Precipitation of withdrawal in morphine tolerant/dependent rats by either naloxone or the partial agonist ZK 48491 caused a significant increase in the contraction of striatal DA, which persisted for at least 1 h. During the same time the probenecid-induced accumulation of HVA and DOPAC was reduced in the striatum in relation to probenecid-treated tolerant/dependent controls. 20 min after precipitation of withdrawal by naloxone, the striatal concentration of 3-methoxytyramine was decreased by about 40%, while the activity of the DA metabolizing enzymes, MAO and COMT, remained unchanged. Naloxone-precipitated withdrawal was, further, found to delay the depletion of striatal DA caused by inhibition of synthesis 90 min after α-methyl-p-tyrosine treatment. All these results provide evidence for a decreased release of DA from the striatum during precipitated morphine withdrawal. © 1977.
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