A patch-clamp study of the single channel properties of native N-methyl-D-aspartate (NMDA) receptors in neurons from the rat central nervous system

Native subtypes of NMDA receptors with distinct developmental and anatomical distribution are present in the brain. Knowledge of their function and pharmacology would greatly help in understanding NMDA receptor involvement in physiological and pathological processes in the central nervous system. The single-channel properties of native NMDA receptors were studied in outside-out patches excised from neuronal cell bodies in hippocampal and striatal slices from 0-day old rats. Steady state channel activations produced by glutamate (50 - 100 nM) or NMDA (1 - 5 |LIM) and glycine (3 - 10 fxM) were studied at -60 mV in an extracellular solution containing 1 mM Ca2+ and no added Mg2+. In patches from striatal neurons (n=9), glutamate and NMDA produced a single pattern of channel activity characterized by the presence of two distinct unitary currents of 54 pS (83 1 %) and 44 pS (17 1 %). Frequent and symmetric direct transitions between 44 and 54 pS currents suggested a common channel population obeying microscopic reversibility. In the presence of a saturating glycine concentration , spermine (100 |xM) produced an overall increase in receptor activity; a property described only in recombinant NMDA receptors containing NRla and NR2B subunits. The functional and pharmacological properties of PO striatum NMDA receptors were consistent with these being a NR2B-like NMDA receptor population. In dentate gyrus neurons, glutamate and NMDA produced two patterns of single-channel activity. One similar to that of PO striatum NMDA receptors and characterized by only high conductance (42 and 51 pS) unitary currents and a second pattern showing, in addition to high conductance (42 and 49 pS) currents, low conductance (17 and 42 pS) single-channel currents. Direct transitions between 42 and 51 pS currents were symmetric while those between 17 and 42 pS were asymmetric. No direct transitions linked 17 and 49 pS or 17, 42 and 49 pS currents, Ifenprodil produced a selective reduction of single-channel activity mediated by high conductance NMDA channels in both patches with only high conductance and patches with high and low conductance single-channel activity. Based on their different functional and pharmacological properties and striking similarities with NRla/NR2B and NRla/NR2D recombinant NMDA receptors, it was concluded that high and low conductance singlechannel activity were the result of the activation of two functionally and pharmacologically different NMDA receptor-channel populations: high conductance ifenprodil-sensitive NR2D-like NMDA receptors and low conductance ifenprodil-insensitive NR2D-like NMDA receptors