An octopus toxin, maculotoxin, selectively blocks sodium current in squid axons.

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Abstract
A low molecular weight, stable, cationic neurotoxin (maculotoxin, MTX) extracted from the posterior salivary glands of the octopus Hapalochlaena maculosa, blocked Na current in voltage-clamped squid axons without affecting K current. The effectiveness of MTX was increased by repetitive, brief, depolarizing pulses but not by a single prolonged depolarization. The potency of MTX decreased at pH 8-9. Effectiveness could be restored by lowering the pH to 7.1 again; MTX is active in its cationic form. MTX affected Na conductance kinetics, slowing the turn-on of Na current. This effect was most noticeable with small depolarizations but became progressively less with larger depolarizations. Neither the turn-off of Na current nor Na inactivation kinetics were affected by the toxin. MTX inhibited Na current without inhibiting Na gating current. The effectiveness of MTX was not detectably changed when Ca concentration was varied from 50-10 mM, or Na concentration from 225-750 mM.