Addictive agents and intracranial stimulation (ICS): novel antagonists and agonists of morphine and pressing for ICS.

Rats fixed with chronically indwelling bipolar electrodes pressed for intracranial stimulation (ICS) of the lateral hypothalamus during daily sessions. The effects of two antagonists of morphine (Win 44,441 and naloxone) were then assessed. Naloxone (10 mg/kg) produced its characteristic reduction in pressing. Win 44, 441 produced a reliable increase in pressing at doses as small as 1 mg/kg. Large doses of morphine (10 mg/kg) produced its characteristic effects: depression in pressing when given 1 hr before the test session and facilitation when given 3 hr before the test session. Win 44,441 antagonized morphine's depressive effects. Other compounds (Win 44,156, Win 42,156), having similar structure to Win 44,441 but having agonist and mixed agonist-antagonist activity with respect to analgesia, also facilitated pressing for ICS. All three compounds' effects on pressing for ICS were antagonized by naloxone. It is inferred that opioids' facilitatory effects on pressing for ICS are separable from opioids' other capabilities such as production of analgesia.