Down syndrome afflicts over 350,000 people in the U.S. In this disorder, chromosome 21 is present in three copies instead of two.

Scientists gave low doses of a human drug to mice bred to mimic the learning and memory problems in people with Down syndrome. Mice in the study had a duplicated piece of chromosome 16 and as well as the Down syndrome sufferers, these animals had malformed facial bones and problems forming new memories.

Researchers tested the drug, pentylenetetrazole (PTZ) and two other compounds—picrotoxin and a gingko biloba extract called bilobalide as they all interfere with tiny ion channels on brain cells (neurons). When activated, these channels, known as GABAA receptors, inhibit the cells, which makes it harder for them to form new synapses, or connections, with neighboring neurons inhibiting normal memory and learning functions. It is thought that the deficits of Down syndrome occur because the brain contains too many such inhibitory signals.

The researchers gave the mice either low doses of PTZ mixed with milk, or low-dose injections of picrotoxin or bilobalide, daily for two to four weeks. After the treatment, the mice memory tests were the same as those in healthy mice and the improvement lasted two months after the treatment had finished. It seems that the treatment allowed the normal properties of neurons to work.

The problem with this drug is that it had been taken off the market 25 years ago after being found to cause dangerous seizures in some people. PTZ is nearly 100 years old and was used to treat psychiatric disorders and later dementia. However, researchers never concluded it was effective.

Clinical trials of PTZ could begin in the next year or two, and evaluating them might take five to 10 years. The outcome is unknown since many compounds that boosted learning in mice failed in human trials.