Opioid control of behaviour in sheep: Effects of morphine and naloxone on food intake, activity and the affective state

Abstract

The affective states of animals are important determinants of welfare, yet they are poorly understood. Here we investigate opioid involvement in the regulation of behaviours that may be indicative of the arousal and valence components of affective states in sheep. Ewes treated with sterile water (C), a low or high dose of the opioid agonist morphine (M1, M2) or opioid antagonist naloxone (N1, N2; 'n' = 8 per treatment) were exposed to a range of different situations, including availability of concentrate feed, grazing in a paddock, novel object test and isolation box test; behavioural indicators, activity and ear postures were assessed. Morphine treated ewes crossed more zones (78.1 ± 5.4 for M1, 'P' = 0.025 and 99.3 ± 5.4 for M2, 'P' = 0.01) compared to C ewes (37.3 ± 5.4) and vocalized more (49.9 ± 6.5 for M1, 'P' < 0.001 and 43.2 ± 6.5 for M2, 'P' = 0.005) compared to C ewes (9.7 ± 6.5) during the novel object test. Morphine treated ewes also attempted to escape more often (3.7 ± 0.6 for M1, 'P' = 0.01 and 4.3 ± 0.6 for M2, 'P' < 0.001) compared to C ewes (0.7 ± 0.6) and showed a higher duration of the backward ear posture (14.9 ± 2.0 s for M1, 'P' = 0.02) compared to C ewes (6.1 ± 2.0 s) during the novel object test. While grazing, morphine treated ewes (M1, 'P' = 0.025 and M2, 'P' < 0.001) also walked more compared to compared to C ewes. Opioid treatment did not affect agitation during the isolation test. Concentrate feed intake (square-root g) was slightly reduced in the N2 ewes (3.6 ± 0.9, 'P' = 0.02) compared to the C ewes (6.6 ± 0.9) at 1.5 h after injection, while morphine had no effect on intake. In conclusion, morphine had a major impact on activity suggesting that the opioid system may be involved in regulating the arousal component of affective states in sheep. Morphine may have reduced the negative experience in the novel object test as indicated by the higher duration of the backward ear posture; however, further investigation is needed to determine the impact of opioid administration on emotional valence.