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RJ Primus, J Yu, J Xu, C Hartnett, M Meyyappan, C Kostas, TV Ramabhadran and DW Gallager
Neurogen Corporation, Branford, Connecticut, USA.
By using the baculovirus expression system, we report decreases in allosteric coupling at individual gamma-aminobutyric acid (GABA)(A) receptor subtypes (alpha-1, beta-2 and gamma-2, alpha-2, beta-3 and gamma-2 and alpha-5, beta-3 and gamma-2) after chronic benzodiazepine exposure that replicate coupling changes measured in rat cortical membranes after in vivo benzodiazepine exposure. The appearance of uncoupling was time-dependent and the magnitude of uncoupling at expressed GABA(A) receptor subtypes after chronic exposure was dependent upon the efficacy of the ligand in a subtype-specific manner. In addition, the expression of uncoupling was not accompanied by changes in benzodiazepine receptor number or affinity at any expressed GABA(A) subtype examined. The specificity of the coupling change was further shown by the ability of a brief exposure to the benzodiazepine receptor antagonist, Ro15-1788, to reverse the uncoupling induced by chronic benzodiazepine exposure. These findings suggest that alterations at the GABA(A) receptor complex after chronic benzodiazepine exposure are mediated directly by agonist effects at the GABA(A) receptor complex and are not the product of the changes in the surrounding neuronal environment. Furthermore, the present study shows that drug efficacy, and not simply affinity, plays a critical role in determining the degree of uncoupling, and perhaps, in the development of tolerance and dependence.
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