The Biology Behind Alcohol-Induced Blackouts

A person who drinks too much alcohol may be able to perform complicated tasks, such as dancing, carrying on a conversation or even driving a car, but later have no memory of those escapades. These periods of amnesia, commonly known as “blackouts,” can last from a few minutes to several hours.

Now, at Washington University School of Medicine in St. Louis, neuroscientists have identified the brain cells involved in blackouts and the molecular mechanism that appears to underlie them. They report July 6, 2011, in The Journal of Neuroscience, that exposure to large amounts of alcohol does not necessarily kill brain cells as once was thought. Rather, alcohol interferes with key receptors in the brain, which in turn manufacture steroids that inhibit long-term potentiation (LTP), a process that strengthens the connections between neurons and is crucial to learning and memory.

Better understanding of what occurs when memory formation is inhibited by alcohol exposure could lead to strategies to improve memory.

“The mechanism involves NMDA receptors that transmit glutamate, which carries signals between neurons,” says Yukitoshi Izumi, MD, PhD, research professor of psychiatry at Washington University School of Medicine in St. Louis. “An NMDA receptor is like a double-edged sword because too much activity and too little can be toxic. We’ve found that exposure to alcohol inhibits some receptors and later activates others, causing neurons to manufacture steroids that inhibit LTP and memory formation.”

Izumi says the various receptors involved in the cascade interfere with synaptic plasticity in the brain’s hippocampus, which is known to be important in cognitive function. Just as plastic bends and can be molded into different shapes, synaptic plasticity is a term scientists use to describe the changeable properties of synapses, the sites where nerve cells connect and communicate. LTP is the synaptic mechanism that underlies memory formation.

The brain cells affected by alcohol are found in the hippocampus and other brain structures involved in advanced cognitive functions. Izumi and first author Kazuhiro Tokuda, MD, research instructor of psychiatry, studied slices of the hippocampus from the rat brain.

When they treated hippocampal cells with moderate amounts of alcohol, LTP was unaffected, but exposing the cells to large amounts of alcohol inhibited the memory formation mechanism.

“It takes a lot of alcohol to block LTP and memory,” says senior investigator Charles F. Zorumski, MD, the Samuel B. Guze Professor and head of the Department of Psychiatry. “But the mechanism isn’t straightforward. The alcohol triggers these receptors to behave in seemingly contradictory ways, and that’s what actually blocks the neural signals that create memories. It also may explain why individuals who get highly intoxicated don’t remember what they did the night before.”

But not all NMDA receptors are blocked by alcohol. Instead, their activity is cut roughly in half.