Comparison of the dose-response effects of morphine on brain amines, analgesia and activity in mice.

1. Noradrenaline, dopamine, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid concentrations in the mouse brain were measured 30 min after subcutaneous injection of doses of morphine ranging from 0.1 to 100 mg/kg: motor activity was also measured.2. The noradrenaline concentration in the mouse brain was reduced by moderate (2 to 20 mg/kg) but not by high (above 20 mg/kg) and low (below 2 mg/kg) doses of morphine.3. The dopamine concentration in the mouse brain was reduced by moderate (1 to 20 mg/kg) doses but was raised by high doses (above 20 mg/kg) of morphine.4. The 5-hydroxytryptamine concentration in the mouse brain was reduced by moderate (1 to 20 mg/kg) doses of morphine but not by high (above 20 mg/kg) and low (below 1 mg/kg) doses of morphine.5. The 5-hydroxyindoleacetic acid concentration was not affected by low doses (0.1 to 2 mg/kg), raised by a dose of 5 mg/kg, lowered by doses of 10-50 mg/kg and not affected by 100 mg/kg of morphine.6. These results are discussed with reference to the possible implication of changes in monoamines for the analgesic and behavioural effects of morphine.