Dopamine D-2 receptor mediation of response suppression learning of young rats

In three experiments, the effects of augmenting or blocking dopamine (DA) D-2 receptor activity on the ontogeny of response suppression learning of preweanling rat pups were determined. In the initial experiment, rat pups were trained to traverse a straight alley for nipple attachment to an anesthetized dam. When footshock (0.2 mA, 0.5 sec) was made contingent on responding, younger (11- and 13-day-olds) rat pups were deficient to older (17- and 19-day-olds) pups at withholding punished responding. In the subsequent experiments, response suppression learning was assessed after injecting 11- and 17-day-old rat pups with the specific DA D-2 agonist, LY 171555 (0.005-, 0.01-, and 0.1-mg/kg, i.p.), or the specific DA D-2 antagonist, sulpiride (5.0-, 15.0-, and 50.0-mg/kg, i.p.). LY 171555 enhanced the punished responding of both the 11- and 17-day-old rat pups; whereas, sulpiride increased the punished responding of the 17-, but not the 11-day-olds. In four additional experiments, the effects of LY 171555 and sulpiride on the locomotor activity, nociception, and reinforcement processes of 17-day-old rat pups was assessed. Rat pups given LY 171555 (0.01 mg/kg, i.p.) exhibited enhanced locomotor activity and a trend towards hyperanalgesia using a hot plate task. Sulpiride (15.0 mg/kg, i.p.) completely antagonized LY 171555's activity enhancing effects and had hyperalgesic properties. In two experiments, sulpiride did not affect the nonpunished appetitive responding of the 17-day-olds; whereas, haloperidol-treated pups responded on fewer reinforced trials than did saline-treated pups. Therefore, these results indicate that the response suppression learning of 17-day-old rat pups is mediated, at least partially, by a DAD-2 receptor system, and that D-2 receptors are also involved in the locomotor activity and nociceptive responses of young rat pups.