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".
Arvid Carlsson
In 1957 Kathleen Montagu demonstrated the presence of dopamine in the human brain; later that same year Carlsson also demonstrated that dopamine was a neurotransmitter in the brain and not just a precursor for norepinephrine. Carlsson went on to develop a method for measuring the amount of dopamine in brain tissues. He found that dopamine levels in the basal ganglia, a brain area important for movement, were particularly high. He then showed that giving animals the drug reserpine caused a decrease in dopamine levels and a loss of movement control. These effects were similar to the symptoms of Parkinson's disease. By administering to these animals L-Dopa, which is the precursor of dopamine, he could alleviate the symptoms. These findings led other doctors to try using L-Dopa in patients with Parkinson's disease, and it was found to alleviate some of the symptoms in the early stages of the disease. L-Dopa is still the basis for most commonly used means of treating Parkinson's disease.