Ca2+ ion permeability and single-channel properties of the metabotropic slow EPSC of rat Purkinje neurons

The slow EPSC (sEPSC) of cerebellar parallel fiber-->Purkinje neuron synapses is mediated by metabotropic glutamate receptor type 1 (mGluR1) activation of nonselective cation channels. Here, the channel properties were studied with uniform calibrated photorelease of L-glutamate with ionotropic receptors blocked, allowing isolation of postsynaptic processes, or with parallel fiber stimulation or mGluR1 agonist application. Evoked current and fluorescence from Ca2+ indicators were recorded. Noise analysis of the mGluR1 current gave a single-channel conductance of 0.6 pS and showed low open probability at maximal mGluR1 activation. Similar small single-channel conductances were obtained with the mGluR1 agonist (S)-dihydroxyphenylglycine, with parallel fiber or climbing fiber stimulation. The mGluR1 current fluctuations were unaffected by potassium channel blockers. Photoreleased L-glutamate triggered a Ca2+ concentration increase in the distal dendrites with a time course similar to that of the mGluR1 current. The proximal dendritic and somatic Ca2+ changes were delayed with respect to the current. Ca2+ channel blockers and the phospholipase Cbeta inhibitor 1-[6-[((17beta)-3-methoxyestra-1,3,5[10]-trien-17-yl) amino] hexyl]-1H-pyrrole-2,5-dione, which inhibits mGluR1-activated intracellular Ca2+ release, did not prevent the dendritic Ca2+ concentration increase. Polyamine naphthylacetyl spermine and cationic adamantanes that block the pore of the channel were used to vary the mGluR1 current over a wide range in each cell but still at maximal mGluR1 activation. The Ca2+ influx was inhibited in parallel with the current. The results show that the mGluR1-activated current and the sEPSC are attributable to small-conductance, low-open probability Ca2+-permeable cation channels that will mediate spine-specific Ca2+ influx during the parallel fiber sEPSP.