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Kevin C. Chan, Ed X. Wu; In vivo Studies of Early Postnatal Development and Plasticity in Retinocollicular Projections using Manganese-enhanced MRI. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6572.
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© ARVO (1962-2015); The Authors (2016-present)
The retinocollicular projection between retina and the superficial layers of superior colliculus (SC) in rodents is a commonly used model system for evaluating the mechanisms of retinotopic map formation, neurodegeneration and plasticity in the brains. However, to date, in vivo, high-resolution imaging of the developmental and plastic changes in retinal organization in SC has not yet been available. In this study, we explore the capability of high-resolution Mn-enhanced MRI (MEMRI) for in vivo assessments of the retinocollicular projection and its reorganization in normal neonatal brains and after early postnatal visual impairments.
Sprague-Dawley rats (N=52) were divided into 6 groups. In the normal groups, 6 rats at postnatal days (P) 1, 5, 10 and 60 each were injected intravitreally with 100mM MnCl2 solution into one eye at a volume of 0.5, 1.0, 1.5 and 2.0 uL respectively. MEMRI was performed 8 hours (Hr8) and 1 day (D1) after Mn2+ administration using a 7T Bruker scanner. Four age-matched rats without Mn2+ injection were scanned in each age group as a control. In the visually impaired groups, neonatal monocular enucleation (ME, n=6) and monocular deprivation (MD, n=6) were performed to the right eyes at P1 and P15, respectively. MEMRI was performed 1 day after Mn2+ administration to the left eyes at P60 at the same dosage as for normal P60 rats. Diffusion tensor imaging (DTI) was also acquired to complement the MEMRI findings at P60.
Intravitreal Mn2+ injection into one eye resulted in T1-weighted hyperintensity in the ipsilateral retina and optic nerve, and the contralateral SC at Hr8 in all age groups. Such enhancement was also observed in the contralateral SC at D1 in adult P60 rats, but was significantly reduced in neonatal P1, P5 and P10 rats. Mild but significant Mn2+ enhancements could also be found in the ipsilateral SC of the neonatal rats at Hr8, and of the adult ME but not MD or normal rats at D1. For DTI, while a lower fractional anisotropy (FA) was observed in ME and MD rats along the visual pathway projected from the enucleated or deprived right eye, a significantly higher FA was found in the left optic nerve of ME rats, likely as a result of the retention of axons from the ipsilaterally uncrossed retinal ganglion cells in the remaining eye.
The current results demonstrated the sensitivity of MEMRI for assessing the retinocollicular organiziations of early postnatal brains in vivo before natural eyelid opening, and for detecting the neural plasticity of the uncrossed retinal projections in SC upon early postnatal visual impairments.
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