Chemogenetic activation of an infralimbic cortex to basolateral amygdala projection promotes resistance to acute social defeat stress

Sex-Dependent Effects of Angiotensin Type 2 Receptor-Expressing Medial Prefrontal Cortex Interneurons in Fear Extinction Learning

Background

The renin-angiotensin system has been identified as a potential therapeutic target for posttraumatic stress disorder, although its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. Therefore, we used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neurons in fear and stess related behavior.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/tdTomato male and female mice were used for immunohistochemistry. mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker were quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using a Cre-expressing adeno-associated virus. Mice then underwent Pavlovian auditory fear conditioning, elevated plus maze, and open field testing.

Results

Immunohistochemistry results revealed that AT2R was densely expressed throughout the mPFC and primarily expressed in somatostatin interneurons in a sex-dependent manner. Following fear conditioning, mPFC-AT2R Cre-lox deletion impaired extinction and increased exploratory behavior in female but not male mice, while locomotion was unaltered by mPFC-AT2R deletion in both sexes.

Conclusions

These results identify mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons and reveal their role in regulating fear learning in a sex-dependent manner, potentially offering insights into novel therapeutic targets for posttraumatic stress disorder.

Sex-dependent regulation of social avoidance by oxytocin signaling in the ventral tegmental area

It has been hypothesized that oxytocin increases the salience of social stimuli, whether the valence is positive or negative, through its interactions with the ventral tegmental area (VTA). Indeed, oxytocin neurons project to the VTA and activate dopamine neurons that are necessary for social experiences with positive valence. Surprisingly, though, there has not been an investigation of the role of oxytocin in the VTA in mediating social experiences with negative valence (e.g., social stress). Given that there are sex differences in how oxytocin regulates the salience of positively-valenced social interactions, we hypothesized that oxytocin acting in the VTA also alters the salience of social stress in a sex-dependent manner. To test this, female and male Syrian hamsters were site-specifically infused with either saline, oxytocin (9 μM), or oxytocin receptor antagonist (90 μM) into the VTA. Subjects were then exposed to either no defeat or a single, 15 min defeat by one RA. The day following social defeat, subjects underwent a 5 min social avoidance test. There was an interaction between sex and drug treatment, such that the oxytocin antagonist increased social avoidance compared to saline treatment in socially stressed females, while oxytocin decreased social avoidance compared to saline treatment in socially stressed males. Contrary to expectations, these results suggest that oxytocin signaling generally acts to decrease social avoidance, regardless of sex. These sex differences in the efficacy of oxytocin and oxytocin receptor antagonists to alter negatively-valenced social stimuli, however, should be considered when guiding pharmacotherapies for disorders involving social deficits.

Projections from infralimbic medial prefrontal cortex glutamatergic outputs to amygdala mediates opioid induced hyperalgesia in male rats

Repeated use of opioid analgesics may cause a paradoxically exacerbated pain known as opioid-induced hyperalgesia (OIH), which hinders effective clinical intervention for severe pain. Currently, little is known about the neural circuits underlying OIH modulation. Previous studies suggest that laterocapsular division of the central nucleus of amygdala (CeLC) is critically involved in the regulation of OIH. Our purpose is to clarify the role of the projections from infralimbic medial prefrontal cortex (IL) to CeLC in OIH. We first produced an OIH model by repeated fentanyl subcutaneous injection in male rats. Immunofluorescence staining revealed that c-Fos-positive neurons were significantly increased in the right CeLC in OIH rats than the saline controls. Then, we used calcium/calmodulin-dependent protein kinase IIα (CaMKIIα) labeling and the patch-clamp recordings with ex vivo optogenetics to detect the functional projections from glutamate pyramidal neurons in IL to the CeLC. The synaptic transmission from IL to CeLC, shown in the excitatory postsynaptic currents (eEPSCs), inhibitory postsynaptic currents (eIPSCs) and paired-pulse ratio (PPR), was observably enhanced after fentanyl administration. Moreover, optogenetic activation of this IL-CeLC pathway decreased c-Fos expression in CeLC and ameliorated mechanical and thermal pain in OIH. On the contrary, silencing this pathway by chemogenetics exacerbated OIH by activating the CeLC. Combined with the electrophysiology results, the enhanced synaptic transmission from IL to CeLC might be a cortical gain of IL to relieve OIH rather than a reason for OIH generation. Scaling up IL outputs to CeLC may be an effective neuromodulation strategy to treat OIH.

Sex-dependent effects of angiotensin type 2 receptor expressing medial prefrontal cortex (mPFC) interneurons in fear extinction learning

Background

The renin-angiotensin system (RAS) has been identified as a potential therapeutic target for PTSD, though its mechanisms are not well understood. Brain angiotensin type 2 receptors (AT2Rs) are a subtype of angiotensin II receptors located in stress and anxiety-related regions, including the medial prefrontal cortex (mPFC), but their function and mechanism in the mPFC remain unexplored. We therefore used a combination of imaging, cre/lox, and behavioral methods to investigate mPFC-AT2R-expressing neuron involvement in fear learning.

Methods

To characterize mPFC-AT2R-expressing neurons in the mPFC, AT2R-Cre/td-Tomato male and female mice were used for immunohistochemistry (IHC). mPFC brain sections were stained with glutamatergic or interneuron markers, and density of AT2R+ cells and colocalization with each marker was quantified. To assess fear-related behaviors in AT2R-flox mice, we selectively deleted AT2R from mPFC neurons using an AAV-Cre virus. Mice then underwent Pavlovian auditory fear conditioning, approach/avoidance, and locomotion testing.

Results

IHC results revealed that AT2R is densely expressed in the mPFC. Furthermore, AT2R is primarily expressed in somatostatin interneurons in females but not males. Following fear conditioning, mPFC-AT2R deletion impaired extinction in female but not male mice. Locomotion was unaltered by mPFC-AT2R deletion in males or females, while AT2R-deleted females had increased exploratory behavior.

Conclusion

These results lend support for mPFC-AT2R+ neurons as a novel subgroup of somatostatin interneurons that influence fear extinction in a sex-dependent manner. This furthers underscores the role of mPFC in top-down regulation and a unique role for peptidergic (ie., angiotensin) mPFC regulation of fear and sex differences.

Sex differences in dominance relationships in Syrian hamsters

Dominance relationships are identified by changes in agonistic behavior toward specific individuals. While there are considerable individual and species differences in dominance relationships, sex differences are poorly understood in rodent models because aggression among female rodents is rare. The aim of this study was to characterize sex differences in the formation and maintenance of dominance relationships in same-sex pairs of male and female Syrian hamsters. We pooled data from multiple projects in our lab to evaluate dominance interactions in 68 male dyads and 88 female dyads. In each project, animals were matched with a partner similar in age, sex, and estrous cycle and we exposed animals to daily social encounters for two weeks in a resident-intruder format. We found that female hamsters were quicker to attack and attacked at higher rates compared to males regardless of dominance status. In addition, resident female hamsters were quicker to attack and attacked at higher rates than intruder females, but aggression in males did not depend on residency status. Female subordinates were quicker to submit and fled at higher rates from their dominant counterparts compared to male subordinates. Intruder subordinate females were quicker to submit and fled at higher rates than resident subordinate females. Females were also more resistant than males to becoming subordinate in that they fought back more consistently and were more likely to reverse their dominance status. These findings indicate that dominance relationships are less stable in females compared to males and that residency status has a larger impact on agonistic behavior in females than males. Overall, differences in how males and females display territorial aggression can lead to sex differences in the establishment and maintenance of dominance relationships.

Dominance status modulates activity in medial amygdala cells with projections to the bed nucleus of the stria terminalis

The medial amygdala (MeA) controls several types of social behavior via its projections to other limbic regions. Cells in the posterior dorsal and posterior ventral medial amygdala (MePD and MePV, respectively) project to the bed nucleus of the stria terminalis (BNST) and these pathways respond to chemosensory cues and regulate aggressive and defensive behavior. Because the BNST is also essential for the display of stress-induced anxiety, a MePD/MePV-BNST pathway may modulate both aggression and responses to stress. In this study we tested the hypothesis that dominant animals would show greater neural activity than subordinates in BNST-projecting MePD and MePV cells after winning a dominance encounter as well as after losing a social defeat encounter. We created dominance relationships in male and female Syrian hamsters (Mesocricetus auratus), used cholera toxin b (CTB) as a retrograde tracer to label BNST-projecting cells, and collected brains for c-Fos staining in the MePD and MePV. We found that c-Fos immunoreactivity in the MePD and MePV was positively associated with aggression in males, but not in females. Also, dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells compared to their same-sex subordinate counterparts. Another set of animals received social defeat stress after acquiring a dominant or subordinate social status and we stained for stress-induced c-Fos expression in the MePD and MePV. We found that dominant males showed a greater proportion of c-Fos+ /CTB+ double-labeled cells in the MePD after social defeat stress compared to subordinates. Also, dominants showed a longer latency to submit during social defeat than subordinates. Further, in males, latency to submit was positively associated with the proportion of c-Fos+ /CTB+ double-labeled cells in the MePD and MePV. These findings indicate that social dominance increases neural activity in BNST-projecting MePD and MePV cells and activity in this pathway is also associated with proactive responses during social defeat stress. In sum, activity in a MePD/MePV-BNST pathway contributes to status-dependent differences in stress coping responses and may underlie experience-dependent changes in stress resilience.

Emotional- and cognitive-like responses induced by social defeat stress in male mice are modulated by the BNST, amygdala, and hippocampus

Introduction

Chronic exposure to social defeat stress (SDS) has been used to investigate the neurobiology of depressive- and anxiety-like responses and mnemonic processes. We hypothesized that these affective, emotional, and cognitive consequences induced by SDS are regulated via glutamatergic neurons located in the bed nucleus of the stria terminalis (BNST), amygdaloid complex, and hippocampus in mice.

Methods

Here, we investigated the influence of chronic SDS on (i) the avoidance behavior assessed in the social interaction test, (ii) the anxiety-like behavior (e.g., elevated plus-maze, and open field tests) (iii) depressive-like behaviors (e.g., coat state, sucrose splash, nesting building, and novel object exploration tests), (iv) the short-term memory (object recognition test), (v) ΔFosB, CaMKII as well as ΔFosB + CaMKII labeling in neurons located in the BNST, amygdaloid complex, dorsal (dHPC) and the ventral (vHPC) hippocampus.

Results

The main results showed that the exposure of mice to SDS (a) increased defensive and anxiety-like behaviors and led to memory impairment without eliciting clear depressive-like or anhedonic effects; (b) increased ΔFosB + CaMKII labeling in BNST and amygdala, suggesting that both areas are strongly involved in the modulation of this type of stress; and produced opposite effects on neuronal activation in the vHPC and dHPC, i.e., increasing and decreasing, respectively, ΔFosB labeling. The effects of SDS on the hippocampus suggest that the vHPC is likely related to the increase of defensive- and anxiety-related behaviors, whereas the dHPC seems to modulate the memory impairment.

Discussion

Present findings add to a growing body of evidence indicating the involvement of glutamatergic neurotransmission in the circuits that modulate emotional and cognitive consequences induced by social defeat stress.

Comparing the ontogeny, neurobiology, and function of social play in hamsters and rats

Syrian hamsters show complex social play behavior and provide a valuable animal model for delineating the neurobiological mechanisms and functions of social play. In this review, we compare social play behavior of hamsters and rats and underlying neurobiological mechanisms. Juvenile rats play by competing for opportunities to pin one another and attack their partner's neck. A broad set of cortical, limbic, and striatal regions regulate the display of social play in rats. In hamsters, social play is characterized by attacks to the head in early puberty, which gradually transitions to the flanks in late puberty. The transition from juvenile social play to adult hamster aggression corresponds with engagement of neural ensembles controlling aggression. Play deprivation in rats and hamsters alters dendritic morphology in mPFC neurons and impairs flexible, context-dependent behavior in adulthood, which suggests these animals may have converged on a similar function for social play. Overall, dissecting the neurobiology of social play in hamsters and rats can provide a valuable comparative approach for evaluating the function of social play.

The impact of social defeat on basomedial amygdala neuronal activity in adult male rats

Social stressors negatively impact social function, and this is mediated by the amygdala across species. Social defeat stress is an ethologically relevant social stressor in adult male rats that increases social avoidance, anhedonia, and anxiety-like behaviors. While amygdala manipulations can mitigate the negative effects of social stressors, the impact of social defeat on the basomedial subregion of the amygdala is relatively unclear. Understanding the role of the basomedial amygdala may be especially important, as prior work has demonstrated that it drives physiological responses to stress, including heart-rate related responses to social novelty. In the present study, we quantified the impact of social defeat on social behavior and basomedial amygdala neuronal responses using anesthetized in vivo extracellular electrophysiology. Socially defeated rats displayed increased social avoidance behavior towards novel Sprague Dawley conspecifics and reduced time initiating social interactions relative to controls. This effect was most pronounced in rats that displayed defensive, boxing behavior during social defeat sessions. We next found that socially defeated rats showed lower overall basomedial amygdala firing and altered the distribution of neuronal responses relative to the control condition. We separated neurons into low and high Hz firing groups, and neuronal firing was reduced in both low and high Hz groups but in a slightly different manner. This work demonstrates that basomedial amygdala activity is sensitive to social stress, displaying a distinct pattern of social stress-driven activity relative to other amygdala subregions.

Insulin and fluoxetine produce opposite actions on lateral septal nucleus-infralimbic region connection responsivity

Some diabetes patients develop depression, the main treatment for which is antidepressants. Pharmacological interactions between insulin and antidepressants (e.g., fluoxetine) are controversial in the literature. Some authors reported hypoglycemic actions of fluoxetine, whereas others reported antidepressant-like actions. In healthy rats, insulin produces an antidespair-like action in rats through an increase in locomotor and exploratory activity, but differences in actions of insulin and fluoxetine on neuronal activity are unknown. The present study evaluated Wistar healthy rats that were treated with saline, insulin, fluoxetine, or fluoxetine + insulin for 3 days (short-term) or 21 days (long-term). The model consisted of electrical stimulation of the lateral septal nucleus (LSN) while we performed single-unit extracellular response recordings in the prelimbic cortex (PL) and infralimbic cortex (IL) subregions of the medial prefrontal cortex (mPFC). Stimulation of the LSN produced an initial brief excitatory paucisynaptic response and then a long-lasting inhibitory afterdischarge in the PL and IL. Treatment with saline and fluoxetine, but not insulin, minimally affected the paucisynaptic response. Differences were found in LSN-IL responsivity. The inhibitory afterdischarge was clearly enhanced in the long-term fluoxetine group but not by insulin alone or fluoxetine + insulin. These findings suggest that insulin produces some actions that are opposite to fluoxetine on LSN-mPFC connection responsivity, with no synergistic actions between the actions of insulin and fluoxetine.

Non-rapid eye movement sleep determines resilience to social stress

Resilience, the ability to overcome stressful conditions, is found in most mammals and varies significantly among individuals. A lack of resilience can lead to the development of neuropsychiatric and sleep disorders, often within the same individual. Despite extensive research into the brain mechanisms causing maladaptive behavioral-responses to stress, it is not clear why some individuals exhibit resilience. To examine if sleep has a determinative role in maladaptive behavioral-response to social stress, we investigated individual variations in resilience using a social-defeat model for male mice. Our results reveal a direct, causal relationship between sleep amount and resilience-demonstrating that sleep increases after social-defeat stress only occur in resilient mice. Further, we found that within the prefrontal cortex, a regulator of maladaptive responses to stress, pre-existing differences in sleep regulation predict resilience. Overall, these results demonstrate that increased NREM sleep, mediated cortically, is an active response to social-defeat stress that plays a determinative role in promoting resilience. They also show that differences in resilience are strongly correlated with inter-individual variability in sleep regulation.

Infralimbic medial prefrontal cortex signalling to calbindin 1 positive neurons in posterior basolateral amygdala suppresses anxiety- and depression-like behaviours

Generalization is a fundamental cognitive ability of organisms to deal with the uncertainty in real-world situations. Excessive fear generalization and impaired reward generalization are closely related to many psychiatric disorders. However, the neural circuit mechanism for reward generalization and its role in anxiety-like behaviours remain elusive. Here, we found a robust activation of calbindin 1-neurons (Calb 1) in the posterior basolateral amygdala (pBLA), simultaneous with reward generalization to an ambiguous cue after reward conditioning in mice. We identify the infralimbic medial prefrontal cortex (IL) to the pBLA^Calb1 (Calb 1 neurons in the pBLA) pathway as being involved in reward generalization for the ambiguity. Activating IL–pBLA inputs strengthens reward generalization and reduces chronic unpredictable mild stress-induced anxiety- and depression-like behaviours in a manner dependent on pBLA^Calb1 neuron activation. These findings suggest that the IL–pBLA^Calb1 circuit could be a target to promote stress resilience via reward generalization and consequently ameliorate anxiety- and depression-like behaviours.

The neural mechanisms for reward generalization are not fully understood. Here the authors investigate the role of posterior basolateral amygdala calbindin-expressing cells in modulating behavioural responses related to reward and aversion.

Depression induced by low-frequency repetitive transcranial magnetic stimulation to ventral medial frontal cortex in monkeys

The medial frontal cortex (MFC), especially its ventral part, has long been of great interest with respect to the pathology of mood disorders. A number of human brain imaging studies have demonstrated the abnormalities of this brain region in patients with mood disorders, however, whether it is critically and causally involved in the pathogenesis of such disorders remains to be fully elucidated. In this study, we examined how the suppression of neural activity in the ventral region of the MFC (vMFC) affects the behavioral and physiological states of monkeys by using repetitive transcranial magnetic stimulation (rTMS). By using low-frequency rTMS (LF-rTMS) as an inhibitory intervention, we found that LF-rTMS targeting the vMFC temporarily induced a depression-like state in monkeys, which was characterized by a reduced movement activity level, impaired sociability, and decreased motivation level, as well as increased plasma cortisol level. On the other hand, no such significant changes in behavioral and physiological states were observed when targeting the other MFC regions, dorsal or posterior. We further found that the administration of an antidepressant agent, ketamine, ameliorated the abnormal behavioral and physiological states induced by the LF-rTMS intervention. These findings causally indicate the involvement of the vMFC in the regulation of mood and the validity of the LF-rTMS-induced dysfunction of the vMFC as a nonhuman primate model of depression.

Neural systems that facilitate the representation of social rank

Across species, animals organize into social dominance hierarchies that serve to decrease aggression and facilitate survival of the group. Neuroscientists have adopted several model organisms to study dominance hierarchies in the laboratory setting, including fish, reptiles, rodents and primates. We review recent literature across species that sheds light onto how the brain represents social rank to guide socially appropriate behaviour within a dominance hierarchy. First, we discuss how the brain responds to social status signals. Then, we discuss social approach and avoidance learning mechanisms that we propose could drive rank-appropriate behaviour. Lastly, we discuss how the brain represents memories of individuals (social memory) and how this may support the maintenance of unique individual relationships within a social group. This article is part of the theme issue 'The centennial of the pecking order: current state and future prospects for the study of dominance hierarchies'.

Structure and function differences in the prelimbic cortex to basolateral amygdala circuit mediate trait vulnerability in a novel model of acute social defeat stress in male mice

Stressful life events are ubiquitous and well-known to negatively impact mental health. However, in both humans and animal models, there is large individual variability in how individuals respond to stress, with some but not all experiencing long-term adverse consequences. While there is growing understanding of the neurobiological underpinnings of the stress response, much less is known about how neurocircuits shaped by lifetime experiences are activated during an initial stressor and contribute to this selective vulnerability versus resilience. We developed a model of acute social defeat stress (ASDS) that allows classification of male mice into "susceptible" (socially avoidant) versus "resilient" (expressing control-level social approach) one hour after exposure to six minutes of social stress. Using circuit tracing and high-resolution confocal imaging, we explored differences in activation and dendritic spine density and morphology in the prelimbic cortex to basolateral amygdala (PL→BLA) circuit in resilient versus susceptible mice. Susceptible mice had greater PL→BLA recruitment during ASDS and activated PL→BLA neurons from susceptible mice had more and larger mushroom spines compared to resilient mice. We hypothesized identified structure/function differences indicate an overactive PL→BLA response in susceptible mice and used an intersectional chemogenetic approach to inhibit the PL→BLA circuit during or prior to ASDS. We found in both cases that this blocked ASDS-induced social avoidance. Overall, we show PL→BLA structure/function differences mediate divergent behavioral responses to ASDS in male mice. These results support PL→BLA circuit overactivity during stress as a biomarker of trait vulnerability and potential target for prevention of stress-induced psychopathology.

Mean Girls: Social Stress Models for Female Rodents

Social stressors are known to have strong negative impacts on mental health. There is a long history of preclinical social defeat stress studies in rodents focusing on males that has produced important insights into the neural mechanisms that modulate depression- and anxiety-related behavior. Despite these impressive results, a historical weakness of rodent social stress models has been an under-representation of studies in females. This is problematic because rates of depression and anxiety are higher in women versus men. Recently there has been a surge of interest in adapting social stress methods for female rodents. Here we review new rodent models that have investigated numerous facets of social stress in females. The different models have different strengths and weaknesses, with some model systems having stronger ethological validity with other models having better access to molecular tools to manipulate neural circuits. Continued use and refinement of these complementary models will be critical for addressing gaps in understanding the function of neural circuits modulating depression- and anxiety-related behavior in females.

Ethanol modulation of cortico-basolateral amygdala circuits: Neurophysiology and behavior

This review highlights literature relating the anatomy, physiology, and behavioral contributions by projections between rodent prefrontal cortical areas and the basolateral amygdala. These projections are robustly modulated by both environmental experience and exposure to drugs of abuse including ethanol. Recent literature relating optogenetic and chemogenetic dissection of these circuits within behavior both compliments and occasionally challenges roles defined by more traditional pharmacological or lesion-based approaches. In particular, cortico-amygdala circuits help control both aversive and reward-seeking. Exposure to pathology-producing environments or abused drugs dysregulates the relative 'balance' of these outcomes. Modern circuit-based approaches have also shown that overlapping populations of neurons within a given brain region frequently govern both aversion and reward-seeking. In addition, these circuits often dramatically influence 'local' cortical or basolateral amygdala excitatory or inhibitory circuits. Our understanding of these neurobiological processes, particularly in relation to ethanol research, has just begun and represents a significant opportunity.

Gonadal steroid hormone receptors in the medial amygdala contribute to experience-dependent changes in stress vulnerability

Social experience can generate neural plasticity that changes how individuals respond to stress. Winning aggressive encounters alters how animals respond to future challenges and leads to increased plasma testosterone concentrations and androgen receptor (AR) expression in the social behavior neural network. In this project, our aim was to identify neuroendocrine mechanisms that account for changes in stress-related behavior following the establishment of dominance relationships over a two-week period. We used a Syrian hamster model in which acute social defeat stress increases anxiety-like responses in a conditioned defeat test in males and in a social avoidance test in females. First, we administered flutamide, an AR antagonist, via intraperitoneal injections daily during the establishment of dominance relationships in male hamsters. We found that pharmacological blockade of AR prevented a reduction in conditioned defeat in dominant males and blocked an upregulation of AR in the posterior dorsal medial amygdala (MePD) and posterior ventral medial amygdala (MePV), but not in the ventral lateral septum. Next, we administered flutamide into the posterior aspects of the medial amygdala (MeP) prior to acute social defeat stress or prior to conditioned defeat testing in males. We found that pharmacological blockade of AR in the MeP prior to social defeat, but not prior to testing, increased the conditioned defeat response in dominant males and did not alter behavior in subordinates. Finally, we developed a procedure to establish dominance relationships in female hamsters and investigated status-dependent changes in plasma steroid hormone concentrations, estrogen receptor alpha (ERα) immunoreactivity, and defeat-induced social avoidance. We found that dominant female hamsters showed reduced social avoidance regardless of social defeat exposure as well as increased ERα expression in the MePD, but no status-dependent changes in the concentration of plasma steroid hormones. Overall, these findings suggest that achieving and maintaining stable social dominance leads to sex-specific neural plasticity in the MeP that underlies status-dependent changes in stress vulnerability.

Exposure to Prenatal Stress Is Associated With an Excitatory/Inhibitory Imbalance in Rat Prefrontal Cortex and Amygdala and an Increased Risk for Emotional Dysregulation

Epidemiological studies have shown that environmental insults and maternal stress during pregnancy increase the risk of several psychiatric disorders in the offspring. Converging lines of evidence from humans, as well as from rodent models, suggest that prenatal stress (PNS) interferes with fetal development, ultimately determining changes in brain maturation and function that may lead to the onset of neuropsychiatric disorders. From a molecular standpoint, transcriptional alterations are thought to play a major role in this context and may contribute to the behavioral phenotype by shifting the expression of genes related to excitatory and inhibitory (E/I) transmission balance. Nevertheless, the exact neurophysiological mechanisms underlying the enhanced vulnerability to psychopathology following PNS exposure are not well understood. In the present study, we used a model of maternal stress in rats to investigate the distal effects of PNS on the expression of genes related to glutamatergic and GABAergic neurotransmissions. We inspected two critical brain regions involved in emotion regulation, namely, the prefrontal cortex (PFC) and the amygdala (AMY), which we show to relate with the mild behavioral effects detected in adult rat offspring. We observed that PNS exposure promotes E/I imbalance in the PFC of adult males only, by dysregulating the expression of glutamatergic-related genes. Moreover, such an effect is accompanied by increased expression of the activity-dependent synaptic modulator gene Npas4 specifically in the PFC parvalbumin (PV)-positive interneurons, suggesting an altered regulation of synapse formation promoting higher PV-dependent inhibitory transmission and increased overall circuit inhibition in the PFC of males. In the AMY, PNS more evidently affects the transcription of GABAergic-related genes, shifting the balance toward inhibition. Collectively, our findings suggest that the E/I dysregulation of the PFC-to-AMY transmission may be a long-term signature of PNS and may contribute to increase the risk for mood disorder upon further stress.

Using social rank as the lens to focus on the neural circuitry driving stress coping styles

Social hierarchy position in humans is negatively correlated with stress-related psychiatric disease risk. Animal models have largely corroborated human studies, showing that social rank can impact stress susceptibility and is considered to be a major risk factor in the development of psychiatric illness. Differences in stress coping style is one of several factors that mediate this relationship between social rank and stress susceptibility. Coping styles encompass correlated groupings of behaviors associated with differential physiological stress responses. Here, we discuss recent insights from animal models that highlight several neural circuits that can contribute to social rank-associated differences in coping style.

Valence processing in the PFC: Reconciling circuit-level and systems-level views

An essential component in animal behavior is the ability to process emotion and dissociate among positive and negative valence in response to a rewarding or aversive stimulus. The medial prefrontal cortex (mPFC)-responsible for higher order executive functions that include cognition, learning, and working memory; and is also involved in sociability-plays a major role in emotional processing and control. Although the amygdala is widely regarded as the "emotional hub," the mPFC encodes for context-specific salience and elicits top-down control over limbic circuitry. The mPFC can then conduct behavioral responses, via cortico-striatal and cortico-brainstem pathways, that correspond to emotional stimuli. Evidence shows that abnormalities within the mPFC lead to sociability deficits, working memory impairments, and drug-seeking behavior that include addiction and compulsive disorders; as well as conditions such as anhedonia. Recent studies investigate the effects of aberrant salience processing on cortical circuitry and neuronal populations associated with these behaviors. In this chapter, we discuss mPFC valence processing, neuroanatomical connections, and physiological substrates involved in mPFC-associated behavior. We review neurocomputational and theoretical models such as "mixed selectivity," that describe cognitive control, attentiveness, and motivational drives. Using this knowledge, we describe the effects of valence imbalances and its influence on mPFC neural pathways that contribute to deficits in social cognition, while understanding the effects in addiction/compulsive behaviors and anhedonia.