Morphine catalepsy in the rat: involvement of mu 1 (high affinity) opioid binding sites.

Since the proposal of multiple classes of opiate receptors, much effort has been devoted to understanding the subtypes mediating specific opioid actions. Using the irreversible antagonist naloxazone, we have established a role for the high affinity (mu 1) sites in analgesia. These sites also appear important in opiate catalepsy. Control rats showed dramatic catalepsy following a high dose of morphine sulfate (100 mg/kg, s.c.). Rats whose high affinity (mu 1) sites had been blocked with naloxazone 24 h earlier, on the other hand, showed no evidence of catalepsy (P less than 0.01). Accordingly, catalepsy appears to involve the same receptors as analgesia, the higher affinity (mu 1) sites.