Fig 2: Antinociceptive effects of the DPP4 inhibitor IPI and vildagliptin in chronic neuropathic condition induced by partial sciatic nerve ligation. DPP4 inhibitors were ineffective in dynamic plantar aesthesiometer (a and d) and noxious cold sensitivity (b and e) tests. In Randall-Selitto test, both IPI and vildagliptin had antihyperalgesic effect which was not antagonized by NTX (c and f). Maximal possible effects (MPE%) of DPP4 inhibitors alone or in combination with subtype specific opioid antagonists is given on bar graphs (g–i). Comparisons were made with two-way ANOVA, Bonferoni post hoc test; +p < 0.05; ++p < 0.01; +++p < 0.001 (a–f) and one-way ANOVA followed by Dunnett’s post hoc test; *p < 0.1 **p < 0.01 (g–i). Data on each curves and bars are given as mean and SEM.

Fig 3: DPP4 immunoreactivity in various types of axon terminals. DPP4 immunolabelling on MAP2-stained dendritic surfaces was associated with synaptophysin (SYN) indicating that not the dendrites but the axon terminals express the receptor (row a). DPP4 is expressed by the majority of peptidergic (CGRP-containing) C primary afferent terminals (row b) and coexpression of DPP4 with SYN and VGLUT2 (row c) or VGAT (row d) suggests that DPP4 is present both in excitatory and inhibitory nerve endings. All the images are single optical sections. Scalebar: 5 μm.

Fig 4: Antihyperalgesic effect of DPP4 inhibitor IPI and vildagliptin in carrageenan-induced inflammation. (a) Antihyperalgesic effect of IPI has been completely abolished by co-administartion of CTAP, but (b) neither TIPP[Ψ] nor (c) gNTI altered IPI-evoked antihyperalgesia. (d) Antihyperalgesic effect of vildagliptin was significantly reduced by CTAP and (f) gNTI, but was completely eliminated when (e) TIPP[Ψ] was co-injected. Inhibitory effects of opioid antagonists on IPI and vildagliptin related antihyperalgesia are summarized on bar graphs (g) and (h) constructed from data recorded 210 min after i.t. drug application. Comparisons were made with two-way ANOVA, Bonferoni post hoc test; +p < 0.05; +++p < 0.001 (a–f) and one-way ANOVA followed by Dunnett’s post hoc test. ***p < 0.001 (g and h). Asterisks always indicate significant differences between the time-matching points of DPP4 inhibitor and DPP4 inhibitor + opioid antagonist curves. Data on each curves and bars are given as mean and SEM.

Fig 5: DPP4 immunoreactivity of different cell types in the spinal dorsal horn. DPP4-immunoreactive puncta appeared in Nissl stained neurons (row a), GFAP labelled astrocytes (row b) and IBA1 positive microglial cells (row c). Co-staining with mouse monoclonal antibody against the full length rat CD26 protein (DPP4mo, column 2) and polyclonal goat DPP4 antibody that was raised against the synthetic peptide C-PPHFDKSKKYP representing the internal region of DPP4 (DPP4gt, column 3) labelled the same puncta in all three cell types demonstrating the specificity of the two antibodies (arrows). All the images are single optical sections. Scalebar: 5 μm.