Development of tolerance to amnesic effects of chlordiazepoxide in relation to GABAergic and cholinergic neuronal systems

Chronic administration of benzodiazepines has been reported to produce tolerance in animals and humans. We investigated whether benzodiazepines produce tolerance to the amnesic effects and effects on benzodiazepine receptors, GABAergic and/or cholinergic neuronal systems of repeated administration of chlordiazepoxide, using a passive avoidance task and autoradiographic techniques. Tolerance developed to the amnesic effect of chlordiazepoxide when the drug was administered at a dose of 30 mg/kg (i.p.) once a day for 14 days. Bicuculline (1.0 and 1.5 mg/kg), a GABA_A receptor antagonist, did not induce amnesia in normal mice, but did so in chlordiazepoxide-tolerant mice. Muscimol (0.25 mg/kg), a GABA_A receptor agonist, in combination with a low dose of chlordiazepoxide, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. Scopolamine, an acetylcholine receptor antagonist, induced amnesia in normal mice, but not in chlordiazepoxide-tolerant mice. In the autoradiographical study, although repeated treatment with chlordiazepoxide had no effect on [³H]flunitrazepam and [³H]Ro 15-4513 binding to benzodiazepine receptors, it decreased [³H]muscimol binding to GABA_A receptors, with a decrease in affinity in the cortex and hippocampus. Furthermore, repeated administration of chlordiazepoxide increased [³H]quinuclidinyl benzilate binding to muscarinic acetylcholine receptors in the hippocampus. These results suggest that tolerance develops to the amnesic effects of chlordiazepoxide, and that tolerance may be due to down-regulation of GABA_A receptors and/or up-regulation of acetylcholine receptors.