FRACTIONAL CALCIUM CURRENT THROUGH NEURONAL AMPA-RECEPTOR CHANNELS WITH A LOW-CALCIUM PERMEABILITY

The Ca2+-permeation properties of AMPA-receptor (AMPA-R) channels in P urkinje neurons in rat cerebellar slices were studied using a combinat ion of whole-cell patch-clamp recordings, Fura-2 fluorometry, and sing le-cell reverse-transcription (RT)-PCR. Several lines of evidence indi cate that Purkinje neurons, at both early and late stages of postnatal development, express exclusively AMPA-R channels with a low Ca2+ perm eability. First, no Ca2+ signal was detected during application of eit her AMPA or kainate to Purkinje neurons loaded with the Ca2+ indicator Fura-2 AM. In contrast, kainate application induced large Ca2+ transi ents in Bergmann glia cells. Second, in ion substitution experiments, when Ca2+ is the only extracellular permeant cation, the reversal pote ntial corresponds to that expected for AMPA-R channels with a low perm eability for Ca2+. Third, using a fluorometric flux-measurement approa ch (Schneggenburger et al., 1993a), we found that the Ca2+ fraction of the total cation current through AMPA-R channels is similar to 0.6%. This value is approximately sixfold lower than that found for recombin ant 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 developm ental stages corresponding to those studied electrophysiologically. Th e expression of GluR2 in all cells tested (n = 14) is consistent with the subunit composition predicted from studies of recombinant AMPA-R c hannels with a low permeability for Ca2+ (Burnashev et al., 1992b). In conclusion, this study establishes that cerebellar Purkinje neurons a t all postnatal developmental stages possess AMPA-R channels with a lo w permeability for Ca2+.