The effect of a peripheral decarboxylase inhibitor (carbidopa) on monoamine and neuroendocrine function in man.

Carbidopa, a selective extracerebral decarboxylase inhibitor, was given to 10 normal volunteers to determine its effects on endogenous catecholamine, indoleamine, and endocrine function. Tryptamine, which is largely extracerebral in origin, was inhibited markedly (80 percent) by the carbidopa; 5-hydroxyindoleacetic acid (5-HIAA) and 3-methoxy-4-hydroxyphenolglycol (MHPG) excretion also were inhibited by the drug but not to the same degree as tryptamine. These differential results may be due partly to the higher central nervous system origin of the 5-HIAA and MHPG but also to a peripheral "stores" effect. In addition, carbidopa resulted in significant increases in plasma prolactin and a small but significant decrease in plasma glucagon.