Distinct behavioral phenotypes in male mice lacking the thyroid hormone receptor α1 or β isoforms

The Type 3 Deiodinase Is a Critical Modulator of Thyroid Hormone Sensitivity in the Fetal Brain

Thyroid hormones (TH) are critical for the development and function of the central nervous system (CNS). Although their effects on the rodent brain peak within 2-3 weeks postnatally, the fetal brain has been found largely insensitive to exogenously administrated TH. To address this issue, here we examined gene expression in brains from mouse fetuses deficient in the type 3 deiodinase (DIO3), the selenoenzyme responsible for clearing TH. At embryonic day E18.5 qPCR determinations indicated a marked increase in the mRNA expression of T3-responsive genes Klf9 and Nrgn. The increased expression of these genes was confirmed by in situ hydridization in multiple areas of the cortex and in the striatum. RNA sequencing revealed 246 genes differentially expressed (70% up-regulated) in the brain of E18.5 Dio3-/- male fetuses. Differential expression of 13 of these genes was confirmed in an extended set of samples that included females. Pathway analyses of differentially expressed genes indicated enrichment in glycolysis and signaling related to axonal guidance, synaptogenesis and hypoxia inducible factor alpha. Additional RNA sequencing identified 588 genes differentially expressed (35% up-regulated) in the brain of E13.5 Dio3-/- male fetuses. Differential expression of 13 of these genes, including Klf9, Hr, and Mgp, was confirmed in an extended set of samples including females. Although pathway analyses of differentially expressed genes at E13.5 also revealed significant enrichment in axonal guidance and synaptogenesis signaling, top enrichment was found for functions related to the cell cycle, aryl hydrocarbon receptor signaling, PCP and kinetochore metaphase signaling pathways and mitotic roles of polo-like kinase. Differential expression at E13.5 was confirmed by qPCR for additional genes related to collagen and extracellular matrix and for selected transcription factors. Overall, our results demonstrate that the rodent fetal brain is sensitive to TH as early as E13.5 of gestational age, and suggest that TH distinctly affects brain developmental programs in early and late gestation. We conclude that DIO3 function is critical to ensure an adequate timing for TH action in the developing brain and is probably the main factor underlying the lack of effects on the fetal brain observed in previous studies after TH administration.

Photoperiod-induced changes in hypothalamic de novo DNA methyltransferase expression are independent of triiodothyronine in female Siberian hamsters (Phodopus sungorus)

Many temperate zone animals engage in seasonal reproductive physiology and behavior as a strategy to maximise the propagation of the species. The hypothalamus integrates environmental cues and hormonal signalling to optimize the timing of reproduction. Recent work has revealed that epigenetic modifications, such as DNA methylation, vary across seasonal reproductive states. Multiple hormones act in the hypothalamus to permit or inhibit reproductive physiology, and the increase in thyroid hormone triiodothyronine (T₃) has been implicated in the initiation of breeding in many species. The objective of this study was to examine the effect of T₃ on the photoperiod-dependent regulation of reproductive physiology and hypothalamic DNA methyltransferase enzyme expression in female Siberian hamsters (Phodopus sungorus). We tested the hypothesis that T₃ in short days (SD) would stimulate hypothalamic Rfrp3 and de novo DNA methyltransferase (Dnmt) expression in female Siberian hamsters. 10 weeks of SD lengths induced a decrease in body and uterine mass. Hamsters maintained in SD were found to express lower levels of GnRH, Rfrp3, Dnmt3a and Dnmt3b. Two weeks of daily T₃ injections did not affect body mass, uterine mass, Gnrh, Rfrp3, Dnmt3a or Dnmt3b expression in neuroendocrine tissues. SD significantly lowered Tshβ mRNA expression and T3 reduced Tshβ in LD hamsters. Our data indicate sex-dependent effects of T₃ for the neuroendocrine regulation of seasonal reproduction in hamsters.

Pubertal fenvalerate exposure impairs cognitive and behavioral development partially through down-regulating hippocampal thyroid hormone receptor signaling

Fenvalerate, a synthetic pyrethroid insecticide, is an environmental endocrine disruptor and neurodevelopmental toxicant. An early report found that pubertal exposure to high-dose fenvalerate impaired cognitive and behavioral development. Here, we aimed to further investigate the effect of pubertal exposure to low-dose fenvalerate on cognitive and behavioral development. Mice were orally administered with fenvalerate (0.2, 1.0 and 5.0 mg/kg) daily from postnatal day (PND) 28 to PND56. Learning and memory were assessed by Morris water maze. Anxiety-related activities were detected by open-field and elevated plus-maze. Increased anxiety activities were observed only in females exposed to fenvalerate. Spatial learning and memory were damaged only in females exposed to fenvalerate. Histopathology observed numerous scattered shrinking neurons and nuclear pyknosis in hippocampal CA1 region. Neuronal density was reduced in hippocampal CA1 region of fenvalerate-exposed mice. Mechanistically, hippocampal thyroid hormone receptor (TR)β1 was down-regulated in a dose-dependent manner in females. In addition, TRα1 was declined only in females exposed to 5.0 mg/kg fenvalerate. Taken together, these suggests that pubertal exposure to low-dose fenvalerate impairs cognitive and behavioral development in a gender-dependent manner. Hippocampal TR signaling may be, at least partially, involved in fenvalerate-induced impairment of cognitive and behavioral development.

Critical Period Regulation by Thyroid Hormones: Potential Mechanisms and Sex-Specific Aspects

Adequate perinatal levels of thyroid hormones (THs) are required for normal brain function and development. Studies in non-mammalian species suggest that TH might be involved in the regulation of critical periods (CPs) of heightened plasticity. Yet, it is largely unknown what mechanisms controlling such CPs might be under TH regulation. Here, we briefly review the influence of TH in early life across evolution. We discuss possible links between TH and known circuit and/or molecular mechanisms determining the timing of CPs of heightened brain plasticity. We focus on the role of parvalbumin-positive (PV) interneurons since their maturation defines CP onset and closure. Specifically, abnormal PV circuits are associated with low perinatal levels of TH, possibly because thyroid hypofunction may increase oxidative stress and/or dysregulate Otx2-mediated maturation of neuroprotective perineuronal nets. In addition, the level of cholinergic transmission is important for CP plasticity. Potentially, TH levels could affect gain changes in cholinergic transmission that can alter brain development. We believe that understanding how TH impacts CPs of circuit refinement will shed light onto the principles underlying normal developmental trajectories. Given that the thyroid gland expresses estrogen and androgen receptors, its activity can potentially be regulated differently between the sexes, contributing to sexually dimorphic behaviors.

Thyroid hormones and learning-associated neuroplasticity

Thyroid hormones (THs) are crucial for brain development and maturation in all vertebrates. Especially during pre- and perinatal development, disruption of TH signaling leads to a multitude of neurological deficits. Many animal models provided insight in the role of THs in brain development, but specific data on how they affect the brain's ability to learn and adapt depending on environmental stimuli are rather limited. In this review, we focus on a number of learning processes like spatial learning, fear conditioning, vocal learning and imprinting behavior and on how abnormal TH signaling during development shapes subsequent performance. It is clear from multiple studies that TH deprivation leads to defects in learning on all fronts, and interestingly, changes in local expression of the TH activator deiodinase type 2 seem to have an important role. Taking into account that THs are regulated in a very space-specific manner, there is thus increasing pressure to investigate more local TH regulators as potential factors involved in neuroplasticity. As these learning processes are also important for proper adult human functioning, further elucidating the role of THs in developmental neuroplasticity in various animal models is an important field for advancing both fundamental and applied knowledge on human brain function.

Aging and estradiol effects on gene expression in the medial preoptic area, bed nucleus of the stria terminalis, and posterodorsal medial amygdala of male rats

Studies on the role of hormones in male reproductive aging have traditionally focused on testosterone, but estradiol (E2) also plays important roles in the control of masculine physiology and behavior. Our goal was to examine the effects of E2 on the expression of genes selected for E2-sensitivity, involvement in behavioral neuroendocrine functions, and impairments with aging. Mature adult (MAT, 5 mo) and aged (AG, 18 mo) Sprague-Dawley male rats were castrated, implanted with either vehicle or E2 subcutaneous capsules, and euthanized one month later. Bilateral punches were taken from the bed nucleus of the stria terminalis (BnST), posterodorsal medial amygdala (MePD) and the preoptic area (POA). RNA was extracted, and expression of 48 genes analyzed by qPCR using Taqman low-density arrays. Results showed that effects of age and E2 were age- and region-specific. In the POA, 5 genes were increased with E2 compared to vehicle, and there were no age effects. By contrast the BnST showed primarily age-related changes, with 6 genes decreasing with age. The MePD had 5 genes that were higher in aged than mature males, and 17 genes with significant interactions between age and E2. Gene families identified in the MePD included nuclear hormone receptors, neurotransmitters and neuropeptides and their receptors. Ten serum hormones were assayed in these same males, with results revealing both age- and E2-effects, in several cases quite profound. These results support the idea that the male brain continues to be highly sensitive to estradiol even with aging, but the nature of the response can be substantially different in mature and aging animals.

Neuronal expression of a thyroid hormone receptor α mutation alters mouse behaviour

In humans, alterations in thyroid hormone signalling are associated with mood and anxiety disorders, but the neural mechanisms underlying such association are poorly understood. The present study investigates the involvement of neuronal thyroid hormone receptor α (TRα) in anxiety, using mouse genetics and Cre/loxP technology to specifically alter TRα signalling in neurons. We evaluated the behaviour of mice expressing a dominant negative, neuron-specific mutation of TRα (TRα^AMI/Cre3 mice), using the elevated-plus maze, light-dark box and open-field tests. In a first experiment, mice were housed individually, and the behaviour of TRα^AMI/Cre3 mice differed significantly from that of control littermates in these 3 tests, suggesting heightened anxiety. In a second experiment, designed to evaluate the robustness of the results with the same 3 tests, mice were housed in groups. In these conditions, the behaviour of TRα^AMI/Cre3 mice differed from that of control littermates only in the light-dark box. Thus, TRα^AMI/Cre3 mice appear to be more likely to develop anxiety under stressful housing conditions than control mice. These results suggest that in adult mice, thyroid hormone signalling in neurons, via TRα, is involved in the control of anxiety behaviour.

Effects of experimentally manipulated yolk thyroid hormone levels on offspring development in a wild bird species

Maternal effects are a crucial mechanism in a wide array of taxa to generate phenotypic variation, thereby affecting offspring development and fitness. Maternally derived thyroid hormones (THs) are known to be essential for offspring development in mammalian and fish models, but have been largely neglected in avian studies, especially in respect to natural variation and an ecological context. We studied, for the first time in a wild species and population, the effects of maternally derived THs on offspring development, behavior, physiology and fitness-related traits by experimental elevation of thyroxine and triiodothyronine in ovo within the physiological range in great tits (Parus major). We found that elevated yolk TH levels had a sex-specific effect on growth, increasing male and decreasing female growth, relative to controls, and this effect was similar throughout the nestling period. Hatching or fledging success, motor coordination behavior, stress reactivity and resting metabolic rate were not affected by the TH treatment. We conclude that natural variation in maternally derived THs may affect some offspring traits in a wild species. As this is the first study on yolk thyroid hormones in a wild species and population, more such studies are needed to investigate its effects on pre-hatching development, and juvenile and adult fitness before generalizations on the importance of maternally derived yolk thyroid hormones can be made. However, this opens a new, interesting avenue for further research in the field of hormone mediated maternal effects.

Progesterone-facilitated lordosis of estradiol-primed mice is attenuated by knocking down expression of membrane progestin receptors in the midbrain

Evidence is emerging of the role of membrane progestin receptors (referred to as mPRs herein: members of Progestin and AdipoQ Receptor (Paqr) family) as a novel brain target in mammals, such as rats. In the present study, the role of mPRs in mice was assessed to further elucidate the conservation of this mechanism across species. The brain target investigated was the midbrain ventral tegmental area (VTA) given its described role for rapid actions of progestins for reproduction. Studies tested the hypothesis that if mPRs are required for progestin-facilitated lordosis through actions in the VTA, then knockdown of mPRs in the VTA will attenuate lordosis. Ovariectomized (OVX) mice were subcutaneously injected with estradiol (E2) and progesterone (P4), and infused with antisense oligodeoxynucleotides (AS-ODNs) to mPRα (Paqr7) and/or mPRβ (Paqr8) or vehicle to the lateral ventricle or VTA. Mice were assessed for reproductive behavior (lordosis and aggression/rejection quotients) in a standard mating task. Results supported our hypothesis. E2+P4-facilitated lordosis was significantly reduced, and aggression/rejection increased, with infusions of mPRα, mPRβ, or mPRαβ AS-ODNs to the lateral ventricle, compared to vehicle. E2+P4-facilitated lordosis was significantly decreased, and aggression/rejection increased, with mPRβ or mPRαβ AS-ODNs to the VTA of C57/BL6 mice. Both mPRɑ and mPRβ AS-ODNs reduced lordosis, and increased aggression/rejection, of wildtype (C57/BL6x129) mice, but not nuclear PR knockout mice. Thus, mPRs may be a novel target of progestins for reproductive behavior of mice.

Thyroid hormones regulate anxiety in the male mouse

Thyroid hormone levels are implicated in mood disorders in the adult human but the mechanisms remain unclear partly because, in rodent models, more attention has been paid to the consequences of perinatal hypo and hyperthyroidism. Thyroid hormones act via the thyroid hormone receptor (TR) α and β isoforms, both of which are expressed in the limbic system. TR's modulate gene expression via both unliganded and liganded actions. Though the thyroid hormone receptor (TR) knockouts and a transgenic TRα1 knock-in mouse have provided us valuable insight into behavioral phenotypes such as anxiety and depression, it is not clear if this is because of the loss of unliganded actions or liganded actions of the receptor or due to locomotor deficits. We used a hypothyroid mouse model and supplementation with tri-iodothyronine (T3) or thyroxine (T4) to investigate the consequences of dysthyroid hormone levels on behaviors that denote anxiety. Our data from the open field and the light-dark transition tests suggest that adult onset hypothyroidism in male mice produces a mild anxiogenic effect that is possibly due to unliganded receptor actions. T3 or T4 supplementation reverses this phenotype and euthyroid animals show anxiety that is intermediate between the hypothyroid and thyroid hormone supplemented groups. In addition, T3 but not T4 supplemented animals have lower spine density in the CA1 region of the hippocampus and in the central amygdala suggesting that T3-mediated rescue of the hypothyroid state might be due to lower neuronal excitability in the limbic circuit.