Thermodynamics and kinetics of t-butylbicyclophosphorothionate binding differentiate convulsant and depressant barbiturate stereoisomers acting via GABAA ionophores

G. Maksay, Péter Molnár, M. Simonyi

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Abstract

The temperature dependence of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites of the GABAA receptor complex was studied in membrane preparations of rat forebrain. Although specific [35S]TBPS binding was maximal around 20°C, the rate constants of dissociation decreased monotonously between 37°C and 2°C. The displacing potencies of the convulsant S(+) enantiomer of 1-methyl-5-phenyl-5-propyl-barbituric acid (MPPB) (IC50 = 1250 ± 30 μM) and the depressant R(-) MPPB (IC50 = 310 ± 5 μM) did not show significant changes between 19°C and 37°C. Therefore barbiturate binding seems to be driven by entropic, rather than enthalpic changes. An excess of MPPB enantiomers elicited accelerated and polyphasic dissociations of [35S]TBPS as compared to the monophasic dissociation by TBPS. Arrhenius analysis was applied to the measurable initial rate constants of dissociation. Arrhenius plots were linear between 2°C and 37°C. Activation parameters were similar when [35S] TBPS dissociation was triggered by the convulsants TBPS and S(+) MPPB. It can be attributed to similar conformations of the closed ionophore complex. In contrast, the depressant R(-) MPPB strongly decreased the activation energy of TBPS dissociation from the open ionophore ternary complex. In whole-cell patch-clamp experiments R(-) MPPB, but not S(+) MPPB, elicited chloride currents in rat primary cortical cultures with an EC50 value of 560 ± 30 μM and a Hill coefficient of 2.9 ± 0.2. These currents were similar to those elicited by GABA and blocked by TBPS. A kinetic scheme is proposed for the dissociation of TBPS and to explain the different effects of MPPB enantiomers. Submillimolar R(-) MPPB is supposed to bind to (about three) barbiturate sites on GABAA-ionophores and to open them in a cooperative manner to result in a decreased activation energy for accelerated displacement of convulsant binding.

Original languageEnglish
Pages (from-to)306-313
Number of pages8
JournalNaunyn-Schmiedeberg's Archives of Pharmacology
Volume353
Issue number3
Publication statusPublished - 1996

Fingerprint

Convulsants
Stereoisomerism
Ionophores
Thermodynamics
Inhibitory Concentration 50
barbituric acid
tert-butylbicyclophosphorothionate
GABA-A Receptors
Prosencephalon
gamma-Aminobutyric Acid
Chlorides

Keywords

  • Arrhenius analysis
  • Barbiturate stereoisomers
  • Dissociation of [S]TBPS binding
  • Electrophysiology of GABA-ionophores
  • GABA receptor-ionophore complex
  • Thermodynamics of barbiturate binding

ASJC Scopus subject areas

  • Pharmacology

Cite this

@article{e2d25291c266455f93efc1d91736a7de,
title = "Thermodynamics and kinetics of t-butylbicyclophosphorothionate binding differentiate convulsant and depressant barbiturate stereoisomers acting via GABAA ionophores",
abstract = "The temperature dependence of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites of the GABAA receptor complex was studied in membrane preparations of rat forebrain. Although specific [35S]TBPS binding was maximal around 20°C, the rate constants of dissociation decreased monotonously between 37°C and 2°C. The displacing potencies of the convulsant S(+) enantiomer of 1-methyl-5-phenyl-5-propyl-barbituric acid (MPPB) (IC50 = 1250 ± 30 μM) and the depressant R(-) MPPB (IC50 = 310 ± 5 μM) did not show significant changes between 19°C and 37°C. Therefore barbiturate binding seems to be driven by entropic, rather than enthalpic changes. An excess of MPPB enantiomers elicited accelerated and polyphasic dissociations of [35S]TBPS as compared to the monophasic dissociation by TBPS. Arrhenius analysis was applied to the measurable initial rate constants of dissociation. Arrhenius plots were linear between 2°C and 37°C. Activation parameters were similar when [35S] TBPS dissociation was triggered by the convulsants TBPS and S(+) MPPB. It can be attributed to similar conformations of the closed ionophore complex. In contrast, the depressant R(-) MPPB strongly decreased the activation energy of TBPS dissociation from the open ionophore ternary complex. In whole-cell patch-clamp experiments R(-) MPPB, but not S(+) MPPB, elicited chloride currents in rat primary cortical cultures with an EC50 value of 560 ± 30 μM and a Hill coefficient of 2.9 ± 0.2. These currents were similar to those elicited by GABA and blocked by TBPS. A kinetic scheme is proposed for the dissociation of TBPS and to explain the different effects of MPPB enantiomers. Submillimolar R(-) MPPB is supposed to bind to (about three) barbiturate sites on GABAA-ionophores and to open them in a cooperative manner to result in a decreased activation energy for accelerated displacement of convulsant binding.",
keywords = "Arrhenius analysis, Barbiturate stereoisomers, Dissociation of [S]TBPS binding, Electrophysiology of GABA-ionophores, GABA receptor-ionophore complex, Thermodynamics of barbiturate binding",
author = "G. Maksay and P{\'e}ter Moln{\'a}r and M. Simonyi",
year = "1996",
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journal = "Naunyn-Schmiedeberg's Archives of Pharmacology",
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TY - JOUR

T1 - Thermodynamics and kinetics of t-butylbicyclophosphorothionate binding differentiate convulsant and depressant barbiturate stereoisomers acting via GABAA ionophores

AU - Maksay, G.

AU - Molnár, Péter

AU - Simonyi, M.

PY - 1996

Y1 - 1996

N2 - The temperature dependence of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites of the GABAA receptor complex was studied in membrane preparations of rat forebrain. Although specific [35S]TBPS binding was maximal around 20°C, the rate constants of dissociation decreased monotonously between 37°C and 2°C. The displacing potencies of the convulsant S(+) enantiomer of 1-methyl-5-phenyl-5-propyl-barbituric acid (MPPB) (IC50 = 1250 ± 30 μM) and the depressant R(-) MPPB (IC50 = 310 ± 5 μM) did not show significant changes between 19°C and 37°C. Therefore barbiturate binding seems to be driven by entropic, rather than enthalpic changes. An excess of MPPB enantiomers elicited accelerated and polyphasic dissociations of [35S]TBPS as compared to the monophasic dissociation by TBPS. Arrhenius analysis was applied to the measurable initial rate constants of dissociation. Arrhenius plots were linear between 2°C and 37°C. Activation parameters were similar when [35S] TBPS dissociation was triggered by the convulsants TBPS and S(+) MPPB. It can be attributed to similar conformations of the closed ionophore complex. In contrast, the depressant R(-) MPPB strongly decreased the activation energy of TBPS dissociation from the open ionophore ternary complex. In whole-cell patch-clamp experiments R(-) MPPB, but not S(+) MPPB, elicited chloride currents in rat primary cortical cultures with an EC50 value of 560 ± 30 μM and a Hill coefficient of 2.9 ± 0.2. These currents were similar to those elicited by GABA and blocked by TBPS. A kinetic scheme is proposed for the dissociation of TBPS and to explain the different effects of MPPB enantiomers. Submillimolar R(-) MPPB is supposed to bind to (about three) barbiturate sites on GABAA-ionophores and to open them in a cooperative manner to result in a decreased activation energy for accelerated displacement of convulsant binding.

AB - The temperature dependence of [35S]-t-butylbicyclophosphorothionate (TBPS) binding to the convulsant sites of the GABAA receptor complex was studied in membrane preparations of rat forebrain. Although specific [35S]TBPS binding was maximal around 20°C, the rate constants of dissociation decreased monotonously between 37°C and 2°C. The displacing potencies of the convulsant S(+) enantiomer of 1-methyl-5-phenyl-5-propyl-barbituric acid (MPPB) (IC50 = 1250 ± 30 μM) and the depressant R(-) MPPB (IC50 = 310 ± 5 μM) did not show significant changes between 19°C and 37°C. Therefore barbiturate binding seems to be driven by entropic, rather than enthalpic changes. An excess of MPPB enantiomers elicited accelerated and polyphasic dissociations of [35S]TBPS as compared to the monophasic dissociation by TBPS. Arrhenius analysis was applied to the measurable initial rate constants of dissociation. Arrhenius plots were linear between 2°C and 37°C. Activation parameters were similar when [35S] TBPS dissociation was triggered by the convulsants TBPS and S(+) MPPB. It can be attributed to similar conformations of the closed ionophore complex. In contrast, the depressant R(-) MPPB strongly decreased the activation energy of TBPS dissociation from the open ionophore ternary complex. In whole-cell patch-clamp experiments R(-) MPPB, but not S(+) MPPB, elicited chloride currents in rat primary cortical cultures with an EC50 value of 560 ± 30 μM and a Hill coefficient of 2.9 ± 0.2. These currents were similar to those elicited by GABA and blocked by TBPS. A kinetic scheme is proposed for the dissociation of TBPS and to explain the different effects of MPPB enantiomers. Submillimolar R(-) MPPB is supposed to bind to (about three) barbiturate sites on GABAA-ionophores and to open them in a cooperative manner to result in a decreased activation energy for accelerated displacement of convulsant binding.

KW - Arrhenius analysis

KW - Barbiturate stereoisomers

KW - Dissociation of [S]TBPS binding

KW - Electrophysiology of GABA-ionophores

KW - GABA receptor-ionophore complex

KW - Thermodynamics of barbiturate binding

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M3 - Article

VL - 353

SP - 306

EP - 313

JO - Naunyn-Schmiedeberg's Archives of Pharmacology

JF - Naunyn-Schmiedeberg's Archives of Pharmacology

SN - 0028-1298

IS - 3

ER -