Research reportThe attribution of incentive salience to an appetitive conditioned cue is not affected by knockout of the serotonin transporter in rats

•Attribution of incentive salience to conditioned stimuli & motivation for reward were studied.•Serotonin transporter knockout rats were compared with wild-type counterparts.•Knockout did not affect conditioned stimulus salience attribution.•Knockout animals showed an increased motivation for reward.

Understanding the neurobiological basis underlying individual differences in conditioned stimulus (CS) sensitivity is pertinent, given that excessive conditioned responses to CSs is a key feature of anxiety-related disorders and drug addiction. We have previously shown that behaviour of serotonin transporter knockout (5-HTT−/−) rats-mimicking the common 5-HTT promoter polymorphism in humans-is strongly driven by Pavlovian CSs. To investigate whether the knockout rats attribute greater incentive salience to CSs, we tested the 5-HTT−/− rats and their wild-type counterparts in the sucrose-reinforced sign-versus goal-tracking task. We also assessed whether motivational properties of the unconditioned stimulus (sucrose pellet) are involved in the individual differences under investigation, by testing the animals in a sucrose-reinforced progressive ratio schedule of reinforcement. We found no genotype differences in sign-versus goal-tracking behavior, despite that progressive ratio responding was increased in 5-HTT−/− rats. In conclusion, the high CS sensitivity in 5-HTT−/− rats cannot be explained by enhanced incentive salience attribution to the CS as measured by the sign- versus goal-tracking paradigm. Rather, 5-HTT−/− rats may be more sensitive to the motivational properties of the unconditioned stimulus.