Adolescent Rats Exposed to Repeated Ethanol Treatment Show Lingering Behavioral Impairments

A scoping review of the relationship between alcohol, memory consolidation and ripple activity: An overview of common methodologies to analyse ripples

Alcohol abuse is not only responsible for 5.3% of the total deaths in the world but also has a substantial impact on neurological and memory disabilities throughout the population. One extensively studied brain area involved in cognitive functions is the hippocampus. Evidence in several rodent models has shown that ethanol produces cognitive impairment in hippocampal-dependent tasks and that the damage is varied according to the stage of development at which the rodent was exposed to ethanol and the dose. To the authors' knowledge, there is a biomarker for cognitive processes in the hippocampus that remains relatively understudied in association with memory impairment by alcohol administration. This biomarker is called sharp wave-ripples (SWRs) which are synchronous neuronal population events that are well known to be involved in memory consolidation. Methodologies for facilitated or automatic identification of ripples and their analysis have been reported for a wider bandwidth than SWRs. This review is focused on communicating the state of the art about the relationship between alcohol, memory consolidation and ripple activity, as well as the use of the common methodologies to identify SWRs automatically.

Behavioral changes and dendritic remodeling of hippocampal neurons in adolescent alcohol-treated rats

Objective: Earlier, we and others have reported that alcohol exposure in adolescent rat impaired performance of a spatial memory task in the Morris water maze. The goal of the present study was to investigate the effects of acute adolescent alcohol treatment on the hippocampus-dependent (contextual fear conditioning) and hippocampus-independent (cued fear) memories. The study also looked at the structural changes in anterior CA1 hippocampal neurons in adolescent alcohol-treated rats. Methods: Adolescent female rats were administered with a single dose of alcohol (1.0, 1.5, or 2.0 g/kg) or vehicle either before training (pre-training) or after training (pre-testing). Experimental and control rats were trained in the fear conditioning paradigm, and 24 h later tested for both contextual fear conditioning as well as cued fear memory. Separate groups of rats were treated with either alcohol (2 g/kg) or vehicle and sacrificed 24 h later. Their brains were harvested and processed for rapid Golgi staining. Randomly selected CA1 pyramidal neurons were analyzed for dendritic branching and dendritic spine density. Results: Pre-training alcohol dose-dependently attenuated acquisition of hippocampus-dependent contextual fear conditioning but had no effect on the acquisition of amygdala-associated cued fear. When administered following training (pre-testing), alcohol did not alter either contextual conditioning or cued fear memory. Golgi stained CA1 pyramidal neurons in alcohol treated female rats had reduced basilar tree branching and less complex dendritic arborization. Conclusion: Alcohol specifically impaired hippocampal learning in adolescent rats but not amygdala-associated cued fear memory. Compared to vehicle-treated rats, CA1 hippocampal pyramidal neurons in alcohol-treated rats had less complex dendritic morphology. Together, these data suggest that adolescent alcohol exposure produces changes in the neuronal organization of the hippocampus, and these changes may be related to impairments in hippocampus-dependent memory formation.

Indomethacin restores loss of hippocampal neurogenesis and cholinergic innervation and reduces innate immune expression and reversal learning deficits in adult male and female rats following adolescent ethanol exposure

BACKGROUND

Adolescent intermittent ethanol (AIE) exposure causes long-term changes in the brain and behavior of adult male rodents, including persistent induction of innate immune pathways, reductions in hippocampal neurogenic and forebrain cholinergic neuronal markers, and reversal learning deficits. The current study tests the hypothesis that proinflammatory induction mediates AIE-induced (1) loss of adult neurogenesis (i.e., doublecortin (DCX) expressing immature neurons), (2) reductions in forebrain and hippocampal cholinergic markers, and (3) reversal learning deficits.

METHODS

Male and female rats underwent AIE (5.0 g/kg/day ethanol or water, i.g., 2 day-on/2 day-off from postnatal day (PND) 25-54), followed by a 2-week regimen of the anti-inflammatory compound indomethacin (4.0 g/kg/day, PND 56-69) or vehicle, after which one cohort was euthanized for immunohistochemical markers (PND 70) and the second underwent the Morris water maze to assess reversal learning.

RESULTS

AIE reduced adult (PND 70) DCX+ immunoreactivity (IR) and increased hippocampal expression of the innate immune signal's high-mobility group box protein 1 (HMGB1 + IR) and cyclooxygenase-2 (COX-2 + IR) in adult male and female rats. AIE also reduced choline acetyltransferase (ChAT+IR) in the basal forebrain and co-labeling of hippocampal vesicular acetylcholine transporter (VAChT+) cholinergic terminals on DCX + IR neurons. Indomethacin treatment after AIE restored molecular endpoints to control levels and rescued AIE-induced reversal learning deficits in the Morris water maze in both sexes. Of note, indomethacin produced several adverse effects selectively in control conditions, highlighting the uniquely beneficial effect of indomethacin in AIE rats.

CONCLUSIONS

These data suggest that in males and females, (1) AIE persistent neuroimmune induction mediates both the loss of adult hippocampal DCX and loss of basal forebrain cholinergic neurons and their innervation to hippocampal targets, and (2) anti-inflammatory indomethacin treatment following AIE that restores these persistent molecular pathologies also restores spatial reversal learning deficits.

Early developmental alcohol exposure alters behavioral outcomes following adolescent re-exposure in a rat model

BACKGROUND

Prenatal alcohol exposure alters brain development, affecting cognitive, motor, and emotional domains, and potentially leading to greater alcohol intake during adolescence. The present study investigated whether early alcohol exposure modifies vulnerability to behavioral alterations associated with adolescent alcohol exposure in a rodent model.

METHODS

Sprague-Dawley rats received ethanol or sham intubations during two developmental periods: (1) the third trimester equivalent of brain development in humans (postnatal days [PD] 4-9) and (2) adolescence (PD 28-42). Both exposures resulted in blood alcohol concentrations around 200 mg/dl. Subjects were tested in the open field (PD 45-48) and on hippocampal and prefrontal cortical (PFC) dependent tasks: the Morris water maze (PD 52-58) and trace fear conditioning (PD 63-64).

RESULTS

Neonatal alcohol exposure reduced forebrain and cerebellar weight, increased open-field activity, and slowed acquisition of trace fear conditioning. Adolescent alcohol exposure did not disrupt learning or significantly induce gross brain pathology, suggesting that 200 mg/dl/day of ethanol disrupts cognitive development during the 3rd trimester equivalent, but not during adolescence. Interestingly, females exposed to alcohol only during adolescence exhibited an increased conditioned fear response and more rapid habituation of locomotor activity in the open field, suggesting alterations in emotional responding. Moreover, subjects exposed to a combination of neonatal and adolescent alcohol exposure spent significantly more time in the center of the open field chamber than other groups. Similarly, males exposed to the combination exhibited less thigmotaxis in the Morris water maze.

CONCLUSIONS

These results indicate that combined exposure to alcohol during these two critical periods reduces anxiety-related behaviors and/or increases risk taking in a sex-dependent manner, suggesting that prenatal alcohol exposure may affect risk for emotional consequences of adolescent alcohol exposure.

Age-related differences in the effect of chronic alcohol on cognition and the brain: a systematic review

Adolescence is an important developmental period associated with increased risk for excessive alcohol use, but also high rates of recovery from alcohol use-related problems, suggesting potential resilience to long-term effects compared to adults. The aim of this systematic review is to evaluate the current evidence for a moderating role of age on the impact of chronic alcohol exposure on the brain and cognition. We searched Medline, PsycInfo, and Cochrane Library databases up to February 3, 2021. All human and animal studies that directly tested whether the relationship between chronic alcohol exposure and neurocognitive outcomes differs between adolescents and adults were included. Study characteristics and results of age-related analyses were extracted into reference tables and results were separately narratively synthesized for each cognitive and brain-related outcome. The evidence strength for age-related differences varies across outcomes. Human evidence is largely missing, but animal research provides limited but consistent evidence of heightened adolescent sensitivity to chronic alcohol's effects on several outcomes, including conditioned aversion, dopaminergic transmission in reward-related regions, neurodegeneration, and neurogenesis. At the same time, there is limited evidence for adolescent resilience to chronic alcohol-induced impairments in the domain of cognitive flexibility, warranting future studies investigating the potential mechanisms underlying adolescent risk and resilience to the effects of alcohol. The available evidence from mostly animal studies indicates adolescents are both more vulnerable and potentially more resilient to chronic alcohol effects on specific brain and cognitive outcomes. More human research directly comparing adolescents and adults is needed despite the methodological constraints. Parallel translational animal models can aid in the causal interpretation of observed effects. To improve their translational value, future animal studies should aim to use voluntary self-administration paradigms and incorporate individual differences and environmental context to better model human drinking behavior.

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

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

Behavioral alterations induced by intermittent ethanol intake and noise exposure in adolescent rats

Alcohol intake and exposure to noise are common activities of human adolescents performed in entertainment contexts worldwide that can induce behavioral disturbances. Therefore, the aim of the present work was to investigate in an experimental model of adolescent animals whether noise exposure and intermittent ethanol intake, when present individually or sequentially, might be able to modify different behaviors. Adolescent Wistar rats of both sexes were subjected to voluntary intermittent ethanol intake for 1 week followed by exposure to noise for 2 h and tested in a battery of behavioral tasks. Data show that males exposed to noise experienced a deficit in associative memory (AM), increase in anxiety-like behaviors (ALB) and altered reaction to novelty (RN) when compared with sham animals, whereas females also showed an increase in risk assessment behaviors (RAB) and a decrease in exploratory activity (EA). In contrast, ethanol intake induced an increase in RAB and RN in males and females, whereas females also showed a deficit in AM and EA as well as an increase in ALB. When ethanol was ingested before noise exposure, most parameters were counteracted both in male and females, but differed among sexes. In consequence, it could be hypothesized that an environmental acute stressor like noise might trigger a behavioral counteracting induced by a previous repeated exposure to a chemical agent such as ethanol, leading to a compensation of a non-adaptive behavior and reaching a better adjustment to the environment.

Primed for addiction: A critical review of the role of microglia in the neurodevelopmental consequences of adolescent alcohol drinking

Alcohol is one of the most widely used recreational substances worldwide, with drinking frequently initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiating alcohol use, often in high doses, and particularly susceptible to alcohol-induced brain changes. Microglia, the brain parenchymal macrophages, have been implicated in mediating some of these effects, though the role that these cells play in the progression from alcohol drinking to dependence remains unclear. Microglia are uniquely positioned to sense and respond to central nervous system insult, and are now understood to exhibit innate immune memory, or "priming," altering their future functional responses based on prior exposures. In alcohol use disorders (AUDs), the role of microglia is debated. Whereas microglial activation can be pathogenic, contributing to neuroinflammation, tissue damage, and behavioral changes, or protective, it can also engage protective functions, providing support and mediating the resolution of damage. Understanding the role of microglia in adolescent AUDs is complicated by the fact that microglia are thought to be involved in developmental processes such as synaptic refinement and myelination, which underlie the functional maturation of multiple brain systems in adolescence. Thus, the role microglia play in the impact of alcohol use in adolescence is likely multifaceted. Long-term sequelae may be due to a failure to recover from EtOH-induced tissue damage, altered neurodevelopmental trajectories, and/or persistent changes to microglial responsivity and function. Here, we review critically the literature surrounding the effects of alcohol on microglia in models of adolescent alcohol misuse. We attempt to disentangle what is known about microglia from other neuroimmune effectors, to which we apply recent discoveries on the role of microglia in development and plasticity. Considered altogether, these studies challenge assumptions that proinflammatory microglia drive addiction. Alcohol priming microglia and thereby perturbing their homeostatic roles in neurodevelopment, especially during critical periods of plasticity such as adolescence, may have more serious implications for the neuropathogenesis of AUDs in adolescents.

Voluntary Adolescent-Onset Alcohol Drinking Fails to Influence Alcohol Consumption or Anxiety-Like Behaviour in Adulthood in Female Alcohol-Preferring Rats

AIMS

Alcohol exposure during adolescence is associated with both increased risk for alcohol use disorders and anxiety in adulthood. Our present experiments examined this association using alcohol-preferring AA (Alko Alcohol) rats selected for high voluntary alcohol drinking.

METHODS

Two groups of female AA rats acquired alcohol drinking at different ages. We gave the adolescent-onset group free choice to 10% alcohol and water for seven weeks, starting on post-natal day 42 (PND 42), whereas the adult-onset group started drinking alcohol on PND 112. After the 7-week drinking, we withdrew the adolescent group from alcohol for two weeks, followed by another voluntary 7-week drinking period, started at the same age as the adult-onset group. We assessed anxiety-like behaviour repeatedly during alcohol drinking with open field and elevated plus maze tests. At the end of alcohol drinking, we also tested the rats using the light/dark box, stress-induced body temperature test and social dominance test.

RESULTS

During the first 7-week alcohol drinking, adolescent rats exhibited significantly slower acquisition of alcohol drinking and lower alcohol preference than the adult-onset group. However, when tested at the same age as the adult-onset rats, they displayed identical alcohol intake and preference. We found no alcohol-induced effects on anxiety- or stress-related behaviour in the experimental groups at any time points.

CONCLUSIONS

These data show that the genetically determined phenotype of high alcohol drinking of the female alcohol-preferring AA rats is not associated with a predisposition to develop anxiety-like behaviour following voluntary alcohol exposure, even when initiated during adolescence.

Adult and adolescent C57BL/6J and DBA/2J mice are differentially susceptible to fear learning deficits after acute ethanol or MK-801 treatment

Ethanol and other drugs of abuse disrupt learning and memory processes, creating problems associated with drug use and addiction. Understanding individual factors that determine susceptibility to drug-induced cognitive deficits, such as genetic background, age, and sex, is important for prevention and treatment. Comparison of adolescent and adult mice of both sexes across inbred mouse strains can reveal age, sex, and genetic contributions to phenotypes. We treated adolescent and adult, male and female, C57BL/6J and DBA/2J inbred mice with ethanol (1 g/kg or 1.5 g/kg) or MK-801 (0.05 mg/kg or 0.1 mg/kg), an NMDA receptor antagonist, prior to fear conditioning training. Contextual and cued fear retention were tested one day and eight or nine days after training. After ethanol exposure, adult C57BL/6J mice experienced greater deficits in contextual learning than adult DBA/2J mice. C57BL/6 J adolescents were less susceptible to ethanol-induced contextual learning disruptions than C57BL/6J adults, and adolescent males of both strains exhibited greater ethanol-induced contextual learning deficits than adolescent females. After MK-801 exposure, adolescent C57BL/6J mice experienced more severe contextual learning deficits than adolescent DBA/2J mice. Both ethanol and MK-801 had greater effects on contextual learning than cued learning. Collectively, we demonstrate that genetic background contributes to contextual and cued learning outcomes after ethanol or MK-801 exposure. Further, we report age-dependent drug sensitivities that are strain-, sex-, and drug-specific, suggesting that age, sex, and genetic background interact to determine contextual and cued learning impairments after ethanol or MK-801 exposure.

Sex difference in the vulnerability to hippocampus plasticity impairment after binge-like ethanol exposure in adolescent rat: Is estrogen the key?

Binge drinking during adolescence induces memory impairments, and evidences suggest that females are more vulnerable than males. However, the reason for such a difference is unclear, whereas preclinical studies addressing this question are lacking. Here we tested the hypothesis that endogenous estrogen level (E2) may explain sex differences in the effects of ethanol on hippocampus plasticity, the cellular mechanism of memory. Long-term depression (LTD) in hippocampus slice of pubertal female rats was recorded 24 h after two ethanol binges (3 g/kg, i.p., 9 h apart). Neither the estrous cycle nor ethanol altered LTD. However, if ethanol was administered during proestrus (i.e., at endogenous E2 peak), LTD was abolished 24 h later, whereas NMDA-fEPSPs response to a GluN2B antagonist increased. The abolition of LTD was not observed in adult female rats. Exogenous E2 combined with ethanol replicated LTD abolition in pubertal, prepubertal female, and in pubertal male rats without changes in ethanol metabolism. In male rats, a higher dose of ethanol was required to abolish LTD at 24-h delay. In pubertal female rats, tamoxifen, an antagonist of estrogen receptors, blocked the impairing effects of endogenous and exogenous E2 on LTD, suggesting estrogen interacts with ethanol through changes in gene expression. In addition, tamoxifen prevented LTD abolition at 24 h but not at 48-h delay. In conclusion, estrogen may explain the increased vulnerability to ethanol-induced plasticity impairment seen in females compared with males. This increased vulnerability of female rats is likely due to changes in the GluN2B subunit that represent a common target between ethanol and estrogen.

Adolescent binge-ethanol accelerates cognitive impairment and β-amyloid production and dysregulates endocannabinoid signaling in the hippocampus of APP/PSE mice

Previous research in rodents suggests that the long-term neurobehavioral disturbances induced by chronic ethanol (EtOH) exposure could be due to endocannabinoid system (ECS) alterations. Moreover, ECS failure has been proposed to mediate the cognitive impairment and β-amyloid production in Alzheimer disease (AD). Thus, in the present study, we evaluated the effects of adolescent EtOH binge drinking on the cognitive disturbances, hippocampal β-amyloid levels, and in the ECS expression on a transgenic mouse model (APP/PSEN, AZ) of AD. We exposed AZ and wild-type mice to a binge-drinking treatment during adolescence. At 6 and 12 months of age, we evaluated hippocampal-dependent learning and memory: β-amyloid concentrations and RNA and protein levels of cannabinoid type-2 receptors (CB2), diacylglycerol lipase-α (DAGLα), and monoacylglycerol lipase (MAGL) in the hippocampus. The results showed that binge-EtOH treatment worsens cognitive function and increases β-amyloid levels in AZ. At 6 months, EtOH heightens CB2 (RNA and protein) and DAGLα (RNA) expression in wild type but not in AZ. On the contrary, EtOH enhances MAGL RNA expression only in AZ. At 12 months, AZ displays increased levels of CB2 (RNA and protein) and DAGLα (protein) compared with control. Similar to what happens at 6 months, EtOH induces an increase in CB2 gene expression in wild type but not in AZ; however, it augments CB2 and DAGLα protein levels in both genotypes. Therefore, we propose that adolescent binge drinking accelerates cognitive deficits associated with aging and AD. It also accelerates hippocampal β-amyloid accumulation in AZ and affects differently the ECS response in wild type and AZ.

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

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

Dentate Gyrus Proliferative Responses After Traumatic Brain Injury and Binge Alcohol in Adult Rats

BACKGROUND

Traumatic brain injury is a significant public health issue that results in serious disability in survivors. Traumatic brain injury patients are often intoxicated with alcohol when admitted to the hospital; however, it is not clear how acute intoxication affects recovery from a traumatic brain injury. Our group has previously shown that binge alcohol prior to traumatic brain injury resulted in long-term impairment in a fine sensorimotor task that was correlated with a decreased proliferative and neuroblast response from the subventricular zone. However, whether binge alcohol prior to traumatic brain injury affects the proliferative response in the hippocampal dentate gyrus is not yet known.

METHODS

Male rats underwent binge alcohol (3 g/kg/day) by gastric gavage for 3 days prior to traumatic brain injury. Cell proliferation was labeled by BrdU injections following traumatic brain injury. Stereological quantification and immunofluorescence confocal analysis of BrdU+ cells in the hippocampal dorsal dentate gyrus was performed at 24 hours, 1 week and 6 weeks post traumatic brain injury.

RESULTS

We found that either traumatic brain injury alone or binge alcohol alone significantly increased dentate gyrus proliferation at 24 hours and 1 week. However, a combined binge alcohol and traumatic brain injury regimen resulted in decreased dentate gyrus proliferation at 24 hours post-traumatic brain injury. At the 6 week time point, binge alcohol overall reduced the number of BrdU+ cells. Furthermore, more BrdU+ cells were found in the dentate hilar region of alcohol traumatic brain injury compared to vehicle traumatic brain injury groups. The location and double-labeling of these mismigrated BrdU+ cells was consistent with hilar ectopic granule cells.

CONCLUSION

The results from this study showed that pre-traumatic brain injury binge alcohol impacts the injury-induced proliferative response in the dentate gyrus in the short-term and may affect the distribution of newly generated cells in the dentate gyrus in the long-term.

Adolescent Alcohol Exposure Produces Protracted Cognitive-Behavioral Impairments in Adult Male and Female Rats

Binge drinking is common in adolescence. Rodent studies modeling adolescent binge drinking find persistent effects on the brain's physiology, including increased expression of neuroimmune genes, impaired neurogenesis, and changes in behavioral flexibility. This study used females and males to investigate the effects of adolescent intermittent ethanol (AIE) on a battery of behaviors assessing spatial navigation using a radial arm water maze, working memory using the Hebb-Williams maze, non-spatial long-term memory using novel object recognition, and dominance using a tube dominance test. Results indicate that AIE impairs adult acquisition in spatial navigational learning with deficits predominantly driven by females. Surprisingly, AIE slowed the transition from random to serial search strategies in both sexes, suggesting AIE impairs flexibility in problem-solving processing. In the Hebb-Williams maze working memory task, adult AIE rats exhibited deficits in problem solving, resulting in more errors across the 12 maze configurations, independent of sex. Conversely, AIE decreased dominance behaviors in female rats, and at 7 months post-alcohol, female AIE rats continued to exhibit deficits in novel object recognition. These results suggest that cognitive-behavioral alterations after adolescent binge drinking persist well into middle age, despite abstinence. Future studies should focus on intervening treatment strategies in both females and males.

Repeated unpredictable stress blunts alcohol-induced memory deficit in adolescent rat

OBJECTIVE

There exists a complex interaction between alcohol and stress on brain and behavior. Alcohol and stress are both known to affect memory. Whether stress and alcohol together can modulate memory functions in adolescent rats is not known. In the present study, effects of repeated unpredictable stress (RUPS) on contextual fear conditioning, a hippocampus-related memory function, were investigated in alcohol-treated adolescent rats.

METHODS

Rats were divided into four experimental groups: group i - saline-treated non-stressed rats (sal no stress), group ii - alcohol-treated non-stressed rats (alc no stress), group iii - saline-treated rats subjected to stress (sal + RUPS), group iv - alcohol-treated rats subjected to stress (alc + RUPS). All rats were trained in the fear conditioning paradigm, and 24 h later were tested for contextual fear conditioning in the conditioning chamber, and nonspecific fear memory in a modified chamber.

RESULTS

Stress, in the presence or absence of alcohol, did not alter nonspecific fear. RUPS exposure did not affect contextual freezing in vehicle-treated adolescent rats. Compared to vehicle-treated non-stressed rats, alcohol-treated non-stressed rats showed significant impairments in contextual freezing. Alcohol-treated RUPS rats performed better in the contextual freezing task than alcohol-treated non-stressed rats.

CONCLUSION

RUPS exposure did not alter contextual fear conditioning in adolescent rats. Alcohol significantly reduced contextual fear memory in non-stressed rats. Alcohol-treated RUPS rats showed significantly better memory than alcohol-treated non-stressed rats. Together, these data suggest resiliency to stress-induced memory impairment in adolescent rats, and RUPS exposure causes blunting of alcohol's negative effects on contextual fear conditioning.

The effects of adolescent alcohol exposure on learning and related neurobiology in humans and rodents

Adolescent alcohol use is a widespread problem in the United States. In both humans and rodents, alcohol can impair learning and memory processes mediated by forebrain areas such as the prefrontal cortex (PFC) and hippocampus (HC). Adolescence is a period in which alcohol use often begins, and it is also a time that can be uniquely sensitive to the detrimental effects of alcohol. Exposure to alcohol during adolescence can cause persisting alterations in PFC and HC neurobiology that are linked to cognitive impairments, including changes in neurogenesis, inflammation, and various neurotransmitter systems in rodent models. Consistent with this, chronic adolescent alcohol exposure can cause PFC-dependent learning impairments that persist into adulthood. Deficits in adult HC-dependent learning after adolescent alcohol exposure have also been reported, but these findings are less consistent. Overall, evidence summarized in this review indicates that adolescent exposure to alcohol can produce long-term detrimental effects on forebrain-dependent cognitive processes.

Effect of Alcohol on Hippocampal-Dependent Plasticity and Behavior: Role of Glutamatergic Synaptic Transmission

Problematic alcohol drinking and alcohol dependence are an increasing health problem worldwide. Alcohol abuse is responsible for approximately 5% of the total deaths in the world, but addictive consumption of it has a substantial impact on neurological and memory disabilities throughout the population. One of the better-studied brain areas involved in cognitive functions is the hippocampus, which is also an essential brain region targeted by ethanol. Accumulated evidence in several rodent models has shown that ethanol treatment produces cognitive impairment in hippocampal-dependent tasks. These adverse effects may be related to the fact that ethanol impairs the cellular and synaptic plasticity mechanisms, including adverse changes in neuronal morphology, spine architecture, neuronal communication, and finally an increase in neuronal death. There is evidence that the damage that occurs in the different brain structures is varied according to the stage of development during which the subjects are exposed to ethanol, and even much earlier exposure to it would cause damage in the adult stage. Studies on the cellular and cognitive deficiencies produced by alcohol in the brain are needed in order to search for new strategies to reduce alcohol neuronal toxicity and to understand its consequences on memory and cognitive performance with emphasis on the crucial stages of development, including prenatal events to adulthood.

Autophagy alleviates ethanol-induced memory impairment in association with anti-apoptotic and anti-inflammatory pathways

Chronic excessive drinking leads to a wide spectrum of neurological disorders, including cognitive deficits, such as learning and memory impairment. However, the neurobiological mechanisms underlying these deleterious changes are still poorly understood. We conducted a comprehensive study to investigate the role and mechanism of autophagy in alcohol-induced memory impairment. To establish an ethanol-induced memory impairment mouse model, we allowed C57BL/6J mice intermittent access to 20% ethanol (four-bottle choice) to escalate ethanol drinking levels. Memory impairment was confirmed by a Morris water maze test. We found that mice exposed to EtOH (ethanol) and EtOH combined with the autophagy inhibitor 3-methyladenine (3-MA) showed high alcohol intake and blood alcohol concentration. We confirmed that the EtOH group exhibited notable memory impairment. Inhibition of autophagy by 3-MA worsened ethanol-induced memory impairment. Ethanol induced autophagy in the hippocampus of mice as indicated by western blotting, electron microscopy, RT-qPCR, and fluorescence confocal microscopy. We determined that the mTOR/BECN1 (S14) pathway is involved in ethanol-induced autophagy in vivo. Further, ethanol-induced autophagy suppressed the NLRP3 inflammatory and apoptosis pathways in the hippocampus in mice and in vitro. These findings suggest that autophagy activation in hippocampal cells alleviates ethanol-induced memory impairment in association with anti-apoptotic and anti-inflammatory pathways.

Regulation of NMDA Receptor Plasticity in the BNST Following Adolescent Alcohol Exposure

Persistent alterations in synaptic plasticity and neurotransmission are thought to underlie the heightened risk of adolescent-onset drinkers to develop alcohol use disorders in adulthood. The bed nucleus of the stria terminalis (BNST) is a compelling region to study the consequences of early alcohol, as it is innervated by cortical structures which undergo continued maturation during adolescence and is critically involved in stress and negative affect-associated relapse. In adult mice, chronic ethanol induces long-term changes in GluN2B-containing NMDA receptors (NMDARs) of the BNST. It remains unclear, however, whether the adolescent BNST is susceptible to such persistent alcohol-induced modifications and, if so, whether they are preserved into adulthood. We therefore examined the short- and long-term consequences of adolescent intermittent ethanol exposure (AIE) on NMDAR transmission and plasticity in the BNST of male and female mice. Whole-cell voltage clamp recordings revealed greater glutamatergic tone in the BNST of AIE-treated males and females relative to air-controls. This change, which corresponded to an increase in presynaptic glutamate release, resulted in altered postsynaptic NMDAR metaplasticity and enhanced GluN2B transmission in males but not females. Only AIE-treated males displayed upregulated GluN2B expression (determined by western blot analysis). While these changes did not persist into adulthood under basal conditions, exposing adult males (but not females) to acute restraint stress reinstated AIE-induced alterations in NMDAR metaplasticity and GluN2B function. These data demonstrate that adolescent alcohol exposure specifically modifies NMDARs in the male BNST, that the plastic changes to NMDARs are long-lasting, and that they can be engaged by stress.

Adolescent but not adult ethanol binge drinking modulates ethanol behavioral effects in mice later in life

BACKGROUND

Alcohol use disorder is a serious illness marked by uncontrollable drinking and a negative withdrawal state when not using. Alcohol is one of the most commonly used drugs among adolescent populations. Given that adolescence is a unique developmental stage during which alcohol has long-term effects on future drug-taking behavior; it is essential to understand how early exposure to ethanol during adolescence may affect the abuse liability of the drug later in life. Our studies focused on characterizing how exposure to alcohol in adolescence alters later adult alcohol dependence behaviors, by using well-established mouse models of ethanol drinking. We hypothesized that early exposure to ethanol leads to increased ethanol intake in adults and other behavioral phenotypes that may lead to dependence.

METHODS

We investigated the impact of ethanol drinking in early adolescent C57BL/6J mice using a modified Drinking in the Dark (DID) model.

RESULTS

Our results showed that exposure to ethanol during adolescence enhanced ethanol intake in adulthood in the DID, and the 2-bottle choice drinking paradigms. In contrast, adult exposure of alcohol did not enhance later alcohol intake. We also conducted tests for ethanol behavioral sensitivity such as loss of righting reflex and anxiety-related behaviors to further elucidate the relationship between adolescent ethanol exposure and enhanced ethanol intake in adult mice.

CONCLUSIONS

Overall, our results suggest that adolescence is a critical period of sensitivity and binge drinking that can lead to lasting changes in ethanol intake in adulthood. Further research will be required in order to more fully examine the neurochemical mechanisms underlying the lasting changes in adulthood.

Long-lasting effects of repeated ketamine administration in adult and adolescent rats

Initiation of ketamine use often occurs in adolescence, yet little is known about long-term consequences when use begins in this developmental period. The current experiments were designed to examine the effects of repeated exposure to ketamine in adolescence on behavior in adulthood. We examined locomotor activity, as well as cognitive function, in animals that received repeated administration of ketamine. Groups of adolescent and adult male rats were treated with ketamine (25 mg/kg) once daily for 10 days. Locomotor activity was assessed following the first injection, following 10 days of injection, and following 20 days of abstinence. Acute locomotor effects and locomotor sensitization were compared in adolescents and adults; cross-sensitization to dextromethorphan, another dissociative with abusive potential, was also examined. In a separate group of animals cognitive deficits were assessed following the 20 day abstinence period in spatial learning and novel object recognition tasks. The locomotor stimulant effect of ketamine was much greater in adolescents than adults. Animals that were repeatedly administered ketamine demonstrated locomotor sensitization immediately after the final injection. However, sensitization only persisted after the abstinence period in animals treated as adults. No cross-sensitization to dextromethorphan was evident. Ketamine failed to produce statistically significant cognitive deficits in either age group, although drug-treated adults showed a trend towards deficits in spatial learning. Repeated use of ketamine produces long-lasting neuroadaptations that may contribute to addiction. Mild lasting memory deficits may occur in adults, although further work is necessary to confirm these findings. The results extend the understanding of potential long-term consequences of ketamine use in adolescents and adults.

Differences in the persistence of spatial memory deficits induced by a chronic stressor in adolescents compared to juveniles

Adolescence is thought of as a stress-sensitive developmental period. While many studies have compared adolescent responses to stress relative to that of adults, a growing body of work has examined stress responses in juveniles. Here we investigated if a chronic stressor has a differential effect on spatial memory in rats depending on whether it occurs during adolescence or the juvenile period. Male rats were exposed to the stress hormone corticosterone (Cort) in their drinking water, a vehicle control (2.5% ethanol), or water, for 7 days before being tested on a novel Object/Place task 6 days or 6 weeks later. Exposure to Cort or ethanol at either age impaired spatial memory at the 6-day test. The ethanol induced impairment was attenuated 6 weeks later. However, rats given Cort during adolescence, but not the juvenile period, were still impaired. Together, these results suggest that adolescence is indeed a stress-sensitive period.

Binge Alcohol Exposure Causes Neurobehavioral Deficits and GSK3β Activation in the Hippocampus of Adolescent Rats

Heavy alcohol exposure causes profound damage to the adolescent brain, particularly the hippocampus, which underlie some behavioral deficits. However, the underlying molecular mechanisms remain inconclusive. The current study sought to determine whether binge alcohol exposure affects the hippocampus-related behaviors and key signaling proteins that may mediate alcohol neurotoxicity in adolescent rats. Alcohol exposure reduced the number of both NeuN-positive and doublecortin-positive cells in the hippocampus. Alcohol also induced neurodegeneration which was confirmed by ultrastructural analysis by electronic microscopy and was accompanied with the activation of microglia. Binge alcohol exposure impaired spatial learning and memory which was evaluated by the Morris water maze. However, alcohol did not alter the spontaneous locomotor activity which was determined by the open field test. GSK3β is a multi-function serine/threonine protein kinase regulating both neuronal survival and neurogenesis and plays an important role in various neurodegenerative disorders. We have previously shown that GSK3β is a key mediator of alcohol-induced neuron apoptosis in the developing brain. We showed here binge alcohol exposure caused GSK3β activation by inducing dephosphorylation at Ser9 without affecting the phosphorylation of Tyr216 in the hippocampus. Thus, GSK3β may be involved in binge alcohol exposure-induced neuronal damage to the adolescent hippocampus.

Voluntary alcohol intake after noise exposure in adolescent rats: Hippocampal-related behavioral alterations

Different physical or chemical agents, such as noise or alcohol, can induce diverse behavioral and biochemical alterations. Considering the high probability of young people to undergo consecutive or simultaneous exposures, the aim of the present work was to investigate in an animal model if noise exposure at early adolescence could induce hippocampal-related behavioral changes that might be modified after alcohol intake. Male Wistar rats (28-days-old) were exposed to noise (95-97 dB, 2 h). Afterwards, animals were allowed to voluntarily drink alcohol (10% ethanol in tap water) for three consecutive days, using the two-bottle free choice paradigm. After that, hippocampal-related memory and anxiety-like behavior tests were performed. Results show that whereas noise-exposed rats presented deficits in habituation memory, those who drank alcohol exhibited impairments in associative memory and anxiety-like behaviors. In contrast, exposure to noise followed by alcohol intake showed increases in exploratory and locomotor activities as well as in anxiety-like behaviors, unlike what was observed using each agent separately. Finally, lower levels of alcohol intake were measured in these animals when compared with those that drank alcohol and were not exposed to noise. Present findings demonstrate that exposure to physical and chemical challenges during early adolescence might induce behavioral alterations that could differ depending on the schedule used, suggesting a high vulnerability of rat developing brain to these socially relevant agents.

Transgenic mice with increased astrocyte expression of CCL2 show altered behavioral effects of alcohol

Emerging research provides strong evidence that activation of CNS glial cells occurs in neurological diseases and brain injury and results in elevated production of neuroimmune factors. These factors can contribute to pathophysiological processes that lead to altered CNS function. Recently, studies have also shown that both acute and chronic alcohol consumption can produce activation of CNS glial cells and the production of neuroimmune factors, particularly the chemokine ligand 2 (CCL2). The consequences of alcohol-induced increases in CCL2 levels in the CNS have yet to be fully elucidated. Our studies focus on the hypothesis that increased levels of CCL2 in the CNS produce neuroadaptive changes that modify the actions of alcohol on the CNS. We utilized behavioral testing in transgenic mice that express elevated levels of CCL2 to test this hypothesis. The increased level of CCL2 in the transgenic mice involves increased astrocyte expression. Transgenic mice and their non-transgenic littermate controls were subjected to one of two alcohol exposure paradigms, a two-bottle choice alcohol drinking procedure that does not produce alcohol dependence or a chronic intermittent alcohol procedure that produces alcohol dependence. Several behavioral tests were carried out including the Barnes maze, Y-maze, cued and contextual conditioned fear test, light-dark transfer, and forced swim test. Comparisons between alcohol naïve, non-dependent, and alcohol-dependent CCL2 transgenic and non-transgenic mice show that elevated levels of CCL2 in the CNS interact with alcohol in tests for alcohol drinking, spatial learning, and associative learning.

Long-Lasting Effects of Chronic Intermittent Alcohol Exposure in Adolescent Mice on Object Recognition and Hippocampal Neuronal Activity

BACKGROUND

Binge drinking is popular and highly prevalent in teenagers. However, the long-term cognitive and neurobiological consequences of such practices are not yet fully understood. In this context, we therefore assessed in mice whether a chronic intermittent alcohol (CIA) exposure in adolescence had long-term consequences on object discrimination and memory performances, emotional behaviors, brain activity, and morphology.

METHODS

C57BL/6JRj mice were treated with either saline or ethanol (EtOH) (2 g/kg/d, i.p., from postnatal days [PND] 30 to PND 44 every other day). The day following the last administration or later in adulthood (PND 71) mice were tested for different behavioral tests (novel object recognition, spontaneous alternation, light-dark box, elevated plus-maze, actimeter test), to assess object recognition, working memory performances, anxiety-like behavior, and locomotor activity. We also investigated neuronal activation of hippocampus, prefrontal and perirhinal cortices, and anatomical changes using immediate-early gene expression and longitudinal brain magnetic resonance imaging.

RESULTS

Our results showed that adolescent mice exposed to CIA present a critical and persistent impairment of short-term object recognition performances. By contrast, spatial working memory was not impaired, nor was anxiety-like behavior. This altered object discrimination was associated with a biphasic change in neuronal activity in the hippocampus but without morphological changes. Indeed, c-Fos expression was specifically increased in the dorsal dentate gyrus (DG) of the hippocampus after the binge exposure, but then became significantly lower in adulthood both in the DG and the CA1 part of the hippocampus compared with adult saline pretreated mice.

CONCLUSIONS

These findings provide evidence for adolescent vulnerability to the effects of intermittent binge EtOH exposure on object discrimination and hippocampal activity with long-lasting consequences.

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.

Neurobiology of Adolescent Substance Use Disorders

There are many facets of the neurobiology of substance use that are distinct in adolescence as compared with adulthood. The adolescent brain is subject to intense subcortical reward processes, but is left with an immature prefrontal control system that is often unable to resist the pull of potentially exciting activities like substance use, even when fully aware of the dangers involved. Peer influences serve only to magnify these effects and foster more sensation-seeking, risky behavior. The unique aspects of neurobiology should be taken into consideration when designing prevention programs and clinical interventions for adolescent substance use disorders.

Adolescent Intermittent Alcohol Exposure: Deficits in Object Recognition Memory and Forebrain Cholinergic Markers

The long-term effects of intermittent ethanol exposure during adolescence (AIE) are of intensive interest and investigation. The effects of AIE on learning and memory and the neural functions that drive them are of particular interest as clinical findings suggest enduring deficits in those cognitive domains in humans after ethanol abuse during adolescence. Although studies of such deficits after AIE hold much promise for identifying mechanisms and therapeutic interventions, the findings are sparse and inconclusive. The present results identify a specific deficit in memory function after AIE and establish a possible neural mechanism of that deficit that may be of translational significance. Male rats (starting at PND-30) received exposure to AIE (5g/kg, i.g.) or vehicle and were allowed to mature into adulthood. At PND-71, one group of animals was assessed using the spatial-temporal object recognition (stOR) test to evaluate memory function. A separate group of animals was used to assess the density of cholinergic neurons in forebrain areas Ch1-4 using immunohistochemistry. AIE exposed animals manifested deficits in the temporal component of the stOR task relative to controls, and a significant decrease in the number of ChAT labeled neurons in forebrain areas Ch1-4. These findings add to the growing literature indicating long-lasting neural and behavioral effects of AIE that persist into adulthood and indicate that memory-related deficits after AIE depend upon the tasks employed, and possibly their degree of complexity. Finally, the parallel finding of diminished cholinergic neuron density suggests a possible mechanism underlying the effects of AIE on memory and hippocampal function as well as possible therapeutic or preventive strategies for AIE.

Spatial learning in men undergoing alcohol detoxification

Alcohol dependence is a major public health problem worldwide. Brain and behavioral disruptions including changes in cognitive abilities are common features of alcohol addiction. Thus, the present study was aimed to investigate spatial learning and memory in 29 alcoholic men undergoing alcohol detoxification by using a virtual Morris maze task. As age-matched controls we recruited 29 men among occasional drinkers without history of alcohol dependence and/or alcohol related diseases and with a negative blood alcohol level at the time of testing. We found that the responses to the virtual Morris maze are impaired in men undergoing alcohol detoxification. Notably they showed increased latencies in the first movement during the trials, increased latencies in retrieving the hidden platform and increased latencies in reaching the visible platform. These findings were associated with reduced swimming time in the target quadrant of the pool where the platform had been during the 4 hidden platform trials of the learning phase compared to controls. Such increasing latency responses may suggest motor control, attentional and motivational deficits due to alcohol detoxification.

Adolescent intermittent alcohol exposure: persistence of structural and functional hippocampal abnormalities into adulthood

BACKGROUND

Human adolescence is a crucial stage of neurological development during which ethanol (EtOH) consumption is often at its highest. Alcohol abuse during adolescence may render individuals at heightened risk for subsequent alcohol abuse disorders, cognitive dysfunction, or other neurological impairments by irreversibly altering long-term brain function. To test this possibility, we modeled adolescent alcohol abuse (i.e., intermittent EtOH exposure during adolescence [AIE]) in rats to determine whether adolescent exposure to alcohol leads to long-term structural and functional changes that are manifested in adult neuronal circuitry.

METHODS

We specifically focused on hippocampal area CA1, a brain region associated with learning and memory. Using electrophysiological, immunohistochemical, and neuroanatomical approaches, we measured post-AIE changes in synaptic plasticity, dendritic spine morphology, and synaptic structure in adulthood.

RESULTS

We found that AIE-pretreated adult rats manifest robust long-term potentiation, induced at stimulus intensities lower than those required in controls, suggesting a state of enhanced synaptic plasticity. Moreover, AIE resulted in an increased number of dendritic spines with characteristics typical of immaturity. Immunohistochemistry-based analysis of synaptic structures indicated a significant decrease in the number of co-localized pre- and postsynaptic puncta. This decrease is driven by an overall decrease in 2 postsynaptic density proteins, PSD-95 and SAP102.

CONCLUSIONS

Taken together, these findings reveal that repeated alcohol exposure during adolescence results in enduring structural and functional abnormalities in the hippocampus. These synaptic changes in the hippocampal circuits may help to explain learning-related behavioral changes in adult animals preexposed to AIE.

Alcohol use across the lifespan: An analysis of adolescent and aged rodents and humans

Adolescence and old age are unique periods of the lifespan characterized by differential sensitivity to the effects of alcohol. Adolescents and the elderly appear to be more vulnerable to many of alcohol's physiological and behavioral effects compared to adults. The current review explores the differential effects of acute alcohol, predominantly in terms of motor function and cognition, in adolescent and aged humans and rodents. Adolescents are less sensitive to the sedative-hypnotic, anxiolytic, and motor-impairing effects of acute alcohol, but research results are less consistent as it relates to alcohol's effects on cognition. Specifically, previous research has shown adolescents to be more, less, and similarly sensitive to alcohol-induced cognitive deficits compared to adults. These equivocal findings suggest that learning acquisition may be differentially affected by ethanol compared to memory, or that ethanol-induced cognitive deficits are task-dependent. Older rodents appear to be particularly vulnerable to the motor- and cognitive-impairing effects of acute alcohol relative to younger adults. Given that alcohol consumption and abuse is prevalent throughout the lifespan, it is important to recognize age-related differences in response to acute and long-term alcohol. Unfortunately, diagnostic measures and treatment options for alcohol dependence are rarely dedicated to adolescent and aging populations. As discussed, although much scientific advancement has been made regarding the differential effects of alcohol between adolescents and adults, research with the aged is underrepresented. Future researchers should be aware that adolescents and the aged are uniquely affected by alcohol and should continue to investigate alcohol's effects at different stages of maturation.

Sex differences in spatial navigation and perception in human adolescents and emerging adults

Males typically outperform females on spatial tasks, beginning early in life and continuing into adulthood. This study aimed to characterize age and sex differences in human spatial ability using a virtual Water Maze Task (vWMT), which is based on the classic Morris water maze spatial navigation task used in rodents. Performance on the vWMT and on a task assessing visuospatial perception, Mental Rotations Test (MRT), was examined in 33 adolescents and 39 emerging adults. For the vWMT, significant effects of age and sex were observed for path length in the target region (narrower spatial sampling), and heading error, with emerging adults performing better than adolescents, and an overall male advantage. For the MRT, males scored higher than females, but only in emerging adulthood. Overall, sex differences in visuospatial perception (MRT) emerge differently from those observed on a classic navigation task, with age and sex-specific superior vWMT performance likely related to the use of more efficient strategies. Importantly, these results extend the developmental timeline of spatial ability characterization to include adolescent males and females performing a virtual version of the classic vWMT.

Chronic periadolescent alcohol consumption produces persistent cognitive deficits in rhesus macaques

Although human alcoholics exhibit lasting cognitive deficits, it can be difficult to definitively rule out pre-alcohol performance differences. For example, individuals with a family history of alcoholism are at increased risk for alcoholism and are also behaviorally impaired. Animal models of controlled alcohol exposure permit balanced group assignment, thereby ruling out the effects of pre-existing differences. Periadolescent male rhesus macaques (N = 5) consumed alcohol during 200 drinking sessions (M-F) across a 10-month period (mean daily alcohol consumption: 1.38 g/kg/day). A control group (N = 5) consumed a fruit-flavored vehicle during the same period. Spatial working memory, visual discrimination learning and retention and response time behavioral domains were assessed with subtests of the Monkey CANTAB (CAmbridge Neuropsychological Test Automated Battery). Spatial working memory performance was impaired in the alcohol group after 120 drinking sessions (6 mo) in a manner that depended on retention interval. The chronic alcohol animals were also impaired in retaining a visual discrimination over 24 hrs when assessed 6-8 weeks after cessation of alcohol drinking. Finally, the presentation of distractors in the response time task impaired the response time and accuracy of the chronic alcohol group more than controls after 6 months of alcohol cessation. Chronic alcohol consumption over as little as 6 months produces cognitive deficits, with some domains still affected after acute (6-8 wks) and lasting (6 mo) discontinuation from drinking. Animals were matched on alcohol preference and behavioral performance prior to exposure, thus providing strong evidence for the causal role of chronic alcohol in these deficits.

Adolescent drinking and motivated decision-making: a cotwin-control investigation with monozygotic twins

The present study used a monozygotic (MZ) cotwin-control (CTC) design to investigate associations between alcohol use and performance on the Iowa gambling task (IGT) in a sample of 96 adolescents (half female). The MZ CTC design is well suited to shed light on whether poor decision-making, as reflected on IGT performance, predisposes individuals to abuse substances or is a consequence of use. Participants completed structural MRI scans as well, from which we derived gray matter volumes for cortical and subcortical regions involved in IGT performance and reduced in adolescents with problematic alcohol use. Drinking was associated with poorer task performance and with reduced volume of the left lateral orbital-frontal cortex. CTC analyses indicated that the former was due to differences between members of twin pairs in alcohol use (suggesting a causal effect of alcohol), whereas the latter was due to factors shared by twins (consistent with a pre-existing vulnerability for use). Although these preliminary findings warrant replication, they suggest that normative levels of alcohol use may diminish the quality of adolescent decision-making and thus have potentially important public health implications.

Persistent loss of hippocampal neurogenesis and increased cell death following adolescent, but not adult, chronic ethanol exposure

Although adolescence is a common age to initiate alcohol consumption, the long-term consequences of exposure to alcohol at this time of considerable brain maturation are largely unknown. In studies utilizing rodents, behavioral evidence is beginning to emerge suggesting that the hippocampus may be persistently affected by repeated ethanol exposure during adolescence, but not by comparable alcohol exposure in adulthood. The purpose of this series of experiments was to explore a potential mechanism of hippocampal dysfunction in adults exposed to ethanol during adolescence. Given that disruption in adult neurogenesis has been reported to impair performance on tasks thought to be hippocampally related, we used immunohistochemistry to assess levels of doublecortin (DCX), an endogenous marker of immature neurons, in the dentate gyrus (DG) of the hippocampus 3-4 weeks after adolescent (postnatal day, PD28-48) or adult (PD70-90) intermittent ethanol exposure to 4 g/kg ethanol administered intragastrically. We also investigated another neurogenic niche, the subventricular zone (SVZ), to determine if the effects of ethanol exposure were region specific. Levels of cell proliferation and cell death were also examined in the DG via assessing Ki67 and cleaved caspase-3 immunoreactivity, respectively. Significantly less DCX was observed in the DG of adolescent (but not adult) ethanol-exposed animals about 4 weeks after exposure when these animals were compared to control age-mates. The effects of adolescent ethanol on DCX immunoreactivity were specific to the hippocampus, with no significant exposure effects emerging in the SVZ. In both the DG and the SVZ there was a significant age-related decline in neurogenesis as indexed by DCX. The persistent effect of adolescent ethanol exposure on reduced DCX in the DG appears to be related to significant increases in cell death, with significantly more cleaved caspase-3-positive immunoreactivity observed in the adolescent ethanol group compared to controls, but no alterations in cell proliferation when indexed by Ki67. These results suggest that a history of adolescent ethanol exposure results in lowered levels of differentiating neurons, probably due at least in part to increased cell death of immature neurons. These effects were evident in adulthood, weeks following termination of the chronic exposure, and may contribute to previously reported behavioral deficits on hippocampal-related tasks after chronic ethanol exposure in adolescence.

Effect of sub-chronic intermittent ethanol exposure on spatial learning and ethanol sensitivity in adolescent and adult rats

It has become clear that adolescence is a period of distinct responsiveness to the acute effects of ethanol on learning and other cognitive functions. However, the effects of repeated intermittent ethanol exposure during adolescence on learning and cognition are less well studied, and other effects of repeated ethanol exposure such as withdrawal and chronic tolerance complicate such experiments. Moreover, few studies have compared the effects of repeated ethanol exposure during adolescence and adulthood, and they have yielded mixed outcomes that may be related to methodological differences and/or secondary effects of ethanol on behavioral performance. One emerging question is whether relatively brief intermittent ethanol exposure (i.e., sub-chronic exposure) during adolescence or adulthood might alter learning at a time after exposure when chronic tolerance would be expected, and whether tolerance to the cognitive effects of ethanol might influence the effect of ethanol on learning at that time. To address this, male adolescent and adult rats were pre-treated with sub-chronic daily ethanol (five doses [4.0 g/kg, i.p.] or saline at 24-h intervals, across 5 days). Two days after the last pre-exposure, spatial learning was assessed on 4 consecutive days using the Morris water maze. Half of the animals from each treatment cell received ethanol (2.0 g/kg, i.p.) 30 min prior to each testing session and half of the animals received saline. Ethanol pre-exposure altered water maze performance in adult animals but not in adolescents, and acute ethanol exposure impaired learning in animals of both ages independent of pre-exposure condition. There was no evidence of cognitive tolerance in animals of either age group. These results indicate that a relatively short period of intermittent ethanol exposure during adulthood, but not adolescence, promotes thigmotaxis in the water maze shortly after pre-exposure but does not induce cognitive tolerance to the effects of ethanol in either age group.

Adolescent alcohol exposure and persistence of adolescent-typical phenotypes into adulthood: a mini-review

Alcohol use is typically initiated during adolescence, which, along with young adulthood, is a vulnerable period for the onset of high-risk drinking and alcohol abuse. Given across-species commonalities in certain fundamental neurobehavioral characteristics of adolescence, studies in laboratory animals such as the rat have proved useful to assess persisting consequences of repeated alcohol exposure. Despite limited research to date, reports of long-lasting effects of adolescent ethanol exposure are emerging, along with certain common themes. One repeated finding is that adolescent exposure to ethanol sometimes results in the persistence of adolescent-typical phenotypes into adulthood. Instances of adolescent-like persistence have been seen in terms of baseline behavioral, cognitive, electrophysiological and neuroanatomical characteristics, along with the retention of adolescent-typical sensitivities to acute ethanol challenge. These effects are generally not observed after comparable ethanol exposure in adulthood. Persistence of adolescent-typical phenotypes is not always evident, and may be related to regionally specific ethanol influences on the interplay between CNS excitation and inhibition critical for the timing of neuroplasticity.

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.

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.

Proteomic approaches and identification of novel therapeutic targets for alcoholism

Recent studies have shown that gene regulation is far more complex than previously believed and does not completely explain changes at the protein level. Therefore, the direct study of the proteome, considerably different in both complexity and dynamicity to the genome/transcriptome, has provided unique insights to an increasing number of researchers. During the past decade, extraordinary advances in proteomic techniques have changed the way we can analyze the composition, regulation, and function of protein complexes and pathways underlying altered neurobiological conditions. When combined with complementary approaches, these advances provide the contextual information for decoding large data sets into meaningful biologically adaptive processes. Neuroproteomics offers potential breakthroughs in the field of alcohol research by leading to a deeper understanding of how alcohol globally affects protein structure, function, interactions, and networks. The wealth of information gained from these advances can help pinpoint relevant biomarkers for early diagnosis and improved prognosis of alcoholism and identify future pharmacological targets for the treatment of this addiction.

Tone conditioning potentiates rather than overshadows context fear in adult animals following adolescent ethanol exposure

We have shown that adults exposed to ethanol during adolescence exhibit a deficit in the retention of context fear, reminiscent of that normally seen in preweanling rats. However, preweanlings have been reported to exhibit a potentiation of context fear when they are conditioned in the presence of a tone. Therefore, this study examined context retention 24 hr after tone or context conditioning in male Sprague-Dawley rats exposed intragastrically to 4 g/kg ethanol or water every 48 hr (total of 11 exposures) during adolescence [Postnatal day (P) 28-48] or adulthood (P70-90). Approximately 3 weeks following exposure, retention of fear to the context in animals exposed to ethanol during adolescence was attenuated after context conditioning, but enhanced after tone conditioning. Comparable adult ethanol exposure groups showed typical overshadowing of context fear retention after tone conditioning. These data suggest that adolescent ethanol exposure may induce an immature pattern of cognitive processing.

Neuroimmune activation and myelin changes in adolescent rats exposed to high-dose alcohol and associated cognitive dysfunction: a review with reference to human adolescent drinking

AIMS

The aim of the study was to assess whether intermittent ethanol administration to adolescent rats activates innate immune response and TLRs signalling causing myelin disruption and long-term cognitive and behavioural deficits.

METHODS

We used a rat model of intermittent binge-like ethanol exposure during adolescence.

RESULTS

Binge-like ethanol administration to adolescent rats increased the gene expression of TLR4 and TLR2 in the prefrontal cortex (PFC), as well as inflammatory cytokines TNFα and IL-1β. Up-regulation of TLRs and inflammatory mediators were linked with alterations in the levels of several myelin proteins in the PFC of adolescent rats. These events were associated with previously reported long-term cognitive dysfunctions. Conversely, the same ethanol treatment did not cause significant changes in either inflammatory mediators or myelin changes in the brain of adult rats.

CONCLUSION

Activation of innate immune receptors TLRs in the PFC appears to be involved in the neuroinflammation and demyelination processes induced by ethanol exposure during adolescence. The findings support the vulnerability of the juvenile brain to the effects of ethanol and the long-term cognitive consequences of binge drinking. In addition, ethanol-induced PFC dysfunctions might underlie the propensity of adolescents for impulsivity and to ignore the negative consequences of their behaviour, both of which could increase the risk of substance abuse.