Life without peripheral serotonin: insights from tryptophan hydroxylase 1 knockout mice reveal the existence of paracrine/autocrine serotonergic networks

Abstract

Since its identification, 75 years ago, the monoamine serotonin (5-HT) has attracted considerable attention toward its role as a neurotransmitter in the central nervous system. Yet, increasing evidence, from a growing number of research groups, substantiates the fact that 5-HT regulates important nonneuronal functions. Peripheral 5-HT, synthesized by the enzyme tryptophan hydroxyase (Tph) in intestinal cells, was assumed to be distributed throughout the entire body by blood platelets and to behave as a pleiotropic hormone. A decade ago, generation of a mouse model devoid of peripheral 5-HT lead to the discovery of a second isoform of the enzyme Tph and also suggested that 5-HT might act as a local regulator in various organs. The objective of this review is to highlight the newly discovered functions played by the monoamine using the Tph1 KO murine model and to outline current findings that led to the discovery of complete serotonergic systems in unexpected organs. Within an organ, both the presence of local Tph enzymatic activity and serotonergic components are of particular importance as they support the view that 5-HT meets the criteria to be qualified as a monoamine with a paracrine/autocrine function.

Figure 1

(A) Presence of 5-HT but absence of Tph activity in the reproductive tract of female mice. The 5-HT content of the oviduct (upper panel) and the uterus (lower panel) was evaluated by HPLC in superovulated WT(blue bar; n = 5) or Tph1 KO mice (red bar; n = 5). The 5-HT content of both the oviduct and the uterus increases after ovulation (8 h postovulation), when compared to tissues harvested before ovulation (8 h preovulation), and is greatly reduced in the reproductive tract of Tph1 KO mice. (B) Tph activity is not detected in the oviduct and uterus of superovulated mice at any stage tested. Tph activity was determined by a radioenzymatic assay. Briefly, tissue homogenate was added to a reaction mixture containing 0.05 mM tryptophan, 50 mM Hepes (pH 7.6), 5 mM DTT, 0.01 mM Fe(NH₄)₂(SO₄)₂, 0.5 mM 6-MPH4, 0.1 mg/mL catalase, and [3H]-tryptophan (1 mCi/reaction). Unreacted tryptophan and the product 5-HTP were adsorbed with charcoal in 1 M HCl at the end of the incubation. The samples were then centrifuged, the supernatant added to scintillation fluid, and the radioactivity was measured by a liquid scintillation counter. (C) Genetic crosses were performed in which WT males were mated with WT (+/+; n = 12), heterozygous (+/–; n = 9), or Tph1 KO (−/–; n = 13) females and revealed a reduction in the number of pups per litter born from Tph1 KO mothers. In (A) and (C), the letters denote values that are statistically different using a one-way ANOVA followed by a Tukey’s multiple comparison test. P < 0.05 was considered significant.