In this study, 48 h administration of 3alpha-OH-5beta-pregnan-20-one (3alpha,5beta-THP) or 17beta-estradiol (E2)+progesterone (P) to female rats increased expression of the delta subunit of the GABA(A) receptor (GABAR) in CA1 hippocampus. Coexpression of alpha4 and delta subunits was suggested by an increased response of isolated pyramidal cells to the GABA agonist 4,5,6,7- tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), following 48 h steroid treatment, and nearly complete blockade by 300 microM lanthanum (La3+). Because alpha4betadelta GABAR are extrasynaptic, we also recorded pharmacologically isolated GABAergic holding current from CA1 hippocampal pyramidal cells in the slice. The La3+-sensitive THIP current, representative of current gated by alpha4betadelta GABAR, was measurable only following 48 h steroid treatment. In contrast, the bicuculline-sensitive current was not altered by steroid treatment, assessed with or without 200 nM gabazine to block synaptic current. However, 48 h steroid treatment resulted in a tonic current insensitive to the benzodiazepine agonists lorazepam (10 microM) and zolpidem (100 nM). These results suggest that 48 h steroid treatment increases expression of alpha4betadelta GABAR which replace the ambient receptor population. Increased anxiolytic effects of THIP were also observed following 48 h steroid treatment. The findings from the present study may be relevant for alterations in mood and benzodiazepine sensitivity reported across the menstrual cycle.