Most dopamine produced in the body is produced in the periphery, not the brain, and previous work from this laboratory has established that much of this peripheral dopamine is produced within the gastrointestinal tract from a source independent of sympathetic nerves. However, the exact non-neuronal sources and the physiological significance of the dopamine produced in peripheral tissues have remained obscure. The general hypothesis that forms the basis of this project is that much of the dopamine produced in the periphery reflects production by a novel non- neuronal dopaminergic system.
The aim of this project is to elucidate the sources and physiological significance of the dopamine produced in the periphery and establish the existence of this dopaminergic paracrine system. At the preclinical level, findings in tyrosine hydroxylase deficient, heterozygous and wild type albino and pigmented mice indicate that tyrosinase, not tyrosine hydroxylase, is a major determinant of peripheral DOPA and dopamine. Concentrations of dopamine in some peripheral tissues (e.g., stomach, pancreas, heart, spleen) of pigmented animals are over 30 times higher than in albinos. Findings that peripheral tissue dopamine is unaffected by deficiency of tyrosine hydroxylase, whereas norepinephrine is severely reduced, further support the contention that peripheral dopamine is produced to a large extent independently and in different cellular compartments from the norepinephrine in sympathetic nerves. Further work is underway to establish whether tyrosinase is restricted to cutaneous melanocytes or is more widely expressed than previously believed. In order to ascertain the potential clinical significance of the above recent findings in mice, studies are planned in which tissue (foreskin and stomach biopsies) and plasma levels and urinary excretion rates of DOPA, dopamine and dopamine metabolites will be examined in individuals with differing pigmentation, including albinos. Other clinical studies have established that most dopamine sulfate -- which is present in plasma and urine at concentrations about 100-fold higher than free dopamine -- originates from the gastrointestinal tract. Western blot analysis of peripheral tissues taken at autopsy has shown that the enzyme responsible for dopamine sulfate production, monoamine sulfating phenolsulfotransferase, is largely concentrated to the tissues of the gastrointestinal tract, particularly the small intestine. Large 45-fold increases in dopamine-sulfate after meal ingestion indicate a potential major source from the diet. However, substantial amounts remaining after a three day fast, and little difference compared to an overnight fast, also indicate an important endogenous source of dopamine sulfate within the gastrointestinal tract. These findings suggest that this metabolite may be a particularly good marker for clinical examination of the postulated peripheral dopamine paracrine system.