Blood-optic Nerve Barrier Research Articles

Borneol: a Promising Monoterpenoid in Enhancing Drug Delivery Across Various Physiological Barriers.

Incorporation of permeation enhancers is one of the most widely employed approaches for delivering drugs across biological membranes. Permeation enhancers aid in delivering drugs across various physiological barriers such as brain capillary endothelium, stratum corneum, corneal epithelium, and mucosal membranes that pose resistance to the entry of a majority of drugs. Borneol is a natural, plant-derived, lipophilic, volatile, bicyclic monoterpenoid belonging to the class of camphene. It has been used under the names "Bing Pian" or "Long Nao" in Traditional Chinese Medicine for more than 1000 years. Borneol has been incorporated predominantly as an adjuvant in the traditional Chinese formulations of centrally acting drugs to improve drug delivery to the brain. This background knowledge and anecdotal evidence have led to extensive research in establishing borneol as a permeation enhancer across the blood-brain barrier. Alteration in cell membrane lipid structures and modulation of multiple ATP binding cassette transporters as well as tight junction proteins are the major contributing factors to blood-brain barrier opening functions of borneol. Owing to these mechanisms of altering membrane properties, borneol has also shown promising potential to improve drug delivery across other physiological barriers as well. The current review focuses on the role of borneol as a permeation enhancer across the blood-brain barrier, mucosal barriers including nasal and gastrointestinal linings, transdermal, transcorneal, and blood optic nerve barrier.

Soluble P-selectin promotes retinal ganglion cell survival through activation of Nrf2 signaling after ischemia injury

Retinal ischemic injuries play an important role in the pathogenesis of several eye disorders. Inflammation and oxidative stress are key players in ischemic injuries. Following retinal ischemia, vascular endothelial cells and leukocytes express several inflammatory adhesion receptors, such as selectins and cell adhesion molecules. P-selectin stimulates leukocyte recruitment to platelet aggregates and has an important role in vascular homeostasis and inflammatory leukocyte extravasation. Soluble P-selectin can be neuroprotective through competitive binding to the receptors of endogenous P-selectin molecules. Here, we demonstrate the neuroprotective effect of a recombinant P-selectin immunoglobin G (P-sel-IgG) chimeric fusion protein in a rat anterior ischemic optic neuropathy (rAION) model. rAION was induced by photodynamic therapy. P-sel-IgG treatment reduced optic nerve edema and stabilized the blood–optic nerve barrier (BONB) in the acute phase of rAION. Further, P-sel-IgG increased the retinal ganglion cell (RGC) survival rate, reduced RGC apoptosis, preserved visual function, maintained retinal nerve fiber layer thickness, and reduced macrophage infiltration in optic nerve tissue in the chronic phase (day 28). Increased NAD(P)H quinone dehydrogenase 1 (NQO1) and heme oxygenase 1(HO-1) expression levels, along with increased transcription factor Nrf2, suggesting an antioxidant role of P-sel-IgG via the Nrf2 signaling pathway. In conclusion, this study is the first to demonstrate that P-sel-IgG treatment promotes RGC survival by stabilizing the BONB and activating the Nrf2 signaling pathway in a rAION model.

Early Methylprednisolone Treatment Can Stabilize the Blood-Optic Nerve Barrier in a Rat Model of Anterior Ischemic Optic Neuropathy (rAION).

We investigated whether methylprednisolone (MP) treatment halting retinal ganglion cell (RGC) death and having anti-inflammatory effect over a narrow therapeutic window affects the integrity of the blood-optic nerve barrier (BOB) in a rat model of ischemic optic neuropathy (rAION). The optic nerve (ON) vascular permeability was determined by Evans blue extravasation. Changes in the levels of TNF-α and IL-1β cytokines were analyzed using quantitative RT-PCR (qRT-PCR) from day 1 to day 5 post-rAION. Rats were treated with MP starting on days 0, 1, 2, and 7 post-rAION. The survival and apoptosis of the RGCs were determined by fluoroGold labeling and TUNEL assay, and the visual function was assessed with flash visual-evoked potentials (FVEPs) 4 weeks postinfarct. Inflammation of the ON was detected by immunohistochemical staining of ED1. Macrophage recruitment in the ON was significantly reduced, which was compatible with the reduction in ON vascular permeability, after MP treatment starting on days 0 and 1 postinsult compared to PBS treatment (both, P < 0.05). There was significant reduction in TNF-α and IL-1β expression in MP-treated rats (all, P < 0.05). The survival number and antiapoptotic effect on RGCs, and the P1-N2 FVEP amplitude significantly improved with MP treatment starting on days 0 and 1 (all, P < 0.05). Early treatment with MP halts RGC death and mitigates macrophage infiltration with decreased expression of proinflammatory cytokines in acute rAION. The very narrow therapeutic window is related to the quick stabilization of the disrupted BOB by early application of MP.

Early applications of granulocyte colony-stimulating factor (G-CSF) can stabilize the blood-optic-nerve barrier and ameliorate inflammation in a rat model of anterior ischemic optic neuropathy (rAION).

ABSTRACTGranulocyte colony-stimulating factor (G-CSF) was reported to have a neuroprotective effect in a rat model of anterior ischemic optic neuropathy (rAION model). However, the therapeutic window and anti-inflammatory effects of G-CSF in a rAION model have yet to be elucidated. Thus, this study aimed to determine the therapeutic window of G-CSF and investigate the mechanisms of G-CSF via regulation of optic nerve (ON) inflammation in a rAION model. Rats were treated with G-CSF on day 0, 1, 2 or 7 post-rAION induction for 5 consecutive days, and a control group were treated with phosphate-buffered saline (PBS). Visual function was assessed by flash visual evoked potentials at 4 weeks post-rAION induction. The survival rate and apoptosis of retinal ganglion cells were determined by FluoroGold labeling and TUNEL assay, respectively. ON inflammation was evaluated by staining of ED1 and Iba1, and ON vascular permeability was determined by Evans Blue extravasation. The type of macrophage polarization was evaluated using quantitative real-time PCR (qRT-PCR). The protein levels of TNF-α and IL-1β were analyzed by western blotting. A therapeutic window during which G-CSF could rescue visual function and retinal ganglion cell survival was demonstrated at day 0 and day 1 post-infarct. Macrophage infiltration was reduced by 3.1- and 1.6-fold by G-CSF treatment starting on day 0 and 1 post-rAION induction, respectively, compared with the PBS-treated group (P<0.05). This was compatible with 3.3- and 1.7-fold reductions in ON vascular permeability after G-CSF treatment compared with PBS treatment (P<0.05). Microglial activation was increased by 3.8- and 3.2-fold in the early (beginning treatment at day 0 or 1) G-CSF-treated group compared with the PBS-treated group (P<0.05). Immediate (within 30 mins of infarct) treatment with G-CSF also induced M2 microglia/macrophage activation. The cytokine levels were lower in the group that received immediate G-CSF treatment compared to those in the later G-CSF treatment group (P<0.05). Early treatment with G-CSF stabilized the blood–ON barrier to reduce macrophage infiltration and induced M2 microglia/macrophage polarization to decrease the expressions of pro-inflammatory cytokines in this rAION model.

The distribution and expression of claudin-5 and occludin at the rat blood–optic nerve barrier after borneol treatment

The purpose of this study is to investigate the distribution and expression of the tight junction membrane proteins, claudin-5 and occludin, in rat blood-optic nerve barrier after borneol treatment. Seventy-two female Wistar rats were randomly divided into the borneol gastric lavage group and the equal volume solvent gastric lavage control group. The bilateral optic nerve from the retrobulbar region to the optic chiasma was collected from the rats in the two groups before gastric lavage and at 30 min, 1, 2, 4, and 8 h after gastric lavage. The distribution and expression of claudin-5 and occludin were detected using immunofluorescence staining, Western blotting, and reverse transcription polymerase chain reaction (RT-PCR). Results showed that claudin-5 translocated from the cell membrane to the cytoplasm at 30 min following initiation of borneol treatment, and this translocation peaked at 1 h. During this period of time, a small amount of occludin also translocated from the cell membrane to the cytoplasm. Four hours after initiation of treatment, claudin-5 and occludin levels in the cytoplasm began to decrease and were restored to their normal pattern 8 h after initiation of treatment. There were no significant differences in the levels of claudin-5 or occludin before or after treatment in either group. It was concluded that claudin-5 and occludin translocate within cells of the rat blood-optic nerve barrier after borneol treatment, and this translocation was reversible. Claudin-5 may play a potential role in permeability of the blood-optic nerve barrier following borneol treatment.

Fine Structure of the Retino-Optic Nerve Junction in Dogs

In most mammals, the optic nerve fibers are myelinated in its extraocular part (EON) but not in its intraocular part (ION) and also in the retina. Transitional zone from the myelinated to unmyelinated optic nerve usually lies in the central part to the lamina cribrosa. It has been known that dogs contain exceptionally myelinated fibers in ION by light microscopy. The aim of this study was to investigate electron microscopically the retino-optic nerve junction in dogs and re-evaluate the barrier to migration of oligodendroblasts into ION. Fourteen adult dogs were used. EON was largely myelinated. In ION the percentage of myelinated fibers decreased gradually toward the retina. A narrow area of ION adjoining the retina was completely unmyelinated. In most mammalian optic nerves, oligodendrocytes are not found in ION. It has been suggested that oligodendroblasts are prevented from migrating from EON into ION; that is to say, there is a barrier to migration of oligodendroblasts. The lamina cribrosa, a dense meshwork of fibrous astrocytic processes, and a defect in the blood optic nerve barrier have been proposed as a candidate for the barrier to migration. Our results suggest, however, that these factors, at least in dogs, would be not involved in the formation of a barrier to migration of oligodendroblasts.

The effect of a chronic breakdown of the blood-optic nerve barrier on the severed optic nerve in adult rat

The changes brought about in the severed optic nerve of an adult rat when the blood-optic nerve barrier was chronically broken down by the local application of a solution of mannitol is described. Axonal labelling indicated that axons did not regenerate across the lesion site, refuting the suggestion that breakdown of the blood-optic nerve barrier can stimulate axonal regeneration. However, we observed that blood vessels were dilated and, between the eye and lesion site, provided a substrate for regenerating axonal sprouts. We suggest that blood-borne agents may directly or indirectly influence the navigation of the growth cones of regrowing axons.

Quantitative Contrast-Enhanced MR Imaging of the Optic Nerve

During the acute stages of optic neuritis damage to the blood-optic nerve barrier can be detected using i.v. paramagnetic contrast-enhanced MR imaging. Quantification of the enhancement pattern of the optic nerve, intraorbital fat and muscle was determined in 15 normal subjects using 3 fat-suppression MR imaging methods: T1-weighted spin-echo and spoiled gradient-echo sequences preceded by a fat-frequency selective pulse (FATSAT + SE and FATSAT + SPGR, respectively) and a pulse sequence combining CHOPPER fat suppression with a fat-frequency selective preparation pulse (HYBRID). Pre- and postcontrast-enhanced studies were acquired for FATSAT + SE and FATSAT + SPGR. There was no significant enhancement of the optic nerve by either method (mean increase of 0.96% and 5.3%, respectively), while there was significant enhancement in muscle (mean 118.2% and 108.2%), respectively; p &lt; 0.005) and fat (mean increase of 13% and 37%, respectively; p &lt; 0.05). Postcontrast optic nerve/muscle signal intensity ratios (mean, SD) were 0.51 (0.07), 0.58 (0.05) and 0.75 (0.05) for FATSAT + SE, FATSAT + SPGR and HYBRID, respectively. These results suggest a practical methodology and range of values for normal signal intensity increases and ratios of tissue signal that can be used as objective measures of optic neuritis for natural history studies and treatment trials.

Quantitative Contrast-Enhanced MR Imaging of the Optic Nerve

During the acute stages of optic neuritis damage to the blood-optic nerve barrier can be detected using i.v. paramagnetic contrast-enhanced MR imaging. Quantification of the enhancement pattern of the optic nerve, intraorbital fat and muscle was determined in 15 normal subjects using 3 fat-suppression MR imaging methods: T1-weighted spin-echo and spoiled gradient-echo sequences preceded by a fat-frequency selective pulse (FATSAT+SE and FATSAT+SPGR, respectively) and a pulse sequence combining CHOPPER fat suppression with a fat-frequency selective preparation pulse (HYBRID). Pre- and postcontrast-enhanced studies were acquired for FATSAT+SE and FATSAT+SPGR. There was no significant enhancement of the optic nerve by either method (mean increase of 0.96% and 5.3%, respectively), while there was significant enhancement in muscle (mean 118.2% and 108.2%, respectively; p < 0.005) and fat (mean increase of 13% and 37%, respectively; p < 0.05). Postcontrast optic nerve/muscle signal intensity ratios (mean, SD) were 0.51 (0.07), 0.58 (0.05) and 0.75 (0.05) for FATSAT+SE, FATSAT+SPGR and HYBRID, respectively. These results suggest a practical methodology and range of values for normal signal intensity increases and ratios of tissue signal that can be used as objective measures of optic neuritis for natural history studies and treatment trials.

THE PATHOPHYSIOLOGY OF ACUTE OPTIC NEURITIS

Eighteen patients with acute optic neuritis underwent optic nerve magnetic resonance imaging (MRI) before and after injection of gadolinium-diethylene triamine pentacetic acid (Gd-DTPA). Ten were re-examined 4 weeks later. Leakage of Gd-DTPA across the blood-optic nerve barrier was a consistent finding in the acute lesion, and its presence was associated with abnormal visual acuity and colour vision, retro-ocular pain on eye movement, and afferent pupillary defect, and a reduced amplitude of the P100 component of the visual evoked potential. Gd-DTPA leakage had ceased in 9/11 nerves when restudied 4 wks later, and this evolution was associated with improved visual acuity and an increased P100 amplitude. Leakage is likely to reflect inflammation, and we conclude that the latter plays an important part in the production of conduction block and clinical deficit, and that its resolution is an important step in clinical remission from acute episodes of demyelination.

Gadolinium-DTPA-enhanced Magnetic Resonance Imaging in Experimental Optic Neuritis

Magnetic resonance imaging (MRI) with gadolinium-diethylenetriaminepentaacetic acid (Gd-DTPA) was used to investigate disruption of the bloodoptic nerve barrier associated with acute autoimmune demyelination. Leakage of Gd-DTPA was seen in the optic nerves and optic chiasm of adult guinea pigs sensitized for acute experimental allergic encephalomyelitis, but not in normal unsensitized animals. This finding occurred as early as 5 to 8 days after antigenic sensitization with the myelin emulsion and before the onset of paralysis or ataxia. Pathologic examination at this early stage of experimental allergic encephalomyelitis showed an absence of demyelination in the optic nerves and optic chiasm, although scant perivascular foci of inflammatory cells were seen. Leakage of Gd-DTPA in the optic nerve before demyelination of this white matter tract illustrates that increased permeability of the blood-optic nerve barrier is an early, if not the initial, event in autoimmune demyelination, and the optic nerve is a common site of central nervous system involvement during the initial phase of acute experimental allergic encephalomyelitis. Findings in this animal model appear comparable with the results of MRI with Gd-DTPA in patients with optic neuritis, and they suggest that disruption of the blood-optic nerve barrier is a common denominator for both disorders of primary demyelination.

Acute and chronic experimental optic neuritis. Alteration in the blood-optic nerve barrier.

Horseradish peroxidase (HRP) tracer studies in experimental allergic optic neuritis showed alterations in the blood-optic nerve barrier. Light and electron microscopic examination of the optic disc disclosed intense leakage of HRP at the lamina scleralis, with extracellular staining of both axonal and glial columns, despite the lack of inflammation at this region. The intraorbital optic nerve showed leakage of HRP at the sites of both acute and chronic demyelination. Ultrastructurally, leakage of HRP was confined to the perivascular and extracellular spaces. The mechanism of leakage seems to be by diffusion through the endothelial cell junctions or vesicular transport across the endothelial cells. The high predilection for involvement of the optic nerve in chronic demyelinating disorders may be due to the inherent weakness of the blood-brain barrier at the optic nerve head.

PERMEABILITY PROPERTIES OF THE TISSUES IN THE OPTIC NERVE HEAD REGION IN THE RABBIT AND THE MONKEY

The distribution of the protein tracer, horseradish peroxidase, has been studied in the optic nerve head region in rabbits and monkeys by light and electron microscopy. Following intravenous injection of the tracer, the eyes were enucleated after varying time intervals. Leaking out of the choroidal capillaries in the peripapillary choroid, peroxidase rapidly spread to the adjacent sclera and to the intraneural connective tissue, that is lamina cribrosa and the connective tissue surrounding the intraneural vessels. The tracer then diffused from both the perineural and intraneural connective tissue into the adjacent optic nerve tissue. This diffusion could take place because the intercellular spaces in the astrocytic cell layer interposed between the connective and neural tissues were freely permeable to the tracer. The present investigation confirms the presence of a defect in the blood optic nerve barrier in the optic nerve head. The clinical importance of this defect in the permeability barrier is not known. It may represent a predilection point for pathological events, e.g. in retrobulbar neuritis.

Kuhnt intermediary tissue as a barrier between the optic nerve and retina.

Utilizing adult albino rats and lanthanum nitrate as a tracer, electron microscopic studies were done to provide additional information on the blood-optic nerve barrier. Following injection into the cervical artery, lanthanum was observed to fill the intercellular space of the optic nerve parenchyma. Diffusion from the prelaminar optic nerve into the juxta-optic nerve retina was, however, prevented by the Kuhnt tissue at the level of the rod and cone layer. In this region, tight junctions were found between the glial cells of the Kuhnt intermediary tissue and were continuous from the tight junctions of the retinal pigment epithelial cells to the junctions (zonulae adhaerentes) of the outer limiting membrane. From the level of the outer limiting membrane to the inner limiting membrane there was no diffusion of lanthanum into the adjacent retina despite the absence of tight junctions, and the lanthanum which had diffused from the choroid was never observed in the area of the inner limiting membrane of the optic nerve head. It is in this region that the functional barrier may exist.

The distribution of intravenously administered peroxidase in the optic nerve head of rabbit and monkey.

Peroxidase was used as an histologic tracer to study the permeability properties of the tissues in the optic nerve head of rabbit and monkey. The distribution of the tracer was observed by light microscopy. Within a short time after intravenous injection peroxidase was located extravascularly in the optic nerve head. The present study confirms earlier reports of a defect in the blood-optic nerve barrier in the region of the optic nerve head. The tracer was found to have reached the connective tissue of the optic nerve head, and to a lesser degree the optic nerve tissue proper. The observations made it probable that peroxidase had reached the connective tissue of the optic nerve head by diffusion from the perineural chorid and sclera.

An electron microscopic study on the blood-optic nerve and fluid-optic nerve barrier.

Utilizing horseradish peroxidase as a tracer, electron microscopic studies were done on the blood-optic nerve and fluid-optic nerve barrier to the peroxidase diffusion. Following intravenous injection the peroxidase was observed to fill the lumen of the capillaries of the laminar, prelaminar and orbital portions of the optic nerve but there was no penetratation of the capillary walls. The obstruction of the tracer diffusion out of capillary walls was attributed to the tight junctions between the endothelial cells. Peroxidase penetration was also absent in the capillaries of the pia and dura mater, however, was observed in pinocytotic vesicles of the endothelial cells. Lateral diffusion from the surrounding choroid into the optic nerve was detected but diffusion from the prelaminar optic nerve into the juxta-optic nerve retina was prevented by the Kuhnt intermediary tissue. Tight junctions which prevented peroxidase diffusion were found between the glial cells of the Kuhnt tissue, and this tissue was the barrier between the prelaminar optic nerve and the juxta-optic nerve retina. Peroxidase which was given into the lateral ventricle of the brain appeared in the subarachnoidal space around the optic nerve and penetrated freely into the optic nerve. The pial surface of the optic nerve possess no barrier activity. Peroxidase could be traced along the intercellular space between glial cells and optic nerve fibers. The basal lamina of the optic nerve capillaries was filled with peroxidase but diffusion into the capillary lumen was obstructured by the tight junctions between the endothelial cells.