Responses to apomorphine of pigs with different coping characteristics

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.

The present study examined whether, as found in rodents, the behavioral response to apomorphine of pigs relates to individual coping characteristics.

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.

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.

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.