Alkyl-modified side chain variants of anatoxin-a: A series of potent nicotinic agonists

The toxicity of cyanobacterial toxins in the mouse: II anatoxin-a

Blooms of cyanobacteria are known to have caused poisoning in fish, waterfowl, animals and man. One of the low molecular weight toxins responsible for this is the neurotoxin anatoxin-a which has been detected in reservoirs used for domestic water supplies. While the acute behaviour of this alkaloid is clear, there is uncertainty regarding the effects on man of ingestion of anatoxin-a at low levels over longer periods. In order to assess this risk, a series of in vitro and in vivo experiments were undertaken to investigate the pharmacology, subacute toxicity, and the teratogenicity of anatoxin-a in the mouse. The results of this work were as follows: (1) Pharmacological screening studies confirmed that anatoxin-a is a potent nicotinic agonist which can produce neuromuscular blockade and death by respiratory arrest. Recovery from a single sub-lethal dose is rapid and complete; (2) Repeated sub-lethal oral administration over 28 days in the mouse did not produce any reliable evidence of treatment-related toxicity; (3) From a preliminary screening study anatoxin-a does not appear to be a developmental toxicant in the mouse. These results indicate that a guideline value for anatoxin-a in drinking water of 1 microg l(-1) would provide an adequate margin of safety.

Anatoxin-a-evoked [3H]dopamine release from rat striatal synaptosomes

Presynaptic nicotinic acetylcholine receptors on striatal nerve terminals modulate the release of dopamine. Using rat striatal synaptosomes loaded with [3H]dopamine, we have characterized the action of the selective nicotinic agonist, (+/-)anatoxin-a, with respect to [3H]dopamine release, in order to explore the mechanisms coupling nicotinic receptor activation to exocytosis. Anatoxin-a evoked [3H]dopamine release in a concentration-dependent and mecamylamine-sensitive manner, EC50 = 0.11 microM. The maximum [3H]dopamine release elicited by anatoxin-a was only 20% of the maximum elicited by KCl depolarization; there was no additivity between anatoxin-a and sub-maximal concentrations of KCl. Both agents stimulated Ca(2+)-dependent release that was equally sensitive to inhibition by 200 microM Cd2+. This result suggests that anatoxin-a-stimulated exocytosis is mediated by Ca2+ influx via voltage-sensitive Ca2+ channels, with little contribution from Ca2+ entering directly through the nicotinic receptor channel. This view is supported by the abolition of anatoxin-a-evoked [3H]dopamine release in Na(+)-depleted medium. A partial (40%) inhibition by tetrodotoxin was observed. These data suggest that activation of presynaptic nicotinic acetylcholine receptors by anatoxin-a results in an influx of Na+, producing sufficient local depolarization to open voltage-sensitive Ca2+ and Na+ channels. The latter may then amplify the response, activating further Ca2+ channels. The particular voltage-sensitive Ca2+ channels involved remain to be determined.

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 tested on isolated bovine adrenal chromaffin cells to determine its ability to evoke secretion of endogenous catecholamines through neuronal-type nicotinic receptor activation. Anatoxin-a was found to act as a potent agonist of the secretory response of chromaffin cells with an EC50 of 1-2 microM, compared with an EC50 of 4-5 microM for nicotine. The cells responded to anatoxin-a and nicotine with bell-shaped concentration-response curves consistent with desensitisation at concentrations of anatoxin-a greater than 5 microM and of nicotine greater than 20 microM. The secretion of catecholamines stimulated by anatoxin-a was completely inhibited in a non-competitive manner by the nicotinic antagonist mecamylamine with an IC50 of 0.4-0.5 microM. In the presence of depolarising concentrations of K+ (15 or 50 mM), anatoxin-a increased the secretion of catecholamines in a concentration-dependent manner up to the same maximum as that achieved by anatoxin-a alone. It is concluded that anatoxin-a acts as a potent and selective nicotinic agonist, capable of evoking secretion of endogenous catecholamines from chromaffin cells via their neuronal-type nicotinic receptor.