Changes in neuronal connectivity after stroke in rats as studied by serial manganese-enhanced MRI
Introduction
Stroke is the leading cause of disability in the western society. Despite acute loss of function after stroke, most patients demonstrate partial functional recovery over time. Interruption and subsequent spontaneous restoration of function have been associated with anatomical and physiological alterations of neuronal networks in the brain (Lee and van Donkelaar, 1995, Seil, 1997, Steinberg and Augustine, 1997, Weiller, 1998, Johansson, 2000). However, the spatial and temporal characteristics of neural reorganization remain largely unresolved.
In recent years, neuroimaging tools, in particular functional magnetic resonance imaging (fMRI), have been successfully applied for in vivo, whole-brain studies on changes in functional activation patterns in stroke patients (see reviews by Cramer and Bastings, 2000, Rijntjes and Weiller, 2002, Calautti and Baron, 2003) and animal models of stroke (Dijkhuizen et al., 2001, Dijkhuizen and Nicolay, 2003). Studies on anatomical alterations in neuronal connectivity after stroke have been mostly confined to invasive axonal tract tracing techniques (Kataoka et al., 1989, Carmichael et al., 2001, Carmichael, 2003). Manganese-enhanced MRI (MEMRI) provides a unique tool to assess changes in neuronal connections in vivo (Pautler et al., 1998). MEMRI is based on the detection of paramagnetic manganese (Mn2+), a calcium analogue that enters active neurons through Ca2+ channels and is transported axonally and transsynaptically (Sloot and Gramsbergen, 1994, Pautler et al., 1998, Saleem et al., 2002). Focal injection of manganese in animal brain is followed by neuronal uptake and subsequent transport along afferent and efferent connective pathways, thereby allowing in vivo mapping of neuronal connections (Pautler, 2004). Allegrini and Wiessner (2003) have recently demonstrated that MEMRI has the potential to detect alterations in brain circuitry after cortical injury in rats.
The goal of our study was to depict changes in neuronal connectivity within the sensorimotor network in a rat stroke model at a time point when ischemic damage is complete and dynamic alterations in sensorimotor function have largely ceased, i.e., 2 weeks after stroke (Kawamata et al., 1997). To that aim, we characterized the spatiotemporal pattern of manganese accumulation by means of serial brain mapping of changes in the longitudinal relaxation rate R1 (1 / T1), which are proportional to the local manganese concentration (Silva et al., 2004). In addition, MEMRI data were compared with a conventional neuronal tract tracing technique based on the immunohistochemical detection of the tracer wheat-germ agglutinin horseradish peroxidase (WGA-HRP) (Gong and LeDoux, 2003).
Section snippets
Animals
All animal procedures were approved by the local ethical committee of Utrecht University and met governmental guidelines. A total of 31 male Wistar rats weighing 250–340 g were included in the study. Rats were divided into two experimental groups. Group 1 animals (n = 20) were subjected to in vivo tract tracing using MEMRI; Group 2 animals (n = 11) were subjected to conventional tract tracing using WGA-HRP immunohistochemistry. In both groups, rats were divided in two subgroups. Experimental stroke
Ischemic damage
In rats with a stroke, the unilateral ischemic lesion was characterized by a prolonged T2 (see Fig. 2C). The mean %HLVe was 12.3 ± 4.8%, with no significant difference in lesion volumes between Group 1 and 2 (13.1 ± 3.2% and 11.0 ± 6.9%, respectively).
Injection site
Lateral coordinates of tracer injection site varied between 1.5 and 3.0 mm from bregma (see Materials and methods section), but were invariably in the forelimb area of the sensorimotor cortex. To determine if variations in injection site influence the
Discussion
In this study, we characterized the spatiotemporal distribution of the paramagnetic neuronal tract tracer manganese using in vivo MRI in order to assess changes in neuronal connectivity within the sensorimotor network at 2 weeks after unilateral stroke in rats. In addition, MEMRI data were compared with results from a conventional tract tracing method based on post-mortem detection of WGA-HRP labeling in the brain.
Manganese-induced R1 changes were detected in distinct regions of the connective
Conclusion
Our study demonstrates that MEMRI allows unique spatiotemporal assessment of alterations in neuronal connectivity after stroke. We have detected decreased and delayed manganese enhancement in brain network regions that are connected to the sensorimotor cortex where manganese was injected. Loss or dysfunction of neuronal connections, even outside the ischemic lesion, may explain the lasting impairment of function. MEMRI thereby provides a unique in vivo tool that can give important new insights
Acknowledgments
Dr. Dijkhuizen is financially supported by a fellowship of the Royal Netherlands Academy of Arts and Sciences.
We thank Gerard van Vliet and Jan-Willem de Groot for excellent technical assistance.
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2014, NeuroImageCitation Excerpt :Apparent increased contralesional intrahemispheric connectivity would be inconsistent with prior animal studies showing no change in dendritic morphology in the intact hemisphere following unilateral stroke (Johnston et al., 2013; Mostany and Portera-Cailliau, 2011). And, while significant increased anatomical connectivity has been observed in the intact hemisphere using manganese-enhanced MRI (van der Zijden et al., 2007, 2008; van Meer et al., 2010b), unlike the present acute study, those studies were performed up to 10 weeks after tMCAO; time points when remodeling and cortical remapping would have either been initialized or more fully matured. Therefore, the GSR estimate of increased connectivity with increased stroke severity appears to be an artifact.