Opioid reward systems may influence willingness to walk for food in a motivation test

Abstract

The measurement of strength of motivation has become a frequently used tool to assess the resources that an animal values and, subsequently, to aid in determining its welfare. However, the relationship between animal motivation and welfare state is not well defined with conclusions based on the assumption that welfare is reduced if a 'valued' resource is not provided. One way to better identify the relationship between the level of motivation and welfare state is by investigating the determinants which underlie changes in motivation. This study therefore aimed to test the hypothesis that manipulation of the opioid system, through administration of the antagonist naltrexone (NTX), would decrease the willingness of sheep to walk for food in a demand test and that this decrease could be modulated by the energy density of the food. Ten sheep were trained in a 50m U-shaped laneway to access a double-sided feeder and gained a reward with each access event. NTX was administered at 1mg/kg and sheep were tested to see how many times in a 20h period they would walk a specific distance for a 4g food reward (high energy (HE) or low energy (LE)). The distance the sheep walked varied between 1.5-70m and each sheep was exposed to four random distances at each of the four treatment levels (NTX + HE, no NTX + HE, NTX + LE, no NTX + LE). Data were analysed using REMLs and the preliminary results indicate that significant effects of distance (p<0.001), food type (HE or LE p<0.005) and NTX (p<0.05) were present 6h after drug administration. Analysis shows that NTX increased the intake of HE food when compared to LE, although the difference did not reach significance (p>0.1). These preliminary results indicate that NTX does appear to decrease the willingness of sheep to walk for food in a demand test and that energy density and the opioid reward system may act independently to alter the motivation of a ruminant for food. However, further research will be necessary to confirm this.