TiPS - March 1990 IVol. 11] The steady-state concentration of active o~subunits again becomes a hyperbolic function of the agonist concentration, provided that PlA and P2A are practically independent of the concentration of XGDP. The latter condition is true if:
• the concentrations of GTP and GDP are kept constant by cell metabolism; • k+aA [GTP], k-lA and k4A are all much greater than k+2A [XGDP]; • k+aA[GTP] is much greater than k-2A;
103 • k-3A [GDP] is negligible. Under these conditions PIA ---( k + l A k+2A)/(k-lA k5 [ G T P ] ) and P2A = k + l A / k - l A . (The various rate constants are defined in Ref. 1.) If the system being studied is better described by model B, then Eqn 2 can be used to derive null equations for the comparison of agonists and for the estimation of apparent affinity using an irreversible antagonist. From these null equations it can be concluded that P1 still determines relative potency, P2 represents apparent
Dangers of 'single-point pharmacology' in receptor studies
GABAA receptor: complex picture confused In a recent TiPS article (December 1989)1, the uncertainty surrounding the structure of the GABA receptor was reviewed. The rapid progress in the cloning of the receptor subunit isoforms has created demand for quick functional assays, and one of the approaches currently being us~ed is functional expression of different subunit isoform combinations. Part of the complexity in this field has since been created by what may be called 'singlepoint pharmacology', in which, for convenience, drug effects are tested at a single concentration.
Such experiments may lead to false or erroneous interpretations. It is especially difficult to compare data obtained in different laboratories using different drug concentrations. Thus, in attempts to define the minimum structural requirements of a functional receptor, detailed experiments using a wide range of drug concentrations should always be carried out. The statement, "chick brain mRNA, expressed in oocytes, forms GABAA receptors in which the benzodiazepine inverse agonist DMCM potentiates rather than reduces
Is the antispastic action of baclofen mediated by GABAs receptors?
GABAB receptors and spasticity It was with great interest that we read Norman Bowery's review on GABAB receptors and their significance in mammalian pharmacology (TIPS, October 1989)1 However, the antispastic action was not mentioned among the actions attributed to GABAB receptor stimulation. The GABAB receptor agonist baclofen given orally or intrathecally successfully reduces spasticity and prevents flexor spasms in various neurological disorders2-4. Baclofea is thought to exert its antispastic action by suppression of spinal m o n o - and polysynaptic reflexes 5. Indeed, a recent electrophysiological study in patients with lower limb spasticity showed that
intrathecal baclofen reduces the amplitude of the spinal Hoffmann reflex in parallel with improving spasticity 6. We have recently investigated the action of baclofen in rodent models of spasticity. Syst.emic or intrathecal baclofen dose-aependently decreases the pathologically increased muscle tone of spastic mutant rats. This action is resistant to bicuculline but is antagonized by coadministration of the GABAB receptor antagonist 6-aminovaleric acid (6-AVA)7. In anaesthetized rats, 'uom .... 6-AVA and the novel GABAB receptor antagonist phaclofen dose-dependently reverse the depression of spinal mono- and polysynaptic
affinity and P1/P2 represents efficacy. In this case, however, the apparent affinity would be expected to be cJose to the true affinity, and the relative efficacy of agonists would depend on the relative magnitudes of k+2. DENNIS MACKAY
Department of Pharmacology, University of Leeds, Leeds LS2 9JT, UK.
Reference
1 Mackay,D. (1990) TrendsPharmacoLSci. 11, 17-22
GABA-activated Ct flux" (Ref. 1) deserves a further comment. In the quoted work 2, DMCM, at concentrations between 1rim and 10 I~M,was in fact shown to inhibit GABA-induced C1- currents. Only at higher concentrations was a weak potentiation detected. Thus, the GABA receptor expressed in the Xenopus oocyte responds to inverse benzodiazepine agonists in the way expected from the native receptor. ERWIN SIGEL
Pharmakologisches Institut der Universititt Bern, Friedb~ihlstrasse 49, CH-3010 Bern, Switzerland.
References 1 Schofield, P. R. (1989) Trends Pharmacol. Sci. 10, 476-478 2 Sigel, E. and Baur, R. (1988) J. Neurosci. 8, 289-295
reflexes induced by baclofen whereas bicuculline has no effects. U. WULLNER AND T. KLOCKGETHER
Department of Neurology, Univer~ify of Tiibiny~en. Hoppc-Seyier 5tr. 3, D-7400 I ubtngen, FRG. "~ References 1 Bowery, N. (1989) Trends Pharmacol. Sci. 10, 401--407 2 Young, R. R. and Delwaide, P. I. (1981) N. EngL J. Med. 304, 28-33 3 Young, R. R. and Delwaide, P. J. (198!) N. Engl. J. Med. 304, 96--99 4 Penn, R. D. and Kroin, J. S. (1985) Lancet ii, 125-127 5 Oavidoff0 R. A. and Sears, E. S (1974) Neurology 24, 957-963 6 MaeDonell, R., Tal!ala, A., Swash, M. and Grundy, D. (1989) J, Neurol. Neurosurg. Psychol. 52, 111~--1112 7 Schwarz, M., Klockgether, T., Wfillner, U, and Turski, L. and Sontag, K-H. (1988) Exp. Brain Res. 70, 618-626 8 Wfillner, U., Klockgether, T. and Sontag, K-H. (1989) Brain Res. 596, 341-344
• the concentrations of GTP and GDP are kept constant by cell metabolism; • k+aA [GTP], k-lA and k4A are all much greater than k+2A [XGDP]; • k+aA[GTP] is much greater than k-2A;
103 • k-3A [GDP] is negligible. Under these conditions PIA ---( k + l A k+2A)/(k-lA k5 [ G T P ] ) and P2A = k + l A / k - l A . (The various rate constants are defined in Ref. 1.) If the system being studied is better described by model B, then Eqn 2 can be used to derive null equations for the comparison of agonists and for the estimation of apparent affinity using an irreversible antagonist. From these null equations it can be concluded that P1 still determines relative potency, P2 represents apparent
Dangers of 'single-point pharmacology' in receptor studies
GABAA receptor: complex picture confused In a recent TiPS article (December 1989)1, the uncertainty surrounding the structure of the GABA receptor was reviewed. The rapid progress in the cloning of the receptor subunit isoforms has created demand for quick functional assays, and one of the approaches currently being us~ed is functional expression of different subunit isoform combinations. Part of the complexity in this field has since been created by what may be called 'singlepoint pharmacology', in which, for convenience, drug effects are tested at a single concentration.
Such experiments may lead to false or erroneous interpretations. It is especially difficult to compare data obtained in different laboratories using different drug concentrations. Thus, in attempts to define the minimum structural requirements of a functional receptor, detailed experiments using a wide range of drug concentrations should always be carried out. The statement, "chick brain mRNA, expressed in oocytes, forms GABAA receptors in which the benzodiazepine inverse agonist DMCM potentiates rather than reduces
Is the antispastic action of baclofen mediated by GABAs receptors?
GABAB receptors and spasticity It was with great interest that we read Norman Bowery's review on GABAB receptors and their significance in mammalian pharmacology (TIPS, October 1989)1 However, the antispastic action was not mentioned among the actions attributed to GABAB receptor stimulation. The GABAB receptor agonist baclofen given orally or intrathecally successfully reduces spasticity and prevents flexor spasms in various neurological disorders2-4. Baclofea is thought to exert its antispastic action by suppression of spinal m o n o - and polysynaptic reflexes 5. Indeed, a recent electrophysiological study in patients with lower limb spasticity showed that
intrathecal baclofen reduces the amplitude of the spinal Hoffmann reflex in parallel with improving spasticity 6. We have recently investigated the action of baclofen in rodent models of spasticity. Syst.emic or intrathecal baclofen dose-aependently decreases the pathologically increased muscle tone of spastic mutant rats. This action is resistant to bicuculline but is antagonized by coadministration of the GABAB receptor antagonist 6-aminovaleric acid (6-AVA)7. In anaesthetized rats, 'uom .... 6-AVA and the novel GABAB receptor antagonist phaclofen dose-dependently reverse the depression of spinal mono- and polysynaptic
affinity and P1/P2 represents efficacy. In this case, however, the apparent affinity would be expected to be cJose to the true affinity, and the relative efficacy of agonists would depend on the relative magnitudes of k+2. DENNIS MACKAY
Department of Pharmacology, University of Leeds, Leeds LS2 9JT, UK.
Reference
1 Mackay,D. (1990) TrendsPharmacoLSci. 11, 17-22
GABA-activated Ct flux" (Ref. 1) deserves a further comment. In the quoted work 2, DMCM, at concentrations between 1rim and 10 I~M,was in fact shown to inhibit GABA-induced C1- currents. Only at higher concentrations was a weak potentiation detected. Thus, the GABA receptor expressed in the Xenopus oocyte responds to inverse benzodiazepine agonists in the way expected from the native receptor. ERWIN SIGEL
Pharmakologisches Institut der Universititt Bern, Friedb~ihlstrasse 49, CH-3010 Bern, Switzerland.
References 1 Schofield, P. R. (1989) Trends Pharmacol. Sci. 10, 476-478 2 Sigel, E. and Baur, R. (1988) J. Neurosci. 8, 289-295
reflexes induced by baclofen whereas bicuculline has no effects. U. WULLNER AND T. KLOCKGETHER
Department of Neurology, Univer~ify of Tiibiny~en. Hoppc-Seyier 5tr. 3, D-7400 I ubtngen, FRG. "~ References 1 Bowery, N. (1989) Trends Pharmacol. Sci. 10, 401--407 2 Young, R. R. and Delwaide, P. I. (1981) N. EngL J. Med. 304, 28-33 3 Young, R. R. and Delwaide, P. J. (198!) N. Engl. J. Med. 304, 96--99 4 Penn, R. D. and Kroin, J. S. (1985) Lancet ii, 125-127 5 Oavidoff0 R. A. and Sears, E. S (1974) Neurology 24, 957-963 6 MaeDonell, R., Tal!ala, A., Swash, M. and Grundy, D. (1989) J, Neurol. Neurosurg. Psychol. 52, 111~--1112 7 Schwarz, M., Klockgether, T., Wfillner, U, and Turski, L. and Sontag, K-H. (1988) Exp. Brain Res. 70, 618-626 8 Wfillner, U., Klockgether, T. and Sontag, K-H. (1989) Brain Res. 596, 341-344
