Direct measurement of fast axonal transport rates in corticospinal axons of the adult rat

Age-related decrease in axonal transport measured by MR imaging in vivo

Axonal transport is a crucial process for neuronal homeostasis and cell functions. In vitro studies have indicated transport rates decrease with age. Disruption of axonal transport has been implicated in age-associated neurodegenerative disorders. We hypothesized that aged rats would show decreased transport in the brain, which could be measured using in vivo manganese-enhanced MR imaging (Mn-MRI) and parametric estimation. Serial T1-weighted images were obtained at pre- and post-administration of MnCl(2) in rats scanned longitudinally (n=4) and in a separate aged group (n=3). Subtraction analysis was performed for group-wise statistical comparison on a pixel-by-pixel basis. Change in intensity over time was plotted for the olfactory bulb and anterior and posterior olfactory tract. Bulk transport of material was estimated over an initial 72 h. Tracer kinetic estimation of time-intensity data, based on a mass transport model, used intensity change in the bulb as input function for subsequent changes in the tract. Time to the peak of Mn(2+) flow was estimated for both anterior and posterior tracts. Results indicated age-related decreases in axonal transport rate and bulk transport of material in the olfactory tract of living rat brains. Longitudinally scanned, mid-age group was decreased by 58% and the aged group by 71% of young rate (neuronal transport=4.07+/-1.24 mm/h, 1.72+/-0.89 mm/h, and 1.16+/-0.18 mm/h for young, mid-age, and aged, respectively). Neuronal transport rate decreases correlated with increased age. The use of kinetic analysis combined with dynamic manganese enhanced MR imaging provides a unique opportunity to study this important neuronal process.

In vivo tracing of major rat brain pathways using manganese-enhanced magnetic resonance imaging and three-dimensional digital atlasing

The magnetic resonance imaging (MRI)-detectable T1 contrast agent manganese (Mn2+) has recently been introduced as a neural tracer in rodents, birds, and monkeys. We have tested to what extent this in vivo method is useful for three-dimensional (3-D) survey of connectivity patterns in the rat somatosensory system. A commonly available 3 T human clinical MRI scanner was used to trace neural pathways following focal injection of manganese chloride (MnCl2) in the somatosensory cortex. Six to 10 h after MnCl2 injection, we found significant signal enhancement in major projection systems, including corticocortical, corticostriatal, corticothalamic, corticotectal, corticopontine, and corticospinal pathways. To facilitate the assignment of anatomic localization to the observed Mn2+ signal enhancement, we registered the MRI data with a 3-D digital reconstruction of a stereotaxic rat brain atlas. Across-animal comparison using the digital model allowed demonstration of a corticothalamic 3-D topographic organization in agreement with previously published two-dimensional topographic schemes based on classical neural tracing data. We conclude that anterograde MnCl2/MRI tracing allows rapid analysis of topographic organization across multiple brain regions. The method allows a higher data throughput for 3-D studies of large-scale brain connectivity than conventional methods based on tissue sectioning.

Three-dimensional MRI of cerebral projections in rat brain in vivo after intracortical injection of MnCl2

In this study we investigated the potential of in vivo MRI detection of axonal Mn2+ transport for tracing neuronal projections originating in the sensorimotor cortex in healthy and lesioned rat brains. Special attention was given to the potential of visualizing neuronal sprouting of central nervous system across the midline. After injecting unchelated MnCl2 into the forelimb area of sensorimotor cortex of 18 healthy and 10 lesioned rats corticofugal projections could be traced through the internal capsule to the cerebral peduncle and the pyramidal decussation. Although the neuronal tract was visible as early as 6 h after MnCl2 injection, best contrast was achieved after 24-48 h. Beside the cortico-spinal tract, the cortico-thalamic fibres were also visualized by anterograde Mn2+ transport. Cortico-striatal fibres were partially masked by the very high signal near the MnCl2 injection site but could be discerned as well. Slight, diffuse signal enhancement of cortical tissue contralateral to the MnCl2 injection site in healthy rat brains suggests interhemispheric connections or passive diffusion of Mn2+. However, enhanced fibre tract contrast connecting both hemispheres was visible 16 weeks after onset of focal photothrombotic cortical injury. In conclusion our study has shown that we were able to visualize reproducibly the main descending corticofugal projections and interhemispheric connections by non-invasive MRI after localized injection of MnCl2. The appearance of interhemispheric Mn2+-enhanced fibres after photothrombotic focal injury indicates that the method may bear potential to follow non-invasively gross plastic changes of connectivity in the brain after injury.