Disclosed are compounds of the formula: ##STR00001## or the pharmaceutically acceptable non-toxic salts thereof wherein the A, B, C, D, X, R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6, are variables defined herein, which compounds are highly selective agonists, antagonists or inverse agonists for GABAa brain receptors or prodrugs of agonists, antagonists or inverse agonists for GABAa brain receptors, and are therefore useful in the diagnosis and treatment of anxiety, Down Syndrome, sleep, cognitive and seizure disorders, depression, overdose with benzodiazepine drugs, and enhancement of memory and alertness.