Benzodiazepine receptor binding and anticonflict activity in a series of 3,6-disubstituted pyridazino[4,3-c]isoquinolines devoid of anticonvulsant properties

A series of 3,6-disubstituted pyridazino[4,3-c]isoquinolines were synthesized and tested for their ability to inhibit the binding of [3H]diazepam to rat brain receptors in vitro. Compounds bearing a phenyl, 4-methoxyphenyl, or methyl group at position 3 and a dialkylamino group at position 6 showed the highest affinity in the binding assay and were subsequently evaluated for their anticonflict and anticonvulsant effects. All of these compounds (5a-1 and 5q) were active in the Vogel rat conflict procedure, but none prevented convulsions in mice induced either by metrazol or bicuculline. 3-Phenyl-6-pyrrolidinylpyridazino[4,3-c]isoquinoline (5d) with a Ki = 11.4 nM in the binding assay exhibited the best potency in the anticonflict assay (MED 5 mg/kg ip) and did not produce neuromuscular impairment at the highest dose tested (50 mg/kg ip).