Abstract

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 γ₂ 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 β₂N265S that abolishes effects by loreclezole. At a concentration of 30 μM, the substances by themselves elicited only tiny currents. Maximal stimulation at α₁β₂γ₂ amounted to 110 to 450% for the three substances, and half-maximal stimulation was observed at concentrations of 1 to 2 μM. Stimulation was subunit composition-dependent and was for the substance MS-1, α₁β₂γ₂ ≈ α₁β₂ ≈ α₃β₂γ₂ > α₂β₂γ₂ > α₅β₂γ₂ ≈ α₁β₃γ₂ ≈ α₆β₂γ₂ > α₁β₁γ₂, for MS-2 α₁β₂γ₂ ≈ α₃β₂γ₂ ≈ α₁β₂ > α₂β₂γ₂ ≈ α₆β₂γ₂ ≈ α₅β₂γ₂ > α₁β₁γ₂, and for MS-4, α₁β₂γ₂ ≈ α₁β₂ ≈ α₅β₂γ₂ ≈ α₃β₂γ₂ ≈ α₂β₂γ₂ > α₆β₂γ₂ ≫ α₁β₁γ₂. Maximal stimulation by MS-1 was 450% at α₁β₂γ₂, 80% at α₁β₁γ₂, and 150% at α₁β₃γ₂. MS-1 was thus specific for receptors containing the β₂ subunit. The reversal potential was unaffected by 10 μM 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.