Aware that extra amounts of the messenger can kill brain cells by over-stimulating them, the scientists thought preventing it from getting into cells might lead to cell death. But blocking one of the messenger's main transporters didn't kill nerve cells; instead, the rats developed epilepsy, complete with periods of staring and "freezing," the scientists report in the August 1 issue of the Journal of Neuroscience.
"We wanted to see how reducing transport of this messenger affected other brain chemicals and the brain itself, but we hadn't expected it to result in epilepsy," says Jeffrey Rothstein, M.D., Ph.D., professor of neurology and neuroscience at Johns Hopkins. "Now we need to track it down in people to see if the same mechanism is involved in the human condition."
Epilepsy has long been tied to reduced levels of a brain messenger called GABA, which dampens cells' likelihood of firing. However, the stimulating messenger the scientists study, called glutamate, had never been linked to epilepsy in animals or people.
Investigating further, the scientists discovered that the glutamate transporter they blocked imports glutamate molecules that are used by the cell to make new GABA molecules. The rats couldn't make new GABA, so over the course of 10 days, their GABA levels dropped and epileptic symptoms developed, says Rothstein.
"We've shown that the synthesis and release of GABA depends heavily on transporting glutamate into nerve cells," says Rothstein, also director of the Robert Packard Center for ALS Research at Johns Hopkins. Previously, only glutamate made inside the cell - rather than brought into it - was thought to contribute to GABA manufacture. While glutamate transport problems may not contribute to human epilepsy, the finding may lead to new ways to treat it. Getting more glutamate into GABA-producing neurons may help calm the excessive electrical firing of brain cells, says Rothstein.
In rats, the glutamate transporter EAAC1 is the main mover, getting the molecule into and out of brain cells and maintaining the appropriate levels of the excitatory chemical. By giving the rats pieces of RNA that complement EAAC1's message, the scientists were able to gradually reduce the amount of EAAC1 the cells produced.
"The RNA binds to EAAC1's message like a zipper, keeping it tied up," says Rothstein. "The transporter stops working because instructions for making new EAAC1 protein aren't available, but normal degradation of existing EAAC1 protein continues."
In the brains of the rats, GABA production was slowed, and levels of the chemical were cut in half in specific areas of the brain compared to normal animals. Studying the electrical activity of the brain revealed rapid spikes at the same time as freezing and staring episodes - "textbook epileptic seizures," says Rothstein.
"Despite recognizing that glutamate and GABA have the same functions in many creatures, no one had established a clean link between the pathways that use them," says Rothstein. "This glutamate transporter, EAAC1, is central to the production of GABA in the normal rat brain, and its human counterpart likely has the same role." Others have shown that knockout mice, which lack the gene and its product throughout development, do not develop epilepsy. Rothstein suggests knockout mice may recruit other glutamate transporters to compensate for the lack of EAAC1 during early development and use them in its place throughout their lives. Adult animals likely are unable to compensate for EAAC1 function as it disappeared, he says.
Interfering with other glutamate transporters (called GLAST-1 and GLT-1) found in other types of neurons did not reduce GABA levels or cause epilepsy, Rothstein says.
Authors on the report are Rothstein, Jehuda Sepkuty, Christine Eccles and Raquelli Ganel, Johns Hopkins School of Medicine; Akiva Cohen and Douglas Coulter, University of Pennsylvania School of Medicine and the Stokes Research Institute of Children's Hospital of Philadelphia; Azhar Raqif, the Medical College of Virginia at Virginia Commonwealth University; and Kevin Behar, Yale University.
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