Differential effects of the selective D2-antagonist raclopride in the nucleus accumbens of the rat on spontaneous and d-amphetamine-induced activity.

The effect of the D2-antagonist raclopride was investigated in two test situations, which are presumed to involve dopamine (DA) transmission within the nucleus accumbens of the rat. Local injection of d-amphetamine sulphate (10 micrograms/0.5 microliter) produced a marked increase in motor activity, measured as motility, locomotion, and rearing, which was dose- and time-dependently antagonised by local injection of raclopride (0.05-5.0 micrograms/0.5 microliter). After an initial decrease, at low doses (0.05-0.25 microgram/0.5 microliter) an apparent enhancement of the d-amphetamine-induced motor activity appeared, which was most clearly seen with rearing. These lower doses, however, did not induce any clear changes in the exploratory activity in a novel environment (i.e., the second test situation). Only the higher doses used (1.0-5.0 micrograms/0.5 microliter) decreased exploratory activity during the first 5-10 min, also measured as motility, locomotion, and rearing. These data are discussed with respect to the role of D2-receptors within the nucleus accumbens of rats in the motor activity induced by a novel environment and d-amphetamine. Overall, the data underline previous notions that raclopride is a potent antagonist of DA-mediated behaviour.