The imidazobenzodiazepinone Ro 15-4513 has been shown previously to bind to central benzodiazepine receptors as well as to a second, uncharacterized class of sites that do not bind diazepam, differentiating them from the normal benzodiazepine-binding site on the gamma-aminobutyric acid (GABA)-A (GABAA) receptor. This study describes the characterization of these unique diazepam-insensitive (DZ-IS) sites. Ro 15-4513 binding to DZ-IS sites was abundant in cerebellum from cow, rat and human and detectable in cortex, hippocampus and striatum by autoradiography on rat brain sections. These sites represented approximately 20% of the total binding in bovine cerebellar membranes, but only 2 to 3% of the total in cortex. Ro 15-4513 binds with the same affinity (Kd approximately 4.5 nM) to both diazepam-sensitive and DZ-IS sites in the cerebellum. A number of compounds which bind to the classical benzodiazepine receptors also bind to the DZ-IS sites. These compounds include: the pyrazoloquinoline CGS 8216, the beta-carbolines methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, ZK 95962, ZK 94326 and ZK 93126, as well as the classical benzodiazepine receptor antagonist, Ro 15-1788. Besides binding diazepam poorly, the DZ-IS sites demonstrate a very low affinity for other benzodiazepines. Ligands which bind to the various drug receptor sites on the GABA receptor complex do not directly modulate the binding of Ro 15-4513 to DZ-IS sites nor does ethanol. However, antagonism of Ro 15-4513 binding to the DZ-IS sites by methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate and by CGS 8216 is modulated by the presence of GABA.(ABSTRACT TRUNCATED AT 250 WORDS)