Rationale: Although leptin receptors are found in hypothalamic nuclei classically associated with homeostatic feeding mechanisms, they are also present in brain regions known to regulate hedonic-based feeding, natural reward processing, and responses to drugs of abuse. The ob/ob mouse is deficient in leptin signaling, and previous work has found altered mesolimbic dopamine signaling and sensitivity to the locomotor activating effects of amphetamine in these mice.
Objectives: We directly assessed responses to three drugs of abuse and non-drug rewards in the leptin-deficient ob/ob mouse.
Methods: Ob/ob mice were tested in assays of sweet preference, novelty seeking, and drug reward/reinforcement.
Results: In assays of novelty seeking, novel open field activity and operant sensation seeking were reduced in ob/ob mice, although novel object interaction and novel environment preference were comparable to wild types. We also found that ob/ob mice had specific phenotypes in regard to cocaine: conditioned place preference for 2.5 mg/kg was increased, while the locomotor response to 10 mg/kg was reduced, and cocaine self-administration was the same as wild types. Ob/ob mice also acquired self-administration of the potent opioid remifentanil, but breakpoints for the drug were significantly reduced. Finally, we found significant differences in ethanol drinking in ob/ob mice that correlated negatively with body weight and positively with operant sensation seeking.
Conclusions: In conclusion, ob/ob mice displayed task-specific deficits in novelty seeking and dissociable differences in reward/reinforcement associated with cocaine, remifentanil, and ethanol.
Keywords: Alcohol; Cocaine; Leptin; Operant sensation seeking; Opioid; Saccharin; Self-administration; Sucrose.