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  • Title: Opioid agonists/antagonists in morphine-tolerant squirrel monkeys.
    Author: Dykstra LA, Picker MJ, Powell KR.
    Journal: Pharmacol Biochem Behav; 1990 Jul; 36(3):639-44. PubMed ID: 1974067.
    Abstract:
    The mu-opioid agonist morphine, the opioid antagonist naloxone and the isomers of the mixed action opioids, cyclazocine, n-allylnormetazocine, and pentazocine were examined in squirrel monkeys responding under a fixed-ratio 30 schedule of food presentation. Dose-effect curves for all drugs were obtained prior to, during, and following a chronic regimen in which monkeys received 6 mg/kg/day of morphine. When compared to the dose-effect curves obtained prior to the chronic regimen, the morphine dose-effect curve obtained during the chronic regimen was shifted to the right 0.5-1.0 log unit, whereas the naloxone dose-effect curve shifted over 3 log units to the left. No changes were observed between the prechronic and chronic dose-effect curves for (+)-cyclazocine, (+)-n-allylnormetazocine, and (+)- or (-)-pentazocine. The (-) isomers of n-allylnormetazocine and cyclazocine shifted 0.6-1.7 log units to the left. These results suggest that the (-) isomers of cyclazocine and n-allylnormetazocine have mu antagonist properties which are revealed during chronic morphine administration.
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