Long-term cognitive enrichment affects opioid receptor expression in the amygdala of domestic pigs

Enriching the housing environment by stimuli that challenge both reward and cognitive mechanisms may enhance behavioural experiences and can improve animal welfare, particularly in farm animals. A newly developed experimental feeding system for domestic pigs using food-rewarded learning of discriminatory and instrumental tasks enabled the animals to successfully master a cognitive challenge and to be rewarded ca. 30 times per day with small food portions. Reward-related behaviour is expected to be modulated by endogenous opioid systems. Furthermore, recent evidence supports a role for the amygdala in processing positive affects by stimulus-reward learning. Hence, the present study investigates mRNA expression of cerebral receptors, which are involved in these processes. In an initial step, reverse transcription-polymerase chain reaction (RT-PCR) provided the first evidence that transcripts of three different opioid receptors (MOR, DOR, KOR), as well as the neuropeptide Y 5 receptor (NPY5R), leptin receptor (LEPR) and proopiomelanocortin (POMC), are expressed in both the porcine amygdala and hypothalamus. Using real-time PCR we could show that the expression of two receptors of the opioid system (amygdala: KOR, DOR), in addition to the expression of NPY5R (hypothalamus) in eight enriched housed pigs was markedly downregulated compared to that of conventionally housed and fed pigs. Focusing on opioid receptors in the amygdala, the present study shows that long-term cognitive enrichment acts as a biologically relevant stimulus that causes modifications of gene expression of reward-sensitive cerebral receptors in domestic pigs.