DISCRIMINATIVE EFFECTS OF MORPHINE IN SQUIRREL-MONKEY

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  • 1 January 1977
    • journal article
    • research article
    • Vol. 201  (1) , 67-75
Abstract
Squirrel monkeys [Saimiri sciureus] were trained in a 2-choice discrete trial avoidance task to discriminate between i.m. injections of saline and 3.0 mg/kg of morphine. Morphine (0.1-10 mg/kg) produced a dose-related increase in the number of trials completed on the morphine-appropriate lever. The stimulus control produced by the discriminative effects of morphine met the following criteria for classification as a specific narcotic effect: morphine-like stimulus control was produced by all other narcotic analgesics tested (fentanyl, oxymorphone, levorphanol, methadone and meperidine); in so doing, these drugs spanned a 900-fold potency range relative to morphine; stimulus control was blocked by the specific narocotic antagonist naloxone; and stereospecificity was a requirement for stimulus control-levorphanol produced stimulus control equivalent to 3.0 mg/kg of morphine but its optical isomer dextrorphan did not. The time course of the stimulus control produced by 3.0 mg/kg of morphine showed that the animals continued to respond on the morphine-appropriate lever up to 14 h after morphine administration. Monkeys administered 0.01 mg/kg of fentanyl responded on the morphine lever for only as long as .5 h after fentanyl administration. Naloxone, d-amphetamine and pentobarbital all failed to substitute for morphine. Previous observations of the discriminative properties of morphine in rats were extended by demonstrating that qualitatively similar data are produced in a 2nd species, the squirrel monkey.