Does neurosteroid modulatory efficacy depend on GABAA receptor subunit composition?

The modulation of GABA activity by 3 alpha-OH-DHP (allopregnanolone, 3 alpha-hydroxy-5 alpha-pregnan-20-one) and PS (pregnenolone sulfate) has been studied in native GABAA receptors of rat cortical neurons in primary cultures and in structurally different recombinant GABAA receptors of rat cortical neurons in primary cultures and in structurally different recombinant GABAA receptors expressed in the 293 human embryonic kidney cell line (HEK 293). In cortical neurons 3 alpha-OH-DHP positively modulates GABA elicited Cl- currents while PS at 10 microM negatively modulates (50% decrease) this GABA response, but at 10 nM PS positively modulates the GABA current (40% increase). Both neurosteroids are equally active on various types of recombinant GABAA receptors, except for alpha 6 beta 1 gamma 2 receptors which are less sensitive to the positive allosteric modulation by 3 alpha-OH-DHP. In contrast the presence of the gamma 1 subunit doubles the efficacy of 3 alpha-OH-DHP. The negative modulation of PS is similar in recombinant GABAA receptors including various molecular forms of alpha or gamma units. A direct activation of Cl- current by 3 alpha-OH-DHP was observed in native and recombinant receptors but its efficacy on the various molecular forms of GABAA receptor tested was always smaller than that of identical concentrations (10 microM) of GABA.