> A "toxin" is something which is toxic to a system. But our knowledge of the brain is so primitive that we can't reasonably claim to know which chemicals are toxic at tiny, long-term dosage levels, unless it leads to death.

If it leads to harm or death of individual neurons, we have lots of knowledge about such things. For example, we know that glutamate is toxic in large doses to neurons,

http://en.wikipedia.org/wiki/Excitotoxicity