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Relationship: 1506

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

TH synthesis, Decreased leads to Impairment, Learning and memory

Upstream event

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

TH synthesis, Decreased

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
Inhibition of Na+/I- symporter (NIS) leads to learning and memory impairment	non-adjacent	High	Moderate	Anna Price (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
human	Homo sapiens	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	High

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

It is widely accepted that the thyroid hormones (TH) play a prominent role in the development and function of the CNS, including hippocampus and neocortex, two critical brain structure closely linked to the cognitive function (Gilbert et al., 2012). Brain concentrations of T4 are dependent on transfer of T4 from serum, through the vascular endothelia, into astrocytes. In astrocytes, T4 is converted to T3 by deiodinase and subsequently transferred to neurons cellular membrane transporters. In the brain T3 controls transcription and translation of genes responsible for normal hippocampal structural and functional development. Normal hippocampal structure and physiology are critical for the development of cognitive function. Thus, there is an indisputable indirect link between TH synthesis, controlling the levels of T4 in serum, and cognitive function, including learning and memory processes.

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

The weight of evidence supporting the relationship between decreased TH synthesis and learning and memory impairments (occurring as a consequence of altered neuronal network and synaptic function) is strong (Vara et al., 2002; Sui and Gilbert, 2003, 2004, 2011;Dong et al., 2005; Sui et al., 2005). This is consistent with the well understood and documented relationship between TH synthesis that is responsible for TH concentrations in serum, and consequently in brain. TH controls brain development and function, including learning and memory processes, in humans and animals.

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

The importance of thyroid hormones (TH) in brain development has been recognised and investigated for many decades (Bernal, 2011; Williams 2008). Several human studies have shown that low levels of circulating maternal TH (as a consequence of a decrease of TH synthesis) can lead to neurophysiological deficits in the offspring, including learning and memory deficits, or even cretinism in most severe cases (Zoeller and Rovet, 2004; Henrichs et al., 2010).

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

A number of studies have consistently reported alterations in synaptic transmission resulting from developmental TH disruption, and leading to decreased cognition. Developmental hypothyroidism reduces the functional integrity in brain regions critical for learning and memory. For example, pyramidal neurons of hypothyroid animals have fewer synapses and an impoverished dendritic arbor (Rami et al., 1986a, Madeira et al., 1992) that would lead to cognitive impairments.

Both hippocampal regions (area CA1 and dentate gyrus) exhibit alterations in excitatory and inhibitory synaptic transmission following reductions in serum TH in the pre and early postnatal period (Vara et al., 2002; Sui and Gilbert, 2003; Sui et al., 2005; Gilbert and Sui, 2006; Taylor et al., 2008; Gilbert, 2011; Gilbert et al., 2016). These deficits persist into adulthood long after recovery to euthyroid status.

Hypothyroidism, induced by different approaches, including inhibition of TH synthesis, during development reduces the capacity for synaptic plasticity in juvenile and adult offspring (Vara et al., 2002; Sui and Gilbert, 2003; Dong et al., 2005; Sui et al., 2005; Gilbert and Sui, 2006; Taylor et al., 2008; Gilbert, 2011; Gilbert et al., 2016). Impairments in synaptic function and plasticity are observed coincident with deficits in learning tasks that require the hippocampus.

Several in vivo studies have reported associations between decrease of TH synthesis (induced by TPO inhibitors) and learning and memory impairments:

- Davenport and Dorcey, 1972; Tamasy et al., 1986: In these in vivo studies, deficits in passive avoidance learning have been reported, but these early observations are often limited to animals suffering fairly severe hormonal deprivation.

- Akaike, 1991: in this in vivo study, temporary hypothyroidism was induced in neonatal rats by 0.02% PTU administration to lactating dams during days 0-19 after delivery. The radial arm maze test started at 13 weeks revealed that the PTU animals required more trials until they showed the first well-performed trial (with 3-fold more errors on day 3). The total number of choices was also larger, with less correct choices. These results suggest an involvement of temporary neonatal hypothyroidism in learning and memory impairment.

- Axelstad et al., 2008: in this in vivo study, radial arm maze deficits were recorded in adult offspring of rat dams treated with high doses of the TPO inhibitor, propylthiouracil (PTU), throughout gestation and lactation. Data showed a ~66% increase in the total number of errors made in the radial arm maze during 3 weeks of testing, in adult male rats perinatally exposed to 1.6 mg/kg/day of PTU from GD7 to PND17.

- Shafiee et al., 2016: This in vivo study investigated the effects of PTU (TPO inhibitor, 100 mg/L) in pregnant rats to evaluate the effects elicited by maternal hypothyroidism in the offspring. PTU was added to the drinking water from gestation day 6 to PND 21. Analysis of hippocampal BDNF levels, and learning and memory tests were performed on PNDs 45-52 on pups. These results indicated that hypothyroidism during the fetal period and the early postnatal period was associated with: (i) ~ 70% reduction of total serum T4, (ii) ~ 5-fold increase of total serum TSH levels (on PND 21; no significant differences could be found at the end of the behavioral testing, on PND 52), (iii) reduction of hippocampal BDNF protein levels (~ 8% decrease vs control), (iv) impairment of spatial learning and memory, in both male and female rat offspring.

- Gilbert et al., 2016: in this in vivo study, exposure to PTU during development produced dose-dependent reductions in mRNA expression of nerve growth factor (Ngf) in whole hippocampus of neonates. These changes in basal expression persisted to adulthood despite the return to euthyroid conditions in blood. Developmental PTU treatment dramatically reduced the activity-dependent expression of neurotrophins and related genes in neonate hippocampus and was accompanied by deficits in hippocampal-based learning (e.g., mean latency to find a hidden platform, at 2^nd trial resulted ~60% higher in rats treated with 10 ppm PTU).

- Gilbert and Sui, 2006: in this in vivo study, administration of 3 or 10 ppm PTU to pregnant and lactating dams via the drinking water from GD6 until PND30 caused a 47% and 65% reduction in serum T4, in the dams of the low and high-dose groups, respectively. Baseline synaptic transmission was impaired in PTU-exposed animals: mean EPSP slope (by ~60% with 10 ppm PTU) and population spike amplitudes (by ~70% with 10 ppm PTU) in the dentate gyrus were reduced in a dose-dependent manner in adult offspring of PTU-treated dams. High-dose animals (10 ppm) demonstrated very little evidence of learning despite 16 consecutive days of training (~5-fold higher mean latency to find the hidden platform, used as an index of learning).

- Gilbert, 2011: in this in vivo study, trace fear conditioning deficits to context and to cue were reported in animals treated with PTU; animals also displayed synaptic transmission and LTP deficits in hippocampus. Baseline synaptic transmission was impaired in PTU-exposed animals (by ~50% in animal treated with 3 ppm PTU). EPSP slope amplitudes in the dentate gyrus were reduced in a dose-dependent manner in adult offspring of PTU-treated dams.

In addition, studies in humans have shown associations between hypothyroidism and decreased memory and learning:

- Oerbeck et al., 2003: This study reported visual and verbal memory deficits in congenitally hypothyroid (CH) children. The CH group attained significantly lower scores than control subjects on intellectual, motor, and school-associated tests (total IQ: 102.4 vs 111.4). All verbal memory tasks were impaired; visual memory domain gave less consistent findings.

- Wheeler et al., 2011: Functional magnetic resonance imaging (fMRI) data from humans support the relationship between decreased serum concentrations of TH (occurring as a consequence of a decrease of TH synthesis) and memory deficits. CH subjects scored significantly below controls on indices of verbal but not visual memory as well as aspects of everyday memory functioning.

- Willoughby et al., 2013: The present study analysed 26 children with early-treated congenital hypothyroidism (CH), 23 children born to women with inadequately treated hypothyroidism during pregnancy (HYPO), and 30 typically developing controls. Results showed that relative to controls, CH and HYPO groups both exhibited weaknesses in episodic autobiographical memory, but not semantic autobiographical memory. In particular, CH and HYPO groups showed difficulty in recalling event details (i.e., the main happenings) and visual details from past experiences, confirming memory deficits.

- Willoughby et al., 2014: Congenitally hypothyroid children and children born to women with high TSH during pregnancy (biomarker of decreased TH synthesis) were weaker in recalling event and perceptual details from past naturally-occurring autobiographical events than age matched controls. HYPO cases scored significantly below controls on one objective and several subjective memory indices, and these were correlated with lower hippocampal volumes (brain region critical for learning and memory).

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

Numerous studies reported that iodine deficiency in critical periods of brain development and growth causes severe and permanent growth and cognitive impairment (cretinism) (Pesce and Kopp, 2014; de Escobar et al., 2007; de Escobar et al., 2008; Zimmermann, 2007; Melse-Boonstra and Jaiswal, 2010; Horn and Heuer, 2010; Zimmermann, 2012). However, direct quantitative correlation between decreased TH synthesis (as a consequence of TPO inhibition) and decreased cognition, in support to this KER, were not assessed in these reports.

Moreover, Wheeler et al., 2012 used fMRI visuospatial memory task to assess hippocampal activation in adolescents with CH (N = 14; age range, 11.5-14.7 years) compared with controls (N = 15; age range, 11.2-15.5 years). Despite, adolescents with congenital hypothyroidism showed both increased magnitude of hippocampal activation relative to controls and bilateral hippocampal activation when only the left was observed in controls, no group differences were recorded in task performance.

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

Temporal concordance between decrease of TH synthesis and disrupted brain development leading to learning and memory impairment has repeatedly been documented by demonstrating that there are critical windows during brain development where permanent changes are triggered. Hormone replacement studies have also demonstrated that structural alterations in brain are reduced or eliminated if T4 (and/or T3) treatment is given during critical developmental windows (Goodman and Gilbert., 2007; Auso et al., 2004; Lavado-Autric et al., 2003; Berbel et al., 2010; Koibuchi and Chin, 2000). However, quantitative data in support of this indirect KER are not available.

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

Deficiencies in learning and memory following developmental hypothyroidism (TH synthesis inhibition) have been documented mainly in rodents and humans.

References

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

Akaike M, Kato, N., Ohno, H., Kobayashi, T. (1991). Hyperactivity and spatial maze learning impairment of adult rats with temporary neonatal hypothyroidism. Neurotoxicol Teratol 13:317-322.

Auso E, Lavado-Autric R, Cuevas E, Del Rey FE, Morreale De Escobar G, Berbel P. (2004). A moderate and transient deficiency of maternal thyroid function at the beginning of fetal neocorticogenesis alters neuronal migration. Endocrinology 145:4037-4047.

Axelstad M, Hansen PR, Boberg J, Bonnichsen M, Nellemann C, Lund SP, Hougaard KS, U H. (2008). Developmental neurotoxicity of Propylthiouracil (PTU) in rats: relationship between transient hypothyroxinemia during development and long-lasting behavioural and functional changes. Toxicol Appl Pharmacol 232:1-13.

Berbel P, Navarro D, Auso E, Varea E, Rodriguez AE, Ballesta JJ, Salinas M, Flores E, Faura CC, de Escobar GM. (2010). Role of late maternal thyroid hormones in cerebral cortex development: an experimental model for human prematurity. Cereb Cortex 20:1462-1475.

Bernal J. (2011). Thyroid hormone transport in developing brain. Curr Opin Endocrinol Diab Obes 18:295–299.

Davenport JW, Dorcey TP. (1972). Hypothyroidism: learning deficit induced in rats by early exposure to thiouracil. Horm Behav 3:97-112.

de Escobar GM, Obregon MJ, del Rey FE. (2007). Iodine deficiency and brain development in the first half of pregnancy. Public Health Nutr.10(12A):1554–1570.

de Escobar GM, Ares S, Berbel P, Obregon MJ, del Rey FE. (2008). The changing role of maternal thyroid hormone in fetal brain development. Semin Perinatol. 32(6):380–386.

Dong J, Yin H, Liu W, Wang P, Jiang Y, Chen J. (2005). Congenital iodine deficiency and hypothyroidism impair LTP and decrease C-fos and C-jun expression in rat hippocampus. Neurotoxicology 26:417-426.

Gilbert ME. (2004). Alterations in synaptic transmission and plasticity in area CA1 of adult hippocampus following developmental hypothyroidism. Brain Res Dev Brain Res 148:11-18.

Gilbert ME, Sui L. (2006). Dose-dependent reductions in spatial learning and synaptic function in the dentate gyrus of adult rats following developmental thyroid hormone insufficiency. Brain Res 1069:10-22.

Gilbert ME. (2011). Impact of low-level thyroid hormone disruption induced by propylthiouracil on brain development and function. Toxicol Sci 124:432-445.

Gilbert ME, Rovet J, Chen Z, Koibuchi N. (2012). Developmental thyroid hormone disruption: prevalence, environmental contaminants and neurodevelopmental consequences. Neurotoxicology 33(4):842-852.

Gilbert ME, Sanchez-Huerta K, Wood C. (2016). Mild Thyroid Hormone Insufficiency During Development Compromises Activity-Dependent Neuroplasticity in the Hippocampus of Adult Male Rats. Endocrinology 157:774-787.

Goodman JH, Gilbert ME. (2007). Modest thyroid hormone insufficiency during development induces a cellular malformation in the corpus callosum: a model of cortical dysplasia. Endocrinology. 2007 Jun;148(6):2593-7.

Henrichs J, Bongers-Schokking JJ, Schenk JJ, Ghassabian A, Schmidt HG, Visser TJ, Hooijkaas H, de Muinck Keizer-Schrama SM, Hofman A, Jaddoe VV, Visser W, Steegers EA, Verhulst FC, de Rijke YB, Tiemeier H. (2010). Maternal thyroid function during early pregnancy and cognitive functioning in early childhood: the generation R study. J Clin Endocrinol Metab 95:4227–4234.

Horn S, Heuer H. (2010). Thyroid hormone action during brain development: more questions than answers. Mol Cell Endocrinol. 315(1–2):19–26.

Koibuchi N, Chin WW. (2000). Thyroid hormone action and brain development. Trends Endocrinol Metab 11:123-128.

Lavado-Autric R, Auso E, Garcia-Velasco JV, Arufe Mdel C, Escobar del Rey F, Berbel P, Morreale de Escobar G. (2003). Early maternal hypothyroxinemia alters histogenesis and cerebral cortex cytoarchitecture of the progeny. J Clin Invest 111:1073-1082.

Madeira MD, Sousa N, Lima-Andrade MT, Calheiros F, Cadete-Leite A, Paula-Barbosa MM. (1992). Selective vulnerability of the hippocampal pyramidal neurons to hypothyroidism in male and female rats. J Comp Neurol 322:501-518.

Melse-Boonstra A, Jaiswal N. (2010). Iodine deficiency in pregnancy, infancy and childhood and its consequences for brain development. Best Pract Res Clin Endocrinol Metab. 24(1):29–38.

Oerbeck B, Sundet K, Kase BF, Heyerdahl S. (2003). Congenital hypothyroidism: influence of disease severity and L-thyroxine treatment on intellectual, motor, and school-associated outcomes in young adults. Pediatrics 112:923-930.

Pesce L, Kopp P. (2014). Iodide transport: implications for health and disease. Int J Pediatr Endocrinol. 2014(1):8.

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.

Shafiee SM, Vafaei AA, Rashidy-Pour A. (2016). Effects of maternal hypothyroidism during pregnancy on learning, memory and hippocampal BDNF in rat pups: Beneficial effects of exercise. Neuroscience. Aug 4;329:151-61.

Sui L, Anderson WL, Gilbert ME. (2005). Impairment in short-term but enhanced long-term synaptic potentiation and ERK activation in adult hippocampal area CA1 following developmental thyroid hormone insufficiency. Toxicol Sci 85:647-656.

Sui L, Gilbert ME. (2003). Pre- and postnatal propylthiouracil-induced hypothyroidism impairs synaptic transmission and plasticity in area CA1 of the neonatal rat hippocampus. Endocrinology 144:4195-4203.

Tamasy V, Meisami E, Vallerga A, Timiras PS. (1986). Rehabilitation from neonatal hypothyroidism: spontaneous motor activity, exploratory behavior, avoidance learning and responses of pituitary--thyroid axis to stress in male rats. Psychoneuroendocrinology 11:91-103.

Taylor MA, Swant J, Wagner JJ, Fisher JW, Ferguson DC. (2008). Lower thyroid compensatory reserve of rat pups after maternal hypothyroidism: correlation of thyroid, hepatic, and cerebrocortical biomarkers with hippocampal neurophysiology. Endocrinology 149:3521-3530.

Vara H, Martinez B, Santos A, Colino A. (2002). Thyroid hormone regulates neurotransmitter release in neonatal rat hippocampus. Neuroscience 110:19-28.

Wheeler SM, McAndrews MP, Sheard ED, Rovet J. (2012). Visuospatial associative memory and hippocampal functioning in congenital hypothyroidism. J Int Neuropsychol Soc 18:49-56.

Wheeler SM, McLelland VC, Sheard E, McAndrews MP, Rovet JF. (2015). Hippocampal Functioning and Verbal Associative Memory in Adolescents with Congenital Hypothyroidism. Front Endocrinol (Lausanne) 6:163.

Wheeler SM, Willoughby KA, McAndrews MP, Rovet JF. (2011). Hippocampal size and memory functioning in children and adolescents with congenital hypothyroidism. J Clin Endocrinol Metab 96:E1427-1434.

Williams GR. (2008). Neurodevelopmental and Neurophysiological Actions of Thyroid Hormone. Journal of Neuroendocrinology , 20, 784–794.

Willoughby KA, McAndrews MP, Rovet J. (2013). Effects of early thyroid hormone deficiency on children's autobiographical memory performance. J Int Neuropsychol Soc 19:419-429.

Willoughby KA, McAndrews MP, Rovet JF. (2014). Effects of maternal hypothyroidism on offspring hippocampus and memory. Thyroid 24:576-584.

Zimmermann MB. (2007). The adverse effects of mild-to-moderate iodine deficiency during pregnancy and childhood: a review. Thyroid. 17(9):829–835.

Zimmermann MB. (2012). The effects of iodine deficiency in pregnancy and infancy. Paediatr Perinat Epidemiol. 26(Suppl 1):108–117.

Zoeller RT, Rovet J. (2004). Timing of thyroid hormone action in the developing brain: clinical observations and experimental findings. J Neuroendocrinol 16:809–818.