DIRECT EFFECTS OF THE ANABOLIC ANDROGENIC STEROIDS, STANOZOLOL AND 17-ALPHA-METHYLTESTOSTERONE, ON BENZODIAZEPINE BINDING TO THE GAMMA-AMINOBUTYRIC ACID(A) RECEPTOR/

Various exogenous and endogenous steroids have been demonstrated to ha ve both enhancing and inhibiting effects on ligand binding to the gamm a-aminobutyric acid(A) receptor (GABA(A) receptor) in previous studies . In the present study we have explored the possibility that an additi onal class of synthetic steroidal compounds, anabolic/androgenic stero ids (AAS), mediate some of their CNS effects through direct interactio n with the GABA(A) receptor. At micromolar concentrations, two AAS, st anozolol and 17 alpha-methyltestosterone (17 alpha-MT), significantly inhibited 1 nM [H-3]flunitrazepam ([H-3]Fln) binding to rat brain cere brocortical membranes. Inhibition of 1 nM [H-3]Fln binding by stanozol ol was similar for both males and females (similar to 50% inhibition a t 50 mu M stanozolol). 17 alpha-MT was much less efficacious, but did significantly inhibit 1 nM [H-3]Fln binding at concentrations >10 mu M . In equilibrium binding assays, stanozolol (50 mu M) raised the appar ent K-D for [H-3]Fln binding. The observed changes in the [H-3]Fln bin ding curve, when analyzed by Rosenthal analysis, reveal complex equili brium binding behavior. In females, the Rosenthal plot was best fit by a two site binding model. Stanozolol (50 mu M) inhibited binding to t he higher affinity site in a manner consistent with competitive inhibi tion, increasing the K-D without changing the B-max. However, the effe ct of stanozolol on the binding to the low affinity site was more comp lex, with an increase in both the K-D and the B-max. In males the data were best fit by a single binding site model. This single site exhibi ted a slight increase in the K-D and a decrease in the B-max in the pr esence of 50 mu M stanozolol. Our results demonstrate that the AAS, st anozolol and 17 alpha-MT, directly interact with the GABA(A) receptor and that their effects on ligand binding to the GABA(A) receptor are d ifferent between the sexes.