Responses to apomorphine of pigs with different coping characteristics

Abstract.

Rationale: Classification of pigs based on the degree of resistance they display in a so-called "backtest" seems, to a certain extent, predictive for their coping strategy. Objective: The present study examined whether, as found in rodents, the behavioral response to apomorphine of pigs relates to individual coping characteristics. Methods: During the suckling period pigs were subjected to the backtest. In this test, each pig is restrained on its back for 1 min and the resistance (i.e. number of escape attempts) is scored. Pigs classified as low-resisting (LR, n=10) or high-resisting (HR, n=10) were selected. At 17–18 weeks of age they received a saline and an apomorphine injection (0.2 mg/kg SC) on 2 consecutive days in a balanced design. Behavior was recorded until 120 min after injection. Results: Apomorphine increased locomotion in all pigs and reduced standing, standing alert and defecating. In addition, apomorphine induced the occurrence of some peculiar activities, rarely seen in saline-treated pigs, which seemed to represent either a transition between different postures or a conflict between hind- and forelimb activities. Apomorphine-treated LR pigs performed significantly more of these activities than HR pigs. However, snout contact with the floor, an oral stereotypy, was significantly increased in apomorphine-treated HR pigs, but not in apomorphine-treated LR pigs. Conclusions: In conclusion, the response to apomorphine of pigs relates to their behavioral response, high-resisting (HR) versus low-resisting (LR), in the backtest. The contrasts in behavioral response to apomorphine suggest a difference in the dopaminergic system between HR and LR pigs.