The patch-clamp technique was used to record single-channel currents from cell-attached patches on rat brain cortical neurons in culture. The composition of the open and shut intervals during bursts of openings was studied in N-methyl-D-aspartate (NMDA) receptors exposed to 1 microM NMDA and 10 microM glycine at a membrane potential of -70 mV. 2. Open interval histograms were constructed for openings at each position (first, second, third, etc.) during all bursts. Distributions from openings 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 the number of openings they contained and duration histograms were obtained for the openings at each position (one to five) during bursts of up to five openings. For bursts containing two or more openings, the distribution of open durations obtained at a given position were similar 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 from the openings at each position in the burst were fitted with two or three exponential components that were similar for each opening. Bursts consisting of a single opening had a different distribution, having a relatively larger component of short duration. 5. Shut intervals during bursts were described by two exponential components with average time 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 distribution was independent of the numbers of openings in the bursts, their position within the burst, and the types of openings (long or short duration) contained within the burst. 6. These results suggest that each 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 consistent with these results for bursts of two or more openings.
