ARS | Publication request: Aggressiveness and brain amines in pigs fed the ß-adrenoreceptor agonist Ractopamine

Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 1, 2009
Publication Date: September 1, 2010
Citation: Poletto, R., Richert, B.T., Meisel, R.L., Cheng, H., Marchant Forde, J.N. 2010. Aggressiveness and brain amines in pigs fed the ß-adrenoreceptor agonist Ractopamine. Journal of Animal Science. 88:3107-3120.

Interpretive Summary: Under farmed conditions, pigs are often kept in non-related groups and in relatively barren space with limited resources. Episodes of aggression are usually associated with the formation and maintenance of social hierarchy and with competition for resources. Aggression can cause injuries, social stress and has negative consequences on the pig’s health and welfare, and can be an economical problem for the swine industry due to lesions and death. Any factor (i.e. feed additive) that may impact the amount of aggression within a farmed system is therefore worthy of investigation. The effect of ractopamine feeding, but also taking into account gender and social rank of finishing pigs on aggressiveness and brain hormones was investigated. The results of this study showed that the frequency of attacks during the resident-intruder test, designed to measure aggressiveness, was higher in ractopamine-fed female pigs, regardless of their social rank, and in dominant pigs fed the control diet. Ractopamine feeding of female pigs lowered serotonin but enhanced dopamine metabolism in the brain area known as amygdala, and this is a common hormonal profile of aggressive individuals. Female pigs also had lower dopamine metabolite and norepinephrine concentrations in the amygdala and another brain area tested the frontal cortex. Hormonal availability in these brain areas plays a major role on anger control and aggression inhibition. The serotonin concentration was also deficient in the frontal cortex of female pigs and in the raphe nuclei (the sole area of serotonin synthesis in the brain) of dominant female pigs. Our findings indicate that the changes in the hormonal profiles (deficient serotonin but enhanced dopamine system) may be associated with greater impulsive aggressive behavior especially in female pigs and they may require differential management to ensure that aggression is not problematic.

Technical Abstract: The objectives of this study were to evaluate the effects of the widely used feed additive Ractopamine (RAC), gender and social rank on aggressiveness and concentrations of brain amines in finishing pigs. Thirty-two barrows and 32 gilts (4 pigs/pen/gender) were fed either control or RAC (5 mg/kg/2 weeks + 10 mg/kg/2 weeks) diets. The dominant and subordinate pigs (16 pigs/gender) in each pen were determined post-mixing and subjected to resident-intruder tests (max. 300 s) to measure aggressiveness. At week 4 of the study, their amygdala, frontal cortex, hypothalamus and raphe nuclei were dissected and analyzed for concentrations of dopamine (DA), serotonin (5-HT), their metabolites DOPAC/HVA and 5-HIAA respectively, norepinephrine (NE) and epinephrine (EPI) using high performance liquid chromatography. Ractopamine-fed gilts performed more attacks at 30 s of testing than all other subgroups (p < 0.05). The RAC-fed gilts, dominant control gilts and barrows performed the greatest percentage of attacks by 300 s (p < 0.05). Gilts had lower NE and DOPAC concentrations in the amygdala and frontal cortex and when fed RAC, gilts had the lowest 5-HIAA concentration and greatest DA turnover in the amygdala (p < 0.05). The 5-HT concentration was lower in the frontal cortex of gilts and in the raphe nuclei of dominant gilts (p < 0.05). Ractopamine effects on aggression may be particularly considered when fed to gilts. Serotonergic deficiency and enhanced dopaminergic metabolism in brain areas essential for aggression inhibition may be related to the impulsive aggressive behavior observed in gilts.