Heterogeneity of Benzodiazepine Receptors: Experimental Differences Between [3H]Flunitrazepam and [3H]Ethyl-β-carboline-3-carboxylate Binding Sites in Rat Brain Membranes

Abstract: This study was designed to analyze possible differences in the binding of ³H]flunitrazepam (³H]FNZP) and ³H]ethyl - β - carboline - 3 - carboxylate (³H]β-CCE), to rat brain membranes, in various experimental conditions. In cerebral cortex, hippocampus, cerebellum, and orain stem the number of binding sites for ³H]β-CCE was higher than for ³H]FNZP; both were displaced by clonazepam. Until the 7th day of postnatal brain development the numbers of ³H]FNZP and ³H]β-CCE sites were equivalent; but later on, the β-carboline sites increased to a higher level. Noradrenergic denervation by 6-hydroxydopamine was followed in the hippocampal formation. Already after 2 days, there was a decrease in ³H]FNZP sites, which reached 70% of control after 14 days. Similar results were obtained with DSP-4 denervation. This change was only in B_max and not in K_D, In contrast, the ³H]β-CCE sites did not change with denervation. Neonatal injection of l - 2,4,5 - trihydroxyphenylalamine or DSP-4 produced in the adult a decrease in ³H]FNZP sites in the cerebral cortex, in parallel with the noradrenergic denervation. On the other hand, there was an increase in the cerebellum and brain stem, in correspondence with the hyperinnervation by sprouting. In these rats, the number of sites for ³H]β-CCE did not change in the different brain regions. With 0.1% Triton X-100, applied to synaptosomal membranes, ³H]FNZP binding was reduced by 35%, while that of ³H]β-CCE was not significantly changed. These results suggest that there is heterogeneity of binding sites for benzodiazepine receptors in rat brain. A tentative interpretation of the experiments involving noradrenergic denervation and hyperinnervation, as well as those with Triton X-100, is that ³H]FNZP binds to pre- and postsynaptic receptors, while ³H]β-CCE binds mainly to postsynaptic benzodiazepine receptors.