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".
Neuropharmacology
The serotonin neurotransmitter has the ability to mediate synaptic transmission through either GPCR's or LGIC receptors. Depending on what part of the brain region serotonin is being acted upon, will depend on whether the output is either increasing or decreasing post-synaptic responses. The most popular and widely used drugs in the regulation of serotonin during depression are known as SSRIs or selective serotonin reuptake inhibitors. These drugs inhibit the transport of serotonin back into the pre-synaptic neuron, leaving more serotonin in the synaptic gap to be used.