γ-aminobutyric acid, (GABA) is the major inhibitory neurotransmitter in the vertebrate brain, and is still of great interest therapeutically because it comprises the myriad of binding sites for pharmaceutically important drugs that interact allosterically with GABA agonist site of the channel. However we still do not have ideal anxiolytic, sedative, or hypnotic drugs for chronic treatments (Korpi et al., 2002). Octyl-O-β-D-glucoside represents a member of a novel class of GABA_A receptor positive modulator. In this study, we have characterized further the GABA_A receptor pharmacology of the octyl-O-β-D-glucoside class using an [S³⁵]-t-butylbicyclophosphoorothionate radioligand binding assay. The assay was performed in 50 mM Tris-buffer supplemented with 0.2 M NaCl (pH 7.4), and a test concentration range of 10^-9M-10^-2M. Well-washed adult male rat Sprague Dawley strain forebrain membranes (100 µg) were incubated in the presence of 25 nM [S³⁵] TBPS radioligand, at 25 Cº for 90 min. Non-specific binding was defined in the presence of 100 µM picrotoxinin. The reactions were terminated by rapid filtration using a Brandell cell harvester, and washed with 3 x 4ml washes with ice-cold sodium phosphate buffer, pH 7.4. Novel data cited are mean ± SD % stimulation of specific [S³⁵] TBPS binding for 3-5 separate experiments (GraphPad Prism 3).

As reported previously, octyl-O-β-D-glucoside elicited a concentration-dependant stimulation of specific [ ³⁵S] TBPS binding (mean E_max= 144 ± 4%; apparent pEC₅₀ = 6.59 ± 0.24) (Abuhamdah et al., 2004). In this present study, the core monosaccharide glucose ( 107 ± 2% at 1mM), octyl--D-glucoside (110 ± 1% at 1mM), nor hexyl-β-D-glucoside ( 94 ± 2% at 1mM) had little or no effect upon control [ ³⁵S] TBPS binding. Previously, we showed that lactose potentiated [³H] TBOB binding to the channel site of the GABA_A receptor, with a maximal effect observed at 10 µM (Rezai et al., 2003). Here we showed that lactose (10 µM) occluded the potentiation by 100 µM octyl--D-glucoside of [ ³⁵S] TBPS binding (144 ± 4% without lactose; 108 ± 9 % with 10 µM lactose), indicating a shared binding site.

This present study provides new evidence that the modulatory effect of octyl- O-β-D -glucoside upon GABA receptor is dependent on the presence of the side chain, the nature of the glycosidic linkage and the side chain length. This work provides a clearer picture of the SAR of this novel class of GABA_A receptor modulator, which warrants further elucidation using electrophysiological and behavioural approaches (Lees, Chazot et al., 2000).

Abhamdah S. et al. (2004) Br. J. Pharmacol.(Suppl.) 2 (2), 22P
Korpi, E.R et al. (2002). Prog. Neurobiol, 67,113-159.
Lees G., Chazot, P.L. et al. (2000) Bioorg. Med. Chem. Letts. 10, 1759-1761.
Rezai, N. et al. (2003) Biochem. Pharmacol. 65, 619-623.

This work is funded, in part, by an Islamic Bank Development scholarship.