Increasing the levels of a message-carrying chemical in the brain may help prevent some of the memory deficits in Down syndrome that hinder learning and make it hard for the brain to develop normally, U.S. researchers said on Wednesday.

They said mice with a rodent version of Down syndrome that were injected with drugs to increase levels of the neurotransmitter norepinephrine - which nerve cells use to communicate - showed improvements in their thinking ability.

The finding points to a new way of trying to improve some of the deficits seen in Down syndrome, which affects 5,000 newborns in the United States each year.

"If you intervene early enough, you will be able to help kids with Down syndrome to collect and modulate information," said Dr Ahmad Salehi of the Veterans Affairs Palo Alto Health Care System, whose study was published in the journal Science Translational Medicine.

"Theoretically, that could lead to an improvement in cognitive functions in these kids," Salehi, who worked on the study while at Stanford University School of Medicine, said in a statement.

Down syndrome occurs when a person has an extra copy of chromosome 21. At birth, children with Down syndrome are not developmentally delayed, but memory deficits linked with the disorder hinder normal brain development.

Salehi and colleagues simulated this in mice that had an extra copy of chromosome 16 that gave them mental disabilities that resemble problems seen in people with Down syndrome.

In the study, the researchers found that the mice with Down syndrome-like dysfunction had lower amounts of norepinephrine in the brain than normal mice. And this problem appeared to occur in a part of the brain called the locus coeruleus.

When this part of the brain broke down, the mice failed to exhibit normal behaviors. When placed in a strange cage, the genetically engineered mice did not build nests, something
normal mice do.

But giving these mice drugs that boost levels of norepinephrine appeared to help. Within a few hours of getting the drugs, the mice built nests that were on par with those of normal mice.

Salehi said he was surprised to see the drug work so fast, but the team noted that the effects did not last long.

Some drugs already on the market for depression and attention deficit hyperactivity disorder target norepinephrine. Salehi said he hopes the findings will lead to new research on these drugs in people with Down syndrome.