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Relationship: 1505


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

TH synthesis, Decreased leads to GABAergic interneurons, Decreased

Upstream event
The causing Key Event (KE) in a Key Event Relationship (KER). More help
Downstream event
The responding Key Event (KE) in a Key Event Relationship (KER). More help

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
Inhibition of Na+/I- symporter (NIS) leads to learning and memory impairment non-adjacent Low Low Arthur Author (send email) Open for citation & comment WPHA/WNT Endorsed

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
rat Rattus norvegicus Moderate NCBI
mouse Mus musculus Moderate 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
During brain development 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

Thyroid hormone synthesis is responsible for physiological TH serum levels that subsequently correlate with TH brain concentrations. It has been shown that TH regulates function of different neuronal subtypes, including GABAergic neurons. TH increases glutamic acid decarboxylase (GAD) activity (responsible for GABA-synthesis) in neonatal brain, and GABA transaminase (responsible for GABA degradation) activity (Shulga and Rivera, 2013). GABAergic interneurons are remarkably diverse and complex in nature and they are believed to play a key role in numerous neurodevelopmental processes (Southwell et al., 2014). During the early cortical network development TH has been shown to regulate the morphology and function of the GABAergic neurons (Westerholz et al., 2010). It is well documented that decreased TH synthesis triggered by TPO and NIS inhibitors affects survival of GABAergic interneurons, as well as their morphology and function.

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

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

TH levels influence the development of cortical GABAergic circuits (Friauf et al., 2008; Westerholz et al., 2010). In hypothyroid rats the expression of parvalbumin, the marker of a subpopulation of GABAergic neurons, is reduced (Gilbert et al., 2007). TH increase glutamic acid decarboxylase (GAD, GABA-synthesizing enzyme) activity in neonatal brain. Also GABA transaminase (GABA-T, GABA-degrading enzyme) activity appears to be increased by TH. Therefore, both GABA synthesis and degradation are increased by TH. This might reflect either the specific regulation of GABA levels, or general regulation of gene expression maintenance by TH, as commented by Shulga and Rivera, 2013. This strongly supports the link between the two KEs described in this indirect KER (decrease of TH synthesis leads to GABAergic interneuron decrease). It was also shown that low concentrations of T3 increase by non-genomic mechanism the depolarization-dependent release of GABA. GABA appears to provide negative feedback to thyroid endocrine axis, as TSH release is inhibited by GABA (Wiens and Trudeau, 2006).

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

While some in vivo studies (Sawano et al., 2013; Shiraki et al., 2012) have shown a decrease of GABAergic cell populations upon induction of hypothyroidism, Saegusa and co-workers (Saegusa et al., 2010) reported about an increase of GABAergic interneurons. In Saegusa's study, rat dams were treated with either PTU or MMI in the drinking water, and male offspring were immunohistochemically examined on PND 20 and at the adult stage (i.e., 11-week-old). MMI and PTU caused in the offspring growth retardation, lasting into the adult stage. All exposure groups showed a sustained increase of GAD67+ cells in the adult stage, indicating an increase in GABAergic interneurons.

It should be noticed that in Saegusa et al., 2010 in vivo study, increase of GAD67+ cells was mainly observed in the adult stage (11-week-old rats) and analysis of GABAergic interneurons.  PV+ cells, which appear to be the GABAergic population most affected by TH dysregulation, was not evaluated. On the opposite, Sawano's and Shiraki's in vivo studies reported a decrease of GABAergic PV+ neurons at earlier stages, respectively on PND 15 and 21 induced by hypothyroidism (Sawano et al., 2013; Shiraki et al., 2012). Discrepancies in results are due to the fact that THs have effects on multiple components of the GABA system. For instance, in the developing brain, hypothyroidism generally decreases enzyme activities and GABA levels, whereas in adult brain, hypothyroidism generally increases enzyme activities and GABA levels.

There are also conflicting results on effects of long term changes in TH levels on GABA reuptake. Therefore, results variability from study to study is due to different experimental study designs, accounting for differences in brain development stages (PND vs adult), times of exposures to chemicals, and regional brain differences (Wiens and Trudeau, 2006).

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

Empirical evidence comes from work with laboratory rodents (rats and mice).


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

Aguado F, Carmona MA, Pozas E, Aguiló A, Martínez-Guijarro FJ, Alcantara S, Borrell V, Yuste R, Ibañez CF, SorianoE. (2003). BDNF regulates spontaneous correlated activity at early developmental stages by increasing synaptogenesis and expression of the K+/Cl–co-transporter KCC2. Development 130:1267-1280.

Blaesse P, Airaksinen MS, Rivera C, Kaila K. (2009). Cation chloride co-transporters and neuronal function. Neuron 61:820–838

Friauf E, Wenz M, Oberhofer M, Nothwang HG, Balakrishnan V, Knipper M, Lohrke S. (2008). Hypothyroidism impairs chloride homeostasis and onset of inhibitory neurotransmission in developing auditory brainstem and hippocampal neurons. Eur J Neurosci, 28, pp. 2371–2380

Gilbert ME, Sui L, Walker MJ, Anderson W, Thomas S, Smoller SN, Schon JP, Phani S, Goodman JH. (2007). Thyroid hormone insufficiency during brain development reduces parvalbumin immunoreactivity and inhibitory function in the hippocampus. Endocrinology. Jan;148(1):92-102.

Madeira MD, Cadete-Leite A, Andrade JP, Paula-Barbosa MM. (1991). Effects of hypothyroidism upon the granular layer of the dentate gyrus in male and female adult rats: a morphometric study. J Comp Neurol 314:171-186.

Rami A, Patel AJ, Rabie A. (1986a). Thyroid hormone and development of the rat hippocampus: morphological alterations in granule and pyramidal cells. Neuroscience 19:1217-1226.

Rami A, Rabie A, Patel AJ. (1986b). Thyroid hormone and development of the rat hippocampus: cell acquisition in the dentate gyrus. Neuroscience 19:1207-1216.

Rami A, Rabie A. (1990). Delayed synaptogenesis in the dentate gyrus of the thyroid-deficient developing rat. Dev Neurosci 12:398-405.

Rivera, C., Li, H., Thomas-Crusells, J., Lahtinen, H., Vilkman, V., Nanobashvili, A., Kokaia, Z., Airaksinen, M.S., Voipio, J., Kaila, K. & Saarma, M. (2002). BDNF-induced TrkB activation down-regulates the K+–Cl− cotransporter KCC2 and impairs neuronal Cl− extrusion. J. Cell Biol., 159, 747–752.

Rivera, C., Voipio, J., Thomas-Crusells, J., Li, H., Emri, Z., Sipilä, S., Payne, J.A., Minichiello, L., Saarma, M. & Kaila, K. (2004). Mechanism of activity-dependent downregulation of the neuron-specific K-Cl cotransporter KCC2. J. Neurosci., 24, 4683–4691.

Sawano E, Takahashi M, Negishi T, Tashiro T. (2013). Thyroid hormone-dependent development of the GABAergic pre- and post-synaptic components in the rat hippocampus. Int J Dev Neurosci. Dec;31(8):751-61.

Shiraki A, Akane H, Ohishi T, Wang L, Morita R, Suzuki K, Mitsumori K, Shibutani M. (2012). Similar distribution changes of GABAergic interneuron subpopulations in contrast to the different impact on neurogenesis between developmental and adult-stage hypothyroidism in the hippocampal dentate gyrus in rats. Arch Toxicol. Oct;86(10):1559-69.

Shulga A, Rivera C. (2013). Interplay between thyroxin, BDNF and GABA in injured neurons. Neuroscience. Jun 3;239:241-52.

Southwell DG, Nicholas CR, Basbaum AI, Stryker MP, Kriegstein AR, Rubenstein JL, Alvarez-Buylla A. (2014). Interneurons from embryonic development to cell-based therapy. Science. 44:1240622.

Wake, H., Watanabe, M., Moorhouse, A.J., Kanematsu, T., Horibe, S., Matsukawa, N., Asai, K., Ojika, K., Hirata, M. & Nabekura, J. (2007). Early changes in KCC2 phosphorylation in response to neuronal stress result in functional downregulation. J. Neurosci., 27, 1642–1650.

Westerholz S, de Lima AD, Voigt T. (2010). Regulation of early spontaneous network activity and GABAergic neurons development by thyroid hormone. Neuroscience 168:573-589.

Westerholz S, de Lima AD, Voigt T. (2013). Thyroid hormone-dependent development of early cortical networks: temporal specificity and the contribution of trkB and mTOR pathways. Front Cell Neurosci 7:121.

Wiens SC, Trudeau VL. (2006). Thyroid hormone and gamma-aminobutyric acid (GABA) interactions in neuroendocrine systems. Comp Biochem Physiol A Mol Integr Physiol. Jul;144(3):332-44. Epub 2006 Mar 9.