Adolescent Social Isolation as a Model of Heightened Vulnerability to Comorbid Alcoholism and Anxiety Disorders

A novel mouse home cage lickometer system reveals sex- and housing-based influences on alcohol drinking

Alcohol use disorder (AUD) is a significant global health issue. Despite historically higher rates among men, AUD prevalence and negative alcohol-related outcomes in women are rising. Loneliness in humans has been associated with increased alcohol use, and traditional rodent drinking models involve single housing, presenting challenges for studying social enrichment. We developed LIQ PARTI (Lick Instance Quantifier with Poly-Animal RFID Tracking Integration), an open-source tool to examine home cage continuous access two-bottle choice drinking behavior in a group-housed setting, investigating the influence of sex and social isolation on ethanol consumption and bout microstructure in C57Bl/6J mice. LIQ PARTI, based on our previously developed single-housed LIQ HD system, accurately tracks drinking behavior using capacitive-based sensors and RFID technology. Group-housed female mice exhibited higher ethanol preference than males, while males displayed a unique undulating pattern of ethanol preference linked to cage changes, suggesting a potential stress-related response. Chronic ethanol intake distinctly altered bout microstructure between male and female mice, highlighting sex and social environmental influences on drinking behavior. Social isolation with the LIQ HD system amplified fluid intake and ethanol preference in both sexes, accompanied by sex- and fluid-dependent changes in bout microstructure. However, these effects largely reversed upon resocialization, indicating the plasticity of these behaviors in response to social context. Utilizing a novel group-housed home cage lickometer device, our findings illustrate the critical interplay of sex and housing conditions in voluntary alcohol drinking behaviors in C57Bl/6J mice, facilitating nuanced insights into the potential contributions to AUD etiology.

A paradoxical switch: the implications of excitatory GABAergic signaling in neurological disorders

Gamma-aminobutyric acid (GABA) is the primary inhibitory neurotransmitter in the central nervous system. In the mature brain, inhibitory GABAergic signaling is critical in maintaining neuronal homeostasis and vital human behaviors such as cognition, emotion, and motivation. While classically known to inhibit neuronal function under physiological conditions, previous research indicates a paradoxical switch from inhibitory to excitatory GABAergic signaling that is implicated in several neurological disorders. Various mechanisms have been proposed to contribute to the excitatory switch such as chloride ion dyshomeostasis, alterations in inhibitory receptor expression, and modifications in GABAergic synaptic plasticity. Of note, the hypothesized mechanisms underlying excitatory GABAergic signaling are highlighted in a number of neurodevelopmental, substance use, stress, and neurodegenerative disorders. Herein, we present an updated review discussing the presence of excitatory GABAergic signaling in various neurological disorders, and their potential contributions towards disease pathology.

Sexually dimorphic effects of a modified adolescent social isolation paradigm on behavioral risk factors of alcohol use disorder in Long Evans Rats

Early life stress (ELS) is a major risk factor for alcohol use disorder (AUD) and comorbid neuropsychiatric conditions. We previously demonstrated that an adolescent social isolation (aSI) model of ELS significantly increased behavioral risk factors for these disorders (e.g. anxiety-like behaviors, alcohol drinking) in male, but not female rats. Since many neurodevelopmental milestones are accelerated in females, we investigated whether an earlier/shorter isolation window (PND 21-38) would yield comparable phenotypes in both sexes. In two experiments, Long Evans rats were socially isolated (SI) or group-housed (GH) on postnatal day (PND) 21 and locomotion was assessed in the open field test (OFT; PND 30). Experiment 1 also assessed behavior on the elevated plus-maze (EPM) (PND 32). In Experiment 2, all rats were single housed on PND 38 to assess home cage alcohol drinking. Experiment 1 revealed that SI females had increased locomotor activity in the OFT but did not differ from GH subjects on the EPM. The OFT results were replicated in both sexes in Experiment 2 and both male and female SI rats had significantly greater ethanol consumption during an eight day continuous access paradigm. In contrast, during subsequent intermittent two-bottle choice drinking, only SI females displayed greater ethanol intake and preference and increased consumption of a quinine-adulterated alcohol solution. These findings demonstrate that early life social isolation can promote AUD vulnerability-related phenotypes in female rats but that there are profound sex differences in the vulnerability window to this early life stressor. Uncovering the neural mechanisms responsible for these sexually dimorphic differences in sensitivity to ELS may shed light on the biological substrates associated with vulnerability to AUD and comorbid disorders of negative emotion in men and women.

Sex differences in ethanol consumption and drinking despite negative consequences following adolescent social isolation stress in male and female rats

Alcohol use disorder (AUD) is a debilitating psychiatric disorder characterized by drinking despite negative social and biological consequences. AUDs make up 71% of substance use disorders, with relapse rates as high as 80%. Current treatments stem from data conducted largely in males and fail to target the psychological distress motivating drinking in stress-vulnerable and at-risk populations. Here we employed a rat model and hypothesized that early life stress would reveal sex differences in ethanol intake and drinking despite negative consequences in adulthood. Rats were group housed or isolated postweaning to evaluate sex and stress effects on ethanol consumption in homecage drinking, self-administration (SA), and punished SA (drinking despite negative consequences) in adulthood. Stressed rats showed elevated homecage ethanol intake, an effect more pronounced in females. During SA, males were more sensitive to stress-induced elevations of drinking over time, but females drank more overall. Stressed rats, regardless of sex, responded more for ethanol than their non-stressed counterparts. Stressed females showed greater resistance to punishment-suppressed SA than stressed males, indicating a more stress-resistant drinking phenotype. Results support our hypothesis that adolescent social isolation stress enhances adult ethanol intake in a sex- and model-dependent manner with females being especially sensitive to early life stress-induced elevations in ethanol intake and punished SA in adulthood. Our findings echo the clinical literature which indicates that stress-vulnerable populations are more likely to 'self-medicate' with substances. Elucidating a potential mechanism that underlies why vulnerable populations 'self-medicate' with alcohol can lead towards developing catered pharmacotherapeutics that could reduce punishment-resistant drinking and relapse.

The maladaptive alcohol self-administration task: An adapted novel model of alcohol seeking with negative consequences

The progression of recreational drinking to alcohol use disorder is characterized by loss of control over seeking, which involves continued use of alcohol despite negative consequences. The present study proposes a novel maladaptive alcohol self-administration task in which animals are trained to withhold alcohol drinking in the presence of an auditory cue signaling consequence (conflict phase) but to drink freely when there is no consequence (neutral phase). These phases are performed within trial; successful performance involves waiting for the conflict phase to end and drinking during the neutral phase. We discuss the background and implementation of the task, its relation to existing models, and its relevance to the field of translational alcohol research. Importantly, we also present evidence of its efficacy. Both male and female Long-Evans rats are capable of performing the maladaptive alcohol self-administration task for both sweetened and unsweetened alcohol solutions. Finally, we show that acute injection of a pharmacological stressor (yohimbine) significantly disrupted performance of the task in both sexes and reinforcers. We suggest the maladaptive alcohol self-administration task may prove particularly useful in models of alcohol use disorder or vulnerability to this disorder where its application may reveal maladaptive neural circuit adaptations responsible for motivational perturbations associated with loss of control over alcohol seeking.

Stress in adolescence as a first hit in stress-related disease development: Timing and context are crucial

The two-hit stress model predicts that exposure to stress at two different time-points in life may increase or decrease the risk of developing stress-related disorders later in life. Most studies based on the two-hit stress model have investigated early postnatal stress as the first hit with adult stress as the second hit. Adolescence, however, represents another highly sensitive developmental window during which exposure to stressful events may affect programming outcomes following exposure to stress in adulthood. Here, we discuss the programming effects of different types of stressors (social and nonsocial) occurring during adolescence (first hit) and how such stressors affect the responsiveness toward an additional stressor occurring during adulthood (second hit) in rodents. We then provide a comprehensive overview of the potential mechanisms underlying interindividual and sex differences in the resilience/susceptibility to developing stress-related disorders later in life when stress is experienced in two different life stages.

Rats choose alcohol over social reward in an operant choice procedure

The interaction between social factors and alcohol addiction is complex, with potential for both positive and negative contributions to drug use and abstinence. Positive social connections are an important component in successful abstinence, and yet the social context of alcohol use can also lead to relapse. Recently it was shown that rats overwhelmingly choose social reward over methamphetamine, cocaine, and heroin in a discrete choice procedure, and that prolonged choice for social reward attenuates incubation of drug craving. The extent to which this effect generalises to rats trained to self-administer alcohol is not known. In this study we aimed to test the effect of social reward on choice for alcohol in male and female rats. We first validated social reward self-administration in both male and female Long-Evans rats, and found that 60 s access to a social partner of the same sex can serve as an operant reinforcer. Next we trained rats to self-administer both social reward and alcohol (20% ethanol in water), and then used discrete choice trial based tests to determine whether there is a choice preference for alcohol or social reward. Our main finding is that both male and female rats showed persistent choice for alcohol over social reward, with only minor differences between the sexes. We also show that choice for alcohol could be reduced via increased response requirement for alcohol, pre-choice alcohol exposure, and also decreasing the alcohol percentage. This study shows that preference for social rewards over drugs may not generalise to rats self-administering alcohol, and we describe several conditions where choice for social reward can be developed. This study highlights the important contribution of social factors to alcohol abuse, and future studies can investigate the neurobiology underlying a shift in preference from alcohol to social rewards.

Adolescent social isolation shifts the balance of decision-making strategy from goal-directed action to habitual response in adulthood via suppressing the excitatory neurotransmission onto the direct pathway of the dorsomedial striatum

Adverse experience, such as social isolation, during adolescence is one of the major causes of neuropsychiatric disorders that extend from adolescence into adulthood, such as substance addiction, obsessive-compulsive disorder, and eating disorders leading to obesity. A common behavioral feature of these neuropsychiatric disorders is a shift in the balance of decision-making strategy from goal-directed action to habitual response. This study has verified that adolescent social isolation directly shifts the balance of decision-making strategy from goal-directed action to habitual response, and that it cannot be reversed by simple regrouping. This study has further revealed that adolescent social isolation induces a suppression in the excitatory neurotransmission onto the direct-pathway medium spiny neurons of the dorsomedial striatum (DMS), and that chemogenetically compensating this suppression effect shifts the balance of decision-making strategy from habitual response back to goal-directed action. These findings suggest that the plasticity in the DMS causes the shift in the balance of decision-making strategy, which would potentially help to develop a general therapy to treat the various neuropsychiatric disorders caused by adolescent social isolation. Such a study is especially necessary under the circumstances that social distancing and lockdown have caused during times of world-wide, society-wide pandemic.

Post-weaning social isolation increases the incentive value of nicotine-related contexts and decreases the accumulation of ΔFosB in nucleus accumbens in adolescent rats

Adolescent social conditions profoundly affect vulnerability to drug abuse. Preclinical studies have shown that preventing social interactions during adolescence increases the rewarding effects of drugs like alcohol, cocaine, or amphetamines, however, little data exist regarding the impact of social isolation on nicotine effects. The current study evaluated the effects of differential rearing conditions during adolescence (isolation or group rearing) on (1) conditioned place preference induced by low nicotine doses (0.1 or 0.3 mg/kg) and (2) sensitization to the locomotor effects of nicotine after sub-chronic administration (3) and accumulation of ΔFosB in nucleus accumbens (NAc). Results showed that nicotine induced place preference in isolated and grouped rats, but the effect was more persistent for the rats reared in isolation. Isolated reared rats also exhibited lower levels of ΔFosB accumulation in NAc. No differences were found in the behavioral sensitization to nicotine effects between rearing conditions. The results suggest that isolation engenders a more robust incentive value of nicotine-related contexts. This effect could be related to the basal expression of ΔFosB: lower levels of this transcription factor seem to impair the motivation of isolated reared rats and increase their vulnerability to the effects of drugs like nicotine.

The effects of the recurrent social isolation stress on fear extinction and dopamine D2 receptors in the amygdala and the hippocampus

BACKGROUND

The present study assessed the influence of recurrent social isolation stress on the aversive memory extinction and dopamine D₂ receptors (D₂R) expression in the amygdala and the hippocampus subnuclei. We also analyzed the expression of epigenetic factors potentially associated with fear extinction: miRNA-128 and miRNA-142 in the amygdala.

METHODS

Male adult fear-conditioned rats had three episodes of 48 h social isolation stress before each fear extinction session in weeks intervals. Ninety minutes after the last extinction session, the D₂R expression in the nuclei of the amygdala and the hippocampus (immunocytochemical technique), and mRNA levels for D₂R in the amygdala were assessed (PCR). Moreover, we evaluated the levels of miRNA-128 and miRNA-142 in the amygdala.

RESULTS

It was found that recurrent social isolation stress decreased the fear extinction rate. The extinguished isolated rats were characterized by higher expression of D₂R in the CA1 area of the hippocampus compared to the extinguished and the control rats. In turn, the isolated group presented higher D₂R immunoreactivity in the CA1 area compared to the extinguished, the control, and the extinguished isolated animals. Moreover, the extinguished animals had higher expression of D₂R in the central amygdala than the control and the extinguished isolated rats. These changes were accompanied by the increase in miRNA-128 level in the amygdala in the extinguished isolated rats compared to the control, the extinguished, and the isolated rats. Moreover, the extinguished rats had lower expression of miRNA-128 compared to the control and the isolated animals.

CONCLUSIONS

Our results suggest that social isolation stress impairs aversive memory extinction and coexists with changes in the D₂R expression in the amygdala and hippocampus and increased expression of miRNA-128 in the amygdala.

Customizable, wireless and implantable neural probe design and fabrication via 3D printing

This Protocol Extension describes the low-cost production of rapidly customizable optical neural probes for in vivo optogenetics. We detail the use of a 3D printer to fabricate minimally invasive microscale inorganic light-emitting-diode-based neural probes that can control neural circuit activity in freely behaving animals, thus extending the scope of two previously published protocols describing the fabrication and implementation of optoelectronic devices for studying intact neural systems. The 3D-printing fabrication process does not require extensive training and eliminates the need for expensive materials, specialized cleanroom facilities and time-consuming microfabrication techniques typical of conventional manufacturing processes. As a result, the design of the probes can be quickly optimized, on the basis of experimental need, reducing the cost and turnaround for customization. For example, 3D-printed probes can be customized to target multiple brain regions or scaled up for use in large animal models. This protocol comprises three procedures: (1) probe fabrication, (2) wireless module preparation and (3) implantation for in vivo assays. For experienced researchers, neural probe and wireless module fabrication requires ~2 d, while implantation should take 30-60 min per animal. Time required for behavioral assays will vary depending on the experimental design and should include at least 5 d of animal handling before implantation of the probe, to familiarize each animal to their handler, thus reducing handling stress that may influence the result of the behavioral assays. The implementation of customized probes improves the flexibility in optogenetic experimental design and increases access to wireless probes for in vivo optogenetic research.

Effects of voluntary adolescent intermittent alcohol exposure and social isolation on adult alcohol intake in male rats

Initiating alcohol use in adolescence significantly increases the likelihood of developing adult alcohol use disorder (AUD). However, it has been difficult to replicate adolescent alcohol exposure leading to increased adult alcohol intake across differing preclinical models. In the present study, differentially housed male rats (group vs. single cages) were used to determine the effects of voluntary intermittent exposure of saccharin-sweetened ethanol during adolescence on adult intake of unsweetened 20% ethanol. Adolescent male rats were assigned to group- or isolated-housing conditions and underwent an intermittent 2-bottle choice in adolescence (water only or water vs. 0.2% saccharin/20% ethanol), and again in adulthood (water vs. 20% ethanol). Intermittent 2-bottle choice sessions lasted for 24 h, and occurred three days per week, for five weeks. Rats were moved from group or isolated housing to single-housing cages for 2-bottle choice tests and returned to their original housing condition on off days. During adolescence, rats raised in isolated-housing conditions consumed significantly more sweetened ethanol than rats raised in group-housing conditions, an effect that was enhanced across repeated exposures. In adulthood, rats raised in isolated-housing conditions and exposed to sweetened ethanol during adolescence also consumed significantly higher levels of unsweetened 20% ethanol compared to group-housed rats. The effect was most pronounced over the first five re-exposure sessions. Housing conditions alone had little effect on adult ethanol intake. These preclinical results suggest that social isolation stress, combined with adolescent ethanol exposure, may play a key role in adult AUD risk.

Developmental differences in amygdala projection neuron activation associated with isolation-driven changes in social preference

Adolescence is a developmental period characterized by brain maturation and changes in social engagement. Changes in the social environment influence social behaviors. Memories of social events, including remembering familiar individuals, require social engagement during encoding. Therefore, existing differences in adult and adolescent social repertoires and environmentally-driven changes in social behavior may impact novel partner preference, associated with social recognition. Several amygdala subregions are sensitive to the social environment and can influence social behavior, which is crucial for novelty preference. Amygdala neurons project to the septum and nucleus accumbens (NAc), which are linked to social engagement. Here, we investigated how the social environment impacts age-specific social behaviors during social encoding and its subsequent impact on partner preference. We then examined changes in amygdala-septal and -NAc circuits that accompany these changes. Brief isolation can drive social behavior in both adults and adolescents and was used to increase social engagement during encoding. We found that brief isolation facilitates social interaction in adolescents and adults, and analysis across time revealed that partner discrimination was intact in all groups, but there was a shift in preference within isolated and non-isolated groups. We found that this same isolation preferentially increases basal amygdala (BA) activity relative to other amygdala subregions in adults, but activity among amygdala subregions was similar in adolescents, even when considering conditions (no isolation, isolation). Further, we identify isolation-driven increases in BA-NAc and BA-septal circuits in both adults and adolescents. Together, these results provide evidence for changes in neuronal populations within amygdala subregions and their projections that are sensitive to the social environment that may influence the pattern of social interaction within briefly isolated groups during development.

Areas of Convergence and Divergence in Adolescent Social Isolation and Binge Drinking: A Review

Adolescence is a critical developmental period characterized by enhanced social interactions, ongoing development of the frontal cortex and maturation of synaptic connections throughout the brain. Adolescents spend more time interacting with peers than any other age group and display heightened reward sensitivity, impulsivity and diminished inhibitory self-control, which contribute to increased risky behaviors, including the initiation and progression of alcohol use. Compared to adults, adolescents are less susceptible to the negative effects of ethanol, but are more susceptible to the negative effects of stress, particularly social stress. Juvenile exposure to social isolation or binge ethanol disrupts synaptic connections, dendritic spine morphology, and myelin remodeling in the frontal cortex. These structural effects may underlie the behavioral and cognitive deficits seen later in life, including social and memory deficits, increased anxiety-like behavior and risk for alcohol use disorders (AUD). Although the alcohol and social stress fields are actively investigating the mechanisms through which these effects occur, significant gaps in our understanding exist, particularly in the intersection of the two fields. This review will highlight the areas of convergence and divergence in the fields of adolescent social stress and ethanol exposure. We will focus on how ethanol exposure or social isolation stress can impact the development of the frontal cortex and lead to lasting behavioral changes in adulthood. We call attention to the need for more mechanistic studies and the inclusion of the evaluation of sex differences in these molecular, structural, and behavioral responses.

Self-reported anxiety level and related factors in senior high school students in China during the outbreak of coronavirus disease 2019

BACKGROUND

The outbreak of COVID-19 has been a big challenge for senior high school students in China who are facing tremendous pressure of the highly competitive College Entrance Examination.

METHODS

To evaluate the psychological impact of the event in the population, we conducted an anonymous online survey among senior high school students in China between 26 Feb and 4 March, 2020. Information collected included demographic characteristics, attitude toward medical study, infection of COVID-19 in acquaintances, anxiety symptoms evaluated using the GAD-7, and health literacy level measured using the IDSHL.

RESULTS

Of 21,085 participants, 3,575 (17.0%), 943 (4.5%) and 448 (2.1%) reported with mild, moderate, and severe anxiety. Female, higher academic year, worse self-evaluated academic performance, negative attitude toward medical study, living in Hubei province and having acquaintance infected with COVID-19 were significantly associated with anxiety level, while higher education level of mother and higher IDSHL score were associated with a lower risk. The score of IDSHL, particularly of the domain "infectious disease prevention", was associated with the GAD-7 score in a linear pattern (β=-0.0371, p<0.01).

LIMITATIONS

Limitations included the cross-sectional study design unable to infer the casual relationship, anonymous survey, selection bias and self-reported anxiety disorder levels.

CONCLUSIONS

The results suggested that COVID-19 outbreak may increase anxiety level in senior high school students in China. The anxiety related factors observed in this study may help to identify vulnerable individuals and develop interventions.

Friend of the Devil: Negative Social Influences Driving Substance Use Disorders

Substance use disorders in humans have significant social influences, both positive and negative. While prosocial behaviors promote group cooperation and are naturally rewarding, distressing social encounters, such as aggression exhibited by a conspecific, are aversive and can enhance the sensitivity to rewarding substances, promote the acquisition of drug-taking, and reinstate drug-seeking. On the other hand, withdrawal and prolonged abstinence from drugs of abuse can promote social avoidance and suppress social motivation, accentuating drug cravings and facilitating relapse. Understanding how complex social states and experiences modulate drug-seeking behaviors as well as the underlying circuit dynamics, such as those interacting with mesolimbic reward systems, will greatly facilitate progress on understanding triggers of drug use, drug relapse and the chronicity of substance use disorders. Here we discuss some of the common circuit mechanisms underlying social and addictive behaviors that may underlie their antagonistic functions. We also highlight key neurochemicals involved in social influences over addiction that are frequently identified in comorbid psychiatric conditions. Finally, we integrate these data with recent findings on (±)3,4-methylenedioxymethamphetamine (MDMA) that suggest functional segregation and convergence of social and reward circuits that may be relevant to substance use disorder treatment through the competitive nature of these two types of reward. More studies focused on the relationship between social behavior and addictive behavior we hope will spur the development of treatment strategies aimed at breaking vicious addiction cycles.

Post-weaning Social Isolation in Male and Female Prairie Voles: Impacts on Central and Peripheral Immune System

The socially monogamous prairie vole (Microtus ochrogaster) offers a unique opportunity to examine the impacts of adolescent social isolation on the brain, immune system, and behavior. In the current study, male and female prairie voles were randomly assigned to be housed alone or with a same-sex cagemate after weaning (i.e., on postnatal day 21-22) for a 6-week period. Thereafter, subjects were tested for anxiety-like and depressive-like behaviors using the elevated plus maze (EPM) and Forced Swim Test (FST), respectively. Blood was collected to measure peripheral cytokine levels, and brain tissue was processed for microglial density in various brain regions, including the Nucleus Accumbens (NAcc), Medial Amygdala (MeA), Central Amygdala (CeA), Bed Nucleus of the Stria Terminalis (BNST), and Paraventricular Nucleus of the Hypothalamus (PVN). Sex differences were found in EPM and FST behaviors, where male voles had significantly lower total arm entries in the EPM as well as lower latency to immobility in the FST compared to females. A sex by treatment effect was found in peripheral IL-1β levels, where isolated males had a lower level of IL-1β compared to cohoused females. Post-weaning social isolation also altered microglial density in a brain region-specific manner. Isolated voles had higher microglial density in the NAcc, MeA, and CeA, but lower microglial density in the dorsal BNST. Cohoused male voles also had higher microglial density in the PVN compared to cohoused females. Taken together, these data suggest that post-weaning social housing environments can alter peripheral and central immune systems in prairie voles, highlighting a potential role for the immune system in shaping isolation-induced alterations to the brain and behavior.

FACTORS CONTRIBUTING TO THE ESCALATION OF ALCOHOL CONSUMPTION

Understanding factors that contribute to the escalation of alcohol consumption is key to understanding how an individual transitions from non/social drinking to AUD and to providing better treatment. In this review, we discuss how the way ethanol is consumed as well as individual and environmental factors contribute to the escalation of ethanol consumption from intermittent low levels to consistently high levels. Moreover, we discuss how these factors are modelled in animals. It is clear a vast array of complex, interacting factors influence changes in alcohol consumption. Some of these factors act early in the acquisition of ethanol consumption and initial escalation, while others contribute to escalation of ethanol consumption at a later stage and are involved in the development of alcohol dependence. There is considerable need for more studies examining escalation associated with the formation of dependence and other hallmark features of AUD, especially studies examining mechanisms, as it is of considerable relevance to understanding and treating AUD.

Early life stress induces anxiety-like behavior during adulthood through dysregulation of neuronal plasticity in the basolateral amygdala

AIMS

Early life stress (ELS) increases the risk of psychiatric diseases such as anxiety disorders and depression in later life. Hyperactivation of the basolateral amygdala (BLA) neurons plays a pivotal role in the pathogenesis of stress-related diseases. However, the functional roles of BLA neurons in ELS-induced anxiety disorders are not completely understood.

MAIN METHODS

Mice were subjected to maternal separation (MS) during postnatal days 3 to 21 to mimic ELS. Anxiety-like behavior was tested by open field test (OFT), elevated plus maze (EPM), and novelty suppressed feeding (NSF). Then, c-fos expression, a proxy for neuronal activity, was evaluated by immunofluorescence. Finally, synaptic transmission and intrinsic excitability were measured by whole-cell patch-clamp recordings.

KEY FINDINGS

MS significantly increased anxiety-like behavior in adulthood, as indicated by less time spent in the center area of the OFT, less time spent in and fewer entries to the open arms of the EPM, and increased latency to feed in NSF. Mechanistically, MS increased the expression of c-fos in BLA. MS enhanced the excitatory, but not inhibitory, synaptic transmission onto BLA projection neurons (PNs), which was caused by enhanced presynaptic glutamate release. Moreover, MS also markedly increased the intrinsic neuronal excitability of BLA PNs, probably due to the reduced medium afterhyperpolarization (mAHP) in BLA PNs.

SIGNIFICANCE

Our results suggest that the changes of neuronal activity and synaptic transmission in the BLA PNs may play a crucial role in ELS-induced anxiety-like behavior, and these findings provide new insights into the pathological mechanisms of stress-related anxiety disorders.

Evaluation of Treatment Options for Ulcerative Dermatitis in the P Rat

Rotational outbred adult rats, phenotypically selected to prefer drinking alcohol ("P" rats) frequently present with selfinflicted wounds and ulcerative dermatitis, similar to that seen in C57BL/6 mice. Historically, veterinary interventions used to address this clinical condition have included triple antibiotic ointment (TABO), Columbia wound powder (CPW), nail trims, or plastic tubes that allow affected animals to hide. More recent studies have suggested that nail trims are the most successful intervention in mice, but this has not been evaluated previously in rats. In this study, we evaluated nail trims in rats and also tested whether placing a pumice stone in the cage would reduce the need for nail trims to reduce self-inflicted wounds. Our hypothesis was that interacting with the pumice stone would dull/trim the rats' nails without causing stress or illness and allow the wounds time to heal. We used 66 P rats that were assigned to 1 of 6 treatment groups (pumice stone, TABO, CWP, huts, nail trims, and an untreated control group) of 11 rats each. Rats were transferred to this study from a colony of experimentally naïve animals that had evidence of dermatitis. The wounds were photographed and measured for 12 wk at 2 wk intervals. At the end of the study, representative skin samples from the site of the wound were collected for histopathologic evaluation of inflammation. Our data showed no significant differences in the inflammation scores. The rats treated with nail trims healed significantly more often than did all of the other treatment groups. This suggests that nail trims are the most effective intervention for treating self-inflicted wounds in P rats.

Enriched environment and social isolation differentially modulate addiction-related behaviors in male offspring of morphine-addicted dams: The possible role of μ-opioid receptors and ΔFosB in the brain reward pathway

Prenatal opioids exposure negatively affects the neurobehavioral abilities of children born from dependence dams. Adolescent housing conditions can buffer the detrimental impacts of early life experiences or contradictory can worsen individual psychosocial functions. The present study investigated the effects of maternal morphine dependence and different rearing conditions on behaviors and protein expression in brain reward circuits of male pups. Female Wistar rats a week before conception, during pregnancy and lactation were injected twice daily with escalating doses of morphine or saline. On a postnatal day 21, male pups were weaned and subjected to three different environments for two months: standard (STD), isolated (ISO), or enriched environment (EE). The anxiety and drug-related reward were measured using elevated plus maze, open field test, and conditioned place preference. Western blotting was used to determine the protein level of ΔFosB and μ-opioid receptor proteins in the striatum and the midbrain of male offspring, respectively. Results showed that maternal morphine administration dramatically increased anxiety-like and morphine place preference behaviors in offspring. Also, ISO condition aggravated these behavioral outcomes. While, rearing in EE could attenuate anxiety and morphine conditioning in pups. At molecular levels, maternal morphine exposure and social isolation markedly increased both of ΔFosB and μ-opioid receptor proteins expression. However, rearing in the EE declined ΔFosB protein expression. Together, these findings help to elucidate long lasting impacts of early life morphine exposure and rearing environment on the behavioral and molecular profile of addicted individuals.

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.

Social isolation at adolescence: a systematic review on behaviour related to cocaine, amphetamine and nicotine use in rats and mice

Adolescence is known for its high level of risk-taking, and neurobiological alterations during this period may predispose to psychoactive drug initiation and progression into more severe use patterns. Stress is a risk factor for drug consumption, and post-weaning social isolation increases drug self-administration in rodents. This review aimed to provide an overview of the effects of adolescent social isolation on cocaine, amphetamine and nicotine use-related behaviours, highlighting the specific period when animals were submitted to stress and these drugs. We wondered if there was a specific period during adolescence that isolation stress would increase drug use vulnerability. A total of 323 publications from the Scopus, Web of Science and PubMed (Medline) electronic databases were identified using the words "social isolation" and "adolescence" and "drug" or "cocaine" or "amphetamine" or "nicotine", resulting in 24 articles after analyses criteria following the PRISMA statement. The main points raised were social isolation during adolescence increased cocaine self-administration, amphetamine and nicotine locomotor activity. We did not observe a pattern of a specific moment during the adolescent period that could lead to an increased vulnerability to drug use. The precise conditions under which adolescent social stress alters drug use parameters are complex and likely depend on several factors.

Early life stress induces hyperactivity but not increased anxiety-like behavior or ethanol drinking in outbred heterogeneous stock rats

Early life stress is known to impact vulnerability to psychopathological disorders in adulthood, including anxiety and alcohol use disorder (AUD), but the mechanisms underlying susceptibility to these outcomes are not fully understood. In the current study, we used adolescent social isolation (ASI) to determine whether Heterogeneous Stock (HS) rats, an outbred model used for genetic fine-mapping, could be used to study the genetics contributing to ASI-induced anxiety- and AUD-like behavior. We isolated (ASI) or group-housed (adolescent group-housed; AGH) 64 male HS rats at 4 weeks of age. After 5 weeks in these housing conditions, multiple anxiety and coping/despair-like behaviors were measured. All rats were then individually housed and assessed for voluntary ethanol self-administration. At euthanasia, synaptoneurosomes were isolated from a subset of brains to examine the expression of two proteins associated with alcohol drinking-related behaviors, GluA1 and SK2, in the dorsal (dHC) and ventral hippocampus (vHC). We found that ASI increased hyperactivity in the open field test relative to AGH, with no changes in other anxiety-like behaviors. Surprisingly, ASI rats demonstrated decreased immobility and increased climbing in the forced swim test relative to AGH. In contrast to prior studies by us and others, we found no difference in self-administration of 20% ethanol, with decreased ethanol self-administration in ASI relative to AGH rats at higher ethanol concentrations. Furthermore, while ASI in Long-Evans rats resulted in decreased SK2 expression in vHC synaptosomes, no differences were seen in vHC synaptosomes for SK2 or GluA1 in HS rats. These results demonstrate that HS rats are protected against many of the negative effects previously seen in response to ASI, namely anxiety-like behavior and increased ethanol self-administration. The current work suggests that a lack of change in SK2 and GluA1 expression levels in the vHC may play a role in conferring this protection.

The environmental enrichment model revisited: A translatable paradigm to study the stress of our modern lifestyle

Mounting evidence shows that physical activity, social interaction and sensorimotor stimulation provided by environmental enrichment (EE) exert several neurobehavioural effects traditionally interpreted as enhancements relative to standard housing (SH) conditions. However, this evidence rather indicates that SH induces many deficits, which could be ameliorated by exposing animals to an environment vaguely mimicking some features of their wild habitat. Rearing rodents in social isolation (SI) can aggravate such deficits, which can be restored by SH or EE. It is not surprising, therefore, that most preclinical stress models have included severe and unnatural stressors to produce a stress response prominent enough to be distinguishable from SH or SI-frequently used as control groups. Although current stress models induce a stress-related phenotype, they may fail to represent the stress of our urban lifestyle characterized by SI, poor housing and working environments, sedentarism, obesity and limited access to recreational activities and exercise. In the following review, we discuss the stress of living in urban areas and how exposures to and performing activities in green environments are stress relievers. Based on the commonalities between human and animal EE, we discuss how models of housing conditions (e.g., SI-SH-EE) could be adapted to study the stress of our modern lifestyle. The housing conditions model might be easy to implement and replicate leading to more translational results. It may also contribute to accomplishing some ethical commitments by promoting the refinement of procedures to model stress, diminishing animal suffering, enhancing animal welfare and eventually reducing the number of experimental animals needed.

Using zebrafish (Danio rerio) models to understand the critical role of social interactions in mental health and wellbeing

Social behavior represents a beneficial interaction between conspecifics that is critical for maintaining health and wellbeing. Dysfunctional or poor social interaction are associated with increased risk of physical (e.g., vascular) and psychiatric disorders (e.g., anxiety, depression, and substance abuse). Although the impact of negative and positive social interactions is well-studied, their underlying mechanisms remain poorly understood. Zebrafish have well-characterized social behavior phenotypes, high genetic homology with humans, relative experimental simplicity and the potential for high-throughput screens. Here, we discuss the use of zebrafish as a candidate model organism for studying the fundamental mechanisms underlying social interactions, as well as potential impacts of social isolation on human health and wellbeing. Overall, the growing utility of zebrafish models may improve our understanding of how the presence and absence of social interactions can differentially modulate various molecular and physiological biomarkers, as well as a wide range of other behaviors.

Problematic Use of Alcohol and Online Gaming as Coping Strategies During the COVID-19 Pandemic: A Mini Review

The COVID-19 (coronavirus disease 2019) pandemic has dramatically changed our daily lives and activities, including those originally intended to serve for leisure and pleasure. Drinking and online gaming became coping behaviors used to rescue ourselves from the stress and restricted lifestyle during the COVID-19 pandemic. However, frequent drinking and gaming can result in the pathological consequences of addiction. Those affected use the stimuli not to obtain pleasure, but rather to avoid the displeasure induced by stress and previous use, often unsuccessfully. This review aims to provide an overview of recent longitudinal cohort studies on alcohol and gaming use during the COVID-19 pandemic, as well as to analyze how the pandemic has affected alcohol and gaming use. There was a substantial risk of alcohol and online gaming overuse during the lockdown, which may depend on the pandemic's duration or overuse patterns. Previous studies have shown that increased alcohol consumption and online gaming are associated with heightened stress and anxiety levels caused by social isolation/quarantine. Over time, frequent or excessive alcohol consumption and gaming could lead to an increased risk of more serious mental health problems. Every effort should be made to mitigate mental health problems and ensure adequate adaptation to these exceptional circumstances. Therefore, it would be helpful to encourage physical activity, social interaction, and collaboration to facilitate psychological and physical health.

Adolescent low-dose ethanol drinking in the dark increases ethanol intake later in life in C57BL/6J, but not DBA/2J mice

Alcohol is the most widely used and abused drug among youth in the United States. Youths aged 12-20 years old drink almost 11% of all alcohol consumed in the United States, and typically these young people are consuming alcohol in the form of binge drinking. Particularly concerning is that the risk of developing an alcohol use disorder over their lifetime increases the younger one begins to drink. Here we investigated the impact of ethanol drinking in early adolescence on adult ethanol intake using C57BL/6J and DBA/2J mice. We modeled low-dose drinking in adolescent mice using a modified Drinking in the Dark (DID) model where the total ethanol intake during adolescence was similar between the strains to specifically ask whether low-dose ethanol exposure in the high-alcohol preferring C57BL/6J strain will also lead to increased ethanol intake in adulthood. Our results show that low-dose ethanol drinking in early adolescence dramatically increases adult intake, but only in the alcohol-preferring C57BL/6J strain. Early adolescent ethanol exposure had no effect on ethanol intake in the alcohol-nonpreferring DBA/2J mice. These data add to the growing evidence that low-dose ethanol exposures, below the pharmacologically relevant dose, can also contribute to increased drinking in adulthood, but the effect may be influenced by genetic background.

The Neural Processes Interlinking Social Isolation, Social Support, and Problem Alcohol Use

BACKGROUND

Subjective feeling of social isolation, as can be measured by perceived burdensomeness (PB), is a major risk factor for alcohol misuse. Heightened PB is associated with elevated stress response and diminished cognitive control, both of which contribute to problem drinking. Here, we sought to identify the neural substrates underlying the relationship between PB and alcohol misuse.

METHODS

We employed resting-state functional magnetic resonance imaging data collected from 61 problem drinkers to characterize the functional connectivity of the hypothalamus and ventral striatum (VS) in relation to PB. We specifically examined whether the connectivities of the hypothalamus and VS were differentially influenced by PB to produce contrasting effects on alcohol use. Finally, we evaluated how individual differences in social support modulate the inter-relationships of social isolation, neural connectivity, and the severity of problem drinking.

RESULTS

Whole-brain multiple regressions show a positive relationship between PB and hypothalamic connectivity with the hippocampus and an inverse pattern for VS connectivity with the middle frontal gyrus. Difference in strength between the 2 connectivities predicted the severity of problem drinking, suggesting an imbalance involving elevated hypothalamic and diminished prefrontal cortical modulation in socially isolated problem drinkers. A path analysis further revealed that the lack of social support was associated with a bias toward low prefrontal connectivity, which in turn increased PB and facilitated problem drinking.

CONCLUSIONS

Altered hypothalamus and VS connectivity may underlie problem drinking induced by social isolation. The current findings also highlight the important role of social support as a potential protective factor against alcohol misuse.

The effects of age, sex, and handling on behavioral parameters in the multivariate concentric square field™ test

The multivariate concentric square field™ (MCSF) is a complex and ethologically relevant apparatus that is designed to measure several behavioral parameters within the same test session including risk-taking, risk-assessment, shelter-seeking (anxiety relieving), exploration, and general activity. While several studies have behaviorally and pharmacologically validated the use of the MCSF in adults, far fewer have used adolescents. Given the well-established link between adolescence and risk-taking, it is important to validate use of the MCSF in adolescence. The present study compared the effects of age, sex, and handling on behavioral categories in the MCSF. In addition, principal component analyses were used to compare the underlying behavioral components in adolescent and adult Sprague-Dawley rats. Results revealed that handling increased risk-taking and reduced shelter-seeking. Females were more exploratory than males, but no compelling age differences in risk-taking or risk-assessment were found. Principal component analyses revealed six major principal components for both adolescents and adults with the first and second components consisting mainly of center/center circle, risk-assessment, and shelter-seeking variables in adolescence, and general activity and center/center circle variables in adults. These results confirm age differences in the underlying behavioral components in the MCSF.

The neural, behavioral, and epidemiological underpinnings of comorbid alcohol use disorder and post-traumatic stress disorder

Alcohol use disorder (AUD) and (PTSD) frequently co-occur and individuals suffering from this dual diagnosis often exhibit increased symptom severity and poorer treatment outcomes than those with only one of these diseases. Although there have been significant advances in our understanding of the neurobiological mechanisms underlying each of these disorders, the neural underpinnings of the comorbid condition remain poorly understood. This chapter summarizes recent epidemiological findings on comorbid AUD and PTSD, with a focus on vulnerable populations, the temporal relationship between these disorders, and the clinical consequences associated with the dual diagnosis. We then review animal models of the comorbid condition and emerging human and non-human animal research that is beginning to identify maladaptive neural changes common to both disorders, primarily involving functional changes in brain reward and stress networks. We end by proposing a neural framework, based on the emerging field of affective valence encoding, that may better explain the epidemiological and neural findings on AUD and PTSD.

Dose-dependent effects of alcohol injections on omission-contingency learning have an inverted-U pattern

Previous examinations of the long-term effects of alcohol exposure on omission-contingency learning have produced mixed results across different age or sex groups, with evidence for faster learning or no effect. However, none of these experiments made comparisons using the same exposure-dose across the age/sex groups. Here, we exposed rats to 6 weeks of alcohol injections (3 days/week, 1.75 or 3.5 g/kg/24-h, i.p. broken up into 2 injections/day) in adolescent/early adult males or females (PND27-66) or adult males (PND62-101). We then tested the rats in autoshaping and omission-contingency tasks. In contrast to our hypotheses, the low 1.75-g/kg/24-h dose led to slower omission learning and the higher 3.5-g/kg/24-h dose had no effect. There were no age- or sex-differences in omission learning. Additionally, during autoshaping training, rats exposed in adolescence/early adulthood had a faster shift to sign-tracking in their sign-tracking/goal-tracking ratios than rats exposed in adulthood, with no consistent effect of alcohol exposure or sex-differences. Our results suggest complex effects of alcohol on the neural substrates of omission-contingency learning at different doses, which will require future investigation.

Social Isolation Stress in Adolescence, but not Adulthood, Produces Hypersocial Behavior in Adult Male and Female C57BL/6J Mice

Chronic stress during the developmental period of adolescence increases susceptibility to many neuropsychiatric diseases in adulthood, including anxiety, affective, and alcohol/substance use disorders. Preclinical rodent models of adolescent stress have produced varying results that are species, strain, sex, and laboratory-dependent. However, adolescent social isolation is a potent stressor in humans that has been reliably modeled in male rats, increasing adult anxiety-like and alcohol drinking behaviors, among others. In this study, we examined the generalizability and sex-dependence of this model in C57BL/6J mice, the most commonly used rodent strain in neuroscience research. We also performed a parallel study using social isolation in adulthood to understand the impact of adult social isolation on basal behavioral phenotypes. We found that 6 weeks of social isolation with minimal handling in adolescence through early adulthood [postnatal day (PD) 28-70] produced a hypersocial phenotype in both male and female mice and an anxiolytic phenotype in the elevated plus-maze in female mice. However, it had no effects in other assays for avoidance behavior or on fear conditioning, alcohol drinking, reward or aversion sensitivity, or novel object exploration in either sex. In contrast, 6 weeks of social isolation in adulthood beginning at PD77 produced an anxiogenic phenotype in the light/dark box but had no effects on any other assays. Altogether, our results suggest that: (1) adolescence is a critical period for social stress in C57BL/6J mice, producing aberrant social behavior in a sex-independent manner; and (2) chronic individual housing in adulthood does not alter basal behavioral phenotypes that may confound interpretation of behavior following other laboratory manipulations.

Intravenous Ethanol Administration and Operant Self-Administration Alter Extracellular Norepinephrine Concentration in the Mesocorticolimbic Systems of Male Long Evans Rats

BACKGROUND

Norepinephrine has been suggested to regulate ethanol (EtOH)-related behaviors, but little is known about the effects of EtOH on norepinephrine release in mesocortical and mesolimbic brain areas that are targets of EtOH actions.

METHODS

We used in vivo microdialysis to examine the effects of EtOH on extracellular norepinephrine concentrations in mesocorticolimbic brain regions of male Long Evans rats. We determined the effects of intravenous infusion of saline or EtOH in the medial prefrontal cortex (mPFC) and the basal forebrain. We also measured dialysate norepinephrine concentrations during operant self-administration of EtOH in the mPFC.

RESULTS

Intravenous infusion (1 or 0.25 ml/min) of 1.0 g/kg EtOH stimulated an increase in dialysate norepinephrine in mPFC and in basal forebrain. In the basal forebrain, an infusion of 0.5 g/kg EtOH did not stimulate dialysate norepinephrine concentrations. In both regions, saline infusions did not increase dialysate norepinephrine concentrations. In the behavioral experiment, 1 week of experience with operant self-administration of sweetened EtOH resulted in an apparent reduction in basal dialysate norepinephrine concentrations in the mPFC relative to the sucrose control. Dialysate norepinephrine increased during the transfer from home cage to the operant chamber in all groups.

CONCLUSIONS

We conclude that acute EtOH stimulates both the locus coeruleus (which projects to the mPFC) and the nucleus tractus solitarius (which projects to the basal forebrain) noradrenergic neurons. Additionally, limited EtOH self-administration experience alters dialysate norepinephrine in the mPFC in a manner consistent with a decrease in tonic norepinephrine release. Further studies are necessary to elucidate the mechanisms by which EtOH exerts these variable effects.

Adolescent forced swim stress increases social anxiety-like behaviors and alters kappa opioid receptor function in the basolateral amygdala of male rats

Adolescence is a developmental period marked by robust neural alterations and heightened vulnerability to stress, a factor that is highly associated with increased risk for emotional processing deficits, such as anxiety. Stress-induced upregulation of the dynorphin/kappa opioid receptor (DYN/KOP) system is thought to, in part, underlie the negative affect associated with stress. The basolateral amygdala (BLA) is a key structure involved in anxiety, and neuromodulatory systems, such as the DYN/KOP system, can 1) regulate BLA neural activity in an age-dependent manner in stress-naïve animals and 2) underlie stress-induced anxiety in adults. However, the role of the DYN/KOP system in modulating stress-induced anxiety in adolescents is unknown. To test this, we examined the impact of an acute, 2-day forced swim stress (FSS - 10 min each day) on adolescent (~postnatal day (P) 35) and adult Sprague-Dawley rats (~P70), followed by behavioral, molecular and electrophysiological assessment 24 h following FSS. Adolescent males, but not adult males or females of either age, demonstrated social anxiety-like behavioral alterations indexed via significantly reduced social investigation and preference when tested 24 h following FSS. Conversely, adult males exhibited increased social preference. While there were no FSS-induced changes in expression of genes related to the DYN/KOP system in the BLA, these behavioral alterations were associated with alterations in BLA KOP function. Specifically, while GABA transmission in BLA pyramidal neurons from non-stressed adolescent males responded variably (potentiated, suppressed, or was unchanged) to the KOP agonist, U69593, U69593 significantly inhibited BLA GABA transmission in the majority of neurons from stressed adolescent males, consistent with the observed anxiogenic phenotype in stressed adolescent males. This is the first study to demonstrate stress-induced alterations in BLA KOP function that may contribute to stress-induced social anxiety in adolescent males. Importantly, these findings provide evidence for potential KOP-dependent mechanisms that may contribute to pathophysiological interactions with subsequent stress challenges.

Short and prolonged maternal separation impacts on ethanol-related behaviors in rats: sex and age differences

Maternal separation (MS) is an animal model widely used to evaluate the influence of early-life stress exposure on ethanol consumption and dependence. The goal of this study was to evaluate the effects of brief and prolonged MS on the pattern of consumption and ethanol conditioned place preference (CPP) in male and female rats during adolescence and adulthood. Wistar rat pups were separated daily from their dams for 15 or 180 minutes during the 2 to 10 postnatal days (PND). In adolescence, half of the litter from each group was evaluated in the ethanol consumption test using the three-bottle test choice paradigm. In addition, using biased procedure, ethanol-conditioned place preference was also evaluated. In adulthood, the other half of the litter was evaluated on the same tests. Our results showed that there are differences in consumption pattern and in alcohol reinforcement between males and females, adolescents and adults. While prolonged MS had no effect on total ethanol consumption in adolescents of both sexes, it induced CPP in these animals. In turn, in adults, previous exposure to prolonged MS increased ethanol consumption without altering ethanol-CPP.Lay summaryGiving the importance of the mother-children (dam-pups when talking about rodents) relationship to proper brain development, the separation of pups from their dam is broadly used as an animal model to study the impact of early-life stress exposure. Here, we used a protocol of brief or prolonged maternal separation to study the impact of early-life stress exposure in the alcohol consumption and conditioned place preference in rats, and how age and sex influence it. We showed that, overall, the prolonged maternal separation increased alcohol consumption in both males and females, but only when animals were tested during the adulthood. In the other hand, prolonged maternal separation increased ethanol conditioned place preference in adolescent rats, both male and female.

The effects of social instability stress and subsequent ethanol consumption in adolescence on brain and behavioral development in male rats

Excessive drinking in adolescence continues to be a problem, and almost a quarter of young Canadians have reported consuming five or more alcoholic drinks in one occasion in recent surveys. The consequences of such drinking may be more pronounced when commenced in adolescence, given the ongoing brain development during this period of life. Here, we investigated the consequences of 3 weeks' intermittent access to ethanol in mid-adolescence to early adulthood in rats, and the extent to which a stress history moderated the negative consequences of ethanol access. In experiment 1, male rats that underwent adolescent social instability stress (SS; daily 1 h isolation + return to unfamiliar cage partner every day from postnatal day [PND] 30-45) did not differ from control (CTL) rats in intake of 10% ethanol sweetened with 0.1% saccharin (access period; PND 47-66). Ethanol drinking reduced proteins relevant for synaptic plasticity (αCaMKII, βCaMKII, and PSD-95) in the dorsal hippocampus, and in CTL rats only in the prefrontal cortex (αCaMKII and PSD 95), attenuating the difference between CTL and SS rats in the water-drinking group. In experiment 2, ethanol also attenuated the difference between SS and CTL rats in a social interaction test by reducing social interaction in SS rats; CTL rats, however, had a higher intake of ethanol than did SS rats during the access period. Ethanol drinking reduced baseline and fear recall recovery concentrations of corticosterone relative to those exposed only to water, although there was no effect of either ethanol or stress history on fear conditioning. Ethanol drinking did not influence intake after 9 days of withdrawal; however, ethanol-naïve SS rats drank more than did CTL rats when given a 24-h access in adulthood. These results reveal a complex relationship between stress history and ethanol intake in adolescence on outcomes in adulthood.

Impact of Social Isolation on the Behavioral, Functional Profiles, and Hippocampal Atrophy Asymmetry in Dementia in Times of Coronavirus Pandemic (COVID-19): A Translational Neuroscience Approach

The impact of COVID-19 on the elderly is devastating, and nursing homes are struggling to provide the best care to the most fragile. The urgency and severity of the pandemic forces the use of segregation in restricted areas and confinement in individual rooms as desperate strategies to avoid the spread of disease and the worst-case scenario of becoming a deadly trap. The conceptualization of the post-COVID-19 era implies strong efforts to redesign all living conditions, care/rehabilitation interventions, and management of loneliness forced by social distance measures. Recently, a study of gender differences in COVID-19 found that men are more likely to suffer more severe effects of the disease and are over twice as likely to die. It is well-known that dementia is associated with increased mortality, and males have worse survival and deranged neuro-immuno-endocrine systems than females. The present study examines the impact of long-term isolation in male 3xTg-AD mice modeling advanced stages of Alzheimer's disease (AD) and as compared to age-matched counterparts with normal aging. We used a battery of ethological and unconditioned tests resembling several areas in nursing homes. The main findings refer to an exacerbated (two-fold increase) hyperactivity and emergence of bizarre behaviors in isolated 3xTg-AD mice, worrisome results since agitation is a challenge in the clinical management of dementia and an important cause of caregiver burden. This increase was consistently shown in gross (activity in most of the tests) and fine (thermoregulatory nesting) motor functions. Isolated animals also exhibited re-structured anxiety-like patterns and coping-with-stress strategies. Bodyweight and kidney weight loss were found in AD-phenotypes and increased by isolation. Spleen weight loss was isolation dependent. Hippocampal tau pathology was not modified, but asymmetric atrophy of the hippocampus, recently described in human patients with dementia and modeled here for the first time in an animal model of AD, was found to increase with isolation. Overall, the results show awareness of the impact of isolation in elderly patients with dementia, offering some guidance from translational neuroscience in these times of coronavirus and post-COVID-19 pandemic. They also highlight the relevance of personalized-based interventions tailored to the heterogeneous and complex clinical profile of the individuals with dementia and to consider the implications on caregiver burden.

A Novel Neighbor Housing Environment Enhances Social Interaction and Rescues Cognitive Deficits from Social Isolation in Adolescence

Adolescence is characterized by high levels of playful social interaction, cognitive development, and increased risk-taking behavior. Juvenile exposure to social isolation or social stress can reduce myelin content in the frontal cortex, alter neuronal excitability, and disrupt hypothalamic pituitary adrenal (HPA) axis function. As compared to group housed animals, social isolation increases anxiety-like phenotypes and reduces social and cognitive performance in adulthood. We designed a neighbor housing environment to alleviate issues related to social isolation that still allowed individual homecages. Neighbor housing consists of four standard mouse cages fused together with semi-permeable ports that allow visual, olfactory, and limited social contact between mice. Adolescent C57BL/6J males and females were group housed (4/cage), single housed (1/cage), or neighbor housed (4/complex). As adults, mice were tested for social, anxiety-like, and cognitive behaviors. Living in this neighbor environment reduced anxiety-like behavior in the social interaction task and in the light-dark task. It also rescued cognitive deficits from single housing in the novel object recognition task. These data suggest that neighbor housing may partially ameliorate the social anxiety and cognitive deficits induced by social isolation. These neighbor cage environments may serve as a conduit by which researchers can house mice in individual cages while still enabling limited social interactions to better model typical adolescent development.

From adolescence to late aging: A comprehensive review of social behavior, alcohol, and neuroinflammation across the lifespan

The passage of time dictates the pace at which humans and other organisms age but falls short of providing a complete portrait of how environmental, lifestyle and underlying biological processes contribute to senescence. Two fundamental features of the human experience that change dramatically across the lifespan include social interactions and, for many, patterns of alcohol consumption. Rodent models show great utility for understanding complex interactions among aging, social behavior and alcohol use and abuse, yet little is known about the neural changes in late aging that contribute to the natural decline in social behavior. Here, we posit that aging-related neuroinflammation contributes to the insipid loss of social motivation across the lifespan, an effect that is exacerbated by patterns of repeated alcohol consumption observed in many individuals. We provide a comprehensive review of (i) neural substrates crucial for the expression of social behavior under non-pathological conditions; (ii) unique developmental/lifespan vulnerabilities that may contribute to the divergent effects of low-and high-dose alcohol exposure; and (iii) aging-associated changes in neuroinflammation that may sit at the intersection between social processes and alcohol exposure. In doing so, we provide an overview of correspondence between lifespan/developmental periods between common rodent models and humans, give careful consideration to model systems used to aptly probe social behavior, identify points of coherence between human and animal models, and point toward a multitude of unresolved issues that should be addressed in future studies. Together, the combination of low-dose and high-dose alcohol effects serve to disrupt the normal development and maintenance of social relationships, which are critical for both healthy aging and quality of life across the lifespan. Thus, a more complete understanding of neural systems-including neuroinflammatory processes-which contribute to alcohol-induced changes in social behavior will provide novel opportunities and targets for promoting healthy aging.

Effects of Single Cage Housing on Stress, Cognitive, and Seizure Parameters in the Rat and Mouse Pilocarpine Models of Epilepsy

Many experimental approaches require housing rodents in individual cages, including in epilepsy research. However, rats and mice are social animals; and individual housing constitutes a stressful situation. The goal of the present study was to determine the effects of individual housing as compared to conditions maintaining social contact on stress markers and epilepsy. Control male mice socially housed during pretest and then transferred to individual cages for six weeks displayed anhedonia, increased anxiety and biological markers of stress as compared to pretest values or mice kept socially housed during six weeks. Pilocarpine (pilo)-treated mice housed together showed increased levels of anhedonia, anxiety and stress markers as well as decreased cognitive performance as compared to the control group. The differences were more significant in pilo-treated mice housed individually. Anxiety correlated linearly with cognitive performance and stress markers independently of the experimental conditions. In the male rat pilo model, seizures were sixteen times more frequent in singly housed animals as compared to animals kept in pairs. Daily interactions with an experimenter in otherwise singly housed animals was sufficient to produce results identical to those found in animals kept in pairs. We propose that social isolation produces a severe phenotype in terms of stress and seizure frequency as compared to animals maintaining social contact (at least in these two models), a factor that needs to be taken into account for data interpretation, in particular for preclinical studies.

Chronic Social Isolation Stress during Peri-Adolescence Alters Presynaptic Dopamine Terminal Dynamics via Augmentation in Accumbal Dopamine Availability

Chronic peri-adolescent stress in humans increases risk to develop a substance use disorder during adulthood. Rats reared in social isolation during peri-adolescence (aSI; 1 rat/cage) period show greater ethanol and cocaine intake compared to group housed (aGH; 4 rats/cage) rats. In addition, aSI rats have a heightened dopamine response in the nucleus accumbens (NAc) to rewarding and aversive stimuli. Furthermore, single pulse electrical stimulation in slices containing NAc core elicits greater dopamine release in aSI rats. Here, we further investigated dopamine release kinetics and machinery following aSI. Dopamine release, across a wide range of stimulation intensities and frequencies, was significantly greater in aSI rats. Interestingly, subthreshold intensity stimulations also resulted in measurable dopamine release in accumbal slices from aSI but not aGH rats. Extracellular [Ca²⁺] manipulations revealed augmented calcium sensitivity of dopamine release in aSI rats. The readily releasable pools of dopamine, examined by bath application of Ro-04-1284/000, a vesicular monoamine transporter 2 (VMAT2) inhibitor, were depleted faster in aGH rats. Western blot analysis of release machinery proteins (VMAT2, Synaptogyrin-3, Syntaxin-1, and Munc13-3) showed no difference between the two groups. Tyrosine hydroxylase (TH) protein expression levels, however, were elevated in aSI rats. The greater dopamine release could potentially be explained by higher levels of TH, the rate-limiting step for dopamine synthesis. This augmented responsivity of the dopamine system and heightened dopamine availability post-aSI may lead to an increased risk of addiction vulnerability.

Access to voluntary running wheel exercise: Prevention of anxiety-like behavior in chronically stressed rats, but potentiation of ethanol intake/preference

The current study tested the effect of voluntary running on future anxiety-like behavior, physiological response to stress, and ethanol intake/preference, while including a chronically stressed group and healthy group housed conspecifics. When given concurrently, voluntary running reduces ethanol intake, though it is unknown what effect voluntary running will have on anxiety-like behavior, corticosterone response to stress, and ethanol intake/preference when exercise is allowed only prior to ethanol access. Adolescent male Long Evans rats arrived in the lab at postnatal day (PND) 21. At PND 27, rats were either socially isolated (SI; n = 1/cage) or group housed (GH; n = 4/cage). Half of each group was allowed access to a running wheel for 30 min for 24 days from PND 35-66, and half of each group was not allowed access to a running wheel. After the housing/running procedure, we tested anxiety-like behavior using the elevated plus maze and stress responsivity by measuring corticosterone (CORT) levels before and after a swim stressor; then, rats were allowed intermittent access to ethanol in two-bottle choice design for four weeks. In accord with our hypothesis, running reduced anxiety-like behavior in SI runners compared to non-runners. Swim stress increased CORT levels but there was no difference in the response among groups. In regard to ethanol intake and preference, running (irrespective of housing group) increased intake at the 30 min time point and preference at the 24 h time point. Altogether, these data show that access to voluntary exercise was successful in reducing anxiety-like behavior, but withdrawal of exercise access appeared to enhance ethanol intake/preference. We suggest that these data reflect hedonic substitution.

Antioxidant treatment after epileptogenesis onset prevents comorbidities in rats sensitized by a past stressful event

OBJECTIVE

Unresolved past stressful events can induce a state of vulnerability to epilepsy and comorbidities. Using an experimental model of stress-induced vulnerability to depression, we tested whether an antioxidant treatment applied after the onset of epileptogenesis was disease modifying and could prevent the occurrence of comorbidities.

METHODS

We used social defeat (SD) to trigger a state of vulnerability in half of the SD-exposed population of rats. One month after SD, we used repeated injections of kainic acid to trigger status epilepticus (SE). One subset of animals was treated after SE during 2 weeks with Tempol, a strong antioxidant. Supradural 24/7 recordings were used to assess the development of epilepsy. We assessed spatial and nonspatial memory as well as a depressionlike profile 6 weeks after SE.

RESULTS

Serum brain-derived neurotrophic factor (BDNF) levels decreased after SD in all animals and recovered to pre-SD levels 1 month later in half of them (SDN group). The other half kept low serum BDNF levels (SDL group). At that stage, SDN and SDL animals do not present a depressionlike profile. The SDL group was more sensitive than the SDN group to epileptogenic conditions. Following SE, the SDL group displayed accelerated epileptogenesis, a depressionlike profile, and severe cognitive deficits as compared to SDN rats. Transient Tempol treatment was disease-modifying, reducing the number of seizures, and prevented the development of comorbidities in the SDL group. Tempol treatment normalized oxidative stress in the SDL group to SDN levels.

SIGNIFICANCE

This study illustrates the disease-modifying effect of antioxidant treatment after the onset of epileptogenesis in a population rendered vulnerable by past stressful events. The transient treatment decreased seizure burden and had long-term effects, preventing the occurrence of a depressionlike profile and cognitive deficits. We propose that vulnerability to comorbidities can be reversed after the onset of epilepsy.

Prenatal ethanol exposure potentiates isolation-induced ethanol consumption in young adult rats

Prenatal and/or early postnatal ethanol exposure (PEE) is associated with significant behavioral and physiological deficits in offspring, including alterations in stress response systems and a greater likelihood of alcohol use disorders. Stress-induced ethanol drinking after PEE, however, has been largely unexplored. The present study analyzed ethanol intake in male Sprague-Dawley rats after protracted prenatal and early postnatal ethanol exposure and tested whether social isolation during the sensitive period of adolescence modulates the effects of PEE on ethanol drinking. The dams were given 10% ethanol (or its vehicle) as the sole drinking fluid from gestational day 0 (GD0) to postnatal day 7 (PD7). On PD21, male offspring were housed individually (isolated housing group) or in pairs in standard cages (standard housing group). From PD56 to PD84, these male rats were tested for ethanol intake in 24-h, intermittent two-bottle choice sessions that were conducted across 4 weeks. Maternal ethanol consumption during gestation and during the first week of life of the offspring averaged 6.10-8.20 g/kg/22 h. Isolation housing during adolescence increased free-choice ethanol drinking in young adulthood. The main novel finding was that this facilitative effect of isolation on absolute and percent ethanol intake was significantly greater in PEE rats than in control counterparts not exposed to the prenatal and early postnatal ethanol exposure (effect sizes [η²p]: 0.24-0.32). The present results suggest that PEE renders the individual sensitive to the facilitative effect of stress exposure on ethanol intake.

Social modulation of drug use and drug addiction

This review aims to demonstrate how social science and behavioral neurosciences have highlighted the influence of social interactions on drug use in animal models. In neurosciences, the effect of global social context that are distal from drug use has been widely studied. For human and other social animals such as monkeys and rodents, positive social interactions are rewarding, can overcome drug reward and, in all, protect from drug use. In contrast, as other types of stress, negative social experiences facilitate the development and maintenance of drug abuse. However, interest recently emerged in the effect of so-called "proximal" social factors, that is, social interactions during drug-taking. These recent studies have characterized the role of the drug considered, the sharing of drug experience and the familiarity of the peer which interaction are made with. We also examine the few studies regarding the sensorial mediator of social behaviors and critically review the neural mediation of social factors on drug use. However, despite considerable characterization of the factors modulating distal influences, the mechanisms for proximal influences on drug use remain largely unknown. This article is part of the Special Issue entitled 'The neuropharmacology of social behavior: from bench to bedside'.

Advances in behavioral animal models of alcohol use disorder

Alcohol use disorder (AUD) is a multifaceted neuropsychiatric disease that combines behavioral, psychosocial, and neurobiological aspects. Over the previous decade, animal models have advanced in modeling the major psychological constructs that characterize AUD. These advances pave the road for more sophisticated behavioral models that capture addiction-related aspects, such as alcohol craving, compulsive seeking and intake, dependence, and relapse. In this review, we survey the recent progress in behavioral animal modeling of five aspects of AUD: alcohol consumption, dependence, and seeking; compulsivity in alcohol intake despite adverse outcomes; vulnerability and resilience factors in alcohol addiction; relapse despite treatment; and relapse prevention by manipulating alcohol-associated memory reconsolidation. These advances represent a general attempt to grasp the complexity and multidimensional nature of AUD, and to focus on behavioral characteristics that better reflect and model this disorder.