Effects of intraaccumbens microinjections of quinopirole on head turning and circling movement in the rat.

This study was designed to evaluate whether nucleus accumbens dopamine D2 receptors are involved in the initiation of the movement, as distinguished from its execution. For this purpose, the effects of the quinpirole-induced increase of nucleus accumbens dopamine D2 receptor activity were observed on specific parameters of the circling behavior and of its first stage, the head-turning (HT) movement. The experiments were performed on rats with unilateral 6-hydroxydopamine (6-OHDA) lesion of the pars compacta of the substantia nigra and d-amphetamine i.p. (3 mg/kg). Bilateral intraaccumbens microinjections of quinpirole (1, 5, and 10 microg/0.5 microl), an agonist of the D2 receptor family, were performed on three groups of animals. Bilateral saline (0.5 microl) was injected in a fourth group as control. An additional control experiment, with quinpirole (10 microg/0.5 microl) bilaterally injected in accumbens without d-amphetamine i.p., was also performed in a further group of 6-OHDA-lesioned animals. By means of a videoanalysis system, HT duration, angle, and speed were analyzed. Modifications of the circling rate (increase), HT duration (decrease), HT angle (decrease or increase according to the dose), and HT speed (increase) were observed. Moreover, a very close head-to-tail position and a very short-diameter type of turn were also evidenced. Similar modifications, even if different in amplitude and in % distribution, were observed following bilateral quinpirole in accumbens without d-amphetamine i.p. The results indicate a close relationship among head-turning speed, type of turn, and position of the animal in the circling motor sequence. We conclude that D2 receptor family in nucleus accumbens is involved in the initiation of movement as distinguished from its execution.