Dopamine on D2-like receptors "reboosts" dopamine D1-like receptor-mediated behavioural activation in rats licking for a isotonic NaCl solution.

RATIONALE

We recently suggested that dopamine on D1-like receptors is involved in the activation of goal-directed responses and the level of response activation is "reboosted" on the basis of an evaluation process involving D2-like receptors assessing "response efficacy". A main piece of evidence in support of this hypothesis was the observation of an "extinction mimicry" effect in the time course of licking bursts after dopamine D2-like receptor blockade in rats licking for sucrose.

OBJECTIVES

The aim of this study was to determine whether the pattern of licking observed with sucrose as a reward could be reproduced in rats licking for a different reward (0.9% NaCl).

MATERIALS AND METHODS

We investigated the effects of the dopamine D1-like receptor antagonist SCH 23390 (0.01-0.04 mg/kg) and of the dopamine D2-like receptor antagonist raclopride (0.025-0.25 mg/kg) on the microstructure of licking for a 0.9% NaCl solution in 12-h water-deprived rats in 30-min sessions.

RESULTS

As previously observed with sucrose as a reward, raclopride reduced the size of licking bursts and produced on the burst number time course an "extinction mimicry" effect, while SCH 23390 reduced licking exclusively by reducing burst number.

CONCLUSIONS

These results are consistent with the proposed hypothesis and provide support to the use of the study of licking microstructure as a valid model not only for the investigation of the mechanisms governing ingestive behaviour but also for the investigation of the mechanisms underlying behavioural activation and the related evaluation processes.