Unilateral injections of a D2 but not D1 agonist into the frontal cortex of rats produce a contralateral directional bias.

Unilateral manipulations of frontocortical dopamine have been found in previous studies to produce a directional bias in the circling behaviour of rats. Agonists produced contralateral circling and antagonists produced ipsilateral turning. To examine the role of dopamine receptor subtypes, the present studies investigated the ability of unilateral intrafrontal cortical microinjections of the D1 agonist, SKF 38393 or the D2 agonist, quinpirole to produce contralateral circling in rats. The antagonist, cis-flupenthixol was also tested and was expected to produce ipsilateral circling. In 3 separate experiments, rats received 7 50-min sessions in a circular arena separated by at least 48 hr. The first and final sessions were preceded by no injection, the second and sixth by saline [or the inactive trans isomer (2.5 micrograms) in the flupenthixol experiment] and the middle 3 sessions by doses of cis-flupenthixol (1, 10, 25 micrograms in 0.5 microliter), quinpirole (3, 6, 12 micrograms) or SKF 38393 (2, 4, 8 micrograms), the order being counterbalanced across rats. cis-Flupenthixol and quinpirole produced dose-dependent ipsi- and contralateral circling, respectively, whereas SKF 38393 was without significant effect. No reliable directional bias was seen in any no-injection, saline or trans-flupenthixol sessions. Results suggested that the D2 receptor may mediate the motor effects of frontal cortical dopamine.