Effect of morphine administration on social and non-social play behaviour in calves

The objective of this study was to evaluate the effect of morphine on social and non-social play behaviour in calves. Twelve calves experienced four treatments in a cross over 2 x 2 factorial design: calves received an intravenous injection of morphine or saline 10 min prior to being tested individually or in pairs in an arena test for 20 min. Play behaviour was continuously recorded in the arena test. Lying times were recorded in the home pen. Cortisol concentrations were measured to evaluate if opioids play a role in regulating the hypothalamus-pituitary-adrenal axis in calves. In the arena test, calves given morphine performed more social play events than calves given saline, however, morphine administration had no effect on locomotor play. Calves given morphine spent less time lying than calves given saline during the first 4 h after returning to the home pen. Cortisol concentrations were supressed in calves given morphine regardless of whether they were tested individually or in pairs. Administration of morphine appeared to increase social play but have no effect on locomotor play in calves. Therefore, increased social rather than changes in locomotor play may be more indicative of a positive affective state in calves.