Figure 4.

Effects of [Cs⁺]_i and [Mg²⁺]_o on single-channel openings of NR1/2D receptors. All data for this figure were collected at −85 mV. A, Examples of Mg_o²⁺-induced NR1/2D receptor single-channel “flicker.” Traces in 0 Mg_o²⁺ (left) and in the indicated [Mg²⁺]_o (right) are shown in 140 Na_o⁺/125 Cs_i⁺ (top) and in 140 Na_o⁺/8 Cs_i⁺ (bottom). Two traces are shown for each condition at different time scales. Including Mg_o²⁺ caused channel flicker at lower [Mg²⁺]_o in 8 mm [Cs⁺]_i than in 125 mm [Cs⁺]_i. B, Open-duration histograms in 140 Na_o⁺/8 Cs_i⁺ from the patch used for A in the indicated [Mg²⁺]_o values. The [Mg²⁺]_o, and value of the time constant (and relative amplitude) of the largest component (τ_o), are as follows: 0 Mg_o²⁺, 0.759 ms (76.4%); 3 μm Mg_o²⁺, 0.603 ms (67.1%); 5 μm Mg_o²⁺, 0.408 ms (77.1%); 10 μm Mg_o²⁺, 0.301 ms (76.4%). The apparent blocking rate constant for Mg_o²⁺ (k_+,app) was estimated to be 179 μm⁻¹s⁻¹ for this patch. C, Closed-duration histograms in 140 Na_o⁺/8 Cs_i⁺ from the patch used for A in the indicated [Mg²⁺]_o values. The [Mg²⁺]_o, and value of the time constant (and relative amplitude) of the component attributable to Mg_o²⁺ block (τ_b), are as follows: 3 μm Mg_o²⁺, 0.375 ms (20.6%); 5 μm Mg_o²⁺, 0.266 ms (34.5%); 10 μm Mg_o²⁺, 0.355 ms (30.9%). The apparent unblocking rate for Mg_o²⁺ (k_−,app) was estimated to be 3081 s⁻¹ for this patch.