Abstract
The cold water tail-flick test in the rat is somewhat unique in that it is sensitive to the analgesic effects of δ- and k- in addition to μ-opioid agonists. The present study was designed to test whether a component of morphine-induced analgesia in this test might be mediated by δ- or k-opioid receptors. Morphine was administered icv in combination with the non-selective opioid antagonist naloxone (NLX), as well as the μ-, δ- and k-selective antagonists. d-Phe-Cys-Tyr-d-Trp-Arg-Thr-Pen-Thr (CTAP), naltrindole (NTI) and norbinaltorphimine (norBNI), respectively. Morphine induced analgesia in a dose related manner. Administration of NLX (1–10 μg) or CTAP (1μg) antagonized morphine in a competitive fashion. Neither NTI (1–10 μg) nor norBNI (0.1 μg) had any effect on the morphine dose-effect curve. Thus, morphine appeared to be a selective μ agonist in the cold water tail-flick test, at least by the icv route.
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