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G Wong, T Lyon and P Skolnick
Laboratory of Neuroscience, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892.
Chronic exposure to benzodiazepines can result in an "uncoupling" of gamma-aminobutyric acid (GABA) receptors and benzodiazepine receptors (BzR) both in primary neuronal cell cultures and in vivo. The effect of chronic exposure to BzR ligands was examined in an engineered cell line (WSS-1) stably expressing "type I" GABAA receptors. Chronic exposure to flurazepam produced a concentration- (EC50, approximately 1.1 microM after a 48-hr exposure) and time-dependent (t1/2, approximately 3 hr at 100 microM) reduction in the efficacy (Emax) of GABA to enhance [3H]flunitrazepam binding to BzR, a characteristic of uncoupling in native GABAA receptor isoforms. Uncoupling of GABAA receptors and BzR without concomitant changes in BzR density was also produced by chronic exposure to other, structurally diverse, BzR ligands, including Ro 15- 1788 and methyl-6,7-dimethoxy-4-ethyl-beta-carboline-3-carboxylate, but was not manifested after exposure to the 5-hydroxytryptamine reuptake blocker fluoxetine. Chronic (12-48-hr) exposure to flurazepam did not remarkably alter levels of alpha 1 and gamma 2 mRNAs, which constitute GABAA receptors in this cell line. Based on these findings, it is hypothesized that uncoupling of GABAA receptors and BzR in this engineered cell line can proceed without the elaboration of additional novel subunits and could involve either post-translational modification of GABAA receptor proteins or changes in subunit stoichiometry.
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