Hellyer_2015_J.Neurochem_135_479

Pinnatoxins are members of the cyclic imine group of marine phycotoxins that are highly toxic in in vivo rodent bioassays, causing rapid death due to respiratory depression. Recent studies have shown that pinnatoxins E, F and G, found in New Zealand and Australian shellfish, act as antagonists at muscle-type nicotinic acetylcholine receptors (nAChRs) at the neuromuscular junction. In the present study, binding affinities and modes of these pinnatoxin isomers at neuronal and muscle nAChRs were assessed using radioligand binding, electrophysiological and molecular modelling techniques. Radioligand-binding studies revealed that all three pinnatoxins bound with high affinity to muscle-type nAChRs, as well as to the alpha7 and alpha4beta2 neuronal receptors, with an order of affinity of muscle type > alpha7 > alpha4beta2. The rank order of potency at all receptors was pinnatoxin F > G > E. Pinnatoxins F and G also antagonized ACh-evoked responses in alpha7 and alpha4beta2 neuronal receptors expressed in Xenopus oocytes. Molecular modelling revealed that pinnatoxins E, F and G make multiple hydrogen bond interactions with the binding site of muscle-type and alpha7 receptors, with few interactions at the alpha4beta2 binding site, reflecting the binding affinity and functional data. This study shows for the first time that pinnatoxins E, F and G bind to, and functionally antagonize neuronal nAChRs, with interactions potentially playing a role in pinnatoxin toxicity. We used a three-pronged approach of radioligand binding, electrophysiological recording and molecular modelling to determine the nicotinic antagonist properties of the marine algal pinnatoxins E, F and G. These high-affinity toxins bind to and antagonize muscle nicotinic receptors, as well as homomeric and heteromeric neuronal nicotinic receptors, and show a preference for homomeric neuronal and muscle-type receptors over heteromeric neuronal receptors. Modelling of pinnatoxin/receptor interactions revealed potential determinants for binding to each of the receptor subtypes. This high-affinity nicotinic binding presumably underlies the neuromuscular and CNS symptoms associated with pinnatoxin intoxication.