The mechanisms of neuropathic pain after spinal cord injury (SCI) are not fully understood, although spinal and peripheral processes are involved. Maladaptive tropomyosin receptor kinase-B (TrkB) signaling has been implicated in pain hypersensitivity after SCI. A-delta-low threshold mechanoreceptors (Aδ-LTMRs) innervate the hairy skin and normally signal directional touch and are identified by their preferential TrkB expression. This study investigated whether Aδ-LTMRs play a role in at-level pain after thoracic contusion SCI. Using a modified light-dark chamber conditioned place aversion (CPA) paradigm, we assessed chamber preferences and transitions between chambers in response to mechanical stimulation, and optogenetic stimulation of Aδ-LTMRs in the trunk skin of adult TrkBCre mice of both sexes. Respiratory rates (RRs) were monitored at baseline and during truncal stimulation. The expression of brain-derived neurotrophic factor (BDNF), TrkB and pERK1/2 in the lesioned spinal cord and skin, and histological changes in Aδ-LTMRs in trunk skin were assessed. In addition, electrophysiological studies examined changes in Aδ-LTMRs membrane and firing properties, and response to bath-applied 7, 8-dihydroxyflavone (7,8-DHF), a TrkB agonist. The results showed that whereas brush stimulation evoked an aversive response at 4 weeks post-SCI that was accompanied by increased RRs, targeted stimulation of Aδ-LTMRs produced an aversive response 7 weeks post-SCI. SCI increased BDNF, TrkB and pERK1/2 expression in the skin, and augmented 7,8-DHF-induced inward current in Aδ-LTMRs. Together, these results suggest that plasticity of Aδ-LTMR, including an increase in TrkB signaling in the periphery, contribute to at-level affective pain following chronic SCI in adult mice.
Keywords: At-level pain; BDNF; Conditioned placed aversion; Low threshold mechanoreceptors; Peripheral; TRkB.
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