The propagation speed eliminated the third known route of epileptic propagationsignaling by diffusion of sodium and potassium ions across brain cell membranes. Diffusion is too slow to be the method of transmission.
The researchers were left with the fourth known way that neurons communicate: electrical fields. When a group of cells fire together, they can generate currents and electric fields that can be large enough to excite their neighbors.
Computer simulations with a large set of neurons confirmed that a weak electrical field could, by itself, spread epileptic signals at the speed that matched their experimental results. The computer model also predicted that signal transmission would change speeds depending on the distance between neurons.
The researchers verified the prediction by injecting drugs that would either expand or decrease the space between cells. The closer the cells, the faster the transmission. The more distant the cells, the slower the transmission untilat a certain distancethe signal failed to propagate.
For some time, doctors have used diuretics to treat some types of epilepsy.Diuretics are known remove water from cells and but their anticonvulsant properties are not known, Durand said.
As water is removed from cells and enters the extracellular space, it increases the space between cells and may be pushing them beyond the reach of the weak electrical field, he said.
The researchers say that this finding does not diminish the importance of synaptic transmission in epilepsy but points to another synchronizing and propagation mechanism.
But "this discovery has implications for epilepsy," Durand said. "If cells in an excited state can talk to or influence each other, they can synchronize to a seizure-like state, propagate to neighbors thereby generating a full-blown seizure."
|Contact: Kevin Mayhood|
Case Western Reserve University