Vol. 295, Issue 2, 689-696, November 2000

Down-Regulation of Benzodiazepine Binding to 5 Subunit-Containing -Aminobutyric Acid_A Receptors in Tolerant Rat Brain Indicates Particular Involvement of the Hippocampal CA1 Region¹

Ming Li, Andras Szabo and Howard C. Rosenberg

Department of Pharmacology and Therapeutics, Medical College of Ohio, Toledo, Ohio

Chronic benzodiazepine treatment can produce tolerance and changes in -aminobutyric acid (GABA)_A receptors. To study the effect of treatment on a selected population of receptors, assays were performed using [³H]RY-80, which is selective for GABA_A receptors with an 5 subunit. Rats were given a flurazepam treatment known to produce tolerance and down-regulation of benzodiazepine binding, or a diazepam treatment shown to produce tolerance but not receptor down-regulation. Quantitative receptor autoradiography using sagittal brain sections bound with [³H]RY-80 showed binding in areas known to express 5 mRNA. Brains from flurazepam-treated rats showed significantly decreased 1 nM [³H]RY-80 binding in hippocampal formation (e.g., 32% decrease in CA1) and superior colliculus, but not other areas. Using 5 nM [³H]RY-80 showed similar decreases in hippocampus. A corresponding 29% decrease in B_max but no change in K_d was found with a filtration binding assay using hippocampal homogenates. Down-regulation of [³H]RY-80 binding had returned to control by 2 days after withdrawing flurazepam treatment. The magnitude of down-regulation of [³H]RY-80 binding suggested that GABA_A receptors with an 5 subunit may play a prominent role in the adaptive responses associated with benzodiazepine tolerance. Chronic diazepam treatment also resulted in decreased [³H]RY-80 binding. However, the regional selectivity was even more pronounced than in flurazepam-treated rats, and only the hippocampal CA1 region showed decreased binding (27%). This localized down-regulation persisted for several days after the end of diazepam treatment. These data indicate that synapses in the hippocampal CA1 region are particularly involved in the adaptive response to chronic benzodiazepine treatments.