Glutamic acid-insensitive [ 3H]kainic acid binding in goldfish brain

Abstract

Kainic acid is supposed to be a specific agonist for a subclass of excitatory glutamate receptors in the vertebrate CNS. An investigation of (2 nM) [ 3H]kainic acid binding sites in goldfish brain, using quantitative autoradiography, has revealed evidence for two types of kainic acid receptors which differ in sensitivity to glutamic acid. l-Glutamic acid (0.1–1 mM) displaced over 95% of specific [ 3H]kainic acid binding elsewhere in the brain but only 10–50% in the cerebellum and cerebellar crest. These structures apparently contain [ 3H]kainic acid binding sites that are extremely insensitive to glutamic acid. The glutamic acid-insensitive [ 3H]kainic acid bindings was not displaced by quisqualic acid kynurenic acid, α-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA), or N-methyl- d-aspartatic acid, but was completely displaced by the kainic acid analogue domoic acid. The data indicate that two types of high affinity binding sites for [ 3H]kainic acid exist in the goldfish brain: glutamic acid-sensitive and glutamic acid-insensitive. High affinity [ 3H]kainic acid binding may therefore not always represent binding to subsets of glutamic acid receptors.