Effects of Gymnodinium breve toxin on the smooth muscle preparation of guinea-pig ileum.

1 The effects of Gymnodinium breve neurotoxin (GT) on smooth muscles were studied using the guinea-pig isolated ileum.2 The toxin caused strong spasmogenic effects at 1-4 mug/ml, characterized by prolonged tonic contraction with superimposed pronounced pendular movements. Tachyphylaxis was observed upon administration of successive doses.3 Atropine blocked the contractile response elicited by GT, whereas mepyramine and hexamethonium failed to do so. These findings tentatively suggested a cholinergic involvement at a post-ganglionic site of action.4 In the presence of tetrodotoxin the effects of GT were abolished, excluding direct action of the toxin on the smooth muscle.5 It is concluded that GT exerts its spasmogenic effects through stimulation of the post-ganglionic cholinergic nerve fibres.