Guanine nucleotides, including GMP, antagonize kainate responses in Xenopus oocytes injected with chick cerebellar membranes.

Abstract: Injection of chick cerebellar membranes, rich in kainate binding sites, into Xenopus oocytes resulted in the structural integration of chick membrane patches into the oocyte plasma membrane that could be easily identified by specific immunofluorescent staining. Application of kainate to the oocyte perfusion medium, under voltage-clamp conditions, induced dose-dependent (EC50= 87 ± 14 μM) inward currents, confirming the functional incorporation to the oocyte of kainate-driven channels. Responses to kainate were consistently nondesensitizing and strongly potentiated by cyclothiazide, suggesting the selective involvement of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-preferring receptors. Binding experiments with (S)-[3H]AMPA confirmed the presence in the chick membrane preparation of low-affinity AMPA receptors (KD= 278 nM) amounting to <2% of the total population of kainate binding sites. A tenfold concentration of guanine nucleotides, with different degrees of phosphorylation, blocked the responses to 100 μM kainate by ∼90%. In the case of GMP, additional concentration-inhibition studies yielded an IC50 of 180 ± 11 μM. Our results illustrate the apparent failure of kainate-binding proteins to form functional channels, even when maintaining their own native membrane environment, and confirm the antagonistic behavior of guanine nucleotides, including GMP, toward glutamate receptors, in agreement with previous results of ligand-binding experiments and, more interestingly, with the marked neuroprotective effects of some guanine nucleotides in different excitotoxicity experimental paradigms.