Solubilization of the picrotoxinin binding receptor from mammalian brain

Solubilization and detergent effects on interactions of some drugs and insecticides with the t-butylbicyclophosphorothionate binding site within the gamma-aminobutyric acid receptor-ionophore complex

Specific binding of [35S]t-butylbicyclophosphorothionate (TBPS) to rat brain membranes (RBM) is enhanced nine-fold by EDTA/water dialysis and 1.3- to 4.2-fold by 50 nM ketosteroid R 5135, or 5 mM 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) or related piperazine-N-alkanesulfonate buffers, or extensive washing with NaCl/Na phosphate or Na phosphate/citrate solution. About one-fifth of the [35S]TBPS binding capacity appears in the soluble fraction whereas the rest remains in particulate form on treatment of the EDTA/water-dialyzed RBM with 20 mM CHAPS. Similar KD values (64-86 nM) are obtained for the original EDTA/water-dialyzed membranes and the CHAPS-treated and/or -solubilized preparations. The Bmax of the EDTA-treated RBM is reduced five-fold on solubilization with CHAPS. The potency for displacement of [35S]TBPS changes in the presence of CHAPS or on CHAPS solubilization: gamma-aminobutyric acid (GABA) and muscimol inhibit specific [35S]TBPS binding more strongly in the absence than in the presence of CHAPS: TBPS, picrotoxinin, and photoheptachlor epoxide are almost equally active with RBM, RBM + CHAPS, and RBM solubilized with CHAPS. Levels of (1R, alpha S)-cis-cypermethrin and dimethylbutylbarbiturate which are inhibitory with RBM are moderately stimulatory after TBPS receptor solubilization. Thus CHAPS defines three regions of the GABA receptor-ionophore complex, i.e., the GABA and benzodiazepine receptors, the TBPS/picrotoxinin/polychlorocycloalkane receptor(s), and the sites at which the alpha-cyano pyrethroid and the barbiturate interact with TBPS binding.

Solubilization of convulsant/barbiturate binding activity on the gamma-aminobutyric acid/benzodiazepine receptor complex

Binding activity of the radioactive cage convulsant [35S]t-butylbicyclophosphorothionate was solubilized from rat brain membranes using the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethylammonio] propanesulfonate. Binding (KD = 26 nM, Bmax = 0.4 pmol/mg protein) was inhibited by picrotoxin and related convulsants and by barbiturates and related depressants that interact with gamma-aminobutyric acid and benzodiazepine receptors via the picrotoxinin binding site. The convulsant/barbiturate binding activity chromatographed on gel filtration as a single peak coinciding with the benzodiazepine/gamma-aminobutyric acid receptor protein complex.

Benzodiazepine-GABA receptor-ionophore complex. Current concepts

The benzodiazepine--gamma-aminobutyric acid (GABA) receptor--ionophore system is an oligomeric complex, composed of at least three interacting components. These three components have been well characterized in vitro by radioreceptor binding assays. A variety of centrally acting anxiolytic, depressant, anticonvulsant and convulsant drugs, which affect GABAergic transmission, bind to one of the sites and modulate the binding of ligands at the other sites. Thus, depressant barbiturates, nonbarbiturate hypnotics (like etomidate) and pyrazolopyridines (like etazolate), while inhibiting the binding of alpha-dihydropicrotoxinin (DHP), enhance the binding of GABA and benzodiazepines. These enhancing effects are blocked by convulsant drugs that inhibit the binding of dihydropicrotoxinin and also by bicuculline. These interactions involving barbiturates and other modulatory drugs, exhibit stereoselectivity, anion dependence and brain regional selectivity. Several classes of drugs which facilitate GABAergic transmission appear to interact with the sites for GABA and benzodiazepines allosterically via the dihydropicrotoxinin site of the oligomeric complex. The GABA system has also been implicated in a variety of pathological conditions, including anxiety, seizure activity, movement disorders, cardiovascular control, pain and in drug dependence. Since most of the GABA agonists do not pass the blood-brain barrier, future trends in the pharmacology of GABA may be the development of drugs that will activate the GABA receptor system via picrotoxinin or benzodiazepine sites.

The picrotoxin-like action of a convulsant benzodiazepine, Ro5-3663

The potency and site of action of Ro5-3663 as a GABA antagonist were investigated in the rat cuneate slice in vitro. A Schild plot for Ro5-3663 was clearly non-linear but enabled a pA2 value of 3.97 to be calculated. Combination studies using the 'classical' GABA antagonists bicuculline and picrotoxin indicated that Ro5-3663 acted at a separate site from bicuculline, but may share a common site with picrotoxin.

The interactions of ethanol with the benzodiazepine-GABA receptor-ionophore complex

Ethanol, barbiturates and benzodiazepines have similar pharmacological effects. All of these drugs facilitate the inhibitory transmission mediated by GABA. Drugs that facilitate GABAergic transmission are effective in alleviating ethanol withdrawal symptoms. Acute ethanol administration increases the number of low affinity GABA receptor sites, while, during ethanol withdrawal, the affinity of this site is decreased. This decreased affinity during withdrawal correlated with the audiogenic seizure activity. These results indicate that in vivo ethanol interacts with GABA receptors, and this interaction may be responsible for some of the effects of ethanol and for some symptoms of withdrawal. Ethanol, like pentobarbital, also enhances [3H]diazepam binding to the Lubrol-solubilized membrane fraction in vitro. This effect was dose-related and was blocked by picrotoxinin and other GABA antagonists. Enhancement of [3H]diazepam binding by various alcohols did not correlate with lipid:water partition coefficients. Ethanol also partially inhibited the binding of [3H]-alpha-dihydropicrotoxinin in the Lubrol-solubilized fraction. These results indicate that ethanol, like pentobarbital, may modulate the benzodiazepine binding component of the benzodiazepine-GABA receptor-ionophore complex via the picrotoxinin site. The possible interpretation of these results, in relation to the GABAergic transmission, is discussed.

Molecular interactions of etazolate with benzodiazepine and picrotoxinin binding sites

Pyrazolopyridines, such as etazolate (SQ 20009), enhance [3H]diazepam binding to a Lubrol-solubilized fraction that has specific binding sites for 3H benzodiazepines, [alpha-3H]dihydropicrotoxinin (DHP) and [3H]muscimol. Etazolate enhancement of [3H]diazepam binding was inhibited by picrotoxinin. Furthermore, etazolate inhibited the [3H]DHP binding in a Lubrol-solubilized fraction with an IC50 value of 6-8 microM. These results provide evidence that etazolate, like pentobarbital, modulates benzodiazepine binding via the DHP-sensitive site of the benzodiazepine-GABA receptor-ionophore complex.

Histidine modification with diethyl pyrocarbonate shows heterogeneity of benzodiazepine receptors

The effect of diethyl pyrocarbonate modification of histidine on the specific binding of [3H]diazepam and its enhancement with muscimol and (+/-)-pentobarbital was investigated. Diethyl pyrocarbonate treatment produced a dose-related inhibition of specific [3H]diazepam binding to rat brain membranes with a maximal inhibition of approximately 40% at 1 mM. Scatchard analysis of the binding data showed that diethyl pyrocarbonate, while having no effect on the affinity (Kd), decreased the binding capacity (Bmax) of diazepam from a control value of 1543 +/- 116 fmol/mg of protein to 789 +/- 79 fmol/mg of protein (mean +/- SD; P less than 0.005; n = 4). Under conditions in which approximately 40% of the diazepam binding sites were modified by diethyl pyrocarbonate treatment, the ability of muscimol and pentobarbital to enhance diazepam binding was not altered. These results suggest that a histidine residue is critical for a part (approximately 40%) of the benzodiazepine binding sites and that there may exist a heterogeneity of benzodiazepine binding sites. Furthermore, these results indicate that perhaps only a portion of the benzodiazepine binding sites are functionally coupled to the gamma-aminobutyric acid receptor-ionophore complex.

Pentobarbital enhances [3H]diazepam binding to soluble receptors at the benzodiazepine--GABA-receptor-ionophore complex

Pentobarbital enhances [3H]diazepam binding to soluble receptors in a dose-dependent and saturable manner. Half-maximal enhancement occurred at 110 micrometers pentobarbital. This enhancement was due to an increase in the affinity of diazepam to its receptor sites. Pentobarbital enhancement was blocked by picrotoxinin and bicuculline. These results are similar to those reported for membranes and suggest that various components of the benzodiazepine--GABA-receptor-ionophore complex are intimately associated.