Analysis of the action of gymnodimine-A and 13-desmethyl spirolide C on the mouse neuromuscular system in vivo.

Gymnodimine-A and 13-desmethyl spirolide C are marine toxins belonging to the cyclic imine group produced by Karenia selliformis and Alexandrium ostenfeldii/peruvianum dinoflagellates, respectively. The aim of this work was to analyze the pharmacological properties of both toxins (at sub-lethal doses) on neuromuscular excitability, when injected locally to isoflurane-anesthetized mice, using a multimodal minimally-invasive in vivo electrophysiological approach. The main effect of both toxins was a marked reversible time- and dose-dependent decrease of the compound muscle action potential recorded from the tail muscle in response to caudal motor nerve stimulation. The dose-response curves of gymnodimine-A and 13-desmethyl spirolide C effect on the maximal amplitude of compound muscle action potential revealed 50% inhibitory doses of 51 ng/mouse (i.e. 1.6 μg/kg or 3.3 nmol/kg mouse) and 0.18 ng/mouse (i.e. 6 ng/kg or 0.01 nmol/kg mouse), respectively. The blocking effect occurred without significant modification of motor nerve excitability parameters. It is concluded that the inhibition of the mouse compound muscle action potential, induced by gymnodimine-A and 13-desmethyl spirolide C, results from an action of these toxins at the level of the skeletal neuromuscular junction, since both cyclic imine toxins are known to interact and block muscle-type nicotinic acetylcholine receptors. In the present in vivo study, 13-desmethyl spirolide C was about 300 fold more active than gymnodimine-A on equimolar basis. The present in vivo approach, associated to recent progress in chemical synthesis of cyclic imine toxins, paves the way for more detailed structure-activity studies to obtain new and more potent synthetic analogs.