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

The cold water tail-flick test in the rat is somewhat unique in that i t is sensitive to the analgesic effects of delta- and kappa- in additi on to mu-opioid agonists. The present study was designed to test wheth er a component of morphine-induced analgesia in this test might be med iated by delta- or kappa-opioid receptors. Morphine was administered i cv in combination with the non-selective opioid antagonist naloxone (N LX), as well as the mu-, delta- and kappa-selective antagonists, D-Phe -Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr (CTAP), naltrindole (NTI) and norbinalt orphimine (norBNI), respectively. Morphine induced analgesia in a dose related manner. Administration of NLX (1-10 mu g) or CTAP (1 mu g) an tagonized morphine in a competitive fashion. Neither NTI (1-10 mu g) n or 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 wate r tail-flick test, at least by the icv route.