Interaction of chronic food restriction and methylphenidate in sensation seeking of rats.

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.