Enhancement of t-[35S]butylbicyclophosphorothionate and [3H]strychnine binding by monovalent anions reveals similarities between gamma-aminobutyric acid- and glycine-gated chloride channels

Allosteric modulation of glycine receptors is more efficacious for partial rather than full agonists

Allosteric modulation of [3H]strychnine binding to glycine receptors (GlyRs) was examined in synaptosomal membranes of rat spinal cord. An allosteric model enabled us to determine the cooperativity factors of the allosteric agents with [3H]strychnine and glycine bindings (alpha and beta, respectively). We modified the allosteric model with a slope factor because the slope values of the displacement curves of partial agonists (beta-alanine, taurine and gamma-aminobutyric acid) were beyond unity. The slope factor was reduced only by 100 microM propofol. Further, propofol showed positive cooperativity (beta < 1) stronger with taurine than with glycine. The extent of the positive cooperativity of propofol was nearly independent from the potencies and structures of partial agonists. The steroidal alphaxalone and minaxolone also potentiated taurine better than glycine. Alphaxalone exerted weak negative cooperativity with [3H]strychnine binding. Displacement by taurine is attenuated by granisetron and m-chlorophenylbiguanide representing negative cooperativity (beta >> 1) greater than with glycine. The results suggest a developmental role of elevated perinatal levels of taurine and neurosteroids as well as a better allosteric modulation of decreased agonist efficacies for impaired glycine receptor-ionophores.

Dual cooperative allosteric modulation of binding to ionotropic glycine receptors

Glycine receptors (GlyRs) were studied via [(3)H]strychnine binding to synaptosomal membranes of rat spinal cord. A ternary allosteric model was applied for the effects of tropeines, alcohols, minaxolone, nitrendipine, Zn(2+), muscarinic and serotonin receptor ligands. It enabled us to determine the dissociation constants of the allosteric agents (K(A)) and their cooperativity factors affecting the dissociation constants of [(3)H]strychnine (alphaK(S)) and glycine (betaK(L)). Cooperativity with [(3)H]strychnine ranged from strong negative for tropeines to weak positive for nitrendipine. Displacement curves of glycine were examined in the presence of allosteric agents. Positive cooperativities with glycine were found for submicromolar concentrations of tropisetron, bemesetron, zatosetron and nitrendipine; for tubocurarine, propofol, butanol, minaxolone, cocaine and 10 microM Zn(2+). Micromolar concentrations of tropisetron and nitrendipine showed weaker cooperativities. Other allosteric agents and 1 mM Zn(2+) displayed negative cooperativity with glycine. Binding parameters K(A) and beta correlate excellently with the activities of the allosteric agents on GlyR-ionophores. Combined inhibitory effects of the allosteric agents suggest that there are different subgroups (tropeines, alcohols and dihydropyridines) binding to distinct sites on GlyRs exerting cooperativity with glycine via a common mechanism. This is the first quantitative analysis of allosteric binding interactions for GlyRs.

Bidirectional allosteric modulation of strychnine-sensitive glycine receptors by tropeines and 5-HT3 serotonin receptor ligands

Specific binding of [3H]strychnine was studied on membranes prepared from rat spinal cord. Several antagonists and agonists of 5-HT3 receptors and tropane derivatives displaced [3H]strychnine binding with micromolar potencies. In the presence of 10 microM glycine a high affinity (nanomolar) component of displacement was also observed for the tropeines zatosetron, bemesetron and tropisetron. The displacing potency of glycine was also enhanced by these agents which are therefore termed glycine-positive. In contrast, atropine, SR 57227A, m-chlorophenylbiguanide, metoclopramide and granisetron are termed glycine-negative, because they decreased the displacing potency of glycine while glycine decreased the displacing potencies of atropine and metoclopramide. The dissociation of [3H]strychnine binding was accelerated in the presence of m-chlorophenylbiguanide, SR 57227A, atropine and zatosetron with a concentration dependence (EC50 values and Hill slopes) similar to their displacing effects. This demonstrates that the displacement of strychnine binding is associated with allosteric interactions between different binding sites. Structure-activity analysis revealed that the tropeine structure is essential for high affinity binding, and its substitutions (in scopolamine and cocaine) or its replacement (in ondansetron and metoclopramide) strongly decrease the potency and/or efficacy of allosteric modulation. High affinity modulatory sites for tropeines appear to be associated with the potentiation of ionophore function, but distinct from the low affinity channel blocking sites as well as from the binding sites of strychnine and glycine.

Differential effects of thiocyanate on the binding thermodynamics of bicuculline methiodide versus SR95531 (gabazine) to the gamma-aminobutyric acid receptor-ionophore complex

The temperature dependence of the binding of [3H]SR 95531 (Gabazine), an antagonist of gamma-aminobutyric acid (GABA(A)) receptors, was studied in synaptosomal membranes of rat brain in the presence of 50 mM KSCN. The displacing potencies of the antagonists bicuculline methiodide and Gabazine were determined at five temperatures between 0 degrees and 37 degrees. Van't Hoff plots of the displacing potencies were analyzed by linear regression in the presence and absence of thiocyanate. Thiocyanate hardly affected the exothermic ionic binding interaction of gabazine. In contrast, thiocyanate strongly potentiated the binding of bicuculline methiodide and deprived it of its exothermic nature. The enhanced binding of bicuculline methiodide in the presence of chaotropic SCN- ions might be reconciled with "entropic trapping" in a sterically constrained hydrophobic binding pocket.

Temperature and anion dependence of allosteric interactions at the gamma-aminobutyric acid-benzodiazepine receptor

The temperature dependence of [3H]flunitrazepam ([3H]FNZ) binding to rat brain membranes was examined in the presence of the anaesthetics, pentobarbitone, alphaxalone and propofol. Van't Hoff plots showed the binding of FNZ to be largely enthalpy driven. Alphaxalone and propofol increased the entropy of the binding reaction but not the enthalpy and therefore did not show temperature dependence in their efficacy. In contrast, pentobarbitone increased the enthalpy of FNZ binding and, therefore, is more efficacious at low temperatures. The EC50 values of all three modulators increased with temperature indicating that their interactions with the receptor may be enthalpy driven. The EC50 values of all three modulators were also anion dependent, showing a decrease in the presence of gamma-aminobutyric acid (GABAA)-channel permeant anions. The efficacies of alphaxalone and pentobarbitone, but not that of propofol, also increased with increasing chloride ion concentration. The results indicate that all three modulators interact with the GABAA receptor at distinct recognition sites.

Pyrethroid insecticides indirectly inhibit GABA-dependent 36Cl- influx in synaptoneurosomes from the trout brain

Rainbow trout (Oncorhynchus mykiss) are extremely sensitive to the neurotoxic activity of pyrethroid insecticides. One possible target for pyrethroids is the GABA(A) receptor of brain of the trout, the function of which can be tested by measurement of influx of 36Cl- into synaptoneurosomes, in response to the application of agonists. gamma-Aminobutyric acid produced a time- and concentration-dependent increase in influx of 36Cl- in synaptoneurosomes from the brain of the trout, which exhibited the pharmacology characteristic of a response mediated by activation of a GABAA receptor. Deltamethrin, (1R alpha S)-cis-cypermethrin and permethrin produced a dose-dependent increase in the basal uptake and a corresponding decrease in GABA-dependent influx, with a maximum inhibition of 70-82%. This effect of pyrethroid was stereospecific, of high potency and inhibited by tetrodotoxin (TTX) and t-butylbicyclophosphorothionate (TBPS). The sensitivity of the effect of the pyrethroid to TTX suggested an activation by pyrethroid of the voltage-dependent sodium channel. Veratridine, a sodium channel activator, elicited similar changes in the basal uptake of chloride, which were TTX-sensitive. Neither deltamethrin nor veratridine had a measurable effect on the efflux of 36Cl- from synaptoneurosomes. Thus, pyrethroid insecticides may interfere with the function of GABAA receptors indirectly through an interaction with the voltage-dependent sodium channel in the brain of the trout and consequently perturb chloride influx, possibly through a voltage-dependent chloride channel.

Dissociation of muscimol, SR 95531, and strychnine from GABAA and glycine receptors, respectively, suggests similar cooperative interactions

Binding to gamma-aminobutyric acid-A (GABAA) receptors was studied in synaptosomal membranes of rat brain. Dissociation of [3H]muscimol and the GABAA antagonist [3H]2-(3-carboxypropyl)-3-amino-6-p-methoxyphenylpyridazinium bromide ([3H]SR 95531) binding elicited by 100-fold dilution was accelerated by excess of GABA or SR 95531. Control dissociation might be retarded by rebinding. The contribution of a rapid first phase of dissociation of the agonist [3H]muscimol was preferentially enhanced by SR 95531. In contrast, the dissociation of [3H]SR 95531 binding was preferentially accelerated by GABA. These opposite preferential accelerations can be explained by negative heterotropic cooperativity and a reversed affinity relationship of agonists and antagonists to GABAA binding sites with different affinities. Modification of the membranes by p-diazobenzenesulfonic acid (DSA) selectively decreased the accelerating effect of GABA on the dissociation of [3H]SR 95531 binding. [3H]Strychnine binding was studied in a membrane preparation of rat spinal cord. The dissociation of the antagonist [3H]strychnine elicited by dilution was preferentially accelerated by glycine. Again, pretreatment with DSA decreased selectively this negative heterotropic (i.e., allosteric) interaction. Chemical modification by DSA might be attributed to tyrosine residues responsible for similar allosteric interactions for the GABA- and glycine-gated chloride channels.

On the GABAA receptor: a molecular modeling approach

Bicuculline methobromide, a complex alkaloid, antagonizes in some unknown manner the action of GABA, an important inhibitory transmitter in the CNS. To help understand the mechanism of this antagonism we have employed molecular modeling techniques to assess the similarity and difference between this antagonist and GABA plus four direct GABA agonists. Topological and electronic charge congruence between these five molecules was examined in great detail. It was found that each of the five molecules has three clearly defined atoms that serve as attachment points at the GABAA receptor site. It is hypothesized that an additional negatively charged atom on bicuculline serves as an additional point of attachment that blocks the nearby chloride ion channel. The model presented offers an explanation of why muscimol acts as a better agonist than GABA as well as rationalizing why (+)-bicuculline acts as an antagonist but (-)-bicuculline does not.