| Most retinal ganglion cells (RGCs) die two weeks after optic nerve (ON) injury because of retrograde axonal degeneration, ruptured cell membrane, calcium influx, oxidative stress, mitochondrial swelling, energy depletion, enzyme inactivity and inflammation. Attenuation of RGC death and reduction of secondary degeneration are the primary steps for the repair of injured ON. Antioxidants, ion channel blockers and inflammation inhibitors can promote RGC survival after ON injury.Polyethylene glycol (PEG) is non-toxic, hydrophilic polymers. It recombines the lipid bilayers of cellular membrane and promotes the membrane fusion to form multinucleated cells. Therefore, PEG has a potential ability to repair damaged cell membrane. As a non-toxic repairing agent of damaged membrane, PEG is widely used in the research of nerve repair. Both systemic and topical administration of PEG could repair injured giant axon of the earthworm and enhance the regenerating axons to reach the distal segment through the damage area and thus restore the limited functions. The ability of PEG to repair spinal cord injury has been demonstrated in many investigations in which PEG could repair ruptured membranes, inhibited mitochondrial permeability transition and reduce oxidation stress injury. Koob and collegues also found that PEG could reduce the accumulation of β-amyloid precursor proteins caused by traumatic axonal injury of the brain. However, whether systemic and topical administrations of PEG can have different protective effects on RGCs remain unclear.The aim of the present study was to explore the neuroprotective effects of systemic and topical administrations of PEG on RGCs after ON transection and underlying mechanisms. It is expected to provide new basic theories for the clinical use of PEG. Adult rats thus received left ON transection and systemic or topical PEG treatment and the effects of PEG through the vein or local application onto the ON stumps were compared.Part I:Comparison of different effects of systemic or topical PEG treatment on the RGC survival after ON transection in adult rats. Seventy-two adult Sprague-Dawley rats received left ON transection1.5mm behind the eyeball, followed by the placement of a small piece of Gelform soaked in FluoroGold onto the ocular ON stump to retrogradely label the surviving RGCs. Immediately after ON transection, these animals received intravenous injection of lml30%(M/W) PEG (IV PEG group) or the same volume of sterile saline (IV saline control group) though tail vein, or the local placement of a small piece of Gelform soaked in50%(M/W) PEG (Local PEG group) or saline (Local saline control group) onto the ON stump. The animals (n=18for each group) survived for2,7or14days (n=6for each time point) before the left eyes were enucleated to count the number and calculate the density of surviving RGCs in each retina. It was revealed that the mean RGC density in IV PEG group (1121.4286±42.69/mm2) was significantly higher than that in IV saline control group (846.6667±58.19/mm2,P<0.05) at7day time point, but no difference in the RGC densities were detected between these2groups at2and14day points (P>0.05). The RGC densities in Local PEG group were similar to those in Local saline control group at all3time points (P>0.05). A significant higher RGC density was found in IV PEG group in comparison with Local PEG group (774.43±50.49/mm2;P<0.05) at7day point. It is concluded that PEG can delay the death of axotomized RGCs for a certain period after ON transection and this neuroprotective effect of PEG rely on the way by which PEG is administered.Part II:Investigation of the mechanisms underlying PEG neuroprotection. Twenty-four adult Sprague-Dawley rats received left ON transection1.5mm behind the eyeball and immediate intravenous injection of1ml30%(M/W) PEG or the same volume of sterile saline (IV saline control group) though tail vein. These animals survived for2or7days (n=6for each time point) before they were decapitated. The left eyes were enucleated and postfixed overnight. Transverse sections of15μm were cut to immuno-stain the marker ED1of activated microglia or macrophage cells and an index of the oxidative stress damage nitrotyrosine (NT). The retinas were homogenized to detect the mRNA level of IL-1β and TNF-α by qRT-PCR. It was showed that PEG significantly reduced ED1-and NT-positive cells (P<0.05)7days after ON transection. However, PEG could reduce neither IL-1β nor TNF-αmRNA expression at all time points (P>0.05). Thus, it is concluded that administered PEG could promote RGC survival by reducing the level of activated microglial cells and macrophages and attenuating oxidative stress injury after ON transection. |
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