N-Methyl-d-aspartate receptors in the retina

The vertebrate retina is a “genuine neural center” (Ramón y Cajal), in which glutamate is a major excitatory neurotransmitter. Both N-methyl-d-aspartate (NMDA) and non-NMDA receptors are expressed in the retina. Although non-NMDA receptors and/or metabotropic glutamate receptors are generally thought to be responsible for mediating the transfer of visual signals in the outer retina, there is recent evidence suggesting that NMDA receptors are also expressed in photoreceptors, as well as horizontal and bipolar cells. In the inner retina, NMDA receptors, in addition to other glutamate receptor subtypes, are abundantly expressed to mediate visual signal transmission from bipolar cells to amacrine and ganglion cells, and could be involved in modulation of inhibitory feedback from amacrine cells to bipolar cells. NMDA receptors are extrasynaptically expressed in ganglion cells (and probably amacrine cells) and may play physiological roles in a special mode. Activity of NMDA receptors may be modulated by neuromodulators, such as d-serine and others. This article discusses retinal excitotoxicity mediated by NMDA receptors.