Distribution of central omega 1 (benzodiazepine1) and omega 2 (benzodiazepine2) receptor subtypes in the monkey and human brain. An autoradiographic study with [3H]flunitrazepam and the omega 1 selective ligand [3H]zolpidem

Abstract

The distribution of the central omega 1 (benzodiazepine1; BZD1) and omega 2 (BZD2) receptor subtypes has been studied autoradiographically in monkey and human brain sections using [3H]flunitrazepam (which binds indiscriminately to omega 1 and omega 2 subtypes) and [3H]zolpidem (which recognizes selectively the omega 1 subtype). Both ligands labeled an homogeneous population of binding sites (Kd values approximately equal to 1.5 nM and approximately equal to 5 nM, respectively) whose pharmacological characteristics were similar to those of central omega (BZD) receptors. Regional displacement studies in monkey brain showed that zolpidem was a more effective displacer of [3H]flunitrazepam from omega 1-than from omega 2-enriched areas. For example, it was 73-fold more potent in the cerebellum (omega 1-enriched) than in the dentate gyrus (omega 2-enriched). Zolpidem thus selectively recognizes omega 1 sites in primate brain. The autoradiographic distribution of [3H]flunitrazepam (omega 1 + omega 2) binding sites in primate brain was highly heterogeneous. Very high densities were observed in lamina IV of the neocortex (with higher densities in the occipital than in the frontal pole), the substantia innominata and molecular layer of the dentate gyrus. Intermediate densities were found in other neocortical laminae, cerebellum (molecular layer), claustrum, globus pallidus, caudate-putamen, nucleus accumbens, dentate gyrus (granular layer) and the majority of thalamic nuclei. Structures displaying low binding densities included the substantia nigra and the subthalamic nucleus. The regional distribution pattern of [3H]zolpidem binding sites was qualitatively similar to that of [3H]flunitrazepam but the relative density of the 3H-ligands differed in several brain regions. The relative density of omega 1 and omega 2 subtypes in each particular primate brain region was evaluated by measuring 1) the ratio of [3H]zolpidem to [3H]flunitrazepam binding; 2) [3H]flunitrazepam binding in the presence and in the absence of 100 nM zolpidem. With both approaches, a preferential enrichment in omega 1 sites was observed in lamina IV of sensorimotor cortical regions and in the extrapyramidal motor system (globus pallidus, ventral thalamic complex, subthalamic nucleus, substantia nigra and cerebellum). In contrast, omega 2 sites predominated in limbic areas (e.g., the dentate gyrus, amygdala, nucleus accumbens, cingulate and parahippocampal gyri) and in the anterior thalamic nucleus and caudate nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)