Humans and animals use the classical five senses of sight, sound, touch, smell and taste to monitor their environment. The very survival of feral animals depends on these sensory perception systems, which is a central theme in scholarly research on comparative aspects of anatomy and physiology. But how do all of us sense and respond to an infection? We cannot see, hear, feel, smell or taste bacterial and viral pathogens, but humans and animals alike are fully aware of symptoms of sickness that are caused by these microbes. Pain, fatigue, altered sleep pattern, anorexia and fever are common symptoms in both sick animals and humans. Many of these physiological changes represent adaptive responses that are considered to promote animal survival, and this constellation of events results in sickness behavior. Infectious agents display a variety of pathogen-associated molecular patterns (PAMPs) that are recognized by pattern recognition receptors (PRRs). These PRR are expressed on both the surface [e.g. Toll-like receptor (TLR)-4] and in the cytoplasm [e.g. nucleotide-binding oligomerization domain (Nod)-like receptors] of cells of the innate immune system, primarily macrophages and dendritic cells. These cells initiate and propagate an inflammatory response by stimulating the synthesis and release of a variety of cytokines. Once an infection has occurred in the periphery, both cytokines and bacterial toxins deliver this information to the brain using both humoral and neuronal routes of communication. For example, binding of PRR can lead to activation of the afferent vagus nerve, which communicates neuronal signals via the lower brain stem (nucleus tractus solitarius) to higher brain centers such as the hypothalamus and amygdala. Blood-borne cytokines initiate a cytokine response from vascular endothelial cells that form the blood-brain barrier (BBB). Cytokines can also reach the brain directly by leakage through the BBB via circumventricular organs or by being synthesized within the brain, thus forming a mirror image of the cytokine milieu in the periphery. Although all cells within the brain are capable of initiating cytokine secretion, microglia have an early response to incoming neuronal and humoral stimuli. Inhibition of proinflammatory cytokines that are induced following bacterial infection blocks the appearance of sickness behaviors. Collectively, these data are consistent with the notion that the immune system communicates with the brain to regulate behavior in a way that is consistent with animal survival.

An impaired ability to regulate the activation of microglia by fractalkine (CX3CL1) leads to persistent neuroinflammation and behavioral alterations following lipopolysaccharide (LPS) challenge. While these responses are usually transient, LPS injection caused prolonged depressive-like behavior in fractalkine receptor deficient mice (CX3CR1(-/-)) that was associated with exaggerated microglial activation and induction of the tryptophan (TRP) degrading enzyme indoleamine 2,3-dioxygenase (IDO). IDO activation and subsequent generation of neuroactive kynurenine metabolites may have a pivotal role in the development of depression. Therefore, the purpose of this study was to determine the extent to which LPS-induced depressive-like behavior in CX3CR1(-/-) mice was dependent on IDO activation. CX3CR1(-/-) mice were implanted prior to LPS challenge with a slow release pellet of 1-methyl-tryptophan (1-MT), a competitive inhibitor of IDO. Here we show that the depressive-like behavior evident in CX3CR1(-/-) mice 72 h after LPS injection was abrogated by inhibition of IDO. LPS also decreased body weight and locomotor activity in CX3CR1(-/-) mice, but these effects were independent of 1-MT. Consistent with the increased metabolism of TRP by IDO, the ratio of 3-hydroxykynurenine (3-HK) to TRP was increased in the brain 72 h after LPS. Increased serotonin (5-HT) turnover was also evident in the brain. The LPS-associated increases in both 3-HK:TRP and 5-HIAA:5-HT ratios were prevented by the inhibition of IDO. Last, IDO blockade attenuated microglial activation in the prefrontal cortex and hippocampus 72 h after LPS. Collectively these data indicate that LPS-induced IDO activation contributes to persistent microglial activation and depressive-like behavior in CX3CR1(-/-) mice.

Peripheral stimulation of the innate immune system with LPS causes exaggerated neuroinflammation and prolonged sickness behavior in aged mice. Regular moderate intensity exercise has been shown to exert anti-inflammatory effects that may protect against inappropriate neuroinflammation and sickness in aged mice. The purpose of this study was to test the hypothesis that voluntary wheel running would attenuate LPS-induced sickness behavior and proinflammatory cytokine gene expression in ~22-month-old C57BL/6J mice. Mice were housed with a running wheel (VWR), locked-wheel (Locked), or no wheel (Standard) for 10 weeks, after which they were intraperitoneally injected with LPS across a range of doses (0.02, 0.08, 0.16, 0.33 mg/kg). VWR mice ran on average 3.5 km/day and lost significantly more body weight and body fat, and increased their forced exercise tolerance compared to Locked and Shoebox mice. VWR had no effect on LPS-induced anorexia, adipsia, weight-loss, or reductions in locomotor activity at any LPS dose when compared to Locked and Shoebox groups. LPS induced sickness behavior in a dose-dependent fashion (0.33>0.02 mg/kg). Twenty-four hours post-injection (0.33 mg/kg LPS or Saline) we found a LPS-induced upregulation of whole brain TNFα, IL-1β, and IL-10 mRNA, and increased IL-1β and IL-6 in the spleen and liver; these effects were not attenuated by VWR. We conclude that VWR does not reduce LPS-induced exaggerated or prolonged sickness behavior in aged animals, or 24h post-injection (0.33 mg/kg LPS or Saline) brain and peripheral proinflammatory cytokine gene expression. The necessity of the sickness response is critical for survival and may outweigh the subtle benefits of exercise training in aged animals.

OBJECTIVE

Peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes prolonged depressive-like behavior in aged mice that is dependent on indoleamine 2,3 dioxygenase (IDO) activation. Regular moderate-intensity exercise training has been shown to exert neuroprotective effects that might reduce depressive-like behavior in aged mice. The purpose of this study was to test the hypothesis that voluntary wheel running (VWR) would attenuate LPS-induced depressive-like behavior and brain IDO gene expression in 4- and 22-month-old C57BL/6J mice.

METHODS

Mice were housed with a running wheel (VWR) or no wheel (standard) for 30 (young adult mice) or 70 days (aged mice), after which they were intraperitoneally injected with LPS (young adult mice: 0.83 mg/kg; aged mice: 0.33 mg/kg).

RESULTS

Young adult VWR mice ran on average 6.9 km/day, while aged VWR mice ran on average 3.4 km/day. Both young adult and aged VWR mice increased their forced exercise tolerance compared to their respective standard control groups. VWR had no effect on LPS-induced anorexia, weight loss, increased immobility in the tail suspension test and decreased sucrose preference in either young adult or aged mice. Four (young adult mice) and 24 h (aged mice) after injection of LPS, mRNA transcripts for TNF-α, IL-1β, IL-6, and IDO were upregulated in the whole brain independently of VWR.

CONCLUSIONS

Prolonged physical exercise has no effect on the neuroinflammatory response to LPS and its behavioral consequences in young adult and aged mice.