Binge-Drinking Study Shows a Dramatic Effect of Alcohol on Teen Monkeys

The teenage years might be the worst time to experiment with booze.

Teen brains by nature go through a huge amount of upheaval, but adding alcohol or marijuana into the equation makes a messy situation even worse. Though binge drinking may feel especially alluring during adolescence, neuroscientists writing in eNeuro now report that the teenage years may be the worst possible time to start experimenting.

Neuroscientists have already uncovered evidence showing that binge drinking can worsen performance of neurons in the prefrontal cortex, which impair memory in teens. That study, released in 2018, was done in mice; the new study, released Monday, was done in monkeys, whose brains behave a lot more like human ones.

In the paper, Tatiana Shnitko, Ph.D., and Christopher Kroenke, Ph.D., both at Oregon Health and Science University, show that binge drinking changes the way that the brain reorganizes itself during adolescence. The effects appear to be long-lasting: These disorganized circuits may form the “neurodevelopmental underpinnings” of alcohol abuse in adulthood, they write.

Heavy drinking during adolescence impacts the way that the brain develops. 

Unsplash / Drew Farwell

As it approaches adulthood, the teenage brain goes through several important volume changes that can be influenced by certain kinds of behavior. The brain starts off bigger than it needs to be, and it naturally gets smaller as it goes through adolescence. Specifically, grey matter volume — mostly neurons — declines as the brain prunes unnecessary connections to make more efficient circuitry. We already know that substance use can impact how grey matter develops. For instance, one of the negative impacts of marijuana on teen brains is that even small amounts lead to increases in grey matter that should be pruned by the brain.

At the same time, another type of brain material called white matter, generally increases during the teenage years and even into early adulthood. White matter is the pale, fatty tissue of the brain and central nervous system that consists of a substance called myelin, which is crucial for neurons to relay messages to each other. The new study on 77 adolescent macaques, which took place over a year, showed that heavy drinking caused that white matter to grow more slowly than it should have as the monkeys aged.

During the year-long study period, 55 of the macaques were allowed to drink as much of a water-alcohol mixture as they wanted for 22 hours per day. The 26 monkeys who became “heavy drinkers” drank nearly twice as much as the non-heavy drinkers, consuming 3 grams of alcohol per kilogram of body weight.

While all of the adolescent monkeys saw increases in the white matter in their brains, the heavy drinkers had only half as much white matter growth as the control group: The control animals had an average in crease in white matter of 4.7 percent, whereas the monkeys who became heavy drinkers had an increase of only 2.6 percent.

More importantly, the authors reported a similar dynamic in the growth of the subcortical thalamus, a brain region where dysfunction is associated with alcohol misuse. Control animals saw increases of about five percent increase in subcortical thalamus growth on average. But the heavy drinkers saw increase of only 1.8 percent over the same time period, suggesting that excessive alcohol use significantly slowed that rate of growth.

Taken together, the authors suggest that alcohol fundamentally changes the way that the brain develops. Altered development, in turn, may lie behind patterns of alcohol abuse later on; for example, previous studies have shown that one of the primary risk factors for developing an alcohol use disorder is drinking at a young age

Of course, a study like this one doesn’t account for the social dynamics that might account for heavy drinking and the role of genetics and is, notably, only directly applicable to monkeys for now.

However, monkey brains, like human ones, go through a significant period of reorganization during adolescence that probably shouldn’t be messed with, and so the data suggest that binge-drinking teens are more at risk for alcohol abuse down the road than they realize. They may sidestep the hangover pain that comes with adulthood, but they may not escape the longer-lasting effects of alcohol on the brain.

The transition from adolescence to adulthood is associated with brain remodeling in the final stages of developmental growth. It is also a period when a large proportion of this age group engages to binge (occasional consumption of 4-5 drinks leading to intoxication) and heavy (binge drinking on ≥5 days in a month) alcohol drinking. Here we report on magnetic resonance imaging of developmental changes in the brain occurring during late adolescence and early adulthood (3.5-7.5 years) in a rhesus macaque model of alcohol self-administration. Monkeys were imaged prior to alcohol exposure, and following ∼6 and ∼12 months of daily (22 hr/day) access to ethanol and water. The results revealed that the brain volume increases by 1 ml per 1.87 years throughout the late adolescence and early adulthood in controls. Heavy alcohol reduced the rate of brain growth by 0.25 ml/year per 1 g/kg of daily ethanol. Cortical volume increased throughout this period with no significant effect of alcohol drinking on the cortical growth rate. In subcortical regions, age-dependent increases in the volumes of globus pallidus, thalamus, brainstem and cerebellum were observed. Heavy drinking attenuated the growth rate of the thalamus. Thus, developmental brain volume changes in the span of late adolescence to young adulthood in macaques is altered by excessive alcohol, an insult that may be linked to the continuation of heavy drinking throughout later adult life.

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