Autoradiographic Localization of [³H]Zolpidem Binding Sites in the Rat CNS: Comparison with the Distribution of [³H]Flunitrazepam Binding Sites

Abstract

The regional distribution of [3H]zolpidem, a novel imidazopyridine hypnotic possessing preferential affinity for the BZD1 (benzodiazepine subtype 1) receptor, has been studied autoradiographically in the rat CNS and compared with that of [3H]flunitrazepam. The binding of [3H]zolpidem to rat brain sections was saturable, specific, reversible, and of high affinity (KD = 6.4 nM). It occurred at a single population of sites whose pharmacological characteristics were similar to those of the benzodiazepine receptors labeled with [3H]flunitrazepam. However, ethyl-beta-carboline-3-carboxylate and CL 218,872 were more potent displacers of [3H]zolpidem than of [3H]flunitrazepam. The autoradiographic brain distribution of [3H]zolpidem binding sites was qualitatively similar to that previously reported for benzodiazepine receptors. The highest levels of [3H]-zolpidem binding sites occurred in the olfactory bulb (glomerular layer), inferior colliculus, ventral pallidum, nucleus of the diagonal band of Broca, cerebral cortex (layer IV), medial septum, islands of Calleja, subthalamic nucleus, and substantia nigra pars reticulata, whereas the lowest densities were found in parts of the thalamus, pons, and medulla. Comparative quantitative autoradiographic analysis of the binding of [3H]zolpidem and [3H]flunitrazepam [a mixed BZD1/BZD2 (benzodiazepine subtype 2) receptor agonist] in the CNS revealed that the relative density of both 3H-labeled ligands differed in several brain areas. Similar levels of binding for both ligands were found in brain regions enriched in BZD1 receptors, e.g., substantia nigra pars reticulata, inferior colliculus, cerebellum, and cerebral cortex lamina IV. The levels of [3H]zolpidem binding were five times lower than those of [3H]flunitrazepam binding in those brain regions enriched in BZD2 receptors, e.g., nucleus accumbens, dentate gyrus, and striatum. Moreover, [3H]zolpidem binding was undetectable in the spinal cord (which contains predominantly BZD2 receptors). Finally, like CL 218,872 and ethyl-beta-carboline-3-carboxylate, zolpidem was a more potent displacer of [3H]flunitrazepam binding in brain regions enriched in BZD1 receptors than in brain areas enriched in BZD2 receptors. The present data add further support to the view that zolpidem, although structurally unrelated to the benzodiazepines, binds to the benzodiazepine receptor and possesses selectivity for the BZD1 receptor subtype.

The distribution of 13 GABAA receptor subunit mRNAs in the rat brain. II. Olfactory bulb and cerebellum

“…Autoradiographic studies examining the distribution of total GABA, and BZ binding sites in the rat olfactory bulb reveal a very high density of both these binding sites in the external plexiform layer Palacios et al, 198 1;Marcel et al, 1986;Bowery et al, 1987;Niddam et al, 1987). A moderate density of both GABA, and BZ sites also occurs in the glomerular layer.…”

“…Collectively, these data suggest the existence of GABA, receptor heterogeneity. Benzodiazepine (BZ) agonists bind predominantly to the molecular layer, whereas GABA, agonists (GABA, muscimol) bind mainly to sites in the granule cell layer (Palacios et al, 1980, 198 1;Young and Kuhar, 1980;Unnerstall et al, 198 1;Richards et al, 1986;Bowery et al, 1987;Niddam et al, 1987;Olsen et al, 1990). Nevertheless, even though highaffinity GABA, agonist sites are relatively scarce in the molecular layer, BZ binding is still enhanced by GABA in this sector, suggesting that these BZ sites are coupled to GABA, receptors (Unnerstall et al,198 1).…”

“…Subsequently, other ligands such as the GABA, antagonist SR 9553 1 have been shown to target a markedly reduced number of sites in cerebellum, with approximately equal binding densities in both granule cell and molecular layers (Bristow and Martin, 1988;Olsen et al, 1990). The majority of BZ binding sites in the molecular layer (greater than 90%) have a selectively high affinity for P-carbolines and CL 218, 872 (Young et al, 198 1;Niddam et al, 1987;Sieghart and Schlerka, 1991) and have been termed BZ I. Tritiated flunitrazepam photolabels only one polypeptide in cerebellum (P51), which exhibits a BZ I binding profile (Sieghart, 1989).…”

“…Although recombinant receptors consisting only of (Y-and &subunits are activated by GABA (Schofield et al, 1987;Sigel et al, 1990;Verdoom et al, 1990), coexpression of a y-subunit is necessary for robust BZ sensitivity (Pritchett et al, 1989bHerb et al, 1992). The borderline level of y2 mRNA in the granule cells, the paucity of detectable immune reaction for y2 (Benke et al,199 lb), and the very reduced levels of high-affinity BZ binding over this area Marcel et al, 1986;Niddam et al, 1987), together suggest that internal granule cells generally do not construct y,-containing GABA, receptors. Any contribution of y, and ys subunits to granule cell receptors would have gone undetected by previous autoradiographic studies, since replacement of these two subunits for y2 lowers the affinity of &3y complexes for BZs by up to two orders of magnitude Herb et al, 1992).…”

A key role of the mammillary body in mediation of the antianxiety action of zopiclone, a cyclopyrrolone derivative.

“…The functional activity of MB, in the mediation of behavioral suppression, does not seem to be regulated by the r-amino butyric acid (GABA) system, as deduced from the following results (14): The injection of GABA and muscimol into ACE produced a dose-dependent increase in the punished responses, but that into MB did not. On the other hand, ZOP has been reported to have about a 2.5-fold higher affinity for BDZ 1 than BDZ 2 (15). It is, therefore, likely that the atypical BDZ 1 with a high and low affinity for 9-CCM and CL 218,872, respec tively, may mediate the anticonflict action of ZOP in MB, in a GABA-independent manner.…”