Reduced apomorphine-induced sedation following chronic stress.

In agreement with previous reports, low doses of apomorphine (10-50 micrograms/kg, 5 min before test) produced a dose-dependent decrease in motor activity of control rats. This effect was markedly attenuated 24 h after a prolonged immobilization stress (2 h daily during 7 days). A single stress session had no effect on motor activity. These behavioral observations suggest a reduced sensitivity of presynaptic dopamine receptors after chronic stress.