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".
Enkephalin
Enkephalin is also considered a neuropeptide, which in the human body performs as an important signaling molecule in the brain. Enkephalins are found in high concentration in the brain as well as in the cells of adrenal medulla. In response to pain, norepinephrine, a hormone that is activated in fight-or-flight response is released along with endorphins. It has been shown that this polypeptide is linked to brain functioning during a stressful response, especially in the hippocampus and prefrontal cortex regions. During a stress response, several Met-enkephalin analogs had increased activity in the hippocampus, while Leu-enkephalin analogs as well as somatostatins were downregulated during stress. This observation leads to a conclusion that stressors impact neuropeptides, and that their action is localized to a specific brain region.