The Vulnerability Study Of Myelinated And Unmyelinated Nerve Fibers In Acute Ocular Hypertension In Rabbit

Objectives:To evaluate the changes of myelinated and unmyelinated nerve fibers on the retinal ischemia-reperfusion injuries caused by acute ocular hypertensions and to study the sequence of their changes.Methods:Eighteen adult healthy New Zealand white rabbits were divided into two groups (hypertension group, hemodynamic group). Hypertension group concluded 6 rabbits and hemodynamic group had 12 (3 rabbits for retinal photography and 9 for Doppler blood flow ultrasonography).Every rabbit was anesthetized by ketamine, and two 25-gauge cannulae inserted to its anterior chamber. One of them was connected to a pressure transducer and the other to a variable-height infusion of 0.9% NaCl solution, so that IOP (intraocular pressure) could be monitored and adjusted. For hemodynamic group we took photos for central retinal artery under different intraocular pressures.And colored Doppler ultrasonography was also used to measure the velocity of optic artery every 10mmHg from 20mmHg to 120mmHg. For hypertension group we regarded right eyes as experimental group and left eyes as control group.Six exact acute ocular hypertension animal models were found and remained steadily for 90min.After that these six rabbits were kept reperfusion for 2 days, and then finally sacrificed with anesthetic overdose. Eyes were immediately enucleated after euthanasia. Immunohistochemistry (IHC) of MBPs was used to recognize the myelin. The apoptosis of the retinal cells was detected by the TdT-dUTP terminal nick-end labeling (TUNEL) method and the number of apoptotic cells was counted.Usage of optical microscopy and electronic microscopy images were quantitatively and morphologically analyzed to investigate changes in both myelinated and unmyelinated nerve fiber. Finally, gray measurement, apoptosis cell counting and SAS 8.2version were used to analyze the changes of these two parts and sequence of the changes under different ischemia-reperfusion conditions.Results:Thanks for the usage of pressure transducer, IOPs of all the rabbits were exactly in our target values and steadily remained. IOPs of Hypertension group were remained in 70.1569±0.9567mmHg, while IOPs of control group were in 7-14mmHg.Colored Doppler ultrasonography illustrated an obviously decline of peak velocity of retinal bloodstream (PSV) and an increase of resistant index (RI) under IOP of 70mmHg, and the IOP of 70mmHg was the peak point of RI. IHC of MBPs was myelin specific.It showed an obvious injure of myelin, which was as sensitive as electronic microscopy. The result of TUNEL showed a significantly higher number of TUNEL-positive retinal cells in experimental group in contrast to the control group.The apoptosis rate of RGCs and bipolar cells, inspire of photoreceptors, in unmyelinated regions were both much higher than that in myelinated regions.The same result could man observed with electronic microscopy as well.Conclusion:These findings provide evidence that the elevation and decline of IOP can bring about ischemia-reperfusion injuries in retina and optic nerve.An IOP of 70mmHg might lead to an incomplete retinal ischemia but was the threshold of RI.IHC of MBPs, TUNEL and electronic microscopy were showed an obvious injure of myelin. The RIR injury in myelinated nerve fiber was much more serious than that in unmyelinated nerve fiber under IOP of 70mmHg, which suggested myelin should play a positive role in acute ocular hypertension caused RIR injury.