Changes in [3H]flunitrazepam binding in the brain of rats made tolerant to and dependent upon pentobarbital

Changes in benzodiazepine binding sites labeled by [3H]flunitrazepan (FNZ) in twenty discrete brain regions of rats made tolerant to and dependent upon pentobarbital were examined. Animals were rendered tolerant by intracerebroventricular (i.c.v.) infusion with pentobarbital (300 micrograms/ 10 microliters/ hr for six days) through pre-implanted cannulae connected to osmotic mini-pumps. The pentobarbital dependence was assessed 24 hr after abrupt withdrawal from pentobarbital. In the tolerant rats, a significant increase in [3H]FNZ binding sites was found in layer IV of frontal cortex and the molecular layer of olfactory bulb. [3H]FNZ binding sites in the pentobarbital dependent rats were significantly increased in layers I-III and V-VI of frontal cortex, caudate-putamen, olfactory tubercle, globus pallidus and ventral pallidum, in addition to those observed in the tolerant group. There was, however, no significant difference in the hippocampus and several regions in the hindbrain in either pentobarbital-treated group. Taken together with characteristics of subtypes of benzodiazepine receptors and changes in GABA-benzodiazepine receptor complexes elucidated in our previous studies, these findings suggest that both types of benzodiazepine receptors are involved in the development of pentobarbital intoxication mediated by GABAA receptors.