Differentiation of the actions of tetrodotoxin and saxitoxin

Abstract

While tetrodotoxin and saxitoxin are very similar in their actions, they can be differentiated biologically by the observation that newts of the genus Taricha are resistant to tetrodotoxin but not to saxitoxin. The resistance of these newts is due to a remarkable insensitivity of the somatic motor nerves to tetrodotoxin. The isolated desheathed sciatic nerve of Taricha torosa is at least 30,000 times more resistant than frog nerve. As seen in electronmicroscopic sections, there is no special enveloping structure around taricha nerves that can be considered as a particularly effective diffusion barrier. The isolated, desheathed brachial nerve of the Atlantic puffer fish, Spheroides maculatus, is about 1000 times more resistant to tetrodotoxin than frog nerve. Both taricha and tetrodon nerves are readily blocked by saxitoxin. Both nerves require external sodium ions for activity since replacement of Na + with choline, tris (hydroxymethyl) aminomethane or dimethyl-diethanol ammonium ion reduced the spike amplitudes. The resistance of these nerves to tetrodotoxin is interpreted as being a result of peculiarities in membrane structures that prevent an effective association with tetrodotoxin.