Figure 5.
Proton binding decreases NR1/NR2B open probability. A, B, Patches were pulled from HEK 293 cells expressing NR1/NR2B and stimulated with a brief (1-4 msec) 1 mm glutamate pulse (top) at pH 7.9, 7.4, and 6.9, as indicated. All responses in A and B are from the same patch, which contained only a single active channel. Responses in B are shown on an expanded time scale. C, Peak open probability, Po(peak), was calculated at each pH by averaging all traces and then dividing by the measured unitary current amplitude (VHOLD, -100 mV). Probability that agonist binding induces a burst, Po(burst), was defined as the proportion of binding events that lead to at least one channel activation and similarly varied with pH. Only patches with one active channel were analyzed; a subset (n = 6) of the patches at pH 7.4 (n = 10) were also described by Banke and Traynelis, 2003. D, Top, The same conductance level was observed at the three pH values tested under our recording conditions (see Materials and Methods). The results are shown for the patch in A as an all-points histogram. The average chord conductance at -100 mV was 68 ± 4.1 pS (pH 6.9), 72 ± 5.2 pS (pH 7.4), and 75 ± 5.8 pS (pH 7.9; n = 4-5; p = 0.640; ANOVA). Bottom, Open-time histograms for the patch in A [square root of the probability density function, or Sqrt(PDF)] from brief concentration jump experiments at pH 7.9 (809 events) and 6.9 (759 events) are superimposed. Fitted time constants for this histogram were τfast 77, 81 μsec (area 34, 26%) and τslow 2.9, 3.3 msec for pH 6.9 and 7.9, respectively.