Figure 2.
Estimation of ambient glutamate concentration. A, NMDA-evoked currents in a CA1 pyramidal neuron (Vh of +40 mV). Bar indicates applications of 1, 3, and 5 μm NMDA. Baseline currents have been zeroed. B, Nucleated patch currents produced by flow-pipe application of 5, 10, 20, 50, and 1000 μm NMDA (Vh of +40 mV). C, Semi-log plot of whole-cell (n = 6; gray circles) and nucleated patch (n = 6) current responses from application of NMDA (black squares) and glutamate (Glut; black circles; 0.5, 1, 3, 5, 100 μm; n = 6). Nucleated patch response were normalized to the maximum response. Whole-cell responses were scaled by the mean nucleated patch response to 5 μm NMDA. Line through NMDA responses is the nonlinear regression fit with Hill equation of nucleated patch dose responses (EC50 of 37.7 μm; nH = 1.3). Line through glutamate responses is the NMDA fit shifted by EC50Glut/EC50NMDA. Inset, Expansion of low concentration portion of NMDA dose–response curve. D, Nucleated patch responses to saturating concentrations of glutamate (Glu; 100 μm) and NMDA (1 mm). E, Dose–response curves from C, with glutamate-fit curve scaled by the efficacy ratio of 1.77. Arrows indicate the concentrations of glutamate and NMDA required to evoke currents of the same amplitude as that induced by ambient glutamate.