Cerebral hypoperfusion in the rat and its consequences

Abstract

Vascular, especially cerebrovascular, dysfunction may be a critical factor in ageing and dementia. Cerebrovascular impairment due to risk factors such as ageing, stroke, smoking, diabetes and cerebral hypoperfusion has a deterious impact on the normal supply of basic nutrients such as oxygen and glucose to the brain; their absence leads inevitably to neuronal death. The cerebral white matter lesions found in most forms of dementia are reportedly the result of chronic cerebral hypoperfusion. However the temporal and spatial evolution of damage remains unclear. Furthermore, any decrease in the integrity of the blood-brain barrier (BBB) has been hypothesised to be a precocious attack on white matter. The “milieu interieure” the most protected in the body, namely the extracellular fluid of the brain, is no longer maintained homeostatically. The cumulation of these various pathophysiological processes alters cerebral function and it has been postulated that, in the most extreme instances, the outcome of this cascade of nefarious events leads to dementia. This thesis examines the supposition that chronic cerebral hypoperfusion could be responsible for the time-related development of white and grey matter pathology and investigates the relationships between the disturbances in the integrity of the BBB and white matter pathology. Three studies addressed these aims. In the first, chronic cerebral hypoperfusion, induced in male Wistar rats by bilateral common carotid artery occlusion (BCCAo), was chosen as the model to study changes in axons, myelin, perikarya as well as microglial activation. The groups of rats that underwent BCCAo were examined at three hours as well as three, seven, 14 and 28 days after the induction of chronic cerebral hypoperfusion. The microscopic examination revealed that, after three hours post BCCAo, damage was detected only in axons and myelin. In contrast, no visible pathology to the neuronal perikarya or enhancement of activated microglia (compared to the sham group) was observable. Injury in both white and grey matter and enhancement of activated microglia was observed from three days post BCCAo and increased with time post BCCAo. The most severe damage to the white and grey matter and enhancement of microglial activation was detected at seven days post BCCAo. These results would indicate that white matter damage precedes grey matter pathology and the enhancement of activated microglia. In the second study, the integrity of the BBB at three hours (when only white matter pathology was found according to the results of the first study) and seven days post BCCAo (when more severe damage to the white and grey matter was shown) was assessed by the use of MRI on T1-weighted image acquisitions with gadolinium as a tracer for BBB permeability. White matter integrity was measured by MTR maps from MTI acquisitions in four brain structures (corpus callosum, caudatoputamen, the external and internal capsules). No differences in white matter integrity were detected between the BCCAo and sham group at three hours and seven days. No differences in signal enhancement of gadolinium were detected three hours post BCCAo. However, a significant signal enhancement of gadolinium was detected at seven days post BCCAo in the caudatoputamen and in the external capsule. Furthermore, immunohistochemistry revealed a significant enhancement of activated microglia seven days post BCCAo compared to the sham group. This functional and immunohistochemical finding, when taken together, might indicate that chronic cerebral hypoperfusion is not in itself responsible for BBB permeability. Rather, the damage to the white matter caused by cerebral hypoperfusion may be responsible for the dysfunction of the BBB over time. Another point of interest was the evidence that the enhancement of activated microglia may play a critical role in the increased permeability of the BBB. The final study in this thesis aimed to investigate the possible pathway and proteins potentially implicated in white matter damage and BBB permeability. To address this question, protein levels and the expression of genes involved in the apoptotic and nonapoptotic hypoxic pathways were compared to the sham groups (at three hours and seven days after BCCAo), in three brain structures (cortex, corpus callosum and caudatoputamen). The levels of HIF-1α, MMP-2, Caspase-3 and VEGF were unchanged compared to the sham group after BCCAo. However, VEGF mRNA expression was found to be significantly different to the sham group seven days post BCCAo in all the three structures examined. An overexpression of HIF-1α and a significant level of Caspase-3 would indicate the activation of the apoptotic pathway. However, neither of these criteria were met and these negative results suggest that the apoptotic pathway is not implicated in the mechanisms that lead to white matter pathology after cerebral hypoperfusion. Finally, the significant expression of VEGF mRNA, compared to the sham group seven days post BCCAo, may contribute to the time-relate increased permeability of the BBB. The results presented within this thesis provide a body of evidence to support the hypothesis that chronic cerebral hypoperfusion is - at least – causal to the damage to different components of the white matter which precedes either early ischaemic changes to the perikarya or enhancement of activated microglia following BCCAo. The increased permeability of the BBB, which can be related to the significant over-expression of VEGF mRNA (compared to the sham group seven days post BCCAo), does not appear to be primarily responsible for white matter pathology, because the MRI investigations indicated that BBB integrity was not affected after three hours of BCCAo. The increased permeability of the BBB, observed seven days post BCCAo with MRI, seems to be the consequence of increased brain damage; thereafter, there is a time-dependent relationship between increasing BBB permeability and increasing brain pathology. Overall, the studies reported herein, strengthen the initial working hypothesis. The conclusion – and direction for future studies – would be that minimising white matter pathology and protecting components of the BBB represent potential targets to decrease then incidence of neuropsychological function or to obtund the cerebral dysfunction in patients who suffer from chronic cerebral hypoperfusion.