Single channel recordings of reconstituted AMPA receptors reveal low and high conductance states.

Glutamate receptors belonging to the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) subclass were partially purified 30- to 60-fold from forebrain of adult rats and incorporated into planar bimolecular lipid membranes. The channel conductance associated with the reconstituted receptors was activated by kainate and AMPA in a manner that suggests cooperative binding of two to three agonist molecules is required to induce channel opening. This conductance was blocked by the specific antagonist DNQX (6,7-dinitroquinoxaline-2,3-dione). When the partially purified AMPA receptors were reconstituted by the tip-dipping method in asymmetric saline conditions ('outside-out configuration'), the addition of 300 nM AMPA to the pseudo-extracellular solution elicited single channel current fluctuations that were also inhibited by DNQX. Analyses of the currents revealed that the ion channels of reconstituted AMPA receptors have two distinct conductance levels of 12 and 60 pS with the great majority of receptors belonging to the former variety. These results suggest that reconstitution may be useful in identifying factors that regulate the binding and conductance properties of AMPA receptors.