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Relationship: 3649

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

Decreased, Gonadotropins leads to prolonged, estrus cycle

Upstream event

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

Decreased, Gonadotropins

Downstream event

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

prolonged, estrus cycle

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
Activation, estrogen receptor alpha leads to prolonged estrus cycle via decreased kisspeptin release	adjacent	High		John Frisch (send email)	Under development: Not open for comment. Do not cite

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
mammals	mammals	High	NCBI

Sex Applicability

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

Sex	Evidence
Female	High

Life Stage Applicability

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

Term	Evidence
Adult, reproductively mature	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

Gonadotropins are hormones in mammals that cue development of reproductive organs to maturity (Casarini and Simoni 2021; Howard 2021) and the different phases of the estrus cycle (Uenoyama et al. 2021). Key gonadotropins include Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH), which are released from the anterior pituitary gland (Howard 2021).

The estrus cycle is a coordinated series of changes that results in fertility in mammals. Changes to the uterus and vagina are coordinated through hormone signaling, including Progesterone, Estradiol, Luteinizing Hormone, and Follicle-Stimulating Hormone in order to progress through metestrus, diestrus, proestrus, and estrous phases over a period of 4-5 days in rodents (Miller and Takahashi 2014; Swift et al. 2014). In proestrus, increased estradiol levels occur, and physiological changes include ovarian follicle development and the thickening of the uterine wall in preparation for potential pregnancy. In estrus, a surge in luteinizing hormone levels occur, and ovulation of the mature egg. Metestrus is a short transition between estrus and diestrus, features an increase in progesterone levels, and development of the corpus luteum begins in preparation for pregnancy. Diestrus includes continued high levels of progesterone and further development of the corpus luteum; if pregnancy does not occur the corpus luteum regresses and resetting of the cycle occurs. Prolonged estrous occurs when the normal estrus cycle progression has been disrupted, generally through increased diestrus.

Primates (including humans) have a menstrual cycle of about 28 days rather than an estrus cycle as the lining of the uterus is shed rather than being reabsorbed.

From Swift et al. (2024)

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

This Key Event Relationship was part of an Environmental Protection Agency effort to develop AOPs that establish scientifically supported causal linkages between alternative endpoints measured using new approach methodologies (NAMs) and guideline apical endpoints measured in Tier 1 and Tier 2 test guidelines (U.S. EPA, 2024) employed by the Endocrine Disruptor Screening Program (EDSP). A series of key events that represent significant, measurable, milestones connecting molecular initiation to apical endpoints indicative of adversity were identified based on scientific review articles and empirical studies. Additionally, scientific evidence supporting the causal relationships between each pair of key events was assembled and evaluated. The present effort focused primarily on empirical studies with laboratory rodents and other mammals.

Cited empirical studies are focused on decreased gonatotropins and resulting prolonged estrus in laboratory mammals, in support of development of AOP 609.

Authors of KER 3649 did a further evaluation of published peer-reviewed literature to provide additional evidence in support of the key event relationship. In addition, search engines were used to target journal articles with terms ‘Gonadotropin’, ‘Luteinizing hormone’, ‘Follicle-stimulating hormone’, and ‘estrous’ or ‘estrus’.

Evidence Supporting this KER

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

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

Decreased gonadotropins and resulting prolonged estrus have been studied in laboratory mammals by toxicants known to increase estrogen receptor activation (Feng et al. 2015; Cao et al. 2018; Wang et al. 2018; Tang et al. 2020; Yin et al. 2021). Ovariectomized animals have been used to show the role of hormones in reproductive development and the estrus cycle, with hormone replacement restoring normal function (Clarkson et al. 2008; Feng et al. 2015; Dubois et al 2015). The role of gonadotropins in cuing the various phases of the estrus cycle has been well studied in laboratory mammals (e.g. Miller and Takahashi 2014; Swift et al. 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

Species	Duration	Dose	Decreased Gonadotropins?	Prolonged estrus?	Summary	Citation
Mice (Mus musculus)	4 months	0.1 mg/kg/bw/d PFOS.	yes	yes	Female mice exposed to PFOS had statistically significant decreased FSH and LH hormone leading to statistically significant prolonged estrus from increased duration of diestrus.	Feng et al. (2015)
Mice (Mus musculus)	50 days	1,10,100 mg/kg/day Triclosan.	yes	yes	Female mice exposed to 10, 100 mg/kg/day Triclosan had statistically significant decreased FSH and LH hormone leading to statistically significant prolonged estrus at 10, 100 mg/kg/day from increased duration of diestrus.	Cao et al. (2018)
Mice (Mus musculus)	30 days	10 mg/kg PFOS.	yes	yes	Female mice exposed to PFOS had statistically significant decreased LH hormone leading to statistically significant prolonged estrus from increased duration of diestrus.	Wang et al. (2018)
Mice (Mus musculus)	60 days	50 ug/kg/bw BPA	yes	yes	Female mice exposed to BPA had statistically significant decreased FSH and LH hormone leading to statistically significant prolonged estrus from increased duration of diestrus.	Tang et al. (2020)
Mice (Mus musculus)	42 days	5 mg/kg/d PFHxS	yes	yes	Female mice exposed to BPA had statistically significant decreased FSH and LH hormone leading to statistically prolonged estrus from increased duration of diestrus.	Yin et al. (2021)

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

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

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

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

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

Life Stage: Applies to adults, reproductively mature and juveniles.

Sex: Applies to females.

Taxonomic: Primarily studied in humans and laboratory rodents. Plausible for most mammals due to shared reproductive physiology and hormones. Primates have menstrual cycles as the lining of the uterus is shed rather than being reabsorbed.

References

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

Cao XY, Hua X, Xiong JW, Zhu WT, Zhang J, Chen L. 2018. Impact of Triclosan on Female Reproduction through Reducing Thyroid Hormones to Suppress Hypothalamic Kisspeptin Neurons in Mice. Frontiers in Molecular Neuroscience 11(6).

Casarini, L. and Simoni M. 2021. Recent advances in understanding gonadotropin signaling. Faculty Reviews 10: 41.

Clarkson J, d'Anglemont de Tassigny X, Moreno AS, Colledge WH, Herbison AE. 2008. Kisspeptin-GPR54 signaling is essential for preovulatory gonadotropin-releasing hormone neuron activation and the luteinizing hormone surge. The Journal of Neuroscience 2008 28(35): 8691-8697.

Dubois SL, Acosta-Martínez M, DeJoseph MR, Wolfe A, Radovick S, Boehm U, Urban JH, Levine JE. 2015. Positive, but not negative feedback actions of estradiol in adult female mice require estrogen receptor α in kisspeptin neurons. Endocrinology 156(3): 1111-1120.

Feng X, Wang X, Cao X, Xia Y, Zhou R, Chen L. 2015. Chronic Exposure of Female Mice to an Environmental Level of Perfluorooctane Sulfonate Suppresses Estrogen Synthesis Through Reduced Histone H3K14 Acetylation of the StAR Promoter Leading to Deficits in Follicular Development and Ovulation. Toxicological Sciences 148(2): 368-379.

Howard, S.R. 2021. Interpretation of reproductive hormones before, during and after the pubertal transition—identifying health and disordered puberty. Clinical Endocrinolology 95: 702-715.

Miller, B.H. and Takahashi, J.S. 2014. Central circadian control of female reproductive function. Frontiers in Endocrinology 4(1): 195.

Swift, K.M., Gary, N.C., and Urbanczyk, P.J. 2024. On the basis of sex and sleep: the influence of the estrous cycle and sex on sleep-wake behavior. Frontiers in Neuroscience 18:1426189.

Tang C, Zhang J, Liu P, Zhou Y, Hu Q, Zhong Y, Wang X, Chen L. 2020. Chronic exposure to low dose of bisphenol A causes follicular atresia by inhibiting kisspeptin neurons in anteroventral periventricular nucleus in female mice. Neurotoxicology 79: 164-176.

Uenoyama, Y., Inoue, N., Nakamura, S., and Tsukamura, H. 2021. Kisspeptin Neurons and Estrogen–Estrogen Receptor α Signaling: Unraveling the Mystery of Steroid Feedback System Regulating Mammalian Reproduction. International Journal of Molecular Sciences 22(17): 9229.

U.S. Environmental Protection Agency. 2004. EDSP Test Guidelines and Guidance Document. https://www.epa.gov/test-guidelines-pesticides-and-toxic-substances/edsp-test-guidelines-and-guidance-document (retrieved 25 July 2025).

Wang X, Bai Y, Tang C, Cao X, Chang F, Chen L. 2018. Impact of Perfluorooctane Sulfonate on Reproductive Ability of Female Mice through Suppression of Estrogen Receptor α-Activated Kisspeptin Neurons. Toxicological Sciences 165(2): 475-486.

Yin X, Di T, Cao X, Liu Z, Xie J, Zhang S. 2021. Chronic exposure to perfluorohexane sulfonate leads to a reproduction deficit by suppressing hypothalamic kisspeptin expression in mice. Journal of Ovarian Research 14(1): 141.

Italics indicate edits from John Frisch October 2025. A full list of updates can be found in the Change Log on the View History page.