Stimulant effects of apomorphine and (+)-amphetamine in rats with varied habituation to test environment

The stimulants (+)-amphetamine and apomorphine are known to increase motility and induce stereotypy in rats. The present study examined the effects of an habituation period immediately prior to injection on these stimulant effects. Male Wistar rats received doses of either drug including 0, 0.01, 0.1, 0.25, 1.0, 2.0 and 4.0 mg/kg in a random order. Activity was assessed in 6 automated chambers where horizontal and vertical activity were tabulated hourly for 4 hours. Initially all rats had equal exposure to the chambers over 5 days. In the subsequent drug phase, habituated rats were placed in the chambers for the 1-hr period prior to each injection whereas non-habituated rats were in their home cages at the corresponding time. (+)-Amphetamine stimulated horizontal activity, although under either condition the effect was not seen until the second hour post-injection, but lasted until the fourth hour. Vertical activity was similarly enhanced, but with habituation there was a significant stimulant effect in the first hour as well. With apomorphine the habituation period resulted in an absence of a significant stimulant effect. Non-habituated rats showed a significant stimulant effect with the highest dose only on vertical activity in the first hour and a stimulant effect with horizontal activity in the second hour. It is suggested that the relative novelty of the environment affected the behavioral response to apomorphine but not to (+)-amphetamine. Furthermore, the activating effects accompanying drug administration should be taken into account as a factor affecting responses to drugs. Such environmental factors may be of particular importance when considering drugs with a short duration of action.