Characterization of l-glutamate and kainate binding sites in the brain of a freshwater fish, Telapilia monsanbica

Abstract

[ 3H]Kainate and l-[ 3H]glutamate binding sites in a rich source of kainate binding sites, fish brain, have been thoroughly analysed here for the purpose of studying the correlation between kainate binding sites and l-glutamate receptors in vertebrate CNS. The brain of a freshwater fish, Telapilia monsanbica, was found to contain three types of kainate binding sites: Type 1 sites ( K d = 1050 ± 380μM, B max = 4 ± 4pmol/mg), Type 2 sites ( K d = 133 ± 20nM, B max = 190 ± 20pmol/mg), and Type 3 sites ( K d = 23 ± 15nM, B max = 28 ± 19pmol/mg). The dissociation constants of l-glutamate to Type 1, 2 and 3 sites were, respectively, 0.28 ± 0.04, 5.5 ± 0.2 and 137 ± 28μM. Pharmacological characterization of these binding sites showed that Type 1 and 2 sites, respectively, corresponded to N-methyl- d-aspartate-subtype l-glutamate receptors and Non-N-methyl- d-aspartate l-glutamate receptors. Autoradiographic studies showed that Type 1 and 2 sites were distributed widely in fish brain, indicating the involvement of l-glutamate receptors in various brain functions. Type 3 sites, on the other hand, were relatively insensitive to most endogenous amino acids and were only found in the molecular layer of cerebellum and torus longitudinalis. Type 3 sites possibly representing a distinctive class of receptor has been suggested by the results.