Possible role of dopamine D1-like and D2-like receptors in behavioural activation and evaluation of response efficacy in the forced swimming test.

Based on the different effects of the dopamine D1-like and D2-like receptor antagonists SCH 23390 and raclopride on the measures of licking microstructure in rats ingesting a sucrose solution, we suggested that the behavioural activation of reward-associated responses depends on dopamine D1-like receptor stimulation, and its level is updated, or "reboosted", on the basis of a dopamine D2-like receptor-mediated evaluation process. The aim of this study was to test this hypothesis on the forced swimming test response. The effects of the dopamine D1-like and D2-like receptor antagonists SCH 23390 (0.01-0.04 mg/kg) and raclopride (0.025-0.25 mg/kg) administered before a 15-min exposure to forced swimming, and the response to a second session performed 24 h later, were examined. SCH 23390 dose-dependently reduced climbing scores in the first session and increased them in the second session, but the within-session decline of this measure was similar to that observed in the control group in both sessions. Raclopride-treated subjects showed a slightly reduced level of climbing scores at the beginning of the first session, but persisted in emitting this costly behavioural response up to the end of the session, while no effects were observed in the second session. These results, along with our results examining licking for sucrose, are consistent with the hypothesis that behavioural activation and response effort allocation are directly mediated by dopamine D1-like receptor stimulation, but the level of this activation is updated, or "reboosted", on the basis of a dopamine D2-like receptor-mediated mechanism of response efficacy evaluation.