Diffusion tensor MRI reveals chronic alterations in white matter despite the absence of a visible ischemic lesion on conventional MRI: a nonhuman primate study.

BACKGROUND AND PURPOSE: The impact of stroke on white matter is poorly described in preclinical investigations mainly based on rodents with a low white (WM)/gray matter ratio. Using diffusion tensor imaging, we evaluated WM alterations and correlated them with sensorimotor deficits after stroke in the marmoset, a nonhuman primate that displays a WM/gray matter ratio close to that of humans. METHODS: Marmosets underwent a transient brain ischemia (3-hour). Eight serial MRI examinations were made during ischemia and up to 45 days after reperfusion. The sensorimotor deficits were evaluated weekly over 45 days. To assess WM alterations, the SD of the angle of the first eigenvector projection was calculated in the cortex and in the internal and external capsules. The fiber-tracking approach was used to measure the number and the length of bundles. RESULTS: Changes in the apparent diffusion coefficient and the fractional anisotropy values were similar during the temporal evolution of the lesion in the marmoset model of ischemia to that reported in patients with stroke. Despite an absence of visible lesions on T2-MRI and diffusion tensor imaging at the chronic stage, diffusion tensor MRI evidenced alterations in WM by the increase in the standard deviation of the angle of the first eigenvector projection in the cortex, internal and external capsules, and the decrease in the number of bundles of fibers tracked. The disruption of WM was strongly correlated with the chronic sensorimotor deficits. CONCLUSIONS: Despite an absence of a visible ischemic lesion at the chronic stage, diffusion tensor MRI revealed disorganization of WM, which probably underlies the persistence of functional deficits.

Intraluminal thread model of focal stroke in the non-human primate.

The common marmoset (Callithrix jacchus), a New World monkey, has recently been used as a model of focal cerebral ischaemia. Here, we sought to develop a stroke model in this species using an intraluminal approach to occlude the middle cerebral artery (MCA). This technically simple procedure allows both transient and permanent ischaemia with minimal morbidity. Ten common marmosets underwent either transient (3 h) or permanent ischaemia by the insertion of a nylon filament through the external carotid artery up to the origin of the MCA. Cerebral blood flow (CBF) was monitored by the laser-Doppler flowmetry technique. Sensorimotor functions were regularly evaluated, and histologic, immunohistochemical, and magnetic resonance imaging analyses were performed 8 days after the occlusion. The surgical procedure was achieved straightforwardly without postoperative mortality or cerebral haemorrhage. All animals displayed a consistent decrease in CBF that remained stable over 3 h. Infarction affected both cortical and subcortical structures. Although not statistically significant, the volume of infarction was smaller in marmosets subjected to transient ischaemia compared to those permanently occluded (237+/-139 and 358+/-118 mm3, respectively). In all the behavioural tests used, reperfused marmosets exhibited fewer neurologic and functional impairments compared to permanently occluded ones. We show the feasibility of the induction of permanent or transient focal cerebral ischaemia in the marmoset using an intraluminal approach with minimal invasion. This model could be suitable as an advanced screening for potential stroke therapies in which behavioural, imaging, and histologic analyses can be compared.

Comparison of the effects of erythropoietin and its carbamylated derivative on behaviour and hippocampal neurogenesis in mice.

Erythropoietin (EPO), a well known haematopoietic growth factor, possesses neuroprotective and neurotrophic effects which have been recently reported to improve cognition and to modulate emotional processing. We investigated the effects of EPO and of its non-erythropoietic carbamylated derivative (CEPO) on memory- and emotion-related behaviour in the adult mouse. Locomotor activity, memory performances (place and object recognition tasks), anxiety- (light/dark transition test) and despair-like behaviours (tail suspension test) were assessed over 6 weeks of repeated EPO or CEPO administration (40 µg/kg, twice a week). Given the potential involvement of hippocampal neurogenesis in memory, we also assessed the effects of EPO and CEPO on neurogenesis in the dentate gyrus. Both treatments improved spatial and non-spatial recognition memory and increased the number of NeuN/BrdU double-labeled cells in the dentate gyrus. These effects seem to be, at least partly, independent from an haematopoietic action since administration of CEPO leads to the similar results. Moreover, CEPO decreased, albeit modestly, despair-related behaviour and tended to decrease anxiety-like behaviour. These results suggest that CEPO is as an attractive molecule for the treatment of neuropsychiatric diseases associating memory and/or emotional disorders.

Permanent or transient chronic ischemic stroke in the non-human primate: behavioral, neuroimaging, histological, and immunohistochemical investigations.

Using multimodal magnetic resonance imaging (MRI), behavioral, and immunohistochemical analyses, we examined pathological changes at the acute, sub-acute, and chronic stages, induced by permanent or temporary ischemia in the common marmoset. Animals underwent either permanent (pMCAO) or 3-h transient (tMCAO) occlusion of the middle cerebral artery (MCAO) by the intraluminal thread approach. MRI scans were performed at 1 h, 8, and 45 days after MCAO. Sensorimotor deficits were assessed weekly up to 45 days after MCAO. Immunohistological studies were performed to examine neuronal loss, astrogliosis, and neurogenesis. Remote lesions were analyzed using retrograde neuronal tracers. At day 8 (D8), the lesion defined on diffusion tensor imaging (DTI)-MRI and T2-MRI was significantly larger in pMCAO as compared with that in the tMCAO group. At D45, the former still displayed abnormal signals in T2-MRI. Post-mortem analyses revealed widespread neuronal loss and associated astrogliosis to a greater extent in the pMCAO group. Neurogenesis was increased in both groups in the vicinity of the lesion. Disconnections between the caudate and the temporal cortex, and between the parietal cortex and the thalamus, were observed. Sensorimotor impairments were more severe and long-lasting in pMCAO relative to tMCAO. The profile of brain damage and functional deficits seen in the marmoset suggests that this model could be suitable to test therapies against stroke.