Differential effect of gamma-aminobutyric acid on benzodiazepine receptor subtypes labeled by [3H]propyl beta-carboline-3-carboxylate in rat brain
Gee, K.W.; Ehlert, F.J.; Yamamura, H.I.
Journal of Pharmacology and Experimental Therapeutics 225(1): 132-137
1983
ISSN/ISBN: 0022-3565 PMID: 6300371 Document Number: 209113
The regulation of benzodiazepine receptor subtypes (BZ1 and BZ2) by GABA was studied by assessing the effect of GABA on flunitrazepam (FLU) binding to [3H]FLU and [3H]propyl .beta.-carboline-3-carboxylate ([3H]-PCC) labeled receptors in various brain regions. In the hippocampus, the BZ1 receptor (high affinity) labeled by a low concentration (0.04 nM) of [3H]PCC at 0.degree. C did not appear to be GABA regulated. This was based upon the observation that GABA(10-4 M) has no significant effect on the FLU/[3H]PCC competition curves in the hippocampus. When [3H]FLU (0.05 nM) or a high concentration of [3H]PCC (0.5 nM) was used to label both BZ1 and BZ2 (low affinity) receptors under similar conditions, GABA (10-4 M) caused a significant increase (1.7-fold) in the affinity of FLU as measured by FLU/[3H]FLU and FLU/[3H]PCC competition experiments. In cerebral cortex, GABA enhancement of FLU inhibition of [3H]PCC (0.04 nM) binding was present but significantly less than the enhancement observed with [3H]FLU (0.05 nM) or a high concentration of [3H]PCC (0.05 nM). An effect of GABA on FLU inhibition was also observed in the cerebellum when either [3H]PCC (0.04 nM) or [3H]FLU (0.05 nM) was used as the ligand. Dissociation kinetics of high (0.5 nM) and low (0.04 nM) concentrations of [3H]PCC in the hippocampus support the notion that 0.04 nM [3H]PCC labels a single binding site, whereas the high concentration of [3H]PCC labels both the high (BZ1) and low affinity (BZ2) sites. In cerebral cortex, a low concentration of [3H]PCC (0.04 nM) predominantly labels the high affinity site with a small proportion of the low affinity site also being labeled. [3H]PCC dissociation curves and PCC/[3H]FLU competition curves in the cerebellum indicate PCC binds predominantly to a single benzodiazepine receptor site in this brain region. GABA regulation may be predominantly associated with the BZ2 site found in the cerebral cortex and hippocampus. BZ1 receptors show a regional difference in the way they are associated with GABA receptors. In terms of GABA regulation, BZ1 receptors in the cerebellum appear to be different from the BZ1 receptor in hippocampus and cerebral cortex. Consequently, functional coupling to a GABA site may be a determinant of benzodiazepine receptor heterogeneity.