Autoradiographic localization and quantitative determination of specific binding sites of anxiolytic homophthalazines (formerly called 2,3-benzodiazepines) in the striato-pallido-nigral system of rats.

Homophthalazines (2,3-benzodiazepin-derivates, such as tofisopam, nerisopam, girisopam) constitute a drug family with strong anxiolytic and antipsychotic potencies. By autoradiography, all of these drugs showed a specific distribution pattern of binding sites exclusively in brain areas which relate to the striato-pallido-nigral system, while no specific label was found in any other brain areas in the rat. Quantitative analyses of the autoradiograms by computerized densitometry, as well as by a receptor binding assay on 32 microdissected brain areas showed very high concentrations of tritiated homophthalazines in the glubus pallidus, caudate nucleus, putamen and the substantia nigra. Relatively high density of binding sites was measured in the nucleus accumbens, the olfactory tubercle, the entopeduncular nucleus and the subthalamic nucleus. Concentrations measured in the cerebral cortical areas, cerebellum or brainstem nuclei did not differ from the background. No significant differences were found between the homophthalazines investigated in terms of the distribution patterns or density of binding sites.