In vivo Mn-enhanced MRI for visuotopic mapping in normal and reorganized brains

Abstract

This study employed localized Mn-enhanced MRI via 3 different routes of Mn2+ administrations for visuotopic brain mapping of retinal, callosal and transsynaptic connections in the normal and reorganized rat brains. In normal brains, fractionated intravitreal Mn2+ injection resulted in Mn enhancements in the contralateral superior colliculus (SC) and lateral geniculate nucleus (LGN) by 45-60% at 1-3 days after initial Mn2+ injection, and in the contralateral primary visual cortex (V1) by about 10% at 2-3 days after initial Mn2+ injection. Direct, single-dose Mn2+ injection to the LGN resulted in Mn enhancement by 8-11% in SC, and 13-21% in V1 of the ipsilateral hemisphere at 8 to 24 hours after Mn2+ administration. Intracortical, single-dose Mn2+ injection to the visual cortex resulted in Mn enhancement by 15-26% in ipsilateral SC, 53-65% in ipsilateral LGN, 17-25% in contralateral V1/V2 transition zone, and 32-34% in the splenium of corpus callosum at 8 to 24 hours after Mn2+ administration. Notably, some patchy patterns were apparent near the V1/V2 border of the contralateral hemisphere. Upon intracortical Mn injection, animals which had undergone neonatal binocular enucleation exhibited larger projection volumes by about 74% near the V1/V2 border of the contralateral hemisphere compared to normal rat brains at 3 months old. The current results demonstrated the sensitivity of MEMRI for assessing the neuroarchitecture of the visual brains in vivo without depth-limitation, and may possess great potentials for studying the basic neural components in the visual system longitudinally during development, plasticity, therapeutic interventions and genetic modifications.