Monosodium glutamate

There's a class of chemicals used as food additives that neuroscientists have dubbed excitotoxins, so named because they over stimulate neurons in the brain. This poses a distinct danger, especially to people with epilepsy. The most common excitotoxins are glutamate, found in monosodium glutamate (MSG), and aspartate, found in aspartame products. When the amount of glutamate and aspartate rises above certain levels, too many neurons get excited and fire. Without inhibitory transmitters to balance their firing, they fire and fire until they are damaged or die. This is especially worrisome because a child's developing brain is four times more sensitive to excitoxins than an adult brain.*

*Olney, J. W. (1988) "Excitoxic Food Additives: Functional Teratological Aspects, " Progressive Brain Research (18), 283-294.