Effects of methamphetamine and methylphenidate on single and paired rat open-field behaviors.

The present study used a video analysis system to investigate the effects of indirect dopamine agonists, methamphetamine and methylphenidate, on single and paired rat open-field behaviors. Experimental rats were injected with saline, methamphetamine, or methylphenidate (0.008, 0.04, 0.2, 1.0, 5.0 mg/kg, IP) and put into an open field for 30 min singly or paired with another rat. The rats' movements were plotted on XY coordinates by a video analysis system every 0.5 s and analyzed by distance and time factors. When paired, the experimental rats moved more than when single, and the total move distance increased according to dose of methamphetamine or methylphenidate. Further analysis showed that under methamphetamine, fleeing distances increased significantly but chasing distances were not affected; under methylphenidate, fleeing distances also increased, but chasing distances decreased significantly. Although two rats in an open field often stand in contact with each other, both methamphetamine and methylphenidate in doses exceeding 1.0 mg/kg tended to keep the two rats separate. These phenomena indicate that the two indirect dopamine agonists induce less interaction and decrease social behavior.