Peripheral pain sensitisers: a potential therapeutic target for chronic pain relief

Chronic neuropathic pain results from nerve dysfunction/lesion and affects approximately 20% of European adults. Due to its complexity neuropathic pain is often misdiagnosed and inadequately treated. Nerve injury induces several changes such as synthesis of neurotoxic mediators and prolonged transcription of inducible nitric oxide synthase (iNOS) leading to increased production of nitric oxide (NO). These mediators are believed to contribute to peripheral and central neuronal sensitisation and their activity correlates with pain severity, signifying their importance in nociception. The central aim of this thesis was to investigate the role of iNOS and cytokine signalling in models of chronic pain, and then identify underlying changes in ion channel transcription that may contribute to DRG neuron hyperexcitability. Using the in vivo L5- spinal nerve axotomy (L5-SNA) model of neuropathic pain, behavioural data showed that pain can be alleviated with the highly specific iNOS inhibitor, 1400W. This data showed that iNOS and NO are involved in causing neuronal hyperexcitability and in turn mechanical and heat hyperalgesia; two hallmarks of chronic pain. Immunohistochemical staining data showed that elevations in IL-1β, TNF-α and MCP1 expression in small-sized DRG neurons can be modified with 1400W, outlining its role as a potential neuroprotective agent. This was further investigated by examining the effect of iNOS modulation of the circulating cytokine content in blood plasma following surgery-induced neuropathic pain. In these studies the role of 1400W was more complex. 1400W up-regulated anti-nociceptive mediators, including IL-1α and IL-10, but also significantly increased levels of the pro-nociceptive mediators, IL-1β, TNFα and RANTES. DRG neuron action potential frequency responses to thermal stimuli were investigated in vitro in a further novel model of chronic pain. Whole-cell patch-clamp was also used to investigate changes in voltage-sensitivity of DRG neurones post-TNFα treatment. Under control conditions, elevated temperatures decreased reversal potential; however, whole cell conductance remained stable. In contrast, the TNFα treatment showed that the hyperpolarisation in response to noxious heat (45oC), measured in the control cells, was sensed at lower temperatures (30oC). This was consistent with thermal hyperalgesia. Using qPCR, TNF-α treatment significantly altered 17 neuronal ion channel transcripts were and this could underlie the neuronal hyperexcitability observed in pain states. This approach highlighted how cytokines may act on multiple DRG neuronal ion channels, which was also demonstrated in action current studies. In conclusion, this thesis emphasises the key role of the neuroimmune functions in nociception and identified 1400W as a promising analgesic agent with neuromodulatory effects. This study also identified some electrophysiological parameters that are affected as well as discovering key ion channels that are dysregulated in pain states and could be targeted for future disease-modifying drugs for the chronic pain