The pharmacology of anatoxin-a(s), a neurotoxin produced by the freshwater cyanobacterium Anabaena flos-aquae NRC 525-17.

Anatoxin-a(s) [antx-a(s)] is produced by Anabaena flos-aquae clone NRC 525-17 and is different from anatoxin-a, a known depolarizing agent produced by A. flos-aquae NRC 44-1. Purification of antx-a(s) from lyophilized cells involved extraction with 1.0 M acetic acid: ethanol (80:20), column chromatography (Sephadex G-15 and CM-Sephadex C-25) and high performance liquid chromatography. Purified toxin has an LD50 (i.p., mouse) of approximately 50 micrograms/kg. Gross pharmacological tests of antx-a(s) on isolated chick biventer cervicis and frog rectus abdominis muscles showed no direct agonistic effect. Instead, antx-a(s) augments the acetylcholine response and antagonizes the actions of d-tubocurarine. Twitch potentiation and tetanic fade were observed on isolated rat phrenic nerve--diaphragm muscle when stimulated indirectly at different frequencies. In acute toxicity tests with mice and rats the signs of poisoning were indicative of excessive cholinergic stimulation. Mice pretreated with atropine sulfate showed longer survival times and no parasympathomimetic signs of toxicity. The mice still died of respiratory arrest with convulsions, which indicated that toxicity is due to more than just the peripheral muscarinic action of antx-a(s). Assays of serum cholinesterase of rats in acute toxicity tests showed complete inactivation of the enzyme at doses of 350 and 600 micrograms/kg. It was concluded that antx-a(s) may be acting as an anticholinesterase, thereby causing toxicity.