Augmentation of GABAA receptor function by chronic exposure to GABA-neutral and GABA-negative benzodiazepine ligands in cultured cortical neurons.

Chronic benzodiazepine agonist administration may lead to decreases in gamma-aminobutyric acidA (GABAA) receptor binding and function, but little information is available concerning chronic GABA-neutral or GABA-negative benzodiazepine exposure. We evaluated effects of chronic exposure to flumazenil (Ro15-1788) and FG 7142 (N-methyl-beta-carboline-3-carboxamide) on GABA-dependent chloride uptake in chick cerebral cortical neurons in primary culture. Acute flumazenil treatment (1 microM) had no effect on chloride uptake, but uptake was increased after 2 days of exposure. Similar increases were observed after 4 and 10 days. Flumazenil, 0.1 microM, had no effect after 10 days, and a 10 microM concentration had a similar effect as the 1 microM concentration. Acute FG 7142 (1 microM) decreased chloride uptake, but uptake was increased markedly after 2, 4, and 10 days of treatment. No effect was observed after treatment for 10 days with 0.1 microM, but a 10 microM concentration showed similar enhancement to the 1 microM concentration. Concurrent treatment with 0.3 microM flumazenil which did not affect chloride uptake and 1 microM FG 7142 for 10 days substantially attenuated the effects of FG 7142, suggesting that FG 7142 effects are mediated at the benzodiazepine site. Benzodiazepine receptor binding was increased in cultures treated for 10 days with 1 microM flumazenil or FG 7142, with an increase in receptor number in both cases but no change in apparent affinity. Neither flumazenil nor FG 7142 (1 microM for 10 days) altered GABA-independent chloride uptake, total cellular protein, protein synthesis or degradation, or neuronal survival. These results indicate that both chronic GABA-neutral and GABA-negative benzodiazepine exposures in cultured cortical neurons lead to increases in GABA-dependent chloride uptake and benzodiazepine binding. Effects of GABA-negative benzodiazepine exposure appear to be greater than those observed with GABA-neutral benzodiazepine exposure.