Effects of morphine on the turnover of brain catecholamines and serotonin in rats-acute morphine administration.

Morphine increased the rate of brain dopamine (DA) depletion when given before alpha-methyl-p-tyrosine (AMT) or alpha-propyl-dopacetamide, but not when given after AMT. No effect of morphine was found on the rate of depletion of brain noradrenaline (NA) or serotonin (5-HT) after the two synthesis inhibitors. The accumulation of homovanillic acid and 5-hydroxy-indoleacetic acid induced by probenecid was significantly increased by morphine pretreatment, whereas the accumulation of 3-methoxy-4-hydroxy-phenylglycol sulphate was not changed. These findings can be best explained by the hypothesis that morphine increases the non-functional intraneuronal catabolism of newly synthesized DA and 5-HT, without much effect on the monoamines already taken up in the synaptic vesicles. NA turnover does not seem to be changed by acute morphine administration.