Drunken monkeys reveal how binge-drinking harms the adolescent brain

Most of us will be all too familiar with the consequences of night of heavy drinking. But alcohol’s effects on our heads go well beyond a mere hangover. The brain suffers too. A penchant for incoherent slurring aside, alcohol abusers tend to show problems with their spatial skills, short-term memory, impulse control and ability to make decisions or prioritise tasks. Many of these skills are heavily influenced by a part of the brain called the hippocampus. Now, Michael Taffe and researchers from the Scripps Research Institute have shown how binge-drinking during adolescence can cause lasting damage to this vital area.

The hippocampus is one of only two parts of the brain that clearly produces new neurons throughout adult life. While other areas must make do with the set they had at birth, the hippocampus continually churns out a fresh supply. This process may be important for learning and memory but it’s seriously hampered by alcohol. Taffe found that not only does heavy boozing kill off the hippocampus’s neurons, it also weakens its ability to produce reinforcements.

Much like natural history documentaries and martial arts, Taffe’s research was inspired by the antics of drunken monkeys. Taffe gave seven adolescent rhesus macaques a tangy citrus alcoholic drink, which increased in strength from 1% to 6% alcohol over 40 days. Having established their alcohol preferences, he allowed four monkeys to stick with the strong cocktail for an hour a day over the next 11 months. The other three went back to a non-alcoholic version of the tangy beverage. For the study’s final two months, all of the monkeys went tee-total.

The blood alcohol limits of the four binge-drinkers clearly showed that they were knocking back their tipples. If they’d been humans, they would probably have been drunk, and certainly well over the legal limit for driving. And their brains revealed more worrying signs of damage.

Chitra Mandyam, who collaborated on the study, found that regular chugs of the demon drink severely slashed the numbers of neural stem cells in the monkeys’ hippocampi. These are the cells responsible for churning out fresh neurons. With alcohol cutting their numbers and compromising their ability to divide into more mature cell types, the monkeys’ production of hippocampal neurons more than halved in the course of 11 months.

Even after two months of complete abstinence, Taffe found that each monkey’s hippocampus had fewer traces of fresh, immature neurons. Worse still, he found signs that the existing supply had started to degenerate. By comparison, the tee-total trio had a healthy turnover of new hippocampal neurons and no detectable sign of neural death.

Studies with rats and mice have hinted at the same effect, but monkeys provide a far deeper understanding of the alcoholic brain. They’re incredibly similar to us, not just in terms of their mental skills, but also in the way that their hippocampuses produce new neurons, their lengthier window of adolescence, and the fact that they’ll happily drink alcohol to the point of drunkenness.

If the same thing happens in humans, it suggests that alcoholism starts to wreak damage in the brain after a relatively short amount of time. It starts to kill off the hippocampus’s neurons while nixing its ability to make more. This double-whammy could explain many of the mental problems that regular binge-drinkers experience. Most intriguingly of all, the turnover of neurons in the hippocampus affects our learning and memory skill, and Taffe suggests that problems with this process could help to explain alcohol’s addictive side.

Reference: PNAS http://dx.doi.org/10.1073/pnas.0912810107

Photo by Pauk

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