3.2 Differential responses of PrL and IL to IDO activation and
inflammation induced by ICV-STZ.
In order to investigate whether the ICV-STZ induced behavioral
alterations are related to the activation of IDO in the PrL and IL, we
examined the levels of kynurenine metabolites, and local injection of
1-MT was used to directly explore the role of IDO in the PrL and IL in
ICV-STZ rats. The cannula placements of PrL and IL are shown in Figure
3A and S1A, B. The body weights of all groups maintained the same
increasing trend to the end of the experiment (Figure 3B). Besides,
ICV-STZ and 1-MT treatment generally had no effect on food and water
consumption, as shown in Figure S1C, D. In the PrL, there was main
effect of 1-MT treatment (F(1,20)=89.37) for the levels
of Trp. Regarding the levels of Kyn, significant effects of ICV-STZ
(F(1,20)=47.29), 1-MT treatment
(F(1,20)=15.16) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=16.88) were observed. For the
ratios of Kyn/Trp, significant effects of ICV-STZ
(F(1,20)=29.37), 1-MT treatment
(F(1,20)=64.32) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=12.75) were also observed. Post
hoc test showed that ICV-STZ did not cause significant alterations in
the levels of Trp (Figure 3C) but induced an up-regulation of Kyn levels
(Figure 3D) and Kyn/Trp ratios (Figure 3E), which are standardly used as
a measure of IDO activity. Intra-PrL injection of 1-MT increased Trp
levels and reversed the up-regulation of Kyn levels and Kyn/Trp ratios( Figure 3C-E) in the PrL induced by ICV-STZ. In the IL, there
was main effects of ICV-STZ (F(1,20)=23.85) for the
levels of Trp. Regarding the levels of Kyn, significant effects of
ICV-STZ (F(1,20)=25.35), 1-MT treatment
(F(1,20)=18.80) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=19.38) were observed. For the
ratios of Kyn/Trp, significant effects of 1-MT treatment
(F(1,20)=16.91) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=16.58) were also observed.
Different with PrL, the post hoc test revealed that ICV-STZ increased
the levels of Trp (Figure 3F), Kyn (Figure 3G) and Kyn/Trp ratios
(Figure 3H) in the IL. Intra-IL injection of 1-MT has no effect on Trp
levels for vehicle and ICV-STZ rats but normalized the up-regulation of
Kyn levels and Kyn/Trp ratios (Figure 3F-H) in the IL induced by
ICV-STZ. These results proposed that ICV-STZ induced behavior
alterations are likely related to activating IDO in the PrL and IL.
Pro-inflammatory cytokines are the main factors that trigger IDO
activation. Hence, we further tested the levels of cytokines to
elucidate the involvement of neuroinflammatory response in the PrL and
IL. Two-way ANOVA revealed significant effects of 1-MT treatment
(F(1,20)=9.711) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=14.92) on the levels of IL-1β in
the IL. And there were also significant effects of 1-MT treatment
(F(1,20)=15.21) and interactions between ICV-STZ and
1-MT treatment (F(1,20)=8.396) on the levels of IL-6 in
the IL. The results showed that the levels of IL-1β (Figure 3I) and IL-6
(Figure 3J) in the PrL were not significantly altered by ICV-STZ and
intra-PrL injection of 1-MT. In contrast, the neuroinflammatory response
in the IL induced by ICV-STZ was characterized by a significant increase
in the levels of IL-1β (Figure 3K) and IL-6 (Figure 3L). Intra-IL
injection of 1-MT prevented the increase of IL-1β and IL-6 levels in the
IL induced by ICV-STZ.