Direct correlation between level of morphine and its biochemical effect on monoamine systems in mouse brain. Evidence for involvement of dopaminergic neurons in the pharmacological action of acute morphine.

The pharmacokinetics (uptake and elimination) and pharmacodynamics (biochemical effects on monoamine systems) of morphine in the CNS were investigated concurrently. ICR mice, weighing about 25 g, were injected intravenously with several doses (2.5-80 mg/kg) of morphine. The animals were killed by microwave irradiation (5 kW, 0.6 sec) at 10 and 30 min, and 1, 2, 4, 8 and 24 hr after the injection. The intracerebral levels of morphine and metabolically related substances consisting of monoamines [noradrenaline, dopamine (DA), 5-hydroxytryptamine (5-HT), 3, 4-dihydroxyphenylacetic acid (DOPAC), 3-methoxy-4-hydroxyphenylacetic acid [homovanillic acid (HVA)], 5-hydroxyindoleacetic acid (5-HIAA), tyrosine and tryptophan] were determined in identical samples by a combination of organic extraction and high-performance liquid chromatography with electrochemical detection. The intracerebral level of morphine was found to depend on the dose injected, and the biological half-life of the drug was estimated to be about 1 hr. The morphine injection (2.5-80 mg/kg) caused significant increases in monoamine metabolites although only slight changes occurred in the concns of parent transmitters. The intracerebral level of morphine was significantly correlated with the ratios DOPAC/DA and HVA/DA (r = 0.7033, P less than 0.0001; and r = 0.6455, P less than 0.0001, respectively). On the other hand, the correlation between the morphine level and 5-HIAA/5-HT was lower than those for DOPAC/DA and HVA/DA. These results suggest that monoamine systems, especially DA, are closely involved in the biochemical effects of morphine. Furthermore, the proposed procedure is demonstrated to be useful as a new approach in biochemical pharmacology, where the direct correlation between the distribution of a drug (pharmacokinetics) and the biochemical effects of the drug (pharmacodynamics) can be measured.