ACTIONS OF PICRODENDRIN ANTAGONISTS ON DIELDRIN-SENSITIVE AND DIELDRIN-RESISTANT DROSOPHILA GABA RECEPTORS

1 A series of terpenoid compounds, recently isolated from Picrodendron baccatum, share a picrotoxane skeleton with picrotoxinin, an antagoni st of ionotropic GABA receptors. Referred to as picrodendrins, they in hibit the binding of [S-35]-tert-butylbicyclophosphorothionate (TBPS) to rat GABAA receptors. Hitherto, their effects on GABA receptors have not been investigated electrophysiologically. Under two-electrode vol tage-clamp, the actions of picrodendrins and related terpenoids have b een assayed on homooligomeric GABA receptors formed by the expression of a Drosophila GABA receptor subunit (RDL(ac)) in Xenopus oocytes. 2 All the terpenoids tested, dose-dependently antagonized currents induc ed by 30 mu M (EC(50)) GABA. 3 Tutin and its analogues (dihydrotutin a nd isohyenanchin) differ in the structure of their axial C4 substituen ts. Of these compounds, tutin, which bears an isopropenyl group at thi s carbon atom, was the most potent antagonist of RDL(ac) homo-oligomer s, whereas isohyenanchin, which bears a hydroxyisopropyl group, was th e least potent antagonist tested. 4 Picrodendrins differ mainly in the structure of their C9 substituents. The IC(50)s of picrodendrins rang ed from 17+/-1.3 nM (picrodendrin-Q) to 1006+/-1.3 nM (picrodendrin-O) . As such, the most potent picrodendrins (Q, A. and B) were approximat ely equipotent with picrotoxinin as antagonists of RDL(ac) homo-oligom ers. 5 Certain picrodendrin compounds effected a use-dependent blockad e of RDL(ac) homo-oligomers. Such a biphasic block was not observed wi th tutin analogues. 6 Picrotoxin-resistant RDL(ac)(A3025) homo-oligome rs, which have a single amino acid substitution (A302S) in the 2nd tra nsmembrane region, were markedly less sensitive to picrodendrin-O than the wild-type, dieldrin-sensitive, homo-oligomers. 7 The relative pot ency of tutin analogues demonstrates that the structure-activity relat ionship of the C4 substituent of picrotoxane-based compounds is conser ved in vertebrates and insects. However, the relative order of potency of picrodendrins on RDL(ac) homo-oligomers is distinctly different fr om that observed in previous radioligand binding studies performed on vertebrate GABA(A) receptors. As picrodendrin compounds differ in the structure of their C9 substituents, these data suggest that the optima l convulsant pharmacophores of vertebrate GABA(A) receptors and RDL(ac ) homo-oligomers differ with respect to this substituent.