Figure 1. Progesterone withdrawal accelerates the deactivation of GABA-gated current.

A, representative traces showing responses to brief (∼ 1 ms) pulses of GABA (10 mm) recorded from outside-out patches of CA1 hippocampal pyramidal cells following progesterone withdrawal (P Wd), 48 h 3α,5β-THP (48 h THP) or sham conditions (Control). Each trace represents the average of 6–10 individual traces. (Fits are shown next to full traces.) The deactivation rate is best described as a biexponential decay, with a τ_fast in the range of 10–22 ms and a τ_slow of 80–145 ms for the control recordings. Following P withdrawal (P Wd), in Group I 60% of the current deactivated with a τ_fast of 3–6 ms (mean = 4.88 ± 0.61 ms), and a τ_slow of 80–120 ms (mean = 87.0 ± 12.0 ms). In Group II, 40% of the current recorded deactivated with a τ_fast of 3–7 ms, and a τ_slow of 30–40 ms. Forty-eight hours of treatment with 3α,5β-THP produced similar acceleration in deactivation times. Note that in both populations, τ_fast is significantly faster than control values, while in Group II τ_slow is also significantly faster than control. Average peak amplitude was unaffected by prior steroid treatment. The top trace indicates the open tip junctional current. (These results are representative of those recorded from 20 to 30 patches/group.) Inset: amplified traces illustrate an accelerated τ_fast following P Wd compared to control. Inset, representative open tip junction potential for a control recording. B, distribution of values for τ_fast and τ_slow for control (Con, left panels), progesterone withdrawal (P Wd, middle panels), and 48 h treatment with 3α,5β-THP (48 h THP, right panels). Values for τ_slow display a bimodal distribution for P Wd and 48 h THP conditions. All other distributions display a single mode.