Environmental and behavioral components of sensitization induced by the dopamine agonist quinpirole

Chronic intermittent injection of quinpirole (0.5 mg/kg) to rats in a large non-standard open field (mirrored glass table without walls, 160 x 160 cm and 60 cm high) induces pronounced behavioral sensitization characterized by a 6-fold increase in locomotor distance and increased rigidity of travel along the same routes (path stereotypy). Experiment 1 showed that equivalent treatment in the home cage induces much less sensitization of locomotor distance and no sensitization of path stereotypy, as evidenced by a test in the open field. In Experiment 2, transferring rats sensitized in one open field to a novel open field resulted in a 50% loss of sensitized locomotor distance and a virtual loss of sensitized path stereotypy. In Experiment 3, rotating the open field in relation to room cues did not affect sensitized responding, suggesting that the behavior is organized in relation to distal rather than local (open field) cues. Finally, an injection of saline in the sensitized environment failed to elicit conditioned locomotion (Experiment 4). These results are taken to indicate that the control of sensitization to quinpirole has components that are environment independent, behavior specific, and context dependent, each having a relatively different contribution and mechanism. It is suggested that under the experimental conditions of this study, the relative contributions to quinpirole sensitization were 50% for the context-dependent component, 30% for the behavior-specific component, and 20% for the environment-independent contribution. The mechanism for the context-dependent component may be related to development of path stereotypy and involve spatial learning.