ΔFosB Enhances the Rewarding Effects of Cocaine While Reducing the Pro-Depressive Effects of the Kappa-Opioid Receptor Agonist U50488

Background: Elevated expression of the transcription factor Delta FosB accompanies repeated exposure to drugs of abuse, particularly in brain areas associated with reward and motivation (e. g., nucleus accumbens). The persistent effects of Delta FosB on target genes might play an important role in the development and expression of behavioral adaptations that characterize addiction. This study examines how Delta FosB influences the responsiveness of the brain reward system to rewarding and aversive drugs. Methods: We used the intracranial self-stimulation paradigm to assess the effects of cocaine in transgenic mice with inducible overexpression of Delta FosB in striatal regions (including nucleus accumbens and dorsal striatum). Mice implanted with lateral hypothalamic stimulating electrodes were trained with the "rate-frequency" procedure for intracranial self-stimulation to determine the frequency at which stimulation becomes rewarding (threshold). Results: A dose-effect analysis of cocaine effects revealed that mice overexpressing Delta FosB show increased sensitivity to the rewarding(threshold-lowering) effects of the drug, compared with littermate control subjects. Interestingly, mice overexpressing Delta FosB were also less sensitive to the pro-depressive (threshold-elevating) effects of U50488, a kappa-opioid agonist known to induce dysphoria and stress-like effects in rodents. Conclusions: These data suggest that induction of Delta FosB in striatal regions has two important behavioral consequences-increased sensitivity to drug reward, and reduced sensitivity to aversion-producing a complex phenotype that shows signs of vulnerability to addiction as well as resilience to stress.