The effect of tail docking in neonatal pigs on the central expression of genes involved in modulating anxiety-like behaviour

Background: Adverse experiences in early life, such as exposure to stress, can have long term detrimental effects on the future physiology and behaviour of the animal. Typically animals exposed to such experiences are more anxious and more reactive to stress in later life. Tail biting is a major problem in modern pig production, both in terms of animal welfare and productivity. Tail docking in early postnatal life is common practice to reduce risk of this problem, but causes pain and may alter pain sensitivity.
Aims: To investigate whether a significant painful experience in early life (tail docking) alters the expression of genes in the amygdala that are linked to an anxiety-prone phenotype. Methods: Eight female piglets (Landrace/Large White * synthetic sireline) were used. Four piglets were tail docked (amputation of approx. 2/3 of the tail) on post-natal day 3 using hot-iron cautery and four sham-docked piglets served as intact controls. On post-natal day 10, pigs were sedated and then euthanized by barbiturate overdose. Brains were removed, the amygdala grossly dissected and frozen on dry ice. 20 mum sections were cut and subsequently processed using in situ hybridisation with radiolabelled probes complementary to corticotropin-releasing hormone receptor-1 (Crhr1) and CRH receptor-2 (Crhr2) mRNA. Results: Crhr1 mRNA expression was significantly greater in the amygdala of tail-docked piglets compared with the sham-docked animals. There was no significant difference detected in Crhr2 expression in the amygdala between the groups. Conclusions: Increased expression of Crhr1 in the amygdala is associated with an anxiety-prone phenotype in rats and pigs, thus it is likely that tail docking in early life leads to enhanced anxiety which may have a negative impact on pig welfare. Ongoing experiments will determine whether these central changes in gene expression are long-lasting. Support: BBSRC/DEFRA, part of ANIWHA ERA-NET initiative.