Structure and Function of Myelinated Nerve Fibers in the Rabbit Eye Following Ischemia/Reperfusion Injury

Effect of vitrectomy with intrasurgical regulation of intraocular pressure in a rabbit model of central retinal artery occlusion

Central retinal artery occlusion (CRAO) is an ophthalmic emergency that causes severe and permanent visual impairment. The effects of conventional treatments on recanalizing retinal arteries and improving visual outcome are equivocal. This study was designed to determine the possible benefits of pars plana vitrectomy (PPV) with intrasurgical regulation of intraocular pressure using intraocular vascular counterpulsation (IVT). CRAO was induced by 532-nm argon green laser activation of auricular intravenous injected rose bengal, a photosensitive dye, in the central retinal arteries (CRA) of eighty-four New Zealand white albino rabbits. CRAO rabbits were randomly assigned to photocoagulation, vitrectomy and counterpulsation groups. Depending on the time intervals between surgery and CRAO induction, vitrectomy and counterpulsation groups were further divided into 2 h (2h), 6 h (6h) and 24 h (24h) subgroups. The proportion of eyes with complete recanalization was significantly higher in the 2h counterpulsation subgroup after three days (P = 0.032) and in all counterpulsation subgroups after one week (P = 0.020). After one month, the 2h and 6h counterpulsation subgroups showed greater oscillatory potential (OPs) responses (F = 3.519, P = 0.049). The 2h counterpulsation subgroup also exhibited greater b-wave amplitude in photopic 3.0 Flicker(F = 4.530, P = 0.044). Histologic evaluation revealed less destruction in the inner retina for the 2h and 6h counterpulsation subgroups. Expression of HSP70 was higher in the 2h and 6h counterpulsation subgroups (F = 48.915,P < 0.001). Levels of HSP90 were lower in all counterpulsation subgroups (F = 30.065,P < 0.001). Levels of TNF-α were lower in the 2h counterpulsation subgroup (F = 14.762,P < 0.001). These results indicate that PPV with IVT was effective to recanalize retinal arteries after CRAO. Early intervention provided better morphologic and functional prognosis for inner retina. The protective effect was related with higher retinal levels of HSP70 and lower levels of HSP90 and TNF-α.

Animal Models Used to Simulate Retinal Artery Occlusion: A Comprehensive Review

Purpose

To present an overview of animal models of retinal artery occlusion (RAO).

Methods

Through a systematic literature search in PubMed and Embase, papers describing methods of inducing RAO in animal models were included. The identified methodologic approaches were presented in a narrative synthesis and compared with RAO in humans.

Results

In total, 83 papers reporting on 88 experiments were included. Six different species were used with rodents and monkeys being the most common, and a minority were performed using cats, dogs, rabbits, or pigs. The anatomy of pigs and monkeys resemble that of humans most closely. The two most frequently used methods were laser-induced occlusion or ligation of the arteries. Other methods included raised intraocular pressure, arterial clamping, administration of vasoconstricting agents, the use of an occluder, embolization, and endovascular approaches to induce occlusion. In general, occlusions lasted for only 30 to 90 minutes, often followed by reperfusion.

Conclusions

Although a broad range of methods have previously been used, they all have limitations. Preferably, the methods should imitate the human disease as closely as possible and avoid damaging other structures. Therefore, monkeys followed by pigs are to be preferred and ligation or clamping may be a suitable model in larger animals as there is a potential to isolate and occlude the retinal artery only. Being less invasive, laser-induced occlusion is another suitable approach.

Translational Relevance

This review aims at assisting researchers in deciding on the most ideal experimental setting, and thereby increase the translational value to human disease.

Vulnerability study of myelinated and unmyelinated nerve fibers in acute ocular hypertension in rabbit

In the current study, it was aimed to evaluate the changes in myelinated and unmyelinated nerve fibers in retinal ischemia-reperfusion injuries caused by acute ocular hypertension and to determine the sequence of these changes. Adult healthy New Zealand white rabbits were randomized to the hemodynamic group [n=12; used to determine the optimal intraocular pressure (IOP) for the subsequent experiments] and the hypertension group (n=6; 70-mmHg hypertension induced in one eye). IOP was adjusted using a cannula and saline. Doppler ultrasound was used to measure the velocity of the optic artery under different intraocular pressures. Immunohistochemistry for myelin basic protein (MBP) was performed. Apoptosis of retinal cells was detected by terminal deoxynucleotidyl transferase biotin-dUTP nick end labeling (TUNEL) assay. Electron microscopy was used to investigate the changes in myelinated and unmyelinated nerve fibers. IOP of the hypertension eyes was maintained at 70.2±1.0 mmHg, while IOP of control eyes was 7–14 mmHg. Doppler ultrasound demonstrated an obvious decline of peak systolic velocity and an increase of resistance index of retinal bloodstream under a 70-mmHg IOP. MBP immunohistochemistry and electron microscopy demonstrated obvious injuries to the myelin fibers. TUNEL indicated a significantly higher apoptosis rate in the hypertension eyes compared with control eyes. The apoptosis rate of retinal ganglion cells and bipolar cells in unmyelinated regions was higher than in myelinated regions. In conclusion, an IOP of 70 mmHg led to incomplete retinal ischemia but was the threshold for retinal ischemia, leading to obvious injuries to the myelin fibers.