Mixed signals to blame for restless legs syndrome
Iron-deficient cells in the brain are mixing up central nervous system signals to the legs and arms causing the irresistible urges to move and creepy-crawly sensations that characterize restless legs syndrome (RLS), a Penn State College of Medicine study reports.
"Our previous studies established a physical cause for RLS showing certain cells in the brain were iron deficient," said James R. Connor, Ph.D., professor and vice chair for neurosurgery, Penn State College of Medicine, Penn State Milton S. Hershey Medical Center. "We have now found a sequence of events that may connect that cellular iron deficiency to the uncontrollable movements of the disorder."
The study was presented today (Oct. 25, 2004) by Xinsheng Wang, M.D., Ph.D., postdoctoral fellow in Connor’s laboratory, at the Society for Neuroscience’s scientific meeting, Neuroscience 2004, held in San Diego. RLS, a syndrome that may affect 5 percent to 10 percent of the U.S. population, causes irresistible urges to move the legs and arms and is often accompanied by creepy-crawly sensations in the limbs. The sensations are only relieved by movement and become worse as the sun goes down, causing night after night of sleeplessness for those with RLS and their partners.
In normal individuals, cells in a portion of the middle brain called the substantia nigra control the production of tyrosine hydroxylase (TH), an enzyme. The cells also determine how much of the TH is phosphorylated, or activated. The active TH regulates the production of dopamine, a substance in the brain that transmits messages from the brain and central nervous system to the body, giving it instructions for normal functioning.
Connor’s team found that people with RLS have very high levels of active TH. Although this should result in more dopamine being made, in fact, the proper regulation of dopamine production is only possible with both active TH and adequate levels of iron. "We think the ’active form’ has lost its feedback mechanism," Connor said. "The cell is getting a signal that more dopamine is needed so TH is made and shifted to the active form, but the activity is compromised because less iron is available. If the iron was present in sufficient amounts, the feedback process would signal the cells to stop or slow TH production."
Connor’s team first made the connection between iron deficiency and elevated TH levels by examining the brains of iron deficient rats. After weaning, the rats were divided into two groups. One group was given a normal diet, and the second, an iron deficient diet. Half of those that were given the iron deficient diet were later put on a normal iron diet. At 65 days, all rats, regardless of diet, had elevated levels of TH drawing the connection between cellular iron deficiency in the brain and elevated TH. However, the TH levels of the rats that had started a normal diet immediately after weaning eventually returned to normal. "This shows us that developmental iron deficiency can be reversed, but that extended iron deficiency cannot be," Connor said. In a second study using a human cell culture model, the research team exposed PC12 cells, which create dopamine, to a substance that removes iron from cells. As more of the substance was added to the cells and, consequently, more iron was removed, the expression of TH grew, connecting the cellular iron deficiency to elevated TH in human cells.
In a third study, brain tissues from eight individuals with RLS were compared to tissues from the brains of five healthy individuals. The brain tissue was acquired through the Restless Legs Syndrome Foundation’s brain collection at the Harvard Brain Bank. As the animal and cell culture models suggested, the autopsy analysis of the brains of those with RLS showed that iron-deficient cells from the middle brain expressed high levels of TH compared to the non-RLS group. "These results continue to support the idea that the brain dopaminergic system is altered in RLS and that the differences in the dopaminergic system are consistent with insufficient iron," Connor said.
These findings explain why some RLS sufferers find relief from taking dopaminergic drugs. Although not FDA-approved for the treatment of RLS, the drugs are used to calm tremors in those with Parkinson’s disease. The dopaminergic agents replace dopamine in the brain and temporarily improve the nerve signal transmission to the body. "Our next steps are to continue investigations of treatment strategies for RLS involving iron supplementation and dopamine agents to attempt to reach the normal balance between iron and dopamine in the brain," Connor said.
In addition to Connor and Wang, other study authors were: John Beard, Ph.D., and Byron Jones, Ph.D., Penn State University; and Christopher J. Earley, M.B., B.Ch., Ph.D., and Richard Allen, Ph.D., Johns Hopkins Bayview Medical Center.
Valerie Gliem | EurekAlert!