Curare Alkaloids From Matis Dart Poison: Comparison With D-Tubocurarine In Interactions With Nicotinic, 5-Ht3 Serotonin And Gaba(A) Receptors

Several novel bisbenzylisoquinoline alkaloids (BBIQAs) have recently been isolated from a Matis tribe arrow poison and shown by two-electrode voltage-clamp to inhibit mouse muscle nicotinic acetylcholine receptors (nAChR). Here, using radioligand assay with Aplysia californica AChBP and radioiodinated a-bungarotoxin ([I-125]-alpha Bgt), we show that BBIQA1, BBIQA2, and d-tubocurarine (d-TC) have similar affinities to nAChR orthosteric site. However, a competition with [I-125]-alpha Bgt for binding to the Torpedo californica muscle-type nAChR revealed that BBIQAs1, 2, and 3 are less potent (IC(50)s = 26.3, 8.75, and 17.0 mu M) than d-TC (IC50 = 0.39 mu M), while with alpha 7 nAChR in GH(4)C(1) cells, BBIQA1 was less potent that d-TC (IC(50)s = 162 mu M and 7.77 mu M, respectively), but BBIQA2 was similar (IC50 = 5.52 mu M). In inhibiting the Ca2+ responses induced by acetylcholine in Neuro2a cells expressing the mouse adult alpha 1 beta 1 epsilon delta nAChR or human a7 nAChR, BBIQAs1 and 2 had similar potencies to d-TC (IC(50)s in the range 0.75-3.08 mu M). Our data suggest that BBIQA1 and BBIQA2 can inhibit adult muscle alpha 1 beta 1 epsilon delta nAChR by both competitive and noncompetitive mechanisms. Further experiments on neuronal alpha 3 beta 2, alpha 4 beta 2, and alpha 9 alpha 10 nAChRs, expressed in Xenopus laevis oocytes, showed that similar potencies for BBIQAs1, 2, and d-TC. With alpha 3 beta 2 gamma 2 GABA(A)R currents were almost completely inhibited by d-TC at a high (100 mu M) concentration, but BBIQAs1 and 2 were less potent (only 40-50% inhibition), whereas in competition with Alexa Fluor 546-alpha-cobratoxin for binding to alpha 1 beta 3 gamma 2 GABA(A)R in Neuro2a cells, d-TC and these analogs had comparable affinities. Especially interesting effects of BBIQAs1 and 2 in comparison with d-TC were observed for 5-HT3AR: BBIQA1 and BBIQA2 were 5- and 87-fold less potent than d-TC (IC50 = 22.63 nM). Thus, our results reveal that these BBIQAs differ from d-TC in their potencies towards certain Cys-loop receptors, and we suggest that understanding the reasons behind this might be useful for future drug design.