Ocular integrity following manganese labeling of the visual system for MRI

Injection of manganese into the eye will enhance the contrast of visual system neuronal pathways imaged by MRI (MEMRI). The present study was undertaken to determine the effect of a range of MnCl2 doses upon the integrity of various ocular structures. Anesthetized mice received ocular anterior chamber injections of 50-500 nmol of MnCl2. One week later, the eyes were fixed, embedded in paraffin, sectioned, and stained with hematoxylin and eosin. Additional animals received 50 nmol of MnCl2 injected into the anterior chamber and were later imaged using T1-weighted 7T MRI. Following 500 and 300nmol MnCl2, the corneal stroma and endothelium were degenerated, the anterior chamber contained a dense fibrin matrix with extensive inflammatory cell infiltration, a plaque often formed on the anterior lens, and significant retinal degeneration was observed. Following 100nmol MnCl2, retinal preservation of ocular structures was significantly better than at higher doses. In addition, there was no difference from vehicle control retina in cell counts within the ganglion cell layer, or in the width of the inner nuclear layer or outer nuclear layer. Also, there was no difference in the thickness of the inner plexiform layer. However, there was thinning of the peripheral outer plexiform layer, as well as in the outer segment layer. Visual system elements labeled in MRI of mice that received 100nmol MnCl2 included the retina, optic nerve, lateral geniculate nucleus, and superior colliculus. The preservation of ganglion cell layer cell counts and inner plexiform layer thickness following 100nmol MnCl2 suggests there was negligible injury to RGCs following this dose. These results support using 100nmol MnCl2 in mouse eyes for in vivo assessment of the integrity of RGC projections to target neurons in the brain.