Glutamic acid
Glutamate is also a neurotransmitter (see below), which makes it one of the most abundant molecules in the brain. Malignant brain tumors known as glioma or glioblastoma exploit this phenomenon by using glutamate as an energy source, especially when these tumors become more dependent on glutamate due to mutations in the gene "IDH1".
Addiction psychology
The neurochemical, glutamate is the most abundant neurochemical in the brain. It is critical in memory consolidation. When an addict discovers an addicting behavior, glutamate plays a role by creating the drug cues. It is the neurochemical in motivation which initiates the drug seeking, thus creating the addiction.