Influence of phospholipase treatments on ligand bindings to a benzodiazepine receptor-GABA receptor-chloride ionophore complex

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

“…This finding is in agreement with the results of Fujimoto and Okabayashi (1983), but appears to be at variance with the findings of Ueno and Kuriyama (1981), who demonstrated that PLA, reduced GABA-enhanced ['Hldiazepam binding. However, because halide ions can potentiate GABA-enhanced ['Hlbenzodiazepine binding (Supavilai et al, 1982) and Cl-enhanced [3H]diazepam binding is inhibited by low concentrations of PLA, (Fujimoto and Okabayashi, A 1983), the sensitivity of dABA-enhanced [3H]diazepam binding to PLA, reported by Ueno and Kuriyama (1981) may reflect the disruption of a potentiating effect of CI-rather than an effect of PLA, on GABA-enhanced 13H]benzodiazepine binding per se. The failure of PLA, to affect GABAenhanced [3H]flunitrazepam binding appears inconsistent with the increase in [3H]muscimol binding observed by Fujimoto and Okabayashi (1983).…”

“…A concentration-dependent reduction in the protein content of membranes was observed at PLA, concentrations of >0.01 U/ml. Fujimoto and Okabayashi (1983) reported a release of both protein and phospholipid from membranes at PLA, concentrations of -0.2 U/ml using a virtually identical incubation procedure. Thus, if the data are expressed as fmol of radioligand boundiassay, rather than fmol of radioligand bound/mg of protein (as shown in Fig.…”

“…Previous studies (Ueno and Kuriyama, 1981;Fujimoto and Okabayashi, 1983) had demonstrated that PLA, increases the apparent affinity of ['Hldiazepam and ['H]flunitrazepam for the CBR. We have observed a similar phenomenon with ['Hlflunitrazepam ( Fig.…”

“…However, because halide ions can potentiate GABA-enhanced ['Hlbenzodiazepine binding (Supavilai et al, 1982) and Cl-enhanced [3H]diazepam binding is inhibited by low concentrations of PLA, (Fujimoto and Okabayashi, A 1983), the sensitivity of dABA-enhanced [3H]diazepam binding to PLA, reported by Ueno and Kuriyama (1981) may reflect the disruption of a potentiating effect of CI-rather than an effect of PLA, on GABA-enhanced 13H]benzodiazepine binding per se. The failure of PLA, to affect GABAenhanced [3H]flunitrazepam binding appears inconsistent with the increase in [3H]muscimol binding observed by Fujimoto and Okabayashi (1983). However, GABA receptors that are functionally coupled to benzodiazepine receptors are probably in a lowaffinity state (Olsen, 1981) and may not be readily distinguished from other populations of GABA receptors with the radioligand binding techniques used by Fujimoto and Okabayashi (1983).…”

“…A change in membrane lipids induced by activation of phospholipase A, (PLA,), a n endogenous membrane constituent, has been proposed as a physiologic mechanism for regulating receptor function Strittmatter e t al., 1979;Mallorga et al, 1980). Several investigators (Ueno and Kuriyama, 1981;Fujimoto and Okabayashi, 1983) have shown that preincubation of brain membranes with PLA, increases the apparent affinity of [3H]diazepam or [3Hlflunitrazepam for benzodiazepine receptors in the CNS. However, two physically and pharmacologically distinct classes of binding sites for benzodiazepines are present in the C N S (Gallager e t al., 1981;Marangos e t al., 1982;Schoemaker e t al., 1983).…”