Brain
There are dozens of other chemical neurotransmitters that are used in more limited areas of the brain, often areas dedicated to a particular function. Serotonin, for example—the primary target of antidepressant drugs and many dietary aids—comes exclusively from a small brainstem area called the raphe nuclei. Norepinephrine, which is involved in arousal, comes exclusively from a nearby small area called the locus coeruleus. Other neurotransmitters such as acetylcholine and dopamine have multiple sources in the brain, but are not as ubiquitously distributed as glutamate and GABA.
Gliotransmitter
The word “glia” illustrates the original belief among scientists that these cells play a passive role in neural signaling, being responsible only for neuronal structure and support within the brain. Glial cells cannot produce action potentials and therefore were not suspected as playing an important and active communicative role in the central nervous system, because synaptic transmission between neurons is initiated with an action potential. However, research shows that these cells express excitability with changes in the intracellular concentrations of Ca. Gliotransmission occurs because of the ability of glial cells to induce excitability with variations in Ca concentrations. Changes in the concentration of Ca correlate with currents from NMDA receptor-mediated neurons which are measured in neighboring neurons of the ventrobasal (VB) thalamus. Because glial cells greatly outnumber neurons in the brain, accounting for over 70% of all cells in the central nervous system, gliotransmitters released by astrocytes have the potential to be very influential and important within the central nervous system, as well as within other neural systems throughout the body. These cells do not simply carry functions of structural support, but can also take part in cell-to-cell communication with neurons, microglia, and other astrocytes by receiving inputs, organizing information, and sending out chemical signals. The Ca signal from the astrocyte may also participate in controlling blood flow in the brain.