Exploration and motor activity after intraventricular ACTH, morphine and naloxone

Intraventricular injection of morphine (5 microgram) and ACTH (2.5 microgram) reduced the time spent head-dipping and the number of head-dips made by rats during a 10-min trial in a holeboard. These effects were reversed by naloxone (2.0 mg/kg, i.p.), in spite of the fact that naloxone alone also reduced these measures of exploration. None of the drugs or drug combinations tested significantly reduced locomotor activity. The results are discussed in relation to the action of ACTH on opiate receptors.