Chronic ethanol exposure during adolescence increases basal dopamine in the nucleus accumbens septi during adulthood

Adolescent ethanol exposure promotes mechanical allodynia and alters dopamine transmission in the nucleus accumbens shell

ABSTRACT

Excessive alcohol consumption in adolescence can disrupt neural development and may augment pain perception. Recent studies have shown that the nucleus accumbens (NAc) shell is involved in mediating pain sensitivity after peripheral inflammation in rodent models of chronic pain and alcohol use disorder. Interestingly, there have been very few studies examining the impact of chronic ethanol exposure during adolescence on pain sensitivity in adulthood. Therefore, in this project, we investigated the impact of adolescent chronic intermittent ethanol (aCIE) exposure on mechanical allodynia. Furthermore, given the involvement of the NAc shell in pain processing and chronic ethanol-mediated changes, we measured changes in accumbal dopamine kinetics during protracted withdrawal. We found that both male and female aCIE rats show mechanical allodynia during withdrawal. Furthermore, male and female aCIE rats show greater evoked tonic dopamine release, maximal rate of dopamine reuptake, and dopamine affinity to the dopamine transporter in the NAc shell compared with controls. With phasic stimulation, aCIE rats also showed greater dopamine release compared with AIR-exposed rats. Inhibition of dopamine transmission targeted in the NAc shell reversed the aCIE-associated facilitation of mechanical allodynia in both sexes. These data suggest that aCIE exposure exacerbates pain sensitivity during withdrawal in an accumbal dopamine-dependent manner.

Adolescent alcohol and nicotine exposure alters the adult response to alcohol use

Adolescence through young adulthood is a unique period of neuronal development and maturation. Numerous agents can alter this process, resulting in long-term neurological and biological consequences. In the clinical literature, it is frequently reported that adolescent alcohol consumption increases the propensity to develop addictions, including alcohol use disorder (AUD), during adulthood. A general limitation of both clinical and human pre-clinical adolescent alcohol research is the high rate of co-using/abusing more than one drug during adolescence, such as co-using/abusing alcohol with nicotine. A primary goal of basic research is elucidating neuroadaptations produced by adolescent alcohol exposure/consumption that promote alcohol and other drug self-administration in adulthood. The long-term goal is to develop pharmacotherapeutics for the prevention or amelioration of these neuroadaptations. This review will focus on studies that have examined the effects of adolescent alcohol and nicotine exposure on adult alcohol consumption, the hypersensitivity of the mesolimbic dopaminergic system, and enhanced responses not only to alcohol but also to nicotine during adulthood. Again, the long-term goal is to identify potential cholinergic agents to prevent or ameliorate the consequences of, peri-adolescent alcohol abuse.

Repeated Cycles of Binge-Like Ethanol Exposure Induces Neurobehavioral Changes During Short- and Long-Term Withdrawal in Adolescent Female Rats

Alcohol consumption is spread worldwide and can lead to an abuse profile associated with severe health problems. Adolescents are more susceptible to addiction and usually consume ethanol in a binge drinking pattern. This form of consumption can lead to cognitive and emotional disorders, however scarce studies have focused on long-term hazardous effects following withdrawal periods after binge drinking in adolescents. Thus, the present study aims at investigating whether behavioral and cognitive changes persist until mid and late adulthood. Female Wistar rats (9-10 animals/group) received intragastric administration of four cycles of ethanol binge-like pattern (3.0 g/kg/day, 20% w/v; 3 days-on/4 days-off) from 35^th to 58^th days old, followed withdrawal checkpoints 1 day, 30 days, and 60 days. At each checkpoint period, behavioral tests of open field, object recognition test, elevated plus maze, and forced swimming test were performed, and blood and hippocampus were collected for oxidative biochemistry and brain-derived neurotrophic factor (BDNF) levels analysis, respectively. The results demonstrated that adolescent rats exposed to binge drinking displayed anxiogenic- and depressive-like phenotype in early and midadulthood, however, anxiety-like profile persisted until late adulthood. Similarly, short-term memory was impaired in all withdrawal periods analysed, including late adult life. These behavioral data were associated with oxidative damage in midadulthood but not BDNF alterations. Taken together, the present work highlights the long-lasting emotional and cognitive alterations induced by ethanol binge drinking during adolescence, even after a long period of abstinence, which might impact adult life.

Adolescent Intermittent Ethanol (AIE) Enhances the Dopaminergic Response to Ethanol within the Mesolimbic Pathway during Adulthood: Alterations in Cholinergic/Dopaminergic Genes Expression in the Nucleus Accumbens Shell

A consistent preclinical finding is that exposure to alcohol during adolescence produces a persistent hyperdopaminergic state during adulthood. The current experiments determine that effects of Adolescent Intermittent Ethanol (AIE) on the adult neurochemical response to EtOH administered directly into the mesolimbic dopamine system, alterations in dendritic spine and gene expression within the nucleus accumbens shell (AcbSh), and if treatment with the HDACII inhibitor TSA could normalize the consequences of AIE. Rats were exposed to the AIE (4 g/kg ig; 3 days a week) or water (CON) during adolescence, and all testing occurred during adulthood. CON and AIE rats were microinjected with EtOH directly into the posterior VTA and dopamine and glutamate levels were recorded in the AcbSh. Separate groups of AIE and CON rats were sacrificed during adulthood and Taqman arrays and dendritic spine morphology assessments were performed. The data indicated that exposure to AIE resulted in a significant leftward and upward shift in the dose-response curve for an increase in dopamine in the AcbSh following EtOH microinjection into the posterior VTA. Taqman array indicated that AIE exposure affected the expression of target genes (Chrna7, Impact, Chrna5). The data indicated no alterations in dendritic spine morphology in the AcbSh or any alteration in AIE effects by TSA administration. Binge-like EtOH exposure during adolescence enhances the response to acute ethanol challenge in adulthood, demonstrating that AIE produces a hyperdopaminergic mesolimbic system in both male and female Wistar rats. The neuroadaptations induced by AIE in the AcbSh could be part of the biological basis of the observed negative consequences of adolescent binge-like alcohol exposure on adult drug self-administration behaviors.

Impact of High Fat Diet and Ethanol Consumption on Neurocircuitry Regulating Emotional Processing and Metabolic Function

The prevalence of psychiatry disorders such as anxiety and depression has steadily increased in recent years in the United States. This increased risk for anxiety and depression is associated with excess weight gain, which is often due to over-consumption of western diets that are typically high in fat, as well as with binge eating disorders, which often overlap with overweight and obesity outcomes. This finding suggests that diet, particularly diets high in fat, may have important consequences on the neurocircuitry regulating emotional processing as well as metabolic functions. Depression and anxiety disorders are also often comorbid with alcohol and substance use disorders. It is well-characterized that many of the neurocircuits that become dysregulated by overconsumption of high fat foods are also involved in drug and alcohol use disorders, suggesting overlapping central dysfunction may be involved. Emerging preclinical data suggest that high fat diets may be an important contributor to increased susceptibility of binge drug and ethanol intake in animal models, suggesting diet could be an important aspect in the etiology of substance use disorders. Neuroinflammation in pivotal brain regions modulating metabolic function, food intake, and binge-like behaviors, such as the hypothalamus, mesolimbic dopamine circuits, and amygdala, may be a critical link between diet, ethanol, metabolic dysfunction, and neuropsychiatric conditions. This brief review will provide an overview of behavioral and physiological changes elicited by both diets high in fat and ethanol consumption, as well as some of their potential effects on neurocircuitry regulating emotional processing and metabolic function.

Chronic Ethanol Exposure during Adolescence Increases Voluntary Ethanol Consumption in Adulthood in Female Sprague Dawley Rats

Early alcohol use is a major concern due to the dramatic rise in alcohol use during adolescence. In humans, adolescent males and females consume alcohol at equivalent rates; however, in adulthood males are more likely to consume harmful levels of alcohol. In animal models, the long-term dose-dependent and sex-dependent effects of alcohol exposure during adolescence have not been readily assessed relative to exposure that is initiated in adulthood. The purpose of the present set of experiments was to determine if adolescent exposure to chronic ethanol would predispose male and female rats to greater ethanol intake in adulthood when compared to animals that were not exposed to chronic ethanol exposure until early adulthood. Male and female rats were chronically administered 0.75 g/kg or 1.5 g/kg ethanol or saline for 21 days during adolescence (postnatal day (PND) 30–50) or adulthood (PND 60–80). All rats subsequently underwent 14-days of abstinence (PND 51–64 or PND 81–94, respectively). Finally, all rats were given 30-min daily access to saccharin-sweetened ethanol or saccharin alone from PND 65–80 for adolescent-exposed rats and PND 95–110 for adult-exposed rats. Exposure to 0.75 g/kg ethanol did not alter ethanol or saccharin intake in adolescent-exposed or adult-exposed rats, regardless of sex. In contrast, chronic exposure to the higher 1.5 g/kg dose during adolescence increased ethanol intake in adulthood in female rats. However, there was no change in saccharin intake in animals exposed to 1.5 g/kg ethanol during adolescence or adulthood, regardless of sex. Additionally, there were no clear age- and ethanol-dependent changes in duration of loss of righting reflex and blood ethanol concentrations to a challenge administration of a higher dose of ethanol. The results of the present set of experiments indicate chronic exposure to a high dose of ethanol during adolescence in female rats did indeed predispose rats to consume more ethanol in adulthood. Given that these effects were only observed in adolescent-exposed female rats, these results support a unique vulnerability to the long-term consequences of adolescent ethanol exposure in female rats, an effect that is not merely mediated by the sweetener used in the ethanol solution.

The reinforcing effects of ethanol within the prelimbic cortex and ethanol drinking: Involvement of local dopamine D2 receptor-mediated neurotransmission

Previous studies have identified important mesolimbic regions in supporting the reinforcing effects of ethanol. However, the involvement of the medial prefrontal cortex (mPFC), another key region within the mesocorticolimbic system, in ethanol reinforcement has been understudied. The objective of the current study was to examine the role of the prelimbic (PL) cortex sub-region of the mPFC in ethanol reinforcement and drinking. Intracranial self-administration was used to examine the reinforcing effects of ethanol within the PL cortex. Quantitative microdialysis was used to measure basal extracellular DA concentrations and clearance in the PL cortex following chronic ethanol drinking. In addition, the involvement of dopamine (DA) D2 receptors within the PL cortex on the reinforcing effects of ethanol and ethanol drinking was determined. Ethanol was dose-dependent self-administered into the PL cortex, with significantly more infusions elicited by 100-200 mg% ethanol than vehicle. Co-infusion of the D2 receptor antagonist sulpiride significantly reduced ethanol self-administration. Chronic ethanol drinking significantly elevated basal extracellular DA concentrations without altering DA clearance. Microinjection of sulpiride into the PL cortex selectively reduced ethanol, but not saccharine, drinking. These results indicate that the PL cortex supported the reinforcing effects of ethanol, and that ethanol drinking enhanced basal DA neurotransmission within the PL cortex. In addition, D2 receptor antagonism within the PL cortex reduced ethanol self-administration and drinking. Collectively, these findings revealed important DA mechanisms within the PL cortex in mediating ethanol reinforcement and drinking.

Adolescent Intermittent Ethanol Exposure Effects on Kappa Opioid Receptor Mediated Dopamine Transmission: Sex and Age of Exposure Matter

Underage alcohol drinking increases the risk of developing alcohol use disorder (AUD). In rodents, adolescent ethanol exposure augments ethanol consumption and anxiety-like behavior while reducing social interaction. However, the underlying mechanisms driving these adaptations are unclear. The dopamine and kappa opioid receptor (KOR) systems in the nucleus accumbens (NAc) are implicated in affective disorders, including AUD, with studies showing augmented KOR function and reduced dopamine transmission in ethanol-dependent adult animals. Thus, here we examine the impact of adolescent intermittent ethanol (AIE) exposure on dopamine transmission and KOR function in the NAc. Rats were exposed to water or ethanol (4 g/kg, intragastrically) every other day during early (postnatal day (PD) 25–45) or late (PD 45–65) adolescence. While AIE exposure during early adolescence (early-AIE) did not alter dopamine release in male and female rats, AIE exposure during late adolescence (late-AIE) resulted in greater dopamine release in males and lower dopamine release in females. To determine the impact of AIE on KOR function, we measured the effect of KOR activation using U50,488 (0.01–1.00 µM) on dopamine release. Early-AIE exposure potentiated KOR-mediated inhibition of dopamine release in females, while late-AIE exposure attenuated this effect in males. Interestingly, no differences in KOR function were observed in early-AIE exposed males and late-AIE exposed females. Together, these data suggest that AIE exposure impact on neural processes is dependent on sex and exposure timing. These differences likely arise from differential developmental timing in males and females. This is the first study to show changes in KOR function following AIE exposure.

Gamma oryzanol impairs alcohol-induced anxiety-like behavior in mice via upregulation of central monoamines associated with Bdnf and Il-1β signaling

Adolescent alcohol exposure may increase anxiety-like behaviors by altering central monoaminergic functions and other important neuronal pathways. The present study was designed to investigate the anxiolytic effect of 0.5% γ-oryzanol (GORZ) and its neurochemical and molecular mechanisms under chronic 10% ethanol consumption. Five-week-old ICR male mice received either control (14% casein, AIN 93 M) or GORZ (14% casein, AIN 93 M + 0.5% GORZ) diets in this study. We showed that GORZ could potentially attenuate alcohol-induced anxiety-like behaviors by significantly improving the main behavioral parameters measured by the elevated plus maze test. Moreover, GORZ treatment significantly restored the alcohol-induced downregulation of 5-hydroxytryptophan and 5-hydroxyindole acetic acid in the hippocampus and improved homovanillic acid levels in the cerebral cortex. Furthermore, a recovery increase in the level of 3-methoxy-4-hydroxyphenylglycol both in the hippocampus and cerebral cortex supported the anxiolytic effect of GORZ. The significant elevation and reduction in the hippocampus of relative mRNA levels of brain-derived neurotrophic factor and interleukin 1β, respectively, also showed the neuroprotective role of GORZ in ethanol-induced anxiety. Altogether, these results suggest that 0.5% GORZ is a promising neuroprotective drug candidate with potential anxiolytic, neurogenic, and anti-neuroinflammatory properties for treating adolescent alcohol exposure.

Effects of Ethanol Exposure During Adolescence or Adulthood on Locomotor Sensitization and Dopamine Levels in the Reward System

Behavioral sensitization is a process of neuroadaptation characterized by a gradual increase in motor behaviors. The major neural substrates involved in the behavioral sensitization lie on the dopaminergic mesocorticolimbic pathway, which is still under development during adolescence. To investigate age-differences in ethanol behavioral sensitization and dopamine levels in distinct brain regions of the reward system, adolescent and adult mice were repeatedly pretreated with saline or ethanol (2.0 g/kg i.p.) during 15 consecutive days and challenged with saline or ethanol 5 days after pretreatment. Dopamine and its metabolites were measured in tissue samples of the prefrontal cortex (PFC), nucleus accumbens (NAc) and striatum by HPLC analysis. While repeated ethanol administration resulted in the development of locomotor sensitization in both adult and adolescent mice, only the adults expressed sensitization to a subsequent ethanol challenge injection. Neurochemical results showed reduced dopamine levels in adolescents compared to adults. Specifically, mice pretreated with ethanol during adolescence displayed lower dopamine levels in the PFC compared to the respective adult group in response to an ethanol challenge injection, and preadolescent mice exhibited lower dopamine levels in the NAc following an acute ethanol treatment compared to adults. These findings suggest that adolescent mice are not only less sensitive to the expression of ethanol-induced sensitization than adults, but also show lower dopamine content after ethanol exposition in the PFC and NAc.

Persistent Alterations of Accumbal Cholinergic Interneurons and Cognitive Dysfunction after Adolescent Intermittent Ethanol Exposure

Adolescent binge drinking renders young drinkers vulnerable to alcohol use disorders in adulthood; therefore, understanding alcohol-induced brain damage and associated cognitive dysfunctions is of paramount importance. Here we investigated the effects of binge-like adolescent intermittent ethanol (AIE) exposure on nonspatial working memory, behavioral flexibility and cholinergic alterations in the nucleus accumbens (NAc) in male and female rats. On postnatal days P25-57 rats were intubated with water or ethanol (at a dose of 5 g/kg) on a 2-day-on/2-day-off cycle and were then tested in adulthood on social recognition and probabilistic reversal learning tasks. During the social recognition task AIE-treated rats spent similar amounts of time interacting with familiar and novel juveniles, indicating an impaired ability to sustain memory of the familiar juvenile. During probabilistic reversal learning, AIE-treated male and female rats showed behavioral inflexibility as indicated by a higher number of trials needed to complete three reversals within a session, longer response latencies for lever selection, and for males, a higher number of errors as compared to water-treated rats. AIE exposure also reduced the number of cholinergic interneurons in the NAc in males and females. These findings indicate AIE-related pathologies of accumbal cholinergic interneurons and long lasting cognitive-behavioral deficits, which may be associated with cortico-striatal hypofunction.

Low doses of methylmercury intoxication solely or associated to ethanol binge drinking induce psychiatric-like disorders in adolescent female rats

Methylmercury (MeHg) is an environmental contaminant that provokes damage to developing brain. Simultaneously, the consumption of ethanol among adolescents has increased. Evidence concerning the effects of MeHg low doses per se or associated with ethanol during adolescence are scarce. Thus, we investigate behavioral disorders resulted from exposure to MeHg low doses and co-intoxicated with ethanol in adolescent rats. Wistar rats received chronic exposure to low doses of MeHg (40 μg/kg/day for 5 weeks) and/or ethanol binge drinking (3 g/kg/day at 3 days per week for 5 weeks). Animals were submitted to behavioral assays to assess emotionality and cognitive function. Total mercury content was evaluated in the brain and hair. Oxidative parameters were analyzed in blood samples. MeHg at low doses or associated to ethanol binge drinking produced psychiatric-like disorders and cognitive impairment. Peripherally, MeHg altered oxidative parameters when associated to ethanol. Ethanol administration reduced brain mercury deposit. We proposed that ethanol reduces the necessity of mercury tissue levels to display psychiatric-like disorders/cognitive impairment.

Ethanol Exposure History and Alcoholic Reward Differentially Alter Dopamine Release in the Nucleus Accumbens to a Reward-Predictive Cue

BACKGROUND

Conditioned stimuli (CS) that predict reward delivery acquire the ability to induce phasic dopamine release in the nucleus accumbens (NAc). This dopamine release may facilitate conditioned approach behavior, which often manifests as approach to the site of reward delivery (called "goal-tracking") or to the CS itself (called "sign-tracking"). Previous research has linked sign-tracking in particular to impulsivity and drug self-administration, and addictive drugs may promote the expression of sign-tracking. Ethanol (EtOH) acutely promotes phasic release of dopamine in the accumbens, but it is unknown whether an alcoholic reward alters dopamine release to a CS. We hypothesized that Pavlovian conditioning with an alcoholic reward would increase dopamine release triggered by the CS and subsequent sign-tracking behavior. Moreover, we predicted that chronic intermittent EtOH (CIE) exposure would promote sign-tracking while acute administration of naltrexone (NTX) would reduce it.

METHODS

Rats received 14 doses of EtOH (3 to 5 g/kg, intragastric) or water followed by 6 days of Pavlovian conditioning training. Rewards were a chocolate solution with or without 10% (w/v) alcohol. We used fast-scan cyclic voltammetry to measure phasic dopamine release in the NAc core in response to the CS and the rewards. We also determined the effect of NTX (1 mg/kg, subcutaneous) on conditioned approach.

RESULTS

Both CIE and alcoholic reward, individually but not together, associated with greater dopamine to the CS than control conditions. However, this increase in dopamine release was not linked to greater sign-tracking, as both CIE and alcoholic reward shifted conditioned approach from sign-tracking behavior to goal-tracking behavior. However, they both also increased sensitivity to NTX, which reduced goal-tracking behavior.

CONCLUSIONS

While a history of EtOH exposure or alcoholic reward enhanced dopamine release to a CS, they did not promote sign-tracking under the current conditions. These findings are consistent with the interpretation that EtOH can stimulate conditioned approach, but indicate that the conditioned response may manifest as goal-tracking.

Effects of adolescent alcohol exposure on stress-induced reward deficits, brain CRF, monoamines and glutamate in adult rats

BACKGROUND

Adolescent alcohol exposure may increase depression vulnerability in adulthood by increasing the anhedonic response to stress.

METHODS

Male Wistar rats (postnatal days 28-53) were exposed to binge-like adolescent intermittent ethanol (AIE) or water. In adulthood, rats were exposed to social defeat, consisting of daily confrontations with an aggressive conspecific, followed by testing of brain reward function in a discrete-trial current-intensity intracranial self-stimulation procedure for 10 consecutive days. Neurochemistry and corticotropin-releasing factor (CRF) and CRF receptor 1 (CRFR1) mRNA levels were assessed in corticolimbic brain areas on day 11 of social defeat stress.

RESULTS

Social defeat elevated reward thresholds in both AIE- and water-exposed rats indicating stress-induced anhedonia. However, AIE-exposed rats were more likely to show threshold elevations after repeated stress compared to water-exposed rats. AIE exposure decreased CRF mRNA levels in the nucleus accumbens and increased CRFR1 mRNA levels in the prefrontal cortex, while stress increased CRF mRNA levels in the central amygdala. In the caudate putamen, AIE exposure decreased dopamine turnover, while stress increased glutamate and serotonin metabolism and turnover.

CONCLUSIONS

These results demonstrate increased risk of repeated stress-induced anhedonia after AIE exposure, an effect that may be due to alterations in brain CRF and dopamine systems. These results suggest that the increased rates of depression reported in people with a history of adolescent alcohol exposure may be related to alterations in brain reward and stress systems that may contribute to increased stress-induced anhedonia.

Effects of adolescent alcohol consumption on the brain and behaviour

Per occasion, alcohol consumption is higher in adolescents than in adults in both humans and laboratory animals, with changes in the adolescent brain probably contributing to this elevated drinking. This Review examines the contributors to and consequences of the use of alcohol in adolescents. Human adolescents with a history of alcohol use differ neurally and cognitively from other adolescents; some of these differences predate the commencement of alcohol consumption and serve as potential risk factors for later alcohol use, whereas others emerge from its use. The consequences of alcohol use in human adolescents include alterations in attention, verbal learning, visuospatial processing and memory, along with altered development of grey and white matter volumes and disrupted white matter integrity. The functional consequences of adolescent alcohol use emerging from studies of rodent models of adolescence include decreased cognitive flexibility, behavioural inefficiencies and elevations in anxiety, disinhibition, impulsivity and risk-taking. Rodent studies have also showed that adolescent alcohol use can impair neurogenesis, induce neuroinflammation and epigenetic alterations, and lead to the persistence of adolescent-like neurobehavioural phenotypes into adulthood. Although only a limited number of studies have examined comparable measures in humans and laboratory animals, the available data provide evidence for notable across-species similarities in the neural consequences of adolescent alcohol exposure, providing support for further translational efforts in this context.

Binge-Like Alcohol Exposure During Adolescence Disrupts Dopaminergic Neurotransmission in the Adult Prelimbic Cortex

Repeated binge-like exposure to alcohol during adolescence has been reported to perturb prefrontal cortical development, yet the mechanisms underlying these effects are unknown. Here we report that adolescent intermittent ethanol exposure induces cellular and dopaminergic abnormalities in the adult prelimbic cortex (PrL-C). Exposing rats to alcohol during early-mid adolescence (PD28-42) increased the density of long/thin dendritic spines of layer 5 pyramidal neurons in the adult PrL-C. Interestingly, although AIE exposure did not alter the expression of glutamatergic proteins in the adult PrL-C, there was a pronounced reduction in dopamine (DA) D₁ receptor modulation of both intrinsic firing and evoked NMDA currents in pyramidal cells, whereas D₂ receptor function was unaltered. Recordings from fast-spiking interneurons also revealed that AIE reduced intrinsic excitability, glutamatergic signaling, and D₁ receptor modulation of these cells. Analysis of PrL-C tissue of AIE-exposed rats further revealed persistent changes in the expression of DA-related proteins, including reductions in the expression of tyrosine hydroxylase and catechol-O-methyltransferase (COMT). AIE exposure was associated with hypermethylation of the COMT promoter at a conserved CpG site in exon II. Taken together, these findings demonstrate that AIE exposure disrupts DA and GABAergic transmission in the adult medial prefrontal cortex (mPFC). As DA and GABA work in concert to shape and synchronize neuronal ensembles in the PFC, these alterations could contribute to deficits in behavioral control and decision-making in adults who abused alcohol during adolescence.

Consequences of adolescent use of alcohol and other drugs: Studies using rodent models

Studies using animal models of adolescent exposure to alcohol, nicotine, cannabinoids, and the stimulants cocaine, 3,4-methylenedioxymethampethamine and methamphetamine have revealed a variety of persisting neural and behavioral consequences. Affected brain regions often include mesolimbic and prefrontal regions undergoing notable ontogenetic change during adolescence, although it is unclear whether this represents areas of specific vulnerability or particular scrutiny to date. Persisting alterations in forebrain systems critical for modulating reward, socioemotional processing and cognition have emerged, including apparent induction of a hyper-dopaminergic state with some drugs and/or attenuations in neurons expressing cholinergic markers. Disruptions in cognitive functions such as working memory, alterations in affect including increases in social anxiety, and mixed evidence for increases in later drug self-administration has also been reported. When consequences of adolescent and adult exposure were compared, adolescents were generally found to be more vulnerable to alcohol, nicotine, and cannabinoids, but generally not to stimulants. More work is needed to determine how adolescent drug exposure influences sculpting of the adolescent brain, and provide approaches to prevent/reverse these effects.

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.

Sex differences in the long-lasting consequences of adolescent ethanol exposure for the rewarding effects of cocaine in mice

RATIONALE

The practice of binge drinking is very common among adolescents of both sexes. It can have long-term consequences with respect to drug consumption during adulthood, but knowledge on these effects in females is limited.

OBJECTIVES

The long-lasting effects of intermittent exposure to ethanol (EtOH) during adolescence on different cocaine-elicited behaviours, including locomotor reactivity, conditioned place preference (CPP) and intravenous self-administration, were evaluated in male and female adult mice. It was hypothesized that an EtOH binge during adolescence would increase sensitivity to the effects of a sub-threshold dose of cocaine and has a differential impact on the drug's effects in males and females.

METHODS

Adolescent OF1 mice (postnatal day (PND) 26) underwent a 2-week pre-treatment schedule consisting of 16 doses of EtOH (2.5 g/kg) or saline (twice daily administrations separated by a 4-h interval i.p.) administered on two consecutive days separated by an interval of 2 days. Three weeks later (PND > 60), we assessed locomotor activity responses induced by an acute injection of different doses of cocaine in experiment 1 and the rewarding effects of cocaine on the CPP (1 mg/kg) and intravenous self-administration (1 mg/kg/infusion) paradigms in experiment 2.

RESULTS

Pre-exposure to EtOH during adolescence altered motor reactivity to cocaine in a dose- and sex-dependent manner, increased sensitivity to cocaine in CPP and enhanced self-administration in adult mice.

CONCLUSIONS

The effects of intermittent exposure to ethanol during adolescence are evident in adulthood, during which greater sensitivity and intake of cocaine is observed and differ in each sex.

Adolescent intermittent ethanol exposure enhances ethanol activation of the nucleus accumbens while blunting the prefrontal cortex responses in adult rat

The brain continues to develop through adolescence when excessive alcohol consumption is prevalent in humans. We hypothesized that binge drinking doses of ethanol during adolescence will cause changes in brain ethanol responses that persist into adulthood. To test this hypothesis Wistar rats were treated with an adolescent intermittent ethanol (AIE; 5 g/kg, i.g. 2 days on-2 days off; P25-P54) model of underage drinking followed by 25 days of abstinence during maturation to young adulthood (P80). Using markers of neuronal activation c-Fos, EGR1, and phophorylated extracellar signal regulated kinase (pERK1/2), adult responses to a moderate and binge drinking ethanol challenge, e.g., 2 or 4 g/kg, were determined. Adult rats showed dose dependent increases in neuronal activation markers in multiple brain regions during ethanol challenge. Brain regional responses correlated are consistent with anatomical connections. AIE led to marked decreases in adult ethanol PFC (prefrontal cortex) and blunted responses in the amygdala. Binge drinking doses led to the nucleus accumbens (NAc) activation that correlated with the ventral tegmental area (VTA) activation. In contrast to other brain regions, AIE enhanced the adult NAc response to binge drinking doses. These studies suggest that adolescent alcohol exposure causes long-lasting changes in brain responses to alcohol that persist into adulthood.

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.

Adolescent intermittent ethanol exposure is associated with increased risky choice and decreased dopaminergic and cholinergic neuron markers in adult rats

BACKGROUND

Binge drinking is prevalent during adolescence and may have effects on the adult brain and behavior. The present study investigated whether adolescent intermittent ethanol exposure alters adult risky choice and prefrontal dopaminergic and forebrain cholinergic neuronal marker levels in male Wistar rats.

METHODS

Adolescent (postnatal day 28-53) rats were administered 5 g/kg of 25% (vol/vol) ethanol 3 times/d in a 2-days-on/2-days-off exposure pattern. In adulthood, risky choice was assessed in the probability discounting task with descending and ascending series of large reward probabilities and after acute ethanol challenge. Immunohistochemical analyses assessed tyrosine hydroxylase, a marker of dopamine and norepinephrine in the prelimbic and infralimbic cortices, and choline acetyltransferase, a marker of cholinergic neurons, in the basal forebrain.

RESULTS

All of the rats preferred the large reward when it was delivered with high probability. When the large reward became unlikely, control rats preferred the smaller, safe reward, whereas adolescent intermittent ethanol-exposed rats continued to prefer the risky alternative. Acute ethanol had no effect on risky choice in either group of rats. Tyrosine hydroxylase (prelimbic cortex only) and choline acetyltransferase immunoreactivity levels were decreased in adolescent intermittent ethanol-exposed rats compared with controls. Risky choice was negatively correlated with choline acetyltransferase, implicating decreased forebrain cholinergic activity in risky choice.

CONCLUSIONS

The decreases in tyrosine hydroxylase and choline acetyltransferase immunoreactivity suggest that adolescent intermittent ethanol exposure has enduring neural effects that may lead to altered adult behaviors, such as increased risky decision making. In humans, increased risky decision making could lead to maladaptive, potentially harmful consequences.

Impact of adolescent ethanol exposure and adult amphetamine self-administration on evoked striatal dopamine release in male rats

RATIONALE

Adolescent binge drinking is common and associated with increased risk of substance use disorders. Transition from recreational to habitual ethanol consumption involves alterations in dorsal striatal function, but the long-term impact of adolescent ethanol exposure upon this region remains unclear.

OBJECTIVES

This study aimed to characterise and describe relationships between adolescent ethanol exposure, amphetamine self-administration and adult dopamine dynamics in dorsal striatum, including response to amphetamine challenge, in male Wistar rats.

METHODS

Ethanol (2 g/kg) or water was administered intragastrically in an episodic binge-like regimen (three continuous days/week) between 4 and 9 weeks of age (i.e. post-natal days 28-59). In adulthood, animals were divided into two groups. In the first, dorsal striatal potassium-evoked dopamine release was examined via chronoamperometry, in the basal state and after a single amphetamine challenge (2 mg/kg, i.v.). In the second, amphetamine self-administration behaviour was studied (i.e. fixed and progressive ratio) before chronoamperometric analysis was conducted as described above.

RESULTS

Adolescent ethanol exposure suppressed locally evoked dopamine response after amphetamine challenge in adulthood, whereas in the basal state, no differences in dopamine dynamics were detected. Ethanol-exposed animals showed no differences in adult amphetamine self-administration behaviour but an abolished effect on dopamine removal in response to a single amphetamine challenge after self-administration.

CONCLUSION

Amphetamine challenges in adult rats revealed differences in in vivo dopamine function after adolescent ethanol exposure. The attenuated drug response in ethanol-exposed animals may affect habit formation and contribute to increased risk for substance use disorders as a consequence of adolescent ethanol.

Reinforcing properties and neurochemical response of ethanol within the posterior ventral tegmental area are enhanced in adulthood by periadolescent ethanol consumption

Alcohol drinking during adolescence is associated with increased alcohol drinking and alcohol dependence in adulthood. Research examining the biologic consequences of adolescent ethanol (EtOH) consumption on the response to EtOH in the neurocircuitry shown to regulate drug reinforcement is limited. The experiments were designed to determine the effects of periadolescent alcohol drinking on the reinforcing properties of EtOH within the posterior ventral tegmental area (pVTA) and the ability of EtOH microinjected into the pVTA to stimulate dopamine (DA) release in the nucleus accumbens shell (AcbSh). EtOH access (24-hour free-choice) by alcohol-preferring rats occurred during postnatal days (PND) 30-60. Animals were tested for their response to EtOH after PND 85. Intracranial self-administration techniques were performed to assess EtOH self-infusion into the pVTA. In the second experiment, rats received microinjections of EtOH into the pVTA, and dialysis samples were collected from the AcbSh. The results indicate that in rats that consumed EtOH during adolescence, the pVTA was more sensitive to the reinforcing effects of EtOH (a lower concentration of EtOH supported self-administration) and the ability of EtOH microinjected into the pVTA to stimulate DA release in the AcbSh was enhanced (sensitivity and magnitude). The data indicate that EtOH consumption during adolescence altered the mesolimbic DA system to be more sensitive and responsive to EtOH. This increase in the response to EtOH within the mesolimbic DA during adulthood could be part of biologic sequelae that are the basis for the deleterious effects of adolescent alcohol consumption on the rate of alcoholism during adulthood.

Adolescent intermittent ethanol exposure diminishes anhedonia during ethanol withdrawal in adulthood

Adolescent alcohol use may interfere with neurodevelopment, increasing the likelihood of adult alcohol use disorders (AUDs). We investigated whether adolescent intermittent ethanol (AIE) exposure alters the adult reward response to ethanol. Adolescent rats were administered ethanol once (moderate exposure; Cohort 1) or three times per day (severe exposure; Cohort 2) in a 2 days on/2 days off pattern. In adulthood, subjects responded for electrical stimulation directed at the posterior lateral hypothalamus in a discrete-trial intracranial self-stimulation (ICSS) procedure that provides current-intensity thresholds as a measure of brain reward function. The effects of ethanol administration and withdrawal were assessed. Control rats showed dose-dependent threshold elevations after acute ethanol, indicating reward deficits. A majority of moderately AIE-exposed rats (Cohort 1) showed threshold lowering after ethanol, suggesting ethanol-induced reward enhancement in this sub-set of rats. Rats exposed to severe AIE (Cohort 2) showed no threshold elevation or lowering, suggesting a blunted affective ethanol response. Daily ethanol induced threshold elevations 24h after administration in control rats but not in either group of AIE-exposed rats, suggesting decreased sensitivity to the negative affective state of ethanol withdrawal. Withdrawal from a 4-day ethanol binge produced robust and enduring threshold elevations in all rats, although threshold elevations were diminished in rats exposed to severe AIE. These results indicate that AIE exposure diminished reward deficits associated with ethanol intoxication and withdrawal and may have increased ethanol-induced reward enhancement in a sub-set of rats. In humans, enhanced ethanol reward accompanied by reduced withdrawal severity may contribute to the development of AUDs.

Dopamine release dynamics change during adolescence and after voluntary alcohol intake

Adolescence is associated with high impulsivity and risk taking, making adolescent individuals more inclined to use drugs. Early drug use is correlated to increased risk for substance use disorders later in life but the neurobiological basis is unclear. The brain undergoes extensive development during adolescence and disturbances at this time are hypothesized to contribute to increased vulnerability. The transition from controlled to compulsive drug use and addiction involve long-lasting changes in neural networks including a shift from the nucleus accumbens, mediating acute reinforcing effects, to recruitment of the dorsal striatum and habit formation. This study aimed to test the hypothesis of increased dopamine release after a pharmacological challenge in adolescent rats. Potassium-evoked dopamine release and uptake was investigated using chronoamperometric dopamine recordings in combination with a challenge by amphetamine in early and late adolescent rats and in adult rats. In addition, the consequences of voluntary alcohol intake during adolescence on these effects were investigated. The data show a gradual increase of evoked dopamine release with age, supporting previous studies suggesting that the pool of releasable dopamine increases with age. In contrast, a gradual decrease in evoked release with age was seen in response to amphetamine, supporting a proportionally larger storage pool of dopamine in younger animals. Dopamine measures after voluntary alcohol intake resulted in lower release amplitudes in response to potassium-chloride, indicating that alcohol affects the releasable pool of dopamine and this may have implications for vulnerability to addiction and other psychiatric diagnoses involving dopamine in the dorsal striatum.

Chronic alcohol intake during adolescence, but not adulthood, promotes persistent deficits in risk-based decision making

BACKGROUND

Adolescent alcohol use is a major public health concern and is strongly correlated with the development of alcohol abuse problems in adulthood. Adolescence is characterized by maturation and remodeling of brain regions implicated in decision making and therefore may be uniquely vulnerable to environmental insults such as alcohol exposure. We have previously demonstrated that voluntary alcohol consumption in adolescence results in maladaptive risk-based decision making in adulthood. However, it is unclear whether this effect on risk-based decision making can be attributed to chronic alcohol use in general or to a selective effect of alcohol use during the adolescent period.

METHODS

Ethanol (EtOH) was presented to adolescent (postnatal day [PND] 30 to 49) and adult rats (PND 80 to 99) for 20 days, either 24 hours or 1 h/d, in a gel matrix consisting of distilled water, gelatin, polycose (10%), and EtOH (10%). The 24-hour time course of EtOH intake was measured and compared between adolescent and adult animals. Following 20 days of withdrawal from EtOH, we assessed risk-based decision making with a concurrent instrumental probability-discounting task. Blood EtOH concentrations (BECs) were taken from trunk blood and assessed using the Analox micro-stat GM7 in separate groups of animals at different time points.

RESULTS

Unlike animals exposed to EtOH during adolescence, animals exposed to alcohol during adulthood did not display differences in risk preference compared to controls. Adolescent and adult rats displayed similar EtOH intake levels and patterns when given either 24- or 1-hour access per day. In addition, while both groups reached significant BEC levels, we failed to find a difference between adult and adolescent animals.

CONCLUSIONS

Here, we show that adolescent, but not adult, EtOH intake leads to a persistent increase in risk preference which cannot be attributed to differences in intake levels or BECs attained. Our findings support previous work implicating adolescence as a time period of heightened susceptibility to the long-term negative effects of alcohol exposure.

Higher long-lasting ethanol sensitization after adolescent ethanol exposure in mice

RATIONALE

Due to their maturing brain, adolescents are suggested to be more vulnerable to the long-term consequences of chronic alcohol use. Increased sensitization to the stimulant effects of ethanol is a possible consequence of ethanol exposure during adolescence.

OBJECTIVES

The aim of this study was to characterize the long-term alterations in the stimulant effects of ethanol and in the rate of ethanol sensitization in mice pre-exposed to ethanol during adolescence in comparison to mice pre-exposed to ethanol in adulthood.

METHODS

Adolescent and adult female Swiss mice were injected with saline or ethanol (2.5 or 4 g/kg) during 14 consecutive days. After a 3-week period of ethanol abstinence, mice were tested as adults before and after a second exposure to daily repeated ethanol injections.

RESULTS

All mice pre-exposed to ethanol as adults or adolescents showed higher stimulant effects when re-exposed to ethanol 3 weeks later. However, this enhanced sensitivity to the stimulant effects of ethanol was of significantly higher magnitude in mice repeatedly injected with high ethanol doses (4 g/kg) during adolescence. Furthermore, the increased expression of ethanol stimulant effects in these mice was maintained even after a second procedure of ethanol sensitization.

CONCLUSIONS

Adolescence is a critical period for the development of a sensitization to ethanol stimulant properties providing that high intermittent ethanol doses are administered. These results might contribute to explain the relationship between age at first alcohol use and risks of later alcohol problems and highlight the dangers of repeated consumption of high alcohol amounts in young adolescents.

Juvenile ethanol exposure increases rewarding properties of cocaine and morphine in adult DBA/2J mice

Convergent data showed that ethanol exposure during adolescence can alter durably ethanol-related behaviour at adulthood. However, the consequences of juvenile ethanol exposure on the reinforcing effects of other drugs of abuse remain unclear. In the present work, we evaluated in adult male DBA/2J mice the effects of early ethanol exposure on the sensitivity to the incentive effects of cocaine and morphine, and on extracellular signal-regulated kinase (ERK) activation in response to cocaine. Juvenile male mice received intragastric administration of ethanol (2×2.5g/kg/day) or water for 5 days starting on postnatal day 28. When reaching adult age (10 week-old), animals were subjected to an unbiased procedure to assess conditioned place preference (CPP) to cocaine or morphine. In addition, activation of ERK in response to an acute injection of cocaine was investigated using immunoblotting in the striatum and the nucleus accumbens. Mice that have been subjected to early ethanol exposure developed CPP to doses of cocaine (5mg/kg) or morphine (10mg/kg) below the threshold doses to induce CPP in water pre-exposed mice. In addition, early ethanol administration significantly increased striatal ERK phosphorylation normally induced by acute cocaine (10 and 20mg/kg) in adult mice. These results show that, in DBA/2J mice, early exposure to ethanol enhanced the perception of the incentive effects of cocaine and morphine. Ethanol pre-exposure also induced a positive modulation of striatal ERK signalling, in line with the inference that juvenile ethanol intake may contribute to the development of addictive behaviour at adult age.

The effects of abused drugs on adolescent development of corticolimbic circuitry and behavior

Adolescence is a period of significant neurobiological change that occurs as individuals transition from childhood to adulthood. Because the nervous system is in a relatively labile state during this stage of development, it may be especially sensitive to experience-induced plasticity. One such experience that is relatively common to adolescents is the exposure to drugs of abuse, particularly alcohol and psychostimulants. In this review, we highlight recent findings on the long-lasting effects of exposure to these drugs during adolescence in humans as well as in animal models. Whenever possible, our focus is on studies that use comparison groups of adolescent- and adult-exposed subjects as this is a more direct test of the hypothesis that adolescence represents a period of enhanced vulnerability to the effects of drug-induced plasticity. Lastly, we suggest areas of future investigation that are needed and methodological concerns that should be addressed.

Impact of pubertal stage at first drink on adult drinking behavior

BACKGROUND

Early alcohol use is one of the strongest predictors of later alcohol use disorders, with early use usually taking place during puberty. Many researchers have suggested drinking during puberty as a potential biological basis of the age at first drink (AFD) effect. However, the influence of the pubertal phase at alcohol use initiation on subsequent drinking in later life has not been examined so far.

METHODS

Pubertal stage at first drink (PSFD) was determined in N = 283 young adults (131 males, 152 females) from an epidemiological cohort study. At ages 19, 22, and 23 years, drinking behavior (number of drinking days, amount of alcohol consumed, hazardous drinking) was assessed using interview and questionnaire methods. Additionally, an animal study examined the effects of pubertal or adult ethanol (EtOH) exposure on voluntary EtOH consumption in later life in 20 male Wistar rats.

RESULTS

PSFD predicted drinking behavior in humans in early adulthood, indicating that individuals who had their first drink during puberty displayed elevated drinking levels compared to those with postpubertal drinking onset. These findings were corroborated by the animal study, in which rats that received free access to alcohol during the pubertal period were found to consume more alcohol as adults, compared to the control animals that first came into contact with alcohol during adulthood.

CONCLUSIONS

The results point to a significant role of stage of pubertal development at first contact with alcohol for the development of later drinking habits. Possible biological mechanisms and implications for prevention are discussed.

Involvement of nucleus accumbens dopamine D1 receptors in ethanol drinking, ethanol-induced conditioned place preference, and ethanol-induced psychomotor sensitization in mice

RATIONALE

Dopamine D1 receptor (D1R) signaling has been associated to ethanol consumption and reward in laboratory animals.

OBJECTIVES

Here, we hypothesize that this receptor, which is located within the nucleus accumbens (NAc) neurons, modulates alcohol reward mechanisms.

METHODS

To test this hypothesis, we measured alcohol consumption and ethanol-induced psychomotor sensitization and conditioned place preference (CPP) in mice that received bilateral microinjections of small interference RNA (siRNA)-expressing lentiviral vectors (LV-siD1R) producing D1R knock-down. The other group received control (LV-Mock) viral vectors into the NAc.

RESULTS

There were no differences in the total fluid consumed and also no differences in the amount of ethanol consumed between groups prior to surgery. However, after surgery, the LV-siD1R group consumed less ethanol than the control group. This difference was not associated to taste neophobia. In addition, results have shown that down-regulation of endogenous D1R using viral-mediated siRNA in the NAc significantly decreased ethanol-induced behavioral sensitization as well as acquisition, but not expression, of ethanol-induced place preference.

CONCLUSIONS

We conclude that decreased D1R expression into the NAc led to reduced ethanol rewarding properties, thereby leading to lower voluntary ethanol consumption. Together, these findings demonstrate that the D1 receptor pathway within the NAc controls ethanol reward and intake.

Increased sensitivity to alcohol induced changes in ERK Map kinase phosphorylation and memory disruption in adolescent as compared to adult C57BL/6J mice

Adolescence is a critical period of brain development that is accompanied by increased probability of risky behavior, such as alcohol use. Emerging research indicates that adolescents are differentially sensitive to the behavioral effects of acute ethanol as compared to adults but the neurobiological mechanisms of this effect remain to be fully elucidated. This study was designed to evaluate effects of acute ethanol on extracellular signal-regulated kinase phosphorylation (p-ERK1/2) in mesocorticolimbic brain regions. We also sought to determine if age-specific effects of ethanol on p-ERK1/2 are associated with ethanol-induced behavioral deficits on acquisition of the hippocampal-dependent novel object recognition (NOR) test. Adolescent and adult C57BL/6J mice were administered acute ethanol (0 0.5, 1, or 3g/kg, i.p.). Brains were removed 30-min post injection and processed for analysis of p-ERK1/2 immunoreactivity (IR). Additional groups of mice were administered ethanol (0 or 1g/kg) prior to the NOR test. Analysis of p-ERK1/2 IR showed that untreated adolescent mice had significantly higher levels of p-ERK1/2 IR in the nucleus accumbens shell, basolateral amygdala (BLA), central amygdala (CeA), and medial prefrontal cortex (mPFC) as compared to adults. Ethanol (1g/kg) selectively reduced p-ERK1/2 IR in the dentate gyrus and increased p-ERK1/2 IR in the BLA only in adolescent mice. Ethanol (3g/kg) produced the same effects on p-ERK1/2 IR in both age groups with increases in CeA and mPFC, but a decrease in the dentate gyrus, as compared to age-matched saline controls. Pretreatment with ethanol (1g/kg) disrupted performance on the NOR test specifically in adolescents, which corresponds with the ethanol-induced inhibition of p-ERK1/2 IR in the hippocampus. These data show that adolescent mice have differential expression of basal p-ERK1/2 IR in mesocorticolimbic brain regions. Acute ethanol produces a unique set of changes in ERK1/2 phosphorylation in the adolescent brain that are associated with disruption of hippocampal-dependent memory acquisition.

Developmental differences in ethanol-induced sensitization using postweanling, adolescent, and adult Swiss mice

RATIONALE

The maturing adolescent brain has been suggested to be more sensitive than the adult brain to ethanol-induced neuroadaptations. In animal studies, sensitization to the stimulant effects of ethanol is used to study the vulnerability to chronic ethanol-induced neurobehavioral alterations.

OBJECTIVES

The aim of the present study was to systematically characterize age-dependent changes in the development and expression of the sensitization to the stimulant effects of a range of ethanol doses in female Swiss mice. Three ages were studied: 21-day-old mice (postweanlings), 35-day-old mice (adolescents), and 63-day-old mice (adults).

METHODS

Postweanling, adolescent, and adult mice were daily injected with saline or various ethanol doses (1.5 to 4 g/kg) for 7 days. They were then tested for acute and sensitized locomotor activity.

RESULTS

Postweanling and adolescent mice were more sensitive than adult mice to the acute stimulant effects of ethanol. In adult mice, daily injections of ethanol at doses between 2.5 and 4 g/kg led to significant sensitization. Higher ethanol doses (3.5 and 4 g/kg) were required to induce sensitization in postweanling and adolescent mice. However, younger mice showed ethanol sensitization of higher magnitude.

CONCLUSIONS

Young mice develop very strong ethanol sensitization at doses that mimic binge drinking in humans. These results might explain why early ethanol drinking during adolescence is related to a higher prevalence of subsequent alcohol disorders.

Chronic ethanol exposure during adolescence alters the behavioral responsiveness to ethanol in adult mice

Alcohol exposure during early adolescence is believed to durably alter the behavioral properties of ethanol, increasing the likelihood of later alcohol-related disorders. The aim of the present experiments was to characterize changes in the behavioral effects of ethanol in adult female Swiss mice after a chronic ethanol exposure during adolescence, extending from postnatal day 28 to postnatal day 42. After a chronic ethanol exposure during adolescence (daily injections of 0, 2.5 or 4 g/kg ethanol for 14 consecutive days), adult mice were tested at postnatal day 63. The locomotor stimulant effects of ethanol, together with ethanol sensitization were tested in experiment 1. In experiment 2, the sedative effects of ethanol were assessed with the loss of righting reflex procedure. Finally, in experiment 3, the anxiolytic effects of ethanol were tested with the light/dark box test. Adult mice chronically exposed to ethanol during adolescence showed a lower basal locomotor activity, but higher locomotor stimulant effects of ethanol than non-exposed mice. Additionally, these adult mice developed higher rates of ethanol sensitization after chronic re-exposure to ethanol in adulthood. Adult mice exposed to ethanol during adolescence also had a stronger tolerance to the sedative effects of high ethanol doses, although they showed no evidence of changes in the anxiolytic effects of ethanol. These results are in agreement with the thesis that chronic alcohol consumption during adolescence, especially in high amounts, increases the risk of later alcohol-related disorders.

Molecular and behavioral aspects of the actions of alcohol on the adult and developing brain

The brain is one of the major target organs of alcohol actions. Alcohol abuse can lead to alterations in brain structure and functions and, in some cases, to neurodegeneration. Cognitive deficits and alcohol dependence are highly damaging consequences of alcohol abuse. Clinical and experimental studies have demonstrated that the developing brain is particularly vulnerable to alcohol, and that drinking during gestation can lead to a range of physical, learning and behavioral defects (fetal alcohol spectrum disorders), with the most dramatic presentation corresponding to fetal alcohol syndrome. Recent findings also indicate that adolescence is a stage of brain maturation and that heavy drinking at this stage can have a negative impact on brain structure and functions causing important short- and long-term cognitive and behavioral consequences. The effects of alcohol on the brain are not uniform; some brain areas or cell populations are more vulnerable than others. The prefrontal cortex, the hippocampus, the cerebellum, the white matter and glial cells are particularly susceptible to the effects of ethanol. The molecular actions of alcohol on the brain are complex and involve numerous mechanisms and signaling pathways. Some of the mechanisms involved are common for the adult brain and for the developing brain, while others depend on the developmental stage. During brain ontogeny, alcohol causes irreversible alterations to the brain structure. It also impairs several molecular, neurochemical and cellular events taking place during normal brain development, including alterations in both gene expression regulation and the molecules involved in cell-cell interactions, interference with the mitogenic and growth factor response, enhancement of free radical formation and derangements of glial cell functions. However, in both adult and adolescent brains, alcohol damages specific brain areas through mechanisms involving excitotoxicity, free radical formation and neuroinflammatory damage resulting from activation of the innate immune system mediated by TLR4 receptors. Alcohol also acts on specific membrane proteins, such as neurotransmitter receptors (e.g. NMDA, GABA-A), ion channels (e.g. L-type Ca²⁺ channels, GIRKs), and signaling pathways (e.g. PKA and PKC signaling). These effects might underlie the wide variety of behavioral effects induced by ethanol drinking. The neuroadaptive changes affecting neurotransmission systems which are more sensitive to the acute effects of alcohol occur after long-term alcohol consumption. Alcohol-induced maladaptations in the dopaminergic mesolimbic system, abnormal plastic changes in the reward-related brain areas and genetic and epigenetic factors may all contribute to alcohol reinforcement and alcohol addiction. This manuscript reviews the mechanisms by which ethanol impacts the adult and the developing brain, and causes both neural impairments and cognitive and behavioral dysfunctions. The identification and the understanding of the cellular and molecular mechanisms involved in ethanol toxicity might contribute to the development of treatments and/or therapeutic agents that could reduce or eliminate the deleterious effects of alcohol on the brain.

Ethanol drinking reduces extracellular dopamine levels in the posterior ventral tegmental area of nondependent alcohol-preferring rats

Moderate ethanol exposure produces neuroadaptive changes in the mesocorticolimbic dopamine (DA) system in nondependent rats and increases measures of DA neuronal activity in vitro and in vivo. Moreover, moderate ethanol drinking and moderate systemic exposure elevates extracellular DA levels in mesocorticolimbic projection regions. However, the neuroadaptive changes subsequent to moderate ethanol drinking on basal DA levels have not been investigated in the ventral tegmental area (VTA). In the present study, adult female alcohol-preferring (P) rats were divided into alcohol-naive, alcohol-drinking, and alcohol-deprived groups. The alcohol-drinking group had continuous access to water and ethanol (15%, vol/vol) for 8 weeks. The alcohol-deprived group had 6 weeks of access followed by 2 weeks of ethanol deprivation, 2 weeks of ethanol re-exposure, followed again by 2 weeks of deprivation. The deprived rats demonstrated a robust alcohol deprivation effect (ADE) on ethanol reinstatement. The alcohol-naïve group had continuous access to water only. In the last week of the drinking protocol, all rats were implanted with unilateral microdialysis probes aimed at the posterior VTA and no-net-flux microdialysis was conducted to quantify extracellular DA levels and DA clearance. Results yielded significantly lower basal extracellular DA concentrations in the posterior VTA of the alcohol-drinking group compared with the alcohol-naive and alcohol-deprived groups (3.8±0.3nM vs. 5.0±0.5nM [P<.02] and 4.8±0.4nM, [P<.05], respectively). Extraction fractions were significantly (P<.0002) different between the alcohol-drinking and alcohol-naive groups (72±2% vs. 46±4%, respectively) and not significantly different (P=.051) between alcohol-deprived and alcohol-naive groups (61±6% for the alcohol-deprived group). The data indicate that reductions in basal DA levels within the posterior VTA occur after moderate chronic ethanol intake in nondependent P rats. This reduction may result, in part, from increased DA uptake and may be important for the maintenance of ethanol drinking. These adaptations normalize with ethanol deprivation and may not contribute to the ADE.

Sex differences in the effects of ethanol pre-exposure during adolescence on ethanol-induced conditioned taste aversion in adult rats

Alcohol use, which typically begins during adolescence and differs between males and females, is influenced by both the rewarding and aversive properties of the drug. One way adolescent alcohol use may modulate later consumption is by reducing alcohol's aversive properties. Here, we used a conditioned taste aversion (CTA) paradigm to determine if pre-exposure to alcohol (ethanol) during adolescence would attenuate ethanol-induced CTA assessed in adulthood in a sex-dependent manner. Male and female Long-Evans rats were given intraperitoneal (i.p.) injections of saline or 3.0g/kg ethanol in a binge-like pattern during postnatal days (PD) 35-45. In adulthood (>PD 100), rats were given access to 0.1% saccharin, followed by saline or ethanol (1.0 or 1.5g/kg, i.p.), over four conditioning sessions. We found sex differences in ethanol-induced CTA, with males developing a more robust aversion earlier in conditioning. Sex differences in the effects of pre-exposure were also evident: males, but not females, showed an attenuated CTA in adulthood following ethanol pre-exposure, which occurred approximately nine weeks earlier. Taken together, these findings indicate that males are more sensitive to the aversive properties of ethanol than females. In addition, the ability of pre-exposure to the ethanol US to attenuate CTA is enhanced in males compared to females.

The effects of pre-pubertal gonadectomy and binge-like ethanol exposure during adolescence on ethanol drinking in adult male and female rats

The pubertal surge in gonadal hormones that occurs during adolescence may impact the long-term effects of early alcohol exposure and sex differences in drinking behavior in adulthood. We investigated this hypothesis by performing sham or gonadectomy surgeries in Long-Evans rats around post-natal day (P) 20. From P35-45, males and females were given saline or 3.0 g/kg ethanol using a binge-like model of exposure (8 injections total). As adults (P100), they were trained to self-administer ethanol via a sucrose-fading procedure and then given access to different unsweetened concentrations (5-20%, w/v) for 5 days/concentration. We found that during adolescence, ethanol-induced intoxication was similar in males and females that underwent sham surgery. In gonadectomized males and females, however, the level of intoxication was greater following the last injection compared to the first. During adulthood, females drank more sucrose per body weight than males and binge-like exposure to ethanol reduced sucrose consumption in both sexes. These effects were not seen in gonadectomized rats. Ethanol consumption was higher in saline-exposed females compared to males, with gonadectomy reversing this sex difference by increasing consumption in males and decreasing it in females. Exposure to ethanol during adolescence augmented ethanol consumption in both sexes, but this effect was statistically significant only in gonadectomized females. Together, these results support a role for gonadal hormones during puberty in the short- and long-term effects of ethanol on behavior and in the development of sex differences in consummatory behavior during adulthood.

Research on alcohol and adolescent brain development: opportunities and future directions

In the past 15 years, both human and animal studies have advanced our understanding of the effects of adolescent alcohol exposure on behavioral and neural development, particularly in the areas of the ontogeny of initial sensitivity and tolerance to alcohol, the consequences of adolescent alcohol exposure on subsequent drinking patterns, as well as cognitive and neural function. Despite these advances, there are still substantial gaps in our understanding of whether heavy adolescent drinking interferes with normal brain development at the cellular and molecular level, and if so, how these changes may translate into patterns of brain connectivity that result in the emergence of alcohol use disorders. This article discusses our current knowledge of the cellular and molecular brain changes that stem from heavy alcohol exposure, including binge patterns, during adolescence. Progress has been made in linking the behavioral effects of adolescent drinking to underlying cellular and molecular mechanisms. However, it is suggested that future research on the etiology and consequences of adolescent drinking use an integrative approach to this problem by combining multiple levels, including genetic, cellular and molecular, systems (neuroimaging), and behavioral, with an emphasis on integrating the different levels of analysis.

Alcohol during adolescence selectively alters immediate and long-term behavior and neurochemistry

Alcohol use increases across adolescence and is a concern in the United States. In humans, males and females consume different amounts of alcohol depending on the age of initiation, and the long-term consequences of early ethanol consumption are not readily understood. The purpose of our work was to better understand the immediate and long-term impact of ethanol exposure during adolescence and the effects it can have on behavior and dopaminergic responsivity. We have assessed sex differences in voluntary ethanol consumption during adolescence and adulthood and the influence of binge ethanol exposure during adolescence. We have observed that males are sensitive to passive social influences that mediate voluntary ethanol consumption, and early ethanol exposure induces long-term changes in responsivity to ethanol in adulthood. Exposure to moderate doses of ethanol during adolescence produced alterations in dopamine in the nucleus accumbens septi during adolescence and later in adulthood. Taken together, all of these data indicate that the adolescent brain is sensitive to the impact of early ethanol exposure during this critical developmental period.

Mechanisms involved in the neurotoxic, cognitive, and neurobehavioral effects of alcohol consumption during adolescence

Studies over the last decade demonstrate that adolescence is a brain maturation period from childhood to adulthood. Plastic and dynamic processes drive adolescent brain development, creating flexibility that allows the brain to refine itself, specialize, and sharpen its functions for specific demands. Maturing connections enable increased communication among brain regions, allowing greater integration and complexity. Compelling evidence has shown that the developing brain is vulnerable to the damaging effects of ethanol. It is possible to infer, therefore, that alcohol exposure during the critical adolescent developmental stages could disrupt the brain plasticity and maturation processes, resulting in behavioral and cognitive deficits. Recent neuroimaging studies have provided evidence of the impact of human adolescent drinking in brain structure and functions. Findings in experimental animals have also given new insight into the potential mechanisms of the toxic effects of ethanol on both adolescent brain maturation and the short- and long-term cognitive consequences of adolescent drinking. Adolescence is also characterized by the rapid maturation of brain systems mediating reward and by changes in the secretion of stress-related hormones, events that might participate in the increasing in anxiety and the initiation pattern of alcohol and drug consumption. Studies in human adolescents demonstrate that drinking at early ages can enhance the likelihood of developing alcohol-related problems. Experimental evidence suggests that early exposure to alcohol sensitizes the neurocircuitry of addiction and affects chromatin remodeling, events that could induce abnormal plasticity in reward-related learning processes that contribute to adolescents' vulnerability to drug addiction. In this article, we review the potential mechanisms by which ethanol impacts brain development and lead to brain impairments and cognitive and behavioral dysfunctions as well as the neurobiological and neurochemical processes underlying the adolescent-specific vulnerability to drug addiction.

Neuromaturation and Adolescent Risk Taking: Why Development Is Not Determinism

In the January 2009 issue of this journal, Males argues that adolescent brain science perpetuates the “myth of adolescent risk taking.” He contends that those who study adolescent neuromaturation are biological determinists who ignore the profound social and environmental forces that influence adolescent behavior to further their own agendas. Males mischaracterizes developmental research and misinterprets public health data. This article analyzes his argument and provides a response based on the evidence. There is significant cross-species evidence that adolescence serves an important developmental function on the road to full maturation and is not merely an oppressive social construction. Research on neuromaturation can help elucidate both the vulnerabilities and tremendous potential of the adolescent brain. It also provides the opportunity to examine the role of social environments in shaping developmental processes and to explore how reasoned understandings of adolescent brain and biological development are being used to inform interventions that scaffold adolescent vulnerabilities.

Repeated ethanol exposure during adolescence alters the developmental trajectory of dopaminergic output from the nucleus accumbens septi

Individuals who begin using alcohol prior to 14 years of age are 4 times more likely to progress to addiction than those who do not initiate use until 21 years of age. The nucleus accumbens septi undergoes dramatic developmental transitions during the adolescent period, and dopaminergic activity within this region has been identified as a central neurochemical mediator of drug reward, addiction and dependence. Thus, alcohol-induced neurochemical alterations in dopaminergic activity within this brain region likely mediate the heightened vulnerability to addiction observed in adolescent alcohol users. To investigate this idea, Sprague-Dawley rats were exposed to intraperitoneal injections of either saline or ethanol (0.5, 1.0 or 2.0 g/kg) twice daily over four days beginning on postnatal day 21, 31, 41 or 56. Cannulas were implanted toward the nucleus accumbens septi, subsequent in vivo microdialysis was used to collect samples, and both basal and ethanol-stimulated dopamine overflow was measured using high performance liquid chromatography with electrochemical detection. A developmental transition in basal levels of dopamine in the nucleus accumbens septi was apparent with peak levels at postnatal day 45. An ethanol challenge produced unique responses across ages, with greater peak effects relative to baseline in younger animals (postnatal day 25 and 35). Following repeated exposure to ethanol, a significant increase in basal dopamine was apparent for all ages, and when these animals were challenged with ethanol, peak effects relative to baseline were decreased in younger animals, but unchanged in older animals (postnatal day 45 and 60). Results indicate that there is a key developmental transition in the ability of rats to adapt to the effects of repeated ethanol exposure, which occurs between postnatal day 35 and 45. This alteration may explain the increased addiction vulnerability observed in individuals who initiate alcohol use during early adolescence.

Repeated alcohol administration during adolescence causes changes in the mesolimbic dopaminergic and glutamatergic systems and promotes alcohol intake in the adult rat

Adolescence is a developmental period which the risk of drug and alcohol abuse increases. Since mesolimbic dopaminergic system undergoes developmental changes during adolescence, and this system is involved in rewarding effects of drugs of abuse, we addressed the hypothesis that ethanol exposure during juvenile/adolescent period over-activates mesolimbic dopaminergic system inducing adaptations which can trigger long-term enduring behavioural effects of alcohol abuse. We treated juvenile/adolescent or adult rats with ethanol (3 g/kg) for two-consecutive days at 48-h intervals over 14-day period. Here we show that intermittent ethanol treatment during the juvenile/adolescence period alters subsequent ethanol intake. In vivo microdialysis demonstrates that ethanol elicits a similar prolonged dopamine response in the nucleus accumbens of both adolescent and adult animals pre-treated with multiple doses of ethanol, although the basal dopamine levels were higher in ethanol-treated adolescents than in adult-treated animals. Repeated ethanol administration also down-regulates the expression of DRD2 and NMDAR2B phosphorylation in prefrontal cortex of adolescent animals, but not of adult rats. Finally, ethanol treatment during adolescence changes the acetylation of histones H3 and H4 in frontal cortex, nucleus accumbens and striatum, suggesting chromatin remodelling changes. In summary, our findings demonstrate the sensitivity of adolescent brain to ethanol effects on dopaminergic and glutamatergic neurotransmission, and suggest that abnormal plasticity in reward-related processes and epigenetic mechanisms could contribute to the vulnerability of adolescents to alcohol addiction.

Ethanol-induced behavioral sensitization in adolescent and adult mice: role of the nNOS gene

BACKGROUND

In the brain, nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in synaptic plasticity. Recent evidence suggests the role of NO in a variety of effects produced by alcohol in the central nervous system. The current study investigated the role of the nNOS gene in the development of behavioral sensitization to ethanol in adolescent and adult mice.

METHODS

Adolescent and adult wild type (WT; B6;129SF2) and nNOS knockout (KO; B6;129S4-Nos1) mice of both sexes received saline or ethanol (1.5 g/kg; intraperitoneally) for 5 consecutive days, and locomotor activity was recorded daily. The locomotor response to challenge ethanol and saline injections was investigated at various time points following withdrawal from ethanol.

RESULTS

Adolescent WT but not nNOS KO mice developed a long-lasting sensitized response to ethanol as well as context-dependent hyperlocomotion (in response to saline) from adolescence through adulthood; sex-dependent differences were not observed. Compared to adolescent WT mice, adult WT males developed a short-term sensitized response to ethanol and context-dependent hyperlocomotion; adult WT females showed only short-term context-dependent hyperlocomotion. Adult nNOS KO males (like their adolescent counterparts) did not develop behavioral sensitization; no significant differences between adult nNOS KO and WT females were observed. Blood ethanol concentrations did not show genotype- or sex-dependent differences.

CONCLUSIONS

(1) The nNOS gene is required for the development of behavioral sensitization to ethanol in adolescent male and female mice. (2) Adolescent exposure to ethanol results in long-lasting behavioral sensitization through adulthood, while adult exposure to ethanol results in a shorter behavioral sensitization. (3) Sex-dependent differences are observed when ethanol exposure begins in adulthood but not in adolescence. (4) Ethanol-induced behavioral sensitization in adulthood is nNOS-dependent in males but not in females. Taken together, results suggest genotype-, ontogeny-, and sex-dependent differences in the development of behavioral sensitization to ethanol.