The anatomy of non-NMDA binding sites for EAA : AB Young, JJ Cha, RL Makowiec, and JB Penney, University of Michigan, Ann Arbor, MI 48104-1687 USA

Abstract

1. Binding of [3H]glutamate to housefly brain and honeybee brain and thoracic muscle membranes as well as to the American cockroach nerve cord was measured in Na+-free Tris-citrate buffer, 2.5mM CaCl2, pH 7.4. The dissociation constants (KDs) ranged from 0.16 to 1.36 μM, and thoracic muscles had 2–4-fold higher density of receptors than brain tissue.2. The potent inhibitors of housefly brain binding were in decreasing order of effectiveness: l-glutamate > l-aspartate = l-cysteate = ibotenate > quisqualate >l-homocysteate > l-apb > l-apv > NMDA > d-apb > D-glutamate, with no inhibition by 100 μM of GDEE, dihydrokainate, d-apv, d-homocysteate or d-aspartate.3. The drug specificity of [3H]glutamate binding sites in housefly brain was generally similar to that of binding sites in housefly muscle, except that the former had a slightly higher affinity for l-APB, l-homocysteate and NMDA.4. [3H]Glutamate binding to insect tissues differed in its drug sensitivity from binding to rat brain. Binding to insect membranes was much less sensitive to l-APB, d-APB, APV, homocysteate, l-cysteate, quisqualate and ibotenate. However, the insect binding site was much more stereoselective for the l than d isomers of glutamate and aspartate, while the rat brain site was more stereoselective for APB.5. It is suggested that the observed [3H]glutamate binding to insect tissue is not to NMDA or kainate receptors. A majority of binding is possibly to a depolarizing-type receptor with a relatively low affinity for quisqualate and a higher affinity for l-aspartate; while some of the binding is Cl−-dependent with low affinity for APB and may represent transport into resealed membranes or binding to a hyperpolarizing glutamate receptor.