Endothelin-1 Levels in Morrison Model of Glaucoma: A Relationship with Astrogliosis and Optic Nerve Damage

Abstract

Abstract: : Purpose: Endothelin-1 (ET-1), a potent vasoactive peptide is elevated in glaucomatous patients (both POAG and NTG) and injection of ET-1 to the retina/optic nerve causes damage to the optic nerve similar to that seen in glaucoma. However, the mechanisms involved in this optic neuropathy are unknown and may involve astrocyte activation. To determine if elevated IOP causes an elevation in ocular ET-1 and affects expression of ET receptors and astroglial markers in a rat model of glaucoma. To also determine the effect of intracamerally injected ¹²⁵I-ET-1 on tissue distribution in rat ocular tissues. Methods: The Morrison elevated IOP rat model of glaucoma was used wherein IOP was raised in one eye and maintained for up to three months. IOP was measured by tonometry. Aqueous humor ET-1 levels were measured using a RIA. ET-1 mRNA expression was detected using Q-RT-PCR. Expression of ET_A and ET_B receptors and markers for astrogliosis (e.g. glial fibrillary acidic protein; GFAP) was detected in the retina and ONH by immunohistochemistry and immunoblotting. For tissue distribution studies, ¹²⁵I-ET-1 (0.4 µCi) was intracamerally (IC) injected and 4 hours post-injection, various ocular tissues were isolated and counted in a gamma counter. Results: The IOP was 31 mm Hg for eyes with elevated IOP (OS) while that for contralateral control (OD) was 21 mm Hg. Aqueous ET-1 was three fold greater in AH of OS compared to that seen in OD or C. Based on Q-RT-PCR data, retinal ET-1 mRNA expression is 200-350% greater in rat eyes with elevated IOP compared to control or sham-injected eyes. There was significant binding of ¹²⁵I-ET-1 in the rat optic nerve head/choroid tissues 4-hour post-IC injection. Initial observations revealed an increase in protein expression of ET_A and ET_B receptors in the ONH of OS compared to OD and C. GFAP expression in ONH was increased, suggestive of astroglial activation and/or proliferation in eyes with elevated IOP compared to control eyes. Conclusions: Elevated IOP could be a major factor for increased ET-1 synthesis in glaucomatous eyes. ET-1 thus produced can act on ET receptors on ONH astrocytes and cause their proliferation, which may lead to astrogliosis as seen in glaucoma.

Keywords: intraocular pressure • neuropeptides • animal model