Amitriptyline neurotoxicity: dose-related pathology after topical application to rat sciatic nerve.

BACKGROUND: Amitriptyline is a tricyclic antidepressant drug used systemically for the management of neuropathic pain. Antidepressants, as a class of drugs with direct neurologic actions, are becoming widely used for the management of chronic pain, although their mechanisms are not entirely understood. Amitriptyline exerts potent effects on reuptake of norepinephrine and serotonin and blocks alpha 2A adrenoreceptors and N-methyl-D-aspartate receptors. Because amitriptyline is also a particularly potent blocker of sodium channels and voltage-gated potassium and calcium channels, it has been recommended as a long-acting local anesthetic agent. Unfortunately, amitriptyline has significant toxic side effects in the central nervous system and cardiovascular system that are dose-related to its systemic administration. Therefore, before amitriptyline can be used clinically as a local anesthetic agent, it should be thoroughly explored with respect to its direct neurotoxic effect in the peripheral nervous system.
METHODS: The left sciatic nerve of Sprague-Dawley rats (12/ group) received a single topical amitriptyline dose of 0.625, 1.25, 2.5, or 5 mg; a saline group (n = 2) was used as control. Neuropathologic evaluations were conducted in separate animals (n = 4) 1, 3, and 7 days later.
RESULTS: Amitriptyline topically applied in vivo to rat sciatic nerve causes a dose-related neurotoxic effect. Drug doses of 0.625-5 mg all caused Wallerian degeneration of peripheral nerve fibers, with the number of affected fibers and the severity of the injury directly related to the dose.
CONCLUSION: Because the effective local anesthetic dose is within this dose range, the authors strongly recommend that amitriptyline not be used as a local anesthetic agent.