|Poletto, Rosangela -|
|Steibel, Juan -|
|Cheng, Heng Wei|
|Meisel, Robert -|
|Richert, Brian -|
Submitted to: Brain Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 8, 2010
Publication Date: March 24, 2011
Citation: Poletto, R., Steibel, J.P., Cheng, H., Meisel, R.L., Richert, B.T., Marchant Forde, J.N. 2011. Gene Expression of Serotonin and Dopamine Receptors and Monoamine Oxidase-A in the Brain of Dominant and Subordinate Pubertal Pigs Fed a ß-Adrenoreceptor Agonist. Brain Research. 1381:11-20. Interpretive Summary: Aggression causes social stress and injuries and negatively affects the health and well-being of those involved in the fight. Brain neurotransmitters such as serotonin (5-HT) and dopamine (DA) are widely implicated in aggression regulation in several animal species, but information on molecular mechanisms mediating aggressive behavior in swine is limited. The feed additive ractopamine (RAC) has been widely used as a swine growth enhancer and increases aggressive behaviors like biting and chasing in female pigs. The objective of this study was to evaluate the effects of sex, social rank (dominant and subordinate), RAC feeding, and their interactions on the expression of the genes that encode the serotonin and dopamine receptors in the brain of adult pigs. These receptors bind the neurotransmitters and mediate their actions. We also examined the gene expression of monoamine oxidase (MAO)-A, the enzyme that deactivates serotonin and dopamine by breaking them down. Thirty-two male and 32 female pigs housed as 4 pigs per pen by gender were fed either the control or RAC added diets continuously for 4 weeks. At the end of the feeding study, specific brain areas that control aggression were collected and tested for gene expression of serotonin receptors (5-HT1B, 5-HT2A, 5-HT2B), the dopamine D1 receptor and MAO-A. Overall, we found that the gene expression of most receptors was reduced in the brain of female pigs compared to the males; and the gene expression of the enzyme that metabolizes the neurotransmitters 5-HT and DA in the brain was lower in the brain of the female pigs. This variation in expression of the genes implicated in regulation of the serotonergic and dopaminergic systems offers further support for understanding the mechanisms controlling aggression and may help explain the differences in aggression seen between male and female pigs.
Technical Abstract: Aggression is a major source of social stress and injuries, negatively affecting the health and well-being of those involved in the fight. The serotonergic and dopaminergic systems are widely implicated in aggression regulation in several animal species, but information on molecular mechanisms mediating aggressive behavior in swine is limited. The ß-adrenoreceptor agonist ractopamine (RAC), has been used as a swine growth enhancer, and increases aggressive behavior in female pigs. The objective of this study was to evaluate the effects of gender, social rank, RAC feeding, and their interactions on the mRNA expression of genes encoding the serotonin and dopamine receptors and the monoamine oxidase (MAO)-A enzyme in the brain of adult pigs. Thirty-two male and 32 female pigs housed as 4 pigs per pen by gender were fed either the control or RAC (5 mg/kg/2 wk followed by 10 mg/kg/2 wk) added diets and the dominant and subordinate pigs (16 pigs per gender) in each pen were determined post-mixing. At d 31, the pigs’ raphe nuclei (RN), amygdala (AMY), frontal cortex (FC), and hypothalamus (HYP) were dissected and the relative abundance of mRNA encoding proteins for the serotonin receptors 5-HT1B, 5-HT2A, 5-HT2B, the dopamine D1 receptor and MAO-A was determined by quantitative real-time RT-PCR. The mRNA levels for 5-HT1B were only suppressed in the AMY of gilts, and 5-HT2B was down-regulated in the RN, FC and HYP of gilts, but also suppressed in the FC of barrows and in the RN of dominant pigs (P < 0.05). Furthermore, MAO-A mRNA expression was suppressed in the RN, AMY and HYP of gilts compared to barrows (P < 0.05). On the other hand, 5-HT2A expression only changed in the RN and was more up-regulated in female than in male pigs (P < 0.05). Meanwhile, the dopamine D1 receptor expression varied in the RN and FC, but always as a function of RAC feeding. This variation in expression of genes implicated in regulation of the serotonergic and dopaminergic systems detected in female pigs tested as aggressive offers further support for understanding the mechanisms controlling aggression.