Positive affective state induced by opioid analgesia in laying hens with bone fractures

Laying hens with keel fractures (n = 35) and control hens with no fractures (n = 12) were trained to associate the colour and position of an environment with the effects of either a subcutaneous injection of an opioid analgesic drug (butorphanol) or a subcutaneous injection of saline in a conditioned place preference experiment. Each hen experienced 12 post-injection 30-min exposures to a specified environment over a period of 3 days to allow an association to form. After a 24 h drug clearance period the procedure was repeated. Hens that had initially been given butorphanol and experienced its effects in an environment of one colour, were now given saline paired with the environment of the alternative colour. Similarly, hens that had initially been given saline now received butorphanol. Following this sequential conditioning procedure all hens made 7 choices in a drug-free state between the two coloured environments previously experienced, using a T-maze preference test. Choice data were analysed using a random effect logistic regression model that accounted for bird identity, batch and fracture status. Birds with healed keel fractures preferred the environment where they had experienced the drug, a significantly different pattern of choice from the non-fractured birds that exhibited no preference. That the conditioned place preference was shown only by the fractured birds suggests it was the analgesic properties of butorphanol that were rewarding, and provides further evidence that healed keel fractures are a source of chronic pain. (C) 2013 Elsevier B.V. All rights reserved.