Operant learning is disrupted when opioid reward pathways are blocked in the domesticated hen

Abstract

There is limited research into mesolimbic function specific to birds and, specifically, how neurobiological reward mechanisms affect learning in domestic laying hens (Gallus gallus domesticus). A more thorough understanding of these mechanisms has implications for the improvement of welfare. Reward is proposed to involve two separate processes of ‘wanting’ regulated by dopamine and ‘liking’ regulated by opioids. This study examined the effect the opioid antagonist, nalmefene, on learning in domesticated hens.Eighteen Isa Brown hens were randomly assigned into either a saline or nalmefene treatment group. Hens in the saline group (N = 9) received 0.9 % saline (0.5 mL/kg pectoral muscle injection). Hens in the nalmefene treatment group (N = 9) were administered an opioid antagonist (0.4 mg/kg nalmefene in 0.9 % saline pectoral muscle injection) to disrupt the reward pathway. Prior to administration of treatments, hens were habituated to the training table, cup and trainer. Training occurred over three consecutive days on a series of three tasks using operant conditioning. Hens were dosed with either saline or nalmefene thirty minutes prior training. Hens were trained for one five minute session each day. Learning tasks included (1) pecking a target, (2) colour stimulus discrimination, and (3) pecking a target on cue presentation. All tasks were trained by the same trainer who was blind to treatment, using clicker training to shape responses. By the final day of training (Day 3), more saline hens had completed Task 1 and Task 2 than hens that received nalmefene (P < 0.0001). No hens in the nalmefene treatment group completed any training task. This study demonstrates that hens treated with an opioid antagonist failed to learn any tasks during operant training sessions, using a food reinforcer, over three days. We show that hens did not learn when the ability to experience reward is blocked, providing evidence that performing operant tasks is rewarding to hens.