AN AUTORADIOGRAPHIC STUDY OF [H-3] FLUNITRAZEPAM BINDING-SITES IN THEBRAIN OF RAT MADE TOLERANT TO AND DEPENDENT ON PENTOBARBITAL

The effects of continuous administration of pentobarbital on the benzo diazepine receptor labeled by [H-3]flunitrazepam were investigated. An imals were made tolerant to pentobarbital by intracerebroventricular ( i.c.v.) infusion with pentobarbital (300 mu g/10 mu l/h) fur 6 days th rough pre-implanted cannulae connected to osmotic mini-pumps. The depe ndent rats were assessed 24 h after cessation of pentobarbital infusio n. Changes in [H-3]flunitrazepam binding were examined in 37 brain reg ions at a concentration of [H-3]flunitrazepam of 1 nM. In subsequent s aturation studies, the binding parameters B-max and K-D were also inve stigated in 17 brain regions, most of which showed significant changes in [H-3]flunitrazepam binding in experiments using a fixed concentrat ion of radioligand. The pentobarbital-tolerant rats showed a significa nt increase in B-max with an increase in K-D for [H-3]flunitrazepam in the ventroposterior nucleus of thalamus. In the dependent rats, a sig nificant increase in B-max for [H-3]flunitrazepam binding, without a c hange in K-D, was observed in all layers of the frontal cortex, the ca udate-putamen, olfactory tubercle, and some nuclei in thalamus, compar ed to those in the control. Increased [H-3]flunitrazepam binding in th e molecular layer of the olfactory bulb, the ventral pallidum, and the cerebellum of the pentobarbital dependent rats at a fixed concentrati on of [H-3]flunitrazepam was also observed. There was no significant c hange in [H-3]flunitrazepam binding in the hippocampus and several nuc lei of the brain stem. These findings suggest that benzodiazepine rece ptors are closely involved in the development of tolerance to and depe ndence on pentobarbital. Further studies on changes in gamma-aminobuty ric acid (GABA)(A) receptor subunit mRNA or the effects of pentobarbit al on GABA(A) receptor phosphorylation would be necessary for an expla nation of the precise mechanisms underlying the development of toleran ce to and dependence on pentobarbital.