Flavonoid modulation of ionic currents mediated by GABAA and GABAC receptors

https://doi.org/10.1016/S0014-2999(03)01309-8Get rights and content

Abstract

The modulation of ionotropic γ-aminobutyric acid (GABA) receptors (GABA-gated Cl channels) by a group of natural and synthetic flavonoids was studied in electrophysiological experiments. Quercetin, apigenin, morine, chrysin and flavone inhibited ionic currents mediated by α1β1γ2s GABAA and ρ1 GABAC receptors expressed in Xenopus laevis oocytes in the micromolar range. α1β1γ2s GABAA and ρ1 GABAC receptors differ largely in their sensitivity to benzodiazepines, but they were similarly modulated by different flavonoids. Quercetin produced comparable actions on currents mediated by α4β2 neuronal nicotinic acetylcholine, serotonin 5-HT3A and glutamate AMPA/kainate receptors. Sedative and anxiolytic flavonoids, like chrysin or apigenin, failed to potentiate but antagonized α1β1γ2s GABAA receptors. Effects of apigenin and quercetin on α1β1γ2s GABAA receptors were insensitive to the benzodiazepine antagonist flumazenil. Results indicate that mechanism/s underlying the modulation of ionotropic GABA receptors by some flavonoids differs from that described for classic benzodiazepine modulation.

Introduction

Flavonoids are substances of low molecular weight found in vascular plants Bohm, 1998, Harborne and Williams, 2000. They show a wide range of biological activities Miksicek, 1993, Middleton and Kandaswami, 1994, Harborne and Williams, 2000, with neuropharmacological actions such as analgesia, effects on motility and sleep Speroni and Minghetti, 1988, Picq et al., 1991, modulation of neuronal oxidative metabolism (Oyama et al., 1994), proconvulsant, anticonvulsant, sedative and anxiolytic effects Speroni and Minghetti, 1988, Medina et al., 1990, Häberlein et al., 1994, Viola et al., 1995, Marder et al., 1995, Medina et al., 1998, Griebel et al., 1999, etc.

Effects of flavonoids on the central nervous system (CNS) are complex and involve different mechanisms Vohora et al., 1980, Chakravarthy et al., 1981, Medina et al., 1998, including actions on synaptic receptors or ionic channels Nielsen et al., 1988, Koh et al., 1994, Ji et al., 1996, Medina et al., 1998, Simmen et al., 1998, Dekermendjian et al., 1999, Avallone et al., 2000, Calvo et al., 2000, Mall et al., 2000, Saponara et al., 2002.

Many studies correlated the affinities of diverse natural and synthetic flavonoids for the benzodiazepine binding sites located to the GABAA receptors (γ-aminobutyric acid: GABA) to their pharmacological properties shown in vivo Medina et al., 1998, Griebel et al., 1999, Avallone et al., 2000, Viola et al., 2000. Based on these and other evidences, a benzodiazepine-like mechanism was proposed for flavonoid modulation of ionotropic GABA receptors Medina et al., 1998, Dekermendjian et al., 1999, Marder and Paladini, 2002. However, experimental support for a benzodiazepine-like modulation of the GABA receptor function by flavonoids is still lacking. Now we tested this hypothesis by studying the effects of a group of flavonoids (chemical structures illustrated in Fig. 1) on ionic currents mediated by two common ionotropic GABA receptor subtypes.

GABAA receptors are heteromeric proteins forming pentameric structures assembled from diverse subunit types (α1−6, β1−4, γ1−3, δ, ε and π) Barnard et al., 1998, Hevers and Luddens, 1998. In contrast, GABAC receptors would be homomeric, exclusively composed by ρ subunits (Enz and Cutting, 1998). α1β1γ2s GABAA receptors are highly expressed in the brain and represent a quite common GABAA receptor subtype (in cerebral cortex, thalamus, etc.) that is strongly potentiated by benzodiazepines (Hevers and Luddens, 1998). ρ1 GABAC receptors are mainly expressed in the retina and mediate benzodiazepine-insensitive responses (Zhang et al., 2001).

We studied the effects of flavonoids on α1β1γ2s GABAA and ρ1 GABAC receptors expressed in Xenopus oocytes. Results indicated that diverse flavonoids antagonized ionic currents mediated by α1β1γ2s GABAA and ρ1 GABAC receptors in a similar way and did not behave as benzodiazepine-like modulators.

Section snippets

RNA preparation for oocyte injections

Full-length cDNAs, encoding the different receptor subunits, cloned in vectors suitable for in vitro transcription, were provided by colleagues (see acknowledgments). Rat GABAA receptor subunits: α1 and β1 in pBluescript SK (Promega, Madison, WI, USA), γ2S in pSP6 (Invitrogen, Groningen, The Netherlands). Human GABAA receptor subunits: α1, β1 and γ2S in pCDM8 (Invitrogen). Human GABAC receptor subunit: ρ1 in pBluescript SK(Promega). Rat neuronal nicotinic acetylcholine receptor subunits α4, β2

Results

The modulation of ionic currents mediated by α1β1γ2s GABAA and ρ1 GABAC receptors by quercetin, chrysin, apigenin, morine, flavone and α-naphthoflavone was examined in electrophysiological experiments. Selectivity of the flavonoid actions was also analyzed by studying in a similar way the effects of quercetin on other ionotropic neurotransmitter receptors, namely, α4β2 neuronal nicotinic acetylcholine, 5-HT3A and glutamate AMPA/kainate receptors.

Discussion

We have characterized the pharmacological actions of a group of flavonoids on ionotropic GABA receptors through electrophysiological studies. A number of naturally occurring and synthetic compounds were selected based on their previously reported effects in binding assays and in vivo pharmacological experiments. The main contribution of the present work is the demonstration that diverse flavonoids modulate the function of GABAA and GABAC receptors and also of other ionotropic receptors

Acknowledgements

We would like to thank people that generously provided us with plasmids carrying the different receptor subunits, Dr. Paul Whiting and Dr. Peter Seeburg for α1β1γ2s GABAA; Dr. Ricardo Miledi and Dr. Ataúlfo Martinez-Torres for ρ1 GABAC and Dr. Stephen Heinemann, Dr. Jim Boulter and Dr. David Johnson for nicotinic α4β2 and 5-HT3A. We also thank Dr. Mariana del Vas and Dr. Marcelo Rubinstein for reading the manuscript. This work was supported by grants from CONICET (PIP 780/98) and FONCyT (PICT99

References (43)

  • L. Movileanu et al.

    Interaction of the antioxidant flavonoid quercetin with planar lipid bilayers

    Int. J. Pharm.

    (2000)
  • M. Nielsen et al.

    High affinity of the naturally-occurring biflavonoid, amentoflavone, to brain benzodiazepine receptors in vitro

    Biochem. Pharmacol.

    (1988)
  • Y. Oyama et al.

    Myricetin and quercetin, the flavonoid constituents of Ginkgo biloba extract, greatly reduce oxidative metabolism in both resting and Ca2+-loaded brain neurons

    Brain Res.

    (1994)
  • K. Pattichis et al.

    5-Hydroxytryptamine release from platelets by different red wines: implications for migraine

    Eur. J. Pharmacol.

    (1995)
  • M. Picq et al.

    Effect of two flavonoid compounds on the central nervous system. Analgesic activity

    Life Sci.

    (1991)
  • J.B. Salgueiro et al.

    Anxiolytic natural and synthetic flavonoid ligands of the central benzodiazepine receptor have no effect on memory tasks in rats

    Pharmacol. Biochem. Behav.

    (1997)
  • U. Simmen et al.

    Hypericum perforatum inhibits the binding of mu- and kappa-opioid receptor expressed with the Semliki Forest virus system

    Pharm. Acta Helv.

    (1998)
  • J.A. van Hooft et al.

    5-HT3 receptors and neurotransmitter release in the CNS: a nerve ending story?

    Trends Neurosci.

    (2000)
  • H. Viola et al.

    6-Chlorine-3′-nitroflavone is a potent central benzodiazepine receptor ligand devoid of intrinsic activity

    Pharmacol. Biochem. Behav.

    (2000)
  • C. Wolfman et al.

    Possible anxiolytic effects of chrysin, a central benzodiazepine receptor ligand isolated from Passiflora coerulea

    Pharmacol. Biochem. Behav.

    (1994)
  • D. Zhang et al.

    Structure and function of GABA(C) receptors: a comparison of native versus recombinant receptors

    Trends Pharmacol. Sci.

    (2001)
  • Cited by (112)

    • Current perspectives on benzoflavone analogues with potent biological activities: A review

      2022, Arabian Journal of Chemistry
      Citation Excerpt :

      Benzoflavones derivatives have been found to perform a function in the development of CNS agents. Goutman et al. (2003) reported that multiple flavonoids can modulate the ionic currents facilitated by GABAC and GABAA receptors, while ANF and flavone showed almost non-activity compared to active derivatives including quercetin, apigenin, morine, and chrysin. SAR analysis showed that hydroxyl groups were crucial for the regulation of ionotropic GABA receptors.

    • Citrus flavonoids effects on human umbilical vein

      2021, Journal of Functional Foods
    View all citing articles on Scopus
    View full text