Thyroid hormones modulate GABA(A) receptor-mediated currents in hippocampal neurons

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

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

Thyroid Axis and Vestibular Physiopathology: From Animal Model to Pathology

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

Suppressive Modulation of the Chick Forebrain Network for Imprinting by Thyroid Hormone: An in Vitro Study

The thyroid hormone 3,5,3'-triiodothyronine (T3) is considered to act acutely in the chick forebrain because focal infusion of T3 to the intermediate medial mesopallium (IMM) causes 4 to 6-day-old hatchlings to become imprintable approximately 30 min after the infusion. To understand the mechanism of this acute T3 action, we examined synaptic responses of IMM neurons in slice preparations in vitro. Extracellular field potential responses to local electrical stimulation were pharmacologically dissociated to synaptic components mediated by AMPA and NMDA receptors, as well as GABA-A and -B receptors. Bath-applied T3 (20-40 μM) enhanced the positive peak amplitude of the field potential, which represented the GABA-A component. Bicuculline induced spontaneous epileptic bursts by NMDA receptor activation, and subsequent application of T3 suppressed the bursting frequency. Pretreatment of slices with T3 failed to influence the synaptic potentiation caused by tetanic stimulation. Intracellular whole-cell recording using a patch electrode confirmed the T3 actions on the GABA-A and NMDA components. T3 enhanced the GABA-A response and suppressed the NMDA plateau potential without changes in the resting membrane potential or the threshold of action potentials. Contrary to our initial expectation, T3 suppressed the synaptic drives of IMM neurons, and did not influence activity-dependent synaptic potentiation. Imprinting-associated T3 influx may act as an acute suppressor of the IMM network.

L-Thyroxine Improves Vestibular Compensation in a Rat Model of Acute Peripheral Vestibulopathy: Cellular and Behavioral Aspects

Unilateral vestibular lesions induce a vestibular syndrome, which recovers over time due to vestibular compensation. The therapeutic effect of L-Thyroxine (L-T4) on vestibular compensation was investigated by behavioral testing and immunohistochemical analysis in a rat model of unilateral vestibular neurectomy (UVN). We demonstrated that a short-term L-T4 treatment reduced the vestibular syndrome and significantly promoted vestibular compensation. Thyroid hormone receptors (TRα and TRβ) and type II iodothyronine deiodinase (DIO2) were present in the vestibular nuclei (VN), supporting a local action of L-T4. We confirmed the T4-induced metabolic effects by demonstrating an increase in the number of cytochrome oxidase-labeled neurons in the VN three days after the lesion. L-T4 treatment modulated glial reaction by decreasing both microglia and oligodendrocytes in the deafferented VN three days after UVN and increased cell proliferation. Survival of newly generated cells in the deafferented vestibular nuclei was not affected, but microglial rather than neuronal differentiation was favored by L-T4 treatment.

Subclinical thyroid dysfunction and major depressive disorder

PURPOSE

This review attempts to investigate the link between subclinical thyroid dysfunction (SCH) and major depressive disorder (MDD). It has been speculated that SCH may be related to MDD through an autoimmune mechanism.

METHODS

A comprehensive literature search was conducted in the PubMed database for relevant research and review articles.

RESULTS

There appears to be an association between an autoimmune mechanism, possibly involving the thyroid gland, and depressive disorders, but the available evidence is so far inconclusive.

CONCLUSION

Lifetime prevalence of depression is significantly higher in patients with SCH, a finding reflecting a possible effect of SCH in lowering the threshold for the emergence of MDD. The relationship between SCH and MDD is, however, not clear, with large and well-designed studies investigating possible links between reference-range thyroid hormone levels and MDD having as yet found no relation between the two.

Theory: Treatments for Prolonged ICU Patients May Provide New Therapeutic Avenues for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)

We here provide an overview of treatment trials for prolonged intensive care unit (ICU) patients and theorize about their relevance for potential treatment of myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS). Specifically, these treatment trials generally target: (a) the correction of suppressed endocrine axes, notably through a "reactivation" of the pituitary gland's pulsatile secretion of tropic hormones, or (b) the interruption of the "vicious circle" between inflammation, oxidative and nitrosative stress (O&NS), and low thyroid hormone function. There are significant parallels in the treatment trials for prolonged critical illness and ME/CFS; this is consistent with the hypothesis of an overlap in the mechanisms that prevent recovery in both conditions. Early successes in the simultaneous reactivation of pulsatile pituitary secretions in ICU patients-and the resulting positive metabolic effects-could indicate an avenue for treating ME/CFS. The therapeutic effects of thyroid hormones-including in mitigating O&NS and inflammation and in stimulating the adreno-cortical axis-also merit further studies. Collaborative research projects should further investigate the lessons from treatment trials for prolonged critical illness for solving ME/CFS.

Thyroid hormones reduce nicotinic receptor mediated currents in SH-SY5Y neuroblastoma cells

BACKGROUND

Thyroid hormones (THs) are crucial for maturation and functioning of mammalian CNS. THs "classical" signaling involves nuclear receptors binding but also their non genomic actions, as rapid modulators of cell activity, are widely recognized. Since THs imbalance affects cognition and the cholinergic system is deeply involved in learning and memory processes we have studied THs effects at the level of the nicotinic acetylcholine receptors (nAchR).

METHODS

We used the patch-clamp technique to analyze T3 and T4 modulation of nicotine (NIC)-mediated current in SH-SY5Y neuroblastoma cells.

RESULTS

Both hormones decreased NIC-evoked current in a dose dependent fashion. The antagonism was reversible, not competitive and not blocked by Tetrac, an integrin αVβ3 receptor antagonist. A similar effect was detected with the endogenous agonist Acetylcholine. THs potencies were higher at 100 μM NIC (IC₅₀ = 4.6 ± 2 μM for T3 and 4.8 ± 2 μM for T4) compared to those measured at 10 μM NIC (IC₅₀ = 10 ± 4 μM for T3 and 8 ± 4 μM for T4). Furthermore, the efficacy of THs reached almost 90% at 100 μM NIC while was about 30 % at 10 μM NIC. THs inhibited nAchR-mediated currents by enhancing receptor desensitization and this effect was more pronounced at high agonist concentrations.

CONCLUSIONS

Our results make light on a new non genomic activity of THs at the level of nAchR. This mechanism of action  of THs  can provide  a new explanation for the cognitive deficits associated with tyroid dysfunction.

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.

Age and cognitive status dependent differences in blood steroid and thyroid hormone concentrations in intact male rats

Background

Age-dependent alterations of hormonal states have been considered to be involved in age related decline of cognitive abilities. Most of the studies in animal models are based on hormonal substitution in adrenal- and/or gonadectomized rodents or infusion of steroid hormones in intact rats. Moreover, the manipulations have been done timely, closely related to test procedures, thus reflecting short-term hormonal mechanisms in the regulation of learning and memory. Here we studied whether more general states of steroid and thyroid hormone profiles, independent from acute experiences, may possibly reflect long-term learning capacity. A large cohort of aged (17–18 months) intact male rats were tested in a spatial hole-board learning task and a subset of inferior and superior learners was included into the analysis. Young male adult rats (16 weeks of age) were also tested. Four to 8 weeks after testing blood plasma samples were taken and hormone concentrations of a variety of steroid hormones were measured by gas chromatography-tandem mass spectrometry or radioimmunoassay (17β-estradiol, thyroid hormones).

Results

Aged good learners were similar to young rats in the behavioral task. Aged poor learners but not good learners showed higher levels of triiodothyronine (T3) as compared to young rats. Aged good learners had higher levels of thyroid stimulating hormone (TSH) than aged poor learning and young rats. Both aged good and poor learners showed significantly reduced levels of testosterone (T), 4-androstenedione (4A), androstanediol-3α,17β (AD), dihydrotestosterone (DHT), 17-hydroxyprogesterone (17OHP), higher levels of progesterone (Prog) and similar levels of 17β-estradiol (E2) as compared to young rats. The learning, but not the memory indices of all rats were significantly and positively correlated with levels of dihydrotestosterone, androstanediol-3α,17β and thyroxine (T4), when the impacts of age and cognitive division were eliminated by partial correlation analyses.

Conclusion

The correlation of hormone concentrations of individuals with individual behavior revealed a possible specific role of these androgen and thyroid hormones in a state of general preparedness to learn.

Prenatal thyroxine treatment promotes anxiolysis in male Swiss mice offspring

The proper functioning of the maternal thyroid plays a crucial role in fetal development. Thus, the aim of our study was to verify how maternal hyperthyroidism is able to change behavioral parameters in mice offspring during adulthood. For this purpose, pregnant Swiss mice (n = 24 and ~35 g) were randomly assigned into two groups: a control and a thyroxine (T4)-treatment group. The control was treated with 0.9% saline, while the treatment group received T4 (200 μg/kg, s.c.) once daily during the entire pregnancy period. After completing 70 days of life, a part of male offspring underwent a battery of tests, including open field, dark-light box, elevated plus maze, marble burying, rotarod and tail suspension tests. The other male pups were euthanized, being hippocampus and serum collected for RNA analysis and hormones measurement, respectively. Statistical analysis was performed using Student's t-test, and the means were considered significantly different when p < 0.05. In adult offspring, a significant decrease was observed for serum T3 in treated group. It was demonstrated that the T4 group had an increase in total distance traveled in an open field test. In the elevated plus maze test, we observed a higher time in opened arms as well as an increased in percentage of entries in these arms. In the hippocampus, T4 offspring had a higher expression of tryptophan hydroxylase 2 (TPH2), serotonin transporter (SERT) and glutamate decarboxylase 67 (GAD 67) in comparison to controls. These findings suggest that prenatal T4 treatment alters hippocampal serotonergic and GABAergic systems, promoting anxiolysis in male adult offspring.

GABA-A and GABA-B Receptors in Filial Imprinting Linked With Opening and Closing of the Sensitive Period in Domestic Chicks (Gallus gallus domesticus)

Filial imprinting of domestic chicks has a well-defined sensitive (critical) period lasting in the laboratory from hatching to day 3. It is a typical model to investigate the molecular mechanisms underlying memory formation in early learning. We recently found that thyroid hormone 3,5,3′-triiodothyronine (T₃) is a determinant of the sensitive period. Rapid increases in cerebral T₃ levels are induced by imprinting training, rendering chicks imprintable. Furthermore, the administration of exogenous T₃ makes chicks imprintable on days 4 or 6 even after the sensitive period has ended. However, how T₃ affects neural transmission to enable imprinting remains mostly unknown. In this study, we demonstrate opposing roles for gamma-aminobutyric acid (GABA)-A and GABA-B receptors in imprinting downstream of T₃. Quantitative reverse transcription polymerase chain reaction and immunoblotting showed that the GABA-A receptor expression increases gradually from days 1 to 5, whereas the GABA-B receptor expression gradually decreases. We examined whether neurons in the intermediate medial mesopallium (IMM), the brain region responsible for imprinting, express both types of GABA receptors. Immunostaining showed that morphologically identified putative projection neurons express both GABA-A and GABA-B receptors, suggesting that those GABA receptors interact with each other in these cells to modulate the IMM outputs. The roles of GABA-A and GABA-B receptors were investigated using various agonists and antagonists. Our results show that GABA-B receptor antagonists suppressed imprinting on day 1, while its agonists made day 4 chicks imprintable without administration of exogenous T₃. By contrast, GABA-A receptor agonists suppressed imprinting on day 1, while its antagonists induced imprintability on day 4 without exogenous T₃. Furthermore, both GABA-A receptor agonists and GABA-B receptor antagonists suppressed T₃-induced imprintability on day 4 after the sensitive period has ended. Our data from these pharmacological experiments indicate that GABA-B receptors facilitate imprinting downstream of T₃ by initiating the sensitive period, while the GABA-A receptor contributes to the termination of the sensitive period. In conclusion, we propose that opposing roles of GABA-A and GABA-B receptors in the brain during development determine the induction and termination of the sensitive period.

Investigation of brain GABA+ in primary hypothyroidism using edited proton MR spectroscopy

OBJECTIVE

Evidence indicates that thyroid hormones have effects on the inhibitory GABAergic system. The aim of this study was to investigate whether brain GABA levels are altered in patients with hypothyroidism compared with healthy controls.

DESIGN/METHODS

Fifteen patients with primary hypothyroidism and 15 matched healthy controls underwent single-voxel MEGA-PRESS magnetic resonance spectroscopy at 3T, to quantify GABA levels in the median prefrontal cortex (mPFC) and posterior cingulate cortex (PCC). All participants underwent thyroid function test. Neuropsychological performances were evaluated by administration of the Montreal Cognitive Assessment (MoCA) and the 21-item Beck Depression Inventory-II (BDI-II).

RESULTS

The patients with hypothyroidism had significantly lower GABA+ levels in the mPFC compared with healthy controls (P = 0·016), whereas no significant difference (P = 0·214) was observed in the PCC. Exploratory analyses revealed that mPFC GABA+ levels were negatively correlated with the BDI-II scores in patient group (r = -0·60, P = 0·018). No correlations were found between GABA+ levels and TSH or fT3 or fT4 levels in either region (all P > 0·05).

CONCLUSION

This study suggests that alteration of GABAergic neurotransmission may play an important role in the pathophysiology of primary hypothyroidism, providing intriguing neurochemical clues to understand thyroid-brain interactions.

Pharmacological Tools to Activate Microglia and their Possible use to Study Neural Network Patho-physiology

BACKGROUND

Microglia are the resident immunocompetent cells of the CNS and also constitute a unique cell type that contributes to neural network homeostasis and function. Understanding microglia cell-signaling not only will reveal their diverse functions but also will help to identify pharmacological and non-pharmacological tools to modulate the activity of these cells.

METHODS

We undertook a search of bibliographic databases for peer-reviewed research literature to identify microglial activators and their cell-specificity. We also looked for their effects on neural network function and dysfunction.

RESULTS

We identified several pharmacological targets to modulate microglial function, which are more or less specific (with the proper control experiments). We also identified pharmacological targets that would require the development of new potent and specific modulators. We identified a wealth of evidence about the participation of microglia in neural network function and their alterations in pathological conditions.

CONCLUSION

The identification of specific microglia-activating signals provides experimental tools to modulate the activity of this heterogeneous cell type in order to evaluate its impact on other components of the nervous system, and it also helps to identify therapeutic approaches to ease some pathological conditions related to microglial dysfunction.

The effects of intra-hippocampal L-thyroxine infusion on long-term potentiation and long-term depression: A possible role for the αvβ3 integrin receptor

Although the effects of long-term experimental dysthyroidism on long-term potentiation (LTP) and long-term depression (LTD) have been documented, the relationship between LTP/LTD and acute administration of L-thyroxine (T4) has not been described. Here, we investigated the effects of intra-hippocampal administration of T4 on synaptic plasticity in the dentate gyrus of the hippocampal formation. After a 15-minute baseline recording, LTP and LTD were induced by application of high- and low-frequency stimulation protocols, respectively. Infusions of saline or T4 and tetraiodothyroacetic acid (tetrac), a T4 analog that inhibits binding of iodothyronines to the integrin αvβ3 receptor, either alone or together, were made during the stimulation protocols. The averages of the excitatory postsynaptic potential (EPSP) slopes and population spike (PS) amplitudes, between 55 to 60 minutes, were used as a measure of the LTP/LTD magnitude and were analyzed by two-way univariate ANOVA with T4 and tetrac as between-subjects factors. The input-output curves of the infusion groups were comparable to each other, as shown by the non significant interaction observed between stimulus intensity and infused drug. The magnitude of the LTP in T4-infused rats was significantly lower as compared to saline-infused rats. Both the PS amplitude and the EPSP slope were depressed more markedly with T4 infusion than with saline, tetrac, and T4 + tetrac infusion. Data of this study provide in vivo evidence that T4 can promote LTD over LTP via the integrin αvβ3 receptor, and that the effect of endogenous T4 on this receptor can be suppressed by tetrac in the hippocampus. © 2016 Wiley Periodicals, Inc.

Effect of non-genomic actions of thyroid hormones on the anaesthetic effect of propofol

Hyperthyroidism is a common disease of the endocrine system and it is known that additional propofol anaesthesia is required during surgery for patients with hyperthyroidism compared with those with normal thyroid function. The aim of the present study was to determine the mechanism through which thyroid hormones (THs) inhibit the effect of propofol anaesthesia. Immunofluorescence techniques were used to verify the difference between the expression quantities of γ-aminobutyric acid type A (GABA_A) receptor subunits α2 and β2 in the dorsal root ganglions (DRGs) of rats with hyperthyroidism and those in normal rats. Perforated patch clamp recordings in the whole-cell mode were performed to detect the GABA-activated current in acutely isolated rat DRG neurons from rats with hyperthyroidism and normal rats. This method was also used to evaluate the change in the GABA-activated currents following the pre-perfusion of propofol with and without 3,3',5-L-triiodothyronine (T₃). Compared with normal rats, rats with hyperthyroidism expressed same quantities of GABA_A receptor α2 and β2 subunits in DRGs. In addition, no difference in GABA-activated currents in the acutely isolated DRG neurons from the two types of rat was observed (P>0.05). T₃ inhibits or minimises the augmentation effect of propofol on the GABA-activated currents (P<0.05). The inhibitory effect of T₃ on propofol was minimised by increasing the propofol concentration (P<0.05). The inhibitory effect of T₃ on the anaesthetic effect of propofol is achieved through the inhibition of the function of GABA_A receptors through the non-genomic actions of the THs, rather than by changing the number of GABA_A receptors. This inhibitory effect can be mitigated by increasing the propofol concentration. In conclusion, rats with hyperthyroidism require a larger dose of propofol to induce anaesthesia since the non-genomic actions of THs suppress GABA receptors, which in turn inhibits the anaesthetic action of propofol.

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 thyroid hormone to the preoptic region of hypothyroid adult male rats on sleep, motor activity and body temperature

Thyroid hormones induce short-latency nongenomic effects in adult brain tissue, suggesting that their acute administration would affect brain activity in intact animals. The influence on EEG-defined sleep of acute restoration of l-3,3'5-triiodothyronine (T3) to a sleep-regulatory brain region, the preoptic region, was examined in hypothyroid rats. Sleep parameters were monitored for 48 h weekly: for 24 h immediately following a control microinjection and for an additional 24h after a second microinjection including a T3 dose to the preoptic region or lateral ventricle. Male albino rats were implanted with EEG and EMG electrodes, abdominal temperature/activity transponders and unilateral lateral ventricle cannulae or bilateral preoptic region cannulae, and were given 0.02% n-propythiouracil (PTU) in their drinking water for 4 weeks. For histologically-confirmed bilateral preoptic region cannula placements (N=7), effects of T3 (especially a 3 μg dose) were apparent within 10h of injection as decreases in REM, NREM and total sleep and increases in waking and activity. Minimal effects of lateral ventricle T3 microinjection were demonstrated (N=5). Significant effects due to the time of day on the experimental measures were seen in both lateral ventricle and preoptic region groups, but these effects did not interact with the effect of administered hormone dose. These effects of T3 microinjection to the preoptic region were demonstrated after acute injections and within hours of injection rather than after chronic administration over days.

Is rapid effect of thyroxine on GABAergic IPSCs purely postsynaptic?

BACKGROUND

Thyroid hormones (THs) are well known for their genomic effects but recently several studies revealed their actions as rapid modulators of membrane receptors. In particular, fast thyroxine effect on GABA(A) receptors have been reported. We addressed question whether presynaptic mechanisms can be also involved in modulation of GABAergic transmission by thyroxine.

METHODS

Using patch-clamp technique we examined fast effects of thyroxine (2 μM) on evoked GABAergic postsynaptic currents.

RESULTS

We found that in addition to the inhibitory effect on IPSC amplitude, thyroxine changed IPSC coefficient of variation (CV).

CONCLUSION

This result suggests involvement of a presynaptic mechanism in thyroxine effect on GABAergic transmission.

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.

Perinatal exposure to environmental polychlorinated biphenyls sensitizes hippocampus to excitotoxicity ex vivo

Ortho-substituted polychlorinated biphenyls (PCBs) are a concern to human developmental health. Rat dams were exposed to an environmentally relevant mixture of PCBs, Aroclor 1254, or pure congener PCB 95 (6 mg/kg/day) during the perinatal period (GD 5 through PD 21). Hippocampal slices prepared from offspring 1-3 weeks post-weaning were tested for persisting changes in sensitivity to the excitotoxicant picrotoxin. Hippocampal slices were placed on multielectrode arrays. Field excitatory postsynaptic potentials (fEPSPs) were recorded from Schaffer Collateral/Commissural fibers in striatum radiatum of the CA1 region in response to single pulse stimuli. After recording baseline excitability, GABA(A) receptors were blocked by inclusion of picrotoxin (100 μM) in the aCSF perfusate. Picrotoxin produced negligible changes in fEPSP slope in slices isolated from offspring exposed to vehicle, whereas slices from either PCB test group invariably showed >200% (p<0.01) synaptic facilitation. Picrotoxin produced prominent after-discharges (epileptic wave forms) in the evoked potentials measured from PCB exposed, but not control, hippocampal slices. These results show that developmental exposure to non-coplanar PCBs is sufficient to produce changes in synaptic plasticity that can be unmasked in the presence of GABA(A) receptor deficits that persist 1-3 weeks after exposure ceased. Developmental exposure to PCBs may sensitize seizure susceptibility postnatally, especially in susceptible populations with GABA(A) receptor signaling deficits.