Morphine tolerance and dependence induced by intraventricular injection.

Injection of small quantities of morphine into the cerebral ventricular system of awake, relatively unrestrained, monkeys depressed or abolished operant food-reinforced lever pressing. After repeated injections progressively higher doses of morphine were needed to depress responding. Also, dependence could be demonstrated in these animals by precipitating specific abstinence signs with an antagonist.