The effect of cold pressor test in man on serum immunoreactive dopamine β hydroxylase and on dopamine β hydroxylase activity

Tropolone, an inhibitor of catecholamine O methyltransferase, α methyltyrosine (AMT), an inhibitor of tyrosine hydroxylase, and U 14,624, an inhibitor of dopamine β hydroxylase were tested for an ability to alter the acute lethal toxicity of morphine sulfate. Tropolone (50 mg/kg, i.p.) reduced the 24 hr i.p. LD50 of morphine in male Holtzman rats from 292 to 54 mg/kg. The other two agents both elevated the LD50 significantly. It is suggested that the potentiation of morphine by tropolone resulted from a reduced inactivation of brain catecholamines released by the action of morphine, while the protection afforded by AMT and U 14,624 pretreatment was a result of their inhibition of brain catecholamine synthesis.