Effect of i.v. administration of lignocaine and tetrodotoxin on sensory units in the tooth of the cat.

A study in cats was designed to investigate the blocking of sensory nerve terminals by i.v. injection of lipid-soluble local anaesthetics such as lignocaine. I.v. injection of lignocaine 15--30 mg kg-1 did not completely block sensory nerve endings in the tooth of the cat, whilst tetrodotoxin (TTX) in large non-toxic doses did so. Based on the i.v. doses of TTX and the relationship between the local anaesthetic potency of the two substances when injected intra-arterially, it was calculated that lethal doses of i.v. administered lignocaine are necessary to block sensory nerve endings. It seems unlikely that peripheral pain pathways can be blocked by systemic i.v. injections of local anaesthetics.