Neuroinflammation and disruption in working memory in aged mice after acute stimulation of the peripheral innate immune system☆
Introduction
Old age is associated with increased brain inflammation (Ye and Johnson, 1999, Lee et al., 2000, Richwine et al., 2005) and stimulation of the peripheral innate immune system with lipopolysaccharide (LPS) leads to an increased inflammatory response in the brain of healthy aged mice (Godbout et al., 2005). This is noteworthy because an increased inflammatory response in the aged brain may cause cognitive deficits that are highly prevalent in elderly patients. For example, acute cognitive impairment (i.e., delirium) is common in elderly emergency department patients (Wofford et al., 1996a, Chiovenda et al., 2002) and frequently occurs in patients with infections unrelated to the CNS (Wofford et al., 1996a). Cognitive impairment complicates diagnosis and is associated with reductions in self-care, increased hospitalization and delayed recovery (Johnston et al., 1987). The severity of dementia is related to mortality of infectious disease, indicating an insidious relationship between infection and cognition, whereby infection in the elderly induces cognitive impairment, and cognitive impairment exacerbates the infection.
Because of the dysregulated linkage between the peripheral immune system and brain in old mice we hypothesized that elevated levels of inflammatory cytokines in the hippocampi of aged animals would be associated with greater deficits in hippocampal-mediated learning and memory after peripheral immune activation. We focused on the hippocampus because it is sensitive to the insults of aging and is likely involved in acute cognitive disorders that are evident in elderly patients with systemic infections. Further, inflammatory cytokines can impair synaptic plasticity in the dentate gyrus (DG) and CA regions of the hippocampus (Bellinger et al., 1993, Cunningham et al., 1996, Tancredi et al., 2000, Hellstrom et al., 2005, Shaw et al., 2005, Perry et al., 2007) as well as disrupt hippocampal-dependent learning and memory (Palin et al., 2004, Rosi et al., 2005, Tanaka et al., 2006). A recent study reported that older rats infected with Escherichia coli had increased hippocampal IL-1β and deficits in hippocampal-dependent memory and learning that were not evident in similarly infected younger cohorts (Barrientos et al., 2006).
In the present study, using a mouse model and a working memory version of the water maze, we found that hippocampal processing is more easily disrupted in old animals than in younger ones after peripheral immune activation. This outcome is independent of a generalized effect on memory consolidation (Sparkman et al., 2005a, Barrientos et al., 2006), and demonstrates an important and novel aspect of infection-related cognitive impairments that is consistent with previous studies of hippocampal dysfunction (Squire, 1992, Avital et al., 2003). Furthermore, using laser capture microdissection (LCM) and immunohistochemistry (IHC), we showed the LPS-induced disruption in working memory in old mice was associated with increased expression of inflammatory cytokines in the hippocampal neuronal cell layers (DG and CA regions) and in surrounding hippocampal tissue. The results suggest that aging can influence the neurobehavioral complications associated with systemic infections probably by allowing the over expression of inflammatory cytokines in brain areas that mediate cognitive processing.
Section snippets
Animals
Young (3–6 months old) and old (22–24 months old) male BALB/c mice from our in-house specific-pathogen-free colony were used. Mice were housed in polypropylene cages and maintained at 23 °C under a reverse phase 12 h light–dark cycle with ad libitum access to water and rodent chow. At the end of each study, mice were examined postmortem for gross signs of disease (e.g. splenomeglia and tumors). Data from mice determined to be unhealthy were excluded from analysis. All procedures were in
Results
Young (3–6 months old) and old (22–24 month old) mice were injected i.p. with saline or LPS and 4 h later they were killed and mRNA encoding several inflammatory cytokines was measured in their hippocampi by real-time PCR. LCM was used to separate the neuronal cell layers (i.e., DG and CA regions) from surrounding hippocampal tissue, thus providing a better analysis of the microenvironment of hippocampal neurons which are known to be sensitive to insults of aging and inflammatory cytokines. Fig.
Discussion
The present study demonstrates that old mice have more microglia that stain for tomato lectin and higher basal levels of several inflammatory cytokine mRNAs in the hippocampus than do young adults. Furthermore, when the peripheral innate immune system was stimulated by LPS to mimic an acute Gram-negative bacterial infection, the expression of inflammatory cytokines in the hippocampus was higher in old mice than in young ones. Using a working memory version of the water maze that required
Acknowledgments
We thank Dr. James Zachary for assisting with LCM and Dr. Masaaki Nakai (University of Illinois) for helping with IHC staining. Dr. Colm Cunningham at University of Southampton (U.K.) also provided helpful advice on IHC staining. This research was supported by NIH grants AG16710 (R.W.J.), AG023580 (R.W.J.), MH069148 (R.W.J.) and DK64862 (G.G.F.). J.B.B. and J.P.G. were supported by a Ruth L. Kirchstein NRSA Postdoctoral Fellowship.
References (47)
- et al.
Pyrogens specifically disrupt the acquisition of a task involving cognitive processing in the rat
Brain Behav. Immun.
(1995) - et al.
Peripheral infection and aging interact to impair hippocampal memory consolidation
Neurobiol. Aging
(2006) - et al.
Interleukin 1 beta inhibits synaptic strength and long-term potentiation in the rat CA1 hippocampus
Brain Res.
(1993) - et al.
Alpha-tocopherol attenuates lipopolysaccharide-induced sickness behavior in mice
Brain Behav. Immun.
(2004) - et al.
Alpha-tocopherol and selenium facilitate recovery from lipopolysaccharide-induced sickness in aged mice
J. Nutr.
(2005) - et al.
Cognitive impairment in elderly ED patients: need for multidimensional assessment for better management after discharge
Am. J. Emerg. Med.
(2002) - et al.
Interleukin-1 beta (IL-1 beta) and tumour necrosis factor (TNF) inhibit long-term potentiation in the rat dentate gyrus in vitro
Neurosci. Lett.
(1996) - et al.
Pneumonia in the very old
Lancet Infect. Dis.
(2004) - et al.
Heterogeneity in the distribution and morphology of microglia in the normal adult mouse brain
Neuroscience
(1990) - et al.
Peripheral administration of lipopolysaccharide induces the expression of cytokine transcripts in the brain and pituitary of mice
Brain Res. Mol. Brain Res.
(1994)
Analysis of relative gene expression data using real-time quantitative PCR and the 2(-Delta Delta C(T)) method
Methods
Glucocorticoids worsen excitotoxin-induced expression of pro-inflammatory cytokines in hippocampal cultures
Exp. Neurol.
Distribution of interleukin 1 beta immunoreactivity within the porcine hypothalamus
Brain Res.
Interleukin-1beta mediates the memory impairment associated with a delayed type hypersensitivity response to bacillus Calmette-Guerin in the rat hippocampus
Brain Behav. Immun.
Improved psychomotor performance in aged mice fed diet high in antioxidants is associated with reduced ex vivo brain interleukin-6 production
Brain Behav. Immun.
Interleukin-6 mRNA expression by cortical neurons in culture: evidence for neuronal sources of interleukin-6 production in the brain
J. Neuroimmunol.
Cyclooxygenase inhibition attenuates endotoxin-induced spatial learning deficits, but not an endotoxin-induced blockade of long-term potentiation
Brain Res.
Messenger RNA levels and methylation patterns of GAPDH and beta-actin genes in rat liver, spleen and brain in relation to aging
Mech. Ageing Dev.
Bacterial endotoxin-induced behavioral alterations in two variations of the Morris water maze
Physiol. Behav.
Peripheral lipopolysaccharide administration impairs two-way active avoidance conditioning in C57BL/6J mice
Physiol. Behav.
Acute cognitive impairment in elderly ED patients: etiologies and outcomes
Am. J. Emerg. Med.
Increased interleukin-6 expression by microglia from brain of aged mice
J. Neuroimmunol.
Regulation of interleukin-6 gene expression in brain of aged mice by nuclear factor kappaB
J Neuroimmunol
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Please see Brief Commnetary by Marina Lynch on page 299 of this issue.
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Current address: Department of Molecular Virology, Immunology and Medical Genetics, The Ohio State University, Columbus, OH 43210, USA.