Novel plant substances acting as β subunit isoform-selective positive allosteric modulators of GABAA receptors

GABA(A) receptors are modulated by a large variety of compounds. A common chemical characteristic of most of these modulators is that they contain a cyclic entity. Three linear molecules of a polyacetylene structure were isolated from the East African medicinal plant Cussonia zimmermannii Harms and shown to allosterically stimulate GABA(A) receptors. Stimulation was not abolished by the absence of the gamma(2) subunit, the benzodiazepine antagonist Ro15-1788 (8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4] benzodiazepine-3-carboxylic acid ethyl ester), or the point mutation beta(2)N265S that abolishes effects by loreclezole. At a concentration of 30 mu M, the substances by themselves elicited only tiny currents. Maximal stimulation at alpha(1)beta(2)gamma(2) amounted to 110 to 450% for the three substances, and half-maximal stimulation was observed at concentrations of 1 to 2 mu M. Stimulation was subunit composition-dependent and was for the substance MS-1, alpha(1)beta(2)gamma(2) approximate to alpha(1)beta(2) approximate to alpha(3)beta(2)gamma(2) > alpha(2)beta(2)gamma(2) > alpha(5)beta(2)gamma(2) approximate to alpha(1)beta(3)gamma(2) approximate to alpha(6)beta(2)gamma(2) > alpha(1)beta(1)gamma(2), for MS-2 alpha(1)beta(2)gamma(2) approximate to alpha(3)beta(2)gamma(2) approximate to alpha(1)beta(2) > alpha(2)beta(2)gamma(2) approximate to alpha(6)beta(2)gamma(2) approximate to alpha(5)beta(2)gamma(2) > alpha(1)beta(1)gamma(2), and for MS-4, alpha(1)beta(2)gamma(2) approximate to alpha(1)beta(2) > alpha(5)beta(2)gamma(2) approximate to alpha(3)beta(2)gamma(2) approximate to alpha(2)beta(2)gamma(2) > alpha(6)beta(2)gamma(2) >> alpha(1)beta(1)gamma(2). Maximal stimulation by MS-1 was 450% at alpha(1)beta(2)gamma(2), 80% at alpha(1)beta(1)gamma(2), and 150% at alpha(1)beta(3)gamma(2). MS-1 was thus specific for receptors containing the beta(2) subunit. The reversal potential was unaffected by 10 mu M MS-1, whereas apparent picrotoxin affinity for current inhibition was increased approximately 3-fold. In summary, these positive allosteric modulators of GABA(A) receptors of plant origin have a novel unusual chemical structure and act at a site independent of that of benzodiazepines and loreclezole.