Abstract
OBJECTIVE: Royal demolition explosive (RDX) can induce seizures in wildlife and humans following release into the environment or after voluntary consumption. During the Vietnam War, RDX intoxication was the most common cause of generalized seizures in US service personnel, and in some sections of the armed forces, eating of RDX has continued as "a dare" to this day. After its mechanism of action was long unknown, RDX was recently shown to be a GABA(A) receptor antagonist. We here determined the GABA(A) receptor subtype-selectivity of RDX and mapped its functional binding site. METHODS: We used whole-cell patch-clamp to determine the potency of RDX on 10 recombinantly expressed GABA(A) receptors and mapped the RDX binding site using a combination of Rosetta molecular modeling and site-directed mutagenesis. RESULTS: RDX was found to reversibly inhibit the α1β2γ2 GABA(A) receptor with an IC(50) of 23 μmol/L (95% CI 15.1-33.3 μmol/L), whereas α4 and α6 containing GABA(A) receptor combinations were 4-10-fold less sensitive. RDX is binding to the noncompetitive antagonist (NCA) site in the pore. In a molecular model based on the cryo-EM structure of the resting state of the α1β2γ2 receptor, RDX forms two hydrogen bonds with the threonines at the T6' ring and makes hydrophobic interactions with the valine and alanine in 2' position of the α1 or β2 subunits. INTERPRETATION: Our findings characterize the mechanism of action of RDX at the atomistic level and suggest that RDX-induced seizures should be susceptible to treatment with GABA(A) modulating drugs such as benzodiazepines, barbiturates, propofol, or neurosteroids.