A comparison of the effects of amphetamine and low doses of apomorphine on operant force production, interresponse times and response duration in rat

Rationale: Low doses of apomorphine (APO), a non-selective dopamine (DA) agonist, are thought to suppress motor activity via the preferential activation of DA autoreceptors, which effectively reduces DA tone. Objectives: The suppressant effects on operant responding of low doses of apomorphine were explored and compared with the effects of amphetamine (AMP), an indirect DA agonist. Methods: In an operant task, rats were trained to press sequentially three separate beams under the following different behavioral requirements: low-force beam (1 g high-force beam (force>50 g), and a long-duration beam (response duration>2 s). Inter-response times and kinetic measures, such as peak force, the rate of rise of force and response duration, were recorded. Following training, performance was assessed after systemic injection of low doses of APO (0.01, 0.03 and 0.1 mg/kg, s.c.) and AMP (0.1, 0.3 and 1.0 mg/kg, i.p.). Results: APO decreased peak force for the high-force and the long-duration beams by decreasing the rate of rise of force, but did not affect performance success on the low-force beam or response duration on the long-duration beam. This indicates that APO impaired the ability to generate high forces but did not interfere with the memory or execution of an overall motor plan. Low doses of APO also increased the times taken to switch from one response to the next and to visit the tray when food was present. In contrast, AMP at 1.0 mg/kg shortened both the time taken to switch between responses and the time spent visiting the food tray. Conclusions: Low doses of APO interfered with response initiation and execution, suggesting that dopamine acts as a ”gating” system, enabling certain processes to be carried out in an efficient and automated manner. Received: 17 April 1998 / Final version: 16 March 1999