Feeding-induced tonic pain suppression in the chicken: reversal by naloxone.

Endogenous analgesia induced by changes in motivation has been identified in the chicken in previous studies but either the motivational changes were difficult to interpret or the motivation was unpredictable. Experimental sodium urate (SU) arthritis of the ankle joint resulted in pain-coping behaviour (one-legged standing or sitting) for a 2-h period in non-food-deprived birds without access to food. Complete analgesia or marked hypoalgesia was observed in birds which had been food deprived overnight and given access to food immediately after SU injection. This analgesia seen during feeding behaviour in the food-deprived bird could be completely reversed by intravenous injection of naloxone. These results demonstrate that feeding motivation can totally suppress, in some animals, the severe tonic pain of SU arthritis and that this analgesia may be opioid mediated.