Figure 3.

Mg²⁺ (0 mm) and picrotoxin-induced epileptiform activity in brain slices stimulates astrocyte-evoked neuronally detected NMDAR-dependent currents. A, Paired patch-clamp recording from two CA1 pyramidal neurons under control conditions (a), during interictal-like epileptiform activity triggered by 0 Mg²⁺ and picrotoxin (100 μm; b, picro), and after subsequent application of TTX (1 μm; c). Note the increased frequency of SICs in cells 1 and 2 (c) compared with the basal condition (a) after epileptiform-like activity induced by 0 Mg²⁺ and picrotoxin. SICs are not synchronous in the two cells because the neuronal somata are >100 μm apart. B, TTX application (1 μm) blocks the epileptiform events and unmasks the presence of SICs. C, Average frequency of SICs is shown during a basal period recorded in ACSF (Basal, gray bars) before perfusion with 0 Mg²⁺ and picrotoxin. After epileptiform activity was initiated by perfusing 0 Mg²⁺ and picrotoxin, TTX (1 μm) was added to this saline to block neuronal activity. Thereafter, SIC frequency was again measured (black bars) in the 9 of 12 CA1, 7 of 14 CA3, and 7 of 18 cortical neurons displaying SICs. D, A similar experiment to that described in C was performed except that the initiation of epileptiform activity was prevented by preincubation with TTX (white bars). Perfusion of 0 Mg²⁺ and picrotoxin in TTX did not significantly increase SIC frequency. The average SIC frequency (black bars) in 5 of 7 CA1, 3 of 6 CA3, and 3 of 7 cortical neurons displaying SICs. E, Superimposition of a typical interictal event (black trace) and an SIC (gray trace) recorded from the same cell showing the distinct rise times of these two events. F, Average frequency of SICs before and after the epileptiform activity in the presence of TTX and CNQX (25 μm) in the 8 of 19 cortical neurons that displayed SICs. G, Left, SICs recorded in TTX and induced by 0 Mg²⁺ and picrotoxin are reversibly blocked by d-AP-5 (50–100 μm). Right, Mean number of SICs per minute under the different experimental conditions (n = 4).