Glutamate is an excitatory neurotransmitter normally found in the brain and in the retina. It acts essentially as other neurotransmitters by being released into a synapse and stimulating or exciting neurons at the postsynaptic junction. This neurotransmitter is both excitatory in its normal role and toxic if found in excess quantities. It is thought that glutamate, and perhaps other excitotoxins, play a significant role in the secondary damage to ganglion cells which may not have been insulted as a result of the primary event at the optic nerve head. Researchers have discovered, both in animal models and in humans with glaucoma, that there is a twofold or greater concentration of glutamate in the posterior vitreous as compared to non-glaucomatous eyes.
Several questions arise as to the role of glutamate in the process of glaucomatous optic atrophy.
The degenerating neurons release their intracellular contents into the environment where they are then available to act upon other non-affected neurons. Another explanation for increased levels of glutamate may be that increased pressure on the cell bodies of neurons causes an alteration in the permeability of the cells to glutamate and results in an escape of glutamate from within the cell to the external environment. The NMDA receptor has also been studied in other central nervous system disorders pertaining to excitotoxicity, including stroke, Parkinson disease, Alzheimer disease, amyotrophic lateral sclerosis, and Huntington disease. Excessive stimulation of the NMDA receptor by glutamate can result in chloride ion and cation influx with subsequent osmotic lysis of the cell. The result is that the cell undergoes programmed cell degeneration and death.