Restraint stress and social defeat: What they have in common

Effects of early life stress on network and behavioral states

BACKGROUND

Adverse childhood experiences (ACEs) are associated with numerous detriments in health, including increased vulnerability to psychiatric illnesses. Early life stress (ELS) in rodents has been shown to effectively model several of the behavioral and endocrine impacts of ACEs and has been utilized to investigate the underlying mechanisms contributing to disease. However, the precise neural mechanisms responsible for mediating the impact of ELS on vulnerability to psychiatric illnesses remain largely unknown.

METHODS

We use behavior, immunoassay, in vivo local field potential (LFP) recording, histology, and patch clamp to describe the effects of ELS on stress-related behaviors, endocrine changes, network states, protein expression, and cellular physiology in male and female mice.

RESULTS

We demonstrate that a murine maternal separation (MS) ELS model causes male-specific alterations in behavioral and hormonal responses following an acute stressor. LFP recordings in the basolateral amygdala (BLA) and frontal cortex (FC) reveal similar sex-specific alterations at baseline and in response to acute ethological stress. Furthermore, altered physiology of BLA principal neurons in males and BLA parvalbumin (PV) interneurons in females suggests a likely mechanism through which these effects may be mediated. These findings support a large body of literature demonstrating that these network states contribute to stress reactivity and vulnerability to psychiatric illnesses.

CONCLUSIONS

Collectively, these results implicate distinct, novel male- and female-specific mechanisms through which ACEs may impact psychiatric health, including altered cellular physiology and network states involved in emotional processing.

Fos expression in the periaqueductal gray, but not the ventromedial hypothalamus, is correlated with psychosocial stress-induced cocaine-seeking behavior in rats

Psychosocial stressors are known to promote cocaine craving and relapse in humans but are infrequently employed in preclinical relapse models. Consequently, the underlying neural circuitry by which these stressors drive cocaine seeking has not been thoroughly explored. Using Fos expression analyses, we sought to examine whether the ventromedial hypothalamus (VMH) or periaqueductal gray (PAG), two critical components of the brain's hypothalamic defense system, are activated during psychosocial stress-induced cocaine seeking. Adult male and female rats self-administered cocaine (0.5 mg/kg/inf IV, fixed-ratio 1 schedule, 2 h/session) over 20 sessions. On sessions 11, 14, 17, and 20, a tactile cue was present in the operant chamber that signaled impending social defeat stress (n=16, 8/sex), footshock stress (n=12, 6/sex), or a no-stress control condition (n=12, 6/sex) immediately after the session's conclusion. Responding was subsequently extinguished, and rats were tested for reinstatement of cocaine seeking during re-exposure to the tactile cue that signaled their impending stress/no-stress post-session event. All experimental groups displayed significant reinstatement of cocaine seeking, but Fos analyses indicated that neural activity within the rostrolateral PAG (rPAGl) was selectively correlated with cocaine-seeking magnitude in the socially-defeated rats. rPAGl activation was also associated with active-defense coping behaviors during social defeat encounters and with Fos expression in prelimbic prefrontal cortex and orexin-negative cells of the lateral hypothalamus/perifornical area in males, but not females. These findings suggest a potentially novel role for the rPAGl in psychosocial stress-induced cocaine seeking, perhaps in a sex-dependent manner.

Excitation-inhibition imbalance in medial preoptic area circuits underlies chronic stress-induced depression-like states

Dysregulation of brain homeostasis is associated with neuropsychiatric conditions such as major depressive disorder. However, underlying neural-circuit mechanisms remain not well-understood. We show in mice that chronic restraint stress (CRS) and social defeat stress (SDS) are both associated with disruption of excitation (E)-inhibition (I) balance, with increased E/I ratios, in medial preoptic area (MPOA) circuits, but through affecting different neuronal types. CRS results in elevated activity in glutamatergic neurons, and their suppression mitigates CRS-induced depressive-like behaviors. Paraventricular hypothalamic input to these neurons contributes to induction but not expression of depressive-like behaviors. Their projections to ventral tegmental area and periaqueductal gray/dorsal raphe suppress midbrain dopaminergic and serotonergic activity, respectively, and mediate expression of divergent depressive-like symptoms. By contrast, SDS results in reduced activity of GABAergic neurons, and their activation alleviates SDS-induced depressive-like behaviors. Thus, E/I imbalance with relatively increased excitation in MPOA circuits may be a general mechanism underlying depression caused by different etiological factors.

Sex-dependent effects of early life stress on network and behavioral states

Background

Adverse childhood experiences (ACEs) are associated with numerous detriments in health, including increased vulnerability to psychiatric illnesses. Early life stress (ELS) in rodents has been shown to effectively model several of the behavioral and endocrine impacts of ACEs and has been utilized to investigate the underlying mechanisms contributing to disease. However, the precise neural mechanisms responsible for mediating the impact of ELS on vulnerability to psychiatric illnesses remain largely unknown.

Methods

We use behavior, immunoassay, in vivo LFP recording, histology, and patch clamp to describe the effects of ELS on stress behaviors, endocrinology, network states, protein expression, and cellular physiology in male and female mice.

Results

We demonstrate that a murine maternal separation (MS) ELS model causes sex-dependent alterations in behavioral and hormonal responses following an acute stressor. Local field potential (LFP) recordings in the basolateral amygdala (BLA) and frontal cortex (FC) reveal similar sex-dependent alterations at baseline, in response to acute ethological stress, and during fear memory extinction, supporting a large body of literature demonstrating that these network states contribute to stress reactivity and vulnerability to psychiatric illnesses. Sex differences were accompanied by altered physiology of BLA principal neurons in males and BLA PV interneurons in females.

Conclusions

Collectively, these results implicate novel, sex-dependent mechanisms through which ACEs may impact psychiatric health, involving altered cellular physiology and network states involved in emotional processing.

Role of the rostral dorsomedial column of the periaqueductal gray during social defeat in rats

Previous studies showed that the dorsal premammillary nucleus of the hypothalamus (PMD) is involved in social passive defensive behaviors likely to be meditated by descending projections to the periaqueductal gray (PAG). We focused on the rostral dorsomedial PAG (rPAGdm) to reveal its putative neural mechanisms involved in mediating social defensive responses. By combining retrograde tracing and FOS expression analysis, we showed that in addition to the PMD, the rPAGdm is influenced by several brain sites active during social defeat. Next, we found that cytotoxic lesions of the rPAGdm drastically reduced passive defense and did not affect active defensive responses. We then examined the rPAGdm's projection pattern and found that the PAGdm projections are mostly restricted to midbrain sites, including the precommissural nucleus, different columns of the PAG, and the cuneiform nucleus (CUN). Also, we found decreased FOS expression in the caudal PAGdm, CUN, and PMD after the rPAGdm was lesioned. The results support that the rPAGdm mediates passive social defensive responses through ascending paths to prosencephalic circuits likely mediated by the CUN. This study provides further support for the role of the PAG in the modulation of behavioral responses by working as a unique hub for influencing prosencephalic sites during the mediation of aversive responses.

A hypothalamic pathway that suppresses aggression toward superior opponents

Aggression is costly and requires tight regulation. Here we identify the projection from estrogen receptor alpha-expressing cells in the caudal part of the medial preoptic area (cMPOAEsr1) to the ventrolateral part of the ventromedial hypothalamus (VMHvl) as an essential pathway for modulating aggression in male mice. cMPOAEsr1 cells increase activity mainly during male-male interaction, which differs from the female-biased response pattern of rostral MPOAEsr1 (rMPOAEsr1) cells. Notably, cMPOAEsr1 cell responses to male opponents correlated with the opponents' fighting capability, which mice could estimate based on physical traits or learn through physical combats. Inactivating the cMPOAEsr1-VMHvl pathway increased aggression, whereas activating the pathway suppressed natural intermale aggression. Thus, cMPOAEsr1 is a key population for encoding opponents' fighting capability-information that could be used to prevent animals from engaging in disadvantageous conflicts with superior opponents by suppressing the activity of VMHvl cells essential for attack behaviors.

The Endogenous Cannabinoid and the Nitricoxidergic Systems in the Modulation of Stress Responses

The effects on stress-induced analgesia (SIA) from endogenous cannabinoid system (ECS) and nitric oxide (NO) interaction after 1 h of restraint stress were evaluated in male Wistar rats. The animals were subjected to 1 h of restraint and then injected with different combinations of cannabinoid receptor type 1 agonist anandamide (AEA) or antagonist AM251 along with an NO donor, NO precursor, or inhibitor of NO synthase. Nociception was evaluated using paw pressure (PP) or hot plate (HP) tests. AEA was administered immediately after the end of restraint-SIA (r-SIA). Administration of NO precursor reversed the pronociceptive effect of the CB1 agonist on r-SIA. Both the CB1 antagonist and the NOS inhibitor neutralized the pro-analgesic effect of L-arginine (L-arg). Administration of an NO donor, instead, increased r-SIA. Our experiments confirmed that the endogenous cannabinoid and the NO-ergic systems interact in the modulation of r-SIA. This interaction probably implies NO as a second messenger of the ECS.

Dynamics in brain activation and behaviour in acute and repeated social defensive behaviour

In nature, confrontations between conspecifics are recurrent and related, in general, due to the lack of resources such as food and territory. Adequate defence against a conspecific aggressor is essential for the individual's survival and the group integrity. However, repeated social defeat is a significant stressor promoting several behavioural changes, including social defence per se. What would be the neural basis of these behavioural changes? To build new hypotheses about this, we here investigate the effects of repeated social stress on the neural circuitry underlying motivated social defence behaviour in male mice. We observed that animals re-exposed to the aggressor three times spent more time in passive defence during the last exposure than in the first one. These animals also show less activation of the amygdalar and hypothalamic nuclei related to the processing of conspecific cues. In turn, we found no changes in the activation of the hypothalamic dorsal pre-mammillary nucleus (PMD) that is essential for passive defence. Therefore, our data suggest that the balance between the activity of circuits related to conspecific processing and the PMD determines the pattern of social defence behaviour. Changes in this balance may be the basis of the adaptations in social defence after repeated social defeat.

Role of CRF1 and CRF2 receptors in the lateral hypothalamus in cardiovascular and anxiogenic responses evoked by restraint stress in rats: Evaluation of acute and chronic exposure

We investigated the role of corticotropin-releasing factor (CRF) neurotransmission within the lateral hypothalamus (LH) in cardiovascular and anxiogenic-like responses evoked by acute and repeated restraint stress in rats. For this, animals were subjected to intra-LH microinjection of a selective CRF₁ (CP376395) or CRF₂ (antisauvagine-30) receptor antagonist before either an acute or the 10th session of restraint stress. Restraint-evoked arterial pressure and heart rate increases, tail skin temperature decrease and anxiogenic-like effect in the elevated plus maze (EPM) were evaluated. We also assessed the effect of 10 daily sessions of restraint on expression of CRF₁ and CRF₂ receptors within the LH. We identified that antagonism of either CRF₁ or CRF₂ receptor within the LH decreased the tachycardia during both the acute and 10th session of restraint, but the effect of the CRF₁ receptor antagonist was more pronounced during the 10th session. Acute restraint stress also caused anxiogenic-like effect, and this response was inhibited in animals treated with either CP376395 or antisauvagine-30. Anxiety-like behaviors were not changed following the 10th session of restraint, and pharmacological treatments did not affect the behavior in the EPM in chronically stressed animals. Repeated restraint also did not change the level of the CRF receptors within the LH. Taken together, the findings indicate that CRF₁ and CRF₂ receptors within the LH are involved in tachycardic and anxiogenic-like responses to aversive stimuli. Control of tachycardia by the CRF₁ receptor is sensitized by previous stressful experience, and this effect seems to be independent of changes in expression of the receptor.

Clozapine prevented social interaction deficits and reduced c-Fos immunoreactivity expression in several brain areas of rats exposed to acute restraint stress

In the present study, we evaluate the effect of acute restraint stress (15 min) of male Wistar rats on social interaction measurements and c-Fos immunoreactivity (c-Fos-ir) expression, a marker of neuronal activity, in areas involved with the modulation of acute physical restraint in rats, i.e., the paraventricular nucleus of the hypothalamus (PVN), median raphe nucleus (MnR), medial prefrontal cortex (mPFC), cingulate prefrontal cortex (cPFC), nucleus accumbens (NaC), hippocampus (CA3), lateral septum (LS) and medial amygdala (MeA). We considered the hypothesis that restraint stress exposure could promote social withdrawal induced by the activation of the hypothalamic-pituitary-adrenocortical (HPA) axis, and increase c-Fos expression in these limbic forebrain areas investigated. In addition, we investigated whether pretreatment with the atypical antipsychotic clozapine (5 mg/kg; I.P.) could attenuate or block the effects of restraint on these responses. We found that restraint stress induced social withdrawal, and increased c-Fos-ir in these areas, demonstrating that a single 15 min session of physical restraint of rats effectively activated the HPA axis, representing an effective tool for the investigation of neuronal activity in brain regions sensitive to stress. Conversely, pretreatment with clozapine, prevented social withdrawal and reduced c-Fos expression. We suggest that treatment with clozapine exerted a preventive effect in the social interaction deficit, at least in part, by blocking the effect of restraint stress in brain regions that are known to regulate the HPA-axis, including the cerebral cortex, hippocampus, hypothalamus, septum and amygdala. Further experiments will be done to confirm this hypothesis.

Corticotropin-releasing factor neurotransmission in the lateral hypothalamus modulates the tachycardiac response during acute emotional stress in rats

The lateral hypothalamus (LH) is implicated in the physiological and behavioral responses during stressful events. However, the local neurochemical mechanisms related to control of stress responses by this hypothalamic area are not completely understood. Therefore, in this study we evaluated the involvement of CRFergic neurotransmission acting through the CRF₁ receptor within the LH in cardiovascular responses evoked by an acute session of restraint stress in rats. For this, we investigated the effect of bilateral microinjection of different doses (0.01, 0.1 and 1 nmol/100 nL) of the selective CRF₁ receptor antagonist CP376395 into the LH on arterial pressure and heart rate increases and decrease in tail skin temperature evoked by acute restraint stress. We found that all doses of the CRF₁ receptor antagonist microinjected into the LH decreased the restraint-evoked tachycardia, but without affecting the arterial pressure and tail skin temperature responses. Additionally, treatment of the LH with CP376395 at the doses of 0.1 and 1 nmol/100 nL increased the basal values of both heart rate and arterial pressure, whereas the dose of 0.1 nmol/100 nL decreased the skin temperature. Taken together, these findings indicate that CRFergic neurotransmission in the LH, acting through activation of local CRF₁ receptors, plays a facilitatory role in the tachycardia observed during aversive threats, but without affecting the pressor and tail skin temperature responses. Our results also provide evidence that LH CRFergic neurotransmission in involved in tonic maintenance of cardiovascular function.

Defensive behaviors and brain regional activation changes in rats confronting a snake

In the present study, we examined behavioral and brain regional activation changes of rats). To a nonmammalian predator, a wild rattler snake (Crotalus durissus terrificus). Accordingly, during snake threat, rat subjects showed a striking and highly significant behavioral response of freezing, stretch attend, and, especially, spatial avoidance of this threat. The brain regional activation patterns for these rats were in broad outline similar to those of rats encountering other predator threats, showing Fos activation of sites in the amygdala, hypothalamus, and periaqueductal gray matter. In the amygdala, only the lateral nucleus showed significant activation, although the medial nucleus, highly responsive to olfaction, also showed higher activation. Importantly, the hypothalamus, in particular, was somewhat different, with significant Fos increases in the anterior and central parts of the ventromedial hypothalamic nucleus (VMH), in contrast to patterns of enhanced Fos expression in the dorsomedial VMH to cat predators, and in the ventrolateral VMH to an attacking conspecific. In addition, the juxtodorsalmedial region of the lateral hypothalamus showed enhanced Fos activation, where inputs from the septo-hippocampal system may suggest the potential involvement of hippocampal boundary cells in the very strong spatial avoidance of the snake and the area it occupied. Notably, these two hypothalamic paths appear to merge into the dorsomedial part of the dorsal premammillary nucleus and dorsomedial and lateral parts of the periaqueductal gray, all of which present significant increases in Fos expression and are likely to be critical for the expression of defensive behaviors in responses to the snake threat.

Mechanisms of Stress-Induced Spermatogenesis Impairment in Male Rats Following Unpredictable Chronic Mild Stress (uCMS)

The negative association between psychological stress and male fertility has been known for many years. This study was aimed at (i) identifying spermatogenesis impairment induced by psychological stress in rats and (ii) exploring the role of glucocorticoid receptor (GR) signaling in these adverse effects (if they exist). Male Sprague Dawley rats were exposed to a six-week period of unpredictable chronic mild stress (uCMS) along with cotreatment of GR antagonist RU486 (1 mg/kg/day). Testicular damage was assessed by testicular pathological evaluation, epididymal sperm concentration, serum testosterone levels, testicular apoptotic cell measurements, and cell cycle progression analyses. Rats in the uCMS group had decreased levels of serum testosterone and decreased epididymal sperm concentration. The uCMS-treated rats also had decreased numbers of spermatids and increased levels of apoptotic seminiferous tubules; additionally, cell cycle progression of spermatogonia was arrested at the G₀/G₁ phase. Furthermore, uCMS exposure caused an increase in serum corticosterone level and activated GR signaling in the testes including upregulated GR expression. RU486 treatment suppressed GR signaling and alleviated the damaging effects of stress, resulting in an increased epididymal sperm concentration. Overall, this work demonstrated for the first time that the activation of GR signaling mediates stress-induced spermatogenesis impairment and that this outcome is related to cell apoptosis and cell cycle arrest in germ cells.

GABAA but not GABAB receptors in the lateral hypothalamus modulate the tachycardic response to emotional stress in rats

The lateral hypothalamus (LH) has been described as one of the hypothalamic areas involved in the behavioral and physiological responses triggered by aversive stimuli. Previous studies indicated involvement of the LH in cardiovascular responses to stress. Despite this evidence, the local neurochemical mechanisms involved in LH control of stress responses is still poorly understood. Therefore, in the present study, we investigated the role of GABAergic neurotransmission within the LH in cardiovascular responses induced by an acute session of restraint stress in rats. For this, we evaluated the effect of bilateral microinjection of selective antagonists of either GABA_A or GABA_B receptors into the LH on arterial pressure increase, heart rate (HR) increase and reduction in tail skin temperature induced by restraint stress. We found that microinjection of the selective GABA_A receptor antagonist SR95531 into the LH decreased the increase in HR caused by restraint stress, but without affecting the increase in arterial pressure increase or the reduction in tail skin temperature. Conversely, LH treatment with the selective GABA_B receptor antagonist CGP35348 did not affect the restraint-evoked cardiovascular changes. These findings indicate that GABAergic neurotransmission in the LH, acting through activation of local GABA_A receptors, plays a facilitatory role in the tachycardic response observed during aversive threats.

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.

Evidence of a Role for the Lateral Hypothalamic Area Juxtadorsomedial Region (LHAjd) in Defensive Behaviors Associated with Social Defeat

Our understanding of the extrinsic connections of the lateral hypothalamic area (LHA) has deepened in recent years. In particular, a series of studies using neural pathway-tracing methods to investigate the macroconnections of histologically differentiated LHA regions, have revealed that the neural connections of these regions are substantially distinct, and have robust connections with neural circuits controlling survival behaviors. To begin testing functional associations suggested by the distinct LHA region neural connections, the present study has investigated the role of the LHA juxtadorsomedial region (LHAjd) in the control of social defeat (a socially-relevant defensive behavior). Male rats received bilateral cytotoxic lesions targeted to the LHAjd. A resident-intruder paradigm was then employed to investigate the effect of these lesions on defensive behavioral responses. Behavioral data were collected during three phases of testing: (1) pre-encounter habituation to testing context; (2) encounter with a dominant conspecific in the testing context; and (3) post-encounter context. Statistical analysis of behavioral measures revealed a significant decrease in risk assessment behaviors during post-encounter context testing in lesioned intruders compared to sham-lesioned and intact rats. However, changes in defensive behavioral measures during the habituation, or during resident-intruder encounters, did not reach significance. We discuss these data in relation to LHAjd (and neighboring LHA region) neural connections, and in relation to current advances in understanding of the neural control of defensive behaviors. A refined model for the neural circuits that are central to the control of socially-relevant defensive behaviors is outlined. We also consider possible broader implications of these data for disorders of behavioral control.

Repeated stress-induced expression pattern alterations of the hippocampal chloride transporters KCC2 and NKCC1 associated with behavioral abnormalities in female mice

The balance of cation-chloride co-transporters, particularly KCC2 and NKCC1, is critical for GABAergic inhibitory signaling. However, KCC2/NKCC1 balance is disrupted in many neurodegenerative diseases. Moreover, correlations between chronic stress, KCC2 and NKCC1 in the hippocampus remain poorly understood. Despite the fact that emotional disorders in humans are far more prevalent in women, there have been relatively few studies about female subjects. Here we investigated behaviors and expression patterns of KCC2 and NKCC1 in the hippocampi of female mice under chronic stress. Repeated stress (RS) was induced in experimental mice by repeated forced water administration. Then, expression patterns of GABAergic signaling molecules were identified by immunohistochemical analysis and performance was assessed using several behavioral tests. The results of semi-quantitative analysis showed that RS decreased KCC2 expression and increased NKCC1 expression in membranes of granular and pyramidal cells in the hippocampus. The novel object recognition (NOR) test and sociability test revealed that RS induced cognitive and sociability deficits, whereas RS increased the time spent in the open arms of the elevated plus maze test and induced attention deficits in other tests. In summary, RS induced alterations in membrane KCC2/NKCC1 balance in the hippocampus of female mice, which may contribute to GABAergic disinhibition associated with cognitional, sociability and attention deficits.