ANATOXIN-A IS A POTENT AGONIST OF THE NICOTINIC ACETYLCHOLINE-RECEPTOR OF BOVINE ADRENAL CHROMAFFIN CELLS

(+)-Anatoxin-a is a neurotoxic alkaloid produced by the cyanobacterium Anabaena flos-aquae. In this study synthetic (+/-)-anatoxin-a was tes ted on isolated bovine adrenal chromaffin cells to determine its abili ty to evoke secretion of endogenous catecholamines through neuronal-ty pe nicotinic receptor activation. Anatoxin-a was found to act as a pot ent agonist of the secretory response of chromaffin cells with an EC(5 0) of 1-2 mu M, compared with an EC(50) of 4-5 mu M for nicotine. The cells responded to anatoxin-a and nicotine with bell-shaped concentrat ion-response curves consistent with desensitisation at concentrations of anatoxin-a greater than 5 mu M and of nicotine greater than 20 mu M . The secretion of catecholamines stimulated by anatoxin-a was complet ely inhibited in a non-competitive manner by the nicotinic antagonist mecamylamine with an IC50 of 0.4-0.5 mu M. In the presence of depolari sing concentrations of K+ (15 or 50 mM), anatoxin-a increased the secr etion of catecholamines in a concentration-dependent manner up to the same maximum as that achieved by anatoxin-a alone. It is concluded tha t anatoxin-a acts as a potent and selective nicotinic agonist, capable of evoking secretion. of endogenous catecholamines from chromaffin ce lls via their neuronal-type nicotinic receptor.