Characterising the role of substance P in acute ischaemic stroke.

Abstract

More than 15 million people worldwide will suffer a stroke each year two thirds will die or be left permanently disabled. Accordingly, stroke represents an enormous financial burden on the community, due to the cost of hospitalisation, treatment and rehabilitation of stroke patients. Despite the significance of this public health problem, a safe and widely applicable stroke therapeutic remains elusive. Cerebral oedema is widely recognised as a common and often fatal complication of stroke that is associated with worsened outcome. However, the exact mechanisms of oedema formation remain unclear, with current therapies largely ineffective in addressing the mechanisms of cerebral swelling, and also being associated with their own negative side-effect profile. This thesis characterises the role of neurogenic inflammation and the neuropeptide, substance P (SP), in mediating the development of blood brain barrier breakdown, cerebral oedema and resultant functional deficits following stroke, using a rodent model of reversible cerebral ischaemia. The findings of this thesis demonstrate that increased SP immunoreactivity, particularly of the penumbral tissue vasculature, is a feature of tissue perfusion following stroke, but not in non-reperfused infarcts. The central role for SP in the breakdown of the BBB following stroke and the associated deleterious effects of such breakdown was confirmed by studies using an NK₁ receptor antagonist. These antagonists conferred a profound attenuation of BBB breakdown, cerebral oedema formation, neuronal death and injury, and the associated development of functional deficits following reversible stroke. Similarly, depletion of all neuropeptides by capsaicin pre-treatment also reduced both histological abnormalities and functional deficits following stroke, confirming the central role of neuropeptides in the secondary injury process after stroke. The NK₁ receptor antagonist was able to be safely combined with the currently approved treatment for stroke, tPA, producing a synergistic effect of greater protection from the ischaemic insult. In particular, histological and functional outcome were markedly improved, as well as a reduction in the risk of intracerebral haemorrhage and death. Furthermore, the NK₁ receptor antagonist was effective even when administered up to 8 h following the onset of ischaemia, and in a variety of stroke severities. We conclude that SP plays a central role in the secondary injury that occurs following stroke, in particular, the genesis of BBB breakdown and cerebral oedema. Accordingly, combination therapy of tPA and an NK₁ receptor antagonist may offer a novel therapeutic strategy for the clinical management of ischaemic stroke of varying severity.