Dopamine on D2-like receptors is involved in reward evaluation in water-deprived rats licking for NaCl and water 1

The analysis of licking microstructure provides measures, size and number of licking bouts, which might reveal, respectively, reward evaluation and behavioural activation. Based on the ability of the dopamine D2-like receptor antagonist raclopride to reduce bout size and to induce an “extinction mimicry effect” on bout number, we suggested that the level of activation of reward-associated responses is updated, or “reboosted”, on the basis of a dopamine D2-like receptor-mediated evaluation process occurring during the consummatory transaction with the reward. Here we investigate the effects of the dopamine D2-like receptor antagonist raclopride (0, 25, 125, and 250 µg/kg) on the microstructure of licking for water and sodium chloride solutions (0.075 M, 0.15 M, and 0.3 M) in 12 h water-deprived rats. In each session, rats were exposed to brief contact tests (1 min) for each solution. Bout size, but not bout number, was decreased at the highest NaCl concentration. Raclopride reduced lick number owing to reduced bout size, while bout number was either not affected or even increased depending on the dose. These results are in agreement with the previous observations on sucrose licking, and suggest the involvement of dopamine D2-like receptors in an evaluation process occurring during the consummatory transaction with the reward.