THE EFFECT OF MORPHINE ON THE TRANSPORT AND METABOLISM OF INTRACISTERNALLY-INJECTED LEUCINE IN THE RAT

—Intracisternally injected l or d-¹⁴C]leucine was retained longer in the brains of morphine-treated rats than in saline-injected control animals. This resulted in higher levels of the labelled leucine and of labelled metabolites of the l-isomer in free pools of brain tissue. However, the absolute levels of brain amino acids and the relative distribution of radioactivity among l-leucine metabolites in brain were unaffected by treatment with morphine, indicating that no disturbance of leucine oxidation through the citric acid cycle was produced by the drug. The inhibition of protein synthesis caused by acute administration of morphine was calculated to be greater than previously reported since morphine treatment increased the specific radioactivity of the free pool of leucine in brain following the intracisternal injection of the labelled amino acid. Possible mechanisms responsible for these morphine effects are discussed.