Stability and variability of locomotor responses of laboratory rodents. IV. The responses of rats and mice to apomorphine and amphetamine.

Both apomorphine and amphetamine produce hypermotility in mice and rats previously adapted to the measuring apparatus. The amphetamine hypermotility, induced by 1.5 mg/kg ip of the drug, began to develop after 20 min and lasted for over 1 h. The response was similar in animals tested at various times of the year. In the investigated populations approx. 20% of rats were very highly stimulated, and approx. 15% did not respond with hypermotility. The individual response was a stable feature of a given rat, as the correlation of results of subsequent tests was significant. Apomorphine (1 mg/kg sc) stimulates the locomotor activity of the rat immediately after the injection. The seasonal variability of response was high. The results of subsequent tests were correlated. The differences between animals were less distinct than those observed after amphetamine. In mice hypermotility induced by apomorphine (1 mg/kg sc) began to evolve after 10 min, the amphetamine hypermotility - after 40 min. There exist subpopulations reacting very strongly to either of the drug. The responses in subsequent tests were well correlated. In mice the second injection of amphetamine (after 4 days) produced a much higher locomotor response than the first injection, while in all other cases (apomorphine in mice, both dopaminomimetics in rats) the magnitude of responses in subsequent tests was similar. No correlations between the type of responses to the drugs and characteristics of central catecholaminergic receptors were found.