Layer specific tracing of corticocortical and thalamocortical connectivity in the rodent using manganese enhanced MRI

Abstract

Information about layer specific connections in the brain comes mainly from classical neuronal tracers that rely on histology. Manganese Enhanced MRI (MEMRI) has mapped connectivity along a number of brain pathways in several animal models. It is not clear at what level of specificity neuronal connectivity measured using MEMRI tracing can resolve. The goal of this work was to determine if neural tracing using MEMRI could distinguish layer inputs of major pathways of the cortex. To accomplish this, tracing was performed between hemispheres of the somatosensory (S1) cortex and between the thalamus and S1 cortex. T(1) mapping and T(1) weighted pulse sequences detected layer specific tracing after local injection of MnCl(2). Approximately 12 h following injections into S1 cortex, maximal T(1) reductions were observed at 0.6+/-0.07 and 1.1+/-0.12 mm from the brain surface in the contralateral S1. These distances correspond to the positions of layer 3 and 5 consistent with the known callosal inputs along this pathway. Four to six hours following injection of MnCl(2) into the thalamus there were maximal T(1) reductions between 0.7+/-0.08 and 0.8+/-0.08 mm from the surface of the brain, which corresponds to layer 4. This is consistent with terminations of the known thalamocortical projections. In order to observe the first synapse projection, it was critical to perform MRI at the right time after injections to detect layer specificity with MEMRI. At later time points, tracing through the cortical network led to more uniform contrast throughout the cortex due to its complex neuronal connections. These results are consistent with well established neuronal pathways within the somatosensory cortex and demonstrate that layer specific somatosensory connections can be detected in vivo using MEMRI.

Mapping somatosensory connectivity in adult mice using diffusion MRI tractography and super-resolution track density imaging

“…These findings would not result from connectivity between adjacent layer IV barrels (Feldmeyer, 2012) and, although there is evidence to suggest a higher concentration of myelin within the septal area compared to barrel hollows (Barrera et al, 2012), myelin is not essential for anisotropic diffusion in nerve fibres, with evidence of minor amounts of anisotropy in rodent and piglet cortex (Hoehn-Berlage et al, 1999;Mori et al, 2001;Thornton et al, 1997). In fact, anisotropy from un-myelinated nerves in garfish (Beaulieu and Allen, 1994), rodent (Seo et al, 1999), zebrafish (Ullmann et al, 2013) and human fetal brain (Tucciarone et al, 2009) suggest the radial alignment of microstructure including cell membranes is a sufficient barrier to diffusion to result in anisotropic diffusivity. In addition, complex biological architecture may influence the degree of anisotropy, including axon thickness, density and extracellular space (Beaulieu, 2002).…”

“…Manganese enhanced-MRI (MEMRI) has also been used to map discrete thalamocortical projections achieving layer specific contrast (Silva et al, 2008;Tucciarone et al, 2009) and functional analysis of disease models (Daoust et al, 2014). The advantage of fibre tracing using MEMRI over ex vivo methods is the ability to map neuronal activity, which can be coupled with functional MRI data providing a valuable in vivo technique ).…”

Quantitative activity-induced manganese-dependent MRI for characterizing cortical layers in the primary somatosensory cortex of the rat

“…Manganese-enhanced MRI (MEMRI) is a powerful technique allowing an enhanced definition of structural, anatomical, and functional details in the rodent's brain (Silva et al 2008;Tucciarone et al 2009;Koretsky and Silva 2004;van der Linden et al 2004;Inoue et al 2011;Serrano et al 2008). Systemic administration of Mn 2?…”

“…Systemic administration of Mn 2? several hours before image acquisition (24-72 h) has proved to be a unique strategy to investigate the architecture of specific brain areas with MRI (Silva et al 2008;Tucciarone et al 2009). Manganese can also be used in different ways for functional investigations.…”

“…several hours before MR image acquisition. Tucciarone (2009) and Yu et al 2012a, b demonstrated for example that MEMRI allowed the direct visualization of thalamo-cortical inputs to the barrel cortex and plasticity changes (Yu et al 2012a) as well as the depiction of cortical layers (Lu et al 2004;Silva et al 2008;Tucciarone et al 2009;Yu et al 2012aYu et al , b, 2014. 3D MRI techniques have been used allowing increased spatial resolution of the brain architecture and of the sensory pathways to specific areas as well as a better investigation of connected areas.…”