Excitatory amino acid receptors of the retina: diversity of subtypes and conductance mechanisms

Excitatory amino acid (EAA) receptors underlie major pathways of synaptic communication in the vertebrate retina, including neurotransmission from photoreceptors in the outer plexiform layer (OPL), and from bipolar cells and perhaps some amacrines in the inner plexiform layer (IPL)¹. Pharmacological studies, combined with intracellular electrophysiological recordings, have provided new insights into EAA receptor subtypes associated with identified neurons. The results obtained have established the retina as a model system for studying both the diverse nature of EAA receptors on different identified neurons and the unique conductance mechanisms that underlie the separation of ON and OFF channels within the OPL. There is now strong evidence for an EAA receptor which, when activated, hyperpolarizes the cell by closing ionic channels^2,3, and for a new receptor subtype in the carp retina^4,5, which hyperpolarizes by opening channels to either K⁺ or Cl^-ions. This brief review describes the retinal pathways that utilize EAA and the evidence for diversity in pharmacology and ionic mechanisms.