Time-Dependent Effects of Elevated Intraocular Pressure on Optic Nerve Head Axonal Transport and Cytoskeleton Proteins

Abstract

To study the time-dependent effects of elevated intraocular pressure (IOP) on axonal transport and cytoskeleton proteins in the porcine optic nerve head. Fifteen pigs were used for this study. Rhodamine-beta-isothiocyanate was injected into the vitreous of each eye to study axonal transport. IOP in the left eye was elevated to 40 to 45 mm Hg, and IOP in the right eye was maintained between 10 and 15 mm Hg. Cerebrospinal fluid pressure was also continually monitored. IOP was elevated for 3 hours (n = 7) or 12 hours (n = 8) before animal euthanatization. Antibodies to phosphorylated neurofilament heavy (NFHp), phosphorylation-independent neurofilament heavy (NFH), neurofilament light, neurofilament medium (NFM), microtubule, and microtubule-associated protein (MAP) were used to study the axonal cytoskeleton. Confocal microscopy was used to compare axonal transport and cytoskeleton change between control and high IOP eyes in different laminar regions and quadrants of the optic nerve head. Results from these experiments were also compared with 6-hour elevated IOP data from an earlier study. Three hours of IOP elevation caused a decrease in NFH, NFHp, and NFM within laminar regions, with no demonstrable change in axonal transport. Changes to MAP and microtubules were only seen after 12 hours of IOP elevation. Axonal transport change occurred in a time-dependent manner with peripheral nerve bundle changes occurring earlier and being greater than central nerve bundle changes. Time-dependent changes in axonal transport and cytoskeletal structure in the optic nerve head provide further pathogenic evidence of axonal damage caused by elevated IOP.