The Effect of N-Allylnormorphine on the Development of Acute Tolerance to the Analgesic and Hypothermic Effects of Morphine in the Rat

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Abstract
N-allylnormorphine (nalorphine) injected systemically in rats blocks both the analgesic and hypothermic effects of a subsequent injection of morphine. Injection of nalorphine into the anterior hypothalamic thermoregulatory centers antagonizes the hypo-thermic action of systemically administered morphine. Rats develop acute tolerance to both these effects of morphine so that a 2nd injection of the drug is ineffective. If nalorphine is given systemically or centrally prior to administration of morphine, acute tolerance does not develop. Tolerance may develop as a result of changes at neuronal receptor sites rather than from a generalized immune reaction.