RECORDINGS OF GLUTAMATE-RECEPTOR CHANNELS IN ISOLATED POSTSYNAPTIC DENSITIES

WE have isolated highly purified rat brain postsynaptic densities (PSD s), that are known to contain glutamate receptors of the AMPA and NMDA types. These PSDs were incorporated into liposomes, and grown, by a c ycle of partial de- and rehydration in 5% ethylene glycol, into giant (5-100 mum in diameter) liposomes. These giant liposomes were then mad e to form Gigaohm (10-20 GOMEGA) seals with conventional patch-clamp e lectrodes, which, when withdrawn, retain an excised patch in an inside -out configuration. When 5-10 muM L-glutamate (or 10 muM NMDA) plus 1 muM glycine were present inside the patch pipette, but not in the exte rnal fluid, a highly complex pattern of currents was seen in about 55% of the cases. This was characterized by very fast kinetics, conductan ces as high as 460 pS and multiple lower levels of 45, 80, 120, 230 an d 340 pS. These currents, when evoked by NMDA plus glycine, were entir ely suppressed by the NMDA antagonist 2-amino-5-phosphonovalerate, APV . However, those activated by L-glutamate plus glycine still appeared in the presence of APV in about 18% of the cases, but with lower condu ctance levels. Current kinetics similar to the latter ones were also i nduced by the AMPA receptor agonist quisqualate (10 muM) in 16% of the cases. This indicated that both NMDA and AMPA receptors were present, in a functionally well preserved state, in isolated postsynaptic dens ities. Indirect evidence also suggested that in our experiments, in wh ich 212 seals were studied, only a single postsynaptic density was pre sent in the patches in which channel activity was found. The complex, high-conductance currents found may thus represent the opening and clo sure of many individual glutamate receptor ion channels present in a s ingle isolated postsynaptic density.