In vivo antagonism by naloxone of morphine, beta-endorphin and a synthetic enkephalin analog.

The in vivo equivalent of pA2 values were determined in rat tail-flick and mouse hot-plate tests for naloxone against morphine, beta-endorphin and a synthetic enkephalin analog, (D-Met2,Pro5)-enkephalinamide, as analgesics. In mice the apparent pA2 value of naloxone against morphine (6.86) was similar to that found by previously and essentially the same values were obtained against the opioid peptides, indicating homogenous receptor population for the analgesics studied. In rats the pA2 of naloxone against morphine (7.17) was lower than against either beta-endorphin (7.55) or (D-Met2,Pro5)-enkephalinamide (7.51), warning that in rats such a homogeneity of the "analgesic" receptor population as was observed for mice may not exist.