The ionophore function of gamma-aminobutyric acid A (GABA(A)) receptors was studied by whole-cell patch clamp electrophysiology in primary cultures of rat cerebellar cortex. Chloride currents elicited by 1 microM GABA were potentiated by allopregnanolone with a plateau of high affinity (EC(50) = 14 nM) and a peak of potentiation around 1 microM allopregnanolone. Furosemide (0.1 mM) eliminated the high affinity phase and increased the EC(50) to 685 nM. GABA(A) receptors of rat cerebellar synaptosomal membranes were labelled with [(3)H]ethynylbicycloorthobenzoate (EBOB). Allopregnanolone displaced [(3)H]EBOB binding with IC(50) = 320 nM. The displacing potency of allopregnanolone was strongly enhanced (IC(50) = 39 nM) in the presence of 400 nM GABA and 60 nM SR 95531. Nanomolar potentiation by allopregnanolone can be associated with cerebellar GABA(A) receptors containing alpha(6), beta(2-3) and delta subunits. This might be suitable for physiological modulation of tonic inhibitory neurotransmission via extrasynaptic GABA(A) receptors in cerebellar granule cells by neurosteroids.