Modification of brain biogenic amines by met5-enkephalin amide in the rat.

1. Gross changes in behaviour and whole brain levels of noradrenaline, dopamine, serotonin and 5-hydroxyindoleacetic acid were examined in rats for 3 min following intracerebral injections of met5-enkephalin amide or morphine. 2. Both the enkephalin and morphine depressed respiration and induced a cataleptic-like state, but only the enkephalin produced wet-shake responses. 3. All doses of met5-enkephalin amide (150,200 and 600 microgram) and morphine (134 and 200 microgram) increased the brain levels of 5-hydroxyindoleacetic acid, whereas only the large doses modified the levels of noradrenaline and serotonin. 4. The similarity of the effects of these two compounds indicates that morphine may act through an enkephalin-like mechanism.