Rationale: It is necessary to understand better how chronic food restriction (CFR) and psychostimulant drugs interact in motivated behavior unrelated to food or energy homeostasis.
Objectives: We examined whether CFR augments methylphenidate (MPH)-potentiated responding reinforced by visual sensation (VS) and whether repeated MPH injections or prolonged CFR further augments such responses.
Methods: Before starting the following experiments, rats on a CFR diet received a limited daily ration in such a way that their body weights decreased to 85-90% of their original weights over 2 weeks. In experiment 1, rats on CFR and ad libitum diet received four injections of varying MPH doses (0, 2.5, 5, and 10 mg/kg). In experiment 2, CFR and ad libitum groups received repeated injections of MPH (2.5 mg/kg). In experiment 3, half of CFR rats received repeated injections of MPH (2.5 mg/kg), and the other half received saline, and following a 7-day abstinence, they all received the 2.5-mg/kg dose of MPH.
Results: CFR rats increased VS-reinforced responding more than ad libitum rats when they received MPH. Repeated injections of MPH with prolonged CFR further increased VS-reinforced responding. We found a double dissociation where prolonged CFR (3 vs. 6 weeks) made VS-reinforced responding, but not locomotor activity, more responsive to MPH, whereas repeated MPH injections made locomotor activity, but not VS-reinforced responding, more responsive to MPH.
Conclusions: CFR markedly potentiates effects of MPH on VS-reinforced responding. The present study demonstrates that the longer CFR continues, the greater psychostimulant drugs augment behavioral interaction with salient stimuli.
Keywords: Apathy; Approach motivation; Behavioral sensitization; Caloric restriction; Instrumental reinforcement; Ritalin; Visual stimulus.