Aggressive encounters alter the activation of serotonergic neurons and the expression of 5-HT1A mRNA in the hamster dorsal raphe nucleus

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. Hippocampal area CA2 is known to play a key role in these behaviors in mice and responds to social stimuli in rats, but CA2 has yet to be characterized in hamsters or voles, which are also used in studies of social behaviors. Here, we used immunofluorescence to determine whether CA2 could be molecularly identified in tissue from voles and hamsters. We found that  staining for many CA2 markers was similar in these three species, with labeling seen in neurons at the distal end of the mossy fibers . In contrast, although perineuronal nets (PNNs) surround CA2 cells in mice, PNN staining differed across species. In voles, both CA2 and CA3 were labeled, whereas in hamsters, labeling was seen primarily in CA3. These results demonstrate that CA2 can be molecularly distinguished from neighboring CA1 and CA3 areas in voles and hamsters with several antibodies commonly used in mice. However, PNN staining is not useful for identifying CA2 in voles or hamsters, suggestive of differing roles for either PNNs or for the hippocampal subregions in social behavior. These findings reveal commonalities across species in the molecular profile of CA2 and should facilitate future studies of CA2 in these species.

Identification of hippocampal area CA2 in hamster and vole brain

Prairie voles (Microtus ochrogaster) and Syrian, or golden, hamsters (Mesocricetus auratus) are closely related to mice (Mus musculus) and rats (Rattus norvegicus, for example) and are commonly used in studies of social behavior including social interaction, social memory, and aggression. The CA2 region of the hippocampus is known to play a key role in social memory and aggression in mice and responds to social stimuli in rats, likely owing to its high expression of oxytocin and vasopressin 1b receptors. However, CA2 has yet to be identified and characterized in hamsters or voles. In this study, we sought to determine whether CA2 could be identified molecularly in vole and hamster. To do this, we used immunofluorescence with primary antibodies raised against known molecular markers of CA2 in mice and rats to stain hippocampal sections from voles and hamsters in parallel with those from mice. Here, we report that, like in mouse and rat, staining for many CA2 proteins in vole and hamster hippocampus reveals a population of neurons that express regulator of G protein signaling 14 (RGS14), Purkinje cell protein 4 (PCP4) and striatal-enriched protein tyrosine phosphatase (STEP), which together delineate the borders with CA3 and CA1. These cells were located at the distal end of the mossy fiber projections, marked by the presence of Zinc Transporter 3 (ZnT-3) and calbindin in all three species. In addition to staining the mossy fibers, calbindin also labeled a layer of CA1 pyramidal cells in mouse and hamster but not in vole. However, Wolframin ER transmembrane glycoprotein (WFS1) immunofluorescence, which marks all CA1 neurons, was present in all three species and abutted the distal end of CA2, marked by RGS14 immunofluorescence. Staining for two stress hormone receptors-the glucocorticoid (GR) and mineralocorticoid (MR) receptors-was also similar in all three species, with GR staining found primarily in CA1 and MR staining enriched in CA2. Interestingly, although perineuronal nets (PNNs) are known to surround CA2 cells in mouse and rat, we found that staining for PNNs differed across species in that both CA2 and CA3 showed staining in voles and primarily CA3 in hamsters with only some neurons in proximal CA2 showing staining. These results demonstrate that, like in mouse, CA2 in voles and hamsters can be molecularly distinguished from neighboring CA1 and CA3 areas, but PNN staining is less useful for identifying CA2 in the latter two species. These findings reveal commonalities across species in molecular profile of CA2, which will facilitate future studies of CA2 in these species. Yet to be determined is how differences in PNNs might relate to differences in social behavior across species.

Prior experience with behavioral control over stress facilitates social dominance

Dominance status has extensive effects on physical and mental health, and an individual's relative position can be shaped by experiential factors. A variety of considerations suggest that the experience of behavioral control over stressors should produce winning in dominance tests and that winning should blunt the impact of later stressors, as does prior control. To investigate the interplay between competitive success and stressor control, we first examined the impact of stressor controllability on subsequent performance in a warm spot competition test modified for rats. Prior experience of controllable, but not physically identical uncontrollable, stress increased later effortful behavior and occupation of the warm spot. Controllable stress subjects consistently ranked higher than did uncontrollable stress subjects. Pharmacological inactivation of the prelimbic (PL) cortex during behavioral control prevented later facilitation of dominance. Next, we explored whether repeated winning experiences produced later resistance against the typical sequelae of uncontrollable stress. To establish dominance status, triads of rats were given five sessions of warm spot competition. The development of stable dominance was prevented by reversible inactivation of the PL or NMDA receptor blockade in the dorsomedial striatum. Stable winning blunted the later stress-induced increase in dorsal raphe nucleus serotonergic activity, as well as prevented uncontrollable stress-induced social avoidance. In contrast, endocrine and neuroimmune responses to uncontrollable stress were unaffected, indicating a selective impact of prior dominance. Together, these data demonstrate that instrumental control over stress promotes later dominance, but also reveal that winning experiences buffer against the neural and behavioral outcomes of future adversity.

Prior experience with behavioral control over stress facilitates social dominance

Dominance status has extensive effects on physical and mental health, and an individual's relative position can be shaped by experiential factors. A variety of considerations suggest that the experience of behavioral control over stressors should produce winning in dominance tests and that winning should blunt the impact of later stressors, as does prior control. To investigate the interplay between competitive success and stressor control, we first examined the impact of stressor controllability on subsequent performance in a warm spot competition test modified for rats. Prior experience of controllable, but not physically identical uncontrollable, stress increased later effortful behavior and occupation of the warm spot. Controllable stress subjects consistently ranked higher than did uncontrollable stress subjects. Pharmacological inactivation of the prelimbic (PL) cortex during behavioral control prevented later facilitation of dominance. Next, we explored whether repeated winning experiences produced later resistance against the typical sequelae of uncontrollable stress. To establish dominance status, triads of rats were given five sessions of warm spot competition. Reversible inactivation of the PL or NMDA receptor blockade in the dorsomedial striatum led to a long-term reduction in social rank. Stable dominance blunted the later stress-induced increase in dorsal raphe nucleus serotonergic activity, as well as prevented stress-induced social avoidance. In contrast, endocrine and neuroimmune responses to uncontrollable stress were unaffected, indicating a selective impact of prior dominance. Together, these data demonstrate that instrumental control over stress promotes later dominance, but also reveal that winning experiences buffer against the neural and behavioral outcomes of future adversity.

Chronic Social Defeat During Adolescence Induces Short- and Long-Term Behavioral and Neuroendocrine Effects in Male Swiss-Webster Mice

Chronic stress exposure during adolescence is a significant risk factor for the development of depression. Chronic social defeat (CSD) in rodents is an animal model of depression with excellent ethological, predictive, discriminative, and face validity. Because the CSD model has not been thoroughly examined as a model of stress-induced depression within the adolescence stage, the present study analyzed the short- and long-term behavioral and neuroendocrine effects of CSD during early adolescence. Therefore, adolescent male Swiss-Webster (SW) mice were exposed to the CSD model from postnatal day (PND) 28 to PND37. Twenty-four hours (mid-adolescence) or 4 weeks (early adulthood) later, mice were tested in two models of depression; the social interaction test (SIT) and forced swimming test (FST); cognitive deficits were evaluated in the Barnes maze (BM). Finally, corticosterone and testosterone content was measured before, during, and after CSD exposure, and serotonin transporter (SERT) autoradiography was studied after CSD in adolescent and adult mice. CSD during early adolescence induced enduring depression-like behaviors as inferred from increased social avoidance and immobility behavior in the SIT and FST, respectively, which correlated in an age-dependent manner with SERT binding in the hippocampus; CSD during early adolescence also induced long-lasting learning and memory impairments in the Barnes maze (BM). Finally, CSD during early adolescence increased serum corticosterone levels in mid-adolescence and early adulthood and delayed the expected increase in serum testosterone levels observed at this age. In conclusion: (1) CSD during early adolescence induced long-lasting depression-like behaviors, (2) sensitivity of SERT density during normal brain development was revealed, (3) CSD during early adolescence induced enduring cognitive deficits, and (4) results highlight the vulnerability of the adolescent brain to social stressors on the adrenal and gonadal axes, which emphasizes the importance of an adequate interaction between both axes during adolescence for normal development of brain and behavior.

Estrogen Withdrawal Increases Postpartum Anxiety via Oxytocin Plasticity in the Paraventricular Hypothalamus and Dorsal Raphe Nucleus

BACKGROUND

Estrogen increases dramatically during pregnancy but quickly drops below prepregnancy levels at birth and remains suppressed during the postpartum period. Clinical and rodent work suggests that this postpartum drop in estrogen results in an estrogen withdrawal state that is related to changes in affect, mood, and behavior. How estrogen withdrawal affects oxytocin (OT) neurocircuitry has not been examined.

METHODS

We used a hormone-simulated pseudopregnancy followed by estrogen withdrawal in Syrian hamsters, a first for this species. Ovariectomized females were given daily injections to approximate hormone levels during gestation and then withdrawn from estrogen to simulate postpartum estrogen withdrawal. These hamsters were tested for behavioral assays of anxiety and anhedonia during estrogen withdrawal. Neuroplasticity in OT-producing neurons in the paraventricular nucleus of the hypothalamus and its efferent targets was measured.

RESULTS

Estrogen-withdrawn females had increased anxiety-like behaviors in the elevated plus maze and open field tests but did not differ from control females in sucrose preference. Furthermore, estrogen-withdrawn females had more OT-immunoreactive cells and OT messenger RNA in the paraventricular nucleus of the hypothalamus and an increase in OT receptor density in the dorsal raphe nucleus. Finally, blocking OT receptors in the dorsal raphe nucleus during estrogen withdrawal prevented the high-anxiety behavioral phenotype in estrogen-withdrawn females.

CONCLUSIONS

Estrogen withdrawal induces OT neuroplasticity in the paraventricular nucleus of the hypothalamus and dorsal raphe nucleus to increase anxiety-like behavior during the postpartum period. More broadly, these experiments suggest Syrian hamsters as a novel organism in which to model the effects of postpartum estrogen withdrawal on the brain and anxiety-like behavior.

Evolution of stress responses refine mechanisms of social rank

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Expression of Repressor Element 1 Silencing Transcription Factor (REST) in Serotonin Neurons in the Adult Male Nile Tilapia (Oreochromis niloticus)

Repressor element-1 silencing transcription factor (REST) is highly expressed in the dorsal raphe where serotonin (5-hydroxytryptamine, 5-HT) neurons are located. REST works as a transcription factor for the 5-HT receptor and tryptophan hydroxylase two-gene expression. We hypothesized that REST is co-expressed in 5-HT neurons, which, if demonstrated, would be useful to understand the mechanism of 5-HT dysfunction-related disorders such as negative emotions and depression. Therefore, the present study was designed to examine the expression of the REST gene in the brain (forebrain, midbrain, and hindbrain) of adult male Nile tilapia (Oreochromis niloticus) using rt-PCR. Besides, using immunocytochemistry, co-localization of the REST gene was examined in 5-HT neurons and with neuronal-/glial-cell markers. We found a high expression of the REST gene in the midbrain region of the dorsal raphe, an area of 5-HT neurons. Double-label immunocytochemistry showed neuron-specific expression of REST co-localized in 5-HT neurons in the dorsal and ventral parts of the periventricular pretectal nucleus, paraventricular organ, and dorsal and medial raphe nucleus. Since midbrain 5-HT neurons express REST, we speculate that REST may control 5-HT neuronal activity related to negative emotions, including depression.

Oxytocin Signaling as a Target to Block Social Defeat-Induced Increases in Drug Abuse Reward

There is huge scientific interest in the neuropeptide oxytocin (OXT) due to its putative capacity to modulate a wide spectrum of physiological and cognitive processes including motivation, learning, emotion, and the stress response. The present review seeks to increase the understanding of the role of OXT in an individual’s vulnerability or resilience with regard to developing a substance use disorder. It places specific attention on the role of social stress as a risk factor of addiction, and explores the hypothesis that OXT constitutes a homeostatic response to stress that buffers against its negative impact. For this purpose, the review summarizes preclinical and clinical literature regarding the effects of OXT in different stages of the addiction cycle. The current literature affirms that a well-functioning oxytocinergic system has protective effects such as the modulation of the initial response to drugs of abuse, the attenuation of the development of dependence, the blunting of drug reinstatement and a general anti-stress effect. However, this system is dysregulated if there is continuous drug use or chronic exposure to stress. In this context, OXT is emerging as a promising pharmacotherapy to restore its natural beneficial effects in the organism and to help rebalance the functions of the addicted brain.

Sex-dependent effects of social status on the regulation of arginine-vasopressin (AVP) V1a, oxytocin (OT), and serotonin (5-HT) 1A receptor binding and aggression in Syrian hamsters (Mesocricetus auratus)

Dominance status in hamsters is driven by interactions between arginine-vasopressin V1a, oxytocin (OT), and serotonin 1A (5-HT1A) receptors. Activation of V1a and OT receptors in the anterior hypothalamus (AH) increases aggression in males, while decreasing aggression in females. In contrast, activation of 5-HT1A receptors in the AH decreases aggression in males and increases aggression in females. The mechanism underlying these differences is not known. The purpose of this study was to determine if dominance status and sex interact to regulate V1a, OT, and 5-HT1A receptor binding. Same-sex hamsters (N = 47) were paired 12 times across six days in five min sessions. Brains from paired and unpaired (non-social control) hamsters were collected immediately after the last interaction and processed for receptor binding using autoradiography. Differences in V1a, OT, and 5-HT1A receptor binding densities were observed in several brain regions as a function of social status and sex. For example, in the AH, there was an interaction between sex and social status, such that V1a binding in subordinate males was lower than in subordinate females and V1a receptor density in dominant males was higher than in dominant females. There was also an interaction in 5-HT1A receptor binding, such that social pairing increased 5-HT1A binding in the AH of males but decreased 5-HT1A binding in females compared with unpaired controls. These results indicate that dominance status and sex play important roles in shaping the binding profiles of key receptor subtypes across the neural circuitry that regulates social behavior.

Activity of a vmPFC-DRN Pathway Corresponds With Resistance to Acute Social Defeat Stress

The ventromedial prefrontal cortex (vmPFC) plays a critical role in stress resilience through top-down inhibition of key stress-sensitive limbic and hindbrain structures, including the dorsal raphe nucleus (DRN). In a model of experience-dependent stress resistance, socially dominant Syrian hamsters display fewer signs of anxiety following acute social defeat when compared to subordinate or control counterparts. Further, dominants activate vmPFC neurons to a greater degree during stress than do subordinates and become stress-vulnerable following pharmacological inhibition of the vmPFC. Dominants also display fewer stress-activated DRN neurons than subordinates do, suggesting that dominance experience gates activation of vmPFC neurons that inhibit the DRN during social defeat stress. To test whether social dominance alters stress-induced activity of a vmPFC-DRN pathway, we injected a retrograde tracer, cholera toxin B (CTB), into the DRN of dominant, subordinate, and control hamsters and used a dual-label immunohistochemical approach to identify vmPFC neurons co-labeled with CTB and the defeat-induced expression of an immediate early gene, cFos. Results indicate that dominant hamsters display more cFos+ and dual-labeled cells in layers V/VI of infralimbic and prelimbic subregions of the vmPFC compared to other animals. Furthermore, vmPFC-DRN activation corresponded directly with proactive behavioral strategies during defeat, which is indicative of stress resilience. Together, results suggest that recruiting the vmPFC-DRN pathway during acute stress corresponds with resistance to the effects of social defeat in dominant hamsters. Overall, these findings indicate that a monosynaptic vmPFC-DRN pathway can be engaged in an experience-dependent manner, which has implications for behavioral interventions aimed at alleviating stress-related psychopathologies.

Exploring the impact of trauma type and extent of exposure on posttraumatic alterations in 5-HT1A expression

The long-term behavioral, psychological, and neurobiological effects of exposure to potentially traumatic events vary within the human population. Studies conducted on trauma-exposed human subjects suggest that differences in trauma type and extent of exposure combine to affect development, maintenance, and treatment of a variety of psychiatric syndromes. The serotonin 1-A receptor (5-HT1A) is an inhibitory G protein-coupled serotonin receptor encoded by the HTR1A gene that plays a role in regulating serotonin release, physiological stress responding, and emotional behavior. Studies from the preclinical and human literature suggest that dysfunctional expression of 5-HT1A is associated with a multitude of psychiatric symptoms commonly seen in trauma-exposed individuals. Here, we synthesize the literature, including numerous preclinical studies, examining differences in alterations in 5-HT1A expression following trauma exposure. Collectively, these findings suggest that the impact of trauma exposure on 5-HT1A expression is dependent, in part, on trauma type and extent of exposure. Furthermore, preclinical and human studies suggest that this observation likely applies to additional molecular targets and may help explain variation in trauma-induced changes in behavior and treatment responsivity. In order to understand the neurobiological impact of trauma, including the impact on 5-HT1A expression, it is crucial to consider both trauma type and extent of exposure.

The serotonin system in the hippocampus CA3 involves in effects of CSDS on social recognition in adult female mandarin voles (Microtus mandarinus)

Chronic social defeat stress (CSDS) exacerbated the development of stress-related psychiatric disorders, and the social recognition dysfunction is the core feature of many psychiatric disorders. However, the effects of CSDS on female social recognition and the underlying neural mechanisms remain unclear. Using highly aggressive adult female mandarin voles (Microtus mandarinus) as animal model, the aim of this work is to investigate the effects of CSDS on social recognition in adult female rodents and the neurobiological mechanisms underlying these effects. Our results indicate the CSDS disrupted the normal social recognition in adult female voles. Meanwhile, defeated voles exhibited increased neural activity in the DG, CA1 and CA3 of the hippocampus. Furthermore, CSDS reduced levels of serotonin (5-HT) and serotonin 1A receptors (5-HT_1AR) in the CA3. We also discovered that microinjection of 8-OH-DPAT into the CA3 effectively reversed the social recognition deficits induced by CSDS, and an infusion of WAY-100635 into the CA3 of control female voles impaired social recognition. Moreover, targeted activation of the 5-HT neuron projection from the DRN to CA3 by long-term administration of CNO significantly prevented the CSDS induced social recognition deficits. Taken together, our study demonstrated that CSDS induced social recognition deficits in adult female voles, and these effects were mediated by the action of 5-HT on the 5-HT_1AR in the hippocampus CA3. The projection from the DRN to CA3 may be involved in social recognition deficits induced by CSDS.

Activation of kappa opioid receptors in the dorsal raphe have sex dependent effects on social behavior in California mice

Kappa opioid receptor activation has been linked to stress and anxiety behavior, thus leading to kappa antagonists being popularized in research as potential anxiolytics. However, while these findings may hold true in standard models, the neuromodulatory effects of social defeat may change the behavioral outcome of kappa opioid receptor activation. Previous research has shown that social defeat can lead to hyperactivity of serotonergic neurons in the dorsal raphe nucleus, and that inhibition of this increase blocks the social deficits caused by defeat. Kappa opioid receptor activation in the dorsal raphe nucleus works to decrease serotonergic activity. We injected the kappa opioid receptor U50,488 directly into the dorsal raphe nucleus of male and female, defeat and control adult California mice. Here we show evidence that U50,488 induces anxiety behavior in control male California mice, but helps relieve it in defeated males. Consistent with previous literature, we find little effect in females adding evidence that there are marked and important sex differences in the kappa opioid system.

Dominance status alters restraint-induced neural activity in brain regions controlling stress vulnerability

Understanding the cellular mechanisms that control resistance and vulnerability to stress is an important step toward identifying novel targets for the prevention and treatment of stress-related mental illness. In Syrian hamsters, dominant and subordinate animals exhibit different behavioral and physiological responses to social defeat stress, with dominants showing stress resistance and subordinates showing stress vulnerability. We previously found that dominant and subordinate hamsters show different levels of defeat-induced neural activity in brain regions that modulate coping with stress, although the extent to which status-dependent differences in stress vulnerability generalize to non-social stressors is unknown. In this study, dominant, subordinate, and control male Syrian hamsters were exposed to acute physical restraint for 30min and restraint-induced c-Fos immunoreactivity was quantified in select brain regions. Subordinate animals showed less restraint-induced c-Fos immunoreactivity in the infralimbic (IL), prelimbic (PL), and ventral medial amygdala (vMeA) compared to dominants, which is consistent with the status-dependent effects of social defeat stress. Subordinate animals did not show increased c-Fos immunoreactivity in the rostroventral dorsal raphe nucleus (rvDRN), which is in contrast to the effects of social defeat stress. These findings indicate that status-dependent changes in neural activity generalize from one stressor to another in a brain region-dependent manner. These findings further suggest that while some neural circuits may support a generalized form of stress resistance, others may provide resistance to specific stressors.

Serotonergic activation during courtship and aggression in the brown anole, Anolis sagrei

The role of serotonin (5-hydroxytryptamine, 5-HT) in social behavior regulation is not fully understood. While 5-HT release in nuclei of the social behavior network has generally been associated with inhibition of aggressive behavior across multiple classes of vertebrates, less is known about its effects on sexual, especially non-copulatory courtship display behaviors. Furthermore, most research has examined effects at 5-HT release sites, while studies examining the behavioral relevance of source cell populations have generated contradictory findings. This study utilized immunohistochemistry to examine the colocalization of 5-HT with Fos, an immediate early gene product and marker of neural activity, in the raphe and superior reticular nuclei of male brown anoles (Anolis sagrei) exposed to either aggression, courtship, or control social interactions. Supporting previous research, copulation was associated with a decrease in 5-HT activity, while a novel link between 5-HT activity and latency to non-copulatory courtship was also found. Within the aggression group, intensity and frequency of behavior were both associated with decreased 5-HT activity. An effect of social context was also seen, with anoles exposed to either courtship or aggression encounters showing decreased 5-HT activity in certain raphe and superior reticular nuclei populations compared to controls. Interestingly, context effects and behavioral effects were seen at separate brain nuclei, suggesting the presence of separate systems with distinct functional roles.

Sex Differences in the Regulation of Offensive Aggression and Dominance by Arginine-Vasopressin

Arginine-vasopressin (AVP) plays a critical role in the regulation of offensive aggression and social status in mammals. AVP is found in an extensive neural network in the brain. Here, we discuss the role of AVP in the regulation of aggression in the limbic system with an emphasis on the critical role of hypothalamic AVP in the control of aggression. In males, activation of AVP V1a receptors (V1aRs) in the hypothalamus stimulates offensive aggression, while in females activation of V1aRs inhibits aggression. Serotonin (5-HT) also acts within the hypothalamus to modulate the effects of AVP on aggression in a sex-dependent manner. Activation of 5-HT1a receptors (5-HT1aRs) inhibits aggression in males and stimulates aggression in females. There are also striking sex differences in the mechanisms underlying the acquisition of dominance. In males, the acquisition of dominance is associated with the activation of AVP-containing neurons in the hypothalamus. By contrast, in females, the acquisition of dominance is associated with the activation of 5-HT-containing neurons in the dorsal raphe. AVP and 5-HT also play critical roles in the regulation of a form of social communication that is important for the maintenance of dominance relationships. In both male and female hamsters, AVP acts via V1aRs in the hypothalamus, as well as in other limbic structures, to communicate social status through the stimulation of a form of scent marking called flank marking. 5-HT acts on 5-HT1aRs as well as other 5-HT receptors within the hypothalamus to inhibit flank marking induced by AVP in both males and females. Interestingly, while AVP and 5-HT influence the expression of aggression in opposite ways in males and females, there are no sex differences in the effects of AVP and 5-HT on the expression of social communication. Given the profound sex differences in the incidence of many psychiatric disorders and the increasing evidence for a relationship between aggressiveness/dominance and the susceptibility to these disorders, understanding the neural regulation of aggression and social status will have significant import for translational studies.

Serotonin and arginine-vasopressin mediate sex differences in the regulation of dominance and aggression by the social brain

There are profound sex differences in the incidence of many psychiatric disorders. Although these disorders are frequently linked to social stress and to deficits in social engagement, little is known about sex differences in the neural mechanisms that underlie these phenomena. Phenotypes characterized by dominance, competitive aggression, and active coping strategies appear to be more resilient to psychiatric disorders such as posttraumatic stress disorder (PTSD) compared with those characterized by subordinate status and the lack of aggressiveness. Here, we report that serotonin (5-HT) and arginine-vasopressin (AVP) act in opposite ways in the hypothalamus to regulate dominance and aggression in females and males. Hypothalamic injection of a 5-HT1a agonist stimulated aggression in female hamsters and inhibited aggression in males, whereas injection of AVP inhibited aggression in females and stimulated aggression in males. Striking sex differences were also identified in the neural mechanisms regulating dominance. Acquisition of dominance was associated with activation of 5-HT neurons within the dorsal raphe in females and activation of hypothalamic AVP neurons in males. These data strongly indicate that there are fundamental sex differences in the neural regulation of dominance and aggression. Further, because systemically administered fluoxetine increased aggression in females and substantially reduced aggression in males, there may be substantial gender differences in the clinical efficacy of commonly prescribed 5-HT-active drugs such as selective 5-HT reuptake inhibitors. These data suggest that the treatment of psychiatric disorders such as PTSD may be more effective with the use of 5-HT-targeted drugs in females and AVP-targeted drugs in males.

Depression and substance use comorbidity: What we have learned from animal studies

Depression and substance use disorders are often comorbid, but the reasons for this are unclear. In human studies, it is difficult to determine how one disorder may affect predisposition to the other and what the underlying mechanisms might be. Instead, animal studies allow experimental induction of behaviors relevant to depression and drug-taking, and permit direct interrogation of changes to neural circuits and molecular pathways. While this field is still new, here we review animal studies that investigate whether depression-like states increase vulnerability to drug-taking behaviors. Since chronic psychosocial stress can precipitate or predispose to depression in humans, we review studies that use psychosocial stressors to produce depression-like phenotypes in animals. Specifically, we describe how postweaning isolation stress, repeated social defeat stress, and chronic mild (or unpredictable) stress affect behaviors relevant to substance abuse, especially operant self-administration. Potential brain changes mediating these effects are also discussed where available, with an emphasis on mesocorticolimbic dopamine circuits. Postweaning isolation stress and repeated social defeat generally increase acquisition or maintenance of drug self-administration, and alter dopamine sensitivity in various brain regions. However, the effects of chronic mild stress on drug-taking have been much less studied. Future studies should consider standardizing stress-induction protocols, including female subjects, and using multi-hit models (e.g. genetic vulnerabilities and environmental stress).

Fighting experience alters brain androgen receptor expression dependent on testosterone status

Contest decisions are influenced by the outcomes of recent fights (winner-loser effects). Steroid hormones and serotonin are closely associated with aggression and therefore probably also play important roles in mediating winner-loser effects. In mangrove rivulus fish, Kryptolebias marmoratus, individuals with higher testosterone (T), 11-ketotestosterone and cortisol levels are more capable of winning, but titres of these hormones do not directly mediate winner-loser effects. In this study, we investigated the effects of winning/losing experiences on brain expression levels of the receptor genes for androgen (AR), oestrogen α/β (ERα/β), glucocorticoid (GR) and serotonin (5-HT1AR). The effect of contest experience on AR gene expression depended on T levels: repeated losses decreased, whereas repeated wins increased AR gene expression in individuals with low T but not in individuals with medium or high T levels. These results lend strong support for AR being involved in mediating winner-loser effects, which, in previous studies, were more detectable in individuals with lower T. Furthermore, the expression levels of ERα/β, 5-HT1AR and GR genes were higher in individuals that initiated contests against larger opponents than in those that did not. Overall, contest experience, underlying endocrine state and hormone and serotonin receptor expression patterns interacted to modulate contest decisions jointly.

Neurobiological mechanisms supporting experience-dependent resistance to social stress

Humans and other animals show a remarkable capacity for resilience following traumatic, stressful events. Resilience is thought to be an active process related to coping with stress, although the cellular and molecular mechanisms that support active coping and stress resistance remain poorly understood. In this review, we focus on the neurobiological mechanisms by which environmental and social experiences promote stress resistance. In male Syrian hamsters, exposure to a brief social defeat stressor leads to increased avoidance of novel opponents, which we call conditioned defeat. Also, hamsters that have achieved dominant social status show reduced conditioned defeat as well as cellular and molecular changes in the neural circuits controlling the conditioned defeat response. We propose that experience-dependent neural plasticity occurs in the prelimbic (PL) cortex, infralimbic (IL) cortex, and ventral medial amygdala (vMeA) during the maintenance of dominance relationships, and that adaptations in these neural circuits support stress resistance in dominant individuals. Overall, behavioral treatments that promote success in competitive interactions may represent valuable interventions for instilling resilience.

Activation of 5-HT2a receptors in the basolateral amygdala promotes defeat-induced anxiety and the acquisition of conditioned defeat in Syrian hamsters

Conditioned defeat is a model in Syrian hamsters (Mesocricetus auratus) in which normal territorial aggression is replaced by increased submissive and defensive behavior following acute social defeat. The conditioned defeat response involves both a fear-related memory for a specific opponent as well as anxiety-like behavior indicated by avoidance of novel conspecifics. We have previously shown that systemic injection of a 5-HT2a receptor antagonist reduces the acquisition of conditioned defeat. Because neural activity in the basolateral amygdala (BLA) is critical for the acquisition of conditioned defeat and BLA 5-HT2a receptors can modulate anxiety but have a limited effect on emotional memories, we investigated whether 5-HT2a receptor modulation alters defeat-induced anxiety but not defeat-related memories. We injected the 5-HT2a receptor antagonist MDL 11,939 (0 mM, 1.7 mM or 17 mM) or the 5-HT2a receptor agonist TCB-2 (0 mM, 8 mM or 80 mM) into the BLA prior to social defeat. We found that injection of MDL 11,939 into the BLA impaired acquisition of the conditioned defeat response and blocked defeat-induced anxiety in the open field, but did not significantly impair avoidance of former opponents in the Y-maze. Furthermore, we found that injection of TCB-2 into the BLA increased the acquisition of conditioned defeat and increased anxiety-like behavior in the open field, but did not alter avoidance of former opponents. Our data suggest that 5-HT2a receptor signaling in the BLA is both necessary and sufficient for the development of conditioned defeat, likely via modulation of defeat-induced anxiety.

Social status differences regulate the serotonergic system of a cichlid fish, Astatotilapia burtoni

Serotonin (5-HT) inhibits aggression and modulates aspects of sexual behaviour in many species, but the mechanisms responsible are not well understood. Here, we exploited the social dominance hierarchy of Astatotilapia burtoni to understand the role of the serotonergic system in long-term maintenance of social status. We identified three populations of 5-HT cells in dorsal and ventral periventricular pretectal nuclei (PPd, PPv), the nucleus of the paraventricular organ (PVO) and raphe. Dominant males had more 5-HT cells than subordinates in the raphe, but the size of these cells did not differ between social groups. Subordinates had higher serotonergic turnover in the raphe and preoptic area (POA), a nucleus essential for hypothalamic-pituitary-gonadal (HPG) axis function. The relative abundance of mRNAs for 5-HT receptor (5-HTR) subtypes 1A and 2A (htr1a, htr2a) was higher in subordinates, a difference restricted to the telencephalon. Because social status is tightly linked to reproductive capacity, we asked whether serotonin turnover and the expression of its receptors correlated with testes size and circulating levels of 11-ketotestosterone (11-KT). We found negative correlations between both raphe and POA serotonin turnover and testes size, as well as between htr1a mRNA levels and circulating 11-KT. Thus, increased serotonin turnover in non-aggressive males is restricted to specific brain nuclei and is associated with increased expression of 5-HTR subtypes 1A and 2A exclusively in the telencephalon.

Social opportunity rapidly regulates expression of CRF and CRF receptors in the brain during social ascent of a teleost fish, Astatotilapia burtoni

In social animals, hierarchical rank governs food availability, territorial rights and breeding access. Rank order can change rapidly and typically depends on dynamic aggressive interactions. Since the neuromodulator corticotrophin releasing factor (CRF) integrates internal and external cues to regulate the hypothalamic-pituitary adrenal (HPA) axis, we analyzed the CRF system during social encounters related to status. We used a particularly suitable animal model, African cichlid fish, Astatotilapia burtoni, whose social status regulates reproduction. When presented with an opportunity to rise in rank, subordinate A. burtoni males rapidly change coloration, behavior, and their physiology to support a new role as dominant, reproductively active fish. Although changes in gonadotropin-releasing hormone (GnRH1), the key reproductive molecular actor, have been analyzed during social ascent, little is known about the roles of CRF and the HPA axis during transitions. Experimentally enabling males to ascend in social rank, we measured changes in plasma cortisol and the CRF system in specific brain regions 15 minutes after onset of social ascent. Plasma cortisol levels in ascending fish were lower than subordinate conspecifics, but similar to levels in dominant animals. In the preoptic area (POA), where GnRH1 cells are located, and in the pituitary gland, CRF and CRF₁ receptor mRNA levels are rapidly down regulated in ascending males compared to subordinates. In the Vc/Vl, a forebrain region where CRF cell bodies are located, mRNA coding for both CRFR₁ and CRFR₂ receptors is lower in ascending fish compared to stable subordinate conspecifics. The rapid time course of these changes (within minutes) suggests that the CRF system is involved in the physiological changes associated with shifts in social status. Since CRF typically has inhibitory effects on the neuroendocrine reproductive axis in vertebrates, this attenuation of CRF activity may allow rapid activation of the reproductive axis and facilitate the transition to dominance.

A low-cost automated apparatus for investigating the effects of social defeat in Syrian hamsters

We describe an automated apparatus that can be used to investigate the effects of defeat in hamsters. It consists of a covered alleyway that leads to a box, or arena, where hamsters can be kept separate or allowed to fight. The alleyway is divided into seven equal-sized chambers. Low-power lasers and laser detectors are used to keep track of a hamster's position in the alleyway. A CFL flood lamp placed over the chamber farthest from the arena generates a light gradient in the alleyway that engenders in the subjects a preference for the darker chambers near the arena. A computer automatically records the interruption of the laser beams and yields three measures: average position, the frequency of visits to each chamber, and the frequency of changes in direction of travel in each chamber. The results of a pilot study indicated that when a dominant hamster was placed behind a screened gate in the arena and a subordinate hamster was placed in the alleyway, the subordinate maintained a significantly greater distance from the dominant than did a nondefeated hamster. The subordinate hamster also changed its direction of travel more frequently than did the nondefeated hamster. The results suggest that conditioned fear was elicited in the defeated hamster by proximity to the dominant hamster, an effect that is consistent with published results in which the data were recorded manually or by using commercially available event-tracking software.

Stress-protective neural circuits: not all roads lead through the prefrontal cortex

Exposure to an uncontrollable stressor elicits a constellation of physiological and behavioral sequel in laboratory rats that often reflect aspects of anxiety and other emotional disruptions. We review evidence suggesting that plasticity within the serotonergic dorsal raphe nucleus (DRN) is critical to the expression of uncontrollable stressor-induced anxiety. Specifically, after uncontrollable stressor exposure subsequent anxiogenic stimuli evoke greater 5-HT release in DRN terminal regions including the amygdala and striatum; and pharmacological blockade of postsynaptic 5-HT(2C) receptors in these regions prevents expression of stressor-induced anxiety. Importantly, the controllability of stress, the presence of safety signals, and a history of exercise mitigate the expression of stressor-induced anxiety. These stress-protective factors appear to involve distinct neural substrates; with stressor controllability requiring the medial prefrontal cortex, safety signals the insular cortex and exercise affecting the 5-HT system directly. Knowledge of the distinct yet converging mechanisms underlying these stress-protective factors could provide insight into novel strategies for the treatment and prevention of stress-related psychiatric disorders.

Day-night differences in neural activation in histaminergic and serotonergic areas with putative projections to the cerebrospinal fluid in a diurnal brain

In nocturnal rodents, brain areas that promote wakefulness have a circadian pattern of neural activation that mirrors the sleep/wake cycle, with more neural activation during the active phase than during the rest phase. To investigate whether differences in temporal patterns of neural activity in wake-promoting regions contribute to differences in daily patterns of wakefulness between nocturnal and diurnal species, we assessed Fos expression patterns in the tuberomammillary (TMM), supramammillary (SUM), and raphe nuclei of male grass rats maintained in a 12:12 h light-dark cycle. Day-night profiles of Fos expression were observed in the ventral and dorsal TMM, in the SUM, and in specific subpopulations of the raphe, including serotonergic cells, with higher Fos expression during the day than during the night. Next, to explore whether the cerebrospinal fluid is an avenue used by the TMM and raphe in the regulation of target areas, we injected the retrograde tracer cholera toxin subunit beta (CTB) into the ventricular system of male grass rats. While CTB labeling was scarce in the TMM and other hypothalamic areas including the suprachiasmatic nucleus, which contains the main circadian pacemaker, a dense cluster of CTB-positive neurons was evident in the caudal dorsal raphe, and the majority of these neurons appeared to be serotonergic. Since these findings are in agreement with reports for nocturnal rodents, our results suggest that the evolution of diurnality did not involve a change in the overall distribution of neuronal connections between systems that support wakefulness and their target areas, but produced a complete temporal reversal in the functioning of those systems.

Social defeat induces changes in histone acetylation and expression of histone modifying enzymes in the ventral hippocampus, prefrontal cortex, and dorsal raphe nucleus

Chronic exposure to stress is associated with a number of psychiatric disorders, but little is known about the epigenetic mechanisms that underlie the stress response or resilience to chronic stress. We investigated histone acetylation in seven different brain regions of rats exposed to chronic social defeat stress: the dorsal hippocampus (dHPC), ventral hippocampus (vHPC), medial prefrontal cortex (mPFC), basolateral amygdala (BLA), locus coeruleus (LC), paraventricular thalamus (PVT), and dorsal raphe (DR) nucleus. This stress paradigm was unique in that it allowed rats to display resilience in the form of an active coping mechanism. We found that there was an increase in acetylation of H3K9/14 (H3K9/14ac) and bulk acetylation of H4K5,8,12,16 (H4K5,8,12,16ac) in the DR nucleus of rats that were less resilient. Less resilient rats also displayed increased levels of H3K18 acetylation (H3K18ac) in the mPFC when compared to non-stressed controls. In the vHPC, there was an increase in H3K18ac and H4K12 (H4K12ac) in rats that were less resilient when compared to non-stressed control rats. In addition, there was a decrease in levels of H4K8 acetylation (H4K8ac) in both resilient and non-resilient rats as compared to controls. We assessed expression of histone modifying enzymes in the vHPC and the mPFC using quantitative real-time polymerase chain reaction (PCR) and found changes in expression of a number of targets. These included changes in Sirt1 and Sirt2 in the vHPC and changes in Kat5 in the mPFC. Overall, these results suggest that changes in histone acetylation and expression of histone modifying enzymes in these regions correlate with the behavioral response to stress in socially defeated rats.

Defeat-induced activation of the ventral medial prefrontal cortex is necessary for resistance to conditioned defeat

The ventral medial prefrontal cortex (vmPFC) controls vulnerability to the negative effects of chronic or uncontrollable stress. Dominance status alters responses to social defeat in the conditioned defeat model, which is a model characterized by loss of territorial aggression and increased submissive and defensive behavior following an acute social defeat. We have previously shown that dominant individuals show a reduced conditioned defeat response and increased defeat-induced neural activation in the vmPFC compared to subordinates. Here, we tested the hypothesis that defeat-induced activation of the vmPFC is necessary to confer resistance to conditioned defeat in dominants. We paired weight-matched male Syrian hamsters (Mesocricetus auratus) in daily 5-min aggressive encounters for 2 weeks and identified dominants and subordinates. Twenty-four hours after the final pairing, animals were bilaterally injected with 200 nl of the GABAA receptor agonist muscimol (1.1 nmol) or 200 nl of saline vehicle 5 min prior to social defeat. Defeat consisted of 3, 5-min encounters with resident aggressor hamsters at 10-min intervals. Twenty-four hours following social defeat, animals received conditioned defeat testing which involved a 5-min social interaction test with a non-aggressive intruder. Muscimol injection prior to social defeat prevented the reduced conditioned defeat response observed in vehicle-treated dominants. Further, there was no effect of muscimol injection on the conditioned defeat response in subordinates or controls. These data support the conclusion that activation of the vmPFC during social defeat is necessary for the protective effects of dominant social status on the acquisition of conditioned defeat.

Treadmill exercise during pregnancy ameliorates post‑traumatic stress disorder‑induced anxiety‑like responses in maternal rats

Post‑traumatic stress disorder (PTSD) is an anxiety disorder triggered by life‑threatening events that cause intense fear. Exercise is known to have protective effects on neuropsychiatric diseases. The present study investigated whether treadmill exercise during pregnancy reduced or alleviated symptoms of PTSD in maternal rats. To induce predator stress in pregnant rats, rats were exposed to a hunting dog in an enclosed room. Exposure time was three 10‑min daily sessions separated by 1 h, starting at week 1 of pregnancy until delivery. Pregnant rats in the exercise group were forced to run on a treadmill for 30 min once a day, starting one week following pregnancy until delivery. Rats receiving predator stress during pregnancy exhibited PTSD anxiety‑like behaviors following delivery. Expression of 5‑hydroxytryptamine (5‑HT) and its synthesizing enzyme tryptophan hydroxylase (TPH) in the dorsal raphe was increased compared with unstressed rats. Expression of c‑Fos and neuronal nitric oxide synthases (nNOS) in the hypothalamus and locus coeruleus were higher in the rats receiving stress during pregnancy compared with unstressed rats. By contrast, treadmill exercise during pregnancy ameliorated anxiety‑like behaviors and reduced the expression of 5‑HT, TPH, c‑Fos and nNOS in the PTSD maternal rats. The results of the present study indicate that exercise during pregnancy is suitable for use as a therapeutic strategy to reduce anxiety‑related disorders, including PTSD.

Evolutionary shift from fighting to foraging in blind cavefish through changes in the serotonin network

BACKGROUND

Within the species Astyanax mexicanus, there are several interfertile populations of river-dwelling sighted fish and cave-dwelling blind fish which have evolved morphological and behavioral adaptations, the origins of which are unknown. Here, we have investigated the neural, genetic, and developmental bases for the evolution of aggressive behavior in this teleost.

RESULTS

We used an intruder-resident behavioral assay to compare aggressiveness quantitatively (attack counts) and qualitatively (pattern and nature of attacks) between the surface and cave populations of Astyanax. Using this paradigm, we characterize aggressive behavior in surface fish, bring support for the genetic component of this trait, and show that it is controlled by raphe serotonergic neurons and that it corresponds to the establishment of dominance between fish. Cavefish have completely lost such aggressive/dominance behavior. The few attacks performed by cavefish during the behavioral test instead correspond to food-seeking behavior, driven by the developmental evolution of their hypothalamic serotonergic paraventricular neurons, itself due to increased Sonic Hedgehog signaling during early forebrain embryogenesis.

CONCLUSIONS

We propose that during evolution and adaptation to their cave habitat, cavefish have undergone a behavioral shift, due to modifications of their serotonergic neuronal network. They have lost the typical aggressive behavior of surface fish and evolved a food-seeking behavior that is probably more advantageous to surviving in the dark. We have therefore demonstrated a link between the development of a neuronal network and the likely adaptive behaviors it controls.

Behavioral characterization of escalated aggression induced by GABA(B) receptor activation in the dorsal raphe nucleus

RATIONALE

Pharmacological activation of GABA(B) receptors in the dorsal raphe nucleus (DRN) can escalate territorial aggression in male mice.

OBJECTIVES

We characterized this escalated aggression in terms of its behavioral and environmental determinants.

METHODS

Aggressive behavior of resident male (CFW or ICR mouse) was assessed in confrontations with a group-housed intruder. Either baclofen (0.06 nmol/0.2 μl) or vehicle (saline) was microinjected into the DRN 10 min before the confrontation. We examined baclofen-heightened aggression in five situations: aggression in a neutral arena and after social instigation (experiment 1), aggression during the light phase of the cycle (experiment 2), aggression without prior fighting experience (experiment 3), aggression toward a female (experiment 4), and aggression after defeat experiences (experiment 5). In addition, we examined the body targets towards which bites are directed and the duration of aggressive bursts after baclofen treatment.

RESULTS

Regardless of the past social experience, baclofen escalated aggressive behaviors. Even in the neutral arena and after defeat experiences, where aggressive behaviors were inhibited, baclofen significantly increased aggression. Baclofen increased attack bites directed at vulnerable body areas of male intruders but not toward a female and only in the dark. Also, baclofen prolonged the duration of aggressive bursts.

CONCLUSIONS

For baclofen to escalate aggression, specific stimulation (male intruder) and tonic level of serotonin (dark cycle) are required. Once aggressive behavior is triggered, intra-DRN baclofen escalates the level of aggression to abnormal levels and renders it difficult to terminate. Also, baclofen counteracts the effects of novelty or past experiences of defeat.

Stress-related serotonergic systems: implications for symptomatology of anxiety and affective disorders

Previous studies have suggested that serotonergic neurons in the midbrain raphe complex have a functional topographic organization. Recent studies suggest that stimulation of a bed nucleus of the stria terminalis-dorsal raphe nucleus pathway by stress- and anxiety-related stimuli modulates a subpopulation of serotonergic neurons in the dorsal part of the dorsal raphe nucleus (DRD) and caudal part of the dorsal raphe nucleus (DRC) that participates in facilitation of anxiety-like responses. In contrast, recent studies suggest that activation of a spinoparabrachial pathway by peripheral thermal or immune stimuli excites subpopulations of serotonergic neurons in the ventrolateral part of the dorsal raphe nucleus/ventrolateral periaqueducal gray (DRVL/VLPAG) region and interfascicular part of the dorsal raphe nucleus (DRI). Studies support a role for serotonergic neurons in the DRVL/VLPAG in inhibition of panic-like responses, and serotonergic neurons in the DRI in antidepressant-like effects. Thus, data suggest that while some subpopulations of serotonergic neurons in the dorsal raphe nucleus play a role in facilitation of anxiety-like responses, others play a role in inhibition of anxiety- or panic-like responses, while others play a role in antidepressant-like effects. Understanding the anatomical and functional properties of these distinct serotonergic systems may lead to novel therapeutic strategies for the prevention and/or treatment of affective and anxiety disorders. In this review, we describe the anatomical and functional properties of subpopulations of serotonergic neurons in the dorsal raphe nucleus, with a focus on those implicated in symptoms of anxiety and affective disorders, the DRD/DRC, DRVL/VLPAG, and DRI.

A double dissociation in the effects of 5-HT2A and 5-HT2C receptors on the acquisition and expression of conditioned defeat in Syrian hamsters

Previous research indicates that serotonin enhances the development of stress-induced changes in behavior, although it is unclear which serotonin receptors mediate this effect. 5-HT2 receptors are potential candidates because activation at these receptors is associated with increased fear and anxiety. In this study, we investigated whether pharmacological treatments targeting 5-HT2 receptors would alter the acquisition and expression of conditioned defeat. Conditioned defeat is a social defeat model in Syrian hamsters in which individuals display increased submissive and defensive behavior and a loss of territorial aggression when tested with a novel intruder 24 hours after an acute social defeat. The nonselective 5-HT2 receptor agonist mCPP (0.0, 0.3, 1.0, or 3.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Also, the 5-HT2A receptor antagonist MDL 11,939 (0.0, 0.5, or 2.0 mg/kg) was injected either prior to social defeat training or prior to conditioned defeat testing. Injection of mCPP prior to testing increased the expression of conditioned defeat, but injection of mCPP prior to training did not alter the acquisition of conditioned defeat. Conversely, injection of MDL 11,939 prior to training reduced the acquisition of conditioned defeat, but injection of MDL 11,939 prior to testing did not alter the expression of conditioned defeat. Our data suggest that mCPP activates 5-HT2C receptors during testing to enhance the display of submissive and defensive behavior, whereas MDL 11,939 blocks 5-HT2A receptors during social defeat to disrupt the development of the conditioned defeat response. In sum, these results suggest that serotonin acts at separate 5-HT2 receptors to facilitate the acquisition and expression of defeat-induced changes in social behavior.

Immunohistochemical localization of serotonin in the brain during natural sex change in the hermaphroditic goby Lythrypnus dalli

The neurotransmitter serotonin (5-HT) may play a central role in the inhibition of socially regulated sex change in fish because of its known modulation of both aggressive and reproductive behavior. This is the first study to use immunohistochemical techniques to examine the morphometry of serotonergic neurons at different times during sex change. Using a model species wherein sex change is socially regulated via agonistic social interactions (the bluebanded goby, Lythrypnus dalli), we sampled brains of males and females with different social status, and of females at different times during sex change. Consistent with previous studies on other teleosts, immunoreactive neurons were found in the posterior periventricular nucleus (NPPv), the nucleus of the lateral recess (NRL), the nucleus of the posterior recess (NRP) and in the raphe nucleus. We measured the total area of NPPv, NRL, NRP, and the number and mean cell area of serotonergic neurons in the raphe nucleus. There was no significant difference in any of the brain regions between males, females or sex changing fish, but there was a slight increase in the number of dorsal raphe neurons in the brain of sex changers 2h after male removal. The results show that in L. dalli the serotonergic system does not present any morphological sex and status differences, nor any dramatic modifications during sex change. These data, together with previous results, do not support the hypothesis that serotonin inhibits socially regulated sex change.

Gene expression in aminergic and peptidergic cells during aggression and defeat: relevance to violence, depression and drug abuse

In this review, we examine how experiences in social confrontations alter gene expression in mesocorticolimbic cells. The focus is on the target of attack and threat due to the prominent role of social defeat stress in the study of coping mechanisms and victimization. The initial operational definition of the socially defeated mouse by Ginsburg and Allee (1942) enabled the characterization of key endocrine, cardiovascular, and metabolic events during the initial response to an aggressive opponent and during the ensuing adaptations. Brief episodes of social defeat stress induce an augmented response to stimulant challenge as reflected by increased locomotion and increased extracellular dopamine (DA) in the nucleus accumbens (NAC). Cells in the ventral tegmental area (VTA) that project to the NAC were more active as indicated by increased expression of c-fos and Fos-immunoreactivity and BDNF. Intermittent episodes of social defeat stress result in increased mRNA for MOR in brainstem and limbic structures. These behavioral and neurobiological indices of sensitization persist for several months after the stress experience. The episodically defeated rats also self-administered intravenous cocaine during continuous access for 24 h ("binge"). By contrast, continuous social stress, particularly in the form of social subordination stress, leads to reduced appetite, compromised endocrine activities, and cardiovascular and metabolic abnormalities, and prefer sweets less as index of anhedonia. Cocaine challenges in subordinate rats result in a blunted psychomotor stimulant response and a reduced DA release in NAC. Subordinate rats self-administer cocaine less during continuous access conditions. These contrasting patterns of social stress result from continuous vs. intermittent exposure to social stress, suggesting divergent neuroadaptations for increased vulnerability to cocaine self-administration vs. deteriorated reward mechanisms characteristic of depressive-like profiles.

Regulation of dorsal raphe nucleus function by serotonin autoreceptors: a behavioral perspective

Neurotransmission by serotonin (5-HT) is tightly regulated by several autoreceptors that fine-tune serotonergic neurotransmission through negative feedback inhibition at the cell bodies (predominantly 5-HT(1A)) or at the axon terminals (predominantly 5-HT(1B)); however, more subtle roles for 5-HT(1D) and 5-HT(2B) autoreceptors have also been detected. This review provides an overview of 5-HT autoreceptors, focusing on their contribution in animal behavioral models of stress and emotion. Experiments targeting 5-HT autoreceptors in awake, behaving animals have generally shown that increasing autoreceptor feedback is anxiolytic and rewarding, while enhanced 5-HT function is aversive and anxiogenic; however, the role of serotonergic activity in behavioral models of helplessness is more complex. The prevailing model suggests that 5-HT autoreceptors become desensitized in response to stress exposure and antidepressant administration, two seemingly opposite manipulations. Thus there are still unresolved questions regarding the role of these receptors-and serotonin in general-in normal and pathological states.

Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders

Exposure to traumatic events can increase the risk for major depressive disorder (MDD) as well as posttraumatic stress disorder (PTSD), and pharmacological treatments for these disorders often involve the modulation of serotonergic (5-HT) systems. Several behavioral paradigms in rodents produce changes in behavior that resemble symptoms of MDD and these behavioral changes are sensitive to antidepressant treatments. Here we review two animal models in which MDD-like behavioral changes are elicited by exposure to an acute traumatic event during adulthood, learned helplessness (LH) and conditioned defeat. In LH, exposure of rats to inescapable, but not escapable, tailshock produces a constellation of behavioral changes that include deficits in fight/flight responding and enhanced anxiety-like behavior. In conditioned defeat, exposure of Syrian hamsters to a social defeat by a more aggressive animal leads to a loss of territorial aggression and an increase in submissive and defensive behaviors in subsequent encounters with non-aggressive conspecifics. Investigations into the neural substrates that control LH and conditioned defeat revealed that increased 5-HT activity in the dorsal raphe nucleus (DRN) is critical for both models. Other key brain regions that regulate the acquisition and/or expression of behavior in these two paradigms include the basolateral amygdala (BLA), central nucleus of the amygdala (CeA) and bed nucleus of the stria terminalis (BNST). In this review, we compare and contrast the role of each of these neural structures in mediating LH and conditioned defeat, and discuss the relevance of these data in developing a better understanding of the mechanisms underlying trauma-related depression. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Repeated social defeat increases reactive emotional coping behavior and alters functional responses in serotonergic neurons in the rat dorsal raphe nucleus

Chronic stress is a vulnerability factor for a number of psychiatric disorders, including anxiety and affective disorders. Social defeat in rats has proven to be a useful paradigm to investigate the neural mechanisms underlying physiologic and behavioral adaptation to acute and chronic stress. Previous studies suggest that serotonergic systems may contribute to the physiologic and behavioral adaptation to chronic stress, including social defeat in rodent models. In order to test the hypothesis that repeated social defeat alters the emotional behavior and the excitability of brainstem serotonergic systems implicated in control of emotional behavior, we exposed adult male rats either to home cage control conditions, acute social defeat, or social defeat followed 24h later by a second social defeat encounter. We then assessed behavioral responses during social defeat as well as the excitability of serotonergic neurons within the dorsal raphe nucleus using immunohistochemical staining of tryptophan hydroxylase, a marker of serotonergic neurons, and the protein product of the immediate-early gene, c-fos. Repeated social defeat resulted in a shift away from proactive emotional coping behaviors, such as rearing (explorative escape behavior), and toward reactive emotional coping behaviors such as freezing. Both acute and repeated defeat led to widespread increases in c-Fos expression in serotonergic neurons in the dorsal raphe nucleus. Changes in behavior following a second exposure to social defeat, relative to acute defeat, were associated with decreased c-Fos expression in serotonergic neurons within the dorsal and ventral parts of the mid-rostrocaudal dorsal raphe nucleus, regions that have been implicated in 1) serotonergic modulation of fear- and anxiety-related behavior and 2) defensive behavior in conspecific aggressive encounters, respectively. These data support the hypothesis that serotonergic systems play a role in physiologic and behavioral responses to both acute and repeated social defeat.

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.

Circadian clock resetting by behavioral arousal: neural correlates in the midbrain raphe nuclei and locus coeruleus

Some procedures for stimulating arousal in the usual daily rest period (e.g., gentle handling, novel wheel-induced running) can phase shift circadian rhythms in Syrian hamsters, while other arousal procedures are ineffective (inescapable stress, caffeine, modafinil). The dorsal and median raphe nuclei (DRN, MnR) have been implicated in clock resetting by arousal and, in rats and mice, exhibit strong regionally specific responses to inescapable stress and anxiogenic drugs. To examine a possible role for the midbrain raphe nuclei in the differential effects of arousal procedures on circadian rhythms, hamsters were aroused for 3 h in the mid-rest period by confinement to a novel running wheel, gentle handling (with minimal activity) or physical restraint (with intermittent, loud compressed air stimulation) and sacrificed immediately thereafter. Regional expression of c-fos and tryptophan hydroxylase (TrpOH) were quantified immunocytochemically in the DRN, MnR and locus coeruleus (LC). Neither gentle handling nor wheel running had a large impact on c-fos expression in these areas, although the manipulations were associated with a small increase in c-Fos in TrpOH-like and TrpOH-negative cells, respectively, in the caudal interfascicular DRN region. By contrast, restraint stress significantly increased c-Fos in both TrpOH-like and TrpOH-negative cells in the rostral DRN and LC. c-Fos-positive cells in the DRN did not express tyrosine hydroxylase. These results reveal regionally specific monoaminergic correlates of arousal-induced circadian clock resetting, and suggest a hypothesis that strong activation of some DRN and LC neurons by inescapable stress may oppose clock resetting in response to arousal during the daily sleep period. More generally, these results complement evidence from other rodent species for functional topographic organization of the DRN.

Adverse early life experience and social stress during adulthood interact to increase serotonin transporter mRNA expression

Anxiety disorders, depression and animal models of vulnerability to a depression-like syndrome have been associated with dysregulation of serotonergic systems in the brain. To evaluate the effects of early life experience, adverse experiences during adulthood, and potential interactions between these factors on serotonin transporter (slc6a4) mRNA expression, we investigated in rats the effects of maternal separation (180 min/day from days 2 to 14 of life; MS180), neonatal handing (15 min/day from days 2 to 14 of life; MS15), or normal animal facility rearing (AFR) control conditions with or without subsequent exposure to adult social defeat on slc6a4 mRNA expression in the dorsal raphe nucleus (DR) and caudal linear nucleus. At the level of specific subdivisions of the DR, there were no differences in slc6a4 mRNA expression between MS15 and AFR rats. Among rats exposed to a novel cage control condition, increased slc6a4 mRNA expression was observed in the dorsal part of the DR in MS180 rats, relative to AFR control rats. In contrast, MS180 rats exposed to social defeat as adults had increased slc6a4 mRNA expression throughout the DR compared to both MS15 and AFR controls. Social defeat increased slc6a4 mRNA expression, but only in MS180 rats and only in the "lateral wings" of the DR. Overall these data demonstrate that early life experience and stressful experience during adulthood interact to determine slc6a4 mRNA expression. These data support the hypothesis that early life experience and major stressful life events contribute to dysregulation of serotonergic systems in stress-related neuropsychiatric disorders.

Behavioral control over shock blocks behavioral and neurochemical effects of later social defeat

Experience with behavioral control over tailshock (escapable shock, ES) has been shown to block the behavioral and neurochemical changes produced by later uncontrollable tail shock (inescapable shock, IS). The present experiments tested, in rats, whether the protective effect of control over tailshock extends beyond reducing the behavioral and neurochemical impact of a subsequent tailshock experience to stressors that are quite different. Social defeat (SD) was chosen as the second stress experience because it has few if any cues in common with tailshock. SD produced shuttlebox escape learning deficits ("learned helplessness") and reduced juvenile social investigation 24 h later, as does IS. IS is notable for inducing a large increase in dorsal raphe nucleus (DRN) serotonergic (5-HT) activity as measured by extracellular levels of 5-HT within the DRN, and SD did so as well. ES occurring 7 days before SD blocked this SD-induced DRN activation, as well as the SD-induced interference with shuttlebox escape and reduction in social investigation. Prior exposure to yoked IS did not reduce the DRN 5-HT activation or later behavioral effects produced by SD, and thus the proactive stress-blunting effects of ES can be attributed to it's controllability. Thus, ES confers a very general protection to the impact of a subsequent stress experience.

A direct relationship between aggressive behavior in the resident/intruder test and cell oxidative status in adult male mice

Disturbances in oxidative metabolism are involved in many acute and chronic diseases, as well as in several other conditions. The objective of the present study was to examine the relationship between the level of intracellular reactive oxygen species in the peripheral blood granulocytes of mice, as evaluated by 2',7'-dichlorofluorescin diacetate (DCFH-DA), a sensor of reactive oxygen species, and the aggressive behavior of these mice, as estimated by the resident/intruder test. Our results showed a significant, linear and positive relationship (P<0.001) between the intracellular redox status of peripheral blood granulocytes and the aggressive behavior levels of adult male mice (correlation coefficients (R(2)) ranging from 0.75 to 0.77). This suggests that the granulocytes of aggressively behaving mice have high levels of oxidative stress.