British journal of pharmacology
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1. The relative spinal effectiveness of mu- and kappa-opioids has been assessed by their intravenous potencies on nociceptive responses (heat and/or pinch) of single motoneurones recorded in alpha-chloralose anaesthetized, spinalized rats. 2. The depressant actions of both mu- and kappa-opioids were reversed by low intravenous doses of naloxone (10 to 100 micrograms kg-1). ⋯ The agonist potency values obtained in this study contrast with those reported for local spinal administration. By this route, the potency of lipophilic opioids (e.g. fentanyl, U-50,488 and tifluadom) relative to hydrophilic opioids (e.g. morphine) is much reduced, implying that activity of intrathecally administered opioids is more dependent on the physico-chemical properties of the agonists used than on the relative abundance in the spinal cord of functional opioid receptors of the mu- and kappa-subtypes. This conclusion indicates that the results with locally applied opioids should not be used to assess spinal opioid receptor function.