Effects of steroids on gamma-aminobutyric acid receptors expressed in Xenopus oocytes by poly(A)+ RNA from mammalian brain and retina.

Electrical recordings were made in Xenopus oocytes to study the modulatory effects of steroids on gamma-aminobutyric acid (GABA) receptors expressed by RNA from mammalian brain and retina. GABA responses expressed by rat cerebral cortex poly(A)+ RNA were bicuculline-sensitive Cl- currents mediated by GABAA receptors. GABA responses expressed by bovine retina poly(A)+ RNA also were Cl- currents but were composed of two pharmacologically distinct components, one mediated by GABAA receptors and the other by GABA receptors with novel properties, which were resistant to bicuculline but were not activated by R(+)-baclofen, a selective agonist of GABAB receptors. As reported in neurons and in other expression systems, GABAA responses expressed in oocytes by cerebral cortex RNA were strongly and stereospecifically potentiated by 5 alpha-pregnan-3 alpha-ol-20-one (3 alpha-OH-DHP) and 5 alpha-pregnan-3 alpha,21-diol-20-one (THDOC). Threshold levels of potentiation were detectable using 1-2 nM steroid, and at concentrations of 50 and 500 nM 3 alpha-OH-DHP shifted the EC50 of cortex GABAA responses from a control value of 92 +/- 20 microM GABA to 40 +/- 4.3 microM and 13 +/- 1.8 microM, respectively. However, even at concentrations as high as 50 microM, 3 alpha-OH-DHP did not itself elicit appreciable membrane current responses through direct activation of the cortex GABAA receptors. In addition to potentiation, 3 alpha-OH-DHP and THDOC caused pronounced increases in the rate of desensitization of GABAA responses expressed by cortex RNA. Decay time courses of currents elicited by 1 mM GABA (90-95% of the maximum response) were fitted by the sum of two exponentials. Under control conditions, the time constant of the fast component was 4.4 +/- 0.6 sec and the slow component, 22.5 +/- 4.8 sec. 3 alpha-OH-DHP at 500 nM and 5 microM reduced the time constant of the fast component by 52 +/- 7% and 84 +/- 5%, respectively, but showed little effect on the slow component. Unlike the potentiation effect, actions of pregnenolones on desensitization did not show stringent stereoselectivity, and 5 microM 5 beta-pregnan-3 beta-ol-20-one (3 beta-OH-DHP) reduced the time constant of the fast component by 59 +/- 11%.(ABSTRACT TRUNCATED AT 400 WORDS)