A novel escapable social interaction test reveals that social behavior and mPFC activation during an escapable social encounter are altered by post-weaning social isolation and are dependent on the aggressiveness of the stimulus rat

Neurophysiological analysis of disadvantageous social inequity: Exploring emotional behavior changes and c-Fos expression in a male rat model

Humans and other animals exhibit aversive behavioral and emotional responses to unequal reward distributions compared with their conspecifics. Despite the significance of this phenomenon, experimental animal models designed to investigate social inequity aversion and delve into the underlying neurophysiological mechanisms are limited. In this study, we developed a rat model to determine the effects of socially equal or unequal reward and stress on emotional changes in male rats. During the training session, the rats were trained to escape when a sound cue was presented, and they were assigned to one of the following groups: all escaping rats [advantageous equity (AE)], freely moving rats alongside a restrained rat [advantageous inequity (AI)], all restrained rats [disadvantageous equity (DE)], and a rat restrained in the presence of freely moving companions [disadvantageous inequity (DI)]. During the test session, rats in the advantageous group (AE and AI) escaped after the cue sound (expected reward acquisition), whereas rats in the disadvantageous group (DE and DI) could not escape despite the cue being presented (expected reward deprivation). Emotional alteration induced by exposure to restraint stress under various social interaction circumstances was examined using an open field test. Notably, the DI group displayed reduced exploration of the center zone during the open field tests compared with the other groups, indicating heightened anxiety-like behaviors in response to reward inequity. Immunohistochemical analysis revealed increased c-Fos expression in the medial prefrontal and orbitofrontal cortices, coupled with reduced c-Fos expression in the striatum and nucleus accumbens under DI conditions, in contrast to the other experimental conditions. These findings provide compelling evidence that rats are particularly sensitive to reward inequity, shedding light on the neurophysiological basis for distinct cognitive processes that manifest when individuals are exposed to social equity and inequity situations.

Oxytocin modulation in the medial prefrontal cortex of pair-exposed rats during fear conditioning

INTRODUCTION

Social buffering is the phenomenon, in which stress and fear reactions caused by exposure to stressful stimuli when animals are exposed to homogeneous relationships are attenuated. Social buffering reduces fear memory behavior such as escape, avoidance, and freezing behavior in rodents due to social existence. Here, we aimed to determine alterations of fear behavior and neural activity in the medial prefrontal cortex (mPFC) in response to the presence of another rat in fear-exposed conditions and to confirm the role of oxytocin in mPFC in regulating social buffering.

METHODS

We performed a passive avoidance test and determined positive c-Fos expression in single- and pair-exposed rats. Anisomycin (a protein synthesis inhibitor) and oxytocin receptor regulators (carbetocin; agonist and atosiban; antagonist) were microinjected into the mPFC to clarify the role of oxytocin in the mPFC.

RESULTS

While single-exposed rats showed a significant increase in both freezing and passive avoidance behaviors compared to control rats, pair-exposed rats showed significantly less fear behavior compared to single-exposed rats. The c-Fos expression in the prelimbic (PL) mPFC was significantly increased in pair-exposed rats compared to that in control and single-exposed rats. The pair-exposed effect was blocked by anisomycin injections into the PL mPFC of pair-exposed rats. Furthermore, when a carbetocin was injected into the PL mPFC in single-exposed rats, fear behavior was decreased, and these changes were blocked by atosiban.

DISCUSSION

Our findings suggest that reduction of fear-related behavior induced by acute pair-exposure is mediated by oxytocin receptors in the PL mPFC. Pair exposure with conspecifics during fear-inducing situations helps coping with fear by significantly increasing the role of oxytocin in the PL mPFC.

Experimental Social Stress: Dopaminergic Receptors, Oxidative Stress, and c-Fos Protein Are Involved in Highly Aggressive Behavior

Aggression is defined as hostile behavior that results in psychological damage, injury and even death among individuals. When aggression presents itself in an exacerbated and constant way, it can be considered escalating or pathological. The association between social stress and the emergence of exacerbated aggressiveness is common and is suggested to be interconnected through very complex neurobiological factors. For example, alterations in the expression of the dopaminergic receptors D1 and D2, reactive oxygen species (ROS) and the c-Fos protein in the cortex have been observed. Our objective was to analyze which factors are involved at the neurobiological level in the highly aggressive response of Swiss Webster adult male mice in a vivarium. In this work, we investigated the relationship among dopaminergic receptors, the production of ROS and the expression of c-Fos. Mice with exacerbated aggression were identified by the model of spontaneous aggression (MSA) based on the grouping of young mice and the regrouping of the same animals in adulthood. During the regrouping, we observed different categories of behavior resulting from social stress, such as (i) highly aggressive animals, (ii) defeated animals, and (iii) harmonic groups. To evaluate the dopaminergic system and the c-Fos protein, we quantified the expression of D1 and D2 dopaminergic receptors by Western blotting and fluorescence immunohistochemistry and that of the c-Fos protein by fluorescence immunohistochemistry. The possible production of ROS was also evaluated through the dihydroethidium (DHE) assay. The results showed that aggressive and subordinate mice showed a reduction in the expression of the D1 receptor, and no significant difference in the expression of the D2 receptor was observed between the groups. In addition, aggressive mice exhibited increased production of ROS and c-Fos protein. Based on our results, we suggest that exacerbated aggression is associated with social stress, dysregulation of the dopaminergic system and exacerbated ROS production, which leads to a state of cellular oxidative stress. The overexpression of c-Fos due to social stress suggests an attempt by the cell to produce antioxidant agents to reduce the toxic cellular concentration of ROS.

Involvement of the dopamine system in the effect of chronic social isolation during adolescence on social behaviors in male C57 mice

In the early stage of life, experiencing social isolation can generate long-lasting deleterious effects on behaviors and brain development. However, the effects of chronic social isolation during adolescence on social behaviors and its underlying neurobiological mechanisms remain unclear. The present study found that four weeks of social isolation during adolescence impaired social recognition ability in the three-chamber test and five-trial social recognition test, and increased aggressive-like behaviors, but reduced environmental exploration, as showed in the social interaction test. Chronic social isolation decreased levels of dopamine D2 receptor in the shell of the nucleus accumbens (NAcc) and medial prefrontal cortex. It also reduced TH in the NAcc. Using in vivo fiber photometry, it was also found that isolated mice displayed a reduction in NAcc shell activity upon exploring unfamiliar social stimuli. An injection of a 100 ng dose of the D2R agonist quinpirole into the shell of the NAcc reversed behavioral abnormalities induced by chronic social isolation. These data suggest that the dopamine system is involved in alterations in social behaviors induced by chronic social isolation. This finding sheds light on the mechanism underlying abnormalities in social behavior induced by adolescent chronic social isolation and provides a promising target to treat mental diseases relevant to social isolation.

Reward-related dynamical coupling between basolateral amygdala and nucleus accumbens

Recognizing reward-related stimuli is crucial for survival. Neuronal projections from the basolateral amygdala (BLA) to the nucleus accumbens (NAc) play an important role in processing reward-related cues. Previous studies revealed synchronization between distant brain regions in reward-sensitive neurocircuits; however, whether the NAc synchronizes with the BLA is unknown. Here, we recorded local field potentials simultaneously from the BLA and NAc of rats during social preference tests and an appetitive conditioning test in which explicit stimuli were associated with food. BLA-NAc coherence in the theta band (5-8 Hz) increased in response to food-associated cues. Meanwhile, the modulatory strength of theta-high gamma (50-110 Hz) phase-amplitude cross-frequency coupling (PAC) in the NAc decreased. Importantly, both of these neuromodulations disappeared upon extinction. In contrast, both theta and gamma power oscillations in each region increased in the presence of social conspecifics or contexts associated with conspecifics, but coherence did not change. To potentially disrupt behavior and associated neural activity, a subgroup of rats was exposed prenatally to valproic acid (VPA), which has been shown to disrupt transcriptome and excitatory/inhibitory balance in the amygdala. VPA-exposed rats demonstrated impulsive-like behavior, but VPA did not affect BLA-NAc coherence. These findings reveal changes in BLA-NAc coherence in response to select reward-related stimuli (i.e., food-predictive cues); the differences between the tasks used here could shed light onto the functional nature of BLA-NAc coherence and are discussed.

A novel social fear conditioning procedure alters social behavior and mTOR signaling in differentially housed adolescent rats

Vulnerabilities to fear-related disorders can be enhanced following early life adversity. This study sought to determine whether post-weaning social isolation (PSI), an animal model of early life adversity, alters the development of social fear in an innovative model of conditioned social fear. Male and female Sprague-Dawley rats underwent either social rearing (SR) or PSI for 4 weeks following weaning. Rats were then assigned to groups consisting of either Footshock only, Social conditioned stimulus (CS) only, or Paired footshock with a social CS. Social behavior was assessed the next day. We observed a novel behavioral response in PSI rats, running in circles, that was rarely observed in SR rats; moreover, this behavior was augmented after Paired treatment in PSI rats. Other social behaviors were altered by both PSI and Paired footshock and social CS. The mammalian target of rapamycin (mTOR) pathway was assessed using immunohistochemistry for phosphorylated ribosomal protein S6 (pS6) in subregions of the prefrontal cortex (PFC) and amygdala. Paired treatment produced opposite effects in the PFC and amygdala in males, but no differences were observed in females. Conditioned social fear produced alterations in social behavior and the mTOR pathway that are dependent upon rearing condition and sex.

Social Isolation Rearing Induces Neuropsychiatric Diseases: Updated Overview

Neuropsychiatric and neurologic diseases cause a great burden for individuals, families, and societies. Social isolation rearing can trigger a variety of psychiatric diseases. New advances suggest that epigenetic factors along with other neurochemical changes can be an important topic in neuropsychiatric diseases. It is thought that the prevention of social isolation rearing that occurs around birth can reduce the occurrence of neuropsychiatric diseases. It has been suggested that the environment can induce epigenetic alternation. So, for the diagnosis of a proportion of neuropsychiatric diseases, assessing epigenetic factors may be helpful. Also, apart from epigenetic factors, new advances have been made about new mechanisms of and treatments for such a disorder.

Monoacylglycerol lipase inhibition alters social behavior in male and female rats after post-weaning social isolation

Post-weaning social isolation (PSI) has been shown to increase aggressive behavior and alter medial prefrontal cortex (mPFC) function in rats. The present study sought to determine whether this phenotype would be normalized by increasing levels of the endocannabinoid 2-arachidonoylglycerol (2-AG) using pharmacological inhibition of monoacylglycerol lipase (MAGL). Male and female Sprague-Dawley rats were exposed to either 4 weeks of PSI or social rearing (SR) starting on postnatal day 21, then underwent a 15 min trial of social interaction with a novel, same-sex juvenile rat. Rats were administered an acute injection of the MAGL inhibitor MJN110 or vehicle prior to the social interaction. Rats received either 0 mg/kg (vehicle), 1 mg/kg, or 5 mg/kg of MJN110. Both doses of MJN110 decreased aggressive grooming, a measure of agonistic behavior, in both males and females, largely driven by decreased aggressive grooming in PSI rats. There were no effects of MJN110 on overall social behavior or play behavior, while modest effects were observed on locomotor activity in SR rats only. While social interaction increased c-Fos expression in the mPFC of both males and females, MJN110 reduced c-Fos preferentially in females. These results suggest that 2-AG can modulate specific social behaviors during adolescence, and may affect mPFC function differentially in males and females.