Lack of specificity in cation effects on solubilized benzodiazepine receptor

gamma-Aminobutyric acid receptor complex of insect CNS: characterization of a benzodiazepine binding site

The specific binding of [N-methyl-3H]flunitrazepam ([3H]FNZP) to a membrane fraction from the supraoesophageal ganglion of the locust (Schistocerca gregaria) has been measured. The ligand binds reversibly with a KD of 47 nM. The binding is Ca2+-dependent, a property not found for the equivalent binding site in vertebrate brain. The pharmacological characteristics of the locust binding site show similarities to both central and peripheral benzodiazepine receptors in mammals. Thus binding is enhanced by gamma-aminobutyric acid (GABA), a feature of mammalian central receptors, whereas the ligand Ro 5-4864 was more effective in displacing [3H]FNZP than was clonazepam, which is the pattern seen in mammalian peripheral receptors. The locust benzodiazepine binding site was photoaffinity-labelled by [3H]FNZP, and two major proteins of Mr 45K and 59K were specifically labelled. In parallel experiments with rat brain membranes a single major protein of Mr 49K was labelled, a finding in keeping with many reports in the literature. We suggest that the FNZP binding site described here is part of the GABA receptor complex of locust ganglia. The insect receptor appears to have the same general organization as its mammalian counterpart but differs significantly in its detailed properties.

Solubilization of the benzodiazepine/gamma-aminobutyric acid receptor complex: comparison of the detergents octylglucopyranoside and 3-[(3-cholamidopropyl)-dimethylammonio]1-propanesulfonate (CHAPS)

Octylglucopyranoside (OCTG) was three times more efficient than 3-[(3-cholamidopropyl)-dimethylammonio]1-propanesulfonate (CHAPS) in solubilizing the benzodiazepine (BDZ)/gamma-aminobutyric acid (GABA) receptor complex from rat cerebellar synaptic membranes. OCTG-solubilized receptor preparations had ligand binding characteristics that were significantly different from those of the CHAPS-solubilized receptors. The inclusion of phospholipids in the solubilization media improved the binding characteristics of both soluble receptor preparations and appeared absolutely necessary for the maintenance of chloride facilitation of flunitrazepam (FNZ) binding to OCTG-solubilized receptors. FNZ and ethyl-beta-carboline-3-carboxylate bound to OCTG-solubilized preparations with equilibrium dissociation constants of 2.2 nM and 1.6 nM, respectively, and chloride (150 mM) and GABA (100 microM) + chloride facilitated the binding of FNZ by 15% and 55%, respectively; these ligand binding characteristics are similar to those of membrane-located BDZ receptors. Cartazolate, a pyrazolopyridine that facilitated the binding of FNZ to membrane-located and CHAPS-solubilized receptors, did not facilitate FNZ binding to OCTG-solubilized receptors. These results are discussed in terms of an interaction between the membrane lipid phosphatidylserine (PS) and cartazolate; PS appears to have the capacity to inhibit the effects of cartazolate on FNZ binding. Storage of the soluble receptor preparations for 24 h at 4 degrees C resulted in the loss of several characteristic BDZ receptor binding properties. Incorporation of the OCTG-solubilized receptor complex into liposomes prevented these losses but this procedure did not protect the CHAPS-solubilized receptors. We conclude that OCTG may have some advantages over CHAPS as the detergent of choice for the solubilization and reconstitution in liposomes of a functional BDZ/GABA receptor-chloride ionophore complex.