Absence of cross-tolerance to heroin in morphine-tolerant mice

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Science  04 Apr 1980:
Vol. 208, Issue 4439, pp. 72-74
DOI: 10.1126/science.7361110


Mice implanted with morphine pellets demonstrated a 30-fold increase in tolerance to subcutaneously administered morphine but showed no cross-tolerance to subcutaneously administered heroin. When given morphine intracerebroventricularly, the mice showed no tolerance to morphine or cross-tolerance to heroin. These observations depended on the presence of the morphine pellet. If the pellets were removed prior to determinations of potency, the expected responses--tolerance to morphine and cross-tolerance to heroin--were obtained. The blood-brain barrier may be a prime site for the expression of morphine tolerance in mice.