Sensitivity to ethanol and other hedonic stimuli in an animal model of adolescence: implications for prevention science?

Understanding alcohol use and alcohol use disorders from a developmental psychopathology perspective: Research advances, challenges, and future directions

As part of the special issue of Development and Psychopathology honoring the remarkable contributions of Dr Dante Cicchetti, the current paper attempts to describe the recent contributions that a developmental psychopathology perspective has made in understanding the development of alcohol use and alcohol-related problems over the lifespan. The paper also identifies some of the future challenges and research directions. Because the scope of this task far exceeds the confines of a journal length article this paper does not attempt a comprehensive review. Rather, it builds on an earlier review and commentary that was published in Development and Psychopathology in 2013, with a similar goal.)Building on that work and updating its conclusions and suggestions for future directions, the current paper emphasizes findings from the research areas that were identified for further study in 2013 and the findings that have been published since that time.

Predictors and Correlates of Positive Urine Drug Screening in a Retrospective Cohort Analysis of Child and Adolescent Psychiatry Inpatients Throughout the COVID-19 Pandemic

BACKGROUND AND OBJECTIVES

Youth substance use is associated with significant psychological, neurological, and medical complications. Risk factors for substance use among children and adolescents in the general population include peer and/or parental substance use, certain psychiatric illnesses (eg, Attention-Deficit/Hyperactivity Disorder, depression), and history of maltreatment. Co-occurring substance use and psychiatric illness have been associated with increased suicidality, but few prior studies have characterized substance use among child/adolescent inpatients. As such, it remains unclear how substance use contributing to acute psychiatric presentations has changed since the start of the COVID-19 pandemic.

METHODS

This is a retrospective cohort study of 816 unique child/adolescent psychiatry inpatients with urine drug screening (UDS) results from a diverse urban setting. Charts of patients hospitalized between June 1, 2018 and November 30, 2021 were reviewed for sociodemographic characteristics, indication for admission, psychiatric history, hospital course, treatment plan, and discharge diagnosis. Differences in sociodemographic and clinical characteristics, such as age, race, and diagnoses, between patients with and without positive UDS were explored throughout various periods of the COVID-19 pandemic. Descriptive and comparative statistics were performed, as well as a logistic regression model to identify the predictors of positive UDS.

RESULTS

Of the study sample, 18% had a positive UDS. Older age, diagnosis of impulsive or behavioral disorder, and a history of violence were found to be predictors of positive UDS. Asian/South Asian or Hispanic/LatinX race and history of a developmental or intellectual disability were found to be negative predictors. The frequency of positive UDS in this population did not change based on COVID-19.

DISCUSSION AND CONCLUSIONS

Multiple factors may predispose children and adolescents to substance use. Though no impact of COVID-19 was found in this sample, longer-term studies are needed.

SCIENTIFIC SIGNIFICANCE

This study identifies independent predictors of active substance use in the child and adolescent psychiatric inpatient population.

Heterozygote Dopamine Transporter Knockout Rats Display Enhanced Cocaine Locomotion in Adolescent Females

Cocaine is a powerful psychostimulant that is one of the most widely used illicit addictive. The dopamine transporter (DAT) plays a major role in mediating cocaine's reward effect. Decreases in DAT expression increase rates of drug abuse and vulnerability to comorbid psychiatric disorders. We used the novel DAT transgenic rat model to study the effects of cocaine on locomotor behaviors in adolescent rats, with an emphasis on sex. Female rats showed higher response rates to cocaine at lower acute and chronic doses, highlighting a higher vulnerability and perceived gender effects. In contrast, locomotor responses to an acute high dose of cocaine were more marked and sustained in male DAT heterozygous (HET) adolescents. The results demonstrate the augmented effects of chronic cocaine in HET DAT adolescent female rats. Knockout (KO) DAT led to a level of hyperdopaminergia which caused a marked basal hyperactivity that was unchanged, consistent with a possible ceiling effect. We suggest a role of alpha synuclein (α-syn) and PICK 1 protein expressions to the increased vulnerability in female rats. These proteins showed a lower expression in female HET and KO rats. This study highlights gender differences associated with mutations which affect DAT expression and can increase susceptibility to cocaine abuse in adolescence.

Heterogeneity of definitions and measurements of binge drinking in research on adolescents and young adults

INTRODUCTION

Binge drinking is a widespread health compromising behaviour among adolescents and young adults and is one of the leading causes of mortality and injuries among this population. The definitions and measurement methods of binge drinking are heterogeneous but constitute a crucial component in the literature on associated factors related to binge drinking. This study focused on how binge drinking is defined and measured in the literature exploring its associated risk factors among adolescents and young adults.

METHODS

The databases PubMed, Embase, PsycINFO and Social Care were searched for articles published between 1 January 2006 and 30 April 2020 using 3 concepts: binge drinking; risk or protective factors; and adolescents/young adults with respective key words. Data were extracted on the main characteristics of studies and the parameters of binge drinking measurements.

RESULTS

173 studies were included, mostly cross-sectional (61 %) and longitudinal (38 %). Only 23 % of the studies explicitly referred to a standardised definition of binge drinking even though 76 % of the studies used a consensual threshold of 5 drinks or more for men. A lower threshold for women was applied in 26 % of the studies. Recall periods ranged between 2 weeks and 1 year in 85 % of the studies and only 16 % presented binge drinking in terms of frequency and/or quantity of drinks.

CONCLUSION

Our results highlight the heterogeneity in the definitions and measurements of binge drinking, raising concerns for meaningful comparisons between studies focused on factors associated with the behaviour. The scientific community needs to be aware of these variations and address the gap of poor stratification and inconsistencies in binge drinking reporting.

Spontaneous head twitches in aged rats: behavioral and molecular study

RATIONALE

We have discovered that rats at the age of 18 months begin to twitch their heads spontaneously (spontaneous head twitching, SHT). To date, no one has described this phenomenon.

OBJECTIVES

The purpose of this study was to characterize SHT pharmacologically and to assess some possible mechanisms underlying SHT.

METHODS

Wistar male rats were used in the study. Animals at the age of 18 months were qualified as HSHT (SHT ≥ 7/10 min observations) or LSHT (SHT < 7/10 min observations). Quantitative real-time PCR with TaqMan low-density array (TLDA) approach was adopted to assess the mRNA expression of selected genes in rat's hippocampus.

RESULTS

HSHT rats did not differ from LSHT rats in terms of survival time, general health and behavior, water intake, and spontaneous locomotor activity. 2,5-dimethoxy-4-iodoamphetamine (DOI) at a dose of 2.5 mg/kg increased the SHT in HSHT and LSHT rats, while ketanserin dose-dependently abolished the SHT in the HSHT rats. The SHT was reduced or abolished by olanzapine, clozapine, risperidone, and pimavanserin. All these drugs have strong 5-HT2A receptor-inhibiting properties. Haloperidol and amisulpride, as antipsychotic drugs with a mostly dopaminergic mechanism of action, did not influence SHT. Similarly, escitalopram did not affect SHT. An in-depth gene expression analysis did not reveal significant differences between the HSHT and the LSHT rats.

CONCLUSIONS

SHT appears in some aging rats (about 50%) and is permanent over time and specific to individuals. The 5-HT2A receptor strongly controls SHT. HSHT animals can be a useful animal model for studying 5-HT2A receptor ligands.

Excitatory/inhibitory balance across ontogeny contributes to age-specific behavioral outcomes of ethanol-like challenge in conditioned taste aversion

Adolescent-typical sensitivities to ethanol (EtOH) are characterized in part by reduced sensitivity to EtOH's aversive effects. Rodent studies have shown that adolescents are less sensitive than adults to aversive properties of EtOH in a conditioned taste aversion (CTA) paradigm. To the extent that EtOH exerts antagonist-like actions upon glutamate receptors and/or agonist-like actions upon γ-aminobutyric acid (GABA) receptors, age differences in excitatory/inhibitory balance may regulate age-specific EtOH sensitivities, such as attenuated sensitivity of adolescents to EtOH aversion. In our experiments, adolescent and adult Sprague-Dawley rats were tested for CTA following challenge with one of the following pharmacological agents: glutamatergic AMPA1 receptor antagonist NBQX, glutamatergic N-methyl-d-aspartate NR2B receptor antagonist ifenprodil, and extrasynaptic GABAA receptor agonist THIP to determine whether these induced age-specific aversive sensitivities like those seen with EtOH. NBQX administration did not induce CTA. The highest dose of extrasynaptic GABAA agonist THIP induced CTA in adolescents but not adults, an opposite ontogenetic profile as seen following EtOH. Ifenprodil exerted an age-specific pattern of CTA similar to that seen with EtOH in males, with adolescents being insensitive to ifenprodil's aversive effects relative to adults. Thus, only antagonism of NR2B receptors in male rats mimicked age-specific sensitivities to the aversive effects of EtOH.

Subjective cannabis effects as part of a developing disorder in adolescents and emerging adults

In light of expanding legalization of cannabis and swelling debate about the potential risks, particularly for younger users, understanding acute cannabis effects among adolescents and emerging adults is more important than ever. Contemporary models of addiction development identify subjective drug responses as central to the developmental unfolding of drug use disorders. Despite this, surprisingly little is known about cannabis's acute subjective effects in human youths. This research utilized ecological momentary assessment (EMA) in the natural environment to identify the typical situational context of cannabis use among 85 frequent cannabis users, ages 15-24 years (M = 19.8, SD = 2.0; 48.2% female). Study aims were to (a) characterize momentary changes in several subjective states (i.e., stimulation, sedation, tension, craving, and high) when not using, just before cannabis use, and after use, and (b) evaluate whether cannabis responses varied with cannabis use disorder (CUD) severity or across the transition from adolescence to emerging adulthood in a correlational manner. Use of cannabis produced measurable reductions in craving and tension, as well as increases in stimulation, sedation, and "high." Participants with more CUD symptoms reported greater relief of craving and increased stimulatory response and high following use. In contrast, emerging adults reported diminished stimulatory response and high following use, relative to adolescents. Results highlight the utility of EMA for characterizing cannabis response as this behavior unfolds in daily life, during a key developmental timeframe in the pathogenesis of cannabis-use pathology. (PsycINFO Database Record

Hypothalamic CCL2/CCR2 Chemokine System: Role in Sexually Dimorphic Effects of Maternal Ethanol Exposure on Melanin-Concentrating Hormone and Behavior in Adolescent Offspring

Clinical and animal studies show that ethanol exposure and inflammation during pregnancy cause similar behavioral disturbances in the offspring. While ethanol is shown to stimulate both neuroimmune and neurochemical systems in adults, little is known about their anatomical relationship in response to ethanol in utero and whether neuroimmune factors mediate ethanol's effects on neuronal development and behavior in offspring. Here we examined in female and male adolescent rats a specific population of neurons concentrated in lateral hypothalamus, which coexpress the inflammatory chemokine C-C motif ligand 2 (CCL2) or its receptor CCR2 with the orexigenic neuropeptide, melanin-concentrating hormone (MCH), that promotes ethanol drinking behavior. We demonstrate that maternal administration of ethanol (2 g/kg/d) from embryonic day 10 (E10) to E15, while having little impact on glia, stimulates expression of neuronal CCL2 and CCR2, increases density of both large CCL2 neurons colocalizing MCH and small CCL2 neurons surrounding MCH neurons, and stimulates ethanol drinking and anxiety in adolescent offspring. We show that these neuronal and behavioral changes are similarly produced by maternal administration of CCL2 (4 or 8 μg/kg/d, E10-E15) and blocked by maternal administration of a CCR2 antagonist INCB3344 (1 mg/kg/d, E10-E15), and these effects of ethanol and CCL2 are sexually dimorphic, consistently stronger in females. These results suggest that this neuronal CCL2/CCR2 system closely linked to MCH neurons has a role in mediating the effects of maternal ethanol exposure on adolescent offspring and contributes to the higher levels of adolescent risk factors for alcohol use disorders described in women.SIGNIFICANCE STATEMENT Ethanol consumption and inflammatory agents during pregnancy similarly increase alcohol intake and anxiety in adolescent offspring. To investigate how neurochemical and neuroimmune systems interact to mediate these disturbances, we examined a specific population of hypothalamic neurons coexpressing the inflammatory chemokine CCL2 and its receptor CCR2 with the neuropeptide, melanin-concentrating hormone. We demonstrate in adolescent offspring that maternal administration of CCL2, like ethanol, stimulates these neurons and increases ethanol drinking and anxiety, and these effects of ethanol are blocked by maternal CCR2 antagonist and consistently stronger in females. This suggests that neuronal chemokine signaling linked to neuropeptides mediates effects of maternal ethanol exposure on adolescent offspring and contributes to higher levels of adolescent risk factors for alcohol use disorders in women.

Adolescent rats fail to demonstrate a LiCl-induced pre-exposure effect: Implications for the balance of drug reward and aversion in adolescence

Adolescents display weaker taste avoidance induced by both abused and non-abused drugs than adults. Drug history attenuates avoidance learning in adults (the drug pre-exposure effect), but little is known about this phenomenon in adolescents. Given that the weaker taste avoidance in adolescence is thought to be a function of their relative insensitivity to the drug's aversive effects, it might be expected that the drug pre-exposure effect would be weaker in adolescents given that for some drugs this effect is mediated by associative blocking that depends on the association of environmental cues with the drug's aversive effects. To address this, in the present studies male adolescent (Experiment 1) and adult (Experiment 2) rats were given five spaced injections of LiCl prior to subsequent taste avoidance conditioning with LiCl. Consistent with past reports, adolescents displayed weaker taste avoidance than adults. While adults displayed attenuated LiCl-induced taste avoidance following LiCl pre-exposure, adolescents showed no evidence of this pre-exposure. This work is consistent with the view that adolescents are relatively insensitive to the aversive effects of drugs, an insensitivity potentially important in subsequent intake of drugs of abuse given that such intake is a function of the balance of their rewarding and aversive effects.

Age-Related Changes in Associations Between Reasons for Alcohol Use and High-Intensity Drinking Across Young Adulthood

OBJECTIVE

Analyses focus on whether self-reported reasons for drinking alcohol change in their associations with high-intensity drinking across the transition to adulthood.

METHOD

Self-report data on high-intensity drinking (10+ drinks) collected from the national Monitoring the Future study in 2005 to 2014 from those ages 18-26 were used (N = 2,664 [60% women] for all drinkers and 1,377 for heavy episodic [5+] drinkers; up to 6,541 person-waves). Time-varying effect modeling examined changes in the direction and magnitude of associations between eight reasons for drinking and high-intensity alcohol use across continuous age.

RESULTS

Four reasons to drink showed quite stable associations with high-intensity drinking across age: drinking to get away from problems, to get high, to relax, and to sleep. Associations between two reasons and high-intensity drinking decreased with age: anger/frustration and to have a good time. The association between drinking because of boredom and high-intensity drinking increased with age. Drinking because it tastes good had a weak association with high-intensity drinking. Among heavy episodic drinkers, reasons for use also differentiated high-intensity drinking, with two exceptions: drinking to have a good time and to relax did not distinguish drinking 10+ drinks from drinking 5-9 drinks.

CONCLUSIONS

Reasons for drinking are differentially associated with high-intensity drinking, compared with any other drinking and compared with lower intensity heavy drinking, across age during the transition to adulthood. Intervention programs seeking to mitigate alcohol-related harms should focus on reasons for use when they are the most developmentally salient.

Chronic Intermittent Ethanol Exposure Modulation of Glutamatergic Neurotransmission in Rat Lateral/Basolateral Amygdala is Duration-, Input-, and Sex-Dependent

The basolateral amygdala (BLA) controls numerous behaviors, like anxiety and reward seeking, via the activity of glutamatergic principal neurons. These BLA neurons receive excitatory inputs primarily via two major anatomical pathways - the external capsule (EC), which contains afferents from lateral cortical structures, and the stria terminalis (ST), containing synapses from more midline brain structures. Chronic intermittent ethanol (CIE) exposure/withdrawal produces distinct alterations in these pathways. Specifically, 10 days of CIE (via vapor inhalation) increases presynaptic function at ST synapses and postsynaptic function at EC synapses. Given that 10-day CIE/withdrawal also increases anxiety-like behavior, we sought to examine the development of these alterations at these inputs using an exposure time-course in both male and female rats. Specifically, using 3, 7, and 10 days CIE exposure, we found that all three durations increase anxiety-like behavior in the elevated plus maze. At BLA synapses, increased presynaptic function at ST inputs required shorter exposure durations relative to post-synaptic alterations at EC inputs in both sexes. But, synaptic alterations in females required longer ethanol exposures compared to males. These data suggest that presynaptic alteration at ST-BLA afferents is an early neuroadaptation during repeated ethanol exposures. And, the similar patterns of presynaptic-then-postsynaptic facilitation across the sexes suggest the former may be required for the latter. These cooperative interactions may contribute to the increased anxiety-like behavior that is observed following CIE-induced withdrawal and may provide novel therapeutic targets to reverse withdrawal-induced anxiety.

A Review of Developmental Considerations in Human Laboratory Alcohol Research

PURPOSE OF REVIEW

Human laboratory studies involving alcohol administration have generated critical knowledge about individual differences in risk for alcohol use disorder (AUD), but have primarily involved adult populations and cross-sectional research designs. Ethical constraints have largely precluded human laboratory alcohol research in adolescence, and prospective studies have been rare. This paper provides an overview of developmental considerations in human laboratory alcohol research, with a focus on studies conducted with youth.

RECENT FINDINGS

Recent human laboratory studies from Europe and Canada have examined aspects of alcohol response during late adolescence, while recent survey studies from the United States have highlighted methods for circumventing alcohol administration in studies of adolescents.

SUMMARY

Across several decades of research, exceedingly few laboratory studies have examined developmental differences in alcohol responses or utilized prospective designs. Efforts to prioritize prospective research would further clarify the role of alcohol sensitivity traits as predictors or markers of AUD onset and progression.

Sex and Adolescent Ethanol Exposure Influence Pavlovian Conditioned Approach

BACKGROUND

Alcohol use among adolescents is widespread and a growing concern due to long-term behavioral deficits, including altered Pavlovian behavior, that potentially contribute to addiction vulnerability. We tested the hypothesis that adolescent intermittent ethanol (AIE) exposure alters Pavlovian behavior in males and females as measured by a shift from goal-tracking to sign-tracking. Additionally, we investigated GLT-1, an astrocytic glutamate transporter, as a potential contributor to a sign-tracking phenotype.

METHODS

Male and female Sprague-Dawley rats were exposed to AIE (5 g/kg, intragastric) or water intermittently 2 days on and 2 days off from postnatal day (P) 25 to 54. Around P70, animals began 20 daily sessions of Pavlovian conditioned approach (PCA), where they learned that a cue predicted noncontingent reward delivery. Lever pressing indicated interaction with the cue, or sign-tracking, and receptacle entries indicated approach to the reward delivery location, or goal-tracking. To test for effects of AIE on nucleus accumbens (NAcc) excitatory signaling, we isolated membrane subfractions and measured protein levels of the glutamate transporter GLT-1 after animals completed behavior as a measure of glutamate homeostasis.

RESULTS

Females exhibited elevated sign-tracking compared to males with significantly more lever presses, faster latency to first lever press, and greater probability to lever press in a trial. AIE significantly increased lever pressing while blunting goal-tracking, as indicated by fewer cue-evoked receptacle entries, slower latency to receptacle entry, and lower probability to enter the receptacle in a trial. No significant sex-by-exposure interactions were observed in sign- or goal-tracking metrics. Moreover, we found no significant effects of sex or exposure on membrane GLT-1 expression in the NAcc.

CONCLUSIONS

Females exhibited enhanced sign-tracking compared to males, while AIE decreased goal-tracking compared to control exposure. Our findings support the hypothesis that adolescent binge ethanol can shift conditioned behavior from goal- to cue-directed in PCA, especially in females.

Subjective alcohol responses in a cross-sectional, field-based study of adolescents and young adults: Effects of age, drinking level, and dependence/consequences

BACKGROUND

Adolescents are physically, cognitively, socially, and emotionally different than adults in ways that may partially explain why alcohol misuse typically develops during this period. Ample animal-science evidence and nascent ecological evidence points toward developmentally limited differences in sensitivity to alcohol's stimulatory and sedative effects. Field-based research methods were used to test for such age-related differences in a sample of adolescents through young adults. Potential moderating influences of estimated blood alcohol content (eBAC), as well as typical consumption and level of dependence/consequences were explored.

METHODS

Subjective alcohol responses were collected from 1,364 participants, aged 17 to 32 years, recruited outside of venues where drinking takes place in a small metropolitan bar district.

RESULTS

Self-reports of stimulatory response to alcohol were age-related, such that younger participants reported increased subjective stimulation at the time of data collection relative to older participants. Age-related differences in stimulatory responses were more pronounced at lower eBACs and among younger participants who typically drank more heavily. Stimulatory responses generally diminished among older than younger participants, although individuals with greater dependence/consequences consistently reported greater stimulation from drinking. Contrastingly, age, typical consumption, and dependence/consequences were not related to sedation in this sample.

CONCLUSIONS

This research provides cross-sectional evidence to support age-, consumption-, and dependence/consequences-related differences in stimulatory alcohol responses among adolescents and young adults assessed within a bar-area context. While cross-sectional, the results of this field-based study provide support for the theory that addiction liability is developmentally linked and associated, in part, with age-related differences in subjective alcohol responses.

Post-weaning Environmental Enrichment, But Not Chronic Maternal Isolation, Enhanced Ethanol Intake during Periadolescence and Early Adulthood

This study analyzed ethanol intake in male and female Wistar rats exposed to maternal separation (MS) during infancy (postnatal days 1-21, PD1-21) and environmental enrichment (EE) during adolescence (PD 21-42). Previous work revealed that MS enhances ethanol consumption during adulthood. It is still unknown if a similar effect is found during adolescence. Several studies, in turn, have revealed that EE reverses stress experiences, and reduces ethanol consumption and reinforcement; although others reported greater ethanol intake after EE. The interactive effects between these treatments upon ethanol's effects and intake have yet to be explored. We assessed chronic ethanol intake and preference (12 two-bottle daily sessions, spread across 30 days, 1st session on PD46) in rats exposed to MS and EE. The main finding was that male - but not female - rats that had been exposed to EE consumed more ethanol than controls given standard housing, an effect that was not affected by MS. Subsequent experiments assessed several factors associated with heightened ethanol consumption in males exposed to MS and EE; namely taste aversive conditioning and hypnotic-sedative consequences of ethanol. We also measured anxiety response in the light-dark box and in the elevated plus maze tests; and exploratory patterns of novel stimuli and behaviors indicative of risk assessment and risk-taking, via a modified version of the concentric square field (CSF) test. Aversive conditioning, hypnosis and sleep time were similar in males exposed or not to EE. EE males, however, exhibited heightened exploration of novel stimuli and greater risk taking behaviors in the CSF test. It is likely that the promoting effect of EE upon ethanol intake was due to these effects upon exploratory and risk-taking behaviors.

Exposure to alcohol during adolescence exerts long-term effects on stress response and the adult brain stress circuits

The hypothalamic-pituitary-adrenal (HPA) axis undergoes critical developments during adolescence. Therefore, stressors experienced during this period potentially have long-term effects on adult HPA axis function. We hypothesized that adolescent intermittent ethanol (AIE) exposure would affect adult HPA axis function, resulting in altered responses to an alcohol challenge in young adults or adults. To test these hypotheses, male rats were exposed to alcohol vapor for 6h per day from post-natal day (PND) 28-42, then acutely challenged with alcohol intragastrically (3.2-4.5g/kg) in young adults (PND 70) or adults (PND 90). Overall, we observed blunted HPA axis responses to an alcohol challenge due to AIE exposure. Specifically, AIE tended to inhibit the alcohol challenge-induced increase in plasma corticosterone (CORT) concentrations in young adult and adult rats. As well, AIE significantly blunted the alcohol challenge-induced arginine vasopressin (Avp) mRNA expression in the paraventricular nucleus (PVN) of the hypothalamus of adult rats. Results of the present study are similar to what we have previously shown, that these changes in PVN responsiveness may result from AIE-induced alterations in adrenergic neurons in brain stem regions C1-C3 known to project to the PVN. AIE elevated the number of colocalized c-fos/phenylethanolamine N-methyltransferase (PNMT)-positive cell bodies in the C1 region of adult rats. Together, these data suggest that AIE exposure produces alterations in male HPA axis responsiveness to administration of an acute alcohol challenge that may be long-lasting.

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.

Emerging Pharmacologic Treatments for Adolescent Substance Use: Challenges and New Directions

Adolescence is a key period in the development of substance use and misuse. Substance use typically begins during adolescence and prevalence rates for many substance use disorders peak before age 21 years. Yet, despite clinical demand, treatments for youth rely almost entirely on psychosocial interventions that yield only modest benefit. One potential way to improve treatment effects is to augment the best available psychosocial interventions with pharmacotherapy. Although pharmacotherapy research has advanced care for adults with substance use disorders, no medication is indicated for adolescents and controlled trials with teenagers are scant. Optimizing treatments for youth will require closing this important gap in medication development research. In this paper we review the paucity of pharmacotherapy research for adolescent substance misuse, and we discuss how we can leverage human laboratory paradigms and technology to advance our understanding regarding if and how medications may improve treatment options for youths.

Age-related effects of chronic restraint stress on ethanol drinking, ethanol-induced sedation, and on basal and stress-induced anxiety response

Adolescents are sensitive to the anxiolytic effect of ethanol, and evidence suggests that they may be more sensitive to stress than adults. Relatively little is known, however, about age-related differences in stress modulation of ethanol drinking or stress modulation of ethanol-induced sedation and hypnosis. We observed that chronic restraint stress transiently exacerbated free-choice ethanol drinking in adolescent, but not in adult, rats. Restraint stress altered exploration patterns of a light-dark box apparatus in adolescents and adults. Stressed animals spent significantly more time in the white area of the maze and made significantly more transfers between compartments than their non-stressed peers. Behavioral response to acute stress, on the other hand, was modulated by prior restraint stress only in adults. Adolescents, unlike adults, exhibited ethanol-induced motor stimulation in an open field. Stress increased the duration of loss of the righting reflex after a high ethanol dose, yet this effect was similar at both ages. Ethanol-induced sleep time was much higher in adult than in adolescent rats, yet stress diminished ethanol-induced sleep time only in adults. The study indicates age-related differences that may increase the risk for initiation and escalation in alcohol drinking.

Chronic Drinking During Adolescence Predisposes the Adult Rat for Continued Heavy Drinking: Neurotrophin and Behavioral Adaptation after Long-Term, Continuous Ethanol Exposure

Previous research has found that adolescent ethanol (EtOH) exposure alters drug seeking behaviors, cognition and neuroplasticity. Using male Sprague Dawley rats, differences in spatial working memory, non-spatial discrimination learning and behavioral flexibility were explored as a function of age at the onset (mid-adolescent vs. adult) of chronic EtOH exposure (CET). Concentrations of mature brain-derived neurotrophic factor (mBDNF) and beta-nerve growth factor (β-NGF) in the prefrontal cortex and hippocampus were also assessed at different time-points: during CET, following acute abstinence (48-hrs), and after protracted abstinence (6-8 wks). Our results revealed that an adolescent onset of CET leads to increased EtOH consumption that persisted into adulthood. In both adult and adolescent onset CET groups, there were significant long-term reductions in prefrontal cortical mBDNF and β-NGF levels. However, only adult onset CET rats displayed decreased hippocampal BDNF levels. Spatial memory, assessed by spontaneous alternation and delayed alternation, was not significantly affected by CET as a function of age of drinking onset, but higher blood-EtOH levels were correlated with lower spontaneous alternation scores. Regardless of the age of onset, EtOH exposed rats were impaired on non-spatial discrimination learning and displayed inflexible behavioral patterns upon reversal learning. Our results indicate that adolescent EtOH exposure changes long-term consumption patterns producing behavioral and neural dysfunctions that persist across the lifespan.

Adolescent binge-like alcohol alters sensitivity to acute alcohol effects on dopamine release in the nucleus accumbens of adult rats

Rationale: Early onset of alcohol drinking has been associated with alcohol abuse in adulthood. The neurobiology of this phenomenon is unclear, but mesolimbic dopamine pathways, which are dynamic during adolescence, may play a role.

OBJECTIVES

We investigated the impact of adolescent binge-like alcohol on phasic dopaminergic neurotransmission during adulthood.

METHODS

Rats received intermittent intragastric ethanol, water, or nothing during adolescence. In adulthood, electrically evoked dopamine release and subsequent uptake were measured in the nucleus accumbens core at baseline and after acute challenge of ethanol or saline.

RESULTS

Adolescent ethanol exposure did not alter basal measures of evoked dopamine release or uptake. Ethanol challenge dose-dependently decreased the amplitude of evoked dopamine release in rats by 30–50 % in control groups, as previously reported, but did not alter evoked release in ethanol-exposed animals. To address the mechanism by which ethanol altered dopamine signaling, the evoked signals were modeled to estimate dopamine efflux per impulse and the velocity of the dopamine transporter. Dopamine uptake was slower in all exposure groups after ethanol challenge compared to saline, while dopamine efflux per pulse of electrical stimulation was reduced by ethanol only in ethanol-naive rats.

CONCLUSIONS

The results demonstrate that exposure to binge levels of ethanol during adolescence blunts the effect of ethanol challenge to reduce the amplitude of phasic dopamine release in adulthood. Large dopamine transients may result in more extracellular dopamine after alcohol challenge in adolescent-exposed rats and may be one mechanism by which alcohol is more reinforcing in people who initiated drinking at an early age.

Application of an alcohol clamp paradigm to examine inhibitory control, subjective responses, and acute tolerance in late adolescence

Individual differences in acute alcohol effects on cognitive control and subjective responses--and acute tolerance to these effects--are implicated in the risk for heavy drinking and alcohol-related harms. Few studies have examined these effects in drinkers under age 21. Additionally, studies of acute tolerance typically involve bolus oral alcohol administration, such that estimates of tolerance are confounded with blood alcohol concentration (BAC) limb. The current study examined cognitive control and subjective responses in young heavy drinkers (n = 88; M = 19.8 years old, SD = 0.8) during a single-session alcohol clamp protocol. Participants completed an intravenous alcohol session comprising an ascending limb (0 to 80 mg% in 20 min) and a BAC plateau (80 mg% for 80 min). Serial assessments included a cued go/no-go task and measures of stimulation, sedation, and craving. Relevant individual difference factors (attention-deficit hyperactivity disorder [ADHD] symptoms and sensation seeking) were examined as moderators. Multilevel modeling demonstrated that response inhibition worsened following initial rise in BAC and showed increasing impairment during the BAC plateau. ADHD symptoms and sensation seeking moderated this effect. Significant within-person associations between stimulation and craving were evident on the ascending limb only. Participants with higher ADHD symptoms reported steeper increases in stimulation during the ascending limb. These findings provide initial information about subjective and behavioral responses during pseudoconstant BAC, and potential moderators of these outcomes, in late adolescence. Additional studies with placebo-controlled designs are necessary to confirm these findings.

Characterizing subjective responses to alcohol among adolescent problem drinkers

Theoretical models of alcoholism emphasize the acute reinforcing properties of alcohol as chief determinants of drinking, and animal research suggests adolescents are uniquely sensitive to these effects. Human studies of these phenomena, however, are virtually nonexistent. We used ecological momentary assessment methods to capture adolescents' subjective responses to alcohol in real time in their natural environments. Adolescent participants were 22 problem drinkers, ages 15 to 19 years (M = 18.3, SD = 0.09; 55% female; 55% alcohol dependent). Participants consumed alcohol on 38% of days during a 1-week monitoring period, with an average of 5 drinks per occasion. Momentary data revealed that adolescents experience decreased stimulation and increased sedation and "high" across the ascending limb of the blood alcohol curve. Notably, greater craving predicted higher volumes of subsequent alcohol consumption during the episode, whereas greater "high" attenuated use. To test for developmental differences in these effects, we pooled these data with data from a similarly ascertained sample of 36 adult heavy drinkers, ages 24 to 64 years (M = 38.1, SD = 11.8; 50% female; 61% alcohol dependent). Adolescents were more sensitive to the stimulant effects of alcohol than adults. This study provides novel data on how adolescent problem drinkers experience alcohol in their natural contexts and illustrates how these effects, which appear to differ from adult problem drinkers, confer liability for future drinking.

Male adolescent rats display blunted cytokine responses in the CNS after acute ethanol or lipopolysaccharide exposure

Alcohol induces widespread changes in cytokine expression, with recent data from our laboratory having demonstrated that, during acute ethanol intoxication, adult rats exhibit consistent increases in interleukin (IL)-6 mRNA expression in several brain regions, while showing reductions in IL-1 and TNFα expression. Given evidence indicating that adolescence may be an ontogenetic period in which some neuroimmune processes and cells may not yet have fully matured, the purpose of the current experiments was to examine potential age differences in the central cytokine response of adolescent (P31-33days of age) and adult (69-71days of age) rats to either an acute immune (lipopolysaccharide; LPS) or non-immune challenge (ethanol). In Experiment 1, male Sprague-Dawley rats were given an intraperitoneal (i.p.) injection of either sterile saline, LPS (250μg/kg), or ethanol (4-g/kg), and then trunk blood and brain tissue were collected 3h later for measurement of blood ethanol concentrations (BECs), plasma endotoxin, and central mRNA expression of several immune-related gene targets. In Experiment 2, the response to intragastrically (i.g.) administered ethanol was examined and compared to animals given tap water (i.g.). Results showed that LPS stimulated robust increases in expression of IL-1, IL-6, TNFα, and IκBα in the hippocampus, PVN, and amygdala, and that these increases were generally less pronounced in adolescents relative to adults. Following an i.p. ethanol challenge, IL-6 and IκBα expression was significantly increased in both ages in the PVN and amygdala, and adults exhibited even greater increases in IκBα than adolescents. I.g. administration of ethanol also increased IL-6 and IκBα expression in all three brain regions, with hippocampal IL-6 elevated even more so in adults compared to adolescents. Furthermore, assessment of plasma endotoxin concentrations revealed (i) whereas robust increases in plasma endotoxin were observed in adults injected with LPS, no corresponding elevations were seen in adolescents after LPS; and (ii) neither adolescents nor adults demonstrated increases in plasma endotoxin concentrations following i.p. or i.g. ethanol administration. Analysis of BECs indicated that, for both routes of exposure, adolescents exhibited lower BECs than adults. Taken together, these data suggest that categorically different mechanisms are involved in the central cytokine response to antigen exposure versus ethanol administration. Furthermore, these findings confirm once again that acute ethanol intoxication is a potent activator of brain cytokines, and calls for future studies to identify the mechanisms underlying age-related differences in the cytokine response observed during ethanol intoxication.

Reduced dopamine release in the nucleus accumbens core of adult rats following adolescent binge alcohol exposure: age and dose-dependent analysis

RATIONALE

Alcohol use in adolescence is one of the most significant predictors of alcohol dependence in adulthood, yet the neurochemical mechanisms underlying this heightened vulnerability remain unknown. Whereas research has focused on characterizing adaptations in the mesolimbic dopamine (DA) system following ethanol exposure in adolescence, whether these changes persist into adulthood has yet to be determined.

OBJECTIVES

The objective of this study is to investigate the effects of binge-intermittent ethanol administration in adolescence (P30-50) or early adulthood (P60-80) on DA in the nucleus accumbens (NAc) core after an ethanol challenge in adulthood following a period of abstinence.

METHODS

Male Sprague Dawley rats (n = 160) were administered intermittent ethanol injections, 1 or 3 g/kg, intraperitoneally (i.p.) every other day for 20 days starting on either P30 or 60. Following an ethanol-free period of either 7, 14, or 28 days, we measured DA efflux following an ethanol challenge (3 g/kg, i.p.) using electrochemical recording electrodes bilaterally implanted into the NAc core.

RESULTS

Moderate-dose ethanol administration (1 g/kg, i.p.) during adolescence significantly decreased ethanol-evoked DA release in adulthood at 7 and 14 days, but not 28 days, following pretreatment exposure compared to saline controls. Relative to rats pretreated with ethanol in adulthood, moderate-dose ethanol in adolescence significantly reduced DA efflux at all time points measured. Additionally, adult rats pretreated with high dose ethanol administration (3 g/kg, i.p.) displayed significantly decreased DA compared to adolescents after 28 days of withdrawal.

CONCLUSIONS

Binge-intermittent ethanol administration during adolescence may induce age-dependent neuroadaptations in the mesolimbic DA system compared to ethanol-treated adults during protracted ethanol withdrawal.

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.

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.

Adolescent and adult rat cortical protein kinase A display divergent responses to acute ethanol exposure

Adolescent rats display reduced sensitivity to many dysphoria-related effects of alcohol (ethanol) including motor ataxia and sedative hypnosis, but the underlying neurobiological factors that contribute to these differences remain unknown. The cyclic adenosine monophosphate (cAMP)-dependent protein kinase A (PKA) pathway, particularly the type II regulatory subunit (RII), has been implicated in ethanol-induced molecular and behavioral responses in adults. Therefore, the current study examined cerebral cortical PKA in adolescent and adult ethanol responses. With the exception of early adolescence, PKA RIIα and RIIβ subunit levels largely did not differ from adult levels in either whole cell lysate or P2 synaptosomal expression. However, following acute ethanol exposure, PKA RIIβ P2 synaptosomal expression and activity were increased in adults, but not in adolescents. Behaviorally, intracerebroventricular administration of the PKA activator Sp-cAMP and inhibitor Rp-cAMP prior to ethanol administration increased adolescent sensitivity to the sedative-hypnotic effects of ethanol compared to controls. Sp-cAMP was ineffective in adults whereas Rp-cAMP suggestively reduced loss of righting reflex (LORR) with paralleled increases in blood ethanol concentrations. Overall, these data suggest that PKA activity modulates the sedative/hypnotic effects of ethanol and may potentially play a wider role in the differential ethanol responses observed between adolescents and adults.

The effect of alcohol consumption on the adolescent brain: A systematic review of MRI and fMRI studies of alcohol-using youth

Background

A large proportion of adolescents drink alcohol, with many engaging in high-risk patterns of consumption, including binge drinking. Here, we systematically review and synthesize the existing empirical literature on how consuming alcohol affects the developing human brain in alcohol-using (AU) youth.

Methods

For this systematic review, we began by conducting a literature search using the PubMED database to identify all available peer-reviewed magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI) studies of AU adolescents (aged 19 and under). All studies were screened against a strict set of criteria designed to constrain the impact of confounding factors, such as co-occurring psychiatric conditions.

Results

Twenty-one studies (10 MRI and 11 fMRI) met the criteria for inclusion. A synthesis of the MRI studies suggested that overall, AU youth showed regional differences in brain structure as compared with non-AU youth, with smaller grey matter volumes and lower white matter integrity in relevant brain areas. In terms of fMRI outcomes, despite equivalent task performance between AU and non-AU youth, AU youth showed a broad pattern of lower task-relevant activation, and greater task-irrelevant activation. In addition, a pattern of gender differences was observed for brain structure and function, with particularly striking effects among AU females.

Conclusions

Alcohol consumption during adolescence was associated with significant differences in structure and function in the developing human brain. However, this is a nascent field, with several limiting factors (including small sample sizes, cross-sectional designs, presence of confounding factors) within many of the reviewed studies, meaning that results should be interpreted in light of the preliminary state of the field. Future longitudinal and large-scale studies are critical to replicate the existing findings, and to provide a more comprehensive and conclusive picture of the effect of alcohol consumption on the developing brain.

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A large proportion of adolescents drink alcohol.

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Studies show how alcohol affects human adolescent brain development.

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This includes a systematic review of MRI and fMRI studies in alcohol-using youth.

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Changes in structure and function are observed in the brain in alcohol-using youth.

Decreases in energy and increases in phase locking of event-related oscillations to auditory stimuli occur during adolescence in human and rodent brain

Synchrony of phase (phase locking) of event-related oscillations (EROs) within and between different brain areas has been suggested to reflect communication exchange between neural networks and as such may be a sensitive and translational measure of changes in brain remodeling that occur during adolescence. This study sought to investigate developmental changes in EROs using a similar auditory event-related potential (ERP) paradigm in both rats and humans. Energy and phase variability of EROs collected from 38 young adult men (aged 18-25 years), 33 periadolescent boys (aged 10-14 years), 15 male periadolescent rats [at postnatal day (PD) 36] and 19 male adult rats (at PD103) were investigated. Three channels of ERP data (frontal cortex, central cortex and parietal cortex) were collected from the humans using an 'oddball plus noise' paradigm that was presented under passive (no behavioral response required) conditions in the periadolescents and under active conditions (where each subject was instructed to depress a counter each time he detected an infrequent target tone) in adults and adolescents. ERPs were recorded in rats using only the passive paradigm. In order to compare the tasks used in rats to those used in humans, we first studied whether three ERO measures [energy, phase locking index (PLI) within an electrode site and phase difference locking index (PDLI) between different electrode sites] differentiated the 'active' from 'passive' ERP tasks. Secondly, we explored our main question of whether the three ERO measures differentiated adults from periadolescents in a similar manner in both humans and rats. No significant changes were found in measures of ERO energy between the active and passive tasks in the periadolescent human participants. There was a smaller but significant increase in PLI but not PDLI as a function of active task requirements. Developmental differences were found in energy, PLI and PDLI values between the periadolescents and adults in both the rats and the human participants. Neuronal synchrony as indexed by PLI and PDLI was significantly higher to the infrequent (target) tone compared to the frequent (nontarget) tone in all brain sites in all of the regions of interest time-frequency intervals. Significantly higher ERO energy and significantly lower synchrony was seen in the periadolescent humans and rats compared to their adult counterparts. Taken together these findings are consistent with the hypothesis that adolescent remodeling of the brain includes decreases in energy and increases in synchrony over a wide frequency range both within and between neuronal networks and that these effects are conserved over evolution.

Ontogeny and adolescent alcohol exposure in Wistar rats: open field conflict, light/dark box and forced swim test

Epidemiological studies have demonstrated that heavy drinking and alcohol abuse and dependence peak during the transition between late adolescence and early adulthood. Studies in animal models have demonstrated that alcohol exposure during adolescence can cause a modification in some aspects of behavioral development, causing the "adolescent phenotype" to be retained into adulthood. However, the "adolescent phenotype" has not been studied for a number of behavioral tests. The objective of the present study was to investigate the ontogeny of behaviors over adolescence/young adulthood in the light/dark box, open field conflict and forced swim test in male Wistar rats. These data were compared to previously published data from rats that received intermittent alcohol vapor exposure during adolescence (AIE) to test whether they retained the "adolescent phenotype" in these behavioral tests. Three age groups of rats were tested (post-natal day (PD) 34-42; PD55-63; PD69-77). In the light/dark box test, younger rats escaped the light box faster than older adults, whereas AIE rats returned to the light box faster and exhibited more rears in the light than controls. In the open field conflict test, both younger and AIE rats had shorter times to first enter the center, spent more time in the center of the field, were closer to the food, and consumed more food than controls. In the forced swim test no clear developmental pattern emerged. The results of the light/dark box and the forced swim test do not support the hypothesis that adolescent ethanol vapor exposure can "lock-in" all adolescent phenotypes. However, data from the open field conflict test suggest that the adolescent and the AIE rats both engaged in more "disinhibited" and food motivated behaviors. These data suggest that, in some behavioral tests, AIE may result in a similar form of behavioral disinhibition to what is seen in adolescence.

Long-term effects of early adolescent methamphetamine exposure on depression-like behavior and the hypothalamic vasopressin system in mice

Methamphetamine (MA) has neurotoxic effects on the adult human brain that can lead to deficits in behavior and cognition. However, relatively little research has examined the behavioral or neurotoxic effects of MA in adolescents. The rising rates of adolescent MA use make it imperative that we understand the long-term effects of MA exposure on the adolescent brain and how these effects may differ from those seen in adults. In this study, the long-term effects of MA exposure during early adolescence on behavior and the vasopressin system in the paraventricular nucleus of the hypothalamus in late adolescent and adult male and female C57BL/6J mice were examined. MA exposure increased depression-like behavior in the Porsolt forced swim test in both late adolescent and adult male and female mice. Late adolescent male mice exposed to MA also showed a decrease in the number of vasopressin-immunoreactive neurons in the paraventricular nucleus compared to sex-matched saline-treated controls. Thus, similar to humans exposed to MA during adolescence, mice exposed to MA during adolescence show increased depression-like behavior later in life. These changes in behavior may be related to MA-induced alterations in vasopressin and the hypothalamic-pituitary-adrenal axis, especially in males.

GABAergic contributions to alcohol responsivity during adolescence: insights from preclinical and clinical studies

There is a considerable body of literature demonstrating that adolescence is a unique age period, which includes rapid and dramatic maturation of behavioral, cognitive, hormonal and neurobiological systems. Most notably, adolescence is also a period of unique responsiveness to alcohol effects, with both hyposensitivity and hypersensitivity observed to the various effects of alcohol. Multiple neurotransmitter systems are undergoing fine-tuning during this critical period of brain development, including those that contribute to the rewarding effects of drugs of abuse. The role of developmental maturation of the γ-amino-butyric acid (GABA) system, however, has received less attention in contributing to age-specific alcohol sensitivities. This review integrates GABA findings from human magnetic resonance spectroscopy studies as they may translate to understanding adolescent-specific responsiveness to alcohol effects. Better understanding of the vulnerability of the GABA system both during adolescent development, and in psychiatric conditions that include alcohol dependence, could point to a putative mechanism, boosting brain GABA, that may have increased effectiveness for treating alcohol use disorders.

Relationships between reward sensitivity, risk-taking and family history of alcoholism during an interactive competitive fMRI task

Background

Individuals with a positive family history for alcoholism (FHP) have shown differences from family-history-negative (FHN) individuals in the neural correlates of reward processing. FHP, compared to FHN individuals, demonstrate relatively diminished ventral striatal activation during anticipation of monetary rewards, and the degree of ventral striatal activation shows an inverse correlation with specific impulsivity measures in alcohol-dependent individuals. Rewards in socially interactive contexts relate importantly to addictive propensities, yet have not been examined with respect to how their neural underpinnings relate to impulsivity-related measures. Here we describe impulsivity measures in FHN and FHP individuals as they relate to a socially interactive functional magnetic resonance imaging (fMRI) task.

Methods

Forty FHP and 29 FHN subjects without histories of Axis-I disorders completed a socially interactive Domino task during functional magnetic resonance imaging and completed self-report and behavioral impulsivity-related assessments.

Results

FHP compared to FHN individuals showed higher scores (p = .004) on one impulsivity-related factor relating to both compulsivity (Padua Inventory) and reward/punishment sensitivity (Sensitivity to Punishment/Sensitivity to Reward Questionnaire). Multiple regression analysis within a reward-related network revealed a correlation between risk-taking (involving another impulsivity-related factor, the Balloon Analog Risk Task (BART)) and right ventral striatum activation under reward >punishment contrast (p<0.05 FWE corrected) in the social task.

Conclusions

Behavioral risk-taking scores may be more closely associated with neural correlates of reward responsiveness in socially interactive contexts than are FH status or impulsivity-related self-report measures. These findings suggest that risk-taking assessments be examined further in socially interactive settings relevant to addictive behaviors.

Growing pains and pleasures: how emotional learning guides development

The nervous system promotes adaptive responding to myriad environmental stimuli by ascribing emotion to specific stimulus domains. This affects the salience of different stimuli, facilitates learning, and likely involves the amygdala. Recent studies suggest a strong homology between adaptive responses that result from learning and those that emerge during development. As in motivated learning, developmental studies have found the salience of different classes of stimulus (e.g., peers) undergoes marked fluctuation across maturation and may involve differential amygdala engagement. In this review, by highlighting the importance of particular stimulus categories during sensitive periods of development, we suggest that variability in amygdala response to different stimulus domains has an active and functional role in shaping emerging cortical circuits across development.

Female rats display enhanced rewarding effects of ethanol that are hormone dependent

BACKGROUND

Ethanol (EtOH) abuse is a major health and economic concern, particularly for females who appear to be more sensitive to the rewarding effects of EtOH. This study compared sex differences to the rewarding and aversive effects of EtOH using place-conditioning procedures in rats.

METHODS

Separate groups of adult (male, female, ovariectomized [OVX] female) and adolescent (male and female) rats received EtOH (0, 0.5, 1.0, 2.0, or 2.5 g/kg, intraperitoneal) and were confined to their initially nonpreferred side of our conditioning apparatus for 30 minutes. On alternate days, they received saline and were confined to the other side. Following 5 drug pairings, the rats were retested for preference behavior. Separate cohorts of the same groups of rats were injected with a similar dose range of EtOH, and blood EtOH levels (BELs) were compared 30 minutes later.

RESULTS

EtOH produced rewarding or aversive effects in a dose-dependent manner. An intermediate dose of EtOH (1.0 g/kg) produced rewarding effects in adult female, but not in male or OVX female rats, suggesting that ovarian hormones facilitate the rewarding effects of EtOH. Similarly, this intermediate dose of EtOH produced rewarding effects in adolescent female, but not in male rats. The highest dose of EtOH (2.5 g/kg) produced aversive effects that were similar across all adult groups. However, the aversive effects of EtOH were lower in adolescents than adults, suggesting that adolescents are less sensitive to the aversive effects of EtOH. The aversive effects of EtOH did not vary across the estrous cycle in intact adult females. There were also no group differences in BELs, suggesting that our results are not related to EtOH metabolism.

CONCLUSION

Our results in rats suggest that human females may be more vulnerable to EtOH abuse due to enhanced rewarding effects of this drug that are mediated by the presence of ovarian hormones.

Changes in CREB activation in the prefrontal cortex and hippocampus blunt ethanol-induced behavioral sensitization in adolescent mice

Drug dependence is a major health problem in adults and has been recognized as a significant problem in adolescents. We previously demonstrated that repeated treatment with a behaviorally sensitizing dose of ethanol in adult mice induced tolerance or no sensitization in adolescents and that repeated ethanol-treated adolescents expressed lower Fos and Egr-1 expression than adult mice in the prefrontal cortex (PFC). In the present work, we investigated the effects of acute and repeated ethanol administration on cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) DNA-binding activity using the electrophoretic mobility shift assay (EMSA) and the phosphorylated CREB (pCREB)/CREB ratio using immunoblotting in both the PFC and hippocampus in adolescent and adult mice. Adult mice exhibited typical locomotor sensitization after 15 days of daily treatment with 2.0 g/kg ethanol, whereas adolescent mice did not exhibit sensitization. Overall, adolescent mice displayed lower CREB binding activity in the PFC compared with adult mice, whereas opposite effects were observed in the hippocampus. The present results indicate that ethanol exposure induces significant and differential neuroadaptive changes in CREB DNA-binding activity in the PFC and hippocampus in adolescent mice compared with adult mice. These differential molecular changes may contribute to the blunted ethanol-induced behavioral sensitization observed in adolescent mice.

Age-dependent effects of stress on ethanol-induced motor activity in rats

RATIONALE

It is important to study age-related differences that may put adolescents at risk for alcohol-related problems. Adolescents seem less sensitive to the aversive effects of ethanol than adults. Less is known of appetitive effects of ethanol and stress modulation of these effects.

OBJECTIVES

This study aims to describe the effects of acute social or restraint stress on ethanol-precipitated locomotor activity (LMA), in adolescent and adult rats. Effects of activation of the kappa system on ethanol-induced LMA were also evaluated.

METHODS

Adolescent or adult rats were restrained for 90 min, exposed to social deprivation stress for 90 or 180 min or administered with the kappa agonist U62,066E before being given ethanol, and assessed for LMA.

RESULTS

Adolescents were significantly more sensitive to the stimulating, and less sensitive to the sedative, effects of ethanol than adults. Basal locomotion was significantly increased by social deprivation stress in adult, but not in adolescent, rats. U62,066E significantly reduced basal and ethanol-induced locomotion in the adolescents. Corticosterone and progesterone levels were significantly higher in adolescents than in adults.

CONCLUSIONS

Adolescents exhibit greater sensitivity to ethanol-induced LMA and reduced sensitivity to ethanol-induced motor sedation than adult rats. Ethanol's effects on motor activity were not affected by acute stress. Unlike adults, adolescents were insensitive to acute restraint and social deprivation stress but exhibited motor depression after activation of the endogenous kappa opioid receptor system.

Controversies about the enhanced vulnerability of the adolescent brain to develop addiction

Adolescence, defined as a transition phase toward autonomy and independence, is a natural time of learning and adjustment, particularly in the setting of long-term goals and personal aspirations. It also is a period of heightened sensation seeking, including risk taking and reckless behaviors, which is a major cause of morbidity and mortality among teenagers. Recent observations suggest that a relative immaturity in frontal cortical neural systems may underlie the adolescent propensity for uninhibited risk taking and hazardous behaviors. However, converging preclinical and clinical studies do not support a simple model of frontal cortical immaturity, and there is substantial evidence that adolescents engage in dangerous activities, including drug abuse, despite knowing and understanding the risks involved. Therefore, a current consensus considers that much brain development during adolescence occurs in brain regions and systems that are critically involved in the perception and evaluation of risk and reward, leading to important changes in social and affective processing. Hence, rather than naive, immature and vulnerable, the adolescent brain, particularly the prefrontal cortex, should be considered as prewired for expecting novel experiences. In this perspective, thrill seeking may not represent a danger but rather a window of opportunities permitting the development of cognitive control through multiple experiences. However, if the maturation of brain systems implicated in self-regulation is contextually dependent, it is important to understand which experiences matter most. In particular, it is essential to unveil the underpinning mechanisms by which recurrent adverse episodes of stress or unrestricted access to drugs can shape the adolescent brain and potentially trigger life-long maladaptive responses.

Repeated restraint stress alters sensitivity to the social consequences of ethanol differentially in early and late adolescent rats

In rats, considerable differences in the social consequences of acute ethanol are seen across ontogeny, with adolescents being more sensitive to low dose ethanol-induced social facilitation and less sensitive to the social inhibition evident at higher ethanol doses relative to adults. Stressor exposure induces social anxiety-like behavior, indexed via decreases in social preference, and alters responsiveness to the social consequences of acute ethanol by enhancing ethanol-associated social facilitation and anxiolysis regardless of age. Given that substantial ontogenetic differences in the social consequences of ethanol are evident even within the adolescent period, the present study was designed to investigate whether similar stress-associated alterations in social behavior and ethanol responsiveness are evident in early and late adolescents. Juvenile-early adolescent [postnatal days (P) 24-28] and mid-late adolescent (P38-42) male and female Sprague-Dawley rats were repeatedly restrained (90 min/day) for 5 days, followed by examination of ethanol-induced (0, 0.25, 0.5, or 1.0 g/kg) alterations in social behaviors on the last day. Responsiveness to restraint stress in terms of both stress-induced behavioral alterations and stress-associated changes in sensitivity to the social consequences of acute ethanol challenge differed drastically at the two ages. Repeated restraint increased anxiety-like behavior in a social context in older adolescents, whereas previously stressed young adolescent males showed substantial increases in play fighting - an effect of stress not evident in P28 females or P42 adolescents of either sex. Unexpectedly, repeated restraint eliminated sensitivity to ethanol-induced social facilitation in P28 adolescent males and made their female counterparts less sensitive to this effect. In contrast, previously stressed late adolescents became sensitive to the socially facilitating and anxiolytic effects of acute ethanol.

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.

Age differences in (±) 3,4-methylenedioxymethamphetamine (MDMA)-induced conditioned taste aversions and monoaminergic levels

Preclinical work indicates that adolescent rats appear more sensitive to the rewarding effects and less sensitive to the aversive effects of abused drugs. The present investigation utilized the conditioned taste aversion (CTA) design to measure the relative aversive effects of (±)3,4-methylenedioxymethamphetamine (MDMA; 0, 1.0, 1.8, or 3.2 mg/kg) in adolescent and adult Sprague-Dawley rats. After behavioral testing was complete, monoamine and associated metabolite levels in discrete brain regions were quantified using high-performance liquid chromatography coupled to electrochemical detection (HPLC-ECD) to determine if adolescent animals displayed a different neurochemical profile than did adult animals after being exposed to subcutaneous low doses of MDMA. Adolescent rats displayed less robust MDMA-induced taste aversions than adults during acquisition and on a final two-bottle aversion test. MDMA at these doses had no consistent effect on monoamine levels in either age group, although levels did vary with age. The relative insensitivity of adolescents to MDMA's aversive effects may engender an increased vulnerability to MDMA abuse in this specific population.

Escalation of i.v. cocaine intake in peri-adolescent vs. adult rats selectively bred for high (HiS) vs. low (LoS) saccharin intake

RATIONALE

Adolescence marks a period of increased vulnerability to the development of substance use disorders. High sweet preference is a genetically mediated behavioral trait that also predicts vulnerability to substances of abuse. Previous research has shown that while adolescent rats selectively bred for high (HiS) saccharin intake acquire cocaine self-administration at the same rate as adult HiS rats, adolescent rats bred for low saccharin intake (LoS) acquire cocaine self-administration faster than adult LoS rats.

OBJECTIVES

This study was conducted to investigate the interaction of the addiction vulnerability factors of peri-adolescence and saccharin preference on cocaine intake using an animal model of escalation of cocaine consumption over 6-h/day sessions.

METHODS

Peri-adolescent and adult HiS and LoS female rats self-administered i.v. cocaine (0.4 mg/kg/inf) during short-access (2-h/day) sessions for 2 days. Next, a long-access (6-h/day) period (LgA) commenced and lasted 16 days. Following LgA, session length was returned to 2-h/day for a second short access phase.

RESULTS

LoS peri-adolescent rats escalated cocaine intake over the LgA period and consumed more drug than LoS adult rats; however, peri-adolescent and adult HiS rats consumed similar amounts of cocaine during this period. Additionally, adult HiS rats self-administered more cocaine than adult LoS rats during the LgA period, while there was no phenotypic difference between the rat lines during peri-adolescence for the LgA period. During the first short-access phase, peri-adolescent rats self-administered more cocaine than adult rats.

CONCLUSIONS

These results emphasize the importance of adolescent drug abuse prevention by illustrating that phenotypic protection from addiction may not be expressed until adulthood.

Refinement of neuronal synchronization with gamma oscillations in the medial prefrontal cortex after adolescence

The marked anatomical and functional changes taking place in the medial prefrontal cortex (PFC) during adolescence set grounds for the high incidence of neuropsychiatric disorders with adolescent onset. Although circuit refinement through synapse pruning may constitute the anatomical basis for the cognitive differences reported between adolescents and adults, a physiological correlate of circuit refinement at the level of neuronal ensembles has not been demonstrated. We have recorded neuronal activity together with local field potentials in the medial PFC of juvenile and adult mice under anesthesia, which allowed studying local functional connectivity without behavioral or sensorial interference. Entrainment of pyramidal neurons and interneurons to gamma oscillations, but not to theta or beta oscillations, was reduced after adolescence. Interneurons were synchronized to gamma oscillations across a wider area of the PFC than pyramidal neurons, and the span of interneuron synchronization was shorter in adults than juvenile mice. Thus, transition from childhood to adulthood is characterized by reduction of the strength and span of neuronal synchronization specific to gamma oscillations in the mPFC. The more restricted and weak ongoing synchronization in adults may allow a more dynamic rearrangement of neuronal ensembles during behavior and promote parallel processing of information.

The Teenage Brain: Self Control

Adolescence refers to the transition from childhood to adulthood that begins with the onset of puberty and ends with successful independence from the parent. A paradox for human adolescence is why, during a time when the individual is probably faster, stronger, of higher reasoning capacity and more resistant to disease, there is such an increase in mortality relative to childhood. These untimely deaths are not due to disease, but rather to preventable forms of death (accidental fatalities, suicide and homicide) associated with adolescents putting themselves in harm's way due, in part, to diminished self control - the ability to suppress inappropriate emotions, desires and actions. This paper highlights how self control varies as a function of age, context and the individual and delineates its neurobiological basis.

Attenuated effects of experimenter-administered heroin in adolescent vs. adult male rats: physical withdrawal and locomotor sensitization

OBJECTIVES

Early onset of heroin use during adolescence might increase chances of later drug addiction. Prior work from our laboratory suggests, however, that adolescent male rats are actually less sensitive than adults to some enduring effects of heroin self-administration. In the present study, we tested two likely correlates of sensitivity to behavioral reinforcement in rats: physical withdrawal and locomotor sensitization.

METHODS

Adolescent (35 days old at start) and adult (79 days old) male Sprague-Dawley rats were administered escalating doses of heroin, increasing from 1.0 to 8.0 mg/kg (i.p.) every 12 h, across 13 days. Somatic signs of spontaneous withdrawal were scored 12 and 24 h after the last injection, and then every 24 h for 5 days; locomotion was recorded concurrently. Challenge injections of heroin (1 mg/kg i.p.) were given at four points: as the first of the escalating doses (day 1), at days 7 and 13 during the escalating regimen, and after 12 days of forced abstinence. Body mass and food intake were measured throughout experimentation.

RESULTS

A heroin withdrawal syndrome was not observed among adolescents as it was among adults, including somatic signs as well as reduced locomotion, body mass, and food intake. On the other hand, heroin-induced locomotor sensitization did not differ across ages.

CONCLUSION

Reduced withdrawal is consistent with the attenuated reinforcing effects of heroin among adolescent male rats that we reported previously. Thus, it is possible that adolescent rats could reveal important neuroprotective factors for use in treatment of heroin dependence.

Escalation of methamphetamine self-administration in adolescent and adult rats

BACKGROUND

Methamphetamine (METH) use has increased substantially in the last 10 years and poses a serious health concern, especially for young populations. Drug abuse primarily begins during adolescence, when uninhibited and excessive and drug intake is a common occurrence; thus, understanding the developmental patterns of addiction during this critical period is an essential step in its prevention. In the present study, the effect of age on the vulnerability to METH abuse was examined using a rat model of bingeing (i.e., escalation).

METHODS

Adolescent and adult rats were compared during short (ShA, 2-h) and long-access (LgA, 6-h) to METH self-administration. On postnatal (PN) days 23 (adolescents) and 90 (adults), rats were implanted with i.v. catheters and trained to lever press for infusions of METH (0.05mg/kg) during 2-h sessions. Once the rats reached a steady rate of METH self-administration, they were divided into ShA or LgA groups and allowed to self-administer METH for 15 additional days.

RESULTS

Results indicated that adolescent rats earned significantly more infusions than adults under the LgA condition, but the age groups did not differ during ShA. Adolescents, but not adults, also significantly increased (i.e., escalated) METH self-administration across the 15 days of testing under the LgA condition. Further analysis indicated excessive responding during infusions in the LgA METH-exposed adolescents compared to the other groups, suggesting elevated impulsivity or motivation for drug.

CONCLUSION

These results demonstrate that adolescents are more vulnerable to the escalation of METH than adults during LgA.

The neglected role of positive emotion in adolescent psychopathology

Adolescence is a developmental period characterized by elevated stress, heightened risky behaviors, and increases in psychopathology. Emotion dysregulation is a hypothesized contributor to negative outcomes and to the onset of psychopathology during adolescence. However, the dysregulation of negative emotion has been the focus of research while the literature on positive emotion in adolescent psychopathology is limited. This review highlights both the development of normative and dysregulated positive emotion during adolescence. First, the literature on normative adolescent emotional development and on negative emotional regulation is briefly reviewed, followed by a discussion of current theories of positive emotion, which are grounded in the adult literature. From a developmental perspective, the dimension of approach motivation within positive emotion is emphasized throughout and frames the review. This conceptualization guides organization of literatures on normative experiences of positive emotion in adolescence and the role of dysregulated positive emotion in adolescent psychopathology, specifically adolescent depression, anxiety, bipolar disorder, externalizing disorders and eating disorders. Last, future directions in the study of adolescent positive emotion and its regulation and the implications of highlighting approach motivation in normative and dysregulated positive emotion in adolescence are detailed.

Periadolescent ethanol exposure reduces adult forebrain ChAT+IR neurons: correlation with behavioral pathology

Substance abuse typically begins in adolescence; therefore, the impact of alcohol during this critical time in brain development is of particular importance. Epidemiological data indicate that excessive alcohol consumption is prevalent among adolescents and may have lasting neurobehavioral consequences. Loss of cholinergic input to the forebrain has been demonstrated following fetal alcohol exposure and in adults with Wernicke-Korsakoff syndrome. In the present study, immunohistochemistry for choline acetyltransferase (ChAT) was determined to assess forebrain cholinergic neurons (Ch1-4), and behavioral changes following periadolescent alcohol exposure. Wistar rats were exposed to intermittent ethanol vapor (14 h on/10 h off/day) for 35 days from postnatal day (PD) 22 to PD 57 (average blood alcohol concentration (BAC): 163 mg%). Rats were withdrawn from vapor and assessed for locomotor activity, startle response, conflict behavior in the open field, and immobility in the forced swim test, as adults. Rats were then sacrificed at day 71/72 and perfused for histochemical analyses. Ethanol vapor-exposed rats displayed: increased locomotor activity 8 h after the termination of vapor delivery for that 24 h period at day 10 and day 20 of alcohol vapor exposure, significant reductions in the amplitude of their responses to prepulse stimuli during the startle paradigm at 24 h withdrawal, and at 2 weeks following withdrawal, less anxiety-like and/or more "disinhibitory" behavior in the open field conflict, and more immobility in the forced swim test. Quantitative analyses of ChAT immunoreactivity revealed a significant reduction in cell counts in the Ch1-2 and Ch3-4 regions of the basal forebrain in ethanol vapor-exposed rats. This reduction in cell counts was significantly correlated with less anxiety-like and/or more "disinhibitory" behavior in the open field conflict test. These studies demonstrate that behavioral measures of arousal, affective state, disinhibitory behavior, and ChAT+IR, are all significantly impacted by periadolescent ethanol exposure and withdrawal in Wistar rats.