Glutamate is undoubtedly the most prevalent transmitter in the brain. This amino acid is probably being used as a signalling substance in a majority of synapses, alone or along with peptides or other neuroactive compounds which colocalize with Glutamate. Notably, the high concentration and relatively even distribution of Glutamate among brain regions was difficult to reconcile with a transmitter role.
The excitatory effect of Glutamate was recognized almost fifty years ago but it took a long time until Glutamate was generally accepted as a neurotransmitter. Notably, the high concentration and relatively even distribution of Glutamate among brain regions was difficult to reconcile with a transmitter role. By the mid 1980ies, Glutamate largely fulfilled the four main criteria for classification as a neurotransmitter: presynaptic localization, release by physiological stimuli, identical action with naturally occurring transmitter, and mechanism for rapid termination of transmitter action.Glutamate is certainly a prevalent transmitter. But this must not be equated with uniformity at the level of synaptic transmission. As the present review has been focused on Glutamate we have not elaborated on the issue of transmitter colocalization. The fact is that many of the fiber systems that contain Glutamate also contain other neuroactive compounds. These may be colocalized with Glutamate or occur in separate fibers. It is also clear that L-aspartate may rival Glutamate as a transmitter in certain fibre systems.