The Differential Effects of Two Sodium Channel Modulators on the Conductive Properties of C-Fibers in Pig Skin In Vivo

BACKGROUND: Axonal sodium channels are attractive targets for chronic pain treatment, and recent evidence suggests that specific targeting of the slow inactivation of sodium channels (NaV) might exert analgesic effects. Using a human-like animal model, the pig, we compared changes in the conductive properties of different C-fiber classes on acute administration of lidocaine (nonselective NaV blocker) and lacosamide (selective enhancer of NaV slow inactivation). METHODS: Single-fiber extracellular recordings from saphenous nerves were performed. We classified C-fibers according to mechanical responsiveness and amount of activity-dependent slowing (ADS) of conduction velocity. Lidocaine (4 mM; 100 mu L), lacosamide (4 mM; 100 mu L), or saline was injected intradermally at the stimulation site, and changes of fibers' conductive properties were assessed. RESULTS: Conduction latencies evoked by lidocaine were more prominent in mechano-sensitive (5.5% +/- 2.1%) than in mechano-insensitive nociceptors (2.5% +/- 1%), whereas lacosamide increased conduction latencies to a greater extent in the mechano-insensitive (3% +/- 1%) than in mechanosensitive C-nociceptors (2% +/- 0.9%). Lidocaine, but not lacosamide, increased electrical thresholds in all mechanosensitive, but not in the mechano-insensitive, C-fibers. Lacosamide blocked conduction and, in addition, reduced ADS in mechano-insensitive nociceptors significantly more than in mechanosensitive nociceptors (Delta ADS: 2.4% +/- 0.5% vs 1.6% +/- 0.5%), whereas lidocaine had opposite effects. Saline had no significant effect on the conductive properties of C-fibers. CONCLUSION: Local application of test compounds in pig skin allows for functional assessment of steady-state and use-dependent modulation of sodium channels in nociceptive and nonnociceptive C-fibers. Increased analgesic specificity might derive from selective enhancement of slow inactivation of sodium channels. (Anesth Analg 2012;115:560-71)