Lipopolysaccharide does not alter behavioral response to successive negative contrast in mice

Reduced motivation is one of the main symptomatic features of inflammation-induced depression. However, the exact nature of inflammation-induced alterations in motivation remains to be fully defined. As inflammation has been shown to increase sensitivity to negative stimuli, the present series of experiments was initiated to determine whether systemic inflammation induced by infra-septic doses of lipopolysaccharide (LPS) in mice influences consummatory and instrumental responding to successive negative contrast.

Successive negative contrast was operationally defined by a shift to a lower value reward than the one mice were trained with. Mice were trained to drink a high sucrose concentration solution and exposed to an acute shift to a lower concentration of sucrose. In another series of experiments, mice were trained to nose poke for chocolate pellets according to a fixed reinforcement schedule 10 (10 nose pokes for the food reinforcement) and exposed to a shift to a lower reward value (decreased number of chocolate pellets or replacement of chocolate pellets by less preferred grain pellets). Lipopolysaccharide (LPS) was administered at the dose of 0.33 1 mg/kg 24 h before the shift.

Mice trained to drink a high sucrose concentration responded to the shift in reward value by a reduction in the volume of sucrose consumed and a decrease in lick numbers and bout durations. Mice trained to nose poke for chocolate pellets responded to the shift by alterations in their total number of nose pokes. In both conditions, LPS had no consistent effect on the response to the shift in reward value.

These findings indicate a high variability in the effects of LPS on successive negative contrast and fail to provide evidence in favor of the hypothesis that LPS increases sensitivity to decreases in expected rewards.