Relations between peripheral and brain serotonin measures and behavioural responses in a novelty test in pigs

Pigs differ in their behavioural responses towards environmental challenges. Individual variation in maladaptive responses such as tail biting, may partly originate from underlying biological characteristics related to (emotional) reactivity to challenges and serotonergic system functioning. Assessing relations between behavioural responses and brain and blood serotonin parameters may help in understanding susceptibility to the development of maladaptive responses. The objective of the current study was, therefore, to assess the relationship between the pigs' serotonergic parameters measured in both blood and brain, and the behaviour of pigs during a novelty test. Pigs (n = 31) were subjected to a novelty test at 11 weeks of age, consisting of 5-min novel environment exposure after which a novel object (a bucket) was introduced for 5 min. Whole blood serotonin, platelet serotonin level, and platelet serotonin uptake were determined at 13 weeks of age. Levels of serotonin, its metabolite and serotonin turnover were determined at 19 weeks of age in the frontal cortex, hypothalamus and hippocampus. The behaviour of the pigs was different during exposure to a novel object compared to the novel environment only, with more fear-related behaviours exhibited during novel object exposure. Platelet serotonin level and brain serotonergic parameters in the hippocampus were interrelated. Notably, the time spent exploring the test arena was significantly correlated with both platelet serotonin level and right hippocampal serotonin activity (turnover and concentration). In conclusion, the existence of an underlying biological trait – possibly fearfulness – may be involved in the pig's behavioural responses toward environmental challenges, and this is also reflected in serotonergic parameters