title: Role of GABAergic antagonism in the neuroprotective effects of bilobalide.
abstract: Bilobalide, a constituent of Ginkgo biloba, has neuroprotective properties. Its mechanism of action is unknown but it was recently found to block GABA(A) receptors. The goal of this study was to test the potential role of a GABAergic mechanism for the neuroprotective activity of bilobalide. In rat hippocampal slices exposed to NMDA, release of choline indicates breakdown of membrane phospholipids. NMDA-induced choline release was almost completely blocked in the presence of bilobalide (10 microM) and under low-chloride conditions. Bicuculline (100 microM), a competitive antagonist at GABA(A) receptors, reduced NMDA-induced choline release to a small extent (-23%). GABA (100 microM) partially antagonized the inhibitory action of bilobalide. Exposure of hippocampal slices to NMDA also caused edema formation as measured by increases of tissue water content. NMDA-induced edema formation was suppressed by bilobalide and by low-chloride conditions. Bicuculline exerted partial protection (by 30%) while GABA reduced bilobalide's effect by about one third. To investigate bilobalide's interaction with GABA(A) receptors directly, we measured binding of [(35)S]-TBPS to rat cortical membranes. TBPS binding was competitively inhibited by bilobalide in the low micromolar range (IC(50)=3.7 microM). As a functional test, we determined (36)chloride flux in rat corticohippocampal synaptoneurosomes. GABA (100 microM) significantly increased (36)chloride flux (+65%), and this increase was blocked by bilobalide, but with low potency (IC(50): 39 microM). We conclude that, while antagonism of GABA(A) receptors may contribute to bilobalide's neuroprotective effects, additional mechanisms must be postulated to fully explain bilobalide's actions.
authors.affiliation: Department of Pharmaceutical Sciences, School of Pharmacy, Texas Tech University Health Science Center, 1300 Coulter Dr., Amarillo, TX 79106, USA.
authors.forename: Cornelia
authors.initials: C
authors.lastname: Kiewert
authors.forename: Vikas
authors.initials: V
authors.lastname: Kumar
authors.forename: Oksana
authors.initials: O
authors.lastname: Hildmann
authors.forename: Misty
authors.initials: M
authors.lastname: Rueda
authors.forename: Joachim
authors.initials: J
authors.lastname: Hartmann
authors.forename: Runa S
authors.initials: RS
authors.lastname: Naik
authors.forename: Jochen
authors.initials: J
authors.lastname: Klein
mesh_terms: D000818:Animals; D019540:Area Under Curve; D001640:Bicuculline / Q000494:pharmacology; D001929:Brain Edema / Q000139:chemically induced / Q000517:prevention & control; D019086:Bridged Bicyclo Compounds, Heterocyclic / Q000378:metabolism; D002794:Choline / Q000378:metabolism; D003517:Cyclopentanes / Q000494:pharmacology*; D004347:Drug Interactions; D018690:Excitatory Amino Acid Agonists / Q000633:toxicity; D005663:Furans / Q000494:pharmacology*; D018756:GABA Antagonists / Q000494:pharmacology; D046934:Ginkgolides / Q000494:pharmacology*; D006624:Hippocampus / Q000187:drug effects* / Q000378:metabolism / Q000648:ultrastructure; D066298:In Vitro Techniques; D008297:Male; D016202:N-Methylaspartate / Q000633:toxicity; D018696:Neuroprotective Agents / Q000494:pharmacology*; D017711:Nonlinear Dynamics; D010852:Picrotoxin / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D013574:Synaptosomes / Q000187:drug effects / Q000378:metabolism; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
chemical_list: D001640:Bicuculline; D019086:Bridged Bicyclo Compounds, Heterocyclic; D002794:Choline; D003517:Cyclopentanes; D018690:Excitatory Amino Acid Agonists; D005663:Furans; D018756:GABA Antagonists; D046934:Ginkgolides; D016202:N-Methylaspartate; D018696:Neuroprotective Agents; D010852:Picrotoxin; D005680:gamma-Aminobutyric Acid; C037476:tert-butylbicyclophosphorothionate; C073710:bilobalide