The effects of dopamine agonists on rotational behavior in non-tolerant and caffeine-tolerant rats

Tolerance develops to caffeine-induced stimulation of both locomotor activity and rotational behavior. The role of dopamine in tolerance to the locomotor stimulant effects of caffeine has been documented. However, the role of dopamine in caffeine-induced turning behavior remains to be elucidated. Therefore, the present study determined the role of dopamine receptors in tolerance to caffeine-induced rotational behavior. Rats with a unilateral lesion of the nigrostriatal tract, induced by 6-hydroxydopamine (6-OHDA), were treated chronically with either caffeine (1.0mg/ml) or with drug-free tap water, by a method of scheduled access. Agonists with and without selectivity for dopamine receptor sub-types were tested in both groups of rats (nonselective: apomorphine, d-amphetamine; D1 selective: SKF-38393, SKF-77434; D2 selective: R(-)-propylnorapomorphine (NPA), quinpirole). All drugs produced dose-dependent increases in turning that, with the exception of quinpirole, were comparable in both groups. Quinpirole produced a smaller effect in rats treated with caffeine than in control rats. Thus, there was significant cross-tolerance only to the effects of quinpirole. The concurrent administration of SKF-38393 with NPA produced a synergistic interaction on rotational behavior in control rats, to which cross-tolerance did not develop in caffeine-treated rats. In contrast to what occurs with locomotor activity, in control rats the selective D1 dopamine receptor antagonist SCH 23390 completely blocked SKF-38393-induced turning behavior and the selective D2 dopamine receptor antagonist eticlopride partially attenuated this effect. NPA-induced turning behavior was blocked only by eticlopride; SCH 23390 was without effect. Both SCH 23390 and eticlopride blocked d-amphetamine-induced rotational behavior. The results of this study suggest that D1 dopamine receptors are not involved in tolerance to caffeine-induced rotational behavior. The role of D2 dopamine receptors in this effect is unresolved. Results obtained from rotational behavior studies generally do not parallel those obtained from locomotor activity studies, suggesting that different mechanisms underlie the effects of caffeine on these two behaviors.