A, representative traces illustrate responses of pyramidal cells to trains of 1 ms GABA (10 mm) pulses applied at frequencies of 2, 8, 20 or 50 Hz. Following progesterone withdrawal (P Wd), desensitization developed at higher agonist application frequencies than seen for control, first apparent at 8 Hz and reaching a maximum of 18% at 50 Hz application frequencies. In contrast, under control conditions, desensitization was apparent with 2 Hz GABA pulses (200 ms interpulse interval) and reached an 84% maximum desensitization at a 50 Hz application. B, deactivation following 50 Hz GABA application was (P < 0.001) faster following P withdrawal (38.2 ± 4.3 ms) compared to control (110.2 ± 5.6 ms). (n = 12–16 cells/group).