Thyroid hormone regulates excitability in central neurons from postnatal rats

Levothyroxine attenuates behavioral impairment and improves oxidative stress and histological alteration 3-nitropropionic acid induced experimental Huntington's disease in rats

Huntington's disease (HD) is a neurodegenerative disorder characterized by degeneration of the striatum; it results in oxidative stress and motor deficits. Thyroid hormones regulate oxidative metabolism. In the present study, we evaluated the effect of administration of levothyroxine (LT-4) on neurobehavioral, oxidative stress, and histological changes in a rat model of HD. Forty-eight Wistar male rats were divided into the following six groups (n = 8): Group 1 (control) received physiological saline intraperitoneally (ip). Groups 2 and 3 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) daily for 7 days. Group 4 (HD) received 3-nitropropionic acid (3-NP) (25 mg/kg, ip) daily for 7 days. Groups 5 and 6 received L-T4,30 and L-T4100 (μg/kg, ip, respectively) 30 min after 3-NP (25 mg/kg, ip) injection for the same duration. On the 8th day, behavioral parameters were evaluated with the Rotarod, Narrow beam walk, and Limb withdrawal tests. Oxidative markers such as Malondialdehyde (MDA) and Glutathione (GSH) levels and Superoxide dismutase (SOD) activity, in striatum tissue were measured. Moreover, striatum tissues were analyzed by Hematoxylin-eosin staining for histological alterations. We found that 3-NP administration caused motor incoordination and induced oxidative stress increased but reduced free radical scavenging. Also, increased amounts of lipid peroxides caused striatal damage as shown by histopathological evaluation. Administration of L-T4 led to increased falling time in the Rotarod, but reduced the time taken in Narrow beam walking and Limb withdrawal test. Furthermore, L-T4 increased antioxidant activity, decreased lipid peroxidation and ameliorated 3-NP-induced degeneration in neurons.

Associations of hyperthyroidism with epilepsy: a Mendelian randomization study

Prior studies have revealed an increased susceptibility to epilepsy in hyperthyroid individuals, but the genetic basis of the hyperthyroidism-epilepsy relationship is not fully comprehended, prompting this study to explore this potential association. We conducted a two-sample Mendelian randomization (TSMR) study to explore the relationship between hyperthyroidism and epilepsy by utilizing aggregated statistics from Genome-Wide Association Studies (GWAS). Data for hyperthyroidism were derived from a GWAS encompassing 462,933 participants, while epilepsy data were sourced from the International League Against Epilepsy (ILAE) consortium. Five distinct methods were employed for TSMR analysis, which included the inverse variance weighting method, MR Egger method, weighted median method, simple model, and weighted model. In our sensitivity analysis, we employed the MR Egger and MR PRESSO methods to assess pleiotropy, and inverse variance weighting and MR Egger in Cochran's Q statistics to assess heterogeneity. In the IEU database, utilizing the MR-Egger method, we obtained an odds ratio (OR) of 2.631 (95% CI 0.608, 9.796) with a p-value of 0.122. Meanwhile, employing the Weighted Median method yielded an OR of 1.813 (95% CI 0.786, 4.181) with a p-value of 0.163. The IVW method exhibited an OR of 1.986 (95% CI 1.127, 3.502) with a p-value of 0.018. In the assessment of heterogeneity, the MR-Egger method produced a Q statistic of 65.205, accompanied by a p-value of 0.087, while the IVW method recorded a Q statistic of 66.668 with a p-value of 0.083. The multifactorial analysis results showed an intercept term with a standard error (SE) value of 0.009 and a p-value of 0.291. In the FinnGen database, employing the MR-Egger method for all epilepsy data, we observed an OR of 0.952 (95% CI 0.831, 1.093) with a p-value of 0.539. Simultaneously, the Weighted Median method produced an OR of 0.986 (95% CI 0.953, 1.021) with a p-value of 0.423. The IVW method indicated an OR of 0.992 (95% CI 0.965, 1.019) with a p-value of 0.541. The MR-Egger method's assessment of heterogeneity resulted in a Q statistic of 2.671, associated with a p-value of 0.445, while the IVW method generated a Q statistic of 3.011 with a p-value of 0.556. The multifactorial analysis results displayed an intercept term with a SE-value of 0.019 and a p-value of 0.601. Sensitivity analysis found no evidence of horizontal pleiotropy or heterogeneity. Hyperthyroidism was found to be causally related to all epilepsy but had no effect on other types of epilepsy.

Neuroprotective effects of levothyroxine on cognition deficits and memory in an experimental model of Huntington's disease in rats: An electrophysiological study

Huntington's disease (HD) is a neurodegenerative disorder characterized by cognitive deficits and motor function. Levothyroxine (L-T4) is a synthetic form of Thyroxine (T4), which can improve cognitive ability. The aim of the present study was to determine the neuroprotective effect of L-T4 administration in rats with 3-nitropropionic acid (3-NP)-induced Huntington's disease. Forty-eight Wistar male rats were divided into six groups (n = 8): Group 1 control group that received physiological saline, Group 2 and 3: which received L-T4 (30 and 100 μg/kg), Group 4: HD group that received 3-NP and Groups 5 and 6: The treatment of the HD rats with L-T4 (30 and 100 μg/kg). Spatial memory, locomotor activity, and frequency of neuronal firing were assessed. After decapitation, the Brain-Derived Neurotrophic Factor (BDNF) and Total antioxidant capacity (TAC) levels in the striatum was measured. The results showed that the indices of spatial memory (mean path length and latency time) and motor dysfunction (immobility time) significantly increased, while time spent in the goal quadrant, swimming speed, spike rate, and striatum levels of BDNF significantly decreased in the HD group compared to the control group. L-T4 treatment significantly enhanced time spent in the goal quadrant, swimming speed, motor activity (number of line crossing and rearing), spike rate and striatal BDNF level. This research showed that L-T4 prevented the disruption of motor activity and cognitive deficiencies induced by 3-NP. The beneficial effects of L-T4 may be due to an increase in the concentration of BDNF and enhancement of the spike rate in the striatum.

The Potential Role of Thyroid Hormone Therapy in Neural Progenitor Cell Differentiation and Its Impact on Neurodevelopmental Disorders

Thyroid hormone (T3) plays a vital role in brain development and its dysregulation can impact behavior, nervous system function, and cognitive development. Large case-cohort studies have associated abnormal maternal T3 during early pregnancy to epilepsy, autism, and attention deficit hyperactivity disorder (ADHD) in children. Recent experimental findings have also shown T3's influence on the fate of neural precursor cells and raise the question of its convergence with embryonic neural progenitors. Our objective was to investigate how T3 treatment affects neuronal development and functionality at the cellular level. In vitro experiments using neural precursor cells (NPCs) measured cell growth and numbers after exposure to varying T3 concentrations. Time points included week 0 (W0) representing NPCs treated with 100 nM T3 for 5 days, and differentiated cortical neurons assessed at weeks 3 (W3), 6 (W6), and 8 (W8). Techniques such as single-cell calcium imaging and whole-cell patch clamp were utilized to evaluate neuronal activity and function. IHC staining detected mature neuron markers, and RNA sequencing enabled molecular profiling. W6 and W8 neurons exhibited higher action potential frequencies, with W6 showing increased peak amplitudes and shortened inter-spike intervals by 50%, indicating enhanced activity. Transcriptomic analysis revealed that W6 T3-treated neurons formed a distinct cluster, suggesting accelerated maturation. Comparison with the whole transcriptome further unveiled a correlation between W6 neurons treated with T3 and neuronal regulatory elements associated with autism and ADHD. These findings provide insights into T3's impact on neuronal development and potential mechanisms of T3 dysregulation and neurodevelopmental disorders.

Nongenomic roles of thyroid hormones and their derivatives in adult brain: are these compounds putative neurotransmitters?

We review the evidence regarding the nongenomic (or non-canonical) actions of thyroid hormones (thyronines) and their derivatives (including thyronamines and thyroacetic acids) in the adult brain. The paper seeks to evaluate these compounds for consideration as candidate neurotransmitters. Neurotransmitters are defined by their (a) presence in the neural tissue, (b) release from neural tissue or cell, (c) binding to high-affinity and saturable recognition sites, (d) triggering of a specific effector mechanism and (e) inactivation mechanism. Thyronines and thyronamines are concentrated in brain tissue and show distinctive patterns of distribution within the brain. Nerve terminals accumulate a large amount of thyroid hormones in mature brain, suggesting a synaptic function. However, surprisingly little is known about the potential release of thyroid hormones at synapses. There are specific binding sites for thyroid hormones in nerve-terminal fractions (synaptosomes). A notable cell-membrane binding site for thyroid hormones is integrin αvβ3. Furthermore, thyronines bind specifically to other defined neurotransmitter receptors, including GABAergic, catecholaminergic, glutamatergic, serotonergic and cholinergic systems. Here, the thyronines tend to bind to sites other than the primary sites and have allosteric effects. Thyronamines also bind to specific membrane receptors, including the trace amine associated receptors (TAARs), especially TAAR1. The thyronines and thyronamines activate specific effector mechanisms that are short in latency and often occur in subcellular fractions lacking nuclei, suggesting nongenomic actions. Some of the effector mechanisms for thyronines include effects on protein phosphorylation, Na+/K+ ATPase, and behavioral measures such as sleep regulation and measures of memory retention. Thyronamines promptly regulate body temperature. Lastly, there are numerous inactivation mechanisms for the hormones, including decarboxylation, deiodination, oxidative deamination, glucuronidation, sulfation and acetylation. Therefore, at the current state of the research field, thyroid hormones and their derivatives satisfy most, but not all, of the criteria for definition as neurotransmitters.

Thyroid Axis and Vestibular Physiopathology: From Animal Model to Pathology

A recent work of our group has shown the significant effects of thyroxine treatment on the restoration of postural balance function in a rodent model of acute peripheral vestibulopathy. Based on these findings, we attempt to shed light in this review on the interaction between the hypothalamic-pituitary-thyroid axis and the vestibular system in normal and pathological situations. Pubmed database and relevant websites were searched from inception through to 4 February 2023. All studies relevant to each subsection of this review have been included. After describing the role of thyroid hormones in the development of the inner ear, we investigated the possible link between the thyroid axis and the vestibular system in normal and pathological conditions. The mechanisms and cellular sites of action of thyroid hormones on animal models of vestibulopathy are postulated and therapeutic options are proposed. In view of their pleiotropic action, thyroid hormones represent a target of choice to promote vestibular compensation at different levels. However, very few studies have investigated the relationship between thyroid hormones and the vestibular system. It seems then important to more extensively investigate the link between the endocrine system and the vestibule in order to better understand the vestibular physiopathology and to find new therapeutic leads.

Deficiency of Thyroid Hormone Reduces Voltage-Gated Na+ Currents as Well as Expression of Na+/K+-ATPase in the Mouse Hippocampus

Mice lacking functional thyroid follicular cells, Pax8^−/− mice, die early postnatally, making them suitable models for extreme hypothyroidism. We have previously obtained evidence in postnatal rat neurons, that a down-regulation of Na⁺-current density could explain the reduced excitability of the nervous system in hypothyroidism. If such a mechanism underlies the development of coma and death in severe hypothyroidism, Pax8^−/− mice should show deficits in the expression of Na⁺ currents and potentially also in the expression of Na⁺/K⁺-ATPases, which are necessary to maintain low intracellular Na⁺ levels. We thus compared Na⁺ current densities in postnatal mice using the patch-clamp technique in the whole-cell configuration as well as the expression of three alpha and two beta-subunits of the Na⁺/K⁺-ATPase in wild type versus Pax8^−/− mice. Whereas the Na⁺ current density in hippocampal neurons from wild type mice was upregulated within the first postnatal week, the Na⁺ current density remained at a very low level in hippocampal neurons from Pax8^−/− mice. Pax8^−/− mice also showed significantly decreased protein expression levels of the catalytic α1 and α3 subunits of the Na⁺/K⁺-ATPase as well as decreased levels of the β2 isoform, with no changes in the α2 and β1 subunits.

Thyroid function in the subacute phase of traumatic brain injury: a potential predictor of post-traumatic neurological and functional outcomes

PURPOSE

That thyroid hormones exert pleiotropic effects and have a contributory role in triggering seizures in patients with traumatic brain injury (TBI) can be hypothesized. We aimed at investigating thyroid function tests as prognostic factors of the development of seizures and of functional outcome in TBI.

METHODS

This retrospective study enrolled 243 adult patients with a diagnosis of mild-to-severe TBI, consecutively admitted to our rehabilitation unit for a 6-month neurorehabilitation program. Data on occurrence of seizures, brain imaging, injury characteristics, associated neurosurgical procedures, neurologic and functional assessments, and death during hospitalization were collected at baseline, during the workup and on discharge. Thyroid function tests (serum TSH, fT4, and fT3 levels) were performed upon admission to neurorehabilitation.

RESULTS

Serum fT3 levels were positively associated with an increased risk of late post-traumatic seizures (LPTS) in post-TBI patients independent of age, sex and TBI severity (OR = 1.85, CI 95% 1.22-2.61, p < 0.01). Measured at admission, fT3 values higher than 2.76 pg/mL discriminated patients with late post-traumatic seizures from those without, with a sensitivity of 74.2% and a specificity of 60.9%. Independently from the presence of post-traumatic epilepsy and TBI severity, increasing TSH levels and decreasing fT3 levels were associated with worse neurological and functional outcome, as well as with higher risk of mortality within 6 months from the TBI event.

CONCLUSIONS

Serum fT3 levels assessed in the subacute phase post-TBI are associated with neurological and functional outcome as well as with the risk of seizure occurrence. Further studies are needed to investigate the mechanisms underlying these associations.

Neocortex saves energy by reducing coding precision during food scarcity

Information processing is energetically expensive. In the mammalian brain, it is unclear how information coding and energy use are regulated during food scarcity. Using whole-cell recordings and two-photon imaging in layer 2/3 mouse visual cortex, we found that food restriction reduced AMPA receptor conductance, reducing synaptic ATP use by 29%. Neuronal excitability was nonetheless preserved by a compensatory increase in input resistance and a depolarized resting potential. Consequently, neurons spiked at similar rates as controls but spent less ATP on underlying excitatory currents. This energy-saving strategy had a cost because it amplified the variability of visually-evoked subthreshold responses, leading to a 32% broadening of orientation tuning and impaired fine visual discrimination. This reduction in coding precision was associated with reduced levels of the fat mass-regulated hormone leptin and was restored by exogenous leptin supplementation. Our findings reveal that metabolic state dynamically regulates the energy spent on coding precision in neocortex.

Anticonvulsant and Neuroprotective Effects of the Thyroid Hormone Metabolite 3-Iodothyroacetic Acid

BACKGROUND

3-Iodothyroacetic acid (TA1) is among the thyroid hormone (T3) metabolites that can acutely modify behavior in mice. This study aimed to investigate whether TA1 is also able to reduce neuron hyper-excitability and protect from excitotoxic damage.

METHODS

CD1 male mice were treated intraperitoneally with saline solution or TA1 (4, 7, 11, or 33 μg/kg) before receiving 90 mg/kg pentylenetrazole subcutaneously. The following parameters were measured: latency to first seizure onset, number of mice experiencing seizures, hippocampal levels of c-fos, and PI3K/AKT activation levels. Organotypic hippocampal slices were exposed to vehicle or to 5 μM kainic acid (KA) in the absence or presence of 0.01-10 μM TA1. In another set of experiments, slices were exposed to vehicle or 5 μM KA in the absence or presence of 10 μM T3, 3,5,3'-triiodothyroacetic acid (TRIAC), T1AM, thyronamine (T0AM), or thyroacetic acid (TA0). Neuronal cell death was measured fluorimetically. The ability of TA1 and T3, TRIAC, T1AM, T0A, and TA0 to activate the PI3K/AKT cascade was evaluated by Western blot. The effect of TA1 on KA-induced currents in CA3 neurons was evaluated by patch clamp recordings on acute hippocampal slices.

RESULTS

TA1 (7 and 11 μg/kg) significantly reduced the number of mice showing convulsions and increased their latency of onset, restored pentylenetrazole-induced reduction of hippocampal c-fos levels, activated the PI3K/AKT, and reduced GSK-3β activity. In rat organotypic hippocampal slices, TA1 reduced KA-induced cell death by activating the PI3K/AKT cascade and increasing GSK-3β phosphorylation levels. Protection against KA toxicity was also exerted by T3 and other T3 metabolites studied. TA1 did not interact at KA receptors. Both the anticonvulsant and neuroprotective effects of TA1 were abolished by pretreating mice or organotypic hippocampal slices with pyrilamine, an histamine type 1 receptor antagonist (10 mg/kg or 1 μM, respectively).

CONCLUSIONS

TA1 exerts anticonvulsant activity and is neuroprotective in vivo and in vitro. These findings extend the current knowledge on the pharmacological profile of TA1 and indicate possible novel clinical use for this T3 metabolite.

Thyroid Storm Presenting as Psychosis

Thyroid storm is a life-threatening endocrine emergency with an incidence rate of 1% to 2%. It is a systemic condition of excessive thyroid hormone production and release leading to thermoregulatory, adrenergic, neuropsychiatric, cardiovascular, and abdominal manifestations. Although it is a rare condition, it carries a significant mortality rate. Hence, knowing the common and uncommon presentations of thyroid storm is important for its prompt diagnosis and treatment. In this article, we present an unusual case of a young woman who presented with psychosis as the manifesting symptom of thyroid storm. She did not respond adequately to conventional medical treatment, requiring plasmapheresis and a definitive thyroidectomy, which ultimately led to the return of patient’s baseline mental status and a dramatic recovery.

Genes linked to species diversity in a sexually dimorphic communication signal in electric fish

Sexually dimorphic behaviors are often regulated by androgens and estrogens. Steroid receptors and metabolism are control points for evolutionary changes in sexual dimorphism. Electric communication signals of South American knifefishes are a model for understanding the evolution and physiology of sexually dimorphic behavior. These signals are regulated by gonadal steroids and controlled by a simple neural circuit. Sexual dimorphism of the signals varies across species. We used transcriptomics to examine mechanisms for sex differences in electric organ discharges (EODs) of two closely related species, Apteronotus leptorhynchus and Apteronotus albifrons, with reversed sexual dimorphism in their EODs. The pacemaker nucleus (Pn), which controls EOD frequency (EODf), expressed transcripts for steroid receptors and metabolizing enzymes, including androgen receptors, estrogen receptors, aromatase, and 5α-reductase. The Pn expressed mRNA for ion channels likely to regulate the high-frequency activity of Pn neurons and for neuromodulator and neurotransmitter receptors that may regulate EOD modulations used in aggression and courtship. Expression of several ion channel genes, including those for Kir3.1 inward-rectifying potassium channels and sodium channel β1 subunits, was sex-biased or correlated with EODf in ways consistent with EODf sex differences. Our findings provide a basis for future studies to characterize neurogenomic mechanisms by which sex differences evolve.

Peripheral and central administration of T3 improved the histological changes, memory and the dentate gyrus electrophysiological activity in an animal model of Alzheimer's disease

The amyloid beta (Aβ) induced Alzheimer's disease (AD) is associated with formation the amyloid plaques, cognitive impairments and decline in spontaneous discharge of neurons. In the current study, we evaluated the effect of subcutaneous (S. C) and intrahippocampal (I. H) administrations of triiodothyronine (T3) on the histological changes, memory and the dentate gyrus (DG) electrophysiological activity in an animal model of AD. Eighty adult male Wistar rats (250-300 g) were divided randomly into five groups: Sham-Operated (Sh-O), AD + Vehicle (S. C), AD + Vehicle (I. H), AD+ T3 (S. C) and AD + T3 (I. H). In order to induce animal model of AD, Aβ (10 ng/μl, bilaterally) were injected intrahippocampally. Rats were treated with T3 and/or normal saline for 10 days. Passive avoidance and spatial memory were evaluated in shuttle box apparatus and Morris water maze, respectively. Neuronal single unit recording was assessed from hippocampal DG. The percent of total time that animals spent in target quarter, the mean latency time (sec), the step through latency and the average number of spikes/bin were decreased significantly in AD rats compared with the Sh-O group (p < 0.001) and were increased significantly in AD groups that have received T3 (S. C and I. H) in compared with AD group (p < 0.01, p < 0.001). Also, formation of amyloid plaques was decreased in AD rats treated with T3.The results showed that T3 injection (S. C and I. H), by reduction of neural damage and increment of neuronal spontaneous activity improved the memory deficits in Aβ-induced AD rats.

Central and peripheral administrations of levothyroxine improved memory performance and amplified brain electrical activity in the rat model of Alzheimer's disease

Alzheimer's disease (AD) is associated with cognitive impairments and a decline in the spontaneous neuronal discharge. In the current study, we evaluated the effect of subcutaneous (S.C.) and intrahippocampal (I.H.) administrations of levothyroxine (LT-4) on the passive avoidance and spatial memory, as well as electrophysiological activity in an animal model of AD. One hundred-sixty male Wistar rats were divided into two main groups. The S.C. group included two Sham and four AD (vehicle or L-T4 25, 50 & 100μg/kg); and the I.H. had consisted of two Sham and two AD (vehicle or L-T4 10μg/kg) subgroups. To make an animal model of AD, amyloid beta (Aβ) plus ibotenic acid (Ibo) were injected I.H. Rats were treated with L-T4 and/or normal saline for ten days. Passive avoidance and spatial memory were evaluated in shuttle box and Morris water maze, respectively. Neuronal single unit recording was assessed from hippocampal dentate gyrus (DG). Results showed that the mean latency time (s) increased significantly (p<0.001) in AD animals and decreased significantly in both S.C. and I.H. L-T4 injected AD animals, compared with the AD group (p<0.001). The percentage of total time that animals spent in goal quarter and the step through latency decreased significantly in AD rats (p<0.001) and increased significantly in both S.C. and I.H. L-T4 injected AD animals in comparison with the AD group (p<0.01, p<0.001). Data showed that the average number of spikes/bin significantly decreased in the AD group (p<0.001). The S.C. and I.H. L-T4 injections in AD rats significantly increased the spike rate in comparison to the AD group (p<0.001). In conclusion, both S.C. and I.H. injections of L-T4 alleviated memory deficits and spontaneous neuronal activity in Aβ-induced AD rats. Also, I.H. microinjection of L-T4 had more beneficial effects on memory and neuronal electrophysiological activity in comparison to S.C. administration.

Changes in Neuronal Excitability by Activated Microglia: Differential Na(+) Current Upregulation in Pyramid-Shaped and Bipolar Neurons by TNF-α and IL-18

Microglia are activated during pathological events in the brain and are capable of releasing various types of inflammatory cytokines. Here, we demonstrate that the addition of 5% microglia activated by 1 μg/ml lipopolysaccharides (LPS) to hippocampal cultures upregulates Na⁺ current densities (I_NavD) of bipolar as well as pyramid-shaped neurons, thereby increasing their excitability. Deactivation of microglia by the addition of 10 ng/ml transforming growth factor-β (TGF-β) decreases I_NavD below control levels suggesting that the residual activated microglial cells influence neuronal excitability in control cultures. Preincubation of hippocampal cultures with 10 ng/ml tumor necrosis factor-α (TNF-α), a major cytokine released by activated microglia, upregulated I_NavD significantly by ~30% in bipolar cells, whereas in pyramid-shaped cells, the upregulation only reached an increase of ~14%. Incubation of the cultures with antibodies against either TNF-receptor 1 or 2 blocked the upregulation of I_NavD in bipolar cells, whereas in pyramid-shaped cells, increases in I_NavD were exclusively blocked by antibodies against TNF-receptor 2, suggesting that both cell types respond differently to TNF-α exposure. Since additional cytokines, such as interleukin-18 (IL-18), are released from activated microglia, we tested potential effects of IL-18 on I_NavD in both cell types. Exposure to 5–10 ng/ml IL-18 for 4 days increased I_NavD in both pyramid-shaped as well as bipolar neurons, albeit the dose–response curves were shifted to lower concentrations in bipolar cells. Our results suggest that by secretion of cytokines, microglial cells upregulate Na⁺ current densities in bipolar and pyramid-shaped neurons to some extent differentially. Depending on the exact cytokine composition and concentration released, this could change the balance between the activity of inhibitory bipolar and excitatory pyramid-shaped cells. Since bipolar cells show a larger upregulation of I_NavD in response to TNF-α as well as respond to smaller concentrations of IL-18, our results offer an explanation for the finding, that in certain conditions of brain inflammations periods of dizziness are followed by epileptic seizures.

Regulation of neuronal excitability by release of proteins from glial cells

Effects of glial cells on electrical isolation and shaping of synaptic transmission between neurons have been extensively studied. Here we present evidence that the release of proteins from astrocytes as well as microglia may regulate voltage-activated Na(+) currents in neurons, thereby increasing excitability and speed of transmission in neurons kept at distance from each other by specialized glial cells. As a first example, we show that basic fibroblast growth factor and neurotrophin-3, which are released from astrocytes by exposure to thyroid hormone, influence each other to enhance Na(+) current density in cultured hippocampal neurons. As a second example, we show that the presence of microglia in hippocampal cultures can upregulate Na(+) current density. The effect can be boosted by lipopolysaccharides, bacterial membrane-derived stimulators of microglial activation. Comparable effects are induced by the exposure of neuron-enriched hippocampal cultures to tumour necrosis factor-α, which is released from stimulated microglia. Taken together, our findings suggest that release of proteins from various types of glial cells can alter neuronal excitability over a time course of several days. This explains changes in neuronal excitability occurring in states of thyroid hormone imbalance and possibly also in seizures triggered by infectious diseases.

Thermal acclimation and thyroxine treatment modify the electric organ discharge frequency in an electric fish, Apteronotus leptorhynchus

In ectotherms, the rate of many neural processes is determined externally, by the influence of the thermal environment on body temperature, and internally, by hormones secreted from the thyroid gland. Through thermal acclimation, animals can buffer the influence of the thermal environment by adjusting their physiology to stabilize certain processes in the face of environmental temperature change. The electric organ discharge (EOD) used by weak electric fish for electrocommunication and electrolocation is highly temperature sensitive. In some temperate species that naturally experience large seasonal fluctuations in environmental temperature, the thermal sensitivity (Q10) of the EOD shifts after long-term temperature change. We examined thermal acclimation of EOD frequency in a tropical electric fish, Apteronotus leptorhynchus that naturally experiences much less temperature change. We transferred fish between thermal environments (25.3 and 27.8 °C) and measured EOD frequency and its thermal sensitivity (Q10) over 11 d. After 6d, fish exhibited thermal acclimation to both warming and cooling, adjusting the thermal dependence of EOD frequency to partially compensate for the small change (2.5 °C) in water temperature. In addition, we evaluated the thyroid influence on EOD frequency by treating fish with thyroxine or the anti-thyroid compound propylthiouricil (PTU) to stimulate or inhibit thyroid activity, respectively. Thyroxine treatment significantly increased EOD frequency, but PTU had no effect. Neither thyroxine nor PTU treatment influenced the thermal sensitivity (Q10) of EOD frequency during acute temperature change. Thus, the EOD of Apteronotus shows significant thermal acclimation and responds to elevated thyroxine.

Gestational and early postnatal hypothyroidism alters VGluT1 and VGAT bouton distribution in the neocortex and hippocampus, and behavior in rats

Thyroid hormones are fundamental for the expression of genes involved in the development of the CNS and their deficiency is associated with a wide spectrum of neurological diseases including mental retardation, attention deficit-hyperactivity disorder and autism spectrum disorders. We examined in rat whether developmental and early postnatal hypothyroidism affects the distribution of vesicular glutamate transporter-1 (VGluT1; glutamatergic) and vesicular inhibitory amino acid transporter (VGAT; GABAergic) immunoreactive (ir) boutons in the hippocampus and somatosensory cortex, and the behavior of the pups. Hypothyroidism was induced by adding 0.02% methimazole (MMI) and 1% KClO4 to the drinking water starting at embryonic day 10 (E10; developmental hypothyroidism) and E21 (early postnatal hypothyroidism) until day of sacrifice at postnatal day 50. Behavior was studied using the acoustic prepulse inhibition (somatosensory attention) and the elevated plus-maze (anxiety-like assessment) tests. The distribution, density and size of VGluT1-ir and VGAT-ir boutons in the hippocampus and somatosensory cortex was abnormal in MMI pups and these changes correlate with behavioral changes, as prepulse inhibition of the startle response amplitude was reduced, and the percentage of time spent in open arms increased. In conclusion, both developmental and early postnatal hypothyroidism significantly decreases the ratio of GABAergic to glutamatergic boutons in dentate gyrus leading to an abnormal flow of information to the hippocampus and infragranular layers of the somatosensory cortex, and alter behavior in rats. Our data show cytoarchitectonic alterations in the basic excitatory hippocampal loop, and in local inhibitory circuits of the somatosensory cortex and hippocampus that might contribute to the delayed neurocognitive outcome observed in thyroid hormone deficient children born in iodine deficient areas, or suffering from congenital hypothyroidism.

Nongenomic effect of levothyroxine on the synchronous electrical activity of the spinal dorsal horn in the rat

Levothyroxine (T4) has a well-known effect on the central nervous system (CNS). This effect requires hours of latency by genetic pathway. We tested for short latency nongenomic effects of T4 superfusion on the spinal dorsal horn (DH) evaluating lumbar somatosensory evoked potentials in rats. T4 increased N and P wave amplitudes and N wave area under the curve, but reduced P wave duration and N-P interval, suggesting that T4 exerts both excitatory and synchronizing effects on DH interneurons in less than 300 s, thus, providing evidence of nongenomic effects of T4 on DH.

Thyroid hormone-dependent development of early cortical networks: temporal specificity and the contribution of trkB and mTOR pathways

Early in neocortical network development, triiodothyronine (T3) promotes GABAergic neurons' population increase, their somatic growth and the formation of GABAergic synapses. In the presence of T3, GABAergic interneurons form longer axons and conspicuous axonal arborizations, with an increased number of putative synaptic boutons. Here we show that the increased GABAergic axonal growth is positively correlated with the proximity to non-GABAergic neurons (non-GABA). A differential innervation emerges from a T3-dependent decrease of axonal length in fields with low density of neuronal cell bodies, combined with an increased bouton formation in fields with high density of neuronal somata. T3 addition to deprived networks after the first 2 weeks of development did not rescue deficits in the GABAergic synaptic bouton distribution, or in the frequency and duration of spontaneous bursts. During the critical 2-week-period, GABAergic signaling is depolarizing as revealed by calcium imaging experiments. Interestingly, T3 enhanced the expression of the potassium-chloride cotransporter 2 (KCC2), and accelerated the developmental shift from depolarizing to hyperpolarizing GABAergic signaling in non-GABA. The T3-related increase of spontaneous network activity was remarkably reduced after blockade of either tropomyosin-receptor kinase B (trkB) or mammalian target of rapamycin (mTOR) pathways. T3-dependent increase in GABAergic neurons' soma size was mediated mainly by mTOR signaling. Conversely, the T3-dependent selective increase of GABAergic boutons near non-GABAergic cell bodies is mediated by trkB signaling only. Both trkB and mTOR signaling mediate T3-dependent reduction of the GABAergic axon extension. The circuitry context is relevant for the interaction between T3 and trkB signaling, but not for the interactions between T3 and mTOR signaling.

Circadian dentate gyrus excitability in a rat model of temporal lobe epilepsy

In human mesial temporal lobe epilepsy (mTLE), seizure occurrence peaks in the late afternoon and early evening. This temporal binding of seizures has been replicated in animal models of mTLE following electrically-induced status epilepticus (SE). We hypothesized that in chronic epilepsy, alterations of circadian excitatory and inhibitory functions of the dentate gyrus (DG), which is believed to regulate the generation of limbic seizures, pathophysiologically contribute to the temporal binding of ictogenesis. We performed electrophysiological single and paired pulse measurements hourly over 24h in the DG of epileptic rats (n=8) 8 weeks after electrically induced SE. Results were compared to individual data obtained before induction of SE and to those of control animals (n=3). Pre and post SE data were analyzed in two distinct phases of the day, i.e. a high-seizure phase between 2p.m. and 10p.m. and a low-seizure phase between 10p.m. and 2p.m. In chronic epileptic animals, latency of evoked potentials was significantly reduced in the high-seizure phase (p=0.027) but not in the low-seizure phase. Compared to baseline values, paired pulse inhibition was significantly increased during the low-seizure phase (interpulse interval (IPI) 25ms, p=0.003; IPI 30ms; p<0.001) but not in the high-seizure phase. Similarly, when compared to controls, inhibition at IPI 20ms was diminished only in the high-seizure phase (p=0.027). Thus, in chronic epileptic animals, DG excitability is increased in the afternoon and early evening possibly contributing to the time of day-dependency of spontaneous seizures in this model system of mTLE. Alterations of circadian DG excitability in epileptic animals may be influenced by changes in hypothalamus-regulated superordinate functions such as excretion of endocrine hormones but further studies are needed.

The prevalence of thyrotoxicosis-related seizures

BACKGROUND

Central nervous system dysfunction, such as hyperexcitation, irritability, and disturbance of consciousness, may occur in patients with thyrotoxicosis. There are also a few case reports of seizures attributed to thyrotoxicosis. The objective of the present study was to determine the prevalence of seizures that appeared to be related to the thyrotoxic state in patients with thyrotoxicosis.

METHODS

We retrospectively determined the prevalence and clinical features of seizures in 3382 patients with hyperthyroidism. Among patients with seizures, we excluded those with other causes of seizures or a history of epilepsy. We did not exclude two patients in whom later work-up showed an abnormal magnetic resonance imaging, as their seizures resolved after they became euthyroid.

RESULTS

Among the 3382 patients with hyperthyroidism, there were seven patients (0.2%) with seizures who met our criteria. Primary generalized tonic-clonic seizures occurred in four patients (57%), complex partial seizures with secondary generalized tonic-clonic seizures occurred in two patients (29%), and one patient had a focal seizure (14%). The initial electroencephalography (EEG) was normal in two patients (29%), had generalized slow activity in four patients (57%), and had diffuse generalized beta activity in one patient (14%). On magnetic resonance imaging, one patient had diffuse brain atrophy, and one had an old basal ganglia infarct. After the patients became euthyroid, the EEG was repeated and was normal in all patients. During follow-up periods ranging from 18 to 24 months, none of the patients had seizures.

CONCLUSIONS

Hyperthyroidism is the precipitating cause of seizures in a small percentage of these patients. In these patients, the prognosis is good if they become euthyroid. The prevalence of thyrotoxicosis-related seizures reported here can be used in conjunction with the prevalence of thyrotoxicosis in the population to estimate the prevalence of thyrotoxicosis-related seizures in populations.

Activity-dependent changes in the electrophysiological properties of regular spiking neurons in the sensorimotor cortex of the rat in vitro

Sensorimotor performance is highly dependent on the level of physical activity. For instance, a period of disuse induces an impairment of motor performance, which is the result of combined muscular, spinal and supraspinal mechanisms. Concerning this latter origin, our hypothesis was that intrinsic properties and input/output coupling of cells within the sensorimotor cortex might participate to the alteration in cortical motor control. The aim of the present study was thus to examine the basic electrophysiological characteristics of cortical cells in control rats and in animals submitted to 14 days of hindlimb unloading, a model of sensorimotor deprivation. Intracellular recordings were obtained in vitro from coronal slices from cortical hindpaw representation area. We have also made an attempt to determine the morphological characteristics as well as the location of the investigated neurons by biocytin labelling. Passive properties of neurons were affected by hindlimb unloading: input resistance and time constant were decreased (-20%), the rheobase was increased (+34%), whereas the resting potential was unchanged. The frequency-current relationships were also modified, the curve being shifted towards right. The size of body area of recorded neurons was unchanged in unloaded rats. Taken together, these data reflect a decrease in excitability of cortical cells in response to a decreased cortical activation.

Effects of thyroid hormone replacement on associative learning and hippocampal synaptic plasticity in adult hypothyroid rats

Activity-dependent changes taking place at the hippocampal perforant pathway-dentate gyrus synapse during classical eyeblink conditioning were recorded in adult thyroidectomized (hypothyroid) and control (euthyroid) rats, and in animals treated with thyroid hormones 20 days after thyroidectomy (recovery rats). The aim was to determine the contribution of thyroid hormones and the consequences of adult-onset hypothyroidism to both associative learning and the physiological potentiation of hippocampal synapses during the actual learning process in alert behaving animals. Control and recovery rats presented similar learning curves, whereas hypothyroid animals presented lower values. A single pulse presented to the perforant pathway during the conditioned-unconditioned inter-stimulus interval evoked a monosynaptic field excitatory postsynaptic potential in dentate granule cells (whose slope was linearly related to the rate of acquisition in the control group), but not in hypothyroid and recovery animals. Input-output relationships and long-term potentiation evoked by train stimulation of the perforant pathway were significantly depressed in hypothyroid animals. Thyroid hormone treatment failed to normalize these two neurophysiological abnormalities observed in hypothyroid animals. In contrast, paired-pulse facilitation was not affected by thyroidectomy. The results indicate that thyroid hormone treatment after a short period of adult hypothyroidism helps to restore some hippocampally dependent functions, such as classical conditioning, but not other hippocampal properties, such as the synaptic plasticity evoked during associative learning and during experimentally induced long-term potentiation. The present results have important clinical implications for the handling of patients with adult-onset thyroid diseases.

Molecular components underlying nongenomic thyroid hormone signaling in embryonic zebrafish neurons

BACKGROUND

Neurodevelopment requires thyroid hormone, yet the mechanisms and targets of thyroid hormone action during embryonic stages remain ill-defined. We previously showed that the thyroid hormone thyroxine (T4) rapidly increases voltage-gated sodium current in zebrafish Rohon-Beard cells (RBs), a primary sensory neuron subtype present during embryonic development. Here, we determined essential components of the rapid T4 signaling pathway by identifying the involved intracellular messengers, the targeted sodium channel isotype, and the spatial and temporal expression pattern of the nongenomic alphaVbeta3 integrin T4 receptor.

RESULTS

We first tested which signaling pathways mediate T4's rapid modulation of sodium current (I(Na)) by perturbing specific pathways associated with nongenomic thyroid hormone signaling. We found that pharmacological blockade of protein phosphatase 1 and the mitogen-activated protein kinase p38 isoform decreased and increased tonic sodium current amplitudes, respectively, and blockade of either occluded rapid responses to acute T4 application. We next tested for the ion channel target of rapid T4 signaling via morpholino knock-down of specific sodium channel isotypes. We found that selective knock-down of the sodium channel alpha-subunit Na(v)1.6a, but not Na(v)1.1la, occluded T4's acute effects. We also determined the spatial and temporal distribution of a nongenomic T4 receptor, integrin alphaVbeta3. At 24 hours post fertilization (hpf), immunofluorescent assays showed no specific integrin alphaVbeta3 immunoreactivity in wild-type zebrafish embryos. However, by 48 hpf, embryos expressed integrin alphaVbeta3 in RBs and primary motoneurons. Consistent with this temporal expression, T4 modulated RB I(Na) at 48 but not 24 hpf. We next tested whether T4 rapidly modulated I(Na) of caudal primary motoneurons, which express the receptor (alphaVbeta3) and target (Na(v)1.6a) of rapid T4 signaling. In response to T4, caudal primary motoneurons rapidly increased sodium current peak amplitude 1.3-fold.

CONCLUSION

T4's nongenomic regulation of sodium current occurs in different neuronal subtypes, requires the activity of specific phosphorylation pathways, and requires both integrin alphaVbeta3 and Na(v)1.6a. Our in vivo analyses identify molecules required for T4's rapid regulation of voltage-gated sodium current.

Thyroid hormone (T3)-induced up-regulation of voltage-activated sodium current in cultured postnatal hippocampal neurons requires secretion of soluble factors from glial cells

We have previously shown that treatment with the thyroid hormone T(3) increases the voltage-gated Na(+)current density (Nav-D) in hippocampal neurons from postnatal rats, leading to accelerated action potential upstrokes and increased firing frequencies. Here we show that the Na(+) current regulation depends on the presence of glial cells, which secrete a heat-instable soluble factor upon stimulation with T(3). The effect of conditioned medium from T(3)-treated glial cells was mimicked by basic fibroblast growth factor (bFGF), known to be released from cerebellar glial cells after T(3) treatment. Neutralization assays of astrocyte-conditioned media with anti-bFGF antibody inhibited the regulation of the Nav-D by T(3). This suggests that the up-regulation of the neuronal sodium current density by T(3) is not a direct effect but involves bFGF release and satellite cells. Thus glial cells can modulate neuronal excitability via secretion of paracrinely acting factors.

Mini-review: Cell surface receptor for thyroid hormone and nongenomic regulation of ion fluxes in excitable cells

Thyroid hormone has been shown experimentally to affect cellular ion fluxes. For example, thyroid hormone-induced modulation has been described of cellular sodium current (I(Na)), inward rectifying potassium current (IKir) and sodium pump (Na, K-ATPase) and of calcium pump (Ca(2+)-ATPase) activities. Certain of these actions appear to reflect nongenomic mechanisms of hormone action that are initiated at the plasma membrane receptor for iodothyronines described on integrin alphavbeta3. One such action is the recent demonstration of enhancement by the hormone of I(Na) in neurons. Nongenomic actions of thyroid hormone initiated at the plasma membrane may be specifically inhibited by tetraiodothyroacetic acid (tetrac), a deaminated thyroid hormone analogue. Important behavioral changes are associated with clinical states of excessive or deficient thyroid function. The molecular basis for these changes has not been established. It is proposed that nongenomic actions of thyroid hormone in neurons-such as that on sodium current-underlie certain of these behaviors. The contribution of such nongenomic actions of the hormone to animal behavioral paradigms possibly relevant to thyroid hormone actions in human subjects may be tested in vivo with tetrac.

Hypothyroidism impairs chloride homeostasis and onset of inhibitory neurotransmission in developing auditory brainstem and hippocampal neurons

Thyroid hormone (TH) deficiency during perinatal life causes a multitude of functional and morphological deficits in the brain. In rats and mice, TH dependency of neural maturation is particularly evident during the first 1-2 weeks of postnatal development. During the same period, synaptic transmission via the inhibitory transmitters glycine and GABA changes from excitatory depolarizing effects to inhibitory hyperpolarizing ones in most neurons [depolarizing-hyperpolarizing (D/H) shift]. The D/H shift is caused by the activation of the K(+)-Cl(-) co-transporter KCC2 which extrudes Cl(-) from the cytosol, thus generating an inward-directed electrochemical Cl(-) gradient. Here we analyzed whether the D/H shift and, consequently, the onset of inhibitory neurotransmission are influenced by TH. Gramicidin perforated-patch recordings from auditory brainstem neurons of experimentally hypothyroid rats revealed depolarizing glycine effects until postnatal day (P)11, i.e. almost 1 week longer than in control rats, in which the D/H shift occurred at approximately P5-6. Likewise, until P12-13 the equilibrium potential E(Gly) in hypothyroids was more positive than the membrane resting potential. Normal E(Gly) could be restored upon TH substitution in P11-12 hypothyroids. These data demonstrate a disturbed Cl(-) homeostasis following TH deficiency and point to a delayed onset of synaptic inhibition. Interestingly, immunohistochemistry demonstrated an unchanged KCC2 distribution in hypothyroids, implying that TH deficiency did not affect KCC2 gene expression but may have impaired the functional status of KCC2. Hippocampal neurons of hypothyroid P16-17 rats also demonstrated an impaired Cl(-) homeostasis, indicating that TH may have promoted the D/H shift and maturation of synaptic inhibition throughout the brain.

Changes in acetylcholinesterase, Na+,K+-ATPase, and Mg2+-ATPase activities in the frontal cortex and the hippocampus of hyper- and hypothyroid adult rats

The thyroid hormones (THs) are crucial determinants of normal development and metabolism, especially in the central nervous system. The metabolic rate is known to increase in hyperthyroidism and decrease in hypothyroidism. The aim of this work was to investigate how changes in metabolism induced by THs could affect the activities of acetylcholinesterase (AChE), (Na+,K+)- and Mg2+-adenosinetriphosphatase (ATPase) in the frontal cortex and the hippocampus of adult rats. Hyperthyroidism was induced by subcutaneous administration of thyroxine (25 microg/100 g body weight) once daily for 14 days, and hypothyroidism was induced by oral administration of propylthiouracil (0.05%) for 21 days. All enzyme activities were evaluated spectrophotometrically in the homogenated brain regions of 10 three-animal pools. A region-specific behavior was observed concerning the examined enzyme activities in hyper- and hypothyroidism. In hyperthyroidism, AChE activity was significantly increased only in the hippocampus (+22%), whereas Na+,K+-ATPase activity was significantly decreased in the hyperthyroid rat hippocampus (-47%) and remained unchanged in the frontal cortex. In hypothyroidism, AChE activity was significantly decreased in the frontal cortex (-23%) and increased in the hippocampus (+21%). Na+,K+-ATPase activity was significantly decreased in both the frontal cortex (-35%) and the hippocampus (-43%) of hypothyroid rats. Mg2+-ATPase remained unchanged in the regions of both hyper- and hypothyroid rat brains. Our data revealed that THs affect the examined adult rat brain parameters in a region- and state-specific way. The TH-reduced Na+,K+-ATPase activity may increase the synaptic acetylcholine release and, thus, modulate AChE activity. Moreover, the above TH-induced changes may affect the monoamine neurotransmitter systems in the examined brain regions.

Postnatal development of masseteric motoneurons in congenital hypothyroid rats

It has been known that an intact thyroid hormone is obligatory for the attainment of the normal masticatory function at the time of weaning. Following induced maternal thyroid hypo-function, the development of masseter motoneurons was determined at postnatal days 1, 7, 15 and 23 (weaning time), using retrograde transport of horseradish peroxidase (HRP) in the normal and hypothyroid pups. Based on the HRP labeling profile (strong and weak), the soma area of the masseteric labeled motoneurons was measured in each group. No significant morphological differences were observed at the end of the first week of life. On day 15, hypothyroid masseteric labeled motoneurons consisted of 76% small and 24% medium-sized neurons compared to 58% and 42% in normal pups, respectively. At the time of weaning (i.e., day 23) the number of large masseter motoneurons reached to 1/3 of normal value with few, short and disoriented dendrites in the hypothyroid pup. There was no statistically significant difference in the uptake of HRP from the neuromuscular junction. These results suggest that neonatal thyroid hormone deficiency considerably postponed the development of feeding behavior from sucking to chewing and biting.