Iodine-125-labeled triiodothyronine in rat brain: evidence for localization in discrete neural systems

Unveiling the nongenomic actions of thyroid hormones in adult mammalian brain: The legacy of Mary B. Dratman

A comprehensive review was conducted to compile the contributions of Mary B. Dratman and studies by other researchers in the field of nongenomic actions of thyroid hormones in adult mammalian brain. Dratman and her collaborators authored roughly half of the papers in this area. It has been almost fifty years since Dratman introduced the novel concept of thyroid hormones as neurotransmitters for the first time. The characterization of unique brain-region specific accumulation of thyroid hormones within the nerve terminals in adult mammals was a remarkable contribution by Dratman. It suggested a neurotransmitter- or neuromodulator-like role of thyroid hormone and/or its derivative, 3-iodothyronamine within adrenergic systems in adult mammalian brain. Several studies by other researchers using synaptosomes as a model system, have contributed to the concept of direct nongenomic actions of thyroid hormones at synaptic regions by establishing that thyroid hormones or their derivatives can bind to synaptosomal membranes, alter membrane functions including enzymatic activities and ion transport, elicit Ca2+/NO-dependent signaling pathways and induce substrate-protein phosphorylation. Such findings can help to explain the physiological and pathophysiological roles of thyroid hormone in psychobehavioral control in adult mammalian brain. However, the exact mode of nongenomic actions of thyroid hormones at nerve terminals in adult mammalian brain awaits further study.

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.

Effect of Levothyroxine Replacement on Cognitive Function Impairment in a Sample of Egyptian Population with Subclinical Hypothyroidism

<b><i>Introduction:</i></b> Subclinical hypothyroidism (SHT) is characterized by a normal range of free thyroxin concentrations together with increased serum TSH levels. SHT is defined as serum thyroid-stimulating hormone (TSH) concentration above the upper limit of the reference range in the face of normal free FT4 and FT3 levels. The effect of SHT on cognitive function has been investigated in several preclinical studies, and a growing body of evidence has suggested a relevant link between thyroid hormones and the central nervous system. <b><i>Objectives:</i></b> This study aimed to investigate the effect of levothyroxine replacement on cognitive impairment in a sample of Egyptian patients with SHT. <b><i>Methods:</i></b> A prospective cohort study conducted on 30 patients with cognitive impairment and SHT attending an endocrine outpatient clinic at the Ain Shams University Hospital to study the effect of levothyroxine supplementation on cognitive impairment in patients with SHT. The study was conducted on 30 patients. All participants were subjected to a full history taking; thorough clinical examination; laboratory investigations including thyroid profile (FT3, FT4, TSH), anti-thyroid peroxidase antibodies, anti-thyroglobulin antibodies, and lipid profile; imaging tests as neck ultrasound, echocardiography, and carotid duplex; and finally Addenbrooke’s questionnaire used to diagnose mild cognitive impairment. <b><i>Results:</i></b> A highly statistically significant difference was found before, 3 months and 6 months after treatment with levothyroxine regarding all clinical data, TSH, LDL, T. cholesterol, FT3, FT4 and HDL, carotid intima-media thickness, and Addenbrooke’s questionnaire. Our study showed a statistically significant inverse correlation between TSH level and mild cognitive impairment before and after treatment with levothyroxine at 3 and 6 months intervals as when TSH increased, results of Addenbrooke’s questionnaire decreased and, so, cognitive impairment increased, while when TSH decreased in response to thyroxine replacement, cognitive impairment improved as detected by an increase in the patient’s score. <b><i>Conclusion:</i></b> SHT has a great effect on cognitive impairment, as normalization in TSH level results in improvement in cognitive function. Also, there was a significant reduction in carotid intima-media thickness, which may contribute to improvement of cognitive function in addition to a great improvement in lipid profile, which in turn positively affects cardiac and cognitive function.

L-3,3',5-triiodothyronine and pregnenolone sulfate inhibit Torpedo nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAChR) is an excitatory pentameric ligand-gated ion channel (pLGIC), homologous to the inhibitory γ-aminobutyric acid (GABA) type A receptor targeted by pharmaceuticals and endogenous sedatives. Activation of the GABA_A receptor by the neurosteroid allopregnanolone can be inhibited competitively by thyroid hormone (L-3,3’,5-triiodothyronine, or T3), but modulation of nAChR by T3 or neurosteroids has not been investigated. Here we show that allopregnanolone inhibits the nAChR from Torpedo californica at micromolar concentrations, as do T3 and the anionic neurosteroid pregnenolone sulfate (PS). We test for the role of protein and ligand charge in mediated receptor inhibition by varying pH in a narrow range around physiological pH. We find that both T3 and PS become less potent with increasing pH, with remarkably similar trends in IC₅₀ when T3 is neutral at pH < 7.3. After deprotonation of T3 (but no additional deprotonation of PS) at pH 7.3, T3 loses potency more slowly with increasing pH than PS. We interpret this result as indicating the negative charge is not required for inhibition but does increase activity. Finally, we show that both T3 and PS affect nAChR channel desensitization, which may implicate a binding site homologous to one that was recently indicated for accelerated desensitization of the GABA_A receptor by PS.

Morning vs. bedtime levothyroxine administration: what is the ideal choice for children?

Background The present study compared the administration of levothyroxine (LT4) before breakfast and bedtime in school children diagnosed with hypothyroidism and analyzed the effects of timing on thyroid functioning and patient satisfaction. Methods A total of 163 children with acquired hypothyroidism (125 females and 38 males) between 8 and 18 years of age and taking LT4 for at least 3 months were enrolled in the study. The timing of administration of the drug of all subjects was shifted to bedtime. The levels of thyroid hormone and blood lipid, anthropometric measurements, Pediatric Quality of Life Inventory, Morisky Medication Adherence Scale and hypothyroidism symptoms scores were analyzed and compared at the beginning of the study and 3 months later after the shift in the timing of drug administration. Results There was no difference between the bedtime and morning regimens of LT4 with respect to thyroid hormone levels, quality of life, drug adherence and symptoms of hypothyroidism. At the end of the study, 45 of 70 new-onset treated subjects preferred the bedtime regimen. Also, drug adherence was found to be better in these patients. Conclusions We found no difference between the bedtime and morning regimens in both new-onset and long-standing treated patients. In naive patients, consideration of patient's preference for timing of drug administration may increase their adherence to medication. Therefore, we suggest that choice of drug administration timing should be based on the preference of patients.

Interactions of L-3,5,3'-Triiodothyronine [corrected], Allopregnanolone, and Ivermectin with the GABAA Receptor: Evidence for Overlapping Intersubunit Binding Modes

Structural mechanisms of modulation of γ-aminobutyric acid (GABA) type A receptors by neurosteroids and hormones remain unclear. The thyroid hormone L-3,5,3’-triiodothyronine (T3) inhibits GABA_A receptors at micromolar concentrations and has common features with neurosteroids such as allopregnanolone (ALLOP). Here we use functional experiments on α₂β₁γ₂ GABA_A receptors expressed in Xenopus oocytes to detect competitive interactions between T3 and an agonist (ivermectin, IVM) with a crystallographically determined binding site at subunit interfaces in the transmembrane domain of a homologous receptor (glutamate-gated chloride channel, GluCl). T3 and ALLOP also show competitive effects, supporting the presence of both a T3 and ALLOP binding site at one or more subunit interfaces. Molecular dynamics (MD) simulations over 200 ns are used to investigate the dynamics and energetics of T3 in the identified intersubunit sites. In these simulations, T3 molecules occupying all intersubunit sites (with the exception of the α-β interface) display numerous energetically favorable conformations with multiple hydrogen bonding partners, including previously implicated polar/acidic sidechains and a structurally conserved deformation in the M1 backbone.

Update on 3-iodothyronamine and its neurological and metabolic actions

3-iodothyronamine (T1AM) is an endogenous amine, that has been detected in many rodent tissues, and in human blood. It has been hypothesized to derive from thyroid hormone metabolism, but this hypothesis still requires validation. T1AM is not a ligand for nuclear thyroid hormone receptors, but stimulates with nanomolar affinity trace amine-associated receptor 1 (TAAR1), a G protein-coupled membrane receptor. With a lower affinity it interacts with alpha2A adrenergic receptors. Additional targets are represented by apolipoprotein B100, mitochondrial ATP synthase, and membrane monoamine transporters, but the functional relevance of these interactions is still uncertain. Among the effects reported after administration of exogenous T1AM to experimental animals, metabolic and neurological responses deserve special attention, because they were obtained at low dosages, which increased endogenous tissue concentration by about one order of magnitude. Systemic T1AM administration favored fatty acid over glucose catabolism, increased ketogenesis and increased blood glucose. Similar responses were elicited by intracerebral infusion, which inhibited insulin secretion and stimulated glucagon secretion. However, T1AM administration increased ketogenesis and gluconeogenesis also in hepatic cell lines and in perfused liver preparations, providing evidence for a peripheral action, as well. In the central nervous system, T1AM behaved as a neuromodulator, affecting adrenergic and/or histaminergic neurons. Intracerebral T1AM administration favored learning and memory, modulated sleep and feeding, and decreased the pain threshold. In conclusion T1AM should be considered as a component of thyroid hormone signaling and might play a significant physiological and/or pathophysiological role. T1AM analogs have already been synthetized and their therapeutical potential is currently under investigation. 3-iodothyronamine (T1AM) is a biogenic amine whose structure is closely related to that of thyroid hormone (3,5,3′-triiodothyronine, or T3). The differences with T3 are the absence of the carboxylate group and the substitution of iodine with hydrogen in 5 and 3′ positions (Figure 1). In this paper we will review the evidence supporting the hypothesis that T1AM is a chemical messenger, namely that it is an endogenous substance able to interact with specific receptors producing significant functional effects. Special emphasis will be placed on neurological and metabolic effects, which are likely to have physiological and pathophysiological importance.

Effects of acute microinjections of the thyroid hormone derivative 3-iodothyronamine to the preoptic region of adult male rats on sleep, thermoregulation and motor activity

The decarboxylated thyroid hormone derivative 3-iodothyronamine (T1AM) has been reported as having behavioral and physiological consequences distinct from those of thyroid hormones. Here, we investigate the effects of T1AM on EEG-defined sleep after acute administration to the preoptic region of adult male rats. Our laboratory recently demonstrated a decrease in EEG-defined sleep after administration of 3,3',5-triiodo-l-thyronine (T3) to the same brain region. After injection of T1AM or vehicle solution, EEG, EMG, activity, and core body temperature were recorded for 24h. Sleep parameters were determined from EEG and EMG data. Earlier investigations found contrasting systemic effects of T3 and T1AM, such as decreased heart rate and body temperature after intraperitoneal T1AM injection. However, nREM sleep was decreased in the present study after injections of 1 or 3 μg T1AM, but not after 0.3 or 10 μg, closely mimicking the previously reported effects of T3 administration to the preoptic region. The biphasic dose-response observed after either T1AM or T3 administration seems to indicate shared mechanisms and/or functions of sleep regulation in the preoptic region. Consistent with systemic administration of T1AM, however, microinjection of T1AM decreased body temperature. The current study is the first to show modulation of sleep by T1AM, and suggests that T1AM and T3 have both shared and independent effects in the adult mammalian brain.

Effects of acute microinjections of thyroid hormone to the preoptic region of euthyroid adult male rats on sleep and motor activity

In adult brain tissue, thyroid hormones are known to have multiple effects which are not mediated by chronic influences of the hormones on heterodimeric thyroid hormone nuclear receptors. Previous work has shown that acute microinjections of l-triiodothyronine (T3) to the preoptic region significantly influence EEG-defined sleep in hypothyroid rats. The current study examined the effects of similar microinjections in euthyroid rats. In 7 rats with histologically confirmed microinjection sites bilaterally placed in the preoptic region, slow-wave sleep time was significantly decreased, but REM and waking were increased as compared to vehicle-injected controls. The EEG-defined parameters were significantly influenced by the microinjections in a biphasic dose-response relationship; the lowest (0.3μg) and highest (10μg) doses tested were without significant effect while intermediate doses (1 and 3μg) induced significant differences from controls. There were significant diurnal variations in the measures, yet no significant interactions between the effect of hormone and time of day were demonstrated. Core body temperature was not significantly altered in the current study. The demonstration of effects of T3 within hours instead of days is consistent with a rapid mechanism of action such as a direct influence on neurotransmission. Since the T3-mediated effects were robust in the current work, euthyroid rats retain thyroid hormone sensitivity which would be needed if sleep-regulatory mechanisms in the preoptic region are continuously modulated by the hormones. This article is part of a Special Issue entitled LInked: BRES-D-12-01552 & BRES-D-12-01363R2.

Drugs in the brain--cellular imaging with receptor microscopic autoradiography

For cell and tissue localization of drugs, receptor microscopic autoradiography is reviewed, including its development history, multiple testing, extensive applications and significant discoveries. This sensitive high-resolution imaging method is based on the use of radiolabeled compounds (esp. tagged with (3)H or (125)I), preservation through freezing of in vivo localization of tissue constituents, cutting thin frozen sections, and close contact with the recording nuclear emulsion. After extensive testing of the utility of this method, the distribution of radiolabeled compounds has been identified and characterized for estradiol, progestagens, adrenal steroids, thyroid hormone, ecdysteroids, vitamin D, retinoic acid, metabolic indicators glucose and 2-deoxyglucose, as well as extracellular space indicators. Target cells and associated tissues have been characterized with special stains, fluorescing compounds, or combined autoradiography-immunocytochemistry with antibodies to dopamine-beta-hydroxylase, GABA, enkephalin, specific receptor proteins, or other cellular products. Blood-brain barrier and brain entries via capillary endothelium, ependyma, or circumventricular recess organs have been visualized for (3)H-dexamethasone, (210)Pb lead, and (3)H-1,25(OH)(2) vitamin D(3). With this histopharmacologic approach, cellular details and tissue integrative overviews can be assessed in the same preparation. As a result, information has been gained that would have been difficult or impossible otherwise. Maps of brain drug distribution have been developed and relevant target circuits have been recognized. Examples include the stria terminalis that links septal-amygdaloid-thalamic-hypothalamic structures and telencephalic limbic system components which extend as the periventricular autonomic-neuroendocrine ABC (Allocortex-Brainstem-Circuitry) system into the mid- and hindbrain. Discoveries with radiolabeled substances challenged existing paradigms, engendering new concepts and providing seminal incentives for further research toward understanding drug actions. Most notable are discoveries made during the 1980s with vitamin D in the brain together with over 50 target tissues that challenged the century-old doctrine of vitamin D's main role as 'the calcitropic hormone', when the new data made it apparent that the main biological function of this multifunctional sunshine hormone rather is maintenance of life and adapting vital functions to the solar environment. In the brain, vitamin D, in close relation to sex and adrenal steroids, participates in the regulation of the secretion of neuro-endocrines, such as, serotonin, dopamine, nerve growth factor, acetyl choline, with importance in prophylaxis and therapy of neuro-psychiatric disorders. Histochemical imaging with high cellular-subcellular resolution is necessary for obtaining detailed information, as this review indicates. New spectrometric methods, like MALDI-MSI, are unlikely to furnish the same information as receptor microautoradiography does, but can provide important correlative molecular information.

3-Monoiodothyronamine: the rationale for its action as an endogenous adrenergic-blocking neuromodulator

The investigations reported here were designed to gain insights into the role of 3-monoiodothyronamine (T1AM) in the brain, where the amine was originally identified and characterized. Extensive deiodinase studies indicated that T1AM was derived from the T4 metabolite, reverse triiodothyronine (revT3), while functional studies provided well-confirmed evidence that T1AM has strong adrenergic-blocking effects. Because a state of adrenergic overactivity prevails when triiodothyronine (T3) concentrations become excessive, the possibility that T3's metabolic partner, revT3, might give rise to an antagonist of those T3 actions was thought to be reasonable. All T1AM studies thus far have required use of pharmacological doses. Therefore we considered that choosing a physiological site of action was a priority and focused on the locus coeruleus (LC), the major noradrenergic control center in the brain. Site-directed injections of T1AM into the LC elicited a significant, dose-dependent neuronal firing rate change in a subset of adrenergic neurons with an EC(50)=2.7 microM, a dose well within the physiological range. Further evidence for its physiological actions came from autoradiographic images obtained following intravenous carrier-free (125)I-labeled T1AM injection. These showed that the amine bound with high affinity to the LC and to other selected brain nuclei, each of which is both an LC target and a known T3 binding site. This new evidence points to a physiological role for T1AM as an endogenous adrenergic-blocking neuromodulator in the central noradrenergic system.

Transport of thyroid hormones is selectively inhibited by 3-iodothyronamine

Thyroid hormone transporters are responsible for the cellular uptake of thyroid hormones, which is a prerequisite for their subsequent metabolism and action at nuclear thyroid hormone receptors. A recently discovered thyroid hormone derivative, 3-iodothyronamine (T(1)AM), has distinct biological effects that are opposite those of thyroid hormone. Here we investigate the effects of T(1)AM on thyroid hormone transporters using COS-1 cells transfected with the multispecific organic anion transporting polypeptides (OATPs) 1A2, 1B3, and 1C1, as well as the specific thyroid hormone transporters MCT8 and MCT10, and show that T(1)AM displays differential inhibition of T(3) and T(4) cellular uptake by these transporters. T(1)AM inhibits T(3) and T(4) transport by OATP1A2 with IC(50) values of 0.27 and 2.1 microM, respectively. T(4) transport by OATP1C1, which is thought to play a key role in thyroid hormone transport across the blood-brain barrier, is inhibited by T(1)AM with an IC(50) of 4.8 microM. T(1)AM also inhibits both T(3) and T(4) uptake via MCT8, the most specific thyroid hormone transporter identified to date, with IC(50) values of 95 and 31 microM, respectively. By contrast, T(1)AM has no effect on thyroid hormone transport by OATP1B3 and MCT10. Given that OATP1A2, OATP1C1, and MCT8 are all present in the brain, T(1)AM may play an important role in modulating thyroid hormone delivery and activity in specific target regions in the central nervous system.

Successful treatment with levothyroxine for idiopathic hypersomnia patients with subclinical hypothyroidism

OBJECTIVE

Our objective was to discuss the effect of levothyroxine on excessive daytime sleepiness (EDS) and a prolonged nocturnal sleep at patients with idiopathic hypersomnia who presented with subclinical hypothyroidism.

METHODS

We present two patients with hypersomnia who complained of EDS and a prolonged nocturnal sleep time. Sleep architecture and subjective daytime sleepiness were estimated by polysomnography (PSG) and Epworth Sleepiness Scale (ESS), respectively. Diagnoses were made using the International Classification of Sleep Disorders, 2nd Edition criteria for idiopathic hypersomnia with long sleep time.

RESULTS

PSG demonstrated a short sleep latency, a prolonged total sleep time and normal proportions of all non-rapid eye movement (REM) and REM sleep stages. Nocturnal PSG excluded other causes of EDS. No medical, neurological and mental disorders were present. Their laboratory data indicated mildly elevated thyrotropin, despite free thyroxine (T4) and triiodothyronine (T3) estimates within their reference ranges, which is a characteristic of latent hypothyroidism. Levothyroxine (25 microg/day) was administrated orally. After treatment with levothyroxine for 8 weeks, the mean daily sleep times decreased. EDS was also improved, and a significant decrease in the ESS score was observed. Levothyroxine was effective for their hypersomnia and well tolerated.

CONCLUSIONS

It should be noted that hypersomnia may be associated with subclinical hypothyroidism, although few abnormalities in physical and neurological examinations are present.

Thyroid hormone action: nongenomic modulation of neuronal excitability in the hippocampus

Years of effort have failed to establish a generally-accepted mechanism of thyroid hormone (TH) action in the mature brain. Recently, both morphological and pharmacological evidence have supported a direct neuroactive role for the hormone and its triiodinated metabolites. However, no direct physiological validation has been available. We now describe electrophysiological studies in vivo in which we observed that local thyroxine (T4) administration promptly inhibited field excitatory postsynaptic potentials recorded in the dentate gyrus (DG) with stimulation of the medial perforant pathway, a result that was found to be especially pronounced in hypothyroid rats. In separate in vitro experiments, we observed more subtle but statistically significant responses of hippocampal slices to treatment with the hormone. The results demonstrate that baseline firing rates of CA1 pyramidal cells were modestly reduced by pulse-perfusion with T4. By contrast, administration of triiodothyronine (T3) was often noted to have modest enhancing effects on CA1 cell firing rates in hippocampal slices from euthyroid animals. Moreover, and more reliably, robust firing rate increases induced by norepinephrine were amplified when preceded by treatment with T3, whereas they were diminished by pretreatment with T4. These studies provide the first direct evidence for functional, nongenomic actions of TH leading to rapid changes in neuronal excitability in adult rat DG studied in vivo and highlight the opposing effects of T4 and T3 on norepinephrine-induced responses of CA1 cells studied in vitro.

The thyroid-brain interaction in thyroid disorders and mood disorders

Thyroid hormones play a critical role in the metabolic activity of the adult brain, and neuropsychiatric manifestations of thyroid disease have long been recognised. However, it is only recently that methodology such as functional neuroimaging has been available to facilitate investigation of thyroid hormone metabolism. Although the role of thyroid hormones in the adult brain is not yet specified, it is clear that without optimal thyroid function, mood disturbance, cognitive impairment and other psychiatric symptoms can emerge. Additionally, laboratory measurements of peripheral thyroid function may not adequately characterise central thyroid metabolism. Here, we review the relationship between thyroid hormone and neuropsychiatric symptoms in patients with primary thyroid disease and primary mood disorders.

Thyronamines inhibit plasma membrane and vesicular monoamine transport

Thyroid hormone has long been known to have important transcriptional regulatory activities. Recently, however, the presence of endogenous derivatives of thyroid hormone, thyronamines, has been reported in various mammalian tissues. These derivatives have potent in vitro activity with a class of orphan G-protein-coupled receptors, the trace amine-associated receptors, and profound in vivo effects when administered to mice. We report here a novel neuromodulatory role for thyronamines. In synaptosomal preparations and heterologous expression systems, thyronamines act as specific dopamine and norepinephrine reuptake inhibitors. Thyronamines also inhibit the transport of monoamines into synaptic vesicles. These observations expand the nontranscriptional role of thyroid hormone derivatives and may help to explain the pharmacological effects of thyronamines in vivo.

Imaging triiodothyronine binding kinetics in rat brain: a model for studies in human subjects

Many lines of evidence indicate a role for thyroid hormones in the expression of cognitive and affective disorders. These conditions constitute a large proportion of the illness burden in the general population. Unfortunately, presently available diagnostic procedures cannot adequately identify these problems. To determine whether imaging studies of thyroid hormone kinetics in brain might be feasible in patients with these disorders, an autoradiographic method for measuring thyroid hormone kinetics was developed. Twenty-five awake adult rats received high specific activity [(125)I]-triiodothyronine (T(3)*). Brains were obtained at intervals from 5 through 300 min after i.v. hormone administration. Every 5th frozen section was thaw mounted and exposed to film. To determine whether T(3) was responsible for the autoradiographic images, the intervening sections were assembled while frozen in regional tissue pools and were extracted and then analyzed by high-performance liquid chromatography. The results demonstrated that radioactivity was almost entirely due to T(3)*( approximately 90%) while small amounts of hormone metabolites, including [(125)I]iodine accounted for the remainder. Regional concentrations of label in autoradiograms were measured by densitometry in hippocampus (CA1, CA2, CA3, and dentate gyrus), cerebellum (molecular and granular cell layers), caudate nucleus, and amygdala. Unexpectedly and interestingly, the results demonstrated that binding through 5 h was mainly irreversible. Regional values of the net uptake rate constant of T(3)* or influx constant, K(i), were determined from the time course of the T(3)* data, showing significant differences among regions. These results suggest that imaging of labeled thyroid hormone ligands by positron emission tomography or single photon emission computed tomography may be feasible and would potentially provide useful information relevant to T(3) processing in the brain during a variety of drug and disease-induced conditions.

Thyroid hormone distribution in the mouse brain: the role of transthyretin

Transthyretin is the major thyroxine-binding protein in the plasma of rodents, and the main thyroxine-binding protein in the cerebrospinal fluid of both rodents and humans. The choroid plexus synthesizes transthyretin and secretes it to the cerebrospinal fluid. Although it was suggested that transthyretin might play an important role in mediating thyroxine transfer from the blood into the brain across the choroid plexus-cerebrospinal fluid barrier, newer findings question this hypothesis. Because thyroid hormone passage across brain barriers is a precondition for its action in the CNS, and because brain is an important target of thyroid hormone action, we investigated the role of transthyretin in mediating thyroid hormone access to and distribution within the brain in a transthyretin-null mouse model system. In this report we describe the results derived from use of film autoradiography, a technique that yields definitive morphological results. Film autoradiograms were prepared at 3 and 19 h after intravenous injection of either high specific activity [(125)I]thyroxine or [(125)I]triiodothyronine. Image analyses were designed to demonstrate regional changes in hormone distribution, and to highlight alterations in iodothyronine delivery from ventricles to brain parenchyma. We find no qualitative or quantitative differences in these parameters between the transthyretin-null and the wild-type mouse brain after either [(125)I]thyroxine or [(125)I]triiodothyronine administration. The data presented here now provide definitive evidence that, under standard laboratory conditions, transthyretin is not required for thyroid hormone access to or distribution within the mouse brain. This study also provides the first map of iodothyronine distribution in the brain of the mouse.

Evidence that 3,3',5-triiodothyronine is concentrated in and delivered from the locus coeruleus to its noradrenergic targets via anterograde axonal transport

Recent immunohistochemical studies of rat brain triiodothyronine reveal heaviest localization in locus coeruleus perikarya. The cellular distribution is similar to that observed in concomitant studies of tyrosine hydroxylase immunohistochemistry: heavy clumps of immunoreactive triiodothyronine are distributed within locus coeruleus cytosol and in cell processes, leaving cell nuclei unstained. At the same time, in locus coeruleus targets, cell nuclei as well as surrounding neuropil are prominently triiodothyronine labeled. These observations, combined with diverse evidence linking thyroid hormone with norepinephrine at many levels of physiological and pathophysiological function, led to the hypothesis that the locus coeruleus binds and accumulates triiodothyronine and delivers the hormone via anterograde axonal transport to postsynaptic locus coeruleus targets, where nuclear triiodothyronine receptors are densely concentrated. Furthermore, the hypothesis predicts that destruction of locus coeruleus nerve terminals would interrupt this neural route of triiodothyronine delivery and prevent or reduce triiodothyronine labeling of nuclear receptors in noradrenergic target cells. To test this formulation, we gave the specific locus coeruleus lesioning agent, N-(2-chloroethyl)-N-2-bromobenzylamine hydrochloride (DSP-4), to adult male rats and examined their brains three, five and seven days thereafter by triiodothyronine and, in alternate sections, tyrosine hydroxylase immunohistochemistry. Treatment with DSP-4 resulted in specific and selective reduction in tyrosine hydroxylase and triiodothyronine immunohistochemical labeling in cell nuclei and in nerve cell processes within the neuropil of the hippocampus and cerebral cortex at all time periods examined. The results demonstrate that full occupancy of locus coeruleus target cells by triiodothyronine requires the presence of intact locus coeruleus projections and supports the proposal that, like norepinephrine, triiodothyronine delivery to noradrenergic targets occurs through delivery by locus coeruleus terminals. These findings provide strong support for earlier proposals that triiodothyronine functions as a co-transmitter with norepinephrine in addition to or as part of its genomic role in the cells receiving noradrenergic innervation.

Thyroid hormones and the treatment of depression: an examination of basic hormonal actions in the mature mammalian brain

Numerous clinical reports indicate that thyroid hormones can influence mood, and a change in thyroid status is an important correlate of depression. Moreover, thyroid hormones have been shown to be effective as adjuncts for traditional antidepressant medications in treatment-resistant patients. In spite of a large clinical literature, little is known about the mechanism by which thyroid hormones elevate mood. The lack of mechanistic insight reflects, in large part, a longstanding bias that the mature mammalian central nervous system is not an important target site for thyroid hormones. Biochemical, physiological, and behavioral evidence is reviewed that provides a clear picture of their importance for neuronal function. This paper offers the hypothesis that the thyroid hormones influence affective state via postreceptor mechanisms that facilitate signal transduction pathways in the adult mammalian brain. This influence is generalizable to widely recognized targets of antidepressant therapies such as noradrenergic and serotonergic neurotransmission.

Testis-dependent and -independent effects of photoperiod on volumes of song control nuclei in American tree sparrows (Spizella arborea)

Songbirds exhibit seasonal changes in the volumes of song control nuclei. Birds on long, spring-like days have larger nuclei than do birds on short, winter-like days. The mechanisms mediating volumetric changes have not been determined unequivocally, but testosterone (T) is probably involved. This study examined whether testicular factors are uniquely responsible for seasonal changes in the song system, or whether photoperiod has testis-independent effects. Male American tree sparrows were exposed to one of three photoperiodic conditions: (1) Photosensitive birds were retained on short days (8L:16D). Plasma T is rarely detected in such birds. (2) Photosensitive birds were moved from short days to long days (20L:4D) and photostimulated for three weeks. Photostimulation elevates circulating T in photosensitive birds. (3) Photorefractory birds were held at least four months on 20L:4D. Such birds seldom have detectable levels of T, even though they are on long days. In each condition, there were both intact and castrated birds. Castration typically removes circulating T in tree sparrows. The volumes of the high vocal center (HVC), nucleus robustus archistriatalis (RA), and area X were measured. Song nuclei were largest in intact photostimulated birds. Other long-day birds (i.e. castrated photostimulated, and intact and castrated photorefractory groups) had larger song nuclei than did short-day intact or castrated photosensitive birds and did not differ from each other. These data indicate that photoperiod has both testis-dependent and -independent effects on the volumes of song control nuclei.

Thyroid hormones as neurotransmitters

During brain development, before the apparatus of neurotransmission has been set into place, many neurotransmitters act as growth regulators. In adult brain, their role in neurotransmission comes to the fore but neuronal plasticity and other growth-related processes are their continuing responsibility. This has been clearly demonstrated for catecholamines. Previous as well as recent evidence now indicates that thyroid hormones may participate in the developing and adult brain through similar mechanisms. Immunohistochemical mapping of brain triiodothyronine (antibody specificity established by numerous appropriate tests) demonstrated that the hormone was concentrated in both noradrenergic centers and noradrenergic projection sites. In the centers (locus coeruleus and lateral tegmental system) triiodothyronine staining, like that of tyrosine hydroxylase, was heavily concentrated in cytosol and cell processes. By contrast, in noradrenergic targets, label was most prominent in cell nuclei. Combined biochemical and morphologic data allows a construct of thyroid hormone circuitry to unfold: The locus coeruleus is conveniently located just beneath the ependyma of the 4th ventricle. Thyroxine, entering the brain via the choroid plexus, is preferentially delivered to subependymal brain structures. High concentrations of locus coeruleus norepinephrine promote active conversion of thyroxine to triiodothyronine, leading to the preeminence of the locus coeruleus as a site of triiodothyronine concentration. Results of treatment with the locus coeruleus neurotoxin DSP-4 established that axonal transport accounts for delivery of both triiodothyronine and norepinephrine from locus coeruleus to noradrenergic terminal fields. The apparatus for transduction of thyronergic and noradrenergic signals at both membrane and nuclear sites resides in the postsynaptic target cells. Upon internalization of hormone in post-synaptic target cells, genomic effects of triiodothyronine, norepinephrine, and/or their second messengers are possible and expected. The evidence establishes a direct morphologic connection between central thyronergic and noradrenergic systems, supporting earlier proposals that triiodothyronine or its proximate metabolites may serve as cotransmitters with norepinephrine in the adrenergic nervous system.

Immunohistochemical mapping of brain triiodothyronine reveals prominent localization in central noradrenergic systems

Many lines of evidence support a close association between thyroid hormones and noradrenergic systems in peripheral tissues. However, there is little certainty regarding interactions of the two systems in brain. We now report that triiodothyronine is concentrated in both nuclei and projection sites of central noradrenergic systems. Immunohistochemical mapping of the hormone revealed the following: (1) Locus coeruleus and all other noradrenergic cell groups identified were the most prominently labeled neural centers in the brain. (2) The hormone was also concentrated in the widely dispersed targets of noradrenergic projections. (3) Triiodothyronine labeling in noradrenergic target cells was most prominent over the cell nuclei, indicating that the hormone was bound to its receptors. Therefore, targets of noradrenergic innervation should be responsive to triiodothyronine. (4) Unlike that in noradrenergic target cells, triiodothyronine staining was decidedly perikaryal in locus coeruleus (A-6) and the other A-1 to A-7 cell groups; the staining pattern in locus coeruleus cytosol and processes was heavy, clumped and similar to that seen in contiguous sections immunostained for tyrosine hydroxylase. Results of radio-immunoassay, immunoabsorption and pharmacological tests demonstrated the specificity of the antibody for triiodothyronine and ruled against cross-reactivity with norepinephrine or its metabolites as the basis for the staining reactions. Although other possibilities consistent with these new observations are given consideration, it appears that the structure and activity of central noradrenergic systems may be major determinants of triiodothyronine distribution patterns and actions in brain. If the noradrenergic system processes both triiodothyronine and norepinephrine and conducts them both to nerve cell groups receiving its terminal arborizations, specific postsynaptic receptors would be available for transduction of both sets of messages. The evidence provides a morphological basis for earlier proposals that triiodothyronine may play a neuromodulatory or neurotransmitter role in the adrenergic nervous system.

Thyroid hormonal modulation of the binding and activity of the GABAA receptor complex of brain

Thyroid hormones, which are known to act by genomic mechanisms in peripheral tissues, were found to influence the binding and function of the GABAA receptor complex in brain membranes. Submicromolar concentrations of triiodothyronine and thyroxine stereospecifically stimulated the binding of [35S]t-butylbicyclophosphorothionate (a convulsant ligand for the GABAA receptor complex) to highly washed rat brain membranes, while higher concentrations of the hormones inhibited radioligand binding. GABA-stimulated 36Cl-flux in isolated brain membrane sacs was inhibited by L-triiodothyronine with a half-maximally inhibitory concentration (IC50) of 10(-7) M. Patch-clamp analysis of recombinant GABAA receptor subunits expressed in human embryonic kidney-293 cells showed an inhibition of chloride currents by thyroid hormones. This effect required only the alpha 1 beta 2 subunits, and was not blocked by the benzodiazepine antagonist flumazenil. Since thyroid hormones are known to be concentrated in nerve terminal preparations and subsequently released, the hormones may have non-genomic mechanisms of action as putative neurotransmitters or neuromodulators in brain and act through GABAA receptors.

Effects of thyroid hormones on central nervous system in aging

Support for the many relationships between thyroid hormones and brain function comes from both laboratory and clinical studies. Studies in laboratory animals provide convincing evidence for a neuroregulatory role of thyroid hormones in the brain, suggesting that they may affect behavior. This notion is supported by human studies which have revealed that the effects of thyroid hormones on brain function are most important during the development and maturation of the brain; thereafter, age does not seem to critically affect brain-thyroid hormone relationships.

Neuroendocrine regulatory mechanisms in the choroid plexus-cerebrospinal fluid system

The CSF is often regarded as merely a mechanical support for the brain, as well as an unspecific sink for waste products from the CNS. New methodology in receptor autoradiography, immunohistochemistry and molecular biology has revealed the presence of many different neuroendocrine substances or their corresponding receptors in the main CSF-forming structure, the choroid plexus. Both older research on the sympathetic nerves and recent studies of peptide neurotransmitters in the choroid plexus support a neurogenic regulation of choroid plexus CSF production and other transport functions. Among the endocrine substances present in blood and CSF, 5-HT, ANP, vasopressin and the IGFs have high receptor concentrations in the choroid plexus and have been shown to influence choroid plexus function. Finally, the choroid plexus produces the growth factor IGF-II and a number of transport proteins, most importantly transthyretin, that might regulate hormone transport from blood to brain. These studies suggest that the choroid plexus-CSF system could constitute an important pathway for neuroendocrine signalling in the brain, although clearcut evidence for such a role is still largely lacking.

3,5,3'-Triiodothyronine binding sites in synaptosomes from brain of chick embryo. Properties and ontogeny

In this study we have demonstrated the presence of specific 3,5,3'-L-triiodothyronine (T3) binding sites in the synaptosomes of chick embryo cerebral cortex and described their ontogeny. Scatchard analyses of binding data obtained with synaptosomal preparations from 17-day-old embryos revealed two T3 binding sites. The first site (N1) had a high affinity and low capacity since its dissociation constant (Kd) was 68 +/- 1.3 nM T3 (mean +/- S.D.; n = 3-5) and its maximal binding capacity (Bmax) was 8.63 +/- 1.59 ng T3/mg of protein, whereas the second site (N2) had a higher Kd of 5.04 +/- 0.5 microM T3 and a larger Bmax of 405 +/- 49 ng T3/mg of protein. The relative affinity of the synaptosomal fraction for T3 and other analogs was the following: T3 greater than T4 (thyroxine) greater than D-T3 (3,5,3'-D-triiodothyronine) = TRIAC (triiodothyroacetic acid) greater than rT3 (reverse T3). Gel chromatography of the [125I]T3 labeled fraction revealed a partially saturable peak with an estimated MW of more than 100 kDa. The ontogenic pattern showed a progressive increase of Kd and Bmax of N1, occurring mainly between the 12 and 19 days of incubation, and a marked fall, particularly of the Bmax, after hatching. The second site did not show any important variation during the embryogenesis. These data indicate the existence of specific T3 binding sites in synaptosomes from cerebral cortex of chick embryo, whose properties and ontogeny are completely different from those of the nuclear receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Transport of iodothyronines from bloodstream to brain: contributions by blood:brain and choroid plexus:cerebrospinal fluid barriers

Thyroid hormone entering the brain from the cerebral circulation must first cross barriers at the the blood:brain and choroid plexus:cerebrospinal fluid interfaces. The route taken after entry through those barriers might bring about selective delivery of hormone to different regions of the brain and those differences might be crucial for the ultimate functional effects of the hormone. To determine whether and how distribution of hormone in the brain might vary according to the route of entry, film autoradiograms of serially sectioned brains were prepared after delivery of a pulse of 125I-labeled thyroid hormone into either the right lateral cerebral ventricle or the femoral vein. The results after intrathecal injection, reflecting the penetration of hormone into brain after crossing the choroid plexus:cerebrospinal fluid barrier, revealed a markedly limited, essentially periventricular distribution of radioactivity at both 3 and 48 h after hormone administration. Results after i.v. administration, which allows hormone access across both barriers, revealed an initial distribution pattern (at 3 h) generally similar to that seen after administration of markers of cerebral blood flow; at 48 h there was strong resolution in selected brain regions never noted to be labeled after intrathecal hormone injection. The functional implications of the differences in results produced by the two different routes of hormone entry are not known. However, ready access to circumventricular organs would appear to be favored by hormone crossing the choroid plexus:cerebrospinal fluid barrier whereas access to the panoply of nuclear triiodothyronine receptors would be favored by hormone crossing the blood:brain barrier. Therefore both routes of barrier transport should be taken into account in assessing the kinetics and actions of thyroid hormones in the central nervous system.

The hippocampal formation: morphological changes induced by thyroid, gonadal and adrenal hormones

The hippocampal formation is of considerable interest due to its proposed role in a number of important functions, including learning and memory processes. Manipulations of thyroid, gonadal and adrenal hormones have been shown to influence hippocampal physiology as well as learning and memory. The cellular events which underlie these hormone-induced functional changes are largely unexplored. However, studies suggest that hormonal manipulations during development and in adulthood result in dramatic morphological changes within the hippocampal formation. Because neuronal physiology has been suggested to depend upon neuronal morphology, we have been determining the morphologic sensitivity of hippocampal neurons to thyroid and steroid hormones in an effort to elucidate possible structural mechanisms to account for differences in hippocampal function. In this review, hormone-induced structural changes in the developing and adult hippocampal formation are discussed, with particular emphasis on their functional relevance. Sex differences, as well as the developmental effects of thyroid hormone and glucocorticoids, are described. Moreover, the effects of ovarian steroids, thyroid hormone and glucocorticoids on neuronal morphology in the hippocampal formation of the adult rat are reviewed. These hormone-induced structural changes may account, at least in part, for previously reported hormone-induced changes in hippocampal function.

A comparison of triiodothyronine and thyroxine in the potentiation of tricyclic antidepressants

Several studies suggest that small amounts of triiodothyronine (T3) will convert tricyclic antidepressant nonresponders into responders within a few days to a few weeks. As thyroxine (T4) is converted to T3 to have its physiological effect, it has been assumed that T4 would have a similar antidepressant effect to that of T3. We carried out a randomized, double-blind evaluation of the antidepressant-potentiating effect of T3 as compared with T4. Significantly more patients responded to 3 weeks of T3 as compared with T4 potentiation. Small but significant differential effects of the two thyroid hormones were noted on the Hamilton Rating Scale for Depression.

[125I] triiodothyronine in the rat brain: evidence for neural localization and axonal transport derived from thaw-mount film autoradiography

Previous thaw-mount light microscopic autoradiographic studies have shown that intravenously administered [125I] triiodothyronine is saturably concentrated and retained for at least 10 hours in discrete neural systems in the rat brain. To survey the brain more completely and to gain information about the time course of labeling, serial thaw-mount film autoradiograms were prepared from rat brains obtained at intervals through 48 hours after intravenous injection of high specific activity [125I] triiodothyronine. Parallel biochemical studies of whole brain homogenate extracts revealed that, at all time intervals, the label in the brain was mainly due to triiodothyronine itself (80%), or other organic iodocompounds (15%), but probably not due to free [125I] iodide (3%), which is rapidly transported out of the brain. The highly reproducible, well-defined labeling patterns seen on film indicated a widespread but selective localization of the hormone. At early times after intravenous injection of [125I] triiodothyronine, label was nonuniformly and prominently concentrated in selected regions of gray matter; evidence for saturability of hormone processing was obtained in competition studies with unlabeled triiodothyronine. Discrete labeling of fiber tracts (usually after 10 hours) left some regions of white matter conspicuously unlabeled. At 48 hours, many originally labeled gray regions showed markedly diminished or virtually complete loss of radioactivity, whereas others became newly or more prominently labeled. At that time, certain fiber tracts were also conspicuously labeled. The observed changing profiles of regional labeling over time are best explained by movement of the hormone from original sites of saturable incorporation in specific nuclei, to terminal fields, through the mechanism of axonal transport.

Development of neuronal markers in aggregating fetal rat telencephalon cells cultured in the presence of triiodothyronine

The concentrations of the general neuronal markers D2-protein (N-CAM), D3-protein and neuron specific enolase (NSE) in reaggregating cultures of fetal rat telencephalon cells were affected by the presence of 30 nM triiodothyronine in the defined culture medium. The extent of normal developmental changes were enhanced by triiodothyronine, as demonstrated by crossed immunoelectrophoresis. From 13 to 19 days in culture, the concentration of D2-protein decreased, and the concentrations of both D3-protein and NSE increased. Nerve growth factor (NGF) was without effect on the development of these general neuronal markers. However, as shown previously both triiodothyronine and NGF increased the activity of choline acetyltransferase, a marker for cholinergic neurons. The results suggest an enhanced overall differentiation of several types of telencephalon neurons in the presence of triiodothyronine, and a specific stimulation of cholinergic telencephalon neurons by NGF.

Intrathecal triiodothyronine administration causes greater heart rate stimulation in hypothyroid rats than intravenously delivered hormone. Evidence for a central nervous system site of thyroid hormone action

To determine whether intracerebrally localized iodothyronines produce thyroid hormone-related functional effects, heart rate responses were compared in conscious hypothyroid rats given triiodothyronine (T3) by either the intrathecal or the intravenous route. A significant increase in heart rate occurred within 18 h after 1.5 nmol T3/100 g body wt was delivered intrathecally through a cannula previously placed in the lateral cerebral ventricle. Injection of the same T3 dose intravenously through an indwelling jugular catheter or injection of vehicle only by either route produced no significant increase in heart rate during the 48-h postinjection period of observation. These differences were observed even though integrated serum T3 concentrations were significantly lower after intrathecal than after intravenous T3 injection. The results indicate that thyroid hormone effects on heart rate are exerted within the brain as well as within the heart.

Regional distribution of nuclear T3 receptors in rat brain and evidence for preferential localization in neurons

We examined the distribution of nuclear T3 in mature rat brain with the aim of determining specific targets of thyroid hormones within this tissue. Saturation experiments, performed in 9 different structures of the brain and in 4 parts of the cortex, revealed the presence of a single class of binding sites with a mean Ka of 0.53 X 10(10) M-1. The highest concentrations of receptors were found in the amygdala (0.523 +/- 0.025 ng T3/mg DNA, Mean +/- SE) and the hippocampus (0.438 +/- 0.071 ng T3/mg DNA) while the lowest were in the brain stem (0.058 +/- 0.003 ng T3/mg DNA) and the cerebellum (0.079 +/- 0.026 ng T3/ml DNA). The receptor was not uniformally distributed within the cerebral cortex, its concentration being relatively high in the central sections and intermediate in the remaining portions. The cell type distribution of the T3 receptor was studied by separating glial and neuronal nuclei on a discontinuous sucrose gradient. There was no detectable specific T3 binding in the fraction of oligodendrocyte nuclei (approximately 95% pure). Conversely, the neuron-enriched fraction (approximately 60%) showed a significant increase in receptor concentration compared to total nuclei (35-40% neurons): 0.857 +/- 0.196 vs 0.511 +/- 0.095 ng T3/mg DNA (p less than 0.01) in the cortex and 0.425 +/- 0.018 vs 0.234 +/- 0.24 ng T3/mg DNA (p less than 0.01) in the forebrain. The absence of nuclear T3 receptors in oligodendrocytes may have important implications on the mechanism of action of thyroid hormone in myelination.