Morphine reduces WIN55,212-12 analgesia in mice that do not express HINT1 proteins. Upper panel: Groups of eight mice each were injected icv with the opioids morphine, DAMGO, DADLE, and clonidine at the indicated doses, and the analgesic effect of WIN55,212-2 was assessed 24 h later. *Significantly different from the control group injected with the vehicle instead of the priming dose of WIN55,212-2 (P < 0.05). Lower panel: The absence of HINT1 enables morphine to reduce the analgesic effects of WIN55,212-2. *Significantly different from the control animals (open bar) or from the group injected with the vehicle instead of the priming dose of morphine (P < 0.05).