BIPHASIC ACTION OF PENTAZOCINE IN MORPHINE-DEPENDENT RATS

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The characteristic actions of pentazocine in morphine-dependent rats were investigated by a drug-admixed food (DAF) method. Pentazocine did not cause evident withdrawal signs when stopped after a continuous administration for 2 months at 3 different dose levels. A state of physical dependency on morphine was produced in rats by feeding them for 3 weeks with food that contained different levels of morphine. Animals exhibiting a moderate degree of morphine-withdrawal signs (17-18 hr after withdrawal) received a s.c. cross-administration with pentazocine at 0, 5, 10, 20, 40, 80, and 150 mg/kg. This drug at 20 mg/kg proved most supressive of morphine-withdrawal signs, being about 1/4 as potent as codeine. In a dose range from 20 to 40 mg/kg, the action of pentazocine on the withdrawal signs was reversed, that is, doses not less than 40 mg/kg exerted a dose-related antagonistic action in all the 3 levels of morphine dependent rats. The challenge with levallorphan (2 mg/kg, s.c.) during the chronic application of pentazocine revealed slight withdrawal signs. These findings show that pentazocine has biphasic action on morphine dependence and suggests that this type of drug must have different properties from morphine type drugs.
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