Abstract
Morphine increased brain concentrations of tryptophan and tyrosine 1–2 hr after administration in a dose-dependent manner in male rats. Concentrations of these amino acids in blood serum decreased 30–45 min post-injection and then rose towards control values. The rise in brain amino acids was antagonized by pretreatment with naloxone. In addicted rats there was only a slight increase in brain tryptophan and no increase in tyrosine. Thirty minutes after naloxone-precipitated withdrawal, tryptophan and tyrosine concentrations were elevated in brain, in contrast to the decline in these amino acids seen after naloxone administration in acutely morphinized rats. These results support the hypothesis that the elevated turnover of brain monoamines induced by morphine administration is related to increased availability of precursor amino acids in morphinized animals.
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