High affinity, heterogeneous displacement of [3H]EBOB binding to cerebellar GABAA receptors by neurosteroids and GABA agonists

Heterogeneous binding interactions of cerebellar GABAA receptors were investigated with GABA agonists and neurosteroids. GABAA receptors of rat cerebellum were labelled with [3H]ethynylbicycloorthobenzoate (EBOB), a convulsant radioligand. Saturation analysis revealed a homogenous, nanomolar population of [3H]EBOB binding. Both GABA and 5α-tetrahydrodeoxycorticosterone (5α-THDOC) displaced [3H]EBOB binding heterogeneously, with nanomolar and micromolar potencies. The nanomolar phase of displacement by GABA was selectively abolished by 100 μM furosemide. Physiological concentrations of allopregnanolone (8 nM) and 5α-THDOC (20 nM) increased the displacing effects of nanomolar GABA. GABA (0.3 μM) and 5α-THDOC (0.3 μM) potentiated the micromolar population of displacement by the other. Taurine inhibited [3H]EBOB binding also heterogeneously, with micromolar and millimolar potencies, and 0.3 μM 5α-THDOC potentiated this inhibition. 5β-THDOC did not affect [3H]EBOB binding significantly but in 1 μM it antagonised selectively the nanomolar displacement by 5α-THDOC. [3H]EBOB binding to hippocampal GABAA receptors was inhibited by GABA and allopregnanolone with low (micromolar) potencies and with slope values higher than unity referring to allosteric interaction. High affinity displacement of cerebellar [3H]EBOB binding by GABA agonists and neurosteroids can be associated with constitutively open α6βδ GABAA receptors, tonic GABAergic inhibitory neurotransmission and its modulation by physiological concentrations of neurosteroids.