Assessing the permeability of the rat sciatic nerve epineural sheath against compounds with local anesthetic activity: an ex vivo electrophysiological study

Abstract

Studies have shown that the sciatic nerve epineural sheath acts as a barrier and has a delaying effect on the diffusion of local anesthetics into the nerve fibers and endoneurium. The purpose of this work is to assess and to quantify the permeability of the epineural sheath. For this purpose, we isolated the rat sciatic nerve in a three-chamber recording bath that allowed us to monitor the constant in amplitude evoked nerve compound action potential (nCAP) for over 24 h. For nerves exposed to the compounds under investigation, we estimated the IT₅₀ the time required to inhibit the nCAP to 50% of its initial value. For desheathed nerves, the half-vitality time was denoted as IT₅₀(−) and for the ensheath normal nerves as IT₅₀(+). There was no significant difference between the IT₅₀ of desheathed and ensheathed nerves exposed to normal saline. The IT₅₀(−) for nerves exposed to 40 mM lidocaine was 12.1 ± 0.95 s (n = 14) and the IT₅₀(+) was 341.4 ± 2.49 s (n = 6). The permeability (P) coefficient of the epineural sheath was defined as the ratio IT₅₀(+)/IT₅₀(−). The P coefficient for 40 mM lidocaine and linalool was 28.2 and 3.48, correspondingly, and for 30 mM 2-heptanone was 4.87. This is an indication that the epineural sheath provided a stronger barrier against lidocaine, compared to natural local anesthetics, linalool and 2-heptanone. The methodology presented here is a useful tool for studying epineural sheath permeability to compounds with local anesthetic properties.

• 2-heptanone
• epineural sheath
• lidocaine
• permeability
• sciatic nerve

Acknowledgements

The authors thank Dr. Carol Everhard, former Teaching Fellow in the School of English, Aristotle University of Thessaloniki, for proofreading the manuscript.