dextro- and levo-morphine attenuate opioid δ and κ receptor agonist produced analgesia in μ-opioid receptor knockout mice

We have demonstrated that the antianalgesia induced by dextro-morphine and levo-morphine is not mediated by the stimulation of μ-opioid receptors in male CD-1 mice. We now report that the dextro-morphine and levo-morphine attenuated antinociception produced by δ-opioid receptor agonist deltorphin II and κ-opioid receptor agonist U50,488H given spinally in the male μ-opioid receptor knockout mice. The tail-flick response was used for the antinociceptive test. Intrathecal injection of levo-morphine (3 nmol) markedly inhibited the tail-flick response in wild type, partially in heterozygous, but not in homozygous μ-opioid receptor knockout mice. Intrathecal pretreatment with dextro-morphine (33 fmol) or levo-morphine (0.3 nmol) for 45 min also attenuated levo-morphine-produced antinociception in wide type mice. Intrathecal pretreatment with dextro-morphine (33 fmol) or levo-morphine (0.3 nmol) for 45 min attenuated the tail-flick inhibition produced by deltorphin II (12.8 nmol) and U50,488H (123.3 nmol) in wide type, heterozygous and homozygous μ-opioid receptor knockout mice. The findings provide additional evidence that μ-opioid receptors are not involved in the antianalgesia induced by dextro-morphine and levo-morphine.