CHRONIC FLURAZEPAM TREATMENT PRODUCES DECREASED EFFICACY OF THE BENZODIAZEPINE LIGANDS AND PENTOBARBITAL WITH GAMMA-AMINOBUTYRIC ACID(A) RECEPTORS IN CORTICAL-NEURONS

The present study was designed to determine whether chronic benzodiaze pine (BZ) agonist treatment alters the ''set point'' of the BZ pharmac ological profile. This was achieved by investigating the modulation of gamma-aminobutyric acid (GABA)-mediated [Cl-36(-)] influx by BZ ligan ds, as well as pentobarbital after chronic flurazepam treatment, in we ll characterized mammalian cortical neurons. Chronic flurazepam treatm ent (5 mu M, 10 days) produced decreased efficacy of BZ agonists (diaz epam and flunitrazepam) and inverse agonists ,7-dimethoxy-4-ethyl-beta -carboline-3'-carboxylate and ethyl-beta-carbotine-3-carboxylate), as measured by GABA-induced [Cl-36(-)] influx. The chronic flurazepam tre atment, although not altering their EC(50)/IC50 values, decreased the E(max)/-E(max) values. Furthermore, the decreased efficacy was reverse d after a 72-hr withdrawal, and by concomitant exposure of the neurons to Ro15-1788 8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5 alpha ][1,4]-BZ-3-carboxylate), a BZ receptor antagonist, but not by R 5135 (3 alpha-hydroxy-16-imino-5 beta-17-androstan-11-one; a GABA receptor antagonist) and pircrotoxin (a channel blocker). The chronic flurazepa m treatment also produced uncoupling between pentobarbital and BZ rece ptor sites, and decreased the efficacy of pentobarbital to enhance GAB A-mediated [Cl-36(-)] influx, events also reversed at 72-hr withdrawal and concomitant exposure to Ro15-1788. Chronic flurazepam treatment-i nduced uncoupling and decreased efficacy of BZ agonists was not revers ed by the GABA(A) receptor antagonist, R 5135, or channel blocker, pic rotoxin. Taken together, these findings suggest that chronic flurazepa m treatment produces the decreased efficacy of BZ agonists and inverse agonists and barbiturates with the GABA receptor gated Cl- channel, a nd these events are mediated selectively via the BZ reception site. Ho wever, the set point was not shifted toward the inverse agonistic spec trum of the pharmacological profile. The decreased efficacy may be res ponsible for the tolerance associated with the chronic BZ treatment. T he molecular basis for the decreased efficacy could be due to an alter ation in the relative levels of various subunits that form the functio nal GABA(A) receptors or due to post-translational modification of the receptors.