Genetic selection for benzodiazepine ataxia produces functional changes in the γ-aminobutyric acid receptor chloride channel complex

The γ-aminobutyric acid (GABA) receptor-operated chloride channel complex was evaluated in mice selected for differential sensitivity to the ataxic effects of diazepam (diazepam-sensitive (DS) and diazepam-resistant (DR) lines). The ataxic effects of several drugs purported to produce some of their actions through the benzodiazepine-GABA receptor complex were examined using the rotarod test. The duration of impairment produced by diazepam, ethanol, 4,5,6,7-tetrahydroisoxazol[5,4-C]pyridine-3-ol (THIP) and phenobarbital was greater in the diazepam-sensitive than in the diazepam-resistant mice. In contrast, pentobarbital produced an equivalent duration of ataxia in the two lines. Muscimol-stimulated 36Cl − influx and the binding of [ 35S] t-butylbiclclophosphorothionate (TBPS) and [ 3H]flunitrazepam were measured using isolated membrane vesicles (microsacs). Depolarization-dependent 45Ca 2+ uptake was measured in whole brain synaptosomes. Muscimol was a more potent stimulator of 36Cl − flux in the DS compared to the DR mice, although no difference betweet the lines was found in muscimol-stimulation of [ 3H]flunitrazepam binding. Flunitrazepam augmented the muscimol-stimulated 36Cl − uptake in the DS but not in the DR mice. However, no differences between the lines of mice were found in either density or affinity of [ 3H]flunitrazepam binding sites. Similarly, no differences in either the density or affinity of [ 35S]TBPS binding sites was found. Ethanol (10–45 mM) potontiated the muscimol-stimulation of 36Cl − in DS, with no effect in DR mice. However, ethanol inhibition of [ 35S]TBPS binding was equivalent in the two lines of mice. Pentobarbital produced an equal potentiation of the muscimol-stimulated 36Cl − flux in the two lines, but phenobarbital potentiated the muscimol-induced 36Cl − influx slightly more in DS mice. Potassium-stimulated uptake of 45Ca 2+ was larger in DS mice, but no difference between the lines was seen in diazepam inhibition of depolarization-dependent 45Ca 2+ uptake. These findings demonstrate differences between the DS and DR mice in the GABA receptor-operated chloride channel function that closely parallel the behavioral differences in ataxia and may represent the neurochemical mechanisms underlying genetic differences in benzodiazepine sensitivity.