Changes of [³H]muscimol binding and GABA(A) receptor β2-subunit mRNA level by tolerance to and withdrawal from pentobarbital in rats

Effects of continuous pentobarbital administration on binding characteristics of [³H]muscimol were examined by autoradiography, and levels of GABA(A) receptor β2-subunit mRNA were investigated by in situ hybridization histochemistry in the rat brain. In order to eliminate the induction of hepatic metabolism by systemic administration of pentobarbital, an i.c.v. infusion model of tolerance to and withdrawal from pentobarbital was used. An experimental model of barbiturate tolerance and withdrawal was developed using i.c.v. infusion of pentobarbital (300 μg/10 μl/hr for 7 days) by osmotic minipumps and abrupt withdrawal from pentobarbital. The levels of [³H]muscimol binding were elevated in cingulate of frontal cortex (46%) and granule layer of cerebellum (32%) of rats 24-hr after withdrawal from pentobarbital, while it was only elevated in cingulate (58%) of tolerant rats. The GABA(A) receptor β2-subunit mRNA was increased in the withdrawal rats only; in the cortex (9-14%), hippocampus (15-21%), inferior colliculus (21%), and granule layer of cerebellum (24%). These results show the involvement of GABA(A) receptor and its β2-subunit up-regulations in pentobarbital withdrawal rats, and suggest that the levels of [³H]muscimol binding and GABA(A) receptor β2-subunit mRNA are altered in a region- specific manner during pentobarbital withdrawal.