BURST KINETICS OF SINGLE NMDA RECEPTOR CURRENTS IN CELL-ATTACHED PATCHES FROM RAT-BRAIN CORTICAL-NEURONS IN CULTURE

1. The patch-clamp technique was used to record single-channel current s from cell-attached patches on rat brain cortical neurons in culture. The composition of the open and shut intervals during bursts of openi ngs was studied in N-methyl-D-aspartate (NMDA) receptors exposed to 1 mu M NMDA and 10 mu M glycine at a membrane potential of -70 mV. 2. Op en interval histograms were constructed for openings at each position (first, second, third, etc.) during all bursts. Distributions from ope nings two to five were fitted with similar (two or three) exponential components. The first opening from all bursts tended to be of shorter duration than the other openings. 3. Bursts were sorted according to t he number of openings they contained and duration histograms were obta ined for the openings at each position (one to five) during bursts of up to five openings. For bursts containing two or more openings, the d istribution of open durations obtained at a given position were simila r to each other regardless of the number of openings in the burst. 4. In bursts of two or more openings (up to five), duration histograms fr om the openings at each position in the burst were fitted with two or three exponential components that were similar for each opening. Burst s consisting of a single opening had a different distribution, having a relatively larger component of short duration. 5. Shut intervals dur ing bursts were described by two exponential components with average t ime constants (and relative areas) (means +/- S.E.M.) of 0.06 +/- 0.01 ms (0.59 +/- 0.02) and 0.64 +/- 0.02 ms (0.41 +/- 0.02). Their distri bution was independent of the numbers of openings in the bursts, their position within the burst, and the types of openings (long or short d uration) contained within the burst. 6. These results suggest that eac h opening in bursts of two or more openings is kinetically similar to every other opening regardless of burst length. Analogously each shut period during a burst was similar to every other. A kinetic model with three open and four closed intraburst states is shown to be consisten t with these results for bursts of two or more openings.