Ionic permeability of the opossum sciatic nerve perineurium, examined using electrophysiological and electron microscopic techniques

Abstract A parallel electrophysiological and electron microscopic study was used to assess the ionic permeability of the sciatic nerve perineurium of the opossum Monodelphis domestica . The electrophysiological method was used to monitor permeability to K + , followed by combined electron microscopy and X-ray probe analysis to monitor permeability to the electron-dense tracer lanthanum. Isolated but intact nerves were mounted in a ‘grease gap’ chamber for extracellular measurement of DC potential and compound action potential (CAP). Challenge with 100 mM [K + ] Ringer was used to assess the K + permeability of the perineurium, since a change in DC potential (ΔDC) under these conditions reflected changes in the axonal resting membrane potential. There was no detectable change in DC potential or CAP to the first K + challenge ( n =71 nerves) indicating negligible K + permeability under control conditions. The inflammatory mediators histamine 0.1–40 mg/ml (1.3–130 mM), bradykinin (0.1–4.7 mM) and 5HT (serotonin) 0.1–5.0 mg/ml (0.5–23.5 mM) caused no measurable ΔDC on subsequent challenge with 100 mM [K + ] Ringer, indicating no effect on perineurial K + permeability. In nerves exposed to the bile salt sodium deoxycholate (DOC, 6 min, 4 mM), challenge with elevated K + Ringer caused a dose-dependent ΔDC in the range 10–100 mM [K + ] (1.67±0.17 mV in 100 mM [K + ], n =20), indicating increased perineurial permeability caused by DOC, but the response was smaller than that previously reported for the frog perineurium. Lanthanum was observed in the outer layers of the perineurium, but was not seen to penetrate the endoneurium in any of the nerves examined ( n =51), even after DOC application. This study shows that the combined electrophysiological and electron microscopic technique for monitoring ionic permeability can be applied to mammalian nerve, and suggests that the opossum perineurium is more resistant to tight junction opening by chemical modulators than is the frog perineurium.