Effects of signalled reward type, food status and a μ-opioid receptor antagonist on cue-induced anticipatory behaviour in laying hens (Gallus domesticus)

Studies using classical conditioning have shown that hens display high frequencies of dopamine-controlled cue-induced anticipatory behaviours in the cue-reward interval when signalling mealworm rewards. However, it is not known whether anticipatory behaviours are reward specific, and whether the opioid system is involved in their control. The purpose of the present study was to investigate (1) effect of incentive value of rewards, and (2) the involvement of μ-opioid receptor activation, on the expression of cue-induced anticipatory behaviours in laying hens. Incentive value was manipulated by reward type (mealworm and whole wheat) and by physiological state (sated and fasted hens). Hens (n = 14) were trained to associate a cue (green or red light) with a reward (whole wheat or mealworms). Blue light served as an unrewarded control stimulus. Cue-induced anticipatory head movements (latency to first head movement after cue presentation, and frequency of head movements in the cue-reward interval), steps (frequency), and pecking at reward (latency), were registered in sated and fasted hens during a 25 s cue-reward interval. An involvement of the opioid system in mediating cue-induced anticipatory behaviours was tested by intraperitoneal injection of the μ-opioid receptor antagonist naloxone at 5.0 mg kg−1. Saline served as control. Injections were administered 30 min before the light cues. Individual hens were tested on all treatment combinations: sated/saline, fasted/saline, sated/naloxone, and fasted/naloxone. Incentive value of signalled reward was differentially reflected by the frequency of cue-induced head movements (P < 0.0001). Hens displayed more head movements in response to signalled mealworms (33.1 ± 0.9) than signalled whole wheat (28.5 ± 1.1) and unrewarded cue (17.2 ± 1.0). The frequency of steps was higher in response to the cue signalling mealworms (12.2 ± 1.0) and whole wheat (9.9 ± 0.7) than to the unrewarded cue (6.3 ± 0.7, P < 0.0001 and P = 0.0003, respectively), but there was no difference between the rewarded cues. Latency to initiate the first head movement was longer in response to the unrewarded cue than to cues signalling whole wheat and mealworms (P = 0.051 (tendency) and P = 0.0017). Hunger amplified the frequency of head movements (P < 0.002) and tended to affect frequency of steps (P < 0.072). No effects of treatment with naloxone were found as tested here. In conclusion, cue-induced anticipatory head movements reflect incentive value of food rewards in laying hens. The role of opioid regulation of reward processes in hens needs to be further investigated.