Characterization of neuronal zebra finch GABA(A) receptors: Steroid effects

Songbirds are widely studied to investigate the hormonal control of behavior. However, little is known about the effects of steroids on neurotransmission in these birds. We used electrophysiological and pharmacological techniques to characterize γ-aminobutyric acid (GABA) type A receptors (GABA(A)) of primary cultured telencephalic and hippocampal neurons from developing zebra finches. Additionally, their modulation by 17β-estradiol(E₂), 5α- and 5β-dihydrotestosterone (DHT), 5α- and 5β-pregnan-3α-ol-20-one, and corticosterone was examined. Whole-cell GABA-evoked currents were inhibited by picrotoxin (10 μmol l^-1) and bicuculline methiodide (10 μmol l^-1) and potentiated by pentobarbital (100 μmol l^-1) and propofol (3 μmol l^-1). Loreclezole (10 μmol l^-1) potentiated GABA-evoked currents, suggesting the presence of 132, 133 and/or β₄ subunits. Diazepam (l μmol l^-1) potentiated currents, while Zn²⁺ (l μmol l^-1) caused no inhibition, indicating the presence of γ subunits. 5α- and 5β-Pregnan-3α-ol-20-one (100 nmol l^-1) potentiated currents, whereas E₂ (1 μmol l^-1), 5α- and 5β-DHT (1 μmol l^-1), and corticosterone (10 μmol 1^-1) had no detectable effect. We conclude that zebra finch telencephalic and hippocampal GABA(A) receptors include α, β, and subunits and are similar to their mammalian counter-parts in both their biophysical and pharmacological properties. Additionally, GABA-evoked currents are greatly potentiated by 5α- and 5β-pregnan-3α-ol-20-one but show little or no acute modulation by sex steroids or corticosterone.