Voluntary alcohol access during adolescence/early adulthood, but not during adulthood, causes faster omission contingency learning

The impact of age on olfactory alcohol cue-reactivity: A functional magnetic resonance imaging study in adolescent and adult male drinkers

BACKGROUND

Adolescence is marked not only by rapid surges in the prevalence of alcohol use disorders (AUDs) but also by remarkable recovery rates, as most adolescent-onset AUDs naturally resolve over time. Little is known about the differential vulnerability of adolescents and adults. Therefore, this study aimed to unravel the moderating role of age by comparing neural alcohol cue-reactivity, an important AUD biomarker, between low-to-high beer-drinking adolescent (n = 50, 16 to 18 years), and adult (n = 51, 30 to 35 years) males matched on drinking severity.

METHODS

Associations between beer odor-induced brain activity and AUD diagnosis, severity of alcohol use-related problems, recent alcohol use, binge-drinking frequency, and task-induced craving were investigated across and between age groups in regions of interest thought to be central in alcohol cue-reactivity: the medial prefrontal cortex, anterior cingulate cortex, and striatal subregions (nucleus accumbens and caudate putamen). These analyses were complemented by exploratory whole-brain analyses.

RESULTS

Pre-task beer craving increased pre-to-post task in adolescents only. Individual differences in alcohol use, binge drinking, and craving did not relate to beer odor-induced activity. Although region-of-interest analyses did not reach significance, whole-brain analyses showed that adolescents with AUD, compared with adolescents without AUD and adults with AUD, had higher beer odor-induced activity in a large mesocorticolimbic cluster encompassing the right caudate, nucleus accumbens, orbitofrontal cortex, and the olfactory sulcus. Activity in the right caudate and putamen was positively associated with the severity of alcohol use-related problems in adolescents but negatively associated in adults.

CONCLUSION

These findings suggest a differential role of alcohol cue-reactivity in adolescents compared with adults with AUD and highlight the need for further studies investigating the role of age in the fundamental processes underlying the development of and recovery from of AUD.

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.

The effects of voluntary adolescent alcohol consumption on alcohol taste reactivity in Long Evans rats

RATIONALE

The relationship between age, ethanol intake, and the hedonic value of ethanol is key to understanding the motivation to consume ethanol.

OBJECTIVE

It is uncertain whether ethanol drinking during adolescence changes ethanol's hedonic value into adulthood.

METHODS

The current study compared voluntary intermittent ethanol consumption (IAE; 2-bottle choice; 20%v/v) among adolescent and adult Long-Evans rats to examine the effects of age and IAE on taste reactivity in adulthood. For taste reactivity, orally infused fluids included water, ethanol (5, 20, and 40%v/v), and sucrose (0.01, 0.1, 1M).

RESULTS

IAE results indicate that adolescents drank more ethanol during IAE but had a lower rate of change in ethanol consumption across time than adults due to initially high adolescent drinking. During taste reactivity testing for ethanol, IAE rats had greater hedonic responding, less aversive responding, and a more positive relationship between hedonic responses and ethanol concentration than water-receiving control rats. Hedonic responses had positive, while aversive responses had negative relationships with ethanol concentration and total ethanol consumed during IAE. Adolescent+IAE rats displayed less hedonic and more aversive responses to ethanol than Adult+IAE rats. Sucrose responding was unrelated to ethanol consumption.

CONCLUSIONS

These results suggest that ethanol consumption influences the future hedonic and aversive value of ethanol in a way that makes ethanol more palatable with greater prior consumption. However, it appears that those drinking ethanol as adolescents may be more resistant to this palatability shift than those first drinking as adults, suggesting different mechanisms of vulnerability to consumption escalation for adolescents and adults.

Alcohol Consumption during Adulthood Does Not Impair Later Go/No-Go Reversal Learning in Male Rats

Reversal learning tasks are used to model flexible decision-making in laboratory animals, and exposure to drugs of abuse can cause long-term impairments in reversal learning. However, the long-term effects of alcohol on reversal learning have varied. We evaluated whether six weeks of voluntary alcohol consumption through chronic intermittent alcohol access (elevated by food restriction) in adult male rats would impair rats in a go/no-go reversal learning task when tested at an interval beyond acute withdrawal. In our go/no-go task, rats were reinforced for pressing one lever or withholding from pressing another lever, and the identities of the two levers were switched twice (once rats reached an accuracy criterion). We found no evidence that prior alcohol consumption altered discrimination or reversal learning in our task. This replicates previous patterns from our laboratory that higher alcohol consumption in food-restricted rats did not impair discrimination or reversal learning in a different go/no-go task and that alcohol consumption in free-fed adolescent/early adult rats did not impair go/no-go discrimination or reversal learning in the same task. It is unclear whether this represents an insensitivity of this task to alcohol exposure generally or whether an alcohol exposure procedure that leads to higher blood ethanol concentration (BEC) levels would impair learning. More research is needed to investigate these possibilities.

Pre-training naltrexone increases conditioned fear learning independent of adolescent alcohol consumption history

Our previous research has shown a relationship between low voluntary alcohol consumption and high conditioned fear in male Long Evans rats. Here, we examined whether differences in the endogenous opioid systems might be responsible for these differences. Rats received 6 weeks of chronic intermittent to 20% alcohol (v/v) or water-only from PND 26-66. Based on their consumption during the last 2 weeks of alcohol access, the alcohol-access rats were divided into high drinking (>2.5 g/kg/24-h) or low drinking (<2 g/kg/24-h). Rats were then given injections of the preferential mu opioid receptor antagonist naltrexone (1 mg/kg, s.c.) or the selective kappa opioid receptor antagonist LY2456302 (10 mg/kg, s.c.) prior to fear conditioning and were then tested for conditioned fear 2 days later. Pre-training naltrexone increased conditioned suppression of lever-pressing during training and testing, with no differences between high versus low alcohol drinkers or between water-only versus alcohol access groups (averaged across drinking levels). There was no effect of LY2456302 on conditioned fear in any comparison. We also found no differences between high and low alcohol drinkers and no reliable effect of prior alcohol access (averaged across drinking levels) on conditioned fear. Our experiment replicates and extends previous demonstrations that a preferential MOR antagonist can increase fear learning using conditioned suppression of lever-pressing as a fear measure. However, additional research is needed to determine the cause of the differences in conditioned fear that we previously observed (as they were not observed in the current experiments).

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

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

Adolescent neurodevelopment and substance use: Receptor expression and behavioral consequences

Adolescence is the transitional period between childhood and adulthood, during which extensive brain development occurs. Since this period also overlaps with the initiation of drug use, it is important to consider how substance use during this time might produce long-term neurobiological alterations, especially against the backdrop of developmental changes in neurotransmission. Alcohol, cannabis, nicotine, and opioids all produce marked changes in the expression and function of the neurotransmitter and receptor systems with which they interact. These acute and chronic alterations also contribute to behavioral consequences ranging from increased addiction risk to cognitive or neuropsychiatric behavioral dysfunctions. The current review provides an in-depth overview and update of the developmental changes in neurotransmission during adolescence, as well as the impact of drug exposure during this neurodevelopmental window. While most of these factors have been studied in animal models, which are the focus of this review, future longitudinal studies in humans that assess neural function and behavior will help to confirm pre-clinical findings. Furthermore, the neural changes induced by each drug should also be considered in the context of other contributing factors, such as sex. Further understanding of these consequences can help in the identification of novel approaches for preventing and reversing the neurobiological effects of adolescent substance use.