A novel specific binding site for homophthalazines in the rat brain.

The specific binding sites of a homophthalazine, girisopam, in rat brain have been localized by qualitative and quantitative autoradiography. This substance exerts strong anxiolytic and antipsychotic effects both in rodents and in humans. High labeling was present in all major components of the extrapyramidal system, such as the caudate-putamen, globus pallidus, subthalamic nucleus, substantia nigra, and the extrapyramidal portion of the accumbens nucleus and the olfactory tubercle, while specific labeling was not seen in any other brain areas including the cerebral cortex, thalamus, cerebellum or brainstem areas. This novel distribution of girisopam is consistent with its antipsychotic effect and anxiolytic properties and may provide a morphological basis for further studies to elucidate the mechanisms of action of homophthalazines in the central nervous system.