Effects of chronic intermittent ethanol exposure during early and late adolescence on anxiety-like behaviors and behavioral flexibility in adulthood

Although both humans and laboratory rodents demonstrate cognitive and affective alterations associated with adolescent alcohol exposure, it is still unknown whether the consequences of early initiation of alcohol use differ from those of later binge drinking within the adolescent developmental period. The present study was designed to assess the effects of early and late AIE on (1) anxiety-like behavior under social (modified social interaction test) and non-social test circumstances (modified light/dark box test, elevated plus maze), and (2) behavioral flexibility, indexed via set shifting in males and females. Early-mid adolescent intermittent exposure (early AIE) occurred between postnatal days (P) 25 and 45, whereas late adolescent intermittent exposure (late AIE) was conducted between P45 and P65, with behavioral testing initiated not earlier than 25 days after repeated exposure to ethanol (4.0 g/kg intragastrically, every other day for a total of 11 exposures). Anxiety-like behavior on the EPM was evident in males and females following early AIE, whereas only males demonstrated non-social anxiety on the EPM following late AIE. Social anxiety-like alterations and deficits in behavioral flexibility were evident only in males following early AIE. Taken together, the results of the present study demonstrate a particular vulnerability of young adolescent males to long-lasting detrimental effects of repeated ethanol and an insensitivity of older adolescent females to the intermittent ethanol exposure paradigm.

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.

Impact of housing conditions on social behavior, neuroimmune markers, and oxytocin receptor expression in aged male and female Fischer 344 rats

Aging is associated with a substantial decline in social behavior, whereas positive social interaction can improve overall health in aged individuals. In laboratory rodents, manipulations of the social environment across the lifespan have been shown to affect social behavior. Therefore, we examined the effects of long-term (5-6 weeks) housing conditions (alone, with one adult, or with two adults) on social behavior and the expression of neuroinflammation-related genes as well as oxytocin receptor (OXTR) gene expression in brain areas associated with social behavior regulation in aged male and female Fischer (F) 344 rats. Single-housed males and females exhibited increased social investigation, relative to pair-housed rats (one aged and one adult). Triple-housed (one aged and two adults) aged males exhibited lower levels of social investigation, relative to triple-housed aged females. Aged females were more socially active that their male counterparts. Although social housing condition significantly affected social behavior in males, it had no impact on cytokine gene expression in the paraventricular nucleus of hypothalamus (PVN), bed nucleus of the stria terminalis (BNST) or medial amygdala (MeA). However, in triple-housed aged females, who exhibited social behavior comparable to their single- and pair-housed counterparts, there was a significant increase in the expression of IL-1β and IL-6 mRNA in the MeA. No changes in cytokine gene expression were observed in the PVN or BNST, indicating that the increased expression of cytokines in the MeA was not a result of a generalized increase in neuroinflammation. Single-housed males and females exhibited elevated OXTR gene expression in the BNST. Taken together, these data indicate that manipulations of the social environment in late aging significantly influenced social interactions with a novel partner and gene expression in social behavior circuits and that these effects are sex-specific.

Stress alters social behavior and sensitivity to pharmacological activation of kappa opioid receptors in an age-specific manner in Sprague Dawley rats

The dynorphin/kappa opioid receptor (DYN/KOR) system has been identified as a primary target of stress due to behavioral effects, such as dysphoria, aversion, and anxiety-like alterations that result from activation of this system. Numerous adaptations in the DYN/KOR system have also been identified in response to stress. However, whereas most studies examining the function of the DYN/KOR system have been conducted in adult rodents, there is growing evidence suggesting that this system is ontogenetically regulated. Likewise, the outcome of exposure to stress also differs across ontogeny. Based on these developmental similarities, the objective of this study was to systematically test effects of a selective KOR agonist, U-62066, on various aspects of social behavior across ontogeny in non-stressed male and female rats as well as in males and females with a prior history of repeated exposure to restraint (90 min/day, 5 exposures). We found that the social consequences of repeated restraint differed as a function of age: juvenile stress produced substantial increases in play fighting, whereas adolescent and adult stress resulted in decreases in social investigation and social preference. The KOR agonist U-62066 dose-dependently reduced social behaviors in non-stressed adults, producing social avoidance at the highest dose tested, while younger animals displayed reduced sensitivity to this socially suppressing effect of U-62066. Interestingly, in stressed animals, the socially suppressing effects of the KOR agonist were blunted at all ages, with juveniles and adolescents exhibiting increased social preference in response to certain doses of U-62066. Taken together, these findings support the hypothesis that the DYN/KOR system changes with age and differentially responds and adapts to stress across development.

Enhanced sensitivity to socially facilitating and anxiolytic effects of ethanol in adolescent Sprague Dawley rats following acute prenatal ethanol exposure

Emerging evidence suggests that deficits in social functioning and social anxiety are associated with adolescent alcohol use. Our previous research has shown that acute exposure to a high dose of ethanol on gestational day (G) 12 produces social alterations in adolescent Sprague Dawley rats. The present study assessed whether these social alterations can affect sensitivity to acute ethanol challenge during adolescence. Pregnant females were exposed intraperitoneally (i.p.) to ethanol (2.5 g/kg followed by 1.25 g/kg in 2 h) or saline on G12, and their male and female offspring were tested on postnatal day (P) 42. Rats were challenged i.p. with one of four ethanol doses (0, 0.5, 0.75, and 1.0 g/kg), and their social behavior was assessed in a modified social interaction test. Social alterations associated with prenatal ethanol exposure and indexed via decreases of social investigation, social preference, and play fighting were evident in males and females challenged with the 0 g/kg ethanol dose. Acute ethanol increased social investigation, social preference, and play fighting in animals prenatally exposed to ethanol. In contrast, rats prenatally exposed to saline, showing no social facilitation, demonstrated significant ethanol-induced (0.75 and 1.0 g/kg) decreases in social behavior. Given that late adolescents demonstrating social alterations induced by prenatal ethanol exposure become sensitive to the socially anxiolytic as well as socially facilitating effects of acute ethanol, it is possible that the attractiveness of ethanol to these adolescents may be based on its ability to alleviate anxiety under social circumstances and facilitate interactions with peers.

Effects of AMPA receptor antagonist, NBQX, and extrasynaptic GABAA agonist, THIP, on social behavior of adolescent and adult rats

Adolescence is characterized by high significance of social interactions, along with a propensity to exhibit social facilitating effects of ethanol while being less sensitive than adults to the inhibition of social behavior that emerges at higher doses of ethanol. Among the neural characteristics of adolescence are generally enhanced levels of glutamatergic (especially NMDA receptor) activity relative to adults, whereas the GABA system is still developmentally immature. Activation of NMDA receptors likely plays a role in modulation of social behavior in adolescent animals as well as in socially facilitating and suppressing effects of ethanol. For instance, adolescent and adult rats differ in their sensitivities to the effects of NMDA antagonists and ethanol on social behavior, with adolescents but not adults demonstrating social facilitation at lower doses of both drugs and adults being more sensitive to the socially suppressing effects evident at higher doses of each. The roles of AMPA and extrasynaptic GABA_A receptors in modulation of social behavior during adolescence and in adulthood are still unknown. The present study was designed to assess whether pharmacological blockade of AMPA receptors and/or activation of extrasynaptic GABA_A receptors results in age-dependent alterations of social behavior. Adolescent and adult male and female Sprague-Dawley rats were injected with an assigned dose of either a selective AMPA antagonist, NBQX (Experiment 1) or extrasynaptic GABA_A agonist, THIP (Experiment 2) and placed into a modified social interaction chamber for a 30-min habituation period prior to a 10-min social interaction test with a novel age- and sex-matched partner. Behaviors such as social investigation, contact behavior and play behavior were scored from video recordings of the interaction tests. In Experiment 1, NBQX produced similar social inhibition at higher doses in both age groups. In Experiment 2, THIP induced inhibition in adolescents, but not adults. No social facilitation was evident following low doses of either drug. Therefore, AMPA and extrasynaptic GABA_A receptors appear to play little role if any in modulation of peer-directed social behavior in adolescence and adulthood and not likely to contribute to previously observed age differences in the social effects of acute ethanol.

Moderate Maternal Alcohol Exposure on Gestational Day 12 Impacts Anxiety-Like Behavior in Offspring

Among the numerous consequences of prenatal alcohol exposure (PAE) is an increase in anxiety-like behavior that can prove debilitating to daily functioning. A significant body of literature has linked gestational day 12 (G12) heavy ethanol exposure with social anxiety, evident in adolescent males and females. However, the association between non-social anxiety-like behavior and moderate alcohol exposure, a more common pattern of drinking in pregnant women, is yet unidentified. To model moderate PAE (mPAE), we exposed pregnant Sprague-Dawley rats to either room air or vaporized ethanol for 6 h on G12. Adolescent offspring were then tested on postnatal days (P) 41-47 in one of the following four anxiety assays: novelty-induced hypophagia (NIH), elevated plus maze (EPM), light-dark box (LDB) and open-field (OF). Our findings revealed significant increases in measures of anxiety-like behavior in male PAE offspring in the NIH, LDB and OF, with no differences observed in females on any test. Additionally, male offspring who demonstrated heightened anxiety-like behavior as adolescents demonstrated decreased anxiety-like behavior in adulthood, as measured by a marble-burying test (MBT), while females continued to be unaffected in adulthood. These results suggest that mPAE leads to dynamic changes in anxiety-like behavior exclusively in male offspring.

Ethanol intake under social circumstances or alone in sprague-dawley rats: impact of age, sex, social activity, and social anxiety-like behavior

BACKGROUND

In human adolescents, heavy drinking is often predicted by high sociability in males and high social anxiety in females. This study assessed the impact of baseline levels of social activity and social anxiety-like behavior in group-housed adolescent and adult male and female Sprague-Dawley rats on ethanol (EtOH) intake when drinking alone or in a social group.

METHODS

Social activity and anxiety-like behavior initially were assessed in a modified social interaction test, followed by 6 drinking sessions that occurred every other day in animals given ad libitum food and water. Sessions consisted of 30-minute access to 10% EtOH in a "supersac" (3% sucrose + 0.1% saccharin) solution given alone as well as in groups of 5 same-sex littermates, with order of the alternating session types counterbalanced across animals.

RESULTS

Adolescent males and adults of both sexes overall consumed more EtOH under social than alone circumstances, whereas adolescent females ingested more EtOH when alone. Highly socially active adolescent males demonstrated elevated levels of EtOH intake relative to their low and medium socially active counterparts when drinking in groups, but not when tested alone. Adolescent females with high levels of social anxiety-like behavior demonstrated the highest EtOH intake under social, but not alone circumstances. Among adults, baseline levels of social anxiety-like behavior did not contribute to individual differences in EtOH intake in either sex.

CONCLUSIONS

The results clearly demonstrate that in adolescent rats, but not their adult counterparts, responsiveness to a social peer predicts EtOH intake in a social setting-circumstances under which drinking typically occurs in human adolescents. High levels of social activity in males and high levels of social anxiety-like behavior in females were associated with elevated social drinking, suggesting that males ingest EtOH for its socially enhancing properties, whereas females ingest EtOH for its socially anxiolytic effects.

Acute prenatal exposure to ethanol on gestational day 12 elicits opposing deficits in social behaviors and anxiety-like behaviors in Sprague Dawley rats

Our previous research has shown that in Long Evans rats acute prenatal exposure to a high dose of ethanol on gestational day (G) 12 produces social deficits in male offspring and elicits substantial decreases in social preference relative to controls, in late adolescents and adults regardless of sex. In order to generalize the observed detrimental effects of ethanol exposure on G12, pregnant female Sprague Dawley rats were exposed to ethanol or saline and their offspring were assessed in a modified social interaction (SI) test as early adolescents, late adolescents, or young adults. Anxiety-like behavior was also assessed in adults using the elevated plus maze (EPM) or the light/dark box (LDB) test. Age- and sex-dependent social alterations were evident in ethanol-exposed animals. Ethanol-exposed males showed deficits in social investigation at all ages and age-dependent alterations in social preference. Play fighting was not affected in males. In contrast, ethanol-exposed early adolescent females showed no changes in social interactions, whereas older females demonstrated social deficits and social indifference. In adulthood, anxiety-like behavior was decreased in males and females prenatally exposed to ethanol in the EPM, but not the LDB. These findings suggest that social alterations associated with acute exposure to ethanol on G12 are not strain-specific, although they are more pronounced in Long Evans males and Sprague Dawley females. Furthermore, given that anxiety-like behaviors were attenuated in a test-specific manner, this study indicates that early ethanol exposure can have differential effects on different forms of anxiety.

Chronic intermittent ethanol exposure during adolescence: Effects on stress-induced social alterations and social drinking in adulthood

We previously observed lasting and sex-specific detrimental consequences of early adolescent intermittent ethanol exposure (AIE), with male, but not female, rats showing social anxiety-like alterations when tested as adults. The present study used Sprague Dawley rats to assess whether social alterations induced by AIE (3.5g/kg, intragastrically, every other day, between postnatal days [P] 25-45) are further exacerbated by stressors later in life. Another aim was to determine whether AIE alone or in combination with stress influenced intake of a sweetened ethanol solution (Experiment 1) or a sweetened solution ("supersac") alone (Experiment 2) under social circumstances. Animals were exposed to restraint on P66-P70 (90min/day) or left nonstressed, with corticosterone (CORT) levels assessed on day 1 and day 5 in Experiment 2. Social anxiety-like behavior emerged after AIE in non-stressed males, but not females, whereas stress-induced social anxiety was evident only in water-exposed males and females. Adult-typical habituation of the CORT response to repeated restraint was not evident in adult animals after AIE, a lack of habituation reminiscent of that normally evident in adolescents. Neither AIE nor stress affected ethanol intake under social circumstances, although AIE and restraint independently increased adolescent-typical play fighting in males during social drinking. Among males, the combination of AIE and restraint suppressed "supersac" intake; this index of depression-like behavior was not seen in females. The results provide experimental evidence associating adolescent alcohol exposure, later stress, anxiety, and depression, with young adolescent males being particularly vulnerable to long-lasting adverse effects of repeated ethanol. This article is part of a Special Issue entitled SI: Adolescent plasticity.

A working model for the assessment of disruptions in social behavior among aged rats: The role of sex differences, social recognition, and sensorimotor processes

Aging results in a natural decline in social behavior, yet little is known about the processes underlying these changes. Engaging in positive social interaction is associated with many health benefits, including reduced stress reactivity, and may serve as a potential buffer against adverse consequences of aging. The goal of these studies was to establish a tractable model for the assessment of social behavior deficits associated with late aging. Thus, in Exp. 1, 1.5-, 3-, and 18-month-old male Fischer 344 (F344) rats were assessed for object investigation, and social interaction with a same-aged partner (novel/familiar), or a different-aged partner, thereby establishing working parameters for studies that followed. Results revealed that 18-month-old males exhibited reductions in social investigation and social contact behavior, with this age-related decline not influenced by familiarity or age of the social partner. Subsequently, Exp. 2 extended assessment of social behavior to both male and female F344 rats at multiple ages (3, 9, 18, and 24 months), after which a series of sensorimotor performance tests were conducted. In this study, both males and females exhibited late aging-related reductions in social interactions, but these changes were more pronounced in females. Additionally, sensorimotor performance was shown to be impaired in 24-month-olds, but not 18-month-olds, with this deficit more evident in males. Finally, Exp. 3 examined whether aging-related inflammation could account for declines in social behavior during late aging by administering naproxen (0, 7, 14, and 28 mg/kg; s.c.)-a non-steroidal anti-inflammatory drug-to 18-month-old females. Results from this study revealed that social behavior was unaffected by acute or repeated (6 days) naproxen, suggesting that aging-related social deficits in females may not be a consequence of a general aging-related inflammation and/or malaise. Together, these findings demonstrate that aging-related declines in social behavior are (i) specific to social stimuli and (ii) not indicative of a general state of aging-related debilitation. Thus, these findings establish working parameters for a highly tractable model in which the neural and hormonal mechanisms underlying aging-related declines in social behavior can be examined.

Sex differences in sensitivity to the social consequences of acute ethanol and social drinking during adolescence

In human adolescents, sociable males frequently drink to enhance positive emotional states, whereas anxious females often drink to avoid negative affective states. This study used a rat model of adolescence to provide information regarding possible sex differences in contributors to social drinking. The effects of ethanol (0, 0.5, and 0.75g/kg) on play fighting and social preference were assessed on P30, P32, and P34 using a within-subject design. Then animals were tested in a social drinking paradigm (P37-P40), with this testing revealing high drinkers and low drinkers. Sex differences in sensitivity to ethanol emerged among high and low drinkers. High socially drinking males, but not females, when tested prior to drinking sessions, showed significant increases in play fighting at both doses. In low drinking males, play fighting was increased by 0.5g/kg ethanol, whereas the higher dose of 0.75g/kg produced significant decreases in play fighting. High drinking females initially showed low levels of social preference than high drinking males and low drinking females and were extremely sensitive to ethanol-induced enhancement of this social measure. Low social drinkers, both males and females, were more sensitive to the suppressing effects of ethanol on social preference following 0.75g/kg ethanol. These findings indicate that during adolescence enhanced sensitivity to the facilitating effects of ethanol on play fighting is associated with heavy drinking among males, whereas low social preference together with high sensitivity to ethanol-induced enhancement of social preference is related to high social drinking in females.

Social consequences of ethanol: Impact of age, stress, and prior history of ethanol exposure

The adolescent period is associated with high significance of interactions with peers, high frequency of stressful situations, and high rates of alcohol use. At least two desired effects of alcohol that may contribute to heavy and problematic drinking during adolescence are its abilities to both facilitate interactions with peers and to alleviate anxiety, perhaps especially anxiety seen in social contexts. Ethanol-induced social facilitation can be seen using a simple model of adolescence in the rat, with normal adolescents, but not their more mature counterparts, demonstrating this ethanol-related social facilitation. Prior repeated stress induces expression of ethanol-induced social facilitation in adults and further enhances socially facilitating effects of ethanol among adolescent rats. In contrast, under normal circumstances, adolescent rats are less sensitive than adults to the social inhibition induced by higher ethanol doses and are insensitive to the socially anxiolytic effects of ethanol. Sensitivity to the socially anxiolytic effects of ethanol can be modified by prior stress or ethanol exposure at both ages. Shortly following repeated restraint or ethanol exposure, adolescents exhibit social anxiety-like behavior, indexed by reduced social preference, and enhanced sensitivity to the socially anxiolytic effects of ethanol, indexed through ethanol-associated reinstatement of social preference in these adolescents. Repeated restraint, but not repeated ethanol, induces similar effects in adults as well, eliciting social anxiety-like behavior and increasing their sensitivity to the socially anxiolytic effects of acute ethanol; the stressor also decreases sensitivity of adults to ethanol-induced social inhibition. The persisting consequences of early adolescent ethanol exposure differ from its immediate consequences, with males exposed early in adolescence, but not females or those exposed later in adolescence, showing social anxiety-like behavior when tested in adulthood. Adult males exposed to ethanol early in adolescence also show enhanced sensitivity to the socially facilitating effects of ethanol, whereas adult males exposed to ethanol during late adolescence demonstrate insensitivity to the socially suppressing effects of ethanol. To the extent that these results are applicable to humans, stressful live events may make alcohol more attractive for stressed adolescents and adults due to its socially facilitating and socially anxiolytic properties, therefore fostering high levels of drinking. Retention of adolescent-typical responsiveness to alcohol in adult males following adolescent alcohol exposure, including enhanced sensitivity to the socially facilitating effects of ethanol following early exposure and insensitivity to the socially inhibiting effects following late adolescent exposure, may put these males at risk for the development of alcohol-related disorders later in life.

Pharmacology of a novel central nervous system-penetrant P2X7 antagonist JNJ-42253432

In the central nervous system, the ATP-gated Purinergic receptor P2X ligand-gated ion channel 7 (P2X7) is expressed in glial cells and modulates neurophysiology via release of gliotransmitters, including the proinflammatory cytokine interleukin (IL)-1β. In this study, we characterized JNJ-42253432 [2-methyl-N-([1-(4-phenylpiperazin-1-yl)cyclohexyl]methyl)-1,2,3,4-tetrahydroisoquinoline-5-carboxamide] as a centrally permeable (brain-to-plasma ratio of 1), high-affinity P2X7 antagonist with desirable pharmacokinetic and pharmacodynamic properties for in vivo testing in rodents. JNJ-42253432 is a high-affinity antagonist for the rat (pKi 9.1 ± 0.07) and human (pKi 7.9 ± 0.08) P2X7 channel. The compound blocked the ATP-induced current and Bz-ATP [2'(3')-O-(4-benzoylbenzoyl)adenosine-5'-triphosphate tri(triethylammonium)]-induced release of IL-1β in a concentration-dependent manner. When dosed in rats, JNJ-42253432 occupied the brain P2X7 channel with an ED50 of 0.3 mg/kg, corresponding to a mean plasma concentration of 42 ng/ml. The compound blocked the release of IL-1β induced by Bz-ATP in freely moving rat brain. At higher doses/exposure, JNJ-42253432 also increased serotonin levels in the rat brain, which is due to antagonism of the serotonin transporter (SERT) resulting in an ED50 of 10 mg/kg for SERT occupancy. JNJ-42253432 reduced electroencephalography spectral power in the α-1 band in a dose-dependent manner; the compound also attenuated amphetamine-induced hyperactivity. JNJ-42253432 significantly increased both overall social interaction and social preference, an effect that was independent of stress induced by foot-shock. Surprisingly, there was no effect of the compound on either neuropathic pain or inflammatory pain behaviors. In summary, in this study, we characterize JNJ-42253432 as a novel brain-penetrant P2X7 antagonist with high affinity and selectivity for the P2X7 channel.

D-Cycloserine ameliorates social alterations that result from prenatal exposure to valproic acid

Prenatal exposure to valproic acid (VPA) alters rodent social interactions in a dose-dependent way: exposure to a high dose of VPA (>500 mg/kg) mid-gestation decreases social interactions whereas a moderate dose of VPA (350 mg/kg) increases peer-directed social behavior. The moderate dose also decreases expression of the mRNA for serine in amygdala and orbitofrontal cortex. In this study, we examined whether d-cycloserine could ameliorate VPA-induced alterations in ultrasonic vocalizations (USVs), social interactions, and locomotor activity. Pregnant Sprague Dawley rats were given intraperintoneal injections of VPA (200mg/kg each) on gestational days 12, 12.5 and 13; controls were injected with saline. Offspring received a subcutaneous injection of saline or d-cycloserine (32 or 64 mg/kg) either acutely (1h prior to testing) or repeatedly (once per day for four days). Social interactions were assessed during late adolescence, and USVs were recorded concomitantly. Male and female rats that were exposed to VPA demonstrated more locomotor activity than control animals during habituation to the testing chamber. VPA-exposed males showed increased play fighting. d-Cycloserine normalized the VPA-induced increase in play fighting in males and also increased social motivation in females. When the pair contained a VPA-exposed rat, significantly fewer USVs were emitted and 16% of the vocalizations were of a novel waveform. These effects were not seen in pairs containing VPA-exposed animals that were treated with d-cycloserine. Overall, these findings are consistent with data from other laboratories suggesting that d-cycloserine may be a promising pharmacotherapeutic compound for improving social behavior disorders.

Chronic intermittent ethanol exposure during adolescence: effects on social behavior and ethanol sensitivity in adulthood

This study assessed long-lasting consequences of repeated ethanol exposure during two different periods of adolescence on 1) baseline levels of social investigation, play fighting, and social preference and 2) sensitivity to the social consequences of acute ethanol challenge. Adult male and female Sprague-Dawley rats were tested 25 days after repeated exposure to ethanol (3.5 g/kg intragastrically [i.g.], every other day for a total of 11 exposures) in a modified social interaction test. Early-mid adolescent intermittent exposure (e-AIE) occurred between postnatal days (P) 25 and 45, whereas late adolescent intermittent exposure (l-AIE) was conducted between P45 and P65. Significant decreases in social investigation and social preference were evident in adult male rats, but not their female counterparts following e-AIE, whereas neither males nor females demonstrated these alterations following l-AIE. In contrast, both e-AIE and l-AIE produced alterations in sensitivity to acute ethanol challenge in males tested 25 days after adolescent exposure. Ethanol-induced facilitation of social investigation and play fighting, reminiscent of that normally seen during adolescence, was evident in adult males after e-AIE, whereas control males showed an age-typical inhibition of social behavior. Males after l-AIE were found to be insensitive to the socially suppressing effects of acute ethanol challenge, suggesting the development of chronic tolerance in these animals. In contrast, females showed little evidence for alterations in sensitivity to acute ethanol challenge following either early or late AIE. The results of the present study demonstrate a particular vulnerability of young adolescent males to long-lasting detrimental effects of repeated ethanol. Retention of adolescent-typical sensitivity to the socially facilitating effects of ethanol could potentially make ethanol especially appealing to these males, therefore promoting relatively high levels of ethanol intake later in life.

Pre-pubertal gonadectomy and the social consequences of acute ethanol in adolescent male and female rats

It has previously been shown that pre-pubertal or adult gonadectomy (GX) increases ethanol intake in male rats. This study examined whether this sex-selective increase reflects a GX-induced maintenance in males of more adolescent-typical responsiveness to ethanol characterized by enhanced sensitivity to positive (e.g., socially facilitating) and a decreased sensitivity to adverse (e.g., socially inhibitory) effects of ethanol. Male and female Sprague-Dawley rats were pre-pubertally GX, sham (SH)-operated, or non-manipulated (NM) at postnatal day (P) 25. During the late adolescent transition into adulthood (P48 - baseline day), rats were given a saline injection, placed alone into a familiar test apparatus for 30min and then exposed for 10min to an unfamiliar partner of the same age and sex. On the following day (P49), similar testing occurred after administration of 0.5, 0.75, 1.0 or 1.25g/kg ethanol. At baseline, GX males and females displayed higher levels of social activity (especially adolescent-typical play and contact behavior) than SH and NM animals, with GX females displaying greater social activity than GX males. Neither males nor females demonstrated social facilitation at lower ethanol doses, regardless of hormonal status. Whereas the social inhibitory effects of higher doses of ethanol were similar across groups among females, SH males were less sensitive than both GX and NM males to ethanol-induced social inhibition. These results suggest that enhanced ethanol intake in GX males is not related to alterations in sensitivity to ethanol's social inhibitory effects. GX, however, results in retention of adolescent-typical social behaviors, with older GX adolescent rats resembling early adolescents in exhibiting elevated social activity-particularly play and contact behavior.

Brief prenatal ethanol exposure alters behavioral sensitivity to the kappa opioid receptor agonist (U62,066E) and antagonist (Nor-BNI) and reduces kappa opioid receptor expression

BACKGROUND

Approximately 10 to 15% of women consume alcohol (ethanol [EtOH]) during pregnancy in the United States. Even low amounts of EtOH consumption during pregnancy can elicit long-term consequences. Prenatal experience with as few as 3 drinks has been associated with increase problem drinking in adulthood. Such effects are corroborated in rodents; however, the underlying neural adaptations contributing to this effect are not clear. In the current set of experiments, we investigated whether changes in EtOH responding following prenatal EtOH exposure involved kappa opioid receptor activation and expression.

METHODS

Sprague-Dawley rats were prenatally exposed to low levels of alcohol (1.0 g/kg) during late gestation (gestational days 17 to 20 [GD17-20]) via intragastric intubation of pregnant dams. Following birth, EtOH intake, kappa- and mu-opioid-induced place conditioning, and kappa opioid receptor expression in mesolimbic brain regions were assessed in infant rats (postnatal days 14 to 15 [PD14-15]) that were offspring of dams given EtOH, vehicle, or untreated, during pregnancy.

RESULTS

Animals exposed to prenatal alcohol drank more alcohol later in life and exhibited significant changes in the kappa opioid system. While control subjects found kappa opioid activation aversive, animals exposed to EtOH prenatally exhibited either no aversion or appetitive responding. Further analysis revealed that synaptosomal kappa opioid receptor expression was significantly decreased in brain areas implicated in responding to EtOH.

CONCLUSIONS

Overall, these data suggest that prenatal EtOH affects kappa opioid function and expression and that these changes may be involved in increased drinking later in life.

Pharmacological activation of kappa opioid receptors: aversive effects in adolescent and adult male rats

RATIONALE

The dynorphin (DYN)/kappa opioid receptor (KOR) system is involved in the dysphoric properties of drugs of abuse. Given that adolescents show reduced sensitivity to aversive effects of many drugs, alterations in the DYN/KOR system may contribute to the prevalence of drug use during adolescence.

OBJECTIVES

The present study was designed to assess dysphoric properties of a selective kappa agonist, U62,066, in adolescent and adult rats using both conditioned taste aversion (CTA) and conditioned place aversion (CPA) paradigms.

METHODS

For CTA, water-restricted rats were administered U62,066 following 30 min access to a saccharin solution, with subsequent saccharin consumption used to index aversion. For CPA, animals were allowed access to both compartments of a two-compartment chamber for a 15-min pre- and post-conditioning test. For conditioning, subjects were administered U62,066 prior to confinement to one side of the chamber and saline prior to confinement to the other side for a total of four pairings.

RESULTS

Overall, adolescents displayed reduced sensitivity to the kappa agonist relative to adults. Adults demonstrated taste aversions to the 0.2 and 0.3 mg/kg doses of U62,066, whereas adolescents did not display aversions to any tested doses. Adults demonstrated a place aversion to the 0.1 and 0.2 mg/kg dose of U62,066 when paired with the preferred side of the conditioning chamber. Adolescents did not display aversions to any of the doses tested.

CONCLUSIONS

Reduced sensitivity to DYN/KOR system activation during adolescence may be a contributing factor to the age-typical insensitivity to aversive properties of drugs commonly abused by adolescents.

Conditioned taste aversion to ethanol in a social context: impact of age and sex

Given that human adolescents place a high value on social interactions-particularly while consuming alcohol-the current study utilized a novel social drinking paradigm to examine rewarding and aversive properties of ethanol in non-water deprived rats that were housed and tested in groups of five same-sex littermates. On postnatal day P34 (adolescents) or P69 (adults), rats were habituated to the testing apparatus for 30 min. On the next day, animals were placed into the test apparatus and given 30 min access to a supersaccharin solution (3% sucrose; 0.125% saccharin), followed immediately by an intraperitoneal injection of ethanol (0, 0.25, 0.5, 1.0, 1.5 g/kg). Subsequent intake of the supersacharrin solution was assessed on three consecutive test days. Adolescent males were less sensitive to ethanol's aversive effects than adult males, with adolescent males maintaining an aversion on all three test days only at the 1.5 g/kg dose, whereas adults demonstrated aversions across test days to 1 and 1.5 g/kg. Adolescent females maintained aversions to 1 and 1.5 g/kg across days, whereas adult females continued to show an aversion to the 1.5 g/kg dose only. These opposite patterns of sensitivity that emerged among males and females at each age in the propensity to maintain an ethanol-induced taste aversion under social conditions may contribute to age- and sex-related differences in ethanol intake. Testing in social groups may be useful for future work when studying rodent models of adolescent alcohol use given the importance that human adolescents place on drinking in social settings.

Acute exposure to ethanol on gestational day 15 affects social motivation of female offspring

Alterations in social behavior are a hallmark of many neurodevelopmental disorders in humans. In rodents, social behavior is affected by prenatal insults. The outcomes are dependent on the timing of the insult as well as the sex and age of the animal tested. The limbic system is particularly important for social behavior, and a peak of neurogenesis within this system occurs on gestational day (G)15. Neurons appear particularly vulnerable to ethanol insult around the time they become post-mitotic. We tested the hypothesis that acute exposure to ethanol on G15 would result in significant social behavior deficits. Accordingly, Long Evans pregnant females were injected with ethanol (2.9 g/kg) or an equivalent volume of saline on G15. Offspring were assessed in a modified social interaction test on postnatal day (P) 28, P42, or P75, i.e., during early adolescence, late adolescence, or young adulthood. Prenatal ethanol exposure decreased social investigation in P28 females and transformed social preference into social avoidance in 75-day-old females. Contact behavior, play fighting, and locomotor activity differed as a function of age, but were not significantly affected by ethanol exposure. Males demonstrated significantly more contact behavior and play fighting at P42 than at P28 or P70, whereas there were no age-related changes in females. Adult females showed more locomotor activity than adult males. Overall, prenatal ethanol exposure on G15 enhanced social anxiety in females, with these effects seen in adulthood only.

Acute prenatal exposure to a moderate dose of valproic acid increases social behavior and alters gene expression in rats

Prenatal exposure to moderate doses of valproic acid (VPA) produces brainstem abnormalities, while higher doses of this teratogen elicit social deficits in the rat. In this pilot study, we examined effects of prenatal exposure to a moderate dose of VPA on behavior and on transcriptomic expression in three brain regions that mediate social behavior. Pregnant Long Evans rats were injected with 350 mg/kg VPA or saline on gestational day 13. A modified social interaction test was used to assess social behavior and social preference/avoidance during early and late adolescence and in adulthood. VPA-exposed animals demonstrated more social investigation and play fighting than control animals. Social investigation, play fighting, and contact behavior also differed as a function of age; the frequency of these behaviors increased in late adolescence. Social preference and locomotor activity under social circumstances were unaffected by treatment or age. Thus, a moderate prenatal dose of VPA produces behavioral alterations that are substantially different from the outcomes that occur following exposure to a higher dose. At adulthood, VPA-exposed subjects exhibited transcriptomic abnormalities in three brain regions: anterior amygdala, cerebellar vermis, and orbitofrontal cortex. A common feature among the proteins encoded by the dysregulated genes was their ability to be modulated by acetylation. Analysis of the expression of individual exons also revealed that genes involved in post-translational modification and epigenetic regulation had particular isoforms that were ubiquitously dysregulated across brain regions. The vulnerability of these genes to the epigenetic effects of VPA may highlight potential mechanisms by which prenatal VPA exposure alters the development of social behavior.

Puberty and gonadal hormones: role in adolescent-typical behavioral alterations

This article is part of a Special Issue "Puberty and Adolescence". Adolescence is characterized by a variety of behavioral alterations, including elevations in novelty-seeking and experimentation with alcohol and other drugs of abuse. Some adolescent-typical neurobehavioral alterations may depend upon pubertal rises in gonadal hormones, whereas others may be unrelated to puberty. Using a variety of approaches, studies in laboratory animals have not revealed clear relationships between pubertal-related changes and adolescent- or adult-typical behaviors that are not strongly sexually dimorphic. Data reviewed suggest surprisingly modest influences of gonadal hormones on alcohol intake, alcohol preference and novelty-directed behaviors. Gonadectomy in males (but not females) increased ethanol intake in adulthood following surgery either pre-pubertally or in adulthood, with these increases in intake largely reversed by testosterone replacement in adulthood, supporting an activational role of androgens in moderating ethanol intake in males. In contrast, neither pre-pubertal nor adult gonadectomy influenced sensitivity to the social inhibitory or aversive effects of ethanol when indexed via conditioned taste aversions, although gonadectomy at either age altered the microstructure of social behavior of both males and females. Unexpectedly, the pre-pubertal surgical manipulation process itself was found to increase later ethanol intake, decrease sensitivity to ethanol's social inhibitory effects, attenuate novelty-directed behavior and lower social motivation, with gonadal hormones being necessary for these long-lasting effects of early surgical perturbations.

Effects of the kappa opioid receptor antagonist, nor-binaltorphimine, on ethanol intake: impact of age and sex

The kappa opioid receptor (KOR) antagonist, nor-binaltorphimine (nor-BNI), was used to investigate the role of the KOR system in mediating ethanol intake. On P25 (adolescent) or P67 (adult) male and female rats were individually housed and given ad libitum access to food and water. The experimental procedure was initiated on P28 or P70: animals were given 30 min/day access to a 10% ethanol/supersaccharin solution every other day (3 baseline exposures). On the day after the final baseline test, rats were injected with nor-BNI (0, 2.5, 5, 10 mg/kg), with testing initiated 24 hr later (30-min access every other day, 3 test exposures). Nor-BNI (10 mg/kg) increased ethanol intake in adult males, whereas the same dose decreased intake in adult females, suggesting pronounced sex differences in KOR-associated mediation of ethanol intake in adulthood. There was no impact of nor-BNI in adolescent animals of either sex, suggesting that the KOR may play less of a role in modulating ethanol intake during adolescence.

Anxiolytic effects of the GABA(A) receptor partial agonist, L-838,417: impact of age, test context familiarity, and stress

The partial α2,3,5 GABA(A) receptor agonist, L-838,417 has been reported to have anxiolytic effects in adult rodents. Although maturational differences exist for the GABA(A) receptor subunits, the anxiolytic effects of L-838,417 have not been tested in younger animals. The goal of the present experiments was to determine whether L-838,417 reverses anxiety-like behavior induced by either an unfamiliar environment (Experiment 1) or repeated restraint stress (Experiment 2) differentially in adolescent and adult, male and female Sprague-Dawley rats using a modified social interaction test. In Experiment 1, rats were injected with 0, 0.5, 1.0, 2.0, or 4.0 mg/kg L-838,417, i.p. and tested 30 min later in an unfamiliar test context for 10 min. In Experiment 2, rats were exposed to restraint stress (90 min daily for 5 days). Immediately after the last restraint session, animals were injected with L-838,417 and placed alone for 30 min in the test apparatus to familiarize them to this context prior to the 10 min social interaction test. In Experiment 1, L-838,417 produced anxiolytic effects in adults at 1.0 mg/kg, as indexed by a transformation of social avoidance into preference and an increase in social investigation. In adolescents, a dose of 2.0 mg/kg eliminated social avoidance, but had no anxiolytic effects on social investigation. Testing under familiar circumstances (Experiment 2) after repeated restraint stress eliminated age differences in sensitivity to L-838,417, with 0.5 mg/kg reversing the anxiogenic effects of prior stress regardless of age, but with doses ≥ 1 mg/kg decreasing social investigation, an effect possibly due in part to locomotor-impairing effects of this compound. Although locomotor activity was suppressed in both experiments, higher doses of L-838,417 were necessary to suppress locomotor activity in Experiment 1. Thus, anxiolytic effects of L-838,417 were found to be context-, age-, and stress-dependent.