Lipopolysaccharide-induced depressive-like behavior is mediated by indoleamine 2,3-dioxygenase activation in mice

Abstract

Although elevated activity of the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO) has been proposed to mediate comorbid depression in inflammatory disorders, its causative role has never been tested. We report that peripheral administration of lipopolysaccharide (LPS) activates IDO and culminates in a distinct depressive-like behavioral syndrome, measured by increased duration of immobility in both the forced-swim and tail suspension tests. Blockade of IDO activation either indirectly with the anti-inflammatory tetracycline derivative minocycline, that attenuates LPS-induced expression of proinflammatory cytokines, or directly with the IDO antagonist 1-methyltryptophan (1-MT), prevents development of depressive-like behavior. Both minocycline and 1-MT normalize the kynurenine/tryptophan ratio in the plasma and brain of LPS-treated mice without changing the LPS-induced increase in turnover of brain serotonin. Administration of L-kynurenine, a metabolite of tryptophan that is generated by IDO, to naive mice dose dependently induces depressive-like behavior. These results implicate IDO as a critical molecular mediator of inflammation-induced depressive-like behavior, probably through the catabolism of tryptophan along the kynurenine pathway.

Fig. 1

Minocycline inhibits LPS-induced depressive-like behavior that is associated with blockade of brain proinflammatory cytokine and IDO upregulation. Mice were injected i.p. with saline or minocycline (50 mg/kg) for 3 successive days. Immediately following the final injection, mice also received either an i.p. injection of non-pyrogenic saline or LPS (0.83 mg/kg). (A) The 24 h change in body weight following LPS administration was measured. The general locomotor activity was measured either 6 or 24 h following LPS injection (B). The duration of immobility during the forced swim test was recorded 24 h following administration of LPS (C). The duration of immobility during the tail suspension test 28 h post-LPS was recorded cumulatively over each 10 min. test (D) or in one min. increments over 10 min (E). Immediately following behavioral testing at 28 h post-LPS, mice were sacrificed, perfused with ice-cold PBS and tissues were collected. Steady-state expression of mRNA transcripts in the brain were measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=11–14 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=35 ± 0.8, TNF-α=26 ± 0.2, IL-1β=26 ± 0.5, and IDO=33 ± 1.1.

Fig. 1

Minocycline inhibits LPS-induced depressive-like behavior that is associated with blockade of brain proinflammatory cytokine and IDO upregulation. Mice were injected i.p. with saline or minocycline (50 mg/kg) for 3 successive days. Immediately following the final injection, mice also received either an i.p. injection of non-pyrogenic saline or LPS (0.83 mg/kg). (A) The 24 h change in body weight following LPS administration was measured. The general locomotor activity was measured either 6 or 24 h following LPS injection (B). The duration of immobility during the forced swim test was recorded 24 h following administration of LPS (C). The duration of immobility during the tail suspension test 28 h post-LPS was recorded cumulatively over each 10 min. test (D) or in one min. increments over 10 min (E). Immediately following behavioral testing at 28 h post-LPS, mice were sacrificed, perfused with ice-cold PBS and tissues were collected. Steady-state expression of mRNA transcripts in the brain were measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=11–14 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=35 ± 0.8, TNF-α=26 ± 0.2, IL-1β=26 ± 0.5, and IDO=33 ± 1.1.

Fig. 1

Minocycline inhibits LPS-induced depressive-like behavior that is associated with blockade of brain proinflammatory cytokine and IDO upregulation. Mice were injected i.p. with saline or minocycline (50 mg/kg) for 3 successive days. Immediately following the final injection, mice also received either an i.p. injection of non-pyrogenic saline or LPS (0.83 mg/kg). (A) The 24 h change in body weight following LPS administration was measured. The general locomotor activity was measured either 6 or 24 h following LPS injection (B). The duration of immobility during the forced swim test was recorded 24 h following administration of LPS (C). The duration of immobility during the tail suspension test 28 h post-LPS was recorded cumulatively over each 10 min. test (D) or in one min. increments over 10 min (E). Immediately following behavioral testing at 28 h post-LPS, mice were sacrificed, perfused with ice-cold PBS and tissues were collected. Steady-state expression of mRNA transcripts in the brain were measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=11–14 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=35 ± 0.8, TNF-α=26 ± 0.2, IL-1β=26 ± 0.5, and IDO=33 ± 1.1.

Fig. 2

1-methyltryptophan abrogates depressive-like behavior in response to LPS without reducing expression of proinflammatory cytokines. Mice were implanted s.c. with a slow release pellet of the IDO competitive inhibitor, 1-MT or placebo. One week later, mice were injected i.p. with either non-pyrogenic saline or LPS (0.83 mg/kg). As in figure 1, the 24 h change in body weight (A) and the reduction in locomotor activity 6 or 24 h after LPS injection (B) were measured. The duration of immobility during the forced swim test was measured 24 h following administration of LPS (C). The duration of immobility during the tail suspension test was recorded 28 h post-LPS and measured either cumulatively (D) or in one min. increments over the 10 min. test (E). Immediately following behavioral testing, mice were sacrificed, perfused with ice-cold PBS and tissue collected. Steady-state expression of mRNA transcripts in the brain was measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=8 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=36 ± 1.7, TNF-α=27 ± 0.4, IL-1β=27 ± 0.3, and IDO=32 ± 1.6.

Fig. 2

1-methyltryptophan abrogates depressive-like behavior in response to LPS without reducing expression of proinflammatory cytokines. Mice were implanted s.c. with a slow release pellet of the IDO competitive inhibitor, 1-MT or placebo. One week later, mice were injected i.p. with either non-pyrogenic saline or LPS (0.83 mg/kg). As in figure 1, the 24 h change in body weight (A) and the reduction in locomotor activity 6 or 24 h after LPS injection (B) were measured. The duration of immobility during the forced swim test was measured 24 h following administration of LPS (C). The duration of immobility during the tail suspension test was recorded 28 h post-LPS and measured either cumulatively (D) or in one min. increments over the 10 min. test (E). Immediately following behavioral testing, mice were sacrificed, perfused with ice-cold PBS and tissue collected. Steady-state expression of mRNA transcripts in the brain was measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=8 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=36 ± 1.7, TNF-α=27 ± 0.4, IL-1β=27 ± 0.3, and IDO=32 ± 1.6.

Fig. 2

1-methyltryptophan abrogates depressive-like behavior in response to LPS without reducing expression of proinflammatory cytokines. Mice were implanted s.c. with a slow release pellet of the IDO competitive inhibitor, 1-MT or placebo. One week later, mice were injected i.p. with either non-pyrogenic saline or LPS (0.83 mg/kg). As in figure 1, the 24 h change in body weight (A) and the reduction in locomotor activity 6 or 24 h after LPS injection (B) were measured. The duration of immobility during the forced swim test was measured 24 h following administration of LPS (C). The duration of immobility during the tail suspension test was recorded 28 h post-LPS and measured either cumulatively (D) or in one min. increments over the 10 min. test (E). Immediately following behavioral testing, mice were sacrificed, perfused with ice-cold PBS and tissue collected. Steady-state expression of mRNA transcripts in the brain was measured by real-time RT-PCR for (F) TNF-α (G) IL-1β (H) IFN-γ and (I) IDO. Data represent means ± SEM (n=8 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Average Ct values for saline + LPS treated mice were, IFNγ=36 ± 1.7, TNF-α=27 ± 0.4, IL-1β=27 ± 0.3, and IDO=32 ± 1.6.

Fig. 3

Minocycline and 1-MT attenuate the LPS-induced increases in the kynurenine/tryptophan ratio in both the plasma and the brain without affecting 5-HT turnover. Mice were treated as in figures 1 & 2 with minocycline or 1-MT prior to i.p. LPS injection. Kynurenine and tryptophan concentrations were determined by HPLC in both the plasma (A, D) and brain (B, E), and the ratio of 5-HIAA/5-HT was measured in brain tissue (C, F). Data represent means ± SEM (n=8–14 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01. Saline vs. LPS groups in 1-MT pretreated mice (D and E) approached significance (P<0.1)

Fig. 4

Kynurenine dose-dependently induces depressive-like behavior. Mice were injected i.p. with saline or increasing amounts of L-kynurenine. Two hours later, mice were tested for changes in locomotor activity (A), immobility in the forced swim (B) or tail suspension tests (C), and the total duration of immobility was measured. Data represent means ± SEM (n=4–8 mice/group). Bars indicate statistical differences among groups. * P<0.05, ** P<0.01.