Laulund, F.C.; (2011) Experimental neuroinflammation: a focus on mitochondria, oxygen and function. Doctoral thesis , UCL (University College London).

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Abstract

There is increasing evidence for "hypoxia-like" conditions in some inflammatory multiple sclerosis (MS) lesions raising the possibility that the tissue is deficient in energy. Accodingly, mitochondrial abnormalities have been described in some MS tissue, including reduced mitochondrial gene transcripts in MS motor cortex and decreased mitochondrial complex IV expression. To explore the role of mitochondrial defects in neuroinflammatory lesions, experimental inflammatory lesions were induced by the intraspinal injection of lipopolysaccharide in rats under general anaesthesia. The tissue was snap-frozen for histochemical assessment of mitochondrial complex II and/or IV activity at various intervals (1-28 days) post injection: the activity of the complexes was compared with porin expression or complex IV subunit I expression (presence of mitochondria or protein respectively). The oxygen concentration within the dorsal column was determined at 24 hours after lesion induction in vivo using an oxygen-sensitive optical probe. EMG potentials were also recorded at the foot dorsum in response to stimulation of the sciatic nerve. Presence of reactive oxygen and nitrogen species were examined by immunohistochemistry and by the fluorescent marker dihydroethidium (DHE) in vivo. Complex IV activity within the motor neurons was decreased 1 day after injection, further decreased by day 2, and returned to pre-injection baseline by day 5. Decreased complex IV activity exactly coincided with a temporary reduction in motor neuron excitability as measured by H reflex and F wave amplitude, which was maximal at day 2. The oxygen concentration within the dorsal columns at the site of the LPS lesion was significantly elevated when compared with the saline-injected and naïve control animals. DHE fluorescence revealed that superoxide production was increased at the lesion site and immmunohistochemistry revealed oxidative stress to DNA, lipids and protein. LPS-induced neuroinflammation results in a reversible and coincident decrease in both complex IV activity and motor neuron excitability, which was strengthened by the result that intraspinal LPS-injections causes hyperoxia, presumably from mitochondrial dysfunction. Nitric oxide has been implicated in neuronal dysfunction and although a causal relationship has not been established, observations support an interpretation that inflammation-mediated NO causes mitochondrial damage, increased oxygen concentration and formation of reactive oxygen species (ROS). These actions illustrate a vicious circle where ROS induce further damage, which results in energy deficiency displayed by reduced neuronal excitability and neurological deficits. The observations are consistent with energy deficiency and reduced function in neuroinflammatory lesions similar to those found in MS.

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