NMDA-receptors on Purkinje cell dendrites in guinea pig cerebellar slices

The Mg2+-dependent depolarizing action of N-methyl-D-aspartate (NMDA) was intrasomatically investigated, in comparison with quisqualate (QA), in Purkinje cells in cerebellar slices from adult guinea pigs. NMDA applied iontophoretically to the proximal dendritic region (about 100 micron from the Purkinje cell soma) induced depolarizations and spike firings in about the half of the Purkinje cells tested in nominal Mg2+-free medium (contaminated with 4-11 microM Mg2+), and 1 mM Mg2+ almost completely blocked this NMDA action. Application of NMDA onto the distal dendritic region (about 200 micron from the soma) caused no depolarization at all even in the Mg2+-free medium. QA applied onto either the proximal or distal dendritic region consistently showed Mg2+-independent depolarizations. The amplitude of NMDA-induced depolarization in the Mg2+-free medium was non-linearly related to the membrane potential, i.e. smaller at a hyperpolarized potential level. 2-Amino-5-phosphonovalerate blocked the NMDA action partially but more selectively than the QA action, while the reverse was the case for glutamic acid diethylester. These results suggest that the Mg2+-dependent, NMDA-sensitive receptor, which is distinct from the QA receptor and probably similar to the well-known NMDA receptor, is present on the proximal dendrite of the cerebellar Purkinje cell of the guinea pig.