Trypsin inhibits the action of tetrodotoxin on neurones

Abstract

EVIDENCE suggests that the molecules responsible for membrane excitability are proteins^1–4, but they have not been isolated in spite of considerable efforts and their identities remain unknown. Some clues have come from study of the effects of enzymes and group-specific chemicals on appropriate electrophysiological properties of the membrane^1–4. In certain nerve and muscle cells the voltage-dependent inward currents which generate action potentials may be carried by Na⁺ or Ca²⁺ (refs 5–13). Tetrodotoxin (TTX) specifically blocks the voltage-dependent inward Na⁺ current in many of these cells¹⁴, including neurones of Aplysia¹⁰ and Helix¹¹ and is widely used for this reason. It also has the advantage of being a relatively simple molecule of known atomic structure¹⁴. But TTX has no effect on Helix neurones when they are isolated from the ganglia in which they are located¹³. The ganglia are first pretreated with trypsin and we wondered whether the lack of TTX-sensitivity in these preparations was due to some action of this enzyme on the TTX receptor. We have now found that exogenous trypsin modifies the TTX-binding properties of the Na channel without affecting its reversal potential or selectivity against Tris⁺ and has no measurable effect on the Ca²⁺ channel. Because trypsin splits polypeptide chains on the carboxyl side of arginine or lysine residues¹⁵, it seems likely that the TTX receptor contains either or both of these amino acids but that the selectivity site does not.