Excitatory amino acid receptors (EAARs) underlie major synaptic pathways in the brain, retina and spinal cord. Several subclasses of EAARs have been proposed, based on pharmacological studies using a variety of agonists and antagonists. Kynurenic acid (Kyn), a metabolite of tryptophan, has been recently proposed as a potent EAAR antagonist. In this report, we show that Kyn can be used to separate two distinct classes of EAAR in the vertebrate retina: it blocks kainic acid (KA) responses but has minimal effects on responses mediated by quisqualate (QQ). At concentrations which block the KA responses, Kyn also blocks the light-evoked synaptic responses of all types of third-order neurons in the retina. These results suggest that KA receptors are the major receptor subtypes which underlie synaptic transmission and that QQ receptors are minimally utilized by light-activated pathways under the conditions of our experiments.
Bibliographical noteFunding Information:
We thank Kathy Richter for typing and Judy Dodge for excellentt echnicala ssistanceT. his work was supported by NEI Grant EY03014 to R.F.M.
- acidic amino acid
- kainic acid
- kynurenic acid