Fractional contribution of calcium to the cation current through glutamate receptor channels

The Ca²⁺ fraction of the ion current flowing through glutamatergic NMDA and AMPA/kainate receptor channels was determined in forebrain neurons of the medial septum. The neurons were overloaded with the Ca²⁺ indicator dye fura-2 (1 mM) via the recording patch pipettes. This approach allowed the direct determination of the Ca²⁺ influx from changes in the Ca²⁺-sensitive fura-2 fluorescence. We found that, at negative membrane potentials and at an extracellular free Ca²⁺ concentration of 1.6 mM, the Ca²⁺ fraction of the current through the NMDA receptor channels is only 6.8%, about 2-fold lower than previously estimated from reversal potential measurements. Interestingly, a quite high fractional Ca²⁺ current of 1.4% was determined for the linearly conducting AMPA/kainate receptor channels found in these neurons.