Identification of a Cl-/Ca2+-dependent glutamate (quisqualate) binding site in bovine pineal organ

The presence of a high concentration of glutamic acid, a transmitter shown to have excitatory action in the pineal organ, prompted us to search for and to characterize glutamate receptor site in the bovine pineal organ. By using 10 nM- 100 microM of labeled and unlabeled L-glutamate and by employing the LIGAND computer program, we found a glutamate binding site with a dissociation equilibrium constant (KD) of 0.534 microM and a receptor density (Bmax) of 4.84 pmol/mg protein. This pH- and temperature-dependent binding site showed stereospecificity, was activated by Ca2+, and displayed affinity for both glutamate agonists and antagonists. The IC50 values for L-glutamate, L-aspartate, L-cysteate, L-cysteine sulfinate, quisqualate, and (+/-) ibotenate were 0.5, 2, 12, 16, 25, and 30 microM, respectively, whereas those for D-aspartate, L-alpha-aminoadipate, L-homocysteate, and DL(+/-) 2-amino-4-phosphonobutyrate were greater than 100 microM. Kainate, N-methyl-D-aspartate, and L-glutamic acid diethyl ester were inactive. Based on these results, the presence of a quisqualate-type, Cl-/Ca2+-dependent glutamate binding site in the pineal organ is suggested, and a possible neuroexcitatory role for glutamic acid, aspartic acid, and certain sulfur-containing amino acids is also implied. The precise nature of this excitatory effect in modulating the function(s) of the pineal organ and the synthesis of its hormone(s) remains to be elucidated.