Vol. 291, Issue 2, 538-546, November 1999

Effects of Delphinium Alkaloids on Neuromuscular Transmission¹

Peter Dobelis² , James E. Madl, James A. Pfister, Gary D. Manners and John P. Walrond

Department of Anatomy and Neurobiology, Colorado State University, Fort Collins, Colorado (P.D., J.E.M., J.P.W.); Poisonous Plants Research Laboratory, U.S. Department of Agriculture-Agricultural Research Service, Logan, Utah (J.A.P.); and Western Regional Research Center, U.S. Department of Agriculture-Agricultural Research Service, Albany, California (G.D.M.)

The Delphinium alkaloids methyllycaconitine (MLA), nudicauline, 14-deacetylnudicauline (14-DN), barbinine, and deltaline were investigated for their effects on neuromuscular transmission in lizards. The substituent at C14 provides the only structural difference among the alkaloids MLA, nudicauline, 14-DN, and barbinine. Deltaline lacks the N-(methylsuccinyl)anthranilic acid at C18 common to the other four alkaloids. Each alkaloid reversibly reduced extracellularly recorded compound muscle action potential (CMAP) amplitudes in a concentration-dependent manner. The IC₅₀ values for CMAP blockade were between 0.32 and 13.2 µM for the N-(methylsuccinimido)anthranoyllycacotonine-type alkaloids and varied with the C14 moiety; the IC₅₀ value for deltaline was 156 µM. The slopes of the concentration-response curves for CMAP blockade were similar for each alkaloid except barbinine, whose shallower curve suggested alternative or additional mechanisms of action. Each alkaloid reversibly reduced intracellularly recorded spontaneous, miniature end-plate potential (MEPP) amplitudes. Alkaloid concentrations producing similar reductions in MEPP amplitude were 0.05 µM for 14-DN, 0.10 µM for MLA, 0.50 µM for barbinine, and 20 µM for deltaline. Only barbinine altered the time constant for MEPP decay, further suggesting additional or alternative effects for this alkaloid. MLA and 14-DN blocked muscle contractions induced by exogenously added acetylcholine. All five alkaloids are likely nicotinic receptor antagonists that reduce synaptic efficacy and block neuromuscular transmission. The substituent at C14 determines the potency and possibly the mechanism of nicotinic acetylcholine receptor blockade for MLA, nudicauline, 14-DN, and barbinine at neuromuscular synapses. The lower potency of deltaline indicates that the N-(methylsuccinyl)anthranilic acid at C18 affects alkaloid interactions with nicotinic acetylcholine receptors at neuromuscular junctions.