Ginkgolides, diterpene trilactones of Ginkgo biloba, as antagonists at recombinant a 1 h 2 g 2L GABA A receptors Shelley H. Huang a , Rujee K. Duke a,b, * , Mary Chebib b , Keiko Sasaki c , Keiji Wada c , Graham A.R. Johnston a,d a Adrien Albert Laboratory of Medicinal Chemistry, Department of Pharmacology, Faculty of Medicine, The University of Sydney, Sydney, NSW 2006, Australia b Pharmaceutical Chemistry, Faculty of Pharmacy, The University of Sydney, NSW 2006, Australia c Department of Hygienic Chemistry, Faculty of Pharmaceutical Sciences, Health Sciences, University of Hokkaido, Ishikari-Tobetsu, Japan d Herbal Medicines Research and Education Centre, Faculty of Pharmacy, The University of Sydney, NSW 2006, Australia Received 24 November 2003; received in revised form 17 March 2004; accepted 30 April 2004 Available online Abstract Ginkgolides A, B, and C are diterpene trilactones and active constituents of the 50:1 Ginkgo biloba leaf extract widely used in the symptomatic treatment of mild to moderate dementia. Using the two-electrode voltage clamp methodology, these ginkgolides were found to be moderately potent antagonists at recombinant human a 1 h 2 g 2L GABA A receptors expressed in Xenopus oocytes. Ginkgolides A, B, and C inhibited the direct action of g-aminobutyric acid (GABA) with K i values of 14.5 F 1.0, 12.7 F 1.7, and 16.3 F 2.4 AM respectively. Antagonism by these ginkgolides at a 1 h 2 g 2L GABA A receptors appears to be noncompetitive as indicated by the nonparallel right shift and reduced maximal GABA response in their GABA concentration – effect curves. D 2004 Elsevier B.V. All rights reserved. Keywords: GABA A receptor; Noncompetitive; Ginkgolide; Picrotoxinin; Two-electrode voltage clamp; Xenopus oocyte 1. Introduction g-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system mediating fast neurotransmission predominantly via the GABA A receptor. GABA A receptors belong to the family of inhibitory transmitter-gated channels that also include GABA C receptors and glycine receptors. The channel of GABA A , GABA C , and glycine receptors opens in response to binding of the respective transmitter, GABA, or glycine enabling Cl À ions to follow its electrochemical gradient into the cells, balancing the effect of neuronal excitation (Jentsch, 2002). The channel of GABA A , GABA C , and glycine receptors is blocked by the plant convulsant pic- rotoxinin (Fig. 1; Zhorov and Bregestovski, 2000). Picrotoxinin is a noncompetitive antagonist of GABA A receptors (Akaike et al., 1985).[ 35 S]t-butylphosphorothio- nate ([ 35 S]TBPS) is a radioligand for picrotoxinin binding sites at GABA A receptors (Olsen et al., 1989). Picrotoxinin and TBPS binding sites have been shown to reside within the channel of GABA A receptors (Jursky et al., 2000; Perret et al., 1999). GABA A receptors also incorporate the binding sites for GABA, barbiturates, benzodiazepines, and steroids. GABA A receptors are heterooligomeric pen- tamers assembled from several possible combinations of protein subunits. To date, at least 16 different subunits for human GABA A receptors have been identified, namely, a 1–6 , h 1–3 , g 1–3 , y, k, and u (Chebib and Johnston, 2000). Most GABA A receptors are formed by the coexpression of a, h, and g subunits with the subunit combination of a 1 h 2 g 2L being the major GABA A receptor subtype in the brain (Upton and Blackburn, 1997). Ginkgolides (Fig. 1) are diterpene trilactones of the maiden hair tree, Ginkgo biloba. Ginkgo leaves have long been used in traditional Chinese medicine. Modern usage utilises the extract from the leaves for the treatment of 0014-2999/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.ejphar.2004.04.051 * Corresponding author. Department of Pharmacology D06, The University of Sydney, Sydney, NSW 2006, Australia. Tel.: +61-2-9351- 6204; fax: +61-2-9351-3868. E-mail address: rujeek@pharmacol.usyd.edu.au (R.K. Duke). www.elsevier.com/locate/ejphar European Journal of Pharmacology 494 (2004) 131 – 138