Fractional calcium current through neuronal AMPA-receptor channels with a low calcium permeability

The Ca²⁺-permeation properties of AMPA-receptor (AMPA-R) channels in Purkinje neurons in rat cerebellar slices were stud ted using a combination of whole-cell patch-clamp recordings, Fura-2 fluorometry, and single-cell reverse-transcription (RT)-PCR. Several lines of evidence indicate that Purkinje neurons, at both early and late stages of postnatal development, express exclusively AMPA-R channels with a low Ca²⁺ permeability. First, no Ca²⁺ signal was detected during application of either AMPA or kainate to Purkinje neurons loaded with the Ca²⁺ indicator Fura-2 AM. In contrast, kainate application induced large Ca²⁺ transients in Bergmann glia cells. Second, in ion substitution experiments, when Ca²⁺ is the only extracellular permeant cation, the reversal potential corresponds to that expected for AMPA-R channels with a low permeability for Ca²⁺. Third, using a fluorometric flux measurement approach (Schneggenburger et al., 1993a), we found that the Ca²⁺ fraction of the total cation current through AMPA-R channels is 0.6%. This value is approximately sixfold lower than that found for recombinant AMPA-R lacking the AMPA R subunit GluR2. Furthermore, single- cell RT-PCR experiments revealed the presence of the AMPA-R subunits GluR1, GluR2, and GluR3 in Purkinje neurons in cerebellar slices at developmental stages corresponding to those studied electrophysiologically. The expression of GluR2 in all cells tested (n = 14) is consistent with the subunit composition predicted from studies of recombinant AMPA-R channels with a low permeability for Ca²⁺ (Burnashev et al., 1992b). In conclusion, this study establishes that cerebellar Purkinje neurons at all postnatal developmental stages possess AMPA-R channels with a low permeability for Ca²⁺.