Manganese-enhanced magnetic resonance imaging of the visual cortex in a rat model of chronic glaucoma

Abstract

Glaucoma is an optic neuropathy in which the optic nerve axons are damaged, with the visual cortex being secondarily affected. Previously, by employing manganese-enhanced magnetic resonance imaging (MEMRI), we have demonstrated the reduction in manganese transport along the rat optic nerve upon intravitreal injection of manganese chloride (MnCl2) in the late stage of chronic glaucoma. However, it is not clear whether MEMRI could detect glaucomatous changes in the visual cortex uniquely. In this study, Sprague-Dawley rats (250-280 g, N=10) were divided into 2 groups. Rats in the disease group (n=6) were induced for ocular hypertension unilaterally in the right eye by photocoagulation of the episcleral and limbal veins using an argon laser. 6 weeks after surgery, MnCl2 solution (45 mg/kg, 100 mM) was applied intraperitoneally, and two-dimensional spin-echo T1W imaging was acquired at a resolution of 125 μm x 125 μm in a 7 Tesla Bruker MRI scanner before and 24 hours after MnCl2 injection. 4 normal rats were scanned as controls. Results showed a significant decrease in T1W signal intensity in the left visual cortex projected from the glaucomatous eye when compared to the contralateral untreated side after MnCl2 injection. No significant difference was observed between bilateral visual cortices in the disease group before MnCl2 injection, or in the control group before and after Mn2+ injection. Given the ability of manganese to act as a calcium analogue and to detect brain activities, these data suggested a reduction in manganese uptake in the visual cortex upon chronic glaucoma, possibly due to the decrease in activations from visual field deficits, or calciumdependent neurodegenerative changes in the visual system.