Fig. S1. Sensitivity to allopregnanolone of the outward tonic current recorded above ECI in the presence of THIP. In this experiment, performed in the presence of kynurenic acid, strychnine, TTX, D-AP-5 and TEA, at −10 mV whereas ECI was −56 mV, addition of 2 μm THIP induced an increase in outward current of 9.1 pA (not shown). After 3 min in the presence of THIP, 100 nm allopregnanolone was added to the THIP-containing solution for 100 s. Addition of allopregnanolone induced a rapid increase in the tonic outward current, whereas wash-out of the steroid in the continuous presence of THIP was correlated with a progressive decrease in outward current, showing the reversibility of the effect of the steroid. Lower panel – current values measured by Gaussian fit of amplitude histograms over 20 s. The effect of the steroid itself (13.1 pA) is indicated by the arrow.

Fig. S2. The activating effect of SR can be detected from 10 μm (A), is larger at 20 or 40 μm (A–C) and persists at postnatal day 15 (D). Experiments performed at −70 mV in high internal Cl (ECl = 0 mV), with kynurenic acid, strychnine, TTX and D-AP-5, in different HMs. (A) Current record and successive mean current values measured by Gaussian fit of amplitude histograms over 20 s. Note the small amplitude (4.5 pA) of the inward current activated by 10 μm SR (arrow) and its rapid reversibility. (B) In this experiment, SR, first applied at 20 μm, blocked the residual synaptic currents and rapidly activated an inward current of 15.2 pA, whereas increasing the SR concentration from 20 to 100 μm did not induce any further current increase. (C) Mean values of the net inward current activated by SR, at 10, 20 and 40 μm in different HMs. The effect increased significantly between 10 and 20 or 40 μm (Student's t test, **< 0.001, ***= 0.004), whereas results obtained at 20 and 40 μm were similar (= 0.17). (D) Experiment performed in a HM from a rat slightly older (P15) than those used in most other experiments (P12–13). As usual, 40 μm SR activated an inward current (of 20.7 pA), which was slowly reversible (complete reversibility required > 4 min wash and is not shown).

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