Serotonin- and Dopamine-Related Gene Expression in db/db Mice Islets and in MIN6 β-Cells Treated with Palmitate and Oleate

Figure 1

Diagram of biosynthetic pathways of monoamines. Serotonin (5-hydroxytryptamine or 5HT) biosynthesis is carried out by two sequential steps: (1) conversion of the essential amino acid tryptophan into 5-hydroxytryptophan (5HTP) by the enzyme tryptophan hydroxylase (TPH) and (2) decarboxylation of 5HTP to serotonin catalyzed by the enzyme Dopa-decarboxylase (DDC). Two isoforms of tryptophan hydroxylase are known: TPH1, which is found in nonneurons cells, and TPH2, which is predominantly found in neurons cells (mainly in CNS) [32], being both subtypes expressed in pancreatic β-cells [33]. Two additional enzymatic steps from serotonin are involved in melatonin synthesis, while DDC is also involved in the synthesis of dopamine from L-Dopa, which in turn is synthetized from the amino acid tyrosine by the action of enzyme tyrosine hydroxylase (TH). Finally, noradrenaline and adrenaline are synthetized from dopamine through the consecutively action of dopamine β-hydroxylase (DBH) and phenylethanolamine N-methyltransferase. The enzymes whose gene has been evaluated for expression were highlighted in italic.