The permeability of perineurium to peroxidase after early undernutrition. An ultrastructural study on rat sciatic nerve

Glucose transporters in rat peripheral nerve: paranodal expression of GLUT1 and GLUT3

Peripheral nerve depends on glucose oxidation to energize the repolarization of excitable axonal membranes following impulse conduction, hence requiring high-energy demands by the axon at the node of Ranvier. To enter the axon at this site, glucose must be transported from the endoneurial space across Schwann cell plasma membranes and the axolemma. Such transport is likely to be mediated by facilitative glucose transporters. Although immunohistochemical studies of peripheral nerves have detected high levels of the transporter GLUT1 in endoneurial capillaries and perineurium, localization of glucose transporters to Schwann cells or peripheral axons in vivo has not been documented. In this study, we demonstrate that the GLUT1 transporter is expressed in the plasma membrane and cytoplasm of myelinating Schwann cells around the nodes of Ranvier and in the Schmidt-Lanterman incisures, making them potential sites of transcellular glucose transport. No GLUT1 was detected in axonal membranes. GLUT3 mRNA was expressed only at low levels, but GLUT3 polypeptide was barely detected by immunocytochemistry or immunoblotting in peripheral nerve from young adult rats. However, in 13-month-old rats, GLUT3 polypeptide was present in myelinated fibers, endoneurial capillaries, and perineurium. In myelinated fibers, GLUT3 appeared to be preferentially expressed in the paranodal regions of Schwann cells and nodal axons, but was also present in the internodal aspects of these structures. The results of the present study suggest that both Schwann cell GLUT1 and axonal and Schwann cell GLUT3 are involved in the transport of glucose into the metabolically active regions of peripheral axons.

A freeze-fracture study of the perineurium in normal and protein-deprived rats

Observations have been made using the freeze-fracture replication technique on the perineurium of normal and protein-deprived rats in which its permeability barrier function is known to be deficient. The perineurial cells of young normal rats possessed belt-like tight junctions (zonulae occludentes) at the borders and maculae occludentes at sites remote from their borders. In older rats, the zonulae occludentes were more prominent and the maculae occludentes relatively less frequent. No abnormalities were detected in the tight junctions of young rats with early induction of protein deficiency but this may have been related to sampling problems. In older severely protein-deficient animals, although many of the tight junctions were normal, some were abnormal and contained focal regions of dispersed strands. The density of caveolae in the surface membrane of the perineurial cells of older rats with severe protein deficiency was significantly greater than in the control animals. This provides support for the view that the pinocytotic-like vesicles of perineurial cells are involved in transport of substances across the cells. The increased numbers of caveolae in the protein deficient rats may reflect increased transcellular traffic. There were considerable differences in the density of P-face IMPs between the different perineurial lamellae, but the results did not allow a decision to be made as to whether there was a polarization of the cells between their endoneurial and epineurial aspects. No differences were detected in the density of P-face IMPs between the young control and protein-deprived rats. In the perineurium of the older rats with protein deficiency, IMP density was significantly greater in the E face than in the controls but not different in the P face. The delay in the development of enzymatic activity in the perineurium of protein-deficient rats that has been demonstrated histochemically is therefore not paralleled by a reduction in IMPs.

Perineurial permeability and endoneurial edema during Wallerian degeneration of the frog peripheral nerve

Perineurial permeabilities to [3H]sucrose and [14C]dextran (MW = 70,000), and water content, conduction velocity (CV) and maximum amplitude (MAP) of the compound action potential, were determined in Wallerian degenerated nerves (sciatic or tibial) of the frog and compared with values in the contralateral uncut nerves. Three days after transection of the lumbosacral plexuses, about 2 cm proximal to the sciatic nerve, mean water content of the sciatic nerve was significantly higher than in the contralateral uncut nerve. After 10 days, the degenerating sciatic nerve showed significant increases in the mean perineurial permeabilities to [3H]sucrose and [14C]dextran when compared to values in the contralateral nerve. Means MAP's and CV's were significantly decreased. At 21 days and after, no compound action potential was detected and perineurial permeability and nerve water content had increased further. Decreases in mean MAP's and CV's and permeability increases of the perineurium were less in degenerating tibial nerves than in degenerating sciatic nerves. It is concluded that following transection, (1) Wallerian degeneration produces an irreversible increase in perineurial permeability, (2) the increase of perineurial permeability follows a proximodistal gradient, and (3) the frog peripheral nerve develops endoneurial edema during Wallerian degeneration as do degenerated nerves of mammals.

A quantitative morphological study of the optic nerve of pre-weanling rats

A quantitative histological estimation of the optic nerve of pre-weanling rats has revealed that the total fiber number decreases rapidly between 7 and 25 days of age.

Histopathological changes following removal of the perineurium

Using microsurgical techniques, the perineurial sheath was stripped off the sciatic nerves of rats over a 0.5 cm length at a point where the nerve consists of a single fascicle. The nerves were excised 0 to 84 days after the injury, and were examined in semi-thin transverse section. A new sheath, closely resembling normal perineurium, became organized during the first 10 days; it appeared uniformly over the length of the injured segment. The new perineurial sheath was probably formed by endoneurial fibroblasts migrating from within the fascicle. In undamaged specimens, the asons immediately beneath the excised perineurium underwent no degenerative changes.

Ionic permeabilities of the frog perineurium

Ionic permeation was investigated across the perineurium of the frog sciatic nerve, under normal conditions and following treatment by hypertonic Ringer, ouabain or amiloride. A cylindrical segment of perineurium removed from the nerve and mounted in vitro on two cannulae was continuously perfused. Permeation rates of 22Na and 42K across the perineurium were the same in either direction and were unaffected by the drugs. The mean 22Na permeability coefficient at the perineurium equaled 1.68 +/- 0.08 (S.E.M.) X 10(-6) cm/sec. Simultaneous measurement of transperineurial fluxes of 22Na, 42K and 36Cl indicated that the K/Na permeability ratio exceeded the ratio of limiting conductances of these ions in free solution, but that the Cl/K permeability ratio did not differ from the respective limiting conductance ratio. Immersion of the perineurial cylinder in Ringer, made hypertonic by addition of NaCl, increased the absolute permeability coefficients of the three ionic tracers but did not affect their permeability ratios. The flux ratio of 22Na/[14C]sucrose, however, was decreased by hypertonic treatment. It is concluded that there is no evidence of active Na or K transport across the perineurium and that the paracellular path in the perineurium exhibits size-dependent permselectivity properties. In addition, the low rates of transperineurial permeation of ions and water-soluble non-electrolytes (e.g. sucrose) are comparable to those in epithelia with tight junctions. These permeability coefficients provide quantitative estimates of the diffusion barrier properties of the perineurium.

Cerebrovascular integrity in protein-deprived rats

Protein-deprivation does not increase the permeability of the blood-brain barrier in rats aged 3.5--12 months. PA, the product of cerebrovascular permeability (P) to 14C-sucrose and of cerebral capillary surface area (A), is very low in mature rats that have been maintained either on an 8% or 25% casein diet, and equals about 8 X 10(-6) sec-1 in both groups. There is a tendency for the calculated distribution volume of 14C-sucrose within the brain to decline in protein-deprived rats. Conscious, partially immobilized, protein-deprived rats have the same mean blood pressure, heart rate, arterial plasma pH and adreno-sympathetic response to stress (as measured by plasma norepinephrine and epinephrine concentrations) as do normally fed animals.

Modification of permeability of frog perineurium to [14C]-sucrose by stretch and hypertonicity

An in vitro method has been developed to determine quantitatively the permeability of the perineurium to radiotracers at room temperature. The permeability to [14C]sucrose of the isolated perineurium of the sciatic nerve of the frog, Rana pipiens, was measured at rest length, when the perineurium was stretched and after the perineurium had been subjected to hypertonic treatment. Mean permeability at rest length was calculated to be 5.6 +/- 0.27 (S.E.M., n = 45) X 10(-7) cm/sec, and both stretch and hypertonic treatment increased the permeability. A 10% stretch increased permeability reversibly, whereas a 20% stretch or immersion of the perineurium in a hypertonic bath increased permeability irreversibly. Altered permeability under these conditions might be related to changes in the ultrastruct of tight junctions in the perineurium.

Barriers and transport properties of the perineurium. An ultrastructural study with 125I-labeled albuminin and horseradish peroxidase in normal and protein-deprived rats

The permeability properties of the perineurium in sciatic nerves of 12-week-old rats were studied. The penetration of 125I-labeled albumin and horseradish peroxidase into the perineurium was investigated electronmicroscopically 10, 30, and 120 min after the local extraneural application of the tracers. The autoradiographic study included age-matched protein-deprived rats. It was concluded that the perineurium acted as a diffusion barrier but also permitted a slow passage of the macromolecules into the endoneurium. The result indicates that this penetration to some extent is due to vesicular transport across the perineurial cells. The significance of these barrier and transport properties of the perineurium is discussed. No obvious differences in perineurial permeability between normal and protein-deprived rats were obtained.

Effects of local extraneural application of diphtheria toxin on the sciatic nerves of normal and protein deprived rats

Diphtheria toxin was locally administered around the sciatic nerves of normal and protein deprived rats aged 3, 6, 12, and 26 weeks in order to investigate the permeability of the barriers enveloping the nerves. At all ages the rats developed a reversible hind limb paralysis linked to a severe segmental demyelination, indicating passage of the toxin into the endoneurium. From 6 weeks of age the pattern of reaction differed between the protein deprived and control rats. The differences are interpreted as being partly dependent on less efficient protective barriers of the sciatic nerves of the protein deprived rats.

The perineurial and blood-nerve barriers in experimental diabetes

The permeabilities of the blood-nerve barrier and the perineurial barrier were investigated in alloxan and streptozotocin diabetic rats and in the mutant diabetic mouse [C57BL/Ks(db/db)]. In the mouse model both fluorescence and electron microscopic techniques were employed using Evans blue albumin (EBA) and horseradish peroxidase (HRP), respectively, as exogeneous tracers. In the rat models only HRP was used. Tracers were applied locally around the sciatic nerve in order to investigate the perineurial barrier, and systemically to detect changes in the blood nerve barrier. None of the models was found to show increased permeability across either of the barriers.

Barriers of peripheral nerve towards exogenous peroxidase in normal and protein deprived rats

The protective barriers of the sciatic nerve in protein deprived and normal rats have been studied. Horseradish peroxidase (HRP) was applied extra-neurally in rats aged between 4 and 26 weeks. The spread of the tracer was investigated 2 h, 24 h, and 3 days after application. Passage of tracer into the endoneurium was found in all rats but decreased with age. Within the endoneurium, HRP was phagocytized by normally occurring endoneurial histiocytes, which constitute part of the protective barrier system of peripheral nerves. The functional development of the nerve protective barriers was retarded in protein deprived rats. Observations made in this study support the concept of a transperineurial vesicular transport of macromolecules.

The distribution of peroxidases in the sciatic nerves of normal and hexachlorophene intoxicated developing rats

Horseradish peroxidase (mol. wt. 40 000) or microperoxidase (mol. wt. 1900) were injected over the sciatic nerve of normal or hexachlorophene (HCP) intoxicated developing rats (3,7,14 and 21 days). Light and electron microscopic studies of nerves after histochemical staining for peroxidase revealed: a) the perineurial barrier to the two peroxidases was established in 21 day normal and HCP intoxicated rats; b) in animals 3-14 days, the perineurial barrier to both peroxidases was not formed and peroxidase staining was observed in the periaxonal space and in the space between paranodal loops of myelin; c) intramyelinic vacuoles, induced by HCP in animals 7-14 days did not show peroxidase staining. HCP-induced intramyelinic vacuolation is due to the separation of the myelin lamellae at the intraperiod line; although these vacuoles are potential extensions of the extracellular space, they are not stained with the extracellular markers horseradish or microperoxidase.