Allosteric potentiation by diazoxide of AMPA receptor currents and synaptic potentials

Abstract

Diazoxide (100–560 μM) reversibly increased the amplitude and duration of excitatory post-synaptic field potentials recorded in the dentate gyrus of hippocampal slices following stimulation of the perforant pathway. In rat cortex mRNA-injected Xenopus oocytes diazoxide (1–1000 μM) alone had little effect on membrane current, but rapidly and reversibly increased (up to 5-fold) current responses to ( R, S)-α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA, 30 μM), l-glutamate (100 μM), quisqualate (3 μM), kainate (100 μM) and domoate (3 μM), an effect that was neither mimicked by other activators of ATP-sensitive potassium channels nor blocked by glibenclamide. Diazoxide increased current amplitudes for all concentrations of the ‘inactivating’ ligands, AMPA, l-glutamate and quisqualate but had little effect on their EC 50 values. In contrast, diazoxide increased the apparent potency of the ‘non-inactivating’ ligands, kainate and domoate, but increased the efficacy of saturating concentrations by only 10–20%. Diazoxide did not modify the competitive inhibition of AMPA and kainate currents by 6-nitro-7-sulfamoylbenzo[f]quinoxaline-2,3-dione (NBQX) and thus does not compete for the agonist site as do AMPA and kainate. Similarly, diazoxide neither inhibited the binding of [ 3H]AMPA or [ 3H]kainate to rat cortical membranes in competition experiments nor consistenty modified the apparent [ 3H]AMPA affinity ( K d) or receptor density ( B max) in saturation experiments. These data suggest that diazoxide acts at an allosteric site on the AMPA receptor/channel to potentiate activation in a manner dependent upon the properties of the excitatory agonist.