed rats. These findings suggest that BNST CRFBP is involved in the modulation of anxiety-like responses induced by social stress.

Effects of a 5-HT1B Receptor Agonist on Locomotion and Reinstatement of Cocaine-Conditioned Place Preference after Abstinence from Repeated Injections in Mice

5-HT_1B receptors (5-HT_1BRs) modulate behavioral effects of cocaine. Here we examined the effects of the 5-HT_1BR agonist 5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine (CP94253) on spontaneous and cocaine-induced locomotion and on cocaine-primed reinstatement of conditioned place preference (CPP) in male mice given daily repeated injections of either saline or cocaine (15 mg/kg, IP) for 20 days. In the locomotor activity experiment, testing occurred both 1 and 20 days after the final injection. In the CPP experiment, mice underwent conditioning procedures while receiving the last of their daily injections, which were given either during or ≥2 h after CPP procedures. The CPP procedural timeline consisted of baseline preference testing (days 12–13 of the chronic regimen), conditioning (days 14–19, 2 daily 30-min sessions separated by 5 h), CPP test (day 21), extinction (days 22–34; no injections), CPP extinction test (day 35), and reinstatement test (day 36). Mice that had not extinguished received additional extinction sessions prior to reinstatement testing on day 42. On test days, mice were pretreated with either saline or CP94253 (10 mg/kg, IP). Testing began 30 min later, immediately after mice were primed with either saline or cocaine (5 mg/kg for locomotion; 15 mg/kg for reinstatement). We found that CP94253 increased spontaneous locomotion in mice receiving repeated injections of either saline or cocaine when tested 1 day after the last injection, but had no effect on spontaneous locomotion after 20 days abstinence from repeated injections. Surprisingly, cocaine-induced locomotion was sensitized regardless of whether the mice had received repeated saline or cocaine. CP94253 attenuated expression of the sensitized locomotion after 20 days abstinence. A control experiment in noninjected, drug-naïve mice showed that CP94253 had no effect on spontaneous or cocaine-induced locomotion. Mice reinstated cocaine-CPP when given a cocaine prime, and CP94253 pretreatment attenuated cocaine reinstatement.The findings suggest that stress from repeated saline injections and/or co-housing with cocaine-injected mice may cross-sensitize with cocaine effects on locomotion and that CP94253 attenuates these effects, as well as reinstatement of cocaine-CPP. This study supports the idea that 5-HT_1BR agonists may be useful anti-cocaine medications.

Social defeat protocol and relevant biomarkers, implications for stress response physiology, drug abuse, mood disorders and individual stress vulnerability: a systematic review of the last decade

INTRODUCTION

Social defeat (SD) in rats, which results from male intraspecific confrontations, is ethologically relevant and useful to understand stress effects on physiology and behavior.

METHODS

A systematic review of studies about biomarkers induced by the SD protocol and published from 2002 to 2013 was carried out in the electronic databases PubMed, Web of Knowledge and ScienceDirect. The search terms were: social defeat, rat, neurotrophins, neuroinflammatory markers, and transcriptional factors.

RESULTS

Classical and recently discovered biomarkers were found to be relevant in stress-induced states. Findings were summarized in accordance to the length of exposure to stress: single, repeated, intermittent and continuous SD. This review found that the brain-derived neurotrophic factor (BDNF) is a distinct marker of stress adaptation. Along with glucocorticoids and catecholamines, BDNF seems to be important in understanding stress physiology.

CONCLUSION

The SD model provides a relevant tool to study stress response features, development of addictive behaviors, clinic depression and anxiety, as well as individual differences in vulnerability and resilience to stress.

Maternal separation induces neuroinflammation and long-lasting emotional alterations in mice

Early life experiences play a key role in brain function and behaviour. Adverse events during childhood are therefore a risk factor for psychiatric disease during adulthood, such as mood disorders. Maternal separation is a validated mouse model for maternal neglect, producing negative early life experiences that result in subsequent emotional alteration. Mood disorders have been found to be associated with neurochemical changes and neurotransmitter deficits such as reduced availability of monoamines in discrete brain areas. Emotional alterations like depression result in reduced serotonin availability and enhanced kynurenine metabolism through the action of indoleamine 2, 3-dioxygenase in response to neuroinflammatory factors. This mechanism involves regulation of the neurotransmitter system by neuroinflammatory agents, linking mood regulation to neuroinmunological reactions. In this context, the aim of this study was to investigate the effects of maternal separation with early weaning on emotional behaviour in mice. We investigated neuroinflammatory responses and the state of the tryptophan-kynurenine metabolic pathway in discrete brain areas following maternal separation. We show that adverse events during early life increase risk of long-lasting emotional alterations during adolescence and adulthood. These emotional alterations are particularly severe in females. Behavioural impairments were associated with microglia activation and disturbed tryptophan-kynurenine metabolism in brain areas related to emotional control. This finding supports the preeminent role of neuroinflammation in emotional disorders.

Differential Rearing Alters Forced Swim Test Behavior, Fluoxetine Efficacy, and Post-Test Weight Gain in Male Rats

Environmental factors play a key role in the etiology of depression. The rodent forced swim test (FST) is commonly used as a preclinical model of depression, with increases in escape-directed behavior reflecting antidepressant effects, and increases in immobility reflecting behavioral despair. Environmental enrichment leads to serotonergic alterations in rats, but it is unknown whether these alterations may influence the efficacy of common antidepressants. Male Sprague-Dawley rats were reared in enriched (EC), standard (SC), or isolated (IC) conditions. Following the rearing period, fluoxetine (10 or 20 mg/kg, i.p.) was administered 23.5 hrs, 5 hrs, and 1 hr before locomotor and FST measures. Following locomotor testing and FST exposure, rats were weighed to assess fluoxetine-, FST-, and environmental condition-induced moderations in weight gain. Results revealed an antidepressant effect of environmental enrichment and a depressant effect of isolation. Regardless of significant fluoxetine effects on locomotor activity, fluoxetine generally decreased swimming and increased immobility in all three environmental conditions, with IC-fluoxetine (10 mg/kg) rats and EC-fluoxetine (20 mg/kg) rats swimming less than vehicle counterparts. Subchronic 20 mg/kg fluoxetine also induced significant weight loss, and differential rearing appeared to moderate weight gain following FST stress. These results suggest that differential rearing has the ability to alter FST behaviors, fluoxetine efficacy, and post-stressor well-being. Moreover, 20 mg/kg fluoxetine, administered subchronically, may lead to atypical effects of those commonly observed in the FST, highlighting the importance and impact of both environmental condition and dosing regimen in common animal models of depression.

Acoustic trauma triggers upregulation of serotonin receptor genes

Hearing loss induces plasticity in excitatory and inhibitory neurotransmitter systems in auditory brain regions. Excitatory-inhibitory balance is also influenced by a range of neuromodulatory regulatory systems, but less is known about the effects of auditory damage on these networks. In this work, we studied the effects of acoustic trauma on neuromodulatory plasticity in the auditory midbrain of CBA/J mice. Quantitative PCR was used to measure the expression of serotonergic and GABAergic receptor genes in the inferior colliculus (IC) of mice that were unmanipulated, sham controls with no hearing loss, and experimental individuals with hearing loss induced by exposure to a 116 dB, 10 kHz pure tone for 3 h. Acoustic trauma induced substantial hearing loss that was accompanied by selective upregulation of two serotonin receptor genes in the IC. The Htr1B receptor gene was upregulated tenfold following trauma relative to shams, while the Htr1A gene was upregulated threefold. In contrast, no plasticity in serotonin receptor gene expression was found in the hippocampus, a region also innervated by serotonergic projections. Analyses in the IC demonstrated that acoustic trauma also changed the coexpression of genes in relation to each other, leading to an overexpression of Htr1B compared to other genes. These data suggest that acoustic trauma induces serotonergic plasticity in the auditory system, and that this plasticity may involve comodulation of functionally-linked receptor genes.

Using Viral-Mediated Gene Transfer to Study Depressive-Like Behavior

The use of viral-mediated gene transfer in animal behavioral experiments has become very popular over the last decade. Altering gene regulation and assessing subsequent behavior can be a useful tool in unraveling a specific gene’s contributions to complex behavioral processes such as depressive-like behavior. However, these types of experiments require detailed planning and designs to avoid pitfalls associated with issues such as surgical procedure or the peak of viral expression. This chapter is intended to be a primer on the design of such experiments and aims to discuss factors that must be considered in the early phases of experimental planning.

Chronic psychosocial stress causes delayed extinction and exacerbates reinstatement of ethanol-induced conditioned place preference in mice

RATIONALE

We have shown previously, using an animal model of voluntary ethanol intake and ethanol-conditioned place preference (EtOH-CPP), that exposure to chronic psychosocial stress induces increased ethanol intake and EtOH-CPP acquisition in mice.

OBJECTIVE

Here, we examined the impact of chronic subordinate colony (CSC) exposure on EtOH-CPP extinction, as well as ethanol-induced reinstatement of CPP.

METHODS

Mice were conditioned with saline or 1.5 g/kg ethanol and were tested in the EtOH-CPP model. In the first experiment, the mice were subjected to 19 days of chronic stress, and EtOH-CPP extinction was assessed during seven daily trials without ethanol injection. In the second experiment and after the EtOH-CPP test, the mice were subjected to 7 days of extinction trials before the 19 days of chronic stress. Drug-induced EtOH-CPP reinstatement was induced by a priming injection of 0.5 g/kg ethanol.

RESULTS

Compared to the single-housed colony mice, CSC mice exhibited increased anxiety-like behavior in the elevated plus maze (EPM) and the open field tests. Interestingly, the CSC mice showed delayed EtOH-CPP extinction. More importantly, CSC mice showed increased alcohol-induced reinstatement of the EtOH-CPP behavior.

CONCLUSION

Taken together, this study indicates that chronic psychosocial stress can have long-term effects on EtOH-CPP extinction as well as drug-induced reinstatement behavior and may provide a suitable model to study the latent effects of chronic psychosocial stress on extinction and relapse to drug abuse.

Social reward requires coordinated activity of nucleus accumbens oxytocin and serotonin

Social behaviours in species as diverse as honey bees and humans promote group survival but often come at some cost to the individual. Although reinforcement of adaptive social interactions is ostensibly required for the evolutionary persistence of these behaviours, the neural mechanisms by which social reward is encoded by the brain are largely unknown. Here we demonstrate that in mice oxytocin acts as a social reinforcement signal within the nucleus accumbens core, where it elicits a presynaptically expressed long-term depression of excitatory synaptic transmission in medium spiny neurons. Although the nucleus accumbens receives oxytocin-receptor-containing inputs from several brain regions, genetic deletion of these receptors specifically from dorsal raphe nucleus, which provides serotonergic (5-hydroxytryptamine; 5-HT) innervation to the nucleus accumbens, abolishes the reinforcing properties of social interaction. Furthermore, oxytocin-induced synaptic plasticity requires activation of nucleus accumbens 5-HT1B receptors, the blockade of which prevents social reward. These results demonstrate that the rewarding properties of social interaction in mice require the coordinated activity of oxytocin and 5-HT in the nucleus accumbens, a mechanistic insight with implications for understanding the pathogenesis of social dysfunction in neuropsychiatric disorders such as autism.

Differential effect of viral overexpression of nucleus accumbens shell 5-HT1B receptors on stress- and cocaine priming-induced reinstatement of cocaine seeking

5-HT1B receptors are densely expressed on terminals of medium spiny neurons projecting from the nucleus accumbens shell (NAccSh) to the ventral tegmental area, where 5-HT1B receptors modulate GABA release directly, and firing of dopaminergic neurons indirectly. While interactions between NAccSh 5-HT1B receptors and stress have been reported in early stages of psychostimulant-induced neuroadaptations, specifically psychomotor sensitization, the effect of this interaction on later stages of drug seeking is currently unknown. Here, we examined the effect of herpes simplex virus (HSV)-mediated overexpression of NAccSh 5-HT1B receptors on reinstatement of cocaine seeking induced by exposure to stress or a cocaine prime. Rats were trained to self-administer cocaine (0.75 mg/kg/infusion) and the operant response was extinguished. Rats were then injected with viral vector expressing 5-HT1B and green fluorescent protein (GFP) or GFP alone into the NAccSh. The effect of 5-HT1B receptor overexpression was assessed on reinstatement induced by intermittent footshock (0.5 mA for 15 min) or a cocaine prime (10mg/kg, ip). Results indicate that NAccSh 5-HT1B receptor overexpression had no effect on footshock reinstatement while significantly decreasing cocaine priming-induced reinstatement. We also found that NAccSh overexpression of 5-HT1B receptors had no effect on saccharin intake following social defeat stress. These results suggest that the efficacy of pharmacological agents targeting 5-HT1B receptors for the treatment of cocaine relapse will depend largely on the nature of the reinstating stimulus. Taken together with previous results, it appears that NAccSh 5-HT1B receptors influence stress responses in early, but not in the later stages of psychostimulant-induced neuroadaptations.