AbstractPeople suffering from chronic widespread pain, which include those diagnosed with non-specific arm pain, whiplash associated disorder, fibromyalgia and complex regional pain syndrome type 1, often complain of neuropathic pain like symptoms in the absence of obvious nerve injury upon routine clinical examination. Many of these patients shown signs of nerve trunk mechanical sensitivity (Tinel’s sign), which can also be detected following a positive response to a neurodynamic test that stretches the peripheral nerve (i.e. movement-evoked pain). Although it is not clear what causes nerve trunk mechanical sensitivity in patients, it is hypothesised that localised nerve inflammation (neuritis) might contribute towards its development. Consistent with this hypothesis, modelling neuritis in the rat causes the development of axonal mechanical sensitivity (AMS) in nociceptive neurons. Previously, our laboratory has inferred a role for inflammatory-induced axonal transport disruption in the development of AMS. Axonal transport disruption is hypothesised to cause an accumulation of anterogradely transported mechanically sensitive ion channels, which are locally inserted into the axonal membrane thereby causing mechanical sensitivity. To examine the contribution of axonal transport to the development of AMS in the absence of inflammation, low doses of vinblastine (0.1mM) can be applied to the rat sciatic nerve. The main aim of this study was to investigate the role of axonal transport disruption in the development of AMS. Specific aims included: investigation of the time course of the development of AMS following vinblastineinduced axonal transport disruption, examination of the properties of mechanically sensitive axons and identification of the mechanically sensitive ion channels hypothesised to be responsible for AMS.
Following vinblastine-treatment, AMS developed rapidly in C-fibre neurons, reached a peak between days 4-5 and had resolved by days 14-15. These changes occurred in the absence of an increase in the development of ongoing activity. Following vinblastine treatment and neuritis, mechanically sensitive hotspots had relatively low firing thresholds and produced graded responses to changes in force. Mechanically sensitive ion channel blockers ruthenium red and FM1-43 attenuated AMS following vinblastine treatment and neuritis, indicating that mechanically sensitive ion channels contribute towards AMS. The anterograde transport of the transient receptor potential channels vaniloid 1 and ankyrin 1 (TPRV1 and TRPA1 respectively) and acid sensing ion channel 3 (ASIC3) were disrupted following vinblastinetreatment and neuritis. Additionally, intraneural injection of OLDA and cinnamaldehyde (TRPV1 and TRPA1 agonists respectively) onto mechanically sensitive hotspots initiated responses in a proportion of neurons with AMS following neuritis. These results indicate an increased presence of functional TRPV1 and TRPA1 ion channels in neurons with AMS following neuritis, which suggests that they are involved in the development of mechanical sensitivity. However, a proportion of neurons that did not have AMS, but were ongoing, responded to cinnamaldehyde following neuritis, suggesting that TRPA1 may also be increased in these neurons and might contribute towards ongoing activity. In contrast, intraneural injections of the same agonists did not cause a response in the majority of neurons with AMS following vinblastine treatment, indicating that there is not an increased presence of functional TRPV1 and TRPA1 in these neurons. This finding suggests that TRPV1 and TRPA1 are not involved in the development of AMS following vinblastine treatment and that other ion channels play a role. In summary, data from this study supports the role of axonal transport disruption in the development of AMS and strongly infers that mechanically sensitive ion channels contribute towards AMS. Whereas the ion channels responsible for AMS following vinblastine treatment were not characterised, this study has revealed that TRPV1 and TRPA1 might contribute towards AMS following neuritis.
|Date of Award||2018|
|Supervisor||Andrew Dilley (Supervisor)|