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

The turnover of brain monoamine was studied in rats in which different degrees of tolerance to and dependence on morphine were induced by pellet implantation. The degree of tolerance to morphine was assessed by measuring the increase in effective dose for an antinociceptive effect (vocalization test). The rate of depletion of brain dopamine (DA) and serotonin (5-ht) after alpha-methyl-p-tyrosine (AMT) or alpha-propyl-dopacetamide (dopacetamide) was not changed by chronic morphine treatment. In contrast, the accumulation of brain homovanillic acid HVA) and 5-hydroxyindoleacetic acid (5-HIAA) after probenecid was significantly increased, but there was no correlation between the biochemical changes and the degree of tolerance/dependence of the animals; at a very high degree of dependence 5-HIAA accumulation even became normal. In rats in which smaller amounts of morphine were repeatedly injected every 8 hr for 1 week the increased accumulation of HVA and 5-HIAA persisted in spite of complete tolerance to the antinociceptive effect. The rate of depletion of brain noradrenaline (NA) after AMT or dopacetamide was not changed and the accumulation of brain 3-methoxy-4-hydroxy-phenylglycol sulphate (MHPG-SO4) after probenecid was not affected in most chronic morphine groups. In the group with the highest degree of tolerance/dependence NA depletion after AMT was even retarded. The results suggest that chronic morphine treatment increases the synthesis and the intraneuronal destruction of newly synthesized DA and 5-HT without changing the rate of functional utilization of the monoamines. It is unlikely that the changes in monoamine metabolism are causally related to processes leading to morphine tolerance/dependence.