Anxiety-like behaviors produced by acute fluoxetine administration in male Fischer 344 rats are prevented by prior exercise

The psychoplastogens ibogaminalog and ibogainalog induce antidepressant-like activity in naïve and depressed mice by mechanisms involving 5-HT2A receptor activation and serotonergic transmission

The antidepressant-like activity of two psychoplastogens, ibogainalog (IBG) and ibogaminalog (DM506), was studied in naïve mice using the forced swim test (FST) and tail suspension test (TST). The behavioral results showed that a single administration of 25 mg/kg DM506 or 10 mg/kg IBG induced antidepressant-like activity in naïve mice in a volinanserin-sensitive manner that persisted for 72 h. Similar results were observed using the chronic immobilization stress (CIS) test, in which depression symptoms were reduced for 48 h. To assess the contribution of serotonergic and/or norepinephrinergic neurotransmission, serotonin (5-HT) and norepinephrine (NE) levels were depleted. The reduction in 5-HT levels, but not NE levels, inhibited the antidepressant-like activity of ibogalogs, suggesting that serotonergic transmission may play a more significant role than norepinephrinergic transmission. Concurrently, DM506, IBG, and TBG (derived from tabernanthine) inhibited monoamine transporters with the following order of selectivity: SERT > NE transporter > dopamine transporter. The IBG exhibited the highest selectivity for SERT. Only TBG inhibited monoamine oxidase A activity, indicating its relatively minor role. Radioligand and functional assays showed that all ibogalogs bind to the 5-HT2 receptor subfamily (DM506 > IBG > TBG) and fully activate 5-HT2A/2C receptors with similar potency in the nM range. However, they act as competitive antagonists of the 5-HT2B receptor, with DM506 as an exception, exhibiting partial but potent agonist activity. In conclusion, ibogalogs induce acute and sustained antidepressant-like activity in naïve and depressed mice through mechanisms involving 5-HT2A receptor activation and serotonergic transmission.

The effect of SSRIs on fear learning: a systematic review and meta-analysis

RATIONALE

Selective serotonin reuptake inhibitors (SSRIs) are considered first-line medication for anxiety-like disorders such as panic disorder, generalized anxiety disorder, and post-traumatic stress disorder. Fear learning plays an important role in the development and treatment of these disorders. Yet, the effect of SSRIs on fear learning are not well known.

OBJECTIVE

We aimed to systematically review the effect of six clinically effective SSRIs on acquisition, expression, and extinction of cued and contextual conditioned fear.

METHODS

We searched the Medline and Embase databases, which yielded 128 articles that met the inclusion criteria and reported on 9 human and 275 animal experiments.

RESULTS

Meta-analysis showed that SSRIs significantly reduced contextual fear expression and facilitated extinction learning to cue. Bayesian-regularized meta-regression further suggested that chronic treatment exerts a stronger anxiolytic effect on cued fear expression than acute treatment. Type of SSRI, species, disease-induction model, and type of anxiety test used did not seem to moderate the effect of SSRIs. The number of studies was relatively small, the level of heterogeneity was high, and publication bias has likely occurred which may have resulted in an overestimation of the overall effect sizes.

CONCLUSIONS

This review suggests that the efficacy of SSRIs may be related to their effects on contextual fear expression and extinction to cue, rather than fear acquisition. However, these effects of SSRIs may be due to a more general inhibition of fear-related emotions. Therefore, additional meta-analyses on the effects of SSRIs on unconditioned fear responses may provide further insight into the actions of SSRIs.

The role of serotonin in modulating common waxbill behaviour

Abstract

Serotonin or 5-hydroxytryptamine (5-HT) is a monoaminergic neurotransmitter that is known to influence behaviour in various animal species. Its actions, however, are complex and not well-understood yet. Here, we tested whether and how two 5-HT receptor agonists and a 5-HT receptor antagonist influence behaviour in common waxbills (Estrilda astrild), focusing on aggression, movement and feeding. We applied acute administration of either 8-OH-DPAT (a 5-HT1A receptor agonist), fluoxetine (a selective serotonin reuptake inhibitor; SSRI) or WAY 100,635 (a 5-HT1A receptor antagonist), and then quantified behaviour in the context of competition for food. Waxbills treated with the SSRI fluoxetine showed an overall decrease of aggressive behaviour, activity and feeding, while we found no significant effects of treatment with the other serotonergic enhancer (8-OH-DPAT) or with the antagonist WAY 100,635. Since both 8-OH-DPAT and WAY 100,635 act mainly on 5-HT1A receptor pathways, while fluoxetine more generally affects 5-HT pathways, our results suggest that receptors other than 5-HT1A are important for serotonergic modulation of waxbill behaviour.

Significance statement

The serotonergic system is of interest for current behavioural research due to its influence on a range of behaviours, including aggression, affiliative behaviour, feeding and locomotion in various species. There are, however, numerous discrepancies regarding the behavioural effects of serotonin across studies. We used acute pharmacological manipulations of the serotonergic system in common waxbills, using two serotonin enhancers (8-OH-DPAT and fluoxetine) and a serotonin blocker (WAY 100,635). Behavioural effects of these pharmacological manipulations on aggressiveness, movement and feeding, during tests of competition over food, indicated an anxiogenic-like effect of fluoxetine, but not of 8-OH-DPAT and WAY 100,635. This suggests a distinct role for different serotonergic pathways on waxbill behaviour.

Exercise reduces the anxiogenic effects of meta-chlorophenylpiperazine: The role of 5-HT2C receptors in the bed nucleus of the stria terminalis

Introduction

Two weeks of voluntary exercise in group-housed mice produces a reduction in anxiety-like behaviors across a number of different measures, including a reduction in the anxiety levels typically produced by the anxiogenic serotonergic drug m-chlorophenylpiperazine (mCPP), an agonist at 5-HT2C/2b receptors. We have previously demonstrated that 2-weeks of voluntary exercise blunted the anxiogenic effects of systemic mCPP, and we have also shown that mCPP infused into the bed nucleus of the stria terminalis (BNST) is anxiogenic. Here we follow up on these reports.

Methods

In Experiment 1 we infused several doses of mCPP into the BNST with or without the 5-HT2C antagonist SB242084. In Experiment 2, we administered mCPP into amygdala subregions and the dorsal hippocampus to investigate site specificity. In Experiment 4 we lesioned the BNST and subsequently infused mCPP systemically, and in Experiment 4 we used RNAscope® to assess BNST 5-HT2C transcripts following wheel running.

Results

BNST mCPP infusion increased acoustic startle responding, which was by 5-HT2C antagonism, while neither mCPP infused into the amygdala nor hippocampus was anxiogenic. Lesions of the BNST prevented the anxiogenic effect of systemically administered mCPP. Lastly, exercise reduced 5-HT2C transcripts in the BNST.

Discussion

These results suggest that the BNST is a critical site of action for the effects of exercise on mCPP. Together these data suggest that exercise may reduce 5-HT2C receptor function in the BNST, which may, in part, explain some of the anxiolytic effects associated with wheel running.

Impact of Fluoxetine on Behavioral Invigoration of Appetitive and Aversively Motivated Responses: Interaction With Dopamine Depletion

Impaired behavioral activation and effort-related motivational dysfunctions like fatigue and anergia are debilitating treatment-resistant symptoms of depression. Depressed people show a bias towards the selection of low effort activities. To determine if the broadly used antidepressant fluoxetine can improve behavioral activation and reverse dopamine (DA) depletion-induced anergia, male CD1 mice were evaluated for vigorous escape behaviors in an aversive context (forced swim test, FST), and also with an exercise preference choice task [running wheel (RW)-T-maze choice task]. In the FST, fluoxetine increased active behaviors (swimming, climbing) while reducing passive ones (immobility). However, fluoxetine was not effective at reducing anergia induced by the DA-depleting agent tetrabenazine, further decreasing vigorous climbing and increasing immobility. In the T-maze, fluoxetine alone produced the same pattern of effects as tetrabenazine. Moreover, fluoxetine did not reverse tetrabenazine-induced suppression of RW time but it reduced sucrose intake duration. This pattern of effects produced by fluoxetine in DA-depleted mice was dissimilar from devaluing food reinforcement by pre-feeding or making the food bitter since in both cases sucrose intake time was reduced but animals compensated by increasing time in the RW. Thus, fluoxetine improved escape in an aversive context but decreased relative preference for active reinforcement. Moreover, fluoxetine did not reverse the anergic effects of DA depletion. These results have implications for the use of fluoxetine for treating motivational symptoms such as anergia in depressed patients.

3,4-methylenedioxymethamphetamine (MDMA) impairs the extinction and reconsolidation of fear memory in rats

Clinical trials have demonstrated that 3,4-methylenedioxymethamphetamine (MDMA) paired with psychotherapy is more effective at reducing symptoms of post-traumatic stress disorder (PTSD) than psychotherapy or pharmacotherapy, alone or in combination. The processes through which MDMA acts to enhance psychotherapy are not well understood. Given that fear memories contribute to PTSD symptomology, MDMA could augment psychotherapy by targeting fear memories. The current studies investigated the effects of a single administration of MDMA on extinction and reconsolidation of cued and contextual fear memory in adult, male Long-Evans rats. Rats were exposed to contextual or auditory fear conditioning followed by systemic administration of saline or varying doses of MDMA (between 1 and 10 mg/kg) either 30 min before fear extinction training or immediately after brief fear memory retrieval (i.e. during the reconsolidation phase). MDMA administered prior to fear extinction training failed to enhance fear extinction memory, and in fact impaired drug-free cued fear extinction recall without impacting later fear relapse. MDMA administered during the reconsolidation phase, but not outside of the reconsolidation phase, produced a delayed and persistent reduction in conditioned fear. These findings are consistent with a general memory-disrupting effect of MDMA and suggest that MDMA could augment psychotherapy by modifying fear memories during reconsolidation without necessarily enhancing their extinction.

On the Run for Hippocampal Plasticity

Accumulating research in rodents and humans indicates that exercise benefits brain function and may prevent or delay onset of neurodegenerative conditions. In particular, exercise modifies the structure and function of the hippocampus, a brain area important for learning and memory. This review addresses the central and peripheral mechanisms underlying the beneficial effects of exercise on the hippocampus. We focus on running-induced changes in adult hippocampal neurogenesis, neural circuitry, neurotrophins, synaptic plasticity, neurotransmitters, and vasculature. The role of peripheral factors in hippocampal plasticity is also highlighted. We discuss recent evidence that systemic factors released from peripheral organs such as muscle (myokines), liver (hepatokines), and adipose tissue (adipokines) during exercise contribute to hippocampal neurotrophin and neurogenesis levels, and memory function. A comprehensive understanding of the body-brain axis is needed to elucidate how exercise improves hippocampal plasticity and cognition.

Does exercise augment operant and Pavlovian extinction: A meta-analysis

BACKGROUND

Exposure therapy, a behavioral approach to reduce symptomology in fear, anxiety, and drug-related psychiatric disorders, is based on learning and memory principles of extinction, and is subject to relapse. As such, it is important to find ways to enhance outcomes. One such way is through exercise.

OBJECTIVES

Identify if exercise augments extinction behavior, and whether this depends on the experimental paradigm used (i.e. operant or Pavlovian) and/or stimulus (i.e. appetitive or aversive). Additionally, determine which moderating variables influence the effects of exercise on extinction learning.

METHODS

A literature search was conducted and a Hedges' g calculation was employed to conduct a meta-analysis (metaSEM) using a structural equation modeling approach. This approach was chosen because of its ability to account for dependencies in effect sizes.

RESULTS

We found a significant effect of exercise as an augmentation over extinction alone (g = 0.37, p < 0.001), with extinction paradigm (but not stimulus) producing a moderating effect (B = 0.43, p = 0.030). Data were then split by extinction paradigm, with operant extinction models having a significant effect (g = 0.55, p < 0.001), and number of extinction sessions moderating aggregate effects. Pavlovian models did not have significant overall effects (g = 0.11, p = 0.3976), but were moderated by the number of animals housed together and exercise after extinction.

CONCLUSIONS

The effects of exercise on extinction learning are differentially modulated by the type of paradigm used, the number of extinction sessions, the timing of when exercise treatment was applied (after extinction), and the housing conditions.

Sleep loss and recovery after administration of drugs related to different arousal systems in rats

Sleep is homeostatically regulated suggesting a restorative function. Sleep deprivation is compensated by an increase in length and intensity of sleep. In this study, suppression of sleep was induced pharmacologically by drugs related to different arousal systems. All drugs caused non-rapid eye movement (NREM) sleep loss followed by different compensatory processes. Apomorphine caused a strong suppression of sleep followed by an intense recovery. In the case of fluoxetine and eserine, recovery of NREM sleep was completed by the end of the light phase due to the biphasic pattern demonstrated for these drugs first in the present experiments. Yohimbine caused a long-lasting suppression of NREM sleep, indicating that either the noradrenergic system has the utmost strength among the examined systems, or that restorative functions occurring normally during NREM sleep were not blocked. Arousal systems are involved in the regulation of various wakefulness-related functions, such as locomotion and food intake. Therefore, it can be hypothesized that activation of the different systems results in qualitatively different waking states which might affect subsequent sleep differently. These differences might give some insight into the homeostatic function of sleep in which the dopaminergic and noradrenergic systems may play a more important role than previously suggested.

Central monoaminergic systems are a site of convergence of signals conveying the experience of exercise to brain circuits involved in cognition and emotional behavior

Physical activity can enhance cognitive function and increase resistance against deleterious effects of stress on mental health. Enhanced cognitive function and stress resistance produced by exercise are conserved among vertebrates, suggesting that ubiquitous mechanisms may underlie beneficial effects of exercise. In the current review, we summarize the beneficial effects of exercise on cognitive function and stress resistance and discuss central and peripheral signaling factors that may be critical for conferring the effects of physical activity to brain circuits involved in cognitive function and stress. Additionally, it is suggested that norepinephrine and serotonin, highly conserved monoamines that are sensitive to exercise and able to modulate behavior in multiple species, could represent a convergence between peripheral and central exercise signals that mediate the beneficial effects of exercise. Finally, we offer the novel hypothesis that thermoregulation during exercise could contribute to the emotional effects of exercise by activating a subset of temperature-sensitive serotonergic neurons in the dorsal raphe nucleus that convey anxiolytic and stress-protective signals to forebrain regions. Throughout the review, we discuss limitations to current approaches and offer strategies for future research in exercise neuroscience.

Serotonin 2C receptor antagonist improves fear discrimination and subsequent safety signal recall

The capacity to discriminate between safety and danger is fundamental for survival, but is disrupted in individuals with posttraumatic stress disorder (PTSD). Acute stressors cause a release of serotonin (5-HT) in the forebrain, which is one mechanism for enhanced fear and anxiety; these effects are mediated by the 5-HT2Creceptor. Using a fear discrimination paradigm where a danger signal conditioned stimulus (CS+) co-terminates with a mild footshock and a safety signal (CS-) indicates the absence of shock, we demonstrate that danger/safety discrimination and fear inhibition develop over the course of 4 daily conditioning sessions. Systemic administration of the 5-HT2Creceptor antagonist SB 242084 (0.25 or 1.0mg/kg) prior to conditioning reduced behavioral freezing during conditioning, and improved learning and subsequent inhibition of fear by the safety signal. Discrimination was apparent in the first recall test, and discrimination during training was evident after 3days of conditioning versus 5days in the vehicle treated controls. These results suggest a novel therapeutic use for 5-HT2Creceptor antagonists to improve learning under stressful circumstances. Potential anatomical loci for 5-HT2Creceptor modulation of fear discrimination learning and cognitive performance enhancement are discussed.

ETHICAL STATEMENT

John P. Christianson and Allison R. Foilb, the authors, verify that animal research was carried out in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals (NIH Publications No. 80-23) and all procedures involving animals were reviewed and approved by the Boston College Animal Care and Use Committee. All efforts were made to limit the number of animals used and their suffering.

Galanin mediates features of neural and behavioral stress resilience afforded by exercise

Exercise promotes resilience to stress and increases galanin in the locus coeruleus (LC), but the question of whether changes in galanin signaling mediate the stress-buffering effects of exercise has never been addressed. To test the contributions of galanin to stress resilience, male Sprague Dawley rats received intracerebroventricular (ICV) cannulation for drug delivery and frontocortical cannulation for microdialysis, and were housed with or without a running wheel for 21d. Rats were acutely injected with vehicle or the galanin receptor antagonist M40 and exposed to a single session of either footshock or no stress. Other groups received galanin, the galanin receptor antagonist M40, or vehicle chronically for 21d prior to the stress session. Microdialysis sampling occurred during stress exposure and anxiety-related behavior was measured on the following day in the elevated plus maze. Dendritic spines were visualized by Golgi impregnation in medial prefrontal cortex (mPFC) pyramidal neurons and quantified. Exercise increased galanin levels in the LC. Under non-stressed conditions, anxiety-related behavior and dopamine levels were comparable between exercised and sedentary rats. In contrast, exposure to stress reduced open arm exploration in sedentary rats but not in exercise rats or those treated chronically with ICV galanin, indicating improved resilience. Both exercise and chronic, ICV galanin prevented the increased dopamine overflow and loss of dendritic spines observed after stress in sedentary rats. Chronic, but not acute M40 administration blocked the resilience-promoting effects of exercise. The results indicate that increased galanin levels promote features of resilience at both behavioral and neural levels.

Repeated exposure to conditioned fear stress increases anxiety and delays sleep recovery following exposure to an acute traumatic stressor

Repeated stressor exposure can sensitize physiological responses to novel stressors and facilitate the development of stress-related psychiatric disorders including anxiety. Disruptions in diurnal rhythms of sleep-wake behavior accompany stress-related psychiatric disorders and could contribute to their development. Complex stressors that include fear-eliciting stimuli can be a component of repeated stress experienced by human beings, but whether exposure to repeated fear can prime the development of anxiety and sleep disturbances is unknown. In the current study, adult male F344 rats were exposed to either control conditions or repeated contextual fear conditioning for 22 days followed by exposure to no, mild (10), or severe (100) acute uncontrollable tail shock stress. Exposure to acute stress produced anxiety-like behavior as measured by a reduction in juvenile social exploration and exaggerated shock-elicited freezing in a novel context. Prior exposure to repeated fear enhanced anxiety-like behavior as measured by shock-elicited freezing, but did not alter social exploratory behavior. The potentiation of anxiety produced by prior repeated fear was temporary; exaggerated fear was present 1 day but not 4 days following acute stress. Interestingly, exposure to acute stress reduced rapid eye movement (REM) and non-REM (NREM) sleep during the hours immediately following acute stress. This initial reduction in sleep was followed by robust REM rebound and diurnal rhythm flattening of sleep/wake behavior. Prior repeated fear extended the acute stress-induced REM and NREM sleep loss, impaired REM rebound, and prolonged the flattening of the diurnal rhythm of NREM sleep following acute stressor exposure. These data suggest that impaired recovery of sleep/wake behavior following acute stress could contribute to the mechanisms by which a history of prior repeated stress increases vulnerability to subsequent novel stressors and stress-related disorders.

Local infusion of citalopram into the basolateral amygdala decreased conditioned fear of rats through increasing extracellular serotonin levels

Selective serotonin reuptake inhibitors (SSRIs) are widely used for the treatment of depressive disorders and anxiety disorders. The anxiolytic mechanism of SSRIs is currently unclear. To investigate the anxiolytic effects of SSRIs, we measured both freezing behavior and extracellular serotonin and dopamine levels in the basolateral amygdala when rats were given conditioned fear stress under local reverse-dialysis of citalopram, an SSRI, into the basolateral amygdala. Local administration of citalopram into the basolateral amygdala significantly decreased freezing behavior induced by conditioned fear stress, and serotonin levels were simultaneously found to be significantly higher. Furthermore, repeated conditioned fear stress under local infusion of citalopram into the basolateral amygdala induced further increases in extracellular dopamine levels. Further studies investigating the role of dopamine in the amygdala for conditioned fear stress will be necessary. These results suggest that the basolateral amygdala is one of the target areas of the anxiolytic effects of citalopram and the increases of extracellular serotonin levels in the basolateral amygdala may be related to the anxiolytic effects.

Fibroblast growth factor 8 deficiency compromises the functional response of the serotonergic system to stress

Functionally heterogeneous populations of serotonergic neurons, located within the dorsal raphe nucleus (DR), play a role in stress-related behaviors and neuropsychiatric illnesses such as anxiety and depression. Abnormal development of these neurons may permanently alter their structure and connections, making the organism more susceptible to anxiety-related disorders. A factor that critically regulates the development of serotonergic neurons is fibroblast growth factor 8 (Fgf8). In this study, we used acute restraint stress followed by behavioral testing to examine whether Fgf8 signaling during development is important for establishing functional stress- and anxiety-related DR neurocircuits in adulthood. Wild-type and heterozygous male mice globally hypomorphic for Fgf8 were exposed to acute restraint stress and then tested for anxiety-like behavior on the elevated plus-maze. Further, we measured c-Fos immunostaining as a marker of serotonergic neuronal activation and tissue 5-hydroxyindoleacetic acid concentrations as a marker of serotonin functional output. Results showed that Fgf8 hypomorphs exhibited 1) an exaggerated response of DR anxiety-promoting circuits and 2) a blunted response of a DR panic-inhibiting circuit to stress, effects that together were associated with increased baseline anxiety-like behavior. Overall, our results provide a neural substrate upon which Fgf8 deficiency could affect stress response and support the hypothesis that developmental disruptions of serotonergic neurons affect their postnatal functional integrity.

A bout of voluntary running enhances context conditioned fear, its extinction, and its reconsolidation

Three experiments used rats to examine the effect of a single bout of voluntary activity (wheel running) on the acquisition, extinction, and reconsolidation of context conditioned fear. In Experiment 1, rats provided with access to a wheel for 3 h immediately before or after a shocked exposure to a context froze more when tested in that context than rats provided with access to the wheels 6 h after the shocked exposure or rats not provided with access to the wheels. In Experiment 2, rats provided with access to the wheels immediately before or after a nonshocked exposure to the conditioned context froze less when tested in that context than rats provided with access to the wheels 6 h after the nonshocked exposure or rats not provided with access to the wheels. In Experiment 3, rats provided with access to wheels immediately after an extended nonshocked exposure to the conditioned context again froze less, whereas rats provided with access to the wheels after a brief nonshocked exposure froze more on the subsequent test than sedentary controls. These results show that a single bout of running can enhance acquisition, extinction, and reconsolidation of context conditioned fear.

Exercise offers anxiolytic potential: a role for stress and brain noradrenergic-galaninergic mechanisms

Although physical activity reduces anxiety in humans, the neural basis for this response is unclear. Rodent models are essential to understand the mechanisms that underlie the benefits of exercise. However, it is controversial whether exercise exerts anxiolytic-like potential in rodents. Evidence is reviewed to evaluate the effects of wheel running, an experimental mode of exercise in rodents, on behavior in tests of anxiety and on norepinephrine and galanin systems in neural circuits that regulate stress. Stress is proposed to account for mixed behavioral findings in this literature. Indeed, running promotes an adaptive response to stress and alters anxiety-like behaviors in a manner dependent on stress. Running amplifies galanin expression in noradrenergic locus coeruleus (LC) and suppresses stress-induced activity of the LC and norepinephrine output in LC-target regions. Thus, enhanced galanin-mediated suppression of brain norepinephrine in runners is supported by current literature as a mechanism that may contribute to the stress-protective effects of exercise. These data support the use of rodents to study the emotional and neurobiological consequences of exercise.

5-HT2C receptors in the basolateral amygdala and dorsal striatum are a novel target for the anxiolytic and antidepressant effects of exercise

Physical activity reduces the incidence and severity of psychiatric disorders such as anxiety and depression. Similarly, voluntary wheel running produces anxiolytic- and antidepressant-like effects in rodent models. The specific neurobiological mechanisms underlying the beneficial properties of exercise, however, remain unclear. One relevant pharmacological target in the treatment of psychiatric disorders is the 5-HT(2C) receptor (5-HT(2C)R). Consistent with data demonstrating the anxiogenic consequences of 5-HT(2C)R activation in humans and rodents, we have previously reported that site-specific administration of the selective 5-HT(2C)R agonist CP-809101 in the lateral/basolateral amygdala (BLA) increases shock-elicited fear while administration of CP-809101 in the dorsal striatum (DS) interferes with shuttle box escape learning. These findings suggest that activation of 5-HT(2C)R in discrete brain regions contributes to specific anxiety- and depression-like behaviors and may indicate potential brain sites involved in the anxiolytic and antidepressant effects of exercise. The current studies tested the hypothesis that voluntary wheel running reduces the behavioral consequences of 5-HT(2C)R activation in the BLA and DS, specifically enhanced shock-elicited fear and interference with shuttle box escape learning. After 6 weeks of voluntary wheel running or sedentary conditions, the selective 5-HT(2C)R agonist CP-809101 was microinjected into either the BLA or the DS of adult Fischer 344 rats, and shock-elicited fear and shuttle box escape learning was assessed. Additionally, in-situ hybridization was used to determine if 6 weeks of voluntary exercise changed levels of 5-HT(2C)R mRNA. We found that voluntary wheel running reduced the behavioral effects of CP-809101 and reduced levels of 5-HT(2C)R mRNA in both the BLA and the DS. The current data indicate that expression of 5-HT(2C)R mRNA in discrete brain sites is sensitive to physical activity status of the organism, and implicates the 5-HT(2C)R as a target for the beneficial effects of physical activity on mental health.

5-HT2 ligands in the treatment of anxiety and depression

INTRODUCTION

One third of depressed patients do not respond adequately to conventional antidepressants including the selective serotonin reuptake inhibitors (SSRIs). Therefore, multi-target drugs or augmentation strategies have been developed for the management of SSRIs-resistant patients. In this context, the 5-HT(2) receptor subtypes represent promising targets but their precise roles have yet to be determined.

AREAS COVERED

The aim of this review is to shed some light on the preclinical evidence supporting the use of 5-HT(2A) and/or 5-HT(2C) receptor antagonists such as antipsychotics, as potential effective adjuncts in SSRIs-resistant depression. This review synthesizes the current literature about the behavioral, electrophysiological and neurochemical effects of 5-HT(2) receptors ligands on the monoaminergic systems but also on adult hippocampal neurogenesis.

EXPERT OPINION

Although studies support the hypothesis that the inactivation of 5-HT(2A) and/or 5-HT(2C) receptors might be of interest to reinforce different facets of the therapeutic activity of SSRIs, this pharmacological strategy remains debatable notably because of the lack of chronic data in relevant animal models. Conversely, emerging evidence suggests that the activation of 5-HT(2B) receptor is required for antidepressant-like activity, opening the way to new therapeutic approaches. However, the potential risks related to the enhancement of monoaminergic neurotransmissions could represent a major concern.

Voluntary exercise offers anxiolytic potential and amplifies galanin gene expression in the locus coeruleus of the rat

Although exercise improves anxiety in humans, it is controversial whether exercise is anxiolytic in rodents. We tested the hypothesis that stress influences the effect of exercise on anxiety-like and defensive behaviors. To explore the neurobiological mechanisms of exercise, we also examined whether exercise alters gene expression for the stress-related peptide galanin. Rats were housed in the presence or absence of a running wheel for 21 d. A subset of these rats were (1) not injected or received a single high, dose of the β-carboline FG7142 (inverse agonist at the benzodiazepine receptor site) immediately prior to testing or (2) were injected repeatedly with vehicle or FG7142 during the last 10d of exercise. On day 22, anxiety-like and defensive behaviors were measured in the elevated plus maze, shock probe defensive burying, and defensive withdrawal tests. Locus coeruleus prepro-galanin mRNA was measured by in situ hybridization. Exercise and sedentary rats that were not injected exhibited similar behavior in all tests, whereas FG7142 injected immediately prior to the test battery produced intense avoidance and immobility consistent with an anxiety-like response. However, exercise produced anxiolytic-like and active defensive behaviors in the test battery relative to the sedentary condition in rats injected repeatedly with vehicle or FG7142. Exercise also increased prepro-galanin mRNA in the locus coeruleus relative to sedentary controls. These data suggest that the emergence of enhanced adaptive behavior after chronic voluntary exercise is influenced by stress. Our data support a role for galanin in the beneficial consequences of wheel running.

The protective effects of voluntary exercise against the behavioral consequences of uncontrollable stress persist despite an increase in anxiety following forced cessation of exercise

Humans who exercise are less likely to suffer from stress-related mood disorders. Similarly, rats allowed voluntary access to running wheels have constrained corticosterone responses to mild stressors and are protected against several behavioral consequences of uncontrollable stress which resemble symptoms of human anxiety and depression, including exaggerated fear and deficits in shuttle box escape learning. Although exercise conveys clear stress resistance, the duration of time the protective effects of exercise against the behavioral consequences of uncontrollable stress persist following exercise cessation is unknown. The current studies investigated (1) whether exercise-induced stress resistance extends to social avoidance, another anxiety-like behavior elicited by uncontrollable stressor exposure, and (2) the duration of time the protective effects of exercise persist following forced cessation of exercise. Six weeks of wheel running constrained the increase in corticosterone elicited by social exploration testing, and prevented the reduction in social exploration, exaggerated shock-elicited fear, and deficits in escape learning produced by uncontrollable stress. The protective effect of voluntary exercise against stress-induced interference with escape learning persisted for 15 days, but was lost by 25 days, following cessation of exercise. An anxiogenic effect, as revealed by a reduction in social exploration and an increase in fear behavior immerged as a function of time following cessation of exercise. Results demonstrate that the protective effect of voluntary exercise against the behavioral consequences of uncontrollable stress extends to include social avoidance, and can persist for several days following exercise cessation despite an increase in anxiety produced by forced cessation of exercise.

Serotonin-2C receptors in the basolateral nucleus of the amygdala mediate the anxiogenic effect of acute imipramine and fluoxetine administration

A growing body of evidence indicates that facilitation of serotonin-2C receptor (5-HT2CR)-mediated neurotransmission in the basolateral nucleus of the amygdala (BLA) is involved in anxiety generation. We investigated here whether BLA 5-HT2CRs exert a differential role in the regulation of defensive behaviours related to generalized anxiety (inhibitory avoidance) and panic (escape) disorders. We also evaluated whether activation of BLA 5-HT2CRs accounts for the anxiogenic effect caused by acute systemic administration of the antidepressants imipramine and fluoxetine. Male Wistar rats were tested in the elevated T-maze after intra-BLA injection of the endogenous agonist 5-HT, the 5-HT2CR agonist MK-212 or the 5-HT2CR antagonist SB-242084. This test allows the measurement of inhibitory avoidance acquisition and escape expression. We also investigated whether intra-BLA administration of SB-242084 interferes with the acute anxiogenic effect caused by imipramine and fluoxetine in the Vogel conflict test, and imipramine in the elevated T-maze. While intra-BLA administration of 5-HT and MK-212 facilitated inhibitory avoidance acquisition, suggesting an anxiogenic effect, SB-242084 had the opposite effect. None of these drugs affected escape performance. Intra-BLA injection of a sub-effective dose of SB-242084 fully blocked the anxiogenic effect caused either by the local microinjection of 5-HT or the systemic administration of imipramine and fluoxetine. Our findings indicate that 5-HT2CRs in BLA are selectively involved in the regulation of defensive behaviours associated with generalized anxiety, but not panic. The results also provide the first direct evidence that activation of BLA 5-HT2CRs accounts for the short-term aversive effect of antidepressants.

Anxiety- and depression-like behaviors are accompanied by an increase in oxidative stress in a rat model of fetal alcohol spectrum disorders: Protective effects of voluntary physical exercise

Prenatal ethanol exposure can damage the developing nervous system, producing long-lasting impairments in both brain structure and function. In this study we analyzed how exposure to this teratogen during the period of brain development affects the intracellular redox state in the brain as well as the development of anxiety- and depressive-like phenotypes. Furthermore, we also tested whether aerobic exercise might have therapeutic potential for fetal alcohol spectrum disorders (FASD) by increasing neuronal antioxidant capacity and/or by alleviating ethanol-induced behavioral deficits. Sprague-Dawley rats were administered ethanol across all three-trimester equivalents (i.e., throughout gestation and during the first 10 days of postnatal life). Ethanol-exposed and control animals were assigned to either sedentary or running groups at postnatal day (PND) 48. Runners had free access to a running wheel for 12 days and at PND 60 anxiety- and depressive-like behaviors were assessed. Perinatal ethanol exposure resulted in the occurrence of depressive and anxiety-like behaviors in adult rats without affecting their locomotor activity. Voluntary wheel running reversed the depressive-like behaviors in ethanol-exposed males, but not in ethanol-exposed females. Levels of lipid peroxidation and protein oxidation were significantly increased in the hippocampus and cerebellum of ethanol-exposed rats, and there was a concomitant reduction in the levels of the endogenous antioxidant glutathione. Voluntary exercise was able to reverse the deficits in glutathione both in ethanol-exposed males and females. Thus, while voluntary physical exercise increased glutathione levels in both sexes, its effects at the behavioral level were sex dependent, with only ethanol-exposed male runners showing a decrease in depressive-like behaviors.

Exercise, stress resistance, and central serotonergic systems

Voluntary exercise reduces the incidence of stress-related psychiatric disorders in humans and prevents serotonin-dependent behavioral consequences of stress in rodents. Evidence reviewed herein is consistent with the hypothesis that exercise increases stress resistance by producing neuroplasticity at multiple sites of the central serotonergic system, which all help to limit the behavioral impact of acute increases in serotonin during stressor exposure.

Uncontrollable, but not controllable, stress desensitizes 5-HT1A receptors in the dorsal raphe nucleus

Uncontrollable stressors produce behavioral changes that do not occur if the organism can exercise behavioral control over the stressor. Previous studies suggest that the behavioral consequences of uncontrollable stress depend on hypersensitivity of serotonergic neurons in the dorsal raphe nucleus (DRN), but the mechanisms involved have not been determined. We used ex vivo single-unit recording in rats to test the hypothesis that the effects of uncontrollable stress are produced by desensitization of DRN 5-HT(1A) autoreceptors. These studies revealed that uncontrollable, but not controllable, tail shock impaired 5-HT(1A) receptor-mediated inhibition of DRN neuronal firing. Moreover, this effect was observed only at time points when the behavioral effects of uncontrollable stress are present. Furthermore, temporary inactivation of the medial prefrontal cortex with the GABA(A) receptor agonist muscimol, which eliminates the protective effects of control on behavior, led even controllable stress to now produce functional desensitization of DRN 5-HT(1A) receptors. Additionally, behavioral immunization, an experience with controllable stress before uncontrollable stress that prevents the behavioral outcomes of uncontrollable stress, also blocked functional desensitization of DRN 5-HT(1A) receptors by uncontrollable stress. Last, Western blot analysis revealed that uncontrollable stress leads to desensitization rather than downregulation of DRN 5-HT(1A) receptors. Thus, treatments that prevent controllable stress from being protective led to desensitization of 5-HT(1A) receptors, whereas treatments that block the behavioral effects of uncontrollable stress also blocked 5-HT(1A) receptor desensitization. These data suggest that uncontrollable stressors produce a desensitization of DRN 5-HT(1A) autoreceptors and that this desensitization is responsible for the behavioral consequences of uncontrollable stress.

5-hydroxytryptamine 2C receptors in the dorsal striatum mediate stress-induced interference with negatively reinforced instrumental escape behavior

Uncontrollable stress can interfere with instrumental learning and induce anxiety in humans and rodents. While evidence supports a role for serotonin (5-HT) and serotonin 2C receptors (5-HT(2C)R) in the behavioral consequences of uncontrollable stress, the specific sites of action are unknown. These experiments sought to delineate the role of 5-HT and 5-HT(2C)R in the dorsal striatum (DS) and the lateral/basolateral amygdala (BLA) in the expression of stress-induced instrumental escape deficits and exaggerated fear, as these structures are critical to instrumental learning and fear behaviors. Using in vivo microdialysis, we first demonstrated that prior uncontrollable, but not controllable, stress sensitizes extracellular 5-HT in the dorsal striatum, a result that parallels prior work in the BLA. Additionally, rats were implanted with bi-lateral cannula in either the DS or the BLA and exposed to uncontrollable tail shock stress. One day later, rats were injected with 5-HT(2C)R antagonist (SB242084) and fear and instrumental learning behaviors were assessed in a shuttle box. Separately, groups of non-stressed rats received an intra-DS or an intra-BLA injection of the 5-HT(2C)R agonist (CP809101) and behavior was observed. Intra-DS injections of the 5-HT(2C)R antagonist prior to fear/escape tests completely blocked the stress-induced interference with instrumental escape learning; a partial block was observed when injections were in the BLA. Antagonist administration in either region did not influence stress-induced fear behavior. In the absence of prior stress, intra-DS administration of the 5-HT(2C)R agonist was sufficient to interfere with escape behavior without enhancing fear, while intra-BLA administration of the 5-HT(2C)R agonist increased fear behavior but had no effect on escape learning. Results reveal a novel role of the 5-HT(2C)R in the DS in the expression of instrumental escape deficits produced by uncontrollable stress and demonstrate that the involvement of 5-HT(2C)R activation in stress-induced behaviors is regionally specific.

Anxiety profile in morphine-dependent and withdrawn rats: effect of voluntary exercise

Withdrawal from chronic opiates is associated with an increase in anxiogenic-like behaviours, but the anxiety profile in the morphine-dependent animals is not clear. Thus, one of the aims of the present study was to examine whether morphine-dependent rats would increase the expression of anxiogenic-like behaviours in novel and stressful conditions. Additionally, recent studies have shown that voluntary exercise can reduce anxiety levels in rodents. Therefore, another aim of this study was to examine the effect of voluntary exercise on the anxiety profile in both morphine-dependent animals and animals experiencing withdrawal. Rats were injected with bi-daily doses (10 mg/kg, at 12 h intervals) of morphine over a period of 10 days in which they were also allowed voluntary exercise. Following these injections, anxiety-like behaviours were tested in the elevated plus-maze (EPM) model and the light/dark (L/D) box. We found reductions in time spent in, and entries into, the EPM open arms and reductions in time spent in the lit side of the L/D box for both sedentary morphine-dependent and withdrawn rats as compared to the sedentary control groups. The exercising morphine-dependent and withdrawn rats exhibited an increase in EPM open arm time and entries and L/D box lit side time as compared with the sedentary control groups. We conclude that voluntary exercise decreases the severity of the anxiogenic-like behaviours in both morphine-dependent and withdrawn rats. Thus, voluntary exercise could be a potential natural method to ameliorate some of the deleterious behavioural consequences of opiate abuse.

A randomized, double-blind study of once-daily extended release quetiapine fumarate (quetiapine XR) monotherapy in patients with generalized anxiety disorder

This study evaluated once-daily, extended-release quetiapine fumarate (quetiapine XR) monotherapy in generalized anxiety disorder (GAD). This was a 10-week (8-week active treatment/2-week posttreatment drug-discontinuation/tapering phase), double-blind, randomized, placebo-controlled study (D1448C00009). Primary end point was change from randomization at week 8 in Hamilton Anxiety Rating Scale (HAM-A) total score. Overall, 951 patients with GAD were randomized (quetiapine XR: 50 mg/d, n = 234; 150 mg/d, n = 241; 300 mg/d, n = 241; placebo, n = 235). At week 8, HAM-A total scores significantly (P < 0.001) improved versus placebo (-11.10) with quetiapine XR 50 mg/d (-13.31) and 150 mg/d (-13.54), but not 300 mg/d (-11.87; P = 0.240). At week 1, HAM-A total scores significantly improved versus placebo (-5.94) with quetiapine XR 50 mg/d (-7.47; P < 0.01), 150 mg/d (-8.19; P < 0.001), and 300 mg/d (-7.23; P < 0.01). Versus placebo at week 8, quetiapine XR 50 and 150 mg/d significantly improved HAM-A psychic (P < 0.01 and P < 0.001, respectively) and somatic (P < 0.001; P < 0.01, respectively) cluster scores, HAM-A response (≥ 50% total score reduction; P < 0.05), and Clinical Global Impression-Improvement categorical changes (P < 0.05). For quetiapine XR 150 mg/d, significant (P < 0.05) improvements were seen for HAM-A remission (total score, ≤ 7) and Clinical Global Impression-Severity of Illness scores. For quetiapine XR 300 mg/d, improvements in these secondary variables were not significantly different versus placebo. Pittsburgh Sleep Quality Index global scores improved with all 3 doses (quetiapine: XR 50 mg/d, -4.07 [P < 0.05]; 150 mg/d, -4.38 [P < 0.05]; 300 mg/d, -3.97 [P < 0.05], versus -3.31 with placebo). Adverse events (>10% with quetiapine XR) were dry mouth, somnolence, sedation, dizziness, headache, and fatigue. Quetiapine XR (50/150 mg/d) monotherapy was effective at week 8 in patients with GAD; symptom improvement was seen at week 1 for all doses (50/150/300 mg/d). Safety and tolerability were consistent with the known profile of quetiapine.

Pharmacological blockade of 5-HT7 receptors as a putative fast acting antidepressant strategy

Current antidepressants still display unsatisfactory efficacy and a delayed onset of therapeutic action. Here we show that the pharmacological blockade of serotonin 7 (5-HT(7)) receptors produced a faster antidepressant-like response than the commonly prescribed antidepressant fluoxetine. In the rat, the selective 5-HT(7) receptor antagonist SB-269970 counteracted the anxiogenic-like effect of fluoxetine in the open field and exerted an antidepressant-like effect in the forced swim test. In vivo, 5-HT(7) receptors negatively regulate the firing activity of dorsal raphe 5-HT neurons and become desensitized after long-term administration of fluoxetine. In contrast with fluoxetine, a 1-week treatment with SB-269970 did not alter 5-HT firing activity but desensitized cell body 5-HT autoreceptors, enhanced the hippocampal cell proliferation, and counteracted the depressive-like behavior in olfactory bulbectomized rats. Finally, unlike fluoxetine, early-life administration of SB-269970, did not induce anxious/depressive-like behaviors in adulthood. Together, these findings indicate that the 5-HT(7) receptor antagonists may represent a new class of antidepressants with faster therapeutic action.

Mental Health Benefits of Strength Training in Adults

This review summarizes evidence from randomized controlled trials to examine whether strength training influences anxiety, chronic pain, cognition, depression, fatigue symptoms, self-esteem, and sleep. The weight of the available evidence supported the conclusion that strength training is associated with reductions in anxiety symptoms among healthy adults (5 trials); reductions in pain intensity among patients with low back pain (5 trials), osteoarthritis (8 trials), and fibromyalgia (4 trials); improvements in cognition among older adults (7 trials); improvements in sleep quality among depressed older adults (2 trials); reductions in symptoms of depression among patients with diagnosed depression (4 trials) and fibromyalgia (2 trials); reductions in fatigue symptoms (10 trials); and improvements in self-esteem (6 trials). The evidence indicates that larger trials with a greater range of patient samples are needed to better estimate the magnitude and the consistency of the relationship between strength training and these mental health outcomes. Plausible social, psychological, and neural mechanisms by which strength training could influence these outcomes rarely have been explored. This review revealed the high-priority research need for animal and human research aimed at better understanding the brain mechanisms underlying mental health changes with strength training.

Rodent models in depression research: classical strategies and new directions

Depression, among other mood disorders, represents one of the most common health problems worldwide, with steadily increasing incidence and major socio-economic consequences. However, since the knowledge about the underlying pathophysiological principles is still very scanty, depression and other mood disorders are currently diagnosed solely on clinical grounds. Currently used treatment modalities would therefore benefit enormously from the development of alternative therapeutic interventions. The implementation of proper animal models is a prerequisite for increasing the understanding of the neurobiological basis of mood disorders and is paving the way for the discovery of novel therapeutic targets. In the past thirty years, since the seminal description of the Forced Swim Test as a system to probe antidepressant activity in rodents, the use of animals to model depression and antidepressant activity has come a long way. In this review we describe some of the most commonly used strategies, ranging from screening procedures, such as the Forced Swim Test and the Tail Suspension Test and animal models, such as those based upon chronic stress procedures, to genetic approaches. Finally we also discuss some of the inherent limitations and caveats that need to be considered when using animals as models for mental disorders in basic research.

5-hydroxytryptamine 2C receptors in the basolateral amygdala are involved in the expression of anxiety after uncontrollable traumatic stress

BACKGROUND

Exposure to uncontrollable stressors often increases anxiety-like behavior in both humans and rodents. In rat, this effect depends on stress-induced activity within the dorsal raphe nucleus (DRN). However, the role of serotonin in DRN projection regions is largely unknown. The goals of this study were to 1) assess the effect of uncontrollable stress on extracellular serotonin in the basolateral amygdala during the anxiety test, 2) determine whether DRN activity during a poststress anxiety test is involved in anxiety-like behavior, and 3) determine the role of the serotonin 2C receptor (5-HT(2C)) in uncontrollable stress-induced anxiety.

METHOD

Rats were exposed to tail shocks that were either controllable or uncontrollable. On the following day, anxiety-like behavior was assessed in a Juvenile Social Exploration (JSE) test. Basolateral amygdala (BLA) extracellular serotonin concentrations were assessed during JSE by in vivo microdialysis 24 hours after uncontrollable stress, controllable stress, or no stress. In separate experiments, drugs were administered before the JSE test to inhibit the DRN or to block 5-HT(2C) receptors.

RESULTS

Exposure to uncontrollable shock reduced later social exploration. Prior uncontrollable stress potentiated serotonin efflux in the BLA during social exploration, but controllable stress did not. Intra-DRN 8-OH-DPAT and systemic and intra-BLA 5-HT(2C) receptor antagonist SB 242,084 prevented the expression of potentiated anxiety in uncontrollably stressed rats. Intra-BLA injection of the 5-HT(2C) agonist CP 809,101 mimicked the effect of stress.

CONCLUSIONS

These results suggest that the anxiety-like behavior observed after uncontrollable stress is mediated by exaggerated 5-HT acting at BLA 5-HT(2C) receptors.

Subchronic SSRI administration reduces insula response during affective anticipation in healthy volunteers

The anterior cingulate cortex (ACC) and insula are important neural substrates for the integration of cognitive, emotional, and physiological information, as well as the coordination of responses to anticipated stimuli. Increased neural activation within these structures has been observed in individuals with anxiety and depressive disorders. Selective serotonin reuptake inhibitors (SSRIs) are among the most effective and frequently prescribed anxiolytic agents, yet it is not known whether ACC or insula underlie the effects of these drugs. We examined whether subchronic administration of a SSRI to healthy volunteers attenuates activation in ACC or insula during anticipation, an important emotional process underlying anxiety. Support for this hypothesis would help to understand where and by what process SSRIs may exert beneficial effects as anxiolytics and would provide further mechanistic evidence for functional magnetic resonance imaging (fMRI) as a biomarker for the development of anxiolytics. Fifteen volunteers participated in a double-blind, placebo-controlled, randomized cross-over study. Participants completed a pleasant and aversive picture-cued anticipation task during fMRI after taking either escitalopram (10 mg) or placebo for 21 d. We found that escitalopram significantly decreased activation in bilateral posterior and middle insula during the anticipation condition irrespective of stimulus valence and in medial prefrontal and ACC during anticipation of aversive vs. pleasant images. Reduced insular and ACC activation in healthy controls during anticipation may be integral to the therapeutic efficacy of SSRIs and may provide a mechanistic approach for the use of pharmaco-fMRI in the identification of novel pharmacotherapeutic agents in patient populations.

Effects of chronic intracerebroventricular 3,4-methylenedioxy-N-methamphetamine (MDMA) or fluoxetine on the active avoidance test in rats with or without exposure to mild chronic stress

In despite the similarity of mechanisms of action between both selective serotonin reuptake inhibitors (SSRI) and MDMA (main compound of "Ecstasy") there are relatively few reports on the effects of the later on animal models of depression. There are many animal models designed to create or to assess depression. Mild chronic stress (MCS) is a procedure designed to create depression. MCS includes the chronic exposure of the animal to several stressors. After that, rats show behavioural changes associated to depression. In the other hand, the active avoidance task (AAT) is an experimental situation in which an animal has to accomplish a particular behaviour in order to avoid the application of a stressor. Animals exhibiting depression fail to acquire avoidance responses as rapidly as normal animals do. In order to assess the effect of MDMA on the acquisition of an active avoidance response, forty-five rats were divided in two groups exposed or not exposed to MCS. Rats also received chronic intracerebroventricular MDMA (0.2microg/microl; 1microl), fluoxetine (2.0microg/microl; 1microl) or saline solution (0.9%; 1microl). Our results showed that the effect of MDMA depends upon the level of stress. MDMA treated animals showed better acquisition (F([2,37])=7.046; P=0.003) and retention (F([2,37])=3.900; P=0.029) of the avoidance response than fluoxetine or saline treated animals when exposed to MCS. This finding suggests that MDMA (and no fluoxetine) was able to change the aversive valence of the stressors maybe enhancing coping strategies. This effect could serve as a protective factor against helplessness and maybe post-traumatic stress.

The effects of the selective 5-HT(2C) receptor antagonist SB 242084 on learned helplessness in male Fischer 344 rats

RATIONALE

Rats exposed to an uncontrollable stressor demonstrate a constellation of behaviors such as exaggerated freezing and deficits in shuttle box escape learning. These behaviors in rats have been called learned helplessness and have been argued to model human stress-related mood disorders. Learned helplessness is thought to be caused by hyperactivation of serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) and a subsequent exaggerated release of 5-HT in DRN projection sites. Blocking 5-HT(2C) receptors in the face of an increase in serotonin can alleviate anxiety behaviors in some animal models. However, specific 5-HT receptor subtypes involved in learned helplessness remain unknown.

OBJECTIVES

The current experiments tested the hypothesis that 5-HT(2C) receptor activation is necessary and sufficient for the expression of learned helplessness.

RESULTS

The selective 5-HT(2C) receptor antagonist SB 242084 (1.0 mg/kg) administered i.p. to adult male Fischer 344 rats prior to shuttle box behavioral testing, but not before stress, blocked stress-induced deficits in escape learning but had no effect on the exaggerated shock-elicited freezing. The selective 5-HT(2C) receptor agonist CP-809101 was sufficient to produce learned helplessness-like behaviors in the absence of prior stress and these effects were blocked by pretreatment with SB 242084.

CONCLUSIONS

Results implicate the 5-HT(2C) receptor subtype in mediating the shuttle box escape deficits produced by exposure to uncontrollable stress and suggest that different postsynaptic 5-HT receptor subtypes underlie the different learned helplessness behaviors.