Question
What is the chemical in the brain that causes blackouts when one consumes alcohol?

Answer
Umm... I think it's called alcohol! Alcohol is an outright poison that crosses the body's own safety barrier between the blood circulation system and the nutrient-rich, delicately balanced fluid that bathes nervous tissue in the brain (white and grey cells). When alcohol gets into the brain, it damages brain tissue, affects the chemical and electrical balance of brain cells (neurones), and stops the cells from doing their job of passing on electrical charges. If enough neurones stop working, especially cells in the deep parts of your brain that control body co-ordination and levels of consciousness, you'll black out. Not all at once: you'll get a warning first as problems develop with co-ordination (including loss of balance, difficulty in walking, and slurred speech) and in levels of consciousness (stupor or sometimes irritability).

So the blackout doesn't happen without warning that toxicity is rising. In really severe overdoses, the person can die because cells controlling respiration are affected, they stop breathing or inhale their own vomit.

Such an overdose is more likely to happen in someone who is not used to drinking and especially a young person who is not fully grown. A seasoned alcohol abuser may suffer chronic brain damage and is more likely to die of organ damage or internal bleeding than from overdose.

People who drink heavily can also experience "memory blackouts" - they might not lose consciousness, but find that they are completely unable to recall what took place while they were intoxicated (another word for poisoned). That's because the part of the brain that manages short term memory and transfers it to long term memory is particularly sensitive to the toxic effects of alcohol. Memory blackouts are an early-warning sign that drinking is getting out of control.

That's a basic layman's view. Perhaps a biochemist could give you a more sophisticated picture of what actually happens in a brain cell when it comes into contact with alcohol; they could also explain the technical mechanisms by which the activity of healthy brain chemicals (the neurotransmitters) are affected by the presence of alcohol in the cerebrospinal fluid. Neurotransmitters are knocked out by alcohol, not only in the short term but also longer term - and this contributes to the mood and sleep disturbances that often form the first warnings of serious alcohol abuse.