Rationale: Basal and diet-induced differences in mesolimbic function, particularly within the nucleus accumbens (NAc), may contribute to human obesity; these differences may be more pronounced in susceptible populations.
Objectives: We examined differences in cocaine-induced behavioral plasticity in rats that are susceptible vs. resistant to diet-induced obesity and basal differences in striatal neuron function in adult and in adolescent obesity-prone and obesity-resistant rats.
Methods: Susceptible and resistant outbred rats were identified based on "junk-food" diet-induced obesity. Then, the induction and expression of cocaine-induced locomotor sensitization, which is mediated by enhanced striatal function and is associated with increased motivation for rewards and reward-paired cues, were evaluated. Basal differences in mesolimbic function were examined in selectively bred obesity-prone and obesity-resistant rats (P70-80 and P30-40) using both cocaine-induced locomotion and whole-cell patch clamping approaches in NAc core medium spiny neurons (MSNs).
Results: In rats that became obese after eating junk-food, the expression of locomotor sensitization was enhanced compared to non-obese rats, with similarly strong responses to 7.5 and 15 mg/kg cocaine. Without diet manipulation, obesity-prone rats were hyper-responsive to the acute locomotor-activating effects of cocaine, and the intrinsic excitability of NAc core MSNs was enhanced by ∼60 % at positive and negative potentials. These differences were present in adult, but not adolescent rats. Post-synaptic glutamatergic transmission was similar between groups.
Conclusions: Mesolimbic systems, particularly NAc MSNs, are hyper-responsive in obesity-prone individuals, and interactions between predisposition and experience influence neurobehavioral plasticity in ways that may promote weight gain and hamper weight loss in susceptible rats.
Keywords: Cocaine; Excitability; Nucleus accumbens; Obesity; Sensitization; Striatum.