Existence of sites for anions and divalent cations in the solubilized gamma-aminobutyric acid/benzodiazepine receptor complex

Peripheral-type benzodiazepine receptors in human cerebral cortex, kidney, and colon

The specific binding of [3H]PK 11195 and [3H]Ro 5-4864 to human cerebral cortex, kidney, and colon membranes was studied in order to determine whether peripheral type benzodiazepine receptors (PBR) characteristics located in human tissues are similar to those located in calf or rat tissues. While [3H]PK 11195 (0.05-10 nM, final concentration) bound with high affinity (KD about 2 nM) to human cerebral cortex, kidney, and colon membranes, yielding maximal numbers of binding sites of 255 +/- 23, 1908 +/- 28, and 1633 +/- 98 fmol/mg protein, respectively, the specific binding of [3H]Ro 5-4864 (1.25-40 nM, final concentration), was barely detectable (nonspecific binding about 90% of the total binding). Furthermore, unlabeled PK 11195 was two orders of magnitude more potent than unlabeled Ro 5-4864 in displacing [3H]PK 11195 specific binding from human cerebral cortex and kidney membranes. These results indicate that PBR binding characteristics located in human tissues are similar (but not identical) to those located in calf tissues, but not to those located in rat tissues.

Species differences and heterogeneity of solubilized peripheral-type benzodiazepine binding sites

The pharmacological characteristics of digitonin-solubilized peripheral-type benzodiazepine binding sites (PBS) from kidney membranes of various species were investigated to determine whether the species differences and heterogeneity observed in membrane-bound binding sites would be maintained after solubilization. [3H]PK 11195 (0.05 to 10 nM) bound with high affinity to rat, guinea pig, calf, and cat kidney solubilized preparations yielding maximal numbers of binding sites (Bmax) of 3,593 +/- 381, 25,645 +/- 1,795, 1,327 +/- 141, and 2,446 +/- 148 fmol/mg protein, respectively, and equilibrium dissociation constant (KD) values of 1.74 +/- 0.18, 2.15 +/- 0.15, 0.85 +/- 0.09, and 1.02 +/- 0.06 nM, respectively. On the other hand, the respective Bmax and KD values for [3H]Ro 5-4864 (1.25 to 40 nM) were 2,688 +/- 275, 14,182 +/- 1,134, 144 +/- 23 and 205 +/- 17 fmol/mg protein (about 75, 55, 11, and 8%, respectively, of that of [3H]PK 11195) and 13.8 +/- 1.5, 14.6 +/- 1.1, 10.6 +/- 1.7, and 19.9 +/- 1.2 nM. Unlabeled Ro 5-4864 was two orders of magnitude more potent in displacing [3H]PK 11195 binding from rat kidney solubilized preparations than from calf kidney solubilized preparations, whereas the potency of unlabeled PK 11195 in displacing [3H]PK 11195 binding from both rat and calf kidney solubilized preparations was almost identical. Analysis of these displacement data revealed that PK 11195 bound to a single population of binding sites (nH approximately equal to 1.0), whereas Ro 5-4864 bound to two populations of binding sites (nH less than 1.0) in both rat and calf kidney solubilized preparations. These results indicate that PBS species differences and heterogeneity observed in membrane-bound binding sites are retained in the soluble state and are probably attributable to variations in the molecular structure of PBS rather than to differences in membrane environment.

Polychlorinated convulsant insecticides potentiate the protective effect of NaCl against heat inactivation of [3H]flunitrazepam binding sites

Six polychlorinated convulsant insecticides that potently inhibit t-[35S]butylbicyclophosphorothionate ([35S]TBPS) binding to rat brain membranes also potentiate the protective effect of NaCl (200 mM) against heat inactivation of [3H]flunitrazepam binding sites on the same membranes. Similar effects were obtained with all "cage" convulsants tested. The rank order of potencies as heat protection potentiators was similar to the rank order of potencies as inhibitors of [35S]TBPS binding (alpha-endosulfan greater than endrin greater than dieldrin greater than toxaphene greater than lindane). alpha-Endosulfan and endrin are more potent in both respects than any previously reported picrotoxin-like (cage) convulsant, but are much less toxic to mammals. The greatly reduced toxicities of alpha-endosulfan and endrin in mammals may reflect partial gamma-aminobutyric acid (GABA)-neutral properties of these compounds. Time courses of heat inactivation of [3H]flunitrazepam binding sites in the presence of 200 mM NaCl plus saturating concentrations of endrin or picrotoxin revealed monophasic components constituting about 88% of the binding sites, suggesting that virtually all [3H]flunitrazepam binding sites are coupled to picrotoxin binding sites in the GABA/benzodiazepine/picrotoxin receptor complex. Protection against heat inactivation constitutes a useful tool for characterizing the various allosterically linked binding sites in neurotransmitter receptor complexes.

Differences in the biophysical properties of the benzodiazepine/gamma-aminobutyric acid receptor chloride channel complex in the long-sleep and short-sleep mouse lines

Significant differences in the thermal stability of benzodiazepine receptors were found in cerebral cortical membranes prepared from the long-sleep (LS) and short-sleep (SS) selected mouse lines. Thus, benzodiazepine receptors from LS mice were heat inactivated (55 degrees C) at a significantly faster rate than those from SS mice. Although gamma-aminobutyric acid (GABA) reduced the rate of heat inactivation in both lines, the more rapid rate of inactivation in the LS line was maintained. Furthermore, the potency of GABA to enhance [3H]flunitrazepam binding decreased threefold in membranes from LS mice as the incubation temperature was increased from 0 degrees to 37 degrees C, but was unaltered in membranes from SS mice. These differences in the biophysical properties of the benzodiazepine/GABA receptor chloride channel complex ("supramolecular complex"), together with a higher KD for t-[35S]butylbicyclophosphorothionate in membranes from LS compared to SS mice, suggest that the supramolecular complex may modulate the differential sensitivity to some depressants and convulsants in these lines.

Effect of chronic haloperidol treatment on peripheral benzodiazepine binding sites in cerebral cortex of rats

The effects of 21 days of haloperidol treatment on central benzodiazepine (BZ) receptors in the cerebral cortex of rats and on peripheral-type BZ binding sites (PBS) in the cerebral cortex and heart of rats were studied. Neuroleptic treatment did not affect the maximal binding capacity or the affinity of the central BZ receptor to 3H-flunitrazepam. Chronic haloperidol treatment resulted in a significant increase of 38% in PBS density in the cerebral cortex, with no alteration in PBS density in the heart. No alteration in PBS affinity for its ligand 3H-PK 11195 was observed, either in the cerebral cortex or in the heart. The modulatory effect of chronic haloperidol administration on PBS density in the brain may be related to some of the neurobehavioral or hormonal effects of the drug.

Differential effect of detergents on [3H]Ro 5-4864 and [3H]PK 11195 binding to peripheral-type benzodiazepine-binding sites

The present study demonstrates a differential effect of various detergent treatments on [3H]Ro 5-4864 and [3H]PK 11195 binding to peripheral benzodiazepine binding sites (PBS). Triton X-100 (0.0125%) caused a decrease of about 70% in [3H]Ro 5-4864 binding to membranes from various peripheral tissues of rat, but had only a negligible effect on [3H]PK 11195 binding. A similar effect of Triton X-100 was observed on guinea pig and rabbit kidney membranes. The decrease in [3H]Ro 5-4864 binding after treatment with Triton X-100 was apparently due to a decrease in the density of PBS, since the affinity remained unaltered. The detergents 3-[(3-cholamidopropyl)-dimethylammonio]-1-propane sulfonate (CHAPS), Tween 20, deoxycholic acid, or digitonin (0.0125%) caused only a minor change in [3H]Ro 5-4864 and [3H]PK 11195 binding to rat kidney membranes; but when concentrations were substantially increased (0.1%), all detergents caused a decrease of at least 50% in [3H]Ro 5-4864 binding, while [3H]PK 11195 binding to rat kidney membranes remained unaffected by the first three detergents, with only a minor decrease (15%) after treatment with digitonin. These results may further support the assumption that Ro 5-4864 and PK 11195 are agonist and antagonist, respectively, of PBS and interact with two different conformations or domains in the peripheral-type benzodiazepine binding site molecule.

Modulatory effects of thyroxine treatment on central and peripheral benzodiazepine receptors in the rat

The effects of 10 days of D-thyroxine (T4) treatment on central benzodiazepine (BZ) receptors in the brain and on peripheral-type BZ binding sites in the heart, kidney, and testis of rats were studied. The experimental hyperthyroidism resulted in an increase in the density of cortical central BZ receptors, without any alteration of the affinity of the receptors to [3H]flunitrazepam. The increase in cortical central BZ receptors was also accompanied by the up-regulation of peripheral BZ binding sites in the heart, kidney, and testis. The affinity of the peripheral BZ binding sites for the ligand [3H]PK 11195 was not affected by T4 treatment in any of these three organs. The increase in the density of brain cortical central BZ receptors was less prominent than the increase in the peripheral BZ binding sites. The modulatory effect of T4 treatment on central and peripheral BZ receptors might be attributed to the direct interaction of the thyroid hormone at these sites or might reflect a physiological compensatory adaptation mechanism to thyrotoxicosis associated with hypermetabolism, anxiety, and stress.