Indeed, in an animal model for vulnerability to drug abuse, animals that exhibit greater motor activity in a novel environment (high responders; HR) are found more sensitive to drugs of abuse and are more likely to self-administer these drugs compared to less reactive animals (low responders; LR). In the light of clinical evidence on comorbidity between drug abuse and mood disorders, this model is widely used to investigate whether individual differences in locomotor reactivity to novelty are related to anxiety- and depression-like responsiveness in rodents.