Properties of the kainate channel in rat brain mRNA injected Xenopus oocytes: ionic selectivity and blockage.

The properties of kainate receptor/channels were studied in Xenopus oocytes injected with mRNA that was isolated from adult rat striatum and cerebellum and partially purified by sucrose gradient fractionation. Kainate (3-1000 microM) induced a smooth inward current that was competitively inhibited by gamma-D-glutamyl-aminomethanesulfonate (GAMS, 300 microM). In striatal mRNA-injected oocytes, the kainate current displayed nearly linear voltage-dependence and mean reversal potential (Er) of -6.1 +/- 0.5 mV. In cerebellar mRNA-injected oocytes; Er was nearly identical (-5.1 +/- 1.2 mV) but there was marked inward rectification of the kainate current. Ion replacement studies reveal that the kainate channel is selective for cations over anions, but relatively non-selective among small monovalent cations. Large monovalent cations such as tetrabutylammonium are impermeant and induce a non-competitive block of kainate current that is strongly voltage-dependent. Divalent cations are relatively impermeant in the kainate channel and Cd++ and other polyvalent metals were shown to block kainate current by a mechanism that is only weakly voltage-dependent. A model of the kainate channel is proposed based upon these observations.