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Abstract

The isolated sciatic nerve of the frog Rana temporaria was used for a parallel electrophysiological and electron microscopic examination of the ionic permeability of the perineurium, one component of the blood-nerve barrier. Nerves mounted in a grease-gap chamber for electrophysiological recording showed negligible changes in DC potential (Delta DC) or compound action potential on challenge with 100 mM K(+) Ringer, evidence that the perineurium was tight to K(+). In preparations then fixed and exposed to 5 mM lanthanum in the fixative, and examined in the electron microscope, electron-dense lanthanum deposits were seen between perineurial lamellae, but lanthanum was not detectable within the endoneurium, confirming that the perineurium was also tight to lanthanum. Absence of lanthanum penetration was confirmed by X-ray analysis of electron microscopic sections. In nerves exposed to 2 mM sodium deoxycholate (DOC) in the recording chamber, then challenged with high [K(+)], a moderate increase in perineurial K(+) permeability (P(K)) was observed, but lanthanum was still excluded. Exposure of nerves to 4 mM DOC caused a greater increase in perineurial potassium permeability, and the two nerves with the greatest permeability (P(K) > 1 x 10(-5) cm x sec(-1)) also showed detectable lanthanum within the endoneurium. The results indicate that DOC causes a dose-dependent increase in tight junctional permeability in the perineurium, and that the electrophysiological monitoring of K(+) penetration is a more sensitive measure of small ion permeability than electron microscopical analysis using lanthanum as tracer. Vesicular profiles observed in perineurial lamellae did not form open channels for ion flux across the perineurium in control nerves, or in those exposed to DOC. In preparations where lanthanum reached the endoneurium, lanthanum was observed in dense deposits in the extracellular spaces around nodes of Ranvier, and in the outer mesaxon cleft, but did not penetrate the internodal periaxonal space, the myelin intraperiod line, or the Schmidt-Lanterman incisures, in contrast to observations in mammalian nerves. The apparent differences in accessibility of the internodal periaxonal space in frog and mammalian axons are discussed in relation to axonal physiology. The study illustrates the value of parallel electrophysiological and electron microscopic examination in elucidating the properties of extracellular ionic pathways and their role in neural function.

Abnormal junctions and permeability of myelin in PMP22‐deficient nerves

“…Similar approaches have been used to differentiate between intact peripheral nerves and nerves whose junctions are disrupted. 12–14 In these studies, pharmacological disruption of peripheral nerve junctions allows ions to penetrate epineurium and reach the periaxonal space to affect axonal potentials. In our studies, due to the tight seal, K + ions did not affect the wild-type nerve CMAP amplitude.…”

“…K + ions were able to reduce action potential propagation after being externally applied to Pmp22 +/− nerves (see Fig 2). Epi-/perineurial junctions, as the initial barrier before K + accesses the periaxonal spaces, 12, 13 would have to become excessively permeable to the ions in Pmp22 +/− epi-/perineurium.…”

Group IV nociceptors develop axonal chemical sensitivity during neuritis and following treatment of the sciatic nerve with vinblastine

“…We attributed this to the diffusion barrier and positive intrafascicular pressure that the perineurium presents (Peltonen et al 2013). We attempted to disrupt this barrier using sodium deoxycholate (Todd et al 2000a(Todd et al , 2000b, but this did not reduce the response latency. Because these recordings are often time-limited, we chose to inject substances subperineurally using a 30-gauge bent needle.…”