Region-specific changes of GABA A receptors by tolerance to and dependence upon pentobarbital

Abstract

An experimental model for inducing tolerance to and dependence upon pentobarbital was characterized. Rats were infused with pentobarbital, 300 μg/10 μ1 per h i.c.v. for 6 or 7 days, through pre-implanted cannulas by osmotic minipumps. Measurement of brain and serum levels showed that pentobarbital remained mainly inside of the brain. Measurements of sleeping time and susceptibility to convulsant-induced seizures indicated a substantial degree of tolerance and dependence (24 h after termination of infusion). Results of GABA A receptor binding assays showed marked regional variations. While [ 35S]t-butylbicyclophosphorothionate (TBPS) binding sites were increased in frontal cortices and striata of pentobarbital-dependent animals, K D was increased in striata of tolerant animals. Dependence upon pentobarbital was correlated with increased [ 3H]flunitrazepam binding sites in all three regions examined. Both high and low affinity [ 3H]muscimol binding sites were increased in dependent animals, but low affinity sites were decreased in frontal cortices of tolerant animals. K Ds of [ 3H]muscimol high affinity sites were increased in cerebellum after animals developed dependence upon pentobarbital. K Ds of [ 3H]muscimol low affinity sites were decreased in striata of pentobarbital tolerant animals. These findings further support the hypothesis that GABA A receptors in discrete areas of the brain have different subunit compositions and are regulated differentially by pharmacological modulators.