Abstract

Dose-response curves for d-amphetamine were determined in the rat for three distinct types of behavior: continuous avoidance responding, locomotor activity and food intake. The effects of d-amphetamine were also evaluated on locomotor activity and food intake in the mouse. d-Amphetamine was tested alone and in combination with naloxone, a specific narcotic antagonist that is almost devoid of agonistic properties. Characteristic amphetamine effects were observed in both species. Avoidance responding and locomotor activity were increased by low doses of d-amphetamine and were disrupted by higher doses. A similar biphasic dose-response curve was generated for food intake in the rat, but d-amphetamine produced only a suppression of food intake in the mouse. Otherwise inactive doses of naloxone significantly and consistently reduced the stimulant effects of d-amphetamine on avoidance responding and locomotor activity in the rat; no consistent drug interactions occurred in the locomotor activity tests in the mouse. There were no interactions between naloxone and d-amphetamine on food intake in either species. However, in the rat, naloxone itself showed pronounced activity in suppressing food intake. The results show that a narcotic antagonist can markedly modify the behavioral effects of a non-opioid psychoactive drug. Furthermore, it appears that some of the actions of naloxone are species dependent.