Imaging nociceptive signaling in peripheral CGRP terminal fibres
In this dissertation I introduce a simple experimental approach for studying afferent pain fibre physiology. I developed an en bloc dural-skull preparation that pairs electrophysiological stimulations, pharmacological manipulations, and the UV photolysis of caged compounds in and around selectively identified individual C-fibre nociceptors with microfluorometric imaging of Ca2+ responses. This allows the observation of physiological functioning in individual nociceptive fibre free nerve endings. I show high-resolution functional imaging of single action potential-evoked fluorescent transients, as well as sub- and supra-threshold calcium signaling events within individual nociceptive fibre terminations. Utilizing the dural-skull preparation I was able to identify a peripheral mechanism of action in the terminals of CGRP nociceptive fibres for an effective migraine therapeutic, the selective 5-HT1 receptor agonist, sumatriptan. I found sumatriptan to cause an approximately 40% reduction in the amplitude of action potential-evoked Ca2+ transients in the peripheral terminals of CGRP nociceptive fibres that was mediated selectively through the inhibition of N-type Ca2+ channels. Observations from this study support a peripheral site of action for sumatriptan in inhibiting the activity of dural pain fibres and adds to our understanding of the mechanisms that underlie the clinical effectiveness of 5-HT1 receptor agonists such as sumatriptan. While μ-opioid receptor agonists remain the most powerful analgesics for the treatment of severe pain, their mechanism of action in peripheral primary afferent pain fibres remain to be established. Further exploiting the dural-skull preparation I found activation of μ-opioid receptors in individual CGRP terminals had a dual modulatory effect; inhibition of N-type Ca2+ channel signaling and a frequency dependent, BKCa channel-mediated, suppression of action potential firing. These results establish possible anti-nociceptive mechanisms of μ-opioid receptor activation in the peripheral terminals of CGRP nociceptive fibres and identify new pathways to target for peripherally mediated analgesia. The development and subsequent testing of the dural-skull preparation in this dissertation displays its utility and opens up a new window for studying nociceptive fibre physiology and pathophysiology.
DegreeDoctor of Philosophy (Ph.D.)
SupervisorMulligan, Sean J.
CommitteeFisher, Thomas; Campanucci, Veronica; West, Nigel; Verge, Valerie
Copyright DateJune 2015