Neurosteroid administration and withdrawal alter GABAA receptor kinetics in CA1 hippocampus of female rats

Withdrawal from the GABA-modulatory steroid 3 alpha-OH-5 alpha-pregnan-20-one (3 alpha,5 alpha-THP) following exposure of female rats to the parent compound progesterone (P) produces a syndrome characterized by behavioural excitability in association with up-regulation of the alpha 4 subunit of the GABA(A) receptor (GABAR) in the hippocampus. Similar changes are seen after 48 h exposure to its stereoisomer, 3 alpha,5 beta-THP. Here, we further characterize the effects of P withdrawal on GABAR kinetics, using brief (1 ms) application of 5-10 mM GABA to outside-out patches from acutely isolated CA1 hippocampal pyramidal cells. Under control conditions, GABA-gated current deactivated biexponentially, with tau(fast) = 12-19 ms (45-60% of the current), and tau(slow) = 80-140 ms. P withdrawal resulted in marked acceleration of deactivation (tau(fast) = 3-7 ms and tau(slow) = 30-100 ms), as did 48 h exposure to 3 alpha,5 beta-THP (tau(fast) = 5-8 ms; tau(slow) = 40-120 ms). When recombinant receptors were tested in HEK-293 cells, a similar acceleration in tau(fast) was observed for alpha 4 beta 2 delta and alpha 4 beta 2 gamma 2 GABARs, compared to alpha 1 beta 2 gamma 2 and alpha 5 beta 2 gamma 2 receptors. In addition, tau(slow) was also accelerated for alpha 4 beta 2 delta receptors, which are increased following steroid withdrawal. As predicted by the Jones-Westbrook model, this change was accompanied by reduced receptor desensitization as well as an acceleration of the rate of recovery from rapid desensitization. A theoretical analysis of the data suggested that steroid treatment leads to receptors with a greater stability of the bound, activatable state. This was achieved by altering multiple parameters, including desensitization and gating rates, within the model. These results suggest that fluctuations in endogenous steroids result in altered GABAR kinetics which may regulate neuronal excitability.