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Relationship: 2839

Title

A descriptive phrase which clearly defines the two KEs being considered and the sequential relationship between them (i.e., which is upstream, and which is downstream). More help

Increase, Pro-Inflammatory Mediators leads to Impairment, Learning and memory

Upstream event

The causing Key Event (KE) in a Key Event Relationship (KER). More help

Increase, Pro-Inflammatory Mediators

Downstream event

The responding Key Event (KE) in a Key Event Relationship (KER). More help

Impairment, Learning and memory

Key Event Relationship Overview

The utility of AOPs for regulatory application is defined, to a large extent, by the confidence and precision with which they facilitate extrapolation of data measured at low levels of biological organisation to predicted outcomes at higher levels of organisation and the extent to which they can link biological effect measurements to their specific causes.Within the AOP framework, the predictive relationships that facilitate extrapolation are represented by the KERs. Consequently, the overall WoE for an AOP is a reflection in part, of the level of confidence in the underlying series of KERs it encompasses. Therefore, describing the KERs in an AOP involves assembling and organising the types of information and evidence that defines the scientific basis for inferring the probable change in, or state of, a downstream KE from the known or measured state of an upstream KE. More help

AOPs Referencing Relationship

AOP Name	Adjacency	Weight of Evidence	Quantitative Understanding	Point of Contact	Author Status	OECD Status
Deposition of Energy Leading to Learning and Memory Impairment	non-adjacent	Moderate	Low	Vinita Chauhan (send email)	Open for citation & comment

Taxonomic Applicability

Latin or common names of a species or broader taxonomic grouping (e.g., class, order, family) that help to define the biological applicability domain of the KER.In general, this will be dictated by the more restrictive of the two KEs being linked together by the KER. More help

Term	Scientific Term	Evidence	Link
human	Homo sapiens	Low	NCBI
mouse	Mus musculus	Moderate	NCBI
rat	Rattus norvegicus	Low	NCBI

Sex Applicability

An indication of the the relevant sex for this KER. More help

Sex	Evidence
Unspecific	Moderate

Life Stage Applicability

An indication of the the relevant life stage(s) for this KER. More help

Term	Evidence
All life stages	Moderate

Key Event Relationship Description

Provides a concise overview of the information given below as well as addressing details that aren’t inherent in the description of the KEs themselves. More help

Inflammatory mediators such as IL-1β, TNF-α, and IL-6, can influence the normal behavior of neuronal cells and their functional connection. Overexpression of pro-inflammatory mediators disrupts the integrity and function of the neuronal network through decreased neurogenesis, synaptic complexity and increased necrosis and demyelination, ultimately impairing learning and memory (Cekanaviciute, Rosi, & Costes, 2018; Fan & Pang, 2017). Impaired short-term and long-term memory, as well as associative learning are consequences of the dysregulated expression of pro-inflammatory cytokines as reported in behavioural paradigms (Donzis & Tronson, 2014).

Under physiological conditions, cytokine levels are low but greatly increased in response to various insults. Cytokines mediate immune response through ligand binding to cell surface receptors, which activate signaling cascades such as the JAK-STAT or MAPK pathways to produce or recruit more cytokines. Once organs initiate inflammatory reactions, the cytokines can modulate different metabolic and molecular pathways that have direct effects on neurons or indirect effects mediated by microglia, astrocytes or vascular endothelial cells (Mousa & Bakhiet, 2013; Prieto & Cotman, 2018). Modulation of these pathways ultimately affects crucial neuronal networks such as that within the hippocampus, which is one of the main brain regions responsible for learning and memory (Barrientos et al., 2015; Bourgognon & Cavanagh, 2020).

Evidence Collection Strategy

Include a description of the approach for identification and assembly of the evidence base for the KER. For evidence identification, include, for example, a description of the sources and dates of information consulted including expert knowledge, databases searched and associated search terms/strings. Include also a description of study screening criteria and methodology, study quality assessment considerations, the data extraction strategy and links to any repositories/databases of relevant references.Tabular summaries and links to relevant supporting documentation are encouraged, wherever possible. More help

The strategy for collating the evidence to support the relationship is described in Kozbenko et al 2022. Briefly, a scoping review methodology was used to prioritize studies based on a population, exposure, outcome, endpoint statement.

Evidence Supporting this KER

Addresses the scientific evidence supporting KERs in an AOP setting the stage for overall assessment of the AOP. More help

Overall Weight of Evidence: Moderate

Biological Plausibility

Addresses the biological rationale for a connection between KEupstream and KEdownstream. This field can also incorporate additional mechanistic details that help inform the relationship between KEs, this is useful when it is not practical/pragmatic to represent these details as separate KEs due to the difficulty or relative infrequency with which it is likely to be measured. More help

Several reviews provide support of biological plausibility between increase in pro-inflammatory mediators and impaired learning and memory. In the central nervous system, cytokines and their receptors are constitutively expressed and affect brain plasticity, which is the ability to modify its activity and connections in response to intrinsic or extrinsic stimuli. In a pathological state, pro-inflammatory, or Th-1 type cytokines become particularly relevant in the brain and these cytokines bind to their receptors to induce a conformational change and activate intracellular signaling pathways (Mousa & Bakhiet, 2013). The main cytokines presenting detrimental effects are IL-1β, TNF-α and IL-6, as these are the most studied.

The mechanism by which these cytokines modify learning and memory processes are not clearly understood due to the complexity of inflammatory signaling, although it involves alterations in the neural circuits that regulate these processes (Bourgognon & Cavanagh, 2020; Pugh et al., 2001). Multiple studies have demonstrated that IL-1β presents a critical role in the formation of hippocampal dependent memory, as IL-1β and its receptor are highly expressed in the hippocampus. Experimentally elevated levels of IL-1β in the hippocampus lead to impaired performance in behavioral paradigms such as spatial memory, contextual learning, and passive avoidance tasks (Donzis & Tronson, 2014; Pugh et al. 2001; Yirmiya & Goshen, 2011).

There are several possible mechanisms for this detrimental effect. One proposed mechanism is through reduced N-methyl-d-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor functions, both of which are involved in long-term potentiation, a process that strengthens synaptic connections between neurons. Other potential mechanisms for the effects of IL-1β on brain plasticity and memory can involve the activation of several pathways such as p38 mitogen-activated protein kinase (MAPK), c-jun NH2-terminal kinase (JNK), caspase 1, nuclear factor kappa B (NF-kB), and brain-derived neurotrophic factor (BDNF). These complex pathways have roles in neuronal health, long-term potentiation, brain plasticity and ultimately learning and memory (Patterson, 2015). The mechanisms of IL-6 and TNF-α in terms of impaired cognition also remain unclear, although the pathways that these cytokines activate are similar to those of IL-1β due to the network of cytokine interactions (Donzis & Tronson, 2014).

Empirical Evidence

Provides specific (citable) evidence that supports the idea of a change in the upstream KE (KEupstream) leading to, or being associated with, a subsequent change in the downstream KE (KEdownstream), assuming the perturbation of KEupstream is sufficient. More help

The empirical evidence collected for this KER comes from in-vivo studies and various methods used to assess the impairment in learning and memory. The behavioral paradigms test associative learning, as well as long and short-term memory (Bourgognon & Cavanagh, 2020).

Various studies also examined the presence of pro-inflammatory mediators induced by lipopolysaccharide (LPS) injections (Sparkman et al., 2006), Escherichia coli (E. coli) injections (Barrientos et al., 2009), direct cytokine injections (Gonzalez et al., 2009; Goshen et al., 2007; Taepavarapruk & Song, 2010), ionizing radiation (Bhat et al., 2020; Jenrow et al., 2013), surgical procedures (Tan et al., 2014) and transgenic models with overexpressed pro-inflammatory cytokines (Hein et al., 2010; Heyser et al., 1997; Moore et al., 2009).

Dose Concordance

Dose concordance relating an increase in pro-inflammatory mediators leading to an impaired learning and memory was demonstrated using various stressors, including ionizing radiation, LPS, IL-1β, and E. coli injections.

A 10 Gy X-ray irradiation in mice led to an increase in IL-6 secretion compared to dimethyl sulfoxide (DMSO) control. Impaired learning and memory were assessed by a decrease in freezing time in fear conditioning tests and a decrease in novel object recognition and object in place discrimination index (DI) (Bhat et al., 2020). A 10 Gy gamma irradiation of rats’ brains similarly indicated impaired cognitive function and an increase in OX-6+ cell density, indicative of inflammation (Jenrow et al., 2013).

Rats injected with 2.5x109 CFU of E. coli showed a significant increase in IL-1β in multiple regions of the brain, spleen and serum, along with impaired memory and learning as indicated by fear conditioning (Barrientos et al., 2009). Mice injected with 100 µg of LPS demonstrated an increase in IL-1β, TNF-α, IL-6 and IL-10 levels, while the performance in a Morris water maze was impaired, indicating impairments in spatial memory (Sparkman et al., 2006)..

Time Concordance

Various studies show that an increase in pro-inflammatory mediators is observed before or at the same time as impaired learning and memory. Some studies observe each event at the same time at specific timepoints. In mice injected with LPS, an increase in IL-1β, TNF-α, IL-6 and IL-10 was observed after 4 hours, while impaired learning and memory were also consistently observed 4 h after LPS injection (Sparkman et al., 2006). Both key events were also found in rats from 4 h to 8 days after E. coli injection (Barrientos et al., 2009). Cohort studies using age as a stressor in humans found increased pro-inflammatory mediators and decreased cognitive function over 6 months (Holmes et al., 2009) and 7 years (Alley et al., 2008). Many studies observe increased pro-inflammatory mediators occurring before impaired learning and memory. Rats with hippocampal injections of IL-1β showed impaired memory 24 h and 7 days later (Gonzalez et al., 2009). Rats showed increased hippocampal IL-1β and IL-6 levels 6 h after surgery to expose the right carotid artery, while learning and memory was impaired 2 weeks later (Tan et al., 2014). When mice were irradiated with X-rays, IL-6 was found to increase 24 h later, while learning and memory was impaired 5 weeks later (Bhat et al., 2020). Rats irradiated with gamma rays showed increased inflammation after 2 months, while memory was impaired after 6 months (Jenrow et al., 2013).

Incidence Concordance

Some evidence also shows that pro-inflammatory mediators increase equal to or greater than the amount that impairs learning and memory at the same stressor severity. Many of these studies were done with transgenic mice expressing IL-1β or IL6 as a stressor and showed increased levels of various pro-inflammatory mediators including ICAM-1, CCL2, IL-1α, COX-1 and MCP-1 from 2- to 147-fold, while learning and memory were decreased a maximum of 0.6-fold (Hein et al., 2010; Heyser et al., 1997; Moore et al., 2009).

Other Evidence

Multiple longitudinal cohort studies followed hundreds to thousands of older adults over the course of several years ranging from four to sixteen years and examined the relationship between pro-inflammatory marker levels and the rate of cognitive change over time. These studies reported linear negative correlations between IL-6 and TNF-α levels and learning and memory ability as the populations aged (Alley et al., 2010; Holmes et al., 2009; Schram et al., 2007).

Essentiality

Studies show that overexpression of pro-inflammatory cytokines affect learning and memory, and several treatments that alter the effects/function of pro-inflammatory mediators preserve cognitive function. The treatments included MW-151, a selective inhibitor of pro-inflammatory cytokine production, lidocaine, an anesthetic with anti-inflammatory properties, ethyl-eicosapentaenoate (E-EPA) and 1-[(4-nitrophenyl)sulfonyl]-4-phenylpiperazine (NSPP), both of which are anti-inflammatory drugs and α-melanocyte stimulating hormone (α-MSH), which antagonizes the effects of pro-inflammatory cytokines through G protein coupled receptors (Bhat et al., 2020; Gonzalez et al., 2009; Jenrow et al., 2013; Taepavarapruk and Song, 2010; Tan et al., 2014). More details are provided in the Modulating Factors section below.

Uncertainties and Inconsistencies

Addresses inconsistencies or uncertainties in the relationship including the identification of experimental details that may explain apparent deviations from the expected patterns of concordance. More help

Due to the indirect linkage between the two key events, there is no clear understanding of how increases in pro-inflammatory mediators cause impaired learning and memory. (Donzis & Tronson, 2014).
A previous prospective population-based cohort study found an association between antihistamine use and increased risk of dementia, which is the loss of cognitive functioning. Therefore, anti-inflammatory medications such as antihistamines could modulate the progression to impaired learning and memory (Gray et al., 2015).

Known modulating factors

This table captures specific information on the MF, its properties, how it affects the KER and respective references.1.) What is the modulating factor? Name the factor for which solid evidence exists that it influences this KER. Examples: age, sex, genotype, diet 2.) Details of this modulating factor. Specify which features of this MF are relevant for this KER. Examples: a specific age range or a specific biological age (defined by...); a specific gene mutation or variant, a specific nutrient (deficit or surplus); a sex-specific homone; a certain threshold value (e.g. serum levels of a chemical above...) 3.) Description of how this modulating factor affects this KER. Describe the provable modification of the KER (also quantitatively, if known). Examples: increase or decrease of the magnitude of effect (by a factor of...); change of the time-course of the effect (onset delay by...); alteration of the probability of the effect; increase or decrease of the sensitivity of the downstream effect (by a factor of...) 4.) Provision of supporting scientific evidence for an effect of this MF on this KER. Give a list of references. More help

Modulating factor	Details	Effects on the KER	References
Drug	NSPP (anti-inflammatory drug)	There was a 0.35-fold decrease in IL-6 levels compared to controls when 10 Gy irradiated mice were treated with NSPP. Mice that were exposed to whole brain irradiation (10 Gy) and treated with NSPP (5 mg/kg) exhibited significantly improved performance in novel object recognition and object in place.	Bhat et al., 2020
Drug	α-MSH (modulator of action of pro-inflammatory cytokines)	α-MSH injected into the hippocampus prevented the IL-1β-induced decrease in contextual fear memory.	Gonzalez et al., 2009
Drug	MW-151 (inhibitor of pro-inflammatory microglial cytokine production)	Treatment decreased the OX-6+ cell density and restored memory.	Jenrow et al., 2013
Drug	Lidocaine (an anti-inflammatory local anesthetic)	Lidocaine treatment restored IL-6 levels and improved memory.	Tan et al., 2014
Age	Young age	Rats aged 3 months showed increased IL-1β levels in the hippocampus for less time than in mice aged 24 months. Hippocampal-dependent memory was impaired in the old mice. The inflammatory response is shorter and less severe in young individuals, leading to reduced cognitive impairment.	Barrientos et al., 2009; Barrientos et al., 2012
Drug	E-EPA (known to improve cognitive function through reducing inflammation)	E-EPA reduced the increase in IL-6 expression and improved memory to control levels.	Taepavarapruk & Song, 2010

Quantitative Understanding of the Linkage

Captures information that helps to define how much change in the upstream KE, and/or for how long, is needed to elicit a detectable and defined change in the downstream KE. More help

The table below provides some representative examples of quantitative linkages between the two key events. It was difficult to identify a general trend across all the studies due to differences in experimental design and reporting of the data. All data is statistically significant unless otherwise stated.

Dose Concordance

Reference	Experiment Description	Result
Sparkman et al., 2006	In-vivo. Three-month-old male C57BL/6 mice were injected with 100 µg of LPS. Pro-inflammatory mediator levels were determined with immunohistochemical staining. Spatial working memory was evaluated through a Morris water maze.	IL-1β increased from 15 pg/mL (control) to 400 pg/mL. TNF-α increased from <5 pg/mL (control) to 360 pg/mL. IL-6 increased from undetectable levels (control) to 300 pg/mL. IL-10 increased from undetectable levels (control) to 260 pg/mL. Performance in the water maze was impaired in distance swam, latency and swim speed after LPS injection, with a maximum 2-fold increased swim distance.
Taepavarapruk & Song, 2010	In-vivo. Male Long-Evans rats were administered 15 ng/µL/day of IL-1β for either 1 or 7 days. IL-1 expression was determined, and memory was assessed with an eight-arm radial maze.	IL-1β administration of 105 ng/µL resulted in a 2.5-fold increase in IL-1 expression and a 1.5-fold increase in number of entries, indicating memory impairment.
Goshen et al., 2007	In-vivo. 2–4-month-old male mice were injected with IL-1β and IL-1ra (IL-1 receptor antagonist) at 1 or 10 ng (injected in 10 µL) doses. IL-1β was assessed by quantitative real time RT-PCR, and hippocampal IL-1ra was determined by ELISA. Contextual fear conditioning was used to assess associative learning and memory. Evaluations were taken after 3 weeks of recovery. Water maze was used for spatial memory testing.	Mice that were injected with a high dose of IL-1β (10 ng) had shorter freezing times, indicating impaired contextual fear conditioning, whereas mice injected with a low dose of IL-1β (1 ng) showed longer freezing times, indicating improved contextual fear conditioning. Spatial memory was impaired in IL-1raTG rats (astrocyte-directed overexpression of IL-1ra) as latency and path length was increased (non-significantly) in the majority of the trials compared to wild type rats.
Gonzalez et al., 2009	In-vivo. Adult male Wistar rats had hippocampal injections of 5 ng/0.25 µL of IL-1β post-conditioning and memory was assessed through freezing behavior.	Rats injected with 5 ng/0.25 µL IL-1β displayed impaired contextual fear memory, where injected rats spent 0.6-fold less time freezing.
Bhat et al., 2020	In-vivo. Female mice were exposed to X-ray irradiation at 0, 2, 4 and 10 Gy (5.519 Gy/min). Novel object recognition, object in place and fear conditioning evaluated memory. IL-6 levels were measured by ELISA.	There was a significant 2.97-fold increase in the secretion of IL-6 in cells irradiated with 10 Gy compared to DMSO control. DMSO was used as a control as treatment mice were also treated with NSPP solubilized in DMSO. Irradiated mice demonstrated a significant decrease in DI for novel object recognition and object in place. Irradiated mice also spent significantly less time freezing in context fear, context gen and pre-tone assessments of fear conditioning.
Jenrow et al., 2013	In-vivo. Adult male Fischer 344 rats’ brains were irradiated with 10 Gy gamma rays. OX-6 levels (indicative of inflammation levels) were determined, and novel object recognition was performed to assess memory.	10 Gy increased OX-6+ cell density from 1493±270 to 1966±218 cells/mm3. Also, after 10 Gy, the discrimination ratio for novel object recognition decreased from 68.76±11.30% to 21.71±10.86%.
Barrientos et al., 2009	In-vivo. Male F344xBN F1 rats, either old (24 months) or young (3 months), received an injection of 2.5x109 CFU of E. coli. IL-1β levels were determined using ELISA and fear conditioning was performed to assess learning and memory.	Significant increases in IL-1β were observed in the hippocampus, hypothalamus, parietal cortex, serum and spleen after injection, with a maximum 4-fold increase. Injected mice also spent 0.6-fold less time freezing.

Time Concordance

Reference	Experiment Description	Result
Sparkman et al., 2006	In-vivo. Three-month-old male C57BL/6 mice were injected with 100 µg of LPS. Pro-inflammatory mediator levels were determined with immunohistochemical staining and measured 4 h after LPS injection. Spatial working memory was evaluated 4 h after LPS injection through a Morris water maze.	IL-1β increased from 15 pg/mL (control) to 400 pg/mL. TNF-α increased from <5 pg/mL (control) to 360 pg/mL. IL-6 increased from undetectable levels (control) to 300 pg/mL. IL-10 increased from undetectable levels (control) to 260 pg/mL. Performance in the water maze was found impaired in distance swam, latency and swim speed 4 h after LPS injection, with a maximum 2-fold increased swim distance.
Gonzalez et al., 2009	In-vivo. Adult male Wistar rats had hippocampal injections of 5 ng/0.25 µL of IL-1β post-conditioning and memory was assessed through freezing behavior.	Rats spent 0.6- to 0.7-fold less time freezing 24 h after injection with IL-1β. Rats also showed impaired long-term memory 7 days after injection when they spent 0.7-fold less time frozen.
Bhat et al., 2020	In-vivo. Female mice were exposed to X-ray irradiation at 0, 2, 4 and 10 Gy (5.519 Gy/min). Novel object recognition, object in place and fear conditioning evaluated memory. IL-6 levels were measured by ELISA.	24 hours after exposure to 10 Gy irradiation, mice showed a significant 2.97-fold increase in the secretion of IL-6 compared to DMSO control (DMSO was used as a control as mice were also treated with NSPP solubilized in DMSO). At week 5, irradiated mice demonstrated a significant decrease in DI for novel object recognition and object in place, and spent less time freezing in context fear, context gen and pre-tone assessments of fear conditioning.
Jenrow et al., 2013	Adult male Fischer 344 rats’ brains were irradiated with 10 Gy 137Cs gamma rays. OX-6 levels (indicative of inflammation levels) were determined, and novel object recognition was performed to assess memory.	After 2 months, OX-6+ cell density increased from 1493±270) to 1966±218 cells/mm3. A similar but smaller increase was found after 9 months. Only measured after 6 months, the discrimination ratio for novel object recognition decreased from 68.76±11.30% to 21.71±10.86%.
Tan et al., 2014	In-vivo. Four-month-old male Fischer 344 rats had 1 cm of right carotid artery dissected free from surrounding tissue in a 15-minute surgery. IL-6 and IL-1β levels were determined by western blot and a Barnes maze was used to test spatial learning and memory.	Surgery increased IL-1β and IL-6 in the hippocampus by 2- to 3-fold compared to controls 6 h after surgery. Additionally, rats in the surgery group took 2- to 3-fold more time to identify the target in the Barnes maze task 2 weeks after surgery.
Barrientos et al., 2009	In-vivo. Male F344xBN F1 rats, either old (24 months) or young (3 months), received an injection of 2.5x109 CFU of E. coli. IL-1β levels were determined using ELISA and fear conditioning was performed to assess hippocampal-dependent learning and memory.	Significant increases in IL-1β were observed in the hippocampus, hypothalamus, parietal cortex, serum and spleen most frequently 4 h after injection, with a maximum 4-fold increase. IL-1β in the hypothalamus was also increased in old mice up to 8 days after injection. Old mice spent less time freezing 4 days after injection, while old injected mice also spent 0.6-fold less time freezing 8 days after injection.
Alley et al., 2008	In-vivo. A cohort study of older adults aged 70-79 years that were tested in 1988, 1991 and 1995 to determine changes in cognitive functioning and IL-6 levels. ELISA was used to measure IL-6 levels. Cognitive function was determined by various tests, including spatial recognition, spatial ability, verbal recall, language and abstraction. Short Portable Mental Status Questionnaire (SPMSQ) was used as a measure of cognitive performance. Participants were re-interviewed at 2.5 and 7 years.	As IL-6 levels increased, mean cognitive scores decreased compared to baseline cognitive scores. A linear inverse association was found between inflammation and general cognitive scores, both measured at the end of the 7-year study.
Holmes et al., 2009	In-vivo. A cohort study of subjects with mild to severe Alzheimer’s disease who were cognitively assessed and tested for inflammatory markers. Cognitive assessments were performed using the Alzheimer's Disease Assessment Scale (ADAS-COG) test. The sandwich immunoassay multiplex cytokine assay measured TNF-α at 2, 4 and 6 months	Subjects with high TNF-α levels (3.2 [standard error (SE) 0.6]) at baseline observed greater changes in ADAS-COG over 6-months compared to subjects with low TNF-α (0.8 [SE 0.8]). The mean change in ADAS-COG score was 2.6 (±7.0) points over the 6 months.

Incidence Concordance

Reference	Experiment Description	Result
Moore et al., 2009	In-vivo. Transgenic mice overexpressing IL-1β (activated by microinjection of FIV-Cre) for 2 weeks underwent spatial and non-spatial behavioral tasks using a Morris water maze. IL-1β, IL-1α and MCP-1 were measured by RT-PCR.	Measurement of IL-1β in hippocampal tissue revealed mRNA levels increased 22.9-fold. The pro-inflammatory mediators IL-1α and MCP-1 mRNA levels were also increased 3.1- and 147-fold, respectively. Overexpression of IL-1β in the hippocampus hindered acquisition and long-term memory retention on the spatial task but did not impact non-spatial learning.
Hein et al., 2010	In-vivo. Male and female IL-1βXAT mice on a C57BL/6 background (containing a dormant human IL-1β gene activated by a virus expressing Cre) were injected with 1.5x104 viral particles of the feline immunodeficiency virus (expresses Cre) in the hippocampus. Fear conditioning and a Morris water maze were performed to assess learning and memory.	IL-1β was increased 15-fold in the hippocampus, and the expression of other pro-inflammatory mediators CCL2, IL-1α and COX-1 were similarly increased. Mice spent 0.6-fold less time freezing and 0.8-fold less time in the target quadrant, indicating impaired learning and memory. No differences between males and females were observed.
Heyser et al., 1997	In-vivo. C57BL/6 x SJL hybrid mice with a GFAP-IL6 fusion gene were tested for avoidance learning and expression of ICAM-1.	Compared to non-transgenic (+/+) mice, ICAM-1 was increased 2-fold at 3 months old and 4-fold at 12 months old in both heterozygous (+/tg) and homozygous (tg/tg) transgenic mice. Also compared to +/+ mice, +/tg mice showed impaired avoidance response at 12 months old, where tg/tg mice showed impaired avoidance at 3, 6 and 12 months old.

Other Evidence

Reference	Experiment Description	Result
Alley et al., 2008	In-vivo. A cohort study of older adults aged 70 to 79 years that were tested in 1988, 1991 and 1995 to determine changes in cognitive functioning and IL-6 levels. ELISA was used to measure IL-6 levels. Cognitive function was determined by various tests, including spatial recognition, spatial ability, verbal recall, language and abstraction. Short Portable Mental Status Questionnaire (SPMSQ) was used as a measure of cognitive performance. Participants were re-interviewed at 2.5 and 7 years.	As IL-6 levels increased, mean cognitive scores decreased compared to baseline cognitive scores. A linear inverse association was found between inflammation and general cognitive scores, both measured over the course of 7 years. Participants in the top IL-6 tertile (IL-6 > 3.8 pg/mL) had 62% increased odds of declines in global cognitive function (Odds Ratio (OR) = 1.62, 95% Confidence Interval (CI), 1.07–2.45). They also had 88% increased odds of cognitive impairment, (OR = 1.88, 95% CI, 1.20–2.94), relative to those with lower levels of IL-6.
Holmes et al., 2009	In-vivo. A cohort study of subjects with mild to severe Alzheimer’s disease were cognitively assessed and tested for inflammatory markers. Cognitive assessments were performed using the Alzheimer's Disease Assessment Scale (ADAS-COG) test. The sandwich immunoassay multiplex cytokine assay measured TNF-α at 2, 4 and 6 months	Subjects with high TNF-α levels (3.2 [SE 0.6]) at baseline observed greater changes in ADAS-COG over 6-months compared to subjects with low TNF-α (0.8 [SE 0.8]). The mean change in ADAS-COG score was 2.6 (SD 7.0) points over the 6 months.
Schram et al., 2007	In-vivo. The Leiden 85-plus Study, performed with participants aged 85 to 90 years (n= 705), assessed memory function and its association with inflammatory markers. IL-6 plasma levels were measured by ELISA. C-reactive protein (CRP) pro-inflammatory mediator was measured by Rate Near Infrared Particle Immunoassay. The 12-Picture Learning Test was used to evaluate memory function.	When levels of pro-inflammatory mediators were higher than baseline, delayed recall memory point estimate was negative, indicating impaired memory.

Response-response Relationship

Provides sources of data that define the response-response relationships between the KEs. More help

Time-scale

Information regarding the approximate time-scale of the changes in KEdownstream relative to changes in KEupstream (i.e., do effects on KEdownstream lag those on KEupstream by seconds, minutes, hours, or days?). More help

Known Feedforward/Feedback loops influencing this KER

Define whether there are known positive or negative feedback mechanisms involved and what is understood about their time-course and homeostatic limits. More help

Not identified.

Domain of Applicability

A free-text section of the KER description that the developers can use to explain their rationale for the taxonomic, life stage, or sex applicability structured terms. More help

Evidence for this relationship comes from human, rat, and mouse models, with most of the evidence in mice. The relationship is not sex or life stage specific.

References

List of the literature that was cited for this KER description. More help

Alley, D. E. et al. (2008), "Inflammation and Rate of Cognitive Change in High-Functioning Older Adults", The Journals of Gerontology Series A: Biological Sciences and Medical Sciences, Vol. 63/1, Oxford University Press, Oxford, https://doi.org/10.1093/gerona/63.1.50.

Barrientos, R. M. et al. (2015), "Neuroinflammation in the normal aging hippocampus", Neuroscience, Vol. 309, Elsevier, Amsterdam, https://doi.org/10.1016/j.neuroscience.2015.03.007.

Barrientos, R. M. et al. (2009), "Time course of hippocampal IL-1 β and memory consolidation impairments in aging rats following peripheral infection", Brain, Behavior, and Immunity, Vol. 23/1, Elsevier, Amsterdam, https://doi.org/10.1016/j.bbi.2008.07.002.

Barrientos, R. M. et al. (2012), "Aging-related changes in neuroimmune-endocrine function: Implications for hippocampal-dependent cognition", Hormones and Behavior, Vol. 62/3, Elsevier, Amsterdam, https://doi.org/10.1016/j.yhbeh.2012.02.010.

Bhat, K. et al. (2020), "1-[(4-Nitrophenyl)sulfonyl]-4-phenylpiperazine treatment after brain irradiation preserves cognitive function in mice", Neuro-Oncology, Vol. 22/10, Oxford University Press, Oxford, https://doi.org/10.1093/neuonc/noaa095.

Bourgognon, J.-M. and J. Cavanagh. (2020), "The role of cytokines in modulating learning and memory and brain plasticity", Brain and Neuroscience Advances, Vol. 4, https://doi.org/10.1177/2398212820979802.

Cekanaviciute, E., S. Rosi and S. Costes. (2018), "Central Nervous System Responses to Simulated Galactic Cosmic Rays", International Journal of Molecular Sciences, Vol. 19/11, MDPI, Basel, https://doi.org/10.3390/ijms19113669.

Donzis, E. J. and N. C. Tronson. (2014), "Modulation of learning and memory by cytokines: Signaling mechanisms and long term consequences", Neurobiology of Learning and Memory, Vol. 115, Elsevier, Amsterdam, https://doi.org/10.1016/j.nlm.2014.08.008.

Fan, L. W. and Y. Pang. (2017), "Dysregulation of neurogenesis by neuroinflammation: key differences in neurodevelopmental and neurological disorders", Neural Regeneration Research, Vol. 12/3, Wolters Kluwer, https://doi.org/10.4103/1673-5374.202926.

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