1. Whole‐cell recordings were made from cerebellar granule cells cultured in high‐K+ medium to induce expression of Ca(2+)‐permeable AMPA receptors. Current‐voltage (I‐V) plots of agonist‐evoked responses showed varying degrees of inward rectification, but became linear within 5‐10 min. 2. Recombinant Ca(2+)‐permeable kainate receptors, composed of GluR6(Q)/KA‐2 subunits, exhibited rectifying whole‐cell I‐V plots that became linear in outside‐out patches. 3. Loss of rectification in granule cells was prevented by including 100 microM spermine in the pipette; the degree of rectification was then correlated with Ca2+ permeability. 4. Spermine also prevented loss of rectification in patches containing GluR6(Q)/KA‐2 receptors (IC50, 1.7 microM). 5. We suggest that spermine, or a similar cellular constituent, may act as a cytoplasmic factor conferring inward rectification on Ca(2+)‐permeable non‐NMDA receptors, and that ‘washout’ of this factor underlies the observed loss of rectification.
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