Generating positive affective states in sheep: The influence of food rewards and opioid administration

This study investigated the involvement of the opioid system in modulating the affective state after the consumption of palatable and unpalatable rewards, using judgement bias as an indicator of the affective state. We hypothesised that consuming a palatable food reward would generate a more optimistic bias compared to receiving unpalatable wood chips. We hypothesised that morphine administration would further enhance the optimistic judgement bias after consumption of a food reward while reducing a pessimistic bias after receiving wood chips. Naloxone administration was expected to prevent the formation of an optimistic bias after consumption of a food reward, and was expected to have little effect after receiving the wood chips. The judgement bias arena consisted of a positive and a negative location cue, associated with conspecifics and a dog, respectively, as well as three non-reinforced ambiguous location cues between the positive and negative locations. Thirty sheep were successfully trained to approach the conspecifics and avoid the dog. Following training, sheep were randomly assigned to one of three treatments; morphine (1 mg/kg), naloxone (2 mg/kg) and control. Within each treatment, half the ewes received a small food reward and the other half wood chips before exposure to each of the five locations. Judgement bias was assessed by recording the latency to approach the five locations. Animals receiving the food reward approached the locations faster than animals receiving the wood chips (0.83 ± 0.04 and 1.00 ± 0.04 log-transformed latencies (s), respectively, P < 0.01). The latency was also near-significantly affected by a treatment × reward type interaction (P = 0.05), mostly due to the morphine treated ewes approaching the locations faster after receiving the food reward (log-transformed latencies (s): 0.82 ± 0.07 for control sheep after consuming food, 0.71 ± 0.07 for morphine treated sheep after food, 0.96 ± 0.07 for naloxone treated sheep after food, 0.94 ± 0.07 for control sheep after wood chips, 1.07 ± 0.07 for morphine sheep after wood chips and 1.00 ± 0.07 for naloxone sheep after wood chips). These results show that consumption of a food reward induced an optimistic judgement bias and suggest that morphine administration further enhanced this optimistic judgement bias. However, animal numbers and statistical power were low in this experiment and further research is necessary to confirm whether the opioid system is involved in the formation of judgement biases.