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Abstract

We examined anterograde labeling of noradrenergic terminals originating from the neurons of brain stem neuromodulatory nucleus Locus Coeruleus (LC), a major course of noradrenaline in the rat forebrain, by means of simultaneous iontophoretic injection of paramagnetic (Mn2+) and classical (fluorescent dextran) tracers in the LC. In order to detect Mn2+ transport, MRI scanning was performed in each rat before and 24h after injection and, subsequently, MR images were compared using voxel-based t-test (voxel size: 0.25x0.25x0.25mm). Fluorescent dextran monosynaptic anterograde transport was analysed 5 days after injection. Iontophoretic injection of Mn2+ did not produce neurotoxic effects as there were no signs of neuronal death or glial inflammatory reaction at the injection site 5 days after injection. Both methods revealed reliable labeling in major subcortical terminal fields of LC neurons (Swanson and Hartman, 1975; Ungerstedt, 1971) including central nucleus of amygdala, internal capsule, anterior part of bed nucleus of the stria terminalis, and mesencephalic region. Consistent with previous studies, labeling was predominantly ipsilateral to the injection site. Classical tracer readily detected terminals like fibers of passage typical for noradrenergic innervation of cortical regions. In contrast, manganese-enhanced MRI (MEMRI) method failed to visualize such dispersed noradrenergic innervation in the cortex. On the other hand, MEMRI might be more sensitive for detecting patterns of functional connectivity. Consistent and strong Mn-labeling in hippocampus was observed, which was not proportional to anatomical connectivity labeled by dextran. Thus, the tract-tracing using MEMRI preferentially maps the target sites of rather strong and highly concentrated projections, but not dispersed terminal fields. Despite the relatively low resolution of MEMRI technique compared to florescent microscopy, this novel tract-tracing method can be successfully applied for visualization of major neural pathways and their reorganization in the same animal in longitudinal studies including those concentrating on development, aging, plasticity, or disease-related neurodegeneration.