Neuropathic pain is caused by lesion or disease of the somatosensory nervous system. It is a multifactorial condition with common symptoms and disease mechanisms (allodynia and hyperalgesia) that causes severe suffering in patients and a high burden to society. We wanted to gain a deeper understanding of disease contributions of the peripheral nervous system with the goal to develop new analgesic substances. We performed chronic constriction injury (CCI) on experimental animals which produced robust mechanical hypersensitivity. Gait analysis was established to provide an additional experimenter-independent read-out for mechanical hypersensitivity following neuropathic injury. Light- and electron microscopy were used to quantify the damage subjected to the peripheral nerve. An ex-vivo skin nerve preparation from the glabrous skin of experimental animals was used to characterize mechanoreceptors and nociceptors in neuropathic skin and uncover possible sensitization effects linking behavioural data with nerve recordings and therefore compile an integrative dataset. STOML3 is upregulated after nerve injury, therefore I additionally investigated the effects of in vivo overexpression of STOML3 on sensory mechanotransduction, as well as stoml3 conditional deletion specifically in sensory neurons. I found that the physiological properties of low-threshold mechanoreceptors (LTMRs) are not altered following CCI, except a diminished static phase activity in slowly-adapting mechanoreceptors (SAMs). However, sensitization following neuropathic injury could be shown in C-fibers, displaying a significant reduction in transduction thresholds as well as emerging dynamic-phase coding properties that resulted from the injury. These alterations in physiological properties could explain in part the phenomenon of mechanical allodynia. Stimuli that would under normal circumstances not induce activity in the nociceptive C-fiber system, now do so following CCI. Following neuropathic injury proportionately more nociceptors are activated by low intensity mechanical stimuli therefore nociceptive input plays a more pronounced role in the overall sensory barrage into the spinal cord in animals with neuropathic pain. The in vivo overexpression of STOML3 has immense effects on the physiological properties of mechanoreceptors and nociceptors. LTMRs (RAMs and SAMs) were clearly sensitized and again C-fibers displayed prominent changes in their mechanosensitivity. Polymodal C-fibers displayed marked sensitization phenotypes: increased activity, lowered mechanical thresholds as well as dynamic-phase coding properties emerged after induced STOML3 overexpression. CMs increased their activity as well but only for the largest suprathreshold force stimuli. Overall, the overexpression of STOML3 in sensory neurons phenocopied the physiological changes emerging following CCI. Animals acquired mechanical hypersensitivity and the changes in the C-fiber nociceptive system were very similar to neuropathic injury. STOML3 is essential for the emerging sensitization of C-fibers following neuropathic injury proven by genetic ablation of stoml3 in sensory neurons. C-fibers recorded from conditional knockouts did not display elevated physiological activity (CMs) or showed intermediate phenotypes (polymodal C-fibers) following CCI. Interestingly, polymodal C-fibers showed increased activity immediately (<1s) after a mechanical stimulus, however in CMs this physiological abnormality was completely abolished. STOML3 seems to be essential for CM sensitization after neuropathic injury and for polymodal C-fibers partially responsible in producing the sensitization phenotype. Finally, I showed that sensitized C-fibers acquired the ability to follow low frequency (5Hz) vibrations. In some cases, to the extent of phase-locking, and in some cases with less accuracy. High stimulation strengths were necessary to induce activity in CMs in the conditional knockouts, polymodal C-fibers lacking STOML3 did not respond to this type of stimulation at all, further evidence that STOML3 is important for onset activity, that STOML3 is essential for peripheral sensitization following neuropathic injury and that the changes in C-fiber physiology might be the driver for mechanical hypersensitivity displayed by experimental animals after induced neuropathy.
