A comparison of the effects of amphetamine and low doses of apomorphine onoperant force production, inter-response times and response duration in rat

Rationale: Low doses of apomorphine (APO), a non-selective dopamine (DA) ag onist, are thought to suppress motor activity via the preferential activati on 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 sequentia lly three separate beams under the following different behavioral requireme nts: low-force beam (1 g<force<3 g), high-force beam (force>50 g), and a lo ng-duration beam (response duration>2 s). Inter-response times and kinetic measures, such as peak force, the rate of rise of force and response durati on, were recorded. Following training, performance was assessed after syste mic 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 h igh-force and the long-duration beams by decreasing the rate of rise of for ce, but did not affect performance success on the low-force beam or respons e 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 ex ecution 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 f ood was present. In contrast, AMP at 1.0 mg/kg shortened both the time take n to switch between responses and the time spent visiting the food tray. Co nclusions: Low doses of APO interfered with response initiation and executi on, suggesting that dopamine acts as a "gating" system, enabling certain pr ocesses to be carried out in an efficient and automated manner.