89. Lipopolysaccharide reduces incentive motivation while boosting preference for high reward in mice

Abstract

Inflammation has been implicated in the development of various disorders, including fatigue and depression. However, the neurobehavioral mechanism involved in this relationship remains elusive. This gap in knowledge may best be filled by evaluating elementary neurobehavioral processes affected by inflammation rather than just behavioral changes in conventional animal tests of depression. To this end the current study used a concurrent choice paradigm to evaluate inflammation-induced alterations in incentive motivation. Mice were food restricted to between 85% and 90% of their free feeding weight and were trained to perform a concurrent choice task where they nose poked for grain rewards on a fixed ratio 1 schedule (low effort/low reward) and chocolate-flavored rewards on a fixed ratio 10 schedule (high effort/high reward). A counterbalanced within subjects design was used. A single injection of 0.33 mg/kg lipopolysaccharide was used to induce peripheral inflammation. Twenty-four hours after lipopolysaccharide administration mice showed a reduction in the total number of nose pokes. A proportionally greater reduction in nose pokes was observed for grain, resulting in an increase in percent chocolate pellets earned. These behavioral changes are not due to reduced appetite as pre-feeding led to a similar increase in percent chocolate pellets earned but without any decrease in responding. These results indicate that inflammation modulates incentive motivation by affecting willingness to exert effort for reward and not by reducing sensitivity to reward.