Solving the 'Brown snake paradox': in vitro characterisation of Australasian snake presynaptic neurotoxin activity

Pseudonaja textilis (Eastern Brown snake) and Oxyuranus scutellatus scutellatus (Coastal taipan) are clinically important Australian elapid snakes, whose potent venoms contain the presynaptic (β) neurotoxins, textilotoxin and taipoxin, respectively, and a number of postsynaptic neurotoxins. However, while taipan envenoming frequently results in neurotoxicity, Brown snake envenoming causes an isolated coagulopathy and neurotoxicity is rare. This phenomenon is called the 'Brown snake paradox'. This study compared the pharmacology of both venoms and their respective presynaptic neurotoxins to investigate this phenomenon. From size-exclusion high performance liquid chromatography (HPLC) analysis textilotoxin represents a significantly smaller proportion (5.7%) of P. textilis venom compared to taipoxin in O. s. scutellatus venom (20.4%). In the chick biventer cervicis nerve-muscle (CBCNM) preparation both venoms caused concentration-dependent neurotoxicity, with P. textilis venom being significantly more potent than O. s. scutellatus venom. Conversely, taipoxin was significantly more potent than textilotoxin when compared at the same concentration. Textilotoxin only partially contributed to the overall neurotoxicity of P. textilis venom, while taipoxin accounted for the majority of the neurotoxicity of O. s. scutellatus venom in the CBCNM preparation. Compared with taipoxin, textilotoxin is less potent and constitutes a smaller proportion of the venom. This is likely to be the reason for the absence of neurotoxicity in envenomed humans thus explaining the 'Brown snake paradox'.