Differential Sensitivity of “Central” and “Peripheral” Type Benzodiazepine Receptors to Phospholipase A₂

Abstract: The effects of preincubating cerebral cortical membranes with phospholipase A₂ (PLA₂) were examined on radioligand binding to benzodiazepine receptors of the “central” and “peripheral” types. PLA₂ (0.005–0.1 U/ml) increased [³H]flunitrazepam and [³H]3‐carbo‐ethoxy‐β‐carboline binding by increasing the apparent affinities of these ligands with no concomitant change in the maximum number of binding sites. In contrast, neither γ‐aminobutyric acid (GABA)‐enhanced [³H]fluni‐trazepam binding nor [³H]Ro 15–1788 binding was altered by preincubation with PLA² at concentrations as high as 2 U/ml. Both pyrazolopyridine (SQ 65,396)‐ and barbiturate (pentobarbital)‐enhanced [³H]flunitrazepam binding and [³⁵S]t‐butylbicyclophosphorothionate (TBPS) binding were markedly reduced by as little as 0.0025–0.005 U/ml of PLA₂. These findings suggest that PLA₂ inactivates the TBPS binding site on the benzodiazepine‐GABA receptor chloride ionophore complex, which results in a selective loss of allosteric “regulation” between the components of this complex. PLA₂ also reduced the apparent affinity of [³H]Ro 5–4864 to peripheral‐type benzodiazepine receptors in cerebral cortical, heart, and kidney membranes, but increased the number of [³H]PK 11195 binding sites with no change in apparent affinity. These data demonstrate that PLA₂ can differentially affect the lipid microenvironment of “central” and “peripheral” types of benzodiazepine receptors.