Complex interactions between the steroid derivative RU 5135 and the GABAA-receptor complex.

The modulation of 35S]t-butylbicyclophosporothionate (35S]TBPS) binding was used to evaluate the actions of the steroid derivative RU 5135 at the gamma-aminobutyric acidA (GABAA) receptor complex. The inhibition of 35S]TBPS binding by GABA in the presence of various concentrations of RU 5135 was consistent with the hypothesis that RU 5135 is a competitive antagonist at the GABAA receptor. Despite common structural features (i.e., 3 alpha-hydroxylated, 5 beta-reduced A ring) with GABAA receptor-active neurosteroids, RU 5135 did not appear to be competitive at the putative steroid site on the GABAA receptor-active, as demonstrated by Schild analysis of 5 alpha-pregnane-3 alpha-ol-20-one (3 alpha,5 alpha-P) modulation of 35S]TBPS binding in the presence of different concentrations of RU 5135. On the other hand, the reduced potency of 3 alpha,5 alpha-P as an inhibitor of 35S]TBPS binding in the presence of RU 5135, as well as blockade of 5 alpha-pregnane-3 alpha-20 alpha-diol (5 alpha-pregnanediol) inhibition of 35S]TBPS binding by RU 5135 provide further support for the GABAA receptor antagonist properties of RU 5135. Moreover, this amidine steroid was able to partially inhibit 35S]TBPS binding independent of GABA with nanomolar potency; yet the mechanism by which this occurs remains to be determined.