The present study was undertaken to reconfirm the previous conclusion that morphine-6-conjugates cause potent analgesia by themselves in mice. For this purpose, inhibitory effect of morphine and its three pairs of 3-and 6-conjugates (glucuronides, ethereal sulfates and phosphates) to nicotine-induced contraction of the isolated guinea-pig ileum was examined. The potency of these conjugates was found to decrease in the following order, morphine-6-sulfate>morphine-6-glucuronide>morphine, morphine-3-and 6-phosphate>morphine-3-glucuronide, morphine-3-sulfate, indicating fairly good accordance with order in their analgesic effects in mice. The present experiment again provided strong suggestion that the above biological effect was attributable to the conjugates themselves.
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Kataoka et al. (1977) studied this question.
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