Classic pharmacological theories predict that opioid antagonists should block the neurochemical and behavioural effects of opioids. Surprisingly, this relationship does not hold true when the antagonist is administered in very low doses (i.e., nanograms instead of milligrams/kg). Coadministration of ultralow doses of naltrexone (antagonist) and morphine (agonist) enhanced morphine’s analgesic effects, which have been attributed to the activation of mu opioid receptors. Morphine inducedcatalepsy, characterized by muscular rigidity and inhibition of postural support systems, is also mediated by mu opioid receptors and can be blocked by standard doses of naltrexone. Our study investigated the hypothesis that ultralow doses of naltrexone will enhance morphineinduced catalepsy. Rats (N = 56) were randomly assigned to six different groups: saline, morphine (10 mg/kg), cotreatments of morphine (10 mg/kg) plus naltrexone (molar ratios of 1 000 000:1, 500 000:1 or 100 000:1) or naltrexone alone. For seven consecutive days, rats were administered one injection daily. Each day, catalepsy and analgesia were assessed 30 and 60 min post injection using the bartest and tailflick test, respectively. Ultralow doses of naltrexone coadministered with morphine did not potentiate catalepsy or attenuate tolerance. In contrast, ultralow doses of naltrexone coadministered with morphine significantly and dosedependently attenuated tolerance to morphine’s analgesic effect in comparison to morphine alone. These data suggest that the enhancement of opioid analgesic effects and attenuated tolerance by ultralow doses of opioid antagonists are not the result of changes in morphineinduced catalepsy. (Funded by NSERC)
C7β-Methyl Analogues of the Orvinols: The Discovery of Kappa Opioid Antagonists with Nociceptin/Orphanin FQ Peptide (NOP) Receptor Partial Agonism and Low, or Zero, Efficacy at Mu Opioid Receptors
