TPH2 and type 1 diabetes mellitus: As shown in Figure 7A, the gene transcripts of Tph-2, the rate-limiting enzyme in methoxyindole pathway, were significantly elevated in the RVM, LSSC and DRG tissues of T1DM rats in comparison to those of control (RVM: F: 11.02, p = 0.0098; LSSC: F: 7.27, p = 0.0249; DRG: F: 10.7, p = 0.0106).