BLOCK OF CONDUCTION BY ACETYLCHOLINE AND d-TUBOCURARINE AFTER TREATMENT OF SQUID AXON WITH COTTONMOUTH MOCCASIN VENOM

¹ Departments of Neurology and Biochemistry, College of Physicians and Surgeons, Columbia University, New York, N. Y.

Attempts were made to reduce the barriers surrounding the squid giant axon by using snake venoms. The potency of these venoms and their ability to render d-tubocurarine (curare) active, were from strongest to weakest as follows: cottonmouth moccasin, hooded cobra, king cobra, Russell's viper and eastern diamond-back rattlesnake venom. After treatment with moccasin venom, 1.4 x 10^-4 M curare and 4.4 x 10^-3 M acetylcholine (ACh) blocked conduction. The combination of physostigmine plus ACh was less effective than ACh alone. Dimethylcurare was also rendered active by moccasin venom, while neither choline, sodium acetate nor neostigmine was active. None of these lipophobic compounds affected conduction in control axons.

Alkaline heated solutions of moccasin and hooded cobra venom were inert upon the squid axon and failed to render curare active while acid heated solutions were 25 to 50% as active as the unheated venoms. The phospholipase A activity of hooded cobra and moccasin venom with lecithin as substrate is about equal, while rattlesnake venom, although it had about half the phospholipase A activity, is at least 40 times weaker on the squid axon. Moccasin venom is more potent on relatively crudely dissected axons than on finely dissected axons. These results indicate that probably the phospholipase A of the venom has reacted with a phosphatide component of the membrane (probably not lecithin) yielding a lysophosphatide which is responsible directly or indirectly for reduction of barriers surrounding the axonal membrane or for alteration of the membrane itself.

Both cottonmouth moccasin and hooded cobra venom depolarize the squid axon. The ability of cottonmouth moccasin venom to render curare active, as measured with internal electrodes and finely dissected axons, was less than had been observed with external recordings of electrical activity on crudely dissected fibers.

These results provide additional support for the chemical theory of nerve activity developed by Nachmansohn.