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

Primary perineuritis, a rare but treatable neuropathy: Review of perineurial anatomy, clinicopathological features, and differential diagnosis

The perineurium surrounds each fascicle in peripheral nerves, forming part of the blood-nerve barrier. We describe its normal anatomy and function. "Perineuritis" refers to both a nonspecific histopathological finding and more specific clinicopathological entity, primary perineuritis (PP). Patients with PP are often assumed to have nonsystemic vasculitic neuropathy until nerve biopsy is performed. We systematically reviewed the literature on PP and developed a differential diagnosis for histopathologically defined perineuritis. We searched PubMed, Embase, Scopus, and Web of Science for "perineuritis." We identified 20 cases (11 M/9F) of PP: progressive, unexplained neuropathy with biopsy showing perineuritis without vasculitis or other known predisposing condition. Patients ranged in age from 18 to 75 (mean 53.7) y and had symptoms 2-24 (median 4.5) mo before diagnosis. Neuropathy was usually sensory-motor (15/20), painful (18/19), multifocal (16/20), and distal-predominant (16/17) with legs more affected than arms. Truncal numbness occurred in 6/17; 10/18 had elevated cerebrospinal fluid (CSF) protein. Electromyography (EMG) and nerve conduction studies (NCS) demonstrated primarily axonal changes. Nerve biopsies showed T-cell-predominant inflammation, widening, and fibrosis of perineurium; infiltrates in epineurium in 10/20 and endoneurium in 7/20; and non-uniform axonal degeneration. Six had epithelioid cells. 19/20 received corticosteroids, 8 with additional immunomodulators; 18/19 improved. Two patients did not respond to intravenous immunoglobulin (IVIg). At final follow-up, 13/16 patients had mild and 2/16 moderate disability; 1/16 died. Secondary causes of perineuritis include leprosy, vasculitis, neurosarcoidosis, neuroborreliosis, neurolymphomatosis, toxic oil syndrome, eosinophilia-myalgia syndrome, and rarer conditions. PP appears to be an immune-mediated, corticosteroid-responsive disorder. It mimics nonsystemic vasculitic neuropathy. Cases with epithelioid cells might represent peripheral nervous system (PNS)-restricted forms of sarcoidosis.

Redox-sensitive structure and function of the first extracellular loop of the cell-cell contact protein claudin-1: lessons from molecular structure to animals

The paracellular cleft within epithelia/endothelia is sealed by tight junction (TJ) proteins. Their extracellular loops (ECLs) are assumed to control paracellular permeability and are targets of pathogenes. We demonstrated that claudin-1 is crucial for paracellular tightening. Its ECL1 is essential for the sealing and contains two cysteines conserved throughout all claudins.

AIMS

We prove the hypothesis that this cysteine motif forms a redox-sensitive intramolecular disulfide bridge and, hence, the claudin-1-ECL1 constitutes a functional structure which is associated to ECLs of this and other TJ proteins.

RESULTS

The structure and function of claudin-1-ECL1 was elucidated by investigating sequences of this ECL as synthetic peptides, C1C2, and as recombinant proteins, and exhibited a β-sheet binding surface flanked by an α-helix. These sequences bound to different claudins, their ECL1, and peptides with nanomolar binding constants. C-terminally truncated C1C2 (-4aaC) opened cellular barriers and the perineurium. Recombinant ECL1 formed oligomers, and bound to claudin-1 expressing cells. Oligomerization and claudin association were abolished by reducing agents, indicating intraloop disulfide bridging and redox sensitivity.

INNOVATION

The structural and functional model based on our in vitro and in vivo investigations suggested that claudin-1-ECL1 constitutes a functional and ECL-binding β-sheet, stabilized by a shielded and redox-sensitive disulfide bond.

CONCLUSION

Since the β-sheet represents a consensus sequence of claudins and further junctional proteins, a general structural feature is implied. Therefore, our model is of general relevance for the TJ assembly in normal and pathological conditions. C1C2-4aaC is a new drug enhancer that is used to improve pharmacological treatment through tissue barriers.

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.

Pharmacology of local anesthetics used in oral surgery

This article provides a comprehensive review of the pharmacology of local anesthetics as a class, and provides details of the individual drugs available in dental cartridges. Maximum recommended doses of local anesthetics and vasoconstrictors are presented for healthy adult and pediatric patients, and for patients with cardiovascular system impairments. Various complications and reasons for failure of local anesthesia effectiveness are discussed, and current and future trends in local anesthesia are presented to provide an overview of current research in local anesthesia.

A peptidomimetic tight junction modulator to improve regional analgesia

The paracellular flux of solutes through tissue barriers is limited by transmembrane tight junction proteins. Within the family of tight junction proteins, claudin-1 seems to be a key protein for tightness formation and integrity. In the peripheral nervous system, the nerve fibers are surrounded with a barrier formed by the perineurium which expresses claudin-1. To enhance the access of hydrophilic pharmaceutical agents via the paracellular route, a claudin-1 specific modulator was developed. For this purpose, we designed and investigated the claudin-1 derived peptide C1C2. It transiently increased the paracellular permeability for ions and high and low molecular weight compounds through a cellular barrier model. Structural studies revealed a β-sheet potential for the functionality of the peptide. Perineurial injection of C1C2 in rats facilitated the effect of hydrophilic antinociceptive agents and raised mechanical nociceptive thresholds. The mechanism is related to the internalization of C1C2 and to a vesicle-like distribution within the cells. The peptide mainly colocalized with intracellular claudin-1. C1C2 decreased membrane-localized claudin-1 of cells in culture and in vivo in the perineurium of rats after perineurial injection. In conclusion, a novel tool was developed to improve the delivery of pharmaceutical agents through the perineurial barrier by transient modulation of claudin-1.

Anesthetic efficacy of combinations of 0.5 mol/L mannitol and lidocaine with epinephrine for inferior alveolar nerve blocks in patients with symptomatic irreversible pulpitis

INTRODUCTION

The purpose of these 2 prospective, randomized, single-blind studies was to determine the anesthetic efficacy of lidocaine with epinephrine compared with a combination lidocaine with epinephrine plus 0.5 mol/L mannitol for inferior alveolar nerve (IAN) blocks in patients experiencing symptomatic irreversible pulpitis.

METHODS

In study one, 55 emergency patients randomly received IAN blocks by using a 3.18-mL formulation containing 63.6 mg of lidocaine with 31.8 μg epinephrine or a 5-mL formulation containing 63.6 mg of lidocaine with 31.8 μg epinephrine (3.18 mL) plus 1.82 mL of 0.5 mol/L mannitol. In study two, 51 emergency patients randomly received IAN blocks by using a 1.9-mL formulation containing 76.4 mg of lidocaine with 36 μg epinephrine or a 3-mL formulation containing 76.4 mg of lidocaine with 36 μg epinephrine (1.9 mL) plus 1.1 mL of 0.5 mol/L mannitol. Endodontic access was begun 15 minutes after the IAN block, and all patients had profound lip numbness. Success was defined as no or mild pain (visual analogue scale recordings) on endodontic access or instrumentation.

RESULTS

The 1.9 mL of lidocaine (76.4 mg) with epinephrine plus 0.5 mol/L mannitol had a significantly (P = .04) better success rate of 39% when compared with the lidocaine formulation without mannitol (13% success rate).

CONCLUSIONS

For mandibular posterior teeth in patients with symptomatic irreversible pulpitis, the addition of 0.5 mol/L mannitol to 1.9 mL of lidocaine (76.4 mg) with epinephrine resulted in a statistically higher success rate. However, the combination lidocaine/mannitol formulation would not result in predictable pulpal anesthesia.

Anesthetic efficacy of combinations of 0.5 m mannitol and lidocaine with epinephrine in inferior alveolar nerve blocks: a prospective randomized, single-blind study

The purpose of this prospective, randomized, single-blind study was to determine the anesthetic efficacy of lidocaine with epinephrine compared to lidocaine with epinephrine plus 0.5 M mannitol in inferior alveolar nerve (IAN) blocks. Forty subjects randomly received an IAN block in 3 separate appointments spaced at least 1 week apart using the following formulations: a 1.8 mL solution of 36 mg lidocaine with 18 µg epinephrine (control solution); a 2.84 mL solution of 36 mg lidocaine with 18 µg epinephrine (1.80 mL) plus 0.5 M mannitol (1.04 mL); and a 5 mL solution of 63.6 mg lidocaine with 32 µg epinephrine (3.18 mL) plus 0.5 M mannitol (1.82 mL). Mandibular teeth were blindly electric pulp tested at 4-minute cycles for 60 minutes postinjection. No response from the subject to the maximum output (80 reading) of the pulp tester was used as the criterion for pulpal anesthesia. Mean percent total pulpal anesthesia was defined as the total of all the times of pulpal anesthesia (80 readings) over the 60 minutes. Pain of solution deposition and postoperative pain were also measured. The results demonstrated that 2.84 mL of lidocaine with epinephrine plus 0.5 M mannitol was significantly better than 1.8 mL of lidocaine with epinephrine for the molars and premolars. The 5 mL of lidocaine with epinephrine plus 0.5 M mannitol was statistically better than 1.8 mL of lidocaine with epinephrine and 2.84 mL of lidocaine with epinephrine plus 0.5 M mannitol for all teeth except the central incisor. Solution deposition pain and postoperative pain were not statistically different among the mannitol formulations and the lidocaine formulation without mannitol. We concluded that adding 0.5 M mannitol to lidocaine with epinephrine formulations significantly improved effectiveness in achieving a greater percentage of total pulpal anesthesia compared with a lidocaine formulation without mannitol for IAN block.

Homeostatic regulation of the endoneurial microenvironment during development, aging and in response to trauma, disease and toxic insult

The endoneurial microenvironment, delimited by the endothelium of endoneurial vessels and a multi-layered ensheathing perineurium, is a specialized milieu intérieur within which axons, associated Schwann cells and other resident cells of peripheral nerves function. The endothelium and perineurium restricts as well as regulates exchange of material between the endoneurial microenvironment and the surrounding extracellular space and thus is more appropriately described as a blood-nerve interface (BNI) rather than a blood-nerve barrier (BNB). Input to and output from the endoneurial microenvironment occurs via blood-nerve exchange and convective endoneurial fluid flow driven by a proximo-distal hydrostatic pressure gradient. The independent regulation of the endothelial and perineurial components of the BNI during development, aging and in response to trauma is consistent with homeostatic regulation of the endoneurial microenvironment. Pathophysiological alterations of the endoneurium in experimental allergic neuritis (EAN), and diabetic and lead neuropathy are considered to be perturbations of endoneurial homeostasis. The interactions of Schwann cells, axons, macrophages, and mast cells via cell-cell and cell-matrix signaling regulate the permeability of this interface. A greater knowledge of the dynamic nature of tight junctions and the factors that induce and/or modulate these key elements of the BNI will increase our understanding of peripheral nerve disorders as well as stimulate the development of therapeutic strategies to treat these disorders.

The blood-nerve barrier: structure and functional significance

The blood-nerve barrier (BNB) defines the physiological space within which the axons, Schwann cells, and other associated cells of a peripheral nerve function. The BNB consists of the endoneurial microvessels within the nerve fascicle and the investing perineurium. The restricted permeability of these two barriers protects the endoneurial microenvironment from drastic concentration changes in the vascular and other extracellular spaces. It is postulated that endoneurial homeostatic mechanisms regulate the milieu intérieur of peripheral axons and associated Schwann cells. These mechanisms are discussed in relation to nerve development, Wallerian degeneration and nerve regeneration, and lead neuropathy. Finally, the putative factors responsible for the cellular and molecular control of BNB permeability are discussed. Given the dynamic nature of the regulation of the permeability of the perineurium and endoneurial capillaries, it is suggested that the term blood-nerve interface (BNI) better reflects the functional significance of these structures in the maintenance of homeostasis within the endoneurial microenvironment.

Antinociception by neutrophil-derived opioid peptides in noninflamed tissue--role of hypertonicity and the perineurium

Inflammatory pain can be controlled by intraplantar opioid injection or by secretion of endogenous opioid peptides from leukocytes in inflamed rat paws. Antinociception requires binding of opioid peptides to opioid receptors on peripheral sensory nerve terminals. In the absence of inflammation, hydrophilic opioid peptides do not penetrate the perineurial barrier and, thus, do not elicit antinociception. This study was designed to examine the conditions under which endogenous, neutrophil-derived hydrophilic opioid peptides (i.e. Met-Enkephalin and beta-endorphin) can raise nociceptive thresholds in noninflamed tissue in rats. Intraplantar injection of the chemokine CXCL2/3 (macrophage inflammatory protein-2) induced selective neutrophil recruitment without overt signs of inflammation or changes in mechanical nociceptive thresholds (paw pressure threshold). Following intraplantar injection of hypertonic saline, the perineurial barrier was permeable for hours and intraplantar injection of opioid peptides increased mechanical nociceptive thresholds. While formyl-Met-Leu-Phe (fMLP) triggered opioid peptide release from neutrophils in vitro, nociceptive thresholds were unchanged in vivo. In vitro, hypertonicity interfered with fMLP-induced p38 mitogen activated kinase (MAPK) phosphorylation and opioid peptide release from neutrophils. These inhibitory effects were fully reversible by washout. In vivo, return to normotonicity occurred within 30min while the perineurium remained permeable for hours. Under these conditions, fMLP triggered MAPK phosphorylation and induced opioid peptide-mediated increases in nociceptive thresholds in the noninflamed paw. Taken together, antinociception mediated by endogenous opioids in noninflamed tissue has two important requirements: (i) opening of the perineurial barrier for opioid peptide access and (ii) opioid peptide release from neutrophils involving p38 MAPK.

Effects of the bile salt sodium deoxycholate, protamine, and inflammatory mediators on the potassium permeability of the frog nerve perineurium

An electrophysiological method was used to measure the potassium permeability (PK) of the perineurium of the sciatic nerve of frogs Rana temporaria and R. pipiens. Isolated but intact nerves were mounted in a grease-gap chamber, and compound action potential and DC potential monitored. Change in the DC potential (delta DC) in response to challenge with 100 mM [K+] Ringer was used to assess the K+ permeability of the perineurium, since change in DC potential under these conditions reflected changes in the axonal resting potential. The permeability of the perineurium was calculated from the published calibration curve relating delta DC to bathing [K+] in desheathed nerves of Abbott et al. (1997). In the control condition, PK was < 1.1 x 10(-6) cm.s-1. The bile salt sodium deoxycholate (DOC, 1-4 mM) caused a dose-dependent increase in PK, which reached a maximum of 1.7 x 10(-5) cm.s-1 after 2-min exposure to 4 mM DOC, but access of K+ to the endoneurial compartment was more restricted after DOC than after desheathing. Protamine phosphate (1 mM) and protamine sulphate (0.1-5 mg/ml equals 0.125-6.25 mM) had no effect on PK. Neither histamine (0.4-40 mg/ml), bradykinin (0.1-5 mg/ml) nor serotonin (5-hydroxytryptamine, 0.1-5 mg/ml) affected PK. The frog nerve perineurium appears to be relatively insensitive to chemical agents and inflammatory mediators, in contrast to the endothelial cells forming the endoneurial blood-nerve barrier and the blood-brain barrier.

An electrophysiological method for measuring the potassium permeability of the nerve perineurium

An electrophysiological method is described for measuring the potassium permeability (PK) of the perineurium of the sciatic nerve of the frog. The method is based on the principle of grease-gap recording, in which an insulating compartment separates two surface recording electrodes. The sciatic nerves of frogs Rana temporaria and R. pipiens were isolated and mounted across a five compartment chamber, with Vaseline grease seals on the partitions between compartments. Compartments #1, #2 and #5 contained frog Ringer solution, #4 was filled with Vaseline and formed the grease gap, and #3 was the test compartment in which solutions could be changed. The nerve was stimulated via platinum electrodes in compartments #1 and #2, and DC potentials and compound action potentials (CAP) were recorded between Ag/AgCl electrodes connected through Ringer-agar bridges to compartments #3 and #5. In nerves with undamaged perineurium, changing from normal Ringer to high [K+] Ringer (100 mM, KCl replacing NaCl) for 2 min caused negligible change in DC potential or CAP, indicating that raised [K+] was not reaching the axon surface, and hence that the perineurium was exerting a diffusional restriction on K+ entry. In nerves damaged by stretching or drying, K+ pulses caused a depolarising change in DC potential (delta DC), and corresponding decline in CAP amplitude, consistent with a leaky perineurium allowing K+ entry and axonal depolarisation. Ringer made hypertonic by the addition of 2.5 M sucrose or 5 M NaCl caused increased perineurial permeability to K+. The method was calibrated by measuring the delta DC in response to raised [K+] in the range 5-100 mM [K+] in desheathed nerves; from this calibration curve relating delta DC to endoneurial [K+] it was possible to calculate the change in endoneurial [K+] occurring in intact preparations. The calculations showed that the undamaged perineurium had a PK of < 6.3 x 10(-7) cm.s-1, similar to the value calculated for in situ nerves using radioisotopic techniques, but less than the value reported for isolated perineurial cylinders. The method gives real-time information on the K+ permeability of the nerve perineurium and its modulation by experimental treatments.

Ultrastructural arrangement of collagen fibrils in the rat facial nerve

We studied the collagen fibril arrangement in the connective tissue sheaths (epineurium, perineurium, and endoneurium) of the intratemporal and extra-temporal portion of the rat facial nerve by transmission electron microscopy after fixing the nerves with tannic acid, and by scanning electron microscopy after digesting cellular elements by sodium hydroxide treatment. These studies revealed that the epineurium consists of thick bundles of collagen fibrils and a coarse meshwork of collagen fibrils, the perineurium consists of a lacework sheet of bundles of collagen fibrils, and the endoneurium consists of a meshwork of fine collagen fibrils in the inner layer and of longitudinally oriented bundles of collagen fibrils in the outer layer. There was little difference in the three-dimensional organization of collagen fibrils between the extra-temporal segment and vertical and horizontal segments of the facial nerve. However, the facial nerve lost the epineurium and perineurium in the central portion of the horizontal segment near the geniculate ganglion and in the labyrinthine portion, i.e. the proximal portion of the intratemporal segment. In these segments, the endoneurium consisted of a meshwork of fine collagen fibrils.

A freeze-fracture study of the perineurium in galactose neuropathy: morphological changes associated with endoneurial oedema

Feeding rats with galactose as 40% of their diet results in peripheral nerve oedema related to the intrafascicular accumulation of galactitol and sodium. In this study, associated changes in the perineurium were examined by the freeze-fracture replication technique. Perineurial cells are linked by tight junctions (zonulae occludentes). In normal animals these are made up of anastomosing strands organized in a belt-like arrangement along the margins of continuous cells. The majority of the tight junctions in the galactose-fed animals displayed structural abnormalities. These ranged from slight separation of the strands to fragmentation and dispersal, with looping of isolated strands. Some of the tight junctions contained large dilated compartments within the junctional network. Short lengths of intramembranous particles, probably representing assembly or disassembly of tight junctional strands, were also observed. The membranes of perineurial cells normally possess numerous openings of caveolae. A quantitative assessment showed that the mean density of these caveolae openings was increased in the galactose-fed rats as compared with controls. The alterations in the tight junctions resemble those that have been produced experimentally in epithelia by subjecting them to abnormal osmotic gradients. They also resemble those seen in human diabetic neuropathy in which osmotic disturbances involving the perineurium have been considered to occur. If the alterations involve the inner layers of the perineurium, they are likely to impair its barrier function. The increased number of caveolae openings in galactose neuropathy may represent a reaction to the endoneurial oedema and indicate that the pinocytotic-like vesicles have a transport function.

Endoneurial blood flow in rat sciatic nerve during development

1. Endoneurial blood flow (EBF) in the sciatic nerve of rats aged 2-12 weeks was studied using microelectrode H2 polarography. 2. EBF is highest in 2-week-old rats and progressively declines during development. Mean arterial pressure (MAP) is low at 2 weeks of age, gradually increases through the next 4 weeks, and is relatively constant thereafter. The decrease in EBF, in spite of an increase in MAP, occurs because the endoneurial vascular resistance is increasing faster than the MAP. 3. The higher EBF in younger rats is not due to the smaller diameter of their nerves. Sural and tibial nerves of 12-week-old rats, with diameters comparable to that of the sciatic nerve of a 3-week-old rat, have EBFs similar to that of the sciatic nerve of a 12-week-old rat. 4. There was no compelling evidence of autoregulation of EBF in 3-week-old rats over a MAP range from -40 to +30 mmHg of the normal value. 5. The increase of nerve vascular resistance with maturation is probably due to a decrease in capillary density and, to a lesser extent, to an increase in plasma viscosity and haematocrit. 6. The higher EBF in immature rats is likely to be a developmentally adaptive mechanism which permits greater blood-nerve exchange of material to accommodate the greater metabolic needs of rapidly elongating and myelinating axons and proliferating Schwann cells.

Perineurium of sciatic nerve in normal and diabetic rodents: freeze-fracture study of intercellular junctional complexes

A comparative study has been carried out using the freeze-fracture technique on the perineurium of the sciatic nerve from normal and diabetic mice (C57Bl/Ks, BALB/c and CD1 strains) and rats of various ages. The replicas showed that tight junctions connected perineurial cells both within the same cell layer (zonulae occludentes) and between adjacent layers (maculae occludentes). In neonates, a number of zonulae occludentes were characterized by short, incomplete or fragmented ridges at various intervals from each other; in adults, tight junctions appeared as 'mature' networks of interconnected, branching and/or anastomosing strands. Zonulae occludentes of diabetic mice also exhibited frequent interruption of the strands and reduction in the branching of strands. Gap junctions occurred in both zonulae and maculae occludentes of normal and diabetic rats at all ages. In the C57Bl/Ks strain such junctions occurred more frequently in zonulae occludentes of diabetic animals. It is suggested that perineurial cells are coupled by gap junctions to allow fast transfer of ions and small-sized molecules across the layers; under pathological conditions, such as diabetes, the increase in cell-to-cell signalling may be important in controlling the abnormal metabolic situation.

Freeze-fracture observations on normal and abnormal human perineurial tight junctions: alterations in diabetic polyneuropathy

Perineurial cells in the human sural nerve possess tight junctions which in freeze-fracture replicas are seen to be composed of networks of branching and anastomosing P face strands and E face grooves. Isolated circular tight junctions (maculae occludentes) may represent attachment devices between adjacent perineurial lamellae. At the overlapping margins of the cells, a belt-like tight junction (zonula occludens) encircles the cells and is believed to comprise a paracellular diffusion barrier. As the permeability of the perineurium has been found to be altered in diabetic polyneuropathy, the zonulae occludentes have been studied. In freeze-fracture replicas from cases of diabetic polyneuropathy a mixed population of structurally normal and abnormal junctions was observed. In some, the strands were abnormally curved with reduced numbers of intersections, the intervening plasma membrane displaying prominent P face concavities and E face convexities. At other sites, the junctions were severely disorganized and represented by fragmented and isolated strands with few intersections and numerous free ends. These abnormalities resemble changes that have been produced experimentally in epithelial tight junctions by osmotic damage. The possibility is considered that similar mechanisms could result in the alterations of the perineurial tight junctions in diabetic polyneuropathy and account for its impaired permeability barrier properties.

Blood-nerve transfer of albumin and its implications for the endoneurial microenvironment

Blood-nerve transfer of plasma albumin was studied by measuring the permeability coefficient-surface area (PS) product of the blood-nerve barrier (BNB) to 125I-albumin in rat sciatic nerve using the i.v. bolus injection method. The calculated PS was 6.3 +/- 0.5 (S.E.M.) x 10(-7) ml.g-1.s-1. This value is smaller by more than an order of magnitude of that measured for sucrose and confirms the relative impermeance of the BNB to blood-borne solutes. From a review of the available evidence, it is concluded that normal blood-nerve exchange occurs predominantly across the endoneurial microvasculature, and the PS of the BNB reflects the permeability of capillaries to a greater extent than that of the perineurium. The only capillaries found to be less permeable than these are the cerebral capillaries. Proximo-distal differences (sciatic vs tibial) of the PS could not be detected. Blood-nerve albumin transfer was calculated at 1.2 mg.g-1.day-1, and the daily turnover of endoneurial albumin to be about 30%. It is postulated that small increases in PS of BNB to albumin lead to an elevation of endoneurial albumin concentration and, through the operation of Starling forces, subsequently produce endoneurial oedema. A major question posed by the results of this study is the identity of pathways for clearance of albumin and other macromolecules from the endoneurium.

Differential localization of alkaline phosphatase in barrier tissues of the frog and rat nervous systems: a cytochemical and biochemical study

We investigated the localization of alkaline phosphatase (AP) in the peripheral and central nervous systems of the frog (Rana pipiens) and rat. In the frog sciatic nerve, AP reaction product was seen as a precipitate within caveolae and vesicular profiles of perineurial cells, and frequently filled the extracellular space. In the rat peripheral nerve, AP reaction product appeared as small tufts on the cell surfaces and within vesicular profiles of endoneurial blood vessels. AP reaction product was not detected in the rat perineurium or in endoneurial blood vessels of the frog. In the frog central nervous system, AP reaction product was detected in the arachnoid membrane adjacent to the subarachnoid space, but not in the cerebral or pial vessels, whereas in the rat it was detected in the outer arachnoid membrane and in the cerebral and pial blood vessels. Biochemical analysis indicated a sevenfold higher AP activity in the frog perineurium over the endoneurium, whereas in the rat, threefold more activity was measured in the endoneurium over the perineurium. Levamisole, an AP inhibitor, decreased the enzyme activity by 95% in rat tissues, and by 70% in frog tissues and in plasma from both animals. Similar decrements were observed cytochemically. This study suggests that: (1) the distribution of AP varies between species, but that it is always present in at least one component of the blood-brain and blood-nerve barriers, (2) because barrier tissues of the nervous system have enzymatic activity, they may biochemically modify the adjacent environment, (3) vesicular profiles and caveolae in the blood vessels and perineurium may function as microenvironments for enzymatic activity, and (4) in the rat and frog, different isozymes of AP may be present.

Permeability of endoneurial capillaries to K, Na and Cl and its relation to peripheral nerve excitability

The permeability coefficient-surface area products (PA) of frog sciatic nerve endoneurial capillaries to K, Na and Cl were measured with an in situ perfusion technique and found to be 40.3, 24.6 and 32.8 X 10(-5) ml . g-1 . s-1, respectively. PAs to [14C]sucrose and 42K, when measured simultaneously, and their ratio were independent of perfusate K concentration (0.1-10.0 mM). Simultaneous measurements with 36Cl and 42K indicated that the Cl/K permeability ratio was significantly smaller than the mobility ratio of these two ions in free solution. On the other hand, comparable experiments with 22Na and 42K revealed that the K/Na permeability ratio was not significantly different from its respective mobility ratio. Thus, these results provide no evidence of facilitated transport of K by endoneurial capillaries, and suggest that K, Na and Cl traverse the endoneurial capillary wall by a paracellular route which is weakly selective for cations. The minimum extracellular K concentration (Ke) capable of producing a depolarization conduction block in frog sciatic nerve was between 12.5 and 15.0 mM. When the vasculature of this nerve was perfused with a hyperkalaemic (20.0 mM) Ringer solution, a conduction block developed in 7.9 min. Comparison of this time with the theoretically predicted rate of change of endoneurial Ke (induced by a comparable change of intravascular K concentration) indicated that an increase of endoneurial Ke is transmitted directly to the paranodal spaces of nerve fibres so as to immediately influence axonal excitability.(ABSTRACT TRUNCATED AT 250 WORDS)

Endoneurial capillary permeability to [14C]sucrose in frog sciatic nerve

Using an in situ perfusion technique, we measured the [14C]sucrose permeability-surface area product (PA) in endoneurial capillaries of 48 frog sciatic nerves, as 6.6 +/- 0.6 (S.E.M.) X 10(-5) s-1, and the vascular space as 1.31 +/- 0.10%. Assuming A = 30 cm2/g, P = 2.2 X 10(-6) cm/s. P for sucrose was greater than P in some barrier tissues with tight junctions, but was less than P in all capillaries examined so far except rat cerebral capillaries. These observations demonstrate that endoneurial capillaries are an effective part of the blood-nerve barrier to water-soluble non-electrolytes. The findings are consistent with capillary impermeability to microperoxidase and with capillary ultrastructure.

Facilitated transport of glucose from blood into peripheral nerve

D-Glucose is the major substrate for energy metabolism in peripheral nerve. The mechanism of transfer of glucose across the blood-nerve barrier is unclarified. In this study an in situ perfusion technique was utilized, in anesthetized rats, to examine monosaccharide transport from blood into peripheral nerve. Unidirectional influxes of D-[14C]glucose, L-[14C]glucose, and [14C]3-O-methyl-D-glucose across capillaries of the tibial nerve were measured at different perfusate concentrations of unlabelled D-glucose. The permeability-surface area product (PA) for D-[14C]glucose and [14C]3-O-methyl-D-glucose decreased, whereas the PA for L-[14C]glucose remained constant, as the perfusate concentration of D-glucose was increased. In the presence of no added unlabelled D-glucose in the perfusate, the PA for L-[14C]glucose equaled one-fifth the PA for D-[14C]glucose. These results demonstrate self-saturation, competitive inhibition, and stereospecificity of glucose transfer, and for the first time show a unidirectional facilitated transport mechanism for D-monosaccharides at capillaries of mammalian peripheral nerve. The data were fit to a model for facilitated transport and passive diffusion. The half-saturation constant and maximal rate of transport for the saturable component of D-glucose influx equaled 23 +/- 11 mumol X ml-1 and 6.6 +/- 3.2 X 10(-3) mumol X s-1 X g-1, respectively. The constant of nonsaturable glucose influx equaled 0.5 +/- 0.1 X 10(-4) s-1. At normal plasma glucose concentrations, the saturable component comprises about 80% of total D-glucose influx into nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphology of endoneurial blood vessels of frog sciatic nerve during vascular perfusion

In order to determine if increased injection pressures can alter the permeability and ultrastructure of blood vessels of the frog blood-nerve barrier, these vessels were examined following perfusion of the iliac artery at rates of 0.21 or 0.82 ml/min. At either perfusion rate, endoneurial blood vessel profiles were clearly evident and the surface area of these vessels amounted to 60% of the surface area of the perineurium. In all vessels a large number of vesicles were present within the endothelial cells. Many were attached by necks to one or the other plasma membrane, but no transcellular channels were evident. At the higher flow rate no changes in vesicles or junctions were seen, but blebs and blisters were evident at the luminal membranes of the endoneurial endothelium. When microperoxidase was perfused at 0.82 ml/min, reaction product frequently flooded the endothelial cells, was found as clumps on the cell surface, and was distributed within the endoneurial space. These changes represent the only ultrastructural evidence of endothelial cell damage and altered permeability in response to increased rate of perfusion.

AC impedance of the perineurium of the frog sciatic nerve

The AC impedance of the isolated perineurium of the frog sciatic nerve was examined at frequencies from 2 Hz to 100 kHz. A Nyquist plot of the imaginary and real components of the impedance demonstrated more than 1 capacitative element, and a DC resistance of 478 +/- 34 (SEM, n = 27) omega cm2. Transperineurial potential in the absence of externally applied current was 0.0 +/- 0.5 mV. The impedance data were fitted by nonlinear least squares to an equation representing the generalized impedance of four equivalent circuits each with two resistive and two capacitative elements. Only two of these circuits were consistent with perineurial morphology, however. In both, the perineurial cells were represented by a resistive and capacitative element in parallel, where capacitance was less than 0.1 microF/cm2. The extracellular matrix and intercellular junctions of the perineurium were represented as single resistive and capacitative elements in parallel or in series, where capacitance exceeded 2 microF/cm2. Immersion of the perineurium in low conductance Ringer's solution increased DC resistive elements as compared with their values in isotonic Ringer's solution, whereas treatment for 10 min with a hypertonic Ringer's solution (containing an additional 1.0 or 2.0 mol NaCl/liter of solution) reduced DC resistive elements, consistent with changes in perineurial permeability. The results indicate that (a) perineurial impedance contains two time constants and can be analyzed in terms of contributions from cellular and extracellular elements, and (b) transperineurial DC resistance, which is intermediate between DC resistance for leaky and nonleaky epithelia, represents intercellular resistance and can be experimentally modified by hypertonicity.

Peripheral nerve as an osmometer: role of the perineurium in frog sciatic nerve

Measurements of volume and hydrostatic pressure in the frog sciatic nerve in vitro demonstrate that the nerve acts as an osmometer, in large part because the perineurium is a semipermeable membrane for water flow. Endoneurial hydrostatic pressure in nerves in isotonic Ringer exceeds bath pressure by about 7 mmHg. In Ringer made hypertonic by addition of sucrose, nerve volume and endoneurial pressure fall linearly in relation to 1/osmolality. The slope of the plot of pressure against volume provides a value for nerve compliance equal to 0.006 mm2/mmHg. Calculations based on the model of the nerve as an osmometer indicate that the nerve has an osmotically "inactive" volume equal to 0.19 mm3/mm, which is about 75% of the total volume of a nerve segment of unit length in normal Ringer. Perineurial hydraulic conductivity (Lp) equals 7.5 x 10(-13) cm3.s-1.dyn-1, a value characteristic of nonleaky epithelia. The perineurium is an elastic tissue with a constant modulus of elasticity equal to 3 x 10(6) dyn/cm2 when not markedly stretched and may limit nerve swelling under pathological conditions of nerve edema.

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.