Diffusion tensor MRI of axonal plasticity in the rat hippocampus

Abstract

The aim of this study was to explore non-invasive imaging methods to detect post-injury structural axonal plasticity. Brain injury was launched by status epilepticus induced by intraperitoneal injection of either kainic acid or pilocarpine. Several months later, ex vivo diffusion tensor magnetic resonance imaging (DTI) showed increased FA in the dentate gyrus of both kainic acid (p < 0.01) and pilocarpine animals (p < 0.01). Importantly, FA changes correlated (p < 0.01) with histologically verified axonal plasticity of myelinated and non-myelinated neuronal fibers. The changes observed in DTI parameters ex vivo in the septal dentate gyrus were also seen by in vivo DTI. As DTI is completely a non-invasive imaging method, these results may pave the way for non-invasive in vivo imaging of axonal plasticity after brain insults.

Introduction

One of the most fascinating features of the brain is its ability to change, a phenomenon referred to as plasticity. A form of plasticity, structural plasticity of axons, is a coordinated sequence of events affecting the formation and organization of neural pathways and connections, most often described in the context of brain development (Goldman-Rakic, 1987, Murakami et al., 1992). More recently axonal plasticity has also been found to be an important response to brain injury, and its role as a part of the disease modifying process has been recognized (Sutula, 2002). One of the best characterized examples of injury-induced axonal plasticity is reorganization of neuronal pathways in the hippocampus as a response to various types of brain trauma (Golarai et al., 2001, Karhunen et al., 2007, Kharatishvili et al., 2006). In particular, the axons of the mossy fiber pathway have been shown to grow new collaterals that extend from the hilus through the granule cell layer to the inner molecular layer of the dentate gyrus (Sutula et al., 1998). This branching, also referred to as mossy fiber sprouting, is commonly observed after induction of status epilepticus or traumatic brain injury and serves as a well-defined model for axonal plasticity (Golarai et al., 2001, Kharatishvili et al., 2006, Nairismagi et al., 2006).

Axonal sprouting can be a part of the recovery process and/or contribute to disease progression after initial damage. As an example, mossy fiber sprouting has been previously shown to precede the occurrence of spontaneous seizures in some animal models of epileptogenesis (Kharatishvili et al., 2006, Nissinen et al., 2001). Undoubtedly, non-invasive imaging of axonal plasticity would provide a great tool for preclinical studies but also for identification of surrogate markers for disease progression or recovery in the clinic. As a proof-of-principle, it was previously shown that axonal plasticity can be detected in vivo using manganese-enhanced magnetic resonance imaging (MEMRI) (Nairismagi et al., 2006). However, toxicity of the MnCl₂ complicates potential human studies, even though use of non-toxic chelated manganese compounds to obtain MEMRI contrast has been reported (Tofts et al., 2009).

Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) method that allows imaging of neuroanatomy and neural connectivity without exogenous contrast agents. DTI is widely used to study white matter integrity in patients with stroke (Pierpaoli et al., 2001), traumatic brain injury (Naganawa et al., 2004, Wieshmann et al., 1999), multiple sclerosis (Bammer et al., 2000), Alzheimer's disease (Taoka et al., 2006) and epilepsy (Eriksson et al., 2001). DTI has also been used to study the song control system of a songbird where plasticity occurs naturally (De Groof et al., 2008) and Ramu et al. (2008) have showed fiber tract plasticity in rat brain after spinal cord injury using in vivo DTI. However, there are currently no in vivo DTI reports of axonal plasticity induced by brain injury. Recently, Kuo et al. detected changes in the dentate gyrus after pilocarpine induced status epilepticus in rat brain ex vivo using diffusion anisotropy (DA) obtained from diffusion spectrum imaging (DSI) and attributed these to mossy fiber sprouting (Kuo et al., 2008).

The present study was designed to demonstrate that trauma induced axonal plasticity can be detected using DTI both ex vivo and, for the first time, in vivo. This was done by acquiring DTI data from the rat dentate gyrus following kainic acid (KA, kainate) and pilocarpine induced status epilepticus, in which prolonged seizure activity triggers neurodegeneration and axonal plasticity. Furthermore, in order to understand which components of structural plasticity contribute to the changes in DTI parameters during epileptogenesis we performed extensive histology including assessment of both mossy fibers and myelinated axons in subregions of the dentate gyrus.