Elsevier

Brain Research

Volume 1303, 25 November 2009, Pages 131-143
Brain Research

Research Report
Detection of early neuron degeneration and accompanying glial responses in the visual pathway in a rat model of acute intraocular hypertension

https://doi.org/10.1016/j.brainres.2009.09.029Get rights and content

Abstract

Transsynaptic degeneration has been implicated in patients with primary open angle glaucoma (POAG) and animal models of chronic intraocular hypertension. Whether the sustained intraocular pressure (IOP) elevation is necessary for the induction of transsynaptic changes in the brain remains unclear. The aim of this study is to characterize the effects of acute and transient intraocular hypertension on the visual pathway of rats. Acute intraocular hypertension was induced in the right eye by anterior chamber perfusion. At 1 day, 3 days, 1 week, 2 weeks and 4 weeks after the operation, neuronal degeneration and glial responses in the retina, dorsal lateral geniculate nucleus (dLGN) and superior colliculus (SC) were assessed using Nissl staining and immunohistochemistry. TUNEL staining was also performed to detect the neuronal apoptosis in the brain. At the first day after the operation, no obvious neuronal changes were detected in the retina or the brain. At 3 days, 46% of the retinal ganglion cells (RGCs) were lost. Atrophy of the contralateral optic tract was also observed. Meanwhile, the cross-sectional area of neurons in the contralateral dLGN and SC was decreased, while cell density in the same regions was increased. Glial activation in the retina occurred much earlier than the RGC loss. Co-expression of glial fibrillary acid protein (GFAP) and glutamine synthetase (GS) was observed in the end-feet and processes of Müller cells at 1 day after the operation. GFAP immunoreactivity was remarkably increased in the contralateral dLGN and SC at 3 days. It also showed a good co-localization with GS. All of aforementioned changes gradually progressed and persisted for the whole observation period. No TUNEL-positive cells were detected in the dLGN and SC at any post-operative time point. Taken together, these results illustrate that acute and transient intraocular hypertension is able to induce early onset and long-lasting neurodegenerative changes and the accompanying glial activation in the visual pathway. Brain changes may occur in parallel with the RGC loss. Reactive glial cells in the brain may participate in the clearance of aberrantly released glutamate and may serve as a sensitive marker of neuronal injury. Neuroprotection of the entire visual pathway and glia-target therapies may bring new insights into the glaucoma treatment.

Introduction

Glaucoma is a neurodegenerative disease characterized by progressive retinal ganglion cell (RGC) loss and the resultant visual field defects. Elevated intraocular pressure (IOP) is considered to be the most important risk factor of RGC death (Weinreb and Khaw, 2004). However, even when IOP is well controlled, deterioration of the visual field may still proceed in some cases. It suggests that factors other than IOP elevation may contribute to the progression of glaucoma. Accumulating evidence demonstrates that glaucomatous neuropathy not only affects the retina, but also damages the central nervous system (CNS) (Gupta and Yücel, 2003, Gupta and Yücel, 2007, Gupta et al., 2007, Yücel and Gupta, 2008). Over a period of IOP elevation, shrinkage and loss of neurons (Gupta et al., 2006, Gupta et al., 2009, Weber et al., 2000, Yücel et al., 2000, Yücel et al., 2001, Yücel et al., 2003), reduced metabolic activity (Crawford et al., 2000, Crawford et al., 2001, Imamura et al., 2009), and changes in the expression patterns of several synaptic plasticity markers (Lam et al., 2003) can be observed in the lateral geniculate nucleus (LGN) and visual cortex of glaucoma patients and primate models. The disease spread in the brain may disturb the visual information processing, and lead to further visual field defects. These findings impact our understanding of the disease and will influence the therapeutic strategies of glaucoma. How the brain changes occur and whether the sustained IOP elevation is necessary for the induction of transsynaptic changes remain unclear. Besides neurons, 90% of the CNS consists of glias. Glial cells maintain an intimate relationship with neurons and play a crucial role in regulating the biochemical environment. Dysfunctional glias, especially astrocytes, are becoming appreciated as key players in the pathogenesis of various CNS disorders (De Keyser et al., 2008), such as epilepsy (Binder and Steinhauser, 2006, Tian et al., 2005), schizophrenia (Matute et al., 2005) and Alzheimer’s disease (Kuchibhotla et al., 2009, Mrak and Griffinbc, 2001). Whether astrocytes participate in the post-retinal abnormalities in glaucoma as well is another question to be addressed. In this study, we attempted to use a rat model of acute and transient intraocular hypertension to investigate the neuronal and glial responses in the retina and its central targets in the brain resulting from an episode of IOP elevation.

Section snippets

Acute intraocular hypertension induced progressive RGC loss

RGCs were time-dependently lost after acute intraocular hypertension (Fig. 1). There was no significant difference in the number of βIII-tubulin-labeled RGCs between the hypertensive eyes (1959 ± 138/mm2) and control eyes at 1 day (2063 ± 179/mm2) following the operation. The number of RGCs in the hypertensive eyes declined sharply to 1114 ± 157/mm2 and 536 ± 121/mm2 at 3 days and 1 week, respectively. At 2 weeks, 79% of the RGCs were lost (433 ± 78/mm2 RGCs survived), and at 4 weeks, the number of RGCs

Neuronal responses in the retina and its central targets

It is generally believed that in rodents most axonal projections from the retina pass to the SC (Hofbauer and Dräger, 1985), with only a small percentage of axons going to the LGN. Therefore, in the present study we investigated the effects of acute intraocular hypertension on these two pathways (retino-geniculate and retino-superior colliculus pathways) in rats. Similar to chronic ocular hypertension (Yücel et al., 2001, Yücel et al., 2003), we found that an episode of 110-mmHg intraocular

Animals

Fifty-four female Wistar rats, 8 weeks of age (200–250 g), were maintained in temperature-controlled rooms on a 12 h light/dark cycle with free access to food and water. All use and handling of animals adhered to the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research and were approved and monitored by the Institutional Animal Care and Use Committee of Capital Medical University. All possible measures were taken to minimize animal suffering and to limit the number of rats

Acknowledgments

The authors thank YanYun Chen, FenFen Xu, Jie Hong, and JingWen Ding for the technical assistance.

This study was supported by the National Natural Science Foundation of China (Grant No. 30571991) and New Star Project on Science & Technology of Beijing Municipal Science & Technology Commission (Grant NO. 2006B51).

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