Radioligand binding studies of caloporoside and novel congeners with contrasting effects upon [35S] TBPS binding to the mammalian GABAA receptor

Caloporoside is a natural active fungal metabolite, which was isolated from Caloporous dichrous and was described to exhibit antibacterial, antifungal and phospholipase C inhibitory activity. We have previously reported evidence that related β-linked compounds, lactose and octyl-β-d-mannoside, bind and functionally modulate rodent GABAA receptors, respectively. We have characterized the binding pharmacology of synthetic caloporoside and two further congeners, 2-hydroxy-6-{[(16R)-(β-d-mannopyranosyloxy)heptadecyl]} benzoic acid and octyl-β-d-glucoside on GABAA receptors using a [35S]-t-butylbicyclophosphoorothionate (TBPS) radioligand binding assay. Caloporoside and 2-hydroxy-6-{[(16R)-(β-d-mannopyranosyloxy)heptadecyl]} benzoic acid produced concentration-dependent complete inhibition of specific [35S] TBPS binding with overall apparent IC50 values of 14.7 ± 0.1 and 14.2 ± 0.1 μM, respectively. In contrast, octyl-β-d-glucoside elicited a concentration-dependent stimulation of specific [35S] TBPS binding (Emax = 144 ± 4%; EC50 = 39.2 ± 22.7 nM). The level of stimulation was similar to that elicited by diazepam (Emax = 147 ± 6%; EC50 = 0.8 ± 0.1 nM), and was occluded by GABA (0.3 μM). However, the three test compounds failed to elicit any significant effect (positive or negative) upon [3H] flunitrazepam or [3H] muscimol binding, indicating that they did not bind directly, or allosterically couple, to the benzodiazepine or agonist binding site of the GABAA receptor, respectively. The constituent monosaccharide, glucose, and both the closely related congeners octyl-β-d-glucoside or hexyl-β-d-glucoside have no significant effect upon [35S] TBPS binding. These data, together, provide strong evidence that a β-glycosidic linkage and chain length are crucial for the positive modulation of [35S] TBPS binding to the GABAA receptor by this novel chemical class.