Tryptophan hydroxylase 2 genotype determines brain serotonin synthesis but not tissue content in C57Bl/6 and BALB/c congenic mice

Abstract

Tryptophan hydroxylase 2 (TPH2) catalyzes the rate-limiting step in the synthesis of brain serotonin (5-HT). In a previous report, a single nucleotide polymorphism in mTph2 (C1473G) reduced 5-HT synthesis by 55%. Mouse strains expressing the 1473C allele, such as C57Bl/6, have higher 5-HT synthesis rates than strains expressing the 1473G allele, such as BALB/c. Many studies have attributed strain differences to Tph2 genotype without ruling out the potential role of alterations in other genes. To test the role of the C1473G polymorphism in strain differences, we generated C57Bl/6 and BALB/c mice congenic for the Tph2 locus. We found that the 1473G allele reduced 5-HT synthesis in C57Bl/6 mice but had no effect on 5-HT tissue content except for a slight reduction (15%) in the frontal cortex. In BALB/c mice, the 1473C allele increased 5-HT synthesis but again did not affect 5-HT tissue content. At the same time, 5-hydroxyindoleacetic acid (5-HIAA) was significantly elevated in BALB/c congenic mice. In C57Bl/6 mice, there was no effect of genotype on 5-HIAA levels. BALB/c mice had lower expression of monoamine oxidase A and B than C57Bl/6 mice, but there was no effect of Tph2 genotype. On the tail suspension test, escitalopram treatment reduced immobility regardless of genotype. These data demonstrate that the C1473G polymorphism determines differences in 5-HT synthesis rates among strains but only minimally affects 5-HT tissue levels.

Figure 1

Rate of 5-HT synthesis in BALB/c and C57Bl/6 mice. The C1473G single nucleotide polymorphism in mTph2 determined the rate of 5-HT synthesis in both strains. Levels of 5-hydroxytryptophan (5-HTP) were measured 1h after treatment with m-hydroxybenzylhydrazine (100 mg/kg) in BALB/c and C57Bl/6 mice congenic at the C1473G locus. In the BALB/c congenic mice (top row), 5-HT synthesis was significantly higher in C/C mice than in wild-type BALB/c mice in the A) frontal cortex, B) hippocampus, and C) striatum. In the C57Bl/6 congenic mice (bottom row), the 5-HT synthesis rate was lower than in wild-type C57Bl/6 mice in the D) frontal cortex, E) hippocampus, but not F) striatum. Data are presented as ng of 5-HTP per mg of wet tissue weight. N=6–9. *, p<.05; ***, p<.001.

Figure 2

Tissue content of 5-HT in BALB/c and C57Bl/6 mice. Tissue content of 5-HT was not significantly affected by the C1473G single nucleotide polymorphism. BALB/c congenic mice (top row) did not have significantly changed levels of 5-HT in the A) frontal cortex, B) hippocampus, or C) striatum. C57Bl/6 mice congenic for the 1473G allele had a 15% reduction in 5-HT levels in the D) frontal cortex, but no changes in 5-HT tissue content in the E) hippocampus or F) striatum. Data are expressed in ng / mg wet tissue for BALB/c G/G (top row) or C57Bl/6 C/C (bottom row) and presented as mean ± SEM. N=6–7. *, p<.05.

Figure 3

5-hydroxyindoleacetic acid (5-HIAA) in Tph2 congenic mice. BALB/c congenic mice (top row) had elevated levels of 5-HIAA in the A) frontal cortex and B) hippocampus, but no significant increase in C) striatum. In the C57Bl/6 mice (bottom row), there were no significant changes in 5-HIAA levels in the D) frontal cortex, E) hippocampus, or F) striatum. Data are expressed in ng / mg wet tissue for BALB/c G/G (top row) or C57Bl/6 C/C (bottom row) and presented as mean ± SEM. N=6–7. *, p<.05.