RECEPTOR SELECTIVITY OF ICV MORPHINE IN THE RAT COLD-WATER TAIL-FLICK TEST

The cold water tail-flick test in the rat is somewhat unique in that it is sensitive to the analgesic effects of delta- and kappa- in addition to mu-opioid agonists. The present study was designed to test whether a component of morphine-induced analgesia in this test might be mediated by delta- or kappa-opioid receptors. Morphine was administered icv in combination with the non-selective opioid antagonist naloxone (NLX), as well as the mu-, delta- and kappa-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 mu g) or CTAP (1 mu g) antagonized morphine in a competitive fashion. Neither NTI (1-10 mu g) nor norBNI (0.1 mu g) had any effect on the morphine dose-effect curve. Thus, morphine appeared to be a selective mu agonist in the cold water tail-flick test, at least by the icv route.