Hyperactivity in children may originate in the inner ear, new research suggests.
Scientists have evidence that inner ear disorders can trigger the neurological changes underlying the behavioural condition.
Two brain proteins are thought to be involved that could provide targets for new treatments.
Many children and teenagers with severe disorders affecting hearing and balance also suffer from behavioural problems such as hyperactivity. Until now, no one has been able to determine for certain if the two are linked.
"Our study provides the first evidence that a sensory impairment, such as inner ear dysfunction, can induce specific molecular changes in the brain that cause maladaptive behaviours traditionally considered to originate exclusively in the brain," said lead researcher Professor Jean Hebert, from the Albert Einstein College of Medicine in New York City.
The investigation began after it was discovered that profoundly deaf laboratory mice with severe inner ear defects were also unusually active.
The animals' inner ear problems were caused by a defective gene affecting the transport of sodium, potassium and chloride molecules in various tissues, including the central nervous system. Humans also possess the gene, said the researchers whose findings are reported in the journal 'Science'.
When the gene was "knocked out" from the inner ears of healthy mice, the animals became hyperactive. To the surprise of the scientists, this only happened when the gene was deleted from the inner ear. The same effect was not triggered by knocking it out of the brain or spinal cord.
Tests revealed a role played by two proteins controlling nerve signalling chemicals, or neurotransmitters. Suppressing one of them with a specific drug restored activity levels to normal.
"Our study also raises the intriguing possibility that other sensory impairments not associated with inner ear defects could cause or contribute to psychiatric or motor disorders that are now considered exclusively of cerebral origin," said Prof Hebert. "This is an area that has not been well-studied."
Meanwhile a separate study has found that bacteria from obese people have a plumping up power that is all their own, scientists have discovered.
Mice given the bugs grow fatter than those receiving microbes from lean individuals. The research adds to growing evidence that bacteria growing in the gut can affect metabolism.
It shows that intestinal microbes can directly transmit a bias towards slimness or obesity. Scientists conducted experiments with bacteria living in the guts of human pairs of twins.
In each pair, one sibling was lean while the other was obese. Bugs from both groups were separately transplanted into the guts of germ-free mice raised under sterile conditions.
"The recipients of the obese twins' microbiota gained more fat than the recipients of the lean twins microbiota," said Dr Jeffrey Gordon, a member of the research team from Washington University.
Transplanting the gut microbes from humans to mice led to metabolic changes in the rodents associated with obesity, said the researchers writing in 'Science'.
But they got a surprise when the two groups of mice that had received bacteria from lean and obese humans were caged together and allowed to exchange microbes.