Positive cooperation of protoberberine type 2 alkaloids from Corydalis cava on the GABAA binding site

Protoberberine alkaloids from the rhizomes of Corydalis cava were investigated with regard to their influence on the GABA(A) receptor using radioreceptor assays. Whereas the protoberberine type 2 alkaloids, isoapocavidine, corydaline, tetrahydropalmatine, scoulerine and isocorypalmine, increased the specific [H-3]BMC-binding in a range of 21 - 49%, the protoberberine type I alkaloids, palmatine, coptisine, dehydroapocavidine, and dehydrocorydaline, had no influence on the binding behaviour of the GABAA receptor. To confirm the modulatory activity of the protoberberine type 2 alkaloids on living cells, GABAA receptor binding studies were performed by fluorescence correlation spectroscopy (FCS) using hippocampal neurons and the fluorescently labelled ligand, muscimol-Alexa (Mu-Alexa). The incubation of hippocampal neurons with 7.5 nM Mu-Alexa showed a specific binding of 5.25 nM (70%). The evaluation of the autocorrelation curve revealed two different mobilities of receptor ligand complexes, D-bound1 = (2.8 +/- 0.91) mum(2)/s for the free lateral mobility and D-bound2 = (0.14 +/- 0.05) mum(2)/s for the hindered mobility. An incubation of hippocampal neurons with 7.5 nM Mu-Alexa and 7.5 nM scoulerine showed a maximal increase of the specific Mu-Alexa binding of approximately 27% by selectively modulating the amount of receptor-ligand complexes with a hindered mobility (9% to 27%).