In the brains of rats without tolerance to morphine, the accumulation of [(14)C]dopamine formed from [(14)C]tyrosine injected intracisternally is increased, reaching a maximum in the hypothalamus and striatum 1 hour after administration of morphine. In tolerant rats, the rate of incorporation of carbon-14 into dopamine and into norepinephrine in these areas is more than twice that in animals that have received only one injection of morphine.
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Clouet et al. (1970) studied this question.
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