Serotoninergic Chemoreceptive Neurons: A Search for a Shared Function

Serotonin Synthesis and CO₂. Primitive cells used the enzyme ribulosebisphosphate (RuBP) carboxylase/oxygenase to fix atmospheric carbon dioxide to produce biologically useful molecules; the same enzyme can alternatively utilize atmospheric oxygen, effectively catalyzing a hydroxylation reaction. The activity of tryptophan hydroxylase (TPH) is the rate-limiting step in 5-HT synthesis and its activity was used by Severson et al. (3) to identify serotoninergic neurons. The second step of 5-HT synthesis involves 5-HT decarboxylase, which cleaves off the alpha carboxyl group to form CO₂. The aromatic decarboxylase enzyme is detected in bacteria and in plants has attracted considerable attention because of its role in the biosynthesis of pharmaceutically important monoterpenoid indole alkaloids (23). Thus, a relationship between CO₂ and serotonin synthesis has long existed.