Aspartame is an excitotoxin.
From
http://www.chem.ox.ac.uk/mom/aspartame/aspartame.h tml:
Aspartame is hydrolysed in the body to three chemicals, aspartic acid (40%), phenylalanine (50%) and methanol (10%). Aspartic acid is an amino acid. Much research has been carried out to determine its behaviour in the body when it is taken in its free form, i.e. unbound to proteins. It is claimed that when it is ingested the blood plasma levels of aspartate and glutamate rise significantly. Both aspartate and glutamate act as neurotransmitters in the brain, carrying information from neuron to neuron. When there is an excess of neurotransmitter, certain neurons are killed by allowing too much calcium into the cells. This influx causes excessive numbers of free radicals to build up which kill the cells. The neural cell damage that is caused by excessive aspartate and glutamate is the reason they are referred to as 'excitotoxins': they 'excite' or stimulate the neural cells to death.
From
http://smart-drugs.net/ias-excitotoxins.htm:
MSG/aspartame defenders also like to point out that glutamate and aspartate are natural constituents of food protein, which is generally considered safe, so why the concern over MSG/aspartame (2)? Yet there is a key difference between food-derived glutamate/aspartate and MSG/aspartame. Food glutamate/aspartate comes in the form of proteins, which contain 20 other amino acids, and take time to digest, slowing the release of protein bound glutamate/aspartate like a "timed-release capsule." This in turn moderates the rise in blood levels of glutamate/aspartate. Also, when glutamate and aspartate are received by the liver (first stop after intestinal absorption) along with 20 other aminos, they are used to make various proteins. This also moderates the rise in blood glutamate/aspartate levels. Yet when the single amino MSG is rapidly absorbed (especially in solution - e.g. soups, sauces and gravies), not requiring digestion, human and animal experiments show rapid rises in glutamate, 5 to 20 times normal blood levels (2). Aspartame is a dipeptide - a union of 2 aminos- and there exist special di-and tripeptide intestinal absorption pathways that allow rapid and efficient absorption (21). The dipeptides are then separated into free aminos, and as with free MSG there will be a rapid rise in blood aspartate. Thus the characteristics of food-bound glutamate/aspartate and MSG/aspartame are completely different.