While the study in the journal Science Translational Medicine offers no direct link to a treatment for humans, researchers are hopeful it may someday offer a path toward future breakthroughs.
There is no cure for Down syndrome, which is caused by the presence of an additional chromosome, leading to extra copies of more than 300 genes and causing intellectual disabilities, distinctive facial features and sometimes other health problems.
The team at Johns Hopkins University used lab mice that were genetically engineered to have extra copies of about half the genes found on human chromosome 21, leading to Down syndrome-like conditions such as smaller brains and difficulty learning to navigate a maze.
On the day the mice were born, scientists injected them with a small molecule known as a sonic hedgehog pathway agonist.
The compound, which has not been proven safe for use in human, is designed to boost normal growth of the brain and body via a gene known as SHH.
The gene provides instructions for making a protein called sonic hedgehog, which is essential for development.
"It worked beautifully," said lead author Roger Reeves of the Johns Hopkins University School of Medicine.
"Most people with Down syndrome have a cerebellum that's about 60 percent of the normal size," he said.
"We were able to completely normalize growth of the cerebellum through adulthood with that single injection."
The injection also led to unexpected benefits in learning and memory, normally handled by a different part of the brain known as the hippocampus.
Researchers found that the treated mice did as well as normal mice on a test of locating a water platform while in a swimming maze.
However, adjusting the treatment for human use would be complicated, since altering the growth of the brain could lead to unintended consequences, like triggering cancer.
"Down syndrome is very complex, and nobody thinks there's going to be a silver bullet that normalizes cognition," he said. "Multiple approaches will be needed."