Tail amputation and traumatic neuroma development in neonatal pigs is not associated with long-term changes in peripheral and spinal nociceptive processing

Aim of Investigation: Concerns exist over the long term consequences for tail stump pain experienced by piglets after tail docking (amputation of approximately two-thirds of the tail), especially in relation to traumatic neuroma development in caudal nerves after docking injury. Traumatic neuroma formation may cause detrimental sensory changes in the tail due to altered spinal and peripheral axonal excitability leading to abnormal sensation or pain. The aim of this investigation was to characterize traumatic neuroma development by histopathological assessment at time intervals up to 16 weeks after tail docking and to measure, in caudal dorsal root ganglia and spinal cord neurons, the expression of key neuropeptides associated with (i) peripheral nerve regeneration: activating transcription factor-3 (ATF3), (ii) inflammatory pain: Calcitonin gene-related peptide (CGRP) and (iii) the maintenance of chronic pain: N-methyl D-aspartate (NMDA) ionotropic glutamate receptor subtype 2B (GRIN2B) at the same time points after tail docking injury. Methods: Thirty-two female piglets (Landrace/Large White x synthetic sireline) were used (16 docked/16 sham-docked). Piglets were tail docked on post-natal day 3 using a gas-heated docking iron. Equivalent sham-docked piglets served as intact controls. Pigs were euthanized by barbiturate overdose at 1, 4, 8 and 16 weeks after sham/tail docking. Tail stumps (2 cm) were collected post-mortem for histopathological assessment. Caudal dorsal root ganglia (Ca1-Ca4+) and associated spinal cord were collected for gene expression analysis by real-time quantitative PCR of mRNA. Results: Non-specific epidermal and dermal changes associated with healing were observed in the distal tail stump after amputation. Mild inflammation, ulceration and oedema were present at 1 week. Traumatic neuroma development in caudal peripheral nerves was a consistently observed feature from 4 weeks after amputation. Neuroma axonal dispersion in the distal tail stump was still on-going 16 weeks after amputation. ATF-3 mRNA was significantly (P<0.05) upregulated in DRG neurons up to 8 weeks after tail amputation, but did not differ at 16 weeks compared with sham controls. Both CGRP and GRIN2B mRNA expression was significantly (P<0.05) upregulated 1 week after tail amputation in caudal spinal cord neurons but were not significantly different from sham-tail amputation pigs thereafter. Conclusions: Histopathological lesions that occurred shortly after tail amputation (beyond 1 week) were not likely to induce or maintain pain. The effects of tail docking on peripheral nerve axonal proliferation and dispersion were relatively short-lived and, although still present, were attenuated by 16 weeks after tail amputation injury. Changes in peripheral and spinal nociceptive processing associated with possible inflammatory and chronic pain appeared to resolve by 4 weeks after tail amputation injury. Acknowledgements: BBSRC/DEFRA (funded as part of the FareWellDock project under the ANIWHA ERA-Net Initiative).