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

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.

Axonal T3 uptake and transport can trigger thyroid hormone signaling in the brain

The development of the brain, as well as mood and cognitive functions, are affected by thyroid hormone (TH) signaling. Neurons are the critical cellular target for TH action, with T3 regulating the expression of important neuronal gene sets. However, the steps involved in T3 signaling remain poorly known given that neurons express high levels of type 3 deiodinase (D3), which inactivates both T4 and T3. To investigate this mechanism, we used a compartmentalized microfluid device and identified a novel neuronal pathway of T3 transport and action that involves axonal T3 uptake into clathrin-dependent, endosomal/non-degradative lysosomes (NDLs). NDLs-containing T3 are retrogradely transported via microtubules, delivering T3 to the cell nucleus, and doubling the expression of a T3-responsive reporter gene. The NDLs also contain the monocarboxylate transporter 8 (Mct8) and D3, which transport and inactivate T3, respectively. Notwithstanding, T3 gets away from degradation because D3's active center is in the cytosol. Moreover, we used a unique mouse system to show that T3 implanted in specific brain areas can trigger selective signaling in distant locations, as far as the contralateral hemisphere. These findings provide a pathway for L-T3 to reach neurons and resolve the paradox of T3 signaling in the brain amid high D3 activity.

Neuropsychiatric Manifestations of Thyroid Diseases

Thyroid disorders are known to cause neuropsychiatric manifestations. Various neuropsychiatric manifestations are depression, dementia, mania, and autoimmune Hashimoto encephalopathy. Numerous investigations carried out in the previous 50-60 years have been evaluated critically. The pathophysiology of neuropsychiatric symptoms of thyroid diseases is described in the current study and its link with autoimmune Hashimoto encephalopathy is also discussed. Furthermore, this paper also describes the association between thyroid-stimulating hormones and cognitive impairment. Hypothyroidism is associated with depression and mania, and hyperthyroidism is linked with dementia and mania. The association between Graves' disease and various mental disorders such as depressive and anxiety disorders is also discussed. The aim of this study is to review the relationship between various neuropsychiatric disorders and thyroid diseases. A literature search from the PubMed database to find various neuropsychiatric manifestations of thyroid disorders in the adult population was conducted. According to the review of the studies, cognitive impairment can result from thyroid disease. It has not been possible to demonstrate how hyperthyroidism can hasten the process of developing dementia. However, subclinical hyperthyroidism, thyroid-stimulating hormone (TSH) levels below the normal range, and high free thyroxine (T4) levels all raise the risk of dementia in the elderly. Additionally, the potential mechanisms underlying this association have been examined. A quick summary of the research on mania as a clinical symptom of hypothyroidism and its likely causes and pathogenesis is also reviewed. There is no dearth of evidence that describes various neuropsychiatric manifestation in thyroid disorders.

Hyperthyroidism and clinical depression: a systematic review and meta-analysis

Hyperthyroidism and clinical depression are common, and there is preliminary evidence of substantial comorbidity. The extent of the association in the general population, however, has not yet been estimated meta-analytically. Therefore we conducted this systematic review and meta-analysis (registered in PROSPERO: CRD42020164791). Until May 2020, Medline (via PubMed), PsycINFO, and Embase databases were systematically searched for studies on the association of hyperthyroidism and clinical depression, without language or date restrictions. Two reviewers independently selected epidemiological studies providing laboratory or ICD-based diagnoses of hyperthyroidism and diagnoses of depression according to operationalized criteria (e.g. DSM) or to cut-offs in established rating scales. All data, including study quality based on the Newcastle-Ottawa Scale, were independently extracted by two authors. Odds ratios for the association of clinical depression and hyperthyroidism were calculated in a DerSimonian-Laird random-effects meta-analysis. Out of 3372 papers screened we selected 15 studies on 239 608 subjects, with 61% women and a mean age of 50. Relative to euthyroid individuals, patients with hyperthyroidism had a higher chance of being diagnosed with clinical depression: OR 1.67 ([95% CI: 1.49; 1.87], I²: 6%; prediction interval: 1.40 to 1.99), a result supported in a number of sensitivity and subgroup analyses. The OR was slightly less pronounced for subclinical as opposed to overt hyperthyroidism (1.36 [1.06; 1.74] vs. 1.70 [1.49; 1.93]). This comorbidity calls for clinical awareness and its reasons need investigation and may include neurobiological mechanisms, common genetic vulnerability and a generally heightened risk for clinical depression in patients with chronic somatic disorders.

Could the Thyroid Gland Dominate the Brain in Obsessive-Compulsive Disorder?

Thyroid hormones have an essential role in brain maturation and neuronal functioning. The comorbidity of thyroid disorders and several mental disturbances is frequently reported. We aimed to evaluate the literature on the potential relationship between thyroid disorders and obsessive-compulsive disorder (OCD) and obsessive-compulsive symptoms (OCS). We searched the literature using PUBMED, ProQuest, Google Scholar, and PsycInfo electronic databases for original studies (cross-sectional, case series, case report) on the association between thyroid dysfunctions and OCD and OCS between 1977 and 2021. Eleven studies met the inclusion criteria. Despite some methodological limitations, the OCD rates in patients with autoimmune thyroid disorders were found to be higher than the normal population in two studies. The findings on thyroid dysfunction in OCD patients were inconclusive. In the light of available data, it could be proposed that there might be a possible association between thyroid disorders and OCD. Some shared immunological mechanisms could play a role in the pathophysiology of both thyroid diseases and OCD. New research is needed to confirm this association and elucidate the underlying common mechanisms between these disorders.

Subclinical hypothyroidism and depression: a meta-analysis

The objective of this study was to evaluate the relationship between subclinical hypothyroidism (SCH) and depression. We also analysed the effect of levothyroxine (L-T4) on depression in SCH patients. We found an insignificant difference for the composite endpoint: standard mean difference (SMD) of 0.23 (95% confidence interval (CI) -0.03, 0.48, P = 0.08, I² = 73.6%). The odds ratio (OR) for depressive patients was 1.75 (95% CI 0.97, 3.17 P = 0.064, I² = 64.6%). Furthermore, sub-group analysis according to age found that SCH was related to depression in younger patients (<60 years old), as defined by the diagnosis of depression: OR of 3.8 (95% CI 1.02, 14.18, P = 0.047, I² = 0.0%) or an increase on the depressive scale: SMD of 0.42 (95% CI 0.03, 0.82, P = 0.036, I² = 66.6%). Meanwhile, SCH did not associate with depression in older patients (≥60 years old), as defined by the diagnosis of depression: OR of 1.53 (95% CI 0.81, 2.90, P = 0.193, I² = 71.3%) or an increase on the depressive scale: SMD of 0.03 (95%CI -0.31, 0.37, P = 0.857, I² = 79.8%). We also found an insignificant difference in the composite endpoint between the L-T4 supplementation group and placebo group in SCH patients. The estimated SMD was 0.26 (95% CI -0.09, 0.62, P = 0.143, I² = 52.9%). This meta-analysis demonstrates that SCH is not connected to depression. However, sub-group analysis according to age found that SCH is related to depression in younger patients, but not in older patients. Furthermore, we failed to find an effect of L-T4 supplementation treatment for SCH on depression.

Understanding the pathophysiology of depression: From monoamines to the neurogenesis hypothesis model - are we there yet?

A number of factors (biogenic amine deficiency, genetic, environmental, immunologic, endocrine factors and neurogenesis) have been identified as mechanisms which provide unitary explanations for the pathophysiology of depression. Rather than a unitary construct, the combination and linkage of these factors have been implicated in the pathogenesis of depression. That is, environmental stressors and heritable genetic factors acting through immunologic and endocrine responses initiate structural and functional changes in many brain regions, resulting in dysfunctional neurogenesis and neurotransmission which then manifest as a constellation of symptoms which present as depression.

Thyroid Status, Quality of Life, and Mental Health in Patients on Hemodialysis

BACKGROUND AND OBJECTIVES

In the general population, there is increasing recognition of the effect of thyroid function on patient-centered outcomes, including health-related quality of life and depression. Although hypothyroidism is highly prevalent in hemodialysis patients, it is unknown whether thyroid status is a risk factor for impaired health-related quality of life or mental health in this population.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We examined the association of thyroid status, defined by serum thyrotropin, with health-related quality of life and depressive symptoms over time in a prospective cohort of 450 patients on hemodialysis from 17 outpatient dialysis facilities from May of 2013 to May of 2015 who underwent protocolized thyrotropin testing, Short-Form 36 surveys, and Beck Depression Inventory-II questionnaires every 6 months. We examined the association of baseline and time-dependent thyrotropin categorized as tertiles and continuous variables with eight Short-Form 36 domains and Beck Depression Inventory-II scores using expanded case mix plus laboratory adjusted linear mixed effects models.

RESULTS

In categorical analyses, the highest baseline thyrotropin tertile was associated with a five-point lower Short-Form 36 domain score for energy/fatigue (P=0.04); the highest time-dependent tertile was associated with a five-point lower physical function score (P=0.03; reference: lowest tertile). In continuous analyses, higher baseline serum thyrotropin levels (+Δ1 mIU/L) were associated with lower role limitations due to physical health (β=-1.3; P=0.04), energy/fatigue (β=-0.8; P=0.03), and pain scores (β=-1.4; P=0.002), equivalent to five-, three-, and five-point lower scores, respectively, for every 1-SD higher thyrotropin. Higher time-dependent thyrotropin levels were associated with lower role limitations due to physical health scores (β=-1.0; P=0.03), equivalent to a three-point decline for every 1-SD higher thyrotropin. Baseline and time-dependent thyrotropin were not associated with Beck Depression Inventory-II scores.

CONCLUSIONS

In patients on hemodialysis, higher serum thyrotropin levels are associated with impaired health-related quality of life across energy/fatigue, physical function, and pain domains. Studies are needed to determine if thyroid-modulating therapy improves the health-related quality of life of hemodialysis patients with thyroid dysfunction.

Psychiatric and physical comorbidities and their impact on the course of bipolar disorder: A prospective, naturalistic 4-year follow-up study

OBJECTIVES

The aim of the present study was to increase the available evidence on how physical and psychiatric comorbidities influence the long-term outcome in bipolar I and II disorder.

METHODS

We examined the prevalence of comorbid physical (metabolic, cardiovascular, thyroid, and neurological) diseases and psychiatric (neurotic, stress-related, somatoform, and personality) disorders and their impact on the risk of relapse in bipolar disorder. A total of 284 consecutively admitted patients with ICD-10 bipolar I (n=161) and II (n=123) disorder were followed up naturalistically over a period of 4 years.

RESULTS

Globally, 22.0% patients had metabolic, 18.8% cardiovascular, 18.8% thyroid, and 7.6% neurological diseases; 15.5% had neurotic, stress-related, and somatoform disorders; 12.0% had personality disorders; and 52.9% had nicotine dependence. We did not find any effect of comorbid metabolic, cardiovascular or neurological diseases or psychiatric disorders on the relapse risk. However, the presence of thyroid diseases, and especially hypothyroidism, was associated with an increased risk of manic relapse in bipolar disorder I (thyroid disease: hazard ratio [HR]=2.7; P=.003; hypothyroidism: HR=3.7;, P<.001). Among patients with hypothyroidism, higher blood levels of baseline thyroid-stimulating hormone (bTSH) were also associated with an increased risk of manic relapse (HR=1.07 per milli-international units per liter; P=.011), whereas blood levels of free triiodothyronine (fT₃ ) or free thyroxine (fT₄ ) were not found to have an influence.

CONCLUSIONS

Our data underline the negative long-term impact of thyroid diseases, and especially hypothyroidism with high blood levels of bTSH, on bipolar disorder with more manic episodes, and the importance of its detection and treatment.

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.

Neural correlates of free T3 alteration after catecholamine depletion in subjects with remitted major depressive disorder and in controls

RATIONALE

Thyroid hormones and their interactions with catecholamines play a potentially important role in alterations of mood and cognition.

OBJECTIVES

This study aimed to examine the neurobiological effects of catecholamine depletion on thyroid hormones by measuring endocrine and cerebral metabolic function in unmedicated subjects with remitted major depressive disorder (RMDD) and in healthy controls.

METHODS

This was a randomized, placebo-controlled, and double-blind crossover trial that included 15 unmedicated RMDD subjects and 13 healthy control subjects. The participants underwent two 3-day-long sessions at 1-week intervals; each participant was randomly administered oral α-methyl-para-tyrosine in one session (catecholamine depletion) and an identical capsule containing hydrous lactose (sham depletion) in the other session prior to a [(18)F]-fluorodeoxyglucose positron emission tomography scan.

RESULTS

Serum concentrations of free T3 (FT3), free T4 (FT4), and TSH were obtained and assessed with respect to their relationship to regional cerebral glucose metabolism. Both serum FT3 (P = 0.002) and FT4 (P = 0.0009) levels were less suppressed after catecholamine depletion compared with placebo treatment in the entire study sample. There was a positive association between both FT3 (P = 0.0005) and FT4 (P = 0.002) and depressive symptoms measured using the Montgomery-Åsberg Depression Rating Scale. The relative elevation in FT3 level was correlated with a decrease in regional glucose metabolism in the right dorsolateral prefrontal cortex (rDLPFC; P < 0.05, corrected).

CONCLUSIONS

This study provided evidence of an association between a thyroid-catecholamine interaction and mood regulation in the rDLPFC.

Hypothyroidism and depression

BACKGROUND

A relationship between hypothyroidism and depression has been assumed for many years; however, the true nature of this association has been difficult to define with many conflicting studies. In recent years, our knowledge in this area has increased significantly with large cohort studies and genetically driven studies being published.

OBJECTIVES

We reviewed the literature on thyroid function and depression to determine if this relationship has been clarified.

METHODS

We performed a search on the Pubmed database using the terms 'thyroid
 ' and 'mental health
 ', 'depression
 ' and 'well-being
 '.

RESULTS

Large epidemiological studies generally suggest no association between thyroid function and depression in subjects without thyroid disease. Subjects on thyroxine have poorer psychological well-being than subjects with no thyroid disease even if biochemically euthyroid, they also show an association between thyroid function and well-being. Whilst there is some early evidence that genetic factors can influence well-being on thyroxine and response to combination therapy, there is also evidence to suggest that much morbidity on thyroxine may be due to initial misdiagnosis and mis-attribution of symptoms.

CONCLUSION

Despite the large number of studies, the relationship between thyroid function and depression remains poorly defined. Clarification of the proportion of subjects on thyroxine incorrectly may assist the large (perhaps genetically driven) studies needed to move forward in this area, as it is expected that they cloud the results.

Effects of DSP4 on the noradrenergic phenotypes and its potential molecular mechanisms in SH-SY5Y cells

Dopamine β-hydroxylase (DBH) and norepinephrine (NE) transporter (NET) are the noradrenergic phenotypes for their functional importance to noradrenergic neurons. It is known that in vivo N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP4) treatment induces degeneration of noradrenergic terminals by interacting with NET and depleting intracellular NE. However, DSP4's precise mechanism of action remains unclear. In this study various biochemical approaches were employed to test the hypothesis that DSP4 down-regulates the expression of DBH and NET, and to determine molecular mechanisms that may be involved. The results showed that treatment of SH-SY5Y neuroblastoma cells with DSP4 significantly decreased mRNA and protein levels of DBH and NET. DSP4-induced reduction of DBH mRNA and proteins, as well as NET proteins showed a time- and concentration-dependent manner. Flow cytometric analysis demonstrated that DSP4-treated cells were arrested predominantly in the S-phase, which was reversible. The arrest was confirmed by several DNA damage response markers (phosphorylation of H2AX and p53), suggesting that DSP4 causes replication stress which triggers cell cycle arrest via the S-phase checkpoints. Moreover, the comet assay verified that DSP4 induced single-strand DNA breaks. In summary, the present study demonstrated that DSP4 down-regulates the noradrenergic phenotypes, which may be mediated by its actions on DNA replication, leading to replication stress and cell cycle arrest. These action mechanisms of DSP4 may account for its degenerative consequence after systematic administration for animal models.

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.

Pathophysiology of depression and mechanisms of treatment

Major depression is a serious disorder of enormous sociological and clinical relevance. The discovery of antidepressant drugs in the 1950s led to the first biochemical hypothesis of depression, which suggested that an impairment in central monoaminergic function was the major lesion underlying the disorder. Basic research in all fields of neuroscience (including genetics) and the discovery of new antidepressant drugs have revolutionized our understanding of the mechanisms underlying depression and drug action. There is no doubt that the monoaminergic system is one of the cornerstones of these mechanisms, but multiple interactions with other brain systems and the regulation of central nervous system function must also be taken into account In spite of all the progress achieved so far, we must be aware that many open questions remain to be resolved in the future.

Selective wheat germ agglutinin (WGA) uptake in the hippocampus from the locus coeruleus of dopamine-β-hydroxylase-WGA transgenic mice

We generated transgenic mice in which a trans-synaptic tracer, wheat germ agglutinin (WGA), was specifically expressed in the locus coeruleus (LC) neurons under the control of the dopamine-β-hydroxylase (DBH) gene promoter. WGA protein was produced in more than 95% of the tyrosine hydroxylase (TH)-positive LC neurons sampled. Transynaptic transfer of WGA was most evident in CA3 neurons of the hippocampus, but appeared absent in CA1 neurons. Faint but significant WGA immunoreactivity was observed surrounding the nuclei of dentate granule cells. Putative hilar mossy cells, identified by the presence of calretinin in the ventral hippocampus, appeared uniformly positive for transynaptically transferred WGA protein. GAD67-positive interneurons in the hilar and CA3 regions tended to be WGA-positive, although a subset of them did not show WGA co-localization. The same mixed WGA uptake profile was apparent when examining co-localization with parvalbumin. The selective uptake of WGA by dentate granule cells, mossy cells, and CA3 pyramidal neurons is consistent with evidence for a large proportion of conventional synapses adjacent to LC axonal varicosities in these regions. The lack of WGA uptake in the CA1 region and its relatively sparse innervation by DBH-positive fibers suggest that a majority of the TH-positive classical synapses revealed by electron microscopy in that region may be producing dopamine. The overall pattern of WGA uptake in these transgenic mice implies a selective role for the granule cell-mossy cell-CA3 network in processing novelty or the salient environmental contingency changes signaled by LC activity.

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.

Neuropsychological functions and metabolic aspects in subclinical hypothyroidism: the effects of L-thyroxine

Thyroid hypofunction is a slowly progressing graded phenomenon [Vanderpump MP, Tunbridge WM, French JM, Appleton D, Bates D, Clark F, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol (Oxf) 1995;43(1):55-68]; subclinical forms (SCH) often represent a laboratory diagnosis in apparently asymptomatic patients. In the absence of adequate parameters for thyroid hormone action in tissues, the level of TSH increase corresponding to negative effects remains unsettled. We studied a wide range of physiological processes in a strictly selected population of 38 female patients (56.4+/-12.6 years) with minor forms of SCH (TSH 6.6+/-1.8 mIU/L), after exclusion of neurological, psychiatric and somatic disorders or confounding conditions. The investigations, performed at admission and after 6 months of l-thyroxine (LT4) treatment, included metabolic evaluation, health status perception and an extensive battery of neuropsychological tests and psychological rating scales. Lipid metabolism improved after LT4 (total cholesterol: 231.9+/-49.6 mg/dl pre- vs 221.0+/-40.0 mg/dl post-treatment; LDL cholesterol: 183.1+/-62.9 vs 162.7+/-53.7 mg/dl; apolipoprotein A1: 183.5+/-64.5 vs 160.9+/-50.3 mg/dl; p<0.05 for all comparisons), while glucose metabolism was unchanged. Health status perception was favourably influenced by the treatment (total SF-36 score 97.8+/-18.4 pre- vs 108.5+/-14.8 post-, p<0.0001); in a matched control group with euthyroid goiter, tested to examine the effects of medical care in the absence of treatment, no significant differences were found in the SF-36 scores at admission and after 6 months (109.3+/-15.1 vs 109+/-14.2, p=0.9). Attention performance improved after LT4; HRSD and HRSA scores did not significantly change, but negative correlations were found between FT3 levels and affective scores at admission, and between the post-treatment changes of affective scores and of FT3. In our study subtle disturbances of health status perception, attention and lipid metabolism associated to SCH of mildest degrees were reverted by LT4 replacement, reinforcing reports of unfavourable consequences of marginal thyroid disease.

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.

Effects of various antidepressants on serum thyroid hormone levels in patients with major depressive disorder

A total of 62 patients with major depressive disorder were analyzed in the study. Patients were evaluated for 11 weeks in an open label design to investigate the differential effects of reboxetine, sertraline and venlafaxine on thyroid hormones. Serum thyrotrophin (TSH), thyroxine (T4) and free (f)T4 levels were measured before and after treatment. All groups showed significant improvement in HAM-D scores. TSH level significantly reduced and T4 level significantly increased in the reboxetine group, however TSH level significantly increased and T4 level significantly reduced in the sertraline group. Percent changes of TSH (p=0.007) and T4 (p=0.001) were significantly different between the reboxetine and sertraline groups. In the sertraline group, baseline TSH levels were correlated with response to treatment as determined by the change in HAM-D scores (p=0.03, r=0.648). There was a significant association between the percent changes in TSH values and the reduction in HAM-D scores in the reboxetine group (p=0.03, r=-0.434). In the whole study group, female patients had lower values of basal T4 compared with men (p=0.043), however percent changes of T4 did not differ between genders. In the treatment-responders significant increase in the reboxetine group and significant decrease in the sertraline group regarding the T4 values were found. We observed that various antidepressants had different effects on thyroid hormone levels and this could be attributed to the different mechanisms of actions of these antidepressants.

Neural response to transcranial magnetic stimulation in adult hypothyroidism and effect of replacement treatment

PURPOSE

Despite clinical evidences that hypothyroidism is often associated with cognitive dysfunction, affective disorders and psychosis, the effects of thyroid hormone deficiency on the adult brain have been largely unexplored. We investigated the hypothesis that hypothyroidism might affect cortical excitability and modulates inhibitory and excitatory cortical circuits by using Transcranial Magnetic Stimulation.

MATERIALS AND METHODS

Cortical excitability was probed in 10 patients with overt hypothyroidism and 10 age-matched healthy controls. We tested motor thresholds and corticospinal excitability, cortical silent period and peripheral silent period, short interval intracortical inhibition, intracortical facilitation. Patients were evaluated at the time of diagnosis, as well as after 3 and 6 months replacement therapy with l-thyroxin.

RESULTS

At baseline, patients showed decreased cortical excitability, with increased resting and active motor threshold and decreased steepness of the motor evoked potential recruitment curves. These changes were paralleled by longer cortical silent period and decreased short interval intracortical inhibition. After 3 months replacement therapy, all the parameters but short interval intracortical inhibition were restored to normal values. Short interval intracortical inhibition returned to normal values only after 6 months of replacement therapy.

CONCLUSIONS

Thyroid hormones are needed to modulate cortical excitability and cortical inhibitory circuits in adults.

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.

Functional neuroanatomy of thyroid hormone feedback in the human hypothalamus and pituitary gland

A major change in thyroid setpoint regulation occurs in various clinical conditions such as critical illness and psychiatric disorders. As a first step towards identifying determinants of these setpoint changes, we have studied the distribution and expression of thyroid hormone receptor (TR) isoforms, type 2 and type 3 deiodinase (D2 and D3), and the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the human hypothalamus and anterior pituitary. Although the post-mortem specimens used for these studies originated from patients who had died from many different pathologies, the anatomical distribution of these proteins was similar in all patients. D2 enzyme activity was detectable in the infundibular nucleus/median eminence (IFN/ME) region coinciding with local D2 immunoreactivity in glial cells. Additional D2 immunostaining was present in tanycytes lining the third ventricle. Thyrotropin-releasing hormone (TRH) containing neurons in the paraventricular nucleus (PVN) expressed MCT8, TRs as well as D3. These findings suggest that the prohormone thyroxine (T4) is taken up in hypothalamic glial cells that convert T4 into the biologically active triiodothyronine (T3) via the enzyme D2, and that T3 is subsequently transported to TRH producing neurons in the PVN. In these neurons, T3 may either bind to TRs or be metabolized into inactive iodothyronines by D3. By inference, local changes in thyroid hormone metabolism resulting from altered hypothalamic deiodinase or MCT8 expression may underlie the decrease in TRH mRNA reported earlier in the PVN of patients with critical illness and depression. In the anterior pituitary, D2 and MCT8 immunoreactivity occurred exclusively in folliculostellate (FS) cells. Both TR and D3 immunoreactivity was observed in gonadotropes and to a lesser extent in thyrotropes and other hormone producing cell types. Based upon these neuroanatomical findings, we propose a novel model for central thyroid hormone feedback in humans, with a pivotal role for hypothalamic glial cells and pituitary FS cells in processing and activation of T4. Production and action of T3 appear to occur in separate cell types of the human hypothalamus and anterior pituitary.

Rapid modulation of TRH-like peptides in rat brain by thyroid hormones

Recent identification of membrane receptors for T4, T3, 3,5-T2, and 3-iodothyronamine that mediate rapid physiologic effects of thyroid hormones suggested that such receptors may supplement the regulation of TRH and TRH-like peptides by nuclear T3 receptors. For this reason 200 g male Sprague-Dawley rats received daily i.p. injections of PTU or T4. Levels of TRH and TRH-like peptides were measured 0, 2 h or 1, 2, 3, or 4 days later. Rapid increases or decreases in TRH and TRH-like peptide levels were observed in response to PTU and T4 treatments in various brain regions involved in mood regulation. Significant effects were measured within 2 h of T4 injection. Nuclear T3 receptor-mediated changes in gene expression altering translation, post-translational processing and constitutive release of peptides require more than 2 h. We conclude that non-genomic mechanisms may contribute to the psychiatric effects of thyroid disease and thyroid hormone adjuvant treatment for major depression.

Multi-target strategies for the improved treatment of depressive states: Conceptual foundations and neuronal substrates, drug discovery and therapeutic application

Major depression is a debilitating and recurrent disorder with a substantial lifetime risk and a high social cost. Depressed patients generally display co-morbid symptoms, and depression frequently accompanies other serious disorders. Currently available drugs display limited efficacy and a pronounced delay to onset of action, and all provoke distressing side effects. Cloning of the human genome has fuelled expectations that symptomatic treatment may soon become more rapid and effective, and that depressive states may ultimately be "prevented" or "cured". In pursuing these objectives, in particular for genome-derived, non-monoaminergic targets, "specificity" of drug actions is often emphasized. That is, priority is afforded to agents that interact exclusively with a single site hypothesized as critically involved in the pathogenesis and/or control of depression. Certain highly selective drugs may prove effective, and they remain indispensable in the experimental (and clinical) evaluation of the significance of novel mechanisms. However, by analogy to other multifactorial disorders, "multi-target" agents may be better adapted to the improved treatment of depressive states. Support for this contention is garnered from a broad palette of observations, ranging from mechanisms of action of adjunctive drug combinations and electroconvulsive therapy to "network theory" analysis of the etiology and management of depressive states. The review also outlines opportunities to be exploited, and challenges to be addressed, in the discovery and characterization of drugs recognizing multiple targets. Finally, a diversity of multi-target strategies is proposed for the more efficacious and rapid control of core and co-morbid symptoms of depression, together with improved tolerance relative to currently available agents.

Hypothalamic thyroid hormone feedback in health and disease

The role of the human hypothalamus in the neuroendocrine response to illness has only recently begun to be explored. Extensive changes in the hypothalamus-pituitary-thyroid (HPT) axis occur within the framework of critical illness. The best-documented change in the HPT axis is a decrease in serum concentrations of the biologically active thyroid hormone triiodothyronine (T3). From studies in post-mortem human hypothalamus it appeared that low serum T3 and thyrotropin (TSH) during illness (nonthyroidal illness, NTI) are paralleled by decreased thyrotropin-releasing hormone (TRH)mRNA expression in the hypothalamic paraventricular nucleus (PVN), pointing to a major alteration in HPT axis setpoint regulation. A strong decrease in TRHmRNA expression is also present in the PVN of patients with major depression as well as in glucocorticoid-treated patients. By inference, hypercortisolism in hospitalized patients with severe depression or in critical illness may induce down-regulation of the HPT axis at the level of the hypothalamus. In order to start defining the determinants and mechanisms of these setpoint changes in various clinical conditions, it is important to note that an increasing number of hypothalamic proteins appears to be involved in central thyroid hormone metabolism. In recent studies, we have investigated the distribution and expression of thyroid hormone receptor (TR) isoforms, type 2 and type 3 deiodinase (D2 and D3), and the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) in the human hypothalamus by a combination of immunocytochemistry, mRNA in situ hybridization and enzyme activity assays. Both D2 and D3 enzyme activities are detectable in the mediobasal hypothalamus. D2 immunoreactivity is prominent in glial cells of the infundibular nucleus/median eminence region and in tanycytes lining the third ventricle. Combined D2, D3, MCT8 or TR immunocytochemistry and TRHmRNA in situ hybridization indicates that D3, MCT8 and TRs are all expressed by TRH neurons in the PVN, whereas D2 is not. Taken together, these results suggest that the prohormone thyroxine (T4) is taken up in glial cells that convert T4 into the biologically active T3 via the enzyme D2; T3 is subsequently transported to TRH producing neurons in the PVN where it may bind to TRs and/or may be degraded into inactive iodothyronines by D3. This model for thyroid hormone action in the human hypothalamus awaits confirmation in future experimental studies.

Cyclic AMP-protein kinase a and protein kinase C mediate in vitro T activation of brain tyrosine hydroxylase in the female catfish Heteropneustes fossilis

The present in vitro study demonstrates an involvement of both cAMP-dependent protein kinase A (PKA) and protein kinase C (PKC) signal transduction mechanisms in the triiodothyronone (T(3))-activation of forebrain (telencephalon and hypothalamus) tyrosine hydroxylase (TH) activity in the female catfish Heteropneustes fossilis. Incubations of the enzyme preparations with different concentrations of T(3) (0.15-2.4 ng/ml) stimulated TH activity over the concentrations. Similarly, coincubations of the enzyme preparations with T(3) and cAMP (1.0 mM) or cAMP-elevating drugs such as 1-methyl-3-isobutylxanthine (1.5 mM) or theophylline (1.5 mM) increased TH activity significantly over that of T(3). The stimulatory effect of TH activity with T(3) or cAMP was coincident with a low apparent K(m) and high V(max) for the cofactor, suggesting a higher affinity of the enzyme. Incubation of the enzyme preparations with PKA (H-89) and PKC (calphostin-C) inhibitors decreased basal enzyme activity significantly, with the inhibition being greater in the former group. The incubations of the enzyme preparations with T(3) or T(3) + cAMP, followed by the different inhibitors, also decreased enzyme activity. Although T(3) could not reverse the inhibitory effect of H-89, it could over-ride the effect of calphostin-C to some extent. The suppressive effect of the inhibitors could be related to a high apparent K(m) and low V(max) for the cofactor. The evidence strongly suggests a nongenomic action of T(3) on TH activity via the cell signalling pathways, for which the cAMP-dependent PKA appears to be the major regulatory mechanism.

A relação entre a função tireoidiana e a depressão: uma revisão

OBJETIVO: O papel da função tireoidiana nas doenças depressivas é pouco claro. Embora existam algumas evidências de que discretas alterações tireoidianas predisponham a casos de depressão, as anormalidades específicas envolvendo a tireóide e os quadros depressivos permanecem pouco conhecidas. Serão destacados nesta revisão os principais achados envolvendo os quadros depressivos e a função tireoidiana, com especial atenção na participação das monoaminas cerebrais nesta relação. MÉTODO: Foram realizados levantamento no sistema Medline e na literatura. RESULTADOS: Existem evidências de atividade alterada do eixo hipotálamo-hipófise-tireóide (HHT) em alguns casos de depressão, que incluem: aumento dos níveis de T4, resposta alterada do TSH pós-estímulo com TRH, presença de anticorpos antitireoidianos e concentração elevada de TRH no LCR. A relação entre estas anormalidades, as principais monoaminas cerebrais e os subtipos de quadros depressivos é complexa e ainda não permite o estabelecimento de hipóteses diretas de compreensão. CONCLUSÕES: Após anos de pesquisas, permanece pouco esclarecida a importância da relação entre o eixo HHT e as depressões, assim como os mecanismos subjacentes às alterações tireoidianas encontradas nos pacientes deprimidos. Portanto, mais pesquisas serão necessárias para uma melhor compreensão do papel do eixo HHT na patogênese e no tratamento dos quadros depressivos.

Thyroid hormone actions on neural cells

1. In addition to its role in cellular metabolic activity, thyroid hormone (TH) is critically involved in growth, development, and function of the central nervous system. In the brain, as in other structures, TH is described to exert its major action by the binding of L-3,5,3'-triiodothyronine (T3), considered as the bioactive form of the hormone, to nuclear thyroid hormone receptors (TR) that function as ligand-dependent transcription factors. 2. The transcription of numerous brain genes was indeed shown to be positively or negatively regulated by TH, turning these TR-mediated effects one explanation for the physiological effects of TH. In this context, the knowledge from TR-knockout studies provides some surprising results, since neonatal hypothyroidism is associated to more significant abnormalities than is TR deficiency. Some (nonexclusive) hypotheses include a permissive effect of TH, allowing derepression of unliganded-TR effects and non-TR-mediated effects of the hormone, further emphasizing the importance of a controlled accessibility of neural cells to TH. 3. On the other hand, T3 was demonstrated to directly act not only on neuronal but also on glial cells proliferation and differentiation, contributing to the harmonious development of the brain. Interestingly, in addition to these direct actions on neuronal and glial cells, several lines of evidence, notably developped in our laboratory, point out the role of thyroid hormone in neuronal-glial interactions.

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.

Dysregulation of the hypothalamic-pituitary-thyroid axis in alcoholism

Thyroid dysfunction is a prominent finding in alcoholism. Subclinical and clinical hypothyroidism have been associated with clinical depression and cognitive impairment and may increase the relapse risk among alcoholics. In spite of these important clinical associations, there is no consensus on thyroid dysfunction in alcoholism in the literature. In this paper, we present a review of the literature and develop a hypothesis that may explain dysfunction of the hypothalamic-pituitary-thyroid axis in alcoholism. Based on a Medline research of the years 1980-2001 we found 33 empirical studies that assessed thyroid function in alcoholism. The most consistent findings were a reduction in total thyroxine and total and free triiodothyronine concentrations during early abstinence. About one-third of all alcoholics also displayed a blunted thyroid stimulation hormone (TSH) response in the thyrotrophin-releasing hormone test (TRH-test). Blunting was observed frequently during detoxification, but was also present in some alcoholics after several weeks of abstinence. We suggest that a reduction in peripheral thyroid hormones may be caused by a direct toxic effect of alcohol on the thyroid gland, which induces a central compensatory activation of the hypothalamic-pituitary axis with an increased TRH release. The TRH release induces a downregulation of pituitary TRH receptors, which manifest as a blunted TSH response to the TRH test. We discuss further additional effects of alcohol on thyroid-hormone metabolizing deiodinases and on monoaminergic systems, which may interact directly with mood states among abstinent alcoholics.

Thyroid hormones, serotonin and mood: of synergy and significance in the adult brain

The use of thyroid hormones as an effective adjunct treatment for affective disorders has been studied over the past three decades and has been confirmed repeatedly. Interaction of the thyroid and monoamine neurotransmitter systems has been suggested as a potential underlying mechanism of action. While catecholamine and thyroid interrelationships have been reviewed in detail, the serotonin system has been relatively neglected. Thus, the goal of this article is to review the literature on the relationships between thyroid hormones and the brain serotonin (5-HT) system, limited to studies in adult humans and adult animals. In humans, neuroendocrine challenge studies in hypothyroid patients have shown a reduced 5-HT responsiveness that is reversible with thyroid replacement therapy. In adult animals with experimentally-induced hypothyroid states, increased 5-HT turnover in the brainstem is consistently reported while decreased cortical 5-HT concentrations and 5-HT2A receptor density are less frequently observed. In the majority of studies, the effects of thyroid hormone administration in animals with experimentally-induced hypothyroid states include an increase in cortical 5-HT concentrations and a desensitization of autoinhibitory 5-HT1A receptors in the raphe area, resulting in disinhibition of cortical and hippocampal 5-HT release. Furthermore, there is some indication that thyroid hormones may increase cortical 5-HT2 receptor sensitivity. In conclusion, there is robust evidence, particularly from animal studies, that the thyroid economy has a modulating impact on the brain serotonin system. Thus it is postulated that one mechanism, among others, through which exogenous thyroid hormones may exert their modulatory effects in affective illness is via an increase in serotonergic neurotransmission, specifically by reducing the sensitivity of 5-HT1A autoreceptors in the raphe area, and by increasing 5-HT2 receptor sensitivity.

Thyroid hormone, neural tissue and mood modulation

The successful treatment of affective disorders with thyroid hormone exemplifies the suggested inter-relationship between endocrine and neuronal systems in these disorders. Thyroid hormones have a profound influence on behaviour and appear to be capable of modulating the phenotypic expression of major affective illness. Specifically, there is good evidence that triiodothyronine (T3) may accelerate the antidepressant response to tricylic antidepressants, and some studies suggest that T3 may augment the therapeutic response to antidepressants in refractory depressed patients. Open studies have also indicated that adjunctive supraphysiological doses of thyroxine (T4) can ameliorate depressive symptomatology and help stabilize the long-term course of illness in bipolar and unipolar patients, especially women refractory to standard medications. Despite acceptance of the essential role of thyroid hormone on brain maturation and differentiation, and the clinical and therapeutic observations in association with mood disorders, the molecular action that may underlie the mood-modulating properties of thyroid hormone in the adult brain has only recently become the focus of research. The identification of nuclear T3 receptors, the region-specific expression of deiodinase isoenzymes and the molecular analyses of thyroid-responsive genes in the adult brain have provided the biological bases for a better understanding of thyroid hormone action in mature neurons. Also the influence of thyroid hormones on the putative neurotransmitter systems that regulate mood and behaviour, serotonin and norepinephrine, may be helpful in explaining their mood-modulating effects.