Abstract

Morphine increases both the locomotor activity of mice and the incorporation of 14-C-tyrosine into 14-C-catecholamines in the mouse brain. To determine whether a relationship exists between increases in locomotor activity and increases in catecholamine synthesis, mice were treated with either morphine (100 mg/kg i.p.) or saline and were forced to run on a treadmill (5.4 m/min). When mice were given saline and forced to run, incorporation of 14-C-tyrosine into 14-C-catecholamines was increased over control values in the heart and spleen but unchanged in the brain. When mice were given morphine, but not forced to run, the incorporation of 14-C-tyrosine into 14-C-catecholamines was increased over control values in the brain but unchanged in the heart and spleen. Morphine decreased the catecholamine content of the mouse brain but not of the heart or spleen. Forced running did not change the catecholamine content of any tissue. The differences between the effects of morphine and forced increases in locomotor activity upon the incorporation of 14-C-tyrosine into 14-C-catecholamines and upon the catecholamine content in various tissues of the mouse indicate that the effects of morphine on catecholamine synthesis are not the result of changes in locomotor activity.