Peripheral Sympathetic Sprouting Drives Neuropathic Pain via Spinal Microglial P2Y12 Activation in a Rat Spinal Nerve Ligation Model

Background: Sympathetically maintained pain (SMP) is heterogeneous, and sympathetic sprouting is not universal across neuropathic pain states. In the rat spinal nerve ligation (SNL) model, peripheral sympathetic-sensory coupling may amplify spinal neuroinflammation. We examined whether peripheral sympathetic drive and spinal microglial P2Y12 signaling operate as parallel pathways or as a coupled peripheral-to-central axis. Methods: Male Sprague-Dawley rats were assigned to Sham, SNL, SNL + 6-OHDA, SNL + intrathecal ticagrelor, ticagrelor-only, or SNL + combined 6-OHDA/ticagrelor groups. Mechanical allodynia, microglial morphology, P2Y12/Iba-1 expression, p38 MAPK phosphorylation, and spinal cytokines were assessed by von Frey testing, immunofluorescence, Western blotting, and ELISA. A pharmacological occlusion paradigm tested pathway additivity. Results: SNL produced marked ipsilateral mechanical allodynia. On postoperative day 7, 6-OHDA increased paw withdrawal threshold from 3.56 to 8.43 g (Hedges’ g=2.56, 95% CI 1.03 to 4.10) and reduced SNL-induced spinal P2Y12, Iba-1, p-p38/t-p38, and TNF-α/IL-1β/IL-6 increases. Intrathecal ticagrelor similarly attenuated allodynia (4.64 to 7.84 g; Hedges’ g=1.45, 95% CI 0.23 to 2.67). In the occlusion experiment, AUCs were higher in SNL + 6-OHDA (50.11± 4.94 g.day), SNL + ticagrelor (55.83± 10.79 g.day), and combined treatment (52.85± 9.28 g.day) than in SNL (25.09± 8.78 g.day), with no additional benefit of combination therapy. Conclusion: These findings support a functional peripheral-to-central neuroimmune axis in which peripheral sympathetic activity amplifies spinal microglial P2Y12-p38 signaling in the SNL model. Purinergic interfaces may represent potential SMP targets, but clinical translation requires further validation.