Maternal stress and tail docking in juvenile female pigs: effects on mechanical force sensitivity and responses to acute inflammatory challenge

The objective of this study was to identify whether an in-utero pre-natal stress (PNS) challenge (social mixing of gilts during 2nd trimester of pregnancy) and early life tissue trauma (tail-docking) caused long-term alterations in nociceptive responses in juvenile pigs (aged c. 8 weeks). Robust and reliable methods for measuring nociceptive responses in juvenile pigs were developed based on mechanical stimulation of the tail base (von Frey filaments) and hind foot (plantar stimulator) . In addition, a model of acute inflammatory pain (intradermal injection 0.2ml capsaicin into tail root) was used to establish if the maternal and neonatal treatments affected nociceptive responses to inflammatory challenge. These approaches were used in a 2x2 study on sixty-one female pigs from four treatment groups: control intact [CI]; control tail docked [CD]; mixed intact [MI]; mixed tail docked [MD]. PNS pigs had higher (p<0.05) response thresholds (lower sensitivity) to punctate mechanical stimulation of the tail root than control pigs, either tail-docked or intact. Baseline punctate mechanical thresholds were 37% and 24% higher in the MI and MD pigs than in their respective control groups. PNS pigs also showed significantly higher (p<0.05) plantar mechanical thresholds following noxious mechanical stimulation of the foot. PNS piglets had significantly attenuated (p<0.05) responses to acute inflammatory challenge with capsaicin, whether intact or tail-docked. Integrated nociceptive threshold responses vs. time (AUC0-240 min) were significantly higher (p<0.05) in both PNS groups than in unmixed control groups after inflammatory challenge (MI vs. CI 54%, MD vs. CD 38%). Plantar mechanical thresholds did not alter over time within any of the treatment groups in response to capsaicin injection into the tail root. Tail-docking had no long term impact on baseline threshold sensitivity to punctate mechanical stimulation measured local to the site of tail injury or on noxious mechanical sensitivity measured on the plantar surface of the hind foot. Tail-docking did not alter the duration or intensity of capsaicin-induced mechanical hyperalgesia. These findings suggest that gilts subjected to stress during gestation produce an offspring phenotype with higher thresholds to noxious mechanical stimulation than those from non-stressed gilts. Acknowledgements: BBSRC grant reference BB/C518965/1