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BEHAVIORAL PHARMACOLOGY

Do Subtype-Selective -Aminobutyric Acid_A Receptor Modulators Have a Reduced Propensity to Induce Physical Dependence in Mice?

NeuroSearch A/S, Ballerup, Denmark

Recent evidence suggests that GABA_A receptors containing an ₁ subunit mediate the sedative effect of diazepam, whereas receptors with an ₂ subunit mediate this benzodiazepine's anxiolytic effect. Thus, compounds selective for GABA_A-₂ receptors may offer advantages, i.e., lack of sedation, over current benzodiazepines. Whether such compounds would offer additional advantages over benzodiazepines is unclear. Here, we address the issue of physical dependence by comparing the GABA_A-₁ affinity-selective drug zolpidem, the novel compounds 7-(1,1-dimethylethyl)-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2,5-difluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (L-838,417) and 6-fluoro-9-methyl-2-phenyl-4-(pyrrolidin-1-yl-carbonyl)-2,9-dihydro-1H-pyridol[3,4-b]indol-1-one (SL651498) with functional selectivity for certain non-₁ GABA_A receptors, nonselective partial agonists [bretazenil, 1-[1-[3-(3-pyridyl)phenyl]benzimidazol-5-yl]ethanone O-ethyloxime (NS2710), and 5-furan-3-yl-1-(3-imidazol-1-phenyl)-1H-benzoimidazole (NS2664)], and nonselective full efficacy benzodiazepines, in a rapid precipitated withdrawal assay using the inverse agonist N-methyl--carboline-3-carboxamide (FG-7142). For all compounds, we determined in vitro IC₅₀ values to displace [³H]flunitrazepam from rat cortex and in vivo ED₅₀ values for displacement of [³H]flunitrazepam from mouse forebrain (including length of in vivo occupancy). In the precipitated withdrawal model, compounds were administered at a dose giving 80% receptor occupancy, obviating major differences in central nervous system bioavailability. Mice were administered compounds twice daily for 4 days and on day 5, 20 h after the final dose, given a dose of FG-7142 (40 mg/kg i.p.) that did not induce seizures in control animals. In mice treated with the three subtype-selective compounds, FG-7142 did not induce seizures. Moreover, there was a low propensity for FG-7142 to induce seizures in animals treated with the partial agonists, whereas seizures were clearly seen in animals treated with most benzodiazepines. Nonetheless, differences among the benzodiazepines themselves, similarities between the partial agonists and subtype-selective compounds, the in vitro/in vivo potency, and in vivo receptor exposure time data suggest a complex interaction among selectivity, efficacy, potency, and receptor exposure in determining physical dependence liability of benzodiazepine site modulators in mice.

Address correspondence to: Naheed R. Mirza, Department of in Vivo Pharmacology, NeuroSearch A/S, 93 Pederstrupvej, DK-2750 Ballerup, Denmark. E-mail: max{at}neurosearch.dk