Interplay between thyroxin, BDNF and GABA in injured neurons

Thyroid hormones and neurological impairments during the first two days after stroke: A pilot study

Despite plenty of studies on the change in thyroid hormones after stroke, there is still no consensus for a comprehensive conclusion and clinical application. This pilot study aimed to investigate the change in thyroid hormones and its association with neurological impairments during the first two days after the stroke. This was a cross-sectional descriptive and analytical study. Neurological deficits were assessed using the National Institutes of Health Stroke Scale (NIHSS). In addition, blood samples were collected for TSH, T3, and T4. P value < 0.05 was considered significant. Fifty patients (42 % men) with a mean age of 69.73 (SD ± 12.31) were included. The majority of patients had a high level of TSH and T4, and a normal level of T3 on the first and second days of hospitalization. 33.3 and 20.9 percent of patients had NIHSS scores >10 on the first and second day respectively. According to the multivariate analysis, high T4 hormone was as a predictor of NIHSS score >10. There was no association between thyroid hormones and NIHSS at the admission. In addition, there was no relationship between thyroid hormones and TSH on the second day. According to the results, stroke affects the levels of thyroid hormones and the pituitary-thyroid axis. Because of the diversity in results of existing studies, further clinical research with the same methodology and data analysis is needed to find consistent results on the association between thyroid hormones and neurological impairments. Monitoring thyroid status in stroke patients may be recommendable.

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.

Posttraumatic midazolam administration does not influence brain damage after experimental traumatic brain injury

BACKGROUND

The benzodiazepine midazolam is a γ-aminobutyric acid (GABA)-A receptor agonist frequently used for sedation or stress control in patients suffering from traumatic brain injury (TBI). However, experimental studies on benzodiazepines have reported divergent results, raising concerns about its widespread use in patients. Some studies indicate that benzodiazepine-mediated potentiation of GABAergic neurotransmission is detrimental in brain-injured animals. However, other experimental investigations demonstrate neuroprotective effects, especially in pretreatment paradigms. This study investigated whether single-bolus midazolam administration influences secondary brain damage post-TBI.

METHODS

Two different midazolam dosages (0.5 and 5 mg/kg BW), a combination of midazolam and its competitive antagonist flumazenil, or vehicle solution (NaCl 0.9%) was injected intravenously to mice 24 h after experimental TBI induced by controlled cortical impact. Mice were evaluated for neurological and motor deficits using a 15-point neuroscore and the rotarod test. Histopathological brain damage and mRNA expression of inflammatory marker genes were analyzed using quantitative polymerase chain reaction three days after insult.

RESULTS

Histological brain damage was not affected by posttraumatic midazolam administration. Midazolam impaired functional recovery, and this effect could not be counteracted by administering the midazolam antagonist flumazenil. An increase in IL-1β mRNA levels due to postinjury application of midazolam was reversible by flumazenil administration. However, other inflammatory parameters were not affected.

CONCLUSIONS

This study merely reports minor effects of a postinjury midazolam application. Further studies focusing on a time-dependent analysis of posttraumatic benzodiazepine administration are required.

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.

Neurochemical changes in the primate lateral geniculate nucleus following lesions of striate cortex in infancy and adulthood: implications for residual vision and blindsight

Following lesions of the primary visual cortex (V1), the lateral geniculate nucleus (LGN) undergoes substantial cell loss due to retrograde degeneration. However, visually responsive neurons remain in the degenerated sector of LGN, and these have been implicated in mediation of residual visual capacities that remain within the affected sectors of the visual field. Using immunohistochemistry, we compared the neurochemical characteristics of LGN neurons in V1-lesioned marmoset monkeys (Callithrix jacchus) with those of non-lesioned control animals. We found that GABAergic neurons form approximately 6.5% of the neuronal population in the normal LGN, where most of these cells express the calcium-binding protein parvalbumin. Following long-term V1 lesions in adult monkeys, we observed a marked increase (~ sevenfold) in the proportion of GABA-expressing neurons in the degenerated sector of the LGN, indicating that GABAergic cells are less affected by retrograde degeneration in comparison with magno- and parvocellular projection neurons. In addition, following early postnatal V1 lesions and survival into adulthood, we found widespread expression of GABA in putative projection neurons, even outside the degenerated sectors (lesion projection zones). Our findings show that changes in the ratio of GABAergic neurons in LGN need to be taken into account in the interpretation of the mechanisms of visual abilities that survive V1 lesions in primates.

Serum BDNF Levels Are Reduced in Patients with Disorders of Consciousness and Are Not Modified by Verticalization with Robot-Assisted Lower-Limb Training

Little is known about plastic changes occurring in the brains of patients with severe disorders of consciousness (DOCs) caused by acute brain injuries at rest and during rehabilitative treatment. Brain-derived neurotrophic factor (BDNF) is a neurotrophin involved in neurogenesis and synaptic plasticity whose production is powerfully modulated by physical exercise. In this study, we compared serum BDNF levels in 18 patients with unresponsive wakefulness syndrome (UWS) and in a minimally conscious state (MCS) with those in 16 sex- and age-matched healthy controls. In 12 patients, serum BDNF levels before and after verticalization with ErigoPro robot-assisted lower-limb training were compared. Serum BDNF levels were significantly lower in patients (median, 1141 pg/ml; 25^th and 75^th percentiles, 1016 and 1704 pg/ml) than in controls (median, 2450 pg/ml; 25^th and 75^th percentiles, 2100 and 2875 pg/ml; p < 0.001). BDNF levels measured before and after verticalization with robot-assisted lower-limb training did not change (p = 0.5). Moreover, BDNF levels did not differ between patients with UWS and MCS (p = 0.2), or between patients with traumatic and nontraumatic brain injuries (p = 0.6). BDNF level correlated positively with the time since brain injury (p = 0.025). In conclusion, serum BDNF levels are reduced in patients with UWS and MCS and cannot be improved by verticalization associated with passive lower-limb training. Additional studies are needed to better understand the mechanisms underlying BDNF reduction in patients with DOCs and to determine the best rehabilitative strategies to promote restorative plastic changes in these patients.

Thyroid hormone levels and structural parameters of thyroid homeostasis are correlated with motor subtype and disease severity in euthyroid patients with Parkinson's disease

Purpose: This study is to investigate the relationship between thyroid function and Parkinson's disease (PD).Materials and Methods: Totally 77 PD patients were included, who were divided into tremor-dominant-type (TDT), akinetic-rigid-type (ART) and mixed-type (MXT) subgroups. Parkinsonism severity and stage was assessed by modified H-Y stage. Thyroid-stimulating hormone (TSH), fT3 and fT4 levels were detected to analyze thyroid function. Parameters of thyroid homeostasis, including thyroid's secretory capacity (SPINA-GT), the total deiodinase activity (SPINA-GD) and Jostel's TSH index and the thyrotroph thyroid hormone sensitivity index (TTSI), were calculated and compared.Results: Thyroid hormone levels in PD patients were lower than normal controls. Patients with TDT/MXT had significantly higher fT4 level than those with ART. TSH levels were 1.73 ± 0.93 and 2.06 ± 1.04 ulU/ml for patients with TDT/MXT and ART, respectively. The patients in the TDT/MXT group had significantly lower SPINA-GD while significantly higher SPINA-GT than ART group. The fT3 level was significantly higher in early group than advanced group. TSH index in the early group was significantly higher than the advanced group. The fT4 level was negatively correlated with UPDRS motor score. Univariate and multivariable logistic regression analysis indicated that fT4 was positively correlated with PD motor subtype, which disappeared after adjusting for confounding factors. The fT3 level was negatively correlated with PD disease severity, even after adjusting for confounding factors. In female PD patients, fT4 level in TDT/MXT group was significantly higher than ART group. Male PD patients had higher fT4 levels in early patients than advanced patients. Percentage of patients exhibiting ART was decreased significantly in higher fT4 level subgroups. With the increase of TSH index and TTSI, the proportion of advanced PD patients gradually decreased. The proportion of PD patients with TDT/MXT motor subtype gradually increased with the quartiles of SPINA-GT.Conclusion: Thyroid hormone levels and structural parameters of thyroid homeostasis are correlated with motor subtype and disease severity in euthyroid patients with PD.

Midazolam Sedation Induces Upper Limb Coordination Deficits That Are Reversed by Flumazenil in Patients with Eloquent Area Gliomas

Editor’s Perspective

What We Already Know about This Topic

What This Article Tells Us That Is New

Background

Midazolam has been found to exacerbate or unmask limb motor dysfunction in patients with brain tumors. This study aimed to determine whether the exacerbated upper limb motor-sensory deficits are mediated through benzodiazepine sites by demonstrating reversibility by flumazenil in patients with gliomas in eloquent areas.

Methods

This was an interventional, parallel assignment, nonrandomized trial. Study subjects were admitted in the operating room. Patients with supratentorial eloquent area gliomas and volunteers of similar age without neurologic disease were sedated with midazolam, but still responsive and cooperative. Motor and sensory functions for upper extremities were evaluated by the Nine-Hole Peg Test before and after midazolam, as well as after flumazenil reversal.

Results

Thirty-two cases were included: 15 in the glioma group and 17 in the control group. The total dose of midazolam and flumazenil were comparable between the groups. In the glioma group, the times to task completion after midazolam in the contralateral hand (P = 0.001) and ipsilateral hand (P = 0.002) were 26.5 (95% CI, 11.3 to 41.7) and 13.7 (95% CI, 5.0 to 22.4) seconds slower than baseline, respectively. After flumazenil reversal, the contralateral hand (P = 0.99) and ipsilateral hand (P = 0.187) performed 1.2 (95% CI, −3.3 to 5.8) and 1.5 (95% CI, −0.5 to 3.5) seconds slower than baseline, respectively. In the control group, the dominant (P &lt; 0.001) and nondominant hand (P = 0.006) were 2.9 (95% CI, 1.4 to 4.3) and 1.7 (95% CI, 0.5 to 2.9) seconds slower than baseline, respectively. After flumazenil, the dominant hand (P = 0.99) and nondominant hand (P = 0.019) performed 0.2 (95% CI, −0.7 to 1.0) and 1.3 (95% CI, −0.2 to 2.4) seconds faster than baseline, respectively.

Conclusions

In patients with eloquent area gliomas, mild sedation with midazolam induced motor coordination deficits in upper limbs. This deficit was almost completely reversed by the benzodiazepine antagonist flumazenil, suggesting that this is a reversible abnormality linked to occupation of the receptor by midazolam.

Bumetanide Prevents Brain Trauma-Induced Depressive-Like Behavior

Brain trauma triggers a cascade of deleterious events leading to enhanced incidence of drug resistant epilepsies, depression, and cognitive dysfunctions. The underlying mechanisms leading to these alterations are poorly understood and treatment that attenuates those sequels are not available. Using controlled-cortical impact as an experimental model of brain trauma in adult mice, we found a strong suppressive effect of the sodium-potassium-chloride importer (NKCC1) specific antagonist bumetanide on the appearance of depressive-like behavior. We demonstrate that this alteration in behavior is associated with an impairment of post-traumatic secondary neurogenesis within the dentate gyrus of the hippocampus. The mechanism mediating the effect of bumetanide involves early transient changes in the expression of chloride regulatory proteins and qualitative changes in GABA(A) mediated transmission from hyperpolarizing to depolarizing after brain trauma. This work opens new perspectives in the early treatment of human post-traumatic induced depression. Our results strongly suggest that bumetanide might constitute an efficient prophylactic treatment to reduce neurological and psychiatric consequences of brain trauma.

Roles of γ-aminobutyric acid on salinity-responsive genes at transcriptomic level in poplar: involving in abscisic acid and ethylene-signalling pathways

γ-Aminobutyric acid (GABA) affected ABA and ethylene metabolic genes and signal components in salt-treated poplar, indicating its potential role in signal pathways of ABA and ethylene during salt stress. GABA is a small signalling molecule that accumulates rapidly in plants exposed to various stresses. However, the relationship between GABA and other signalling molecules, such as hormones, remains unclear. Here, in the poplar woody plant under 200-mM NaCl conditions, the application of low (0.25 mM) and high (10 mM) exogenous GABA, compared to 0 mM, affected the accumulation of hydrogen peroxide and hormones, including ABA and ethylene, in different manners. Transcriptomic analysis demonstrated that 1025 differentially expressed genes (DEGs; |log2Ratio| ≥ 1.5) were widely affected by exogenous GABA under salt stress. A clustering analysis revealed that GABA could rescue or promote the effects of salt stress on gene expression. Among them, 146 genes involved in six hormone-signalling pathways were enriched, including 22 ABA- and 50 ethylene-related genes. Quantitative expression of selected genes involved in hormone-related pathways showed that ABA metabolic genes (ABAG, ABAH2, and ABAH4), ethylene biosynthetic genes (ACO1, ACO2, ACO5, ACOH1, ACS1, and ACS7) and receptor genes (PYL1, PYL2, PYL4, and PYL6) were regulated by exogenous GABA, even at a 0.1 mM level. The production of ABA was negatively correlated with ABAH expression levels at different GABA concentrations. The increase of endogenous GABA, resulting from inhibitor (succinyl phosphonate) of α-ketoglutarate dehydrogenase, affected the PYLs levels. Thus, GABA may be involved in ABA- and ethylene-signalling pathways. Our data provide a better understanding of GABA's roles in the plant responses to environmental stresses.

Thyroid Function Disruptors: from nature to chemicals

The modern concept of thyroid disruptors includes man-made chemicals and bioactive compounds from food that interfere with any aspect of the hypothalamus-pituitary-thyroid axis, thyroid hormone biosynthesis and secretion, blood and transmembrane transport, metabolism and local action of thyroid hormones. This review highlights relevant disruptors that effect populations through their diet: directly from food itself (fish oil and polyunsaturated fatty acids, pepper, coffee, cinnamon and resveratrol/grapes), through vegetable cultivation (pesticides) and from containers for food storage and cooking (bisphenol A, phthalates and polybrominated diphenyl ethers). Due to the vital role of thyroid hormones during every stage of life, we review effects from the gestational period through to adulthood, including evidence from in vitro studies, rodent models, human trials and epidemiological studies.

Posttraumatic Propofol Neurotoxicity Is Mediated via the Pro-Brain-Derived Neurotrophic Factor-p75 Neurotrophin Receptor Pathway in Adult Mice

OBJECTIVES

The gamma-aminobutyric acid modulator propofol induces neuronal cell death in healthy immature brains by unbalancing neurotrophin homeostasis via p75 neurotrophin receptor signaling. In adulthood, p75 neurotrophin receptor becomes down-regulated and propofol loses its neurotoxic effect. However, acute brain lesions, such as traumatic brain injury, reactivate developmental-like programs and increase p75 neurotrophin receptor expression, probably to foster reparative processes, which in turn could render the brain sensitive to propofol-mediated neurotoxicity. This study investigates the influence of delayed single-bolus propofol applications at the peak of p75 neurotrophin receptor expression after experimental traumatic brain injury in adult mice.

DESIGN

Randomized laboratory animal study.

SETTING

University research laboratory.

SUBJECTS

Adult C57BL/6N and nerve growth factor receptor-deficient mice.

INTERVENTIONS

Sedation by IV propofol bolus application delayed after controlled cortical impact injury.

MEASUREMENTS AND MAIN RESULTS

Propofol sedation at 24 hours after traumatic brain injury increased lesion volume, enhanced calpain-induced αII-spectrin cleavage, and increased cell death in perilesional tissue. Thirty-day postinjury motor function determined by CatWalk (Noldus Information Technology, Wageningen, The Netherlands) gait analysis was significantly impaired in propofol-sedated animals. Propofol enhanced pro-brain-derived neurotrophic factor/brain-derived neurotrophic factor ratio, which aggravates p75 neurotrophin receptor-mediated cell death. Propofol toxicity was abolished both by pharmacologic inhibition of the cell death domain of the p75 neurotrophin receptor (TAT-Pep5) and in mice lacking the extracellular neurotrophin binding site of p75 neurotrophin receptor.

CONCLUSIONS

This study provides first evidence that propofol sedation after acute brain lesions can have a deleterious impact and implicates a role for the pro-brain-derived neurotrophic factor-p75 neurotrophin receptor pathway. This observation is important as sedation with propofol and other compounds with GABA receptor activity are frequently used in patients with acute brain pathologies to facilitate sedation or surgical and interventional procedures.

Proneurotrophin Binding to P75 Neurotrophin Receptor (P75ntr) Is Essential for Brain Lesion Formation and Functional Impairment after Experimental Traumatic Brain Injury

Traumatic brain injury (TBI) initiates an excessive mediator release of e.g. neurotrophins, which promote neuronal survival, differentiation, and modulate synaptic plasticity. Paradoxically, mature forms of neurotrophins promote neuronal survival, whereas unprocessed forms of neurotrophins induce cell death through p75 neurotrophin receptor (p75NTR) signaling. p75NTR is widely expressed during synaptogenesis and is subsequently downregulated in adulthood. Repair mechanisms after acute cerebral insults can reactivate its expression. Therefore, the influence of p75NTR on secondary brain damage was addressed. mRNA levels of p75NTR and its ligands were quantified in brain tissue up to 7 days after experimental TBI (controlled cortical impact; CCI). Brain damage, motor function and inflammatory marker gene expression were determined in mice lacking the proneurotrophin-binding site of the p75NTR protein (NGFR(-/-)) and wild type littermates (NGFR(+/+)) 24 h and 5 days after CCI. In addition, the effect of TAT-Pep5 (pharmacological inhibitor of the intracellular p75NTR death domain) on lesion volume was evaluated 24 h after insult. p75NTR mRNA levels were induced nine-fold by TBI. In NGFR(-/-) mice, lesion volume was reduced by 29% at 24 h and by 21% 5 days after CCI. Motor coordination was significantly improved 24 h after trauma compared with the wild type. Pharmacological inhibition of the p75NTR signaling reduced lesion volume by 18%. The present study presents first time evidence that genetic mutation of the neurotrophin interaction site of p75NTR strongly limits post-traumatic cell death. In addition, we revealed pharmacological targeting of the intracellular p75NTR cell death domain as a promising approach to limit acute brain damage.

Decreased thyroid hormone signaling accelerates the reinnervation of the optic tectum following optic nerve crush in adult zebrafish

The regenerative capacity of the adult mammalian central nervous system (CNS) is poor and finding ways to stimulate long distance axonal regeneration in humans remains a challenge for neuroscientists. Thyroid hormones, well known for their key function in CNS development and maturation, more recently also emerged as molecules influencing regeneration. While several studies investigated their influence on peripheral nerve regeneration, in vivo studies on their role in adult CNS regeneration remain scarce. We therefore investigated the effect of lowering T3 signaling on the regeneration of the optic nerve (ON) following crush in zebrafish, a species where full recovery occurs spontaneously. Adult zebrafish were exposed to iopanoic acid (IOP), which lowered intracellular 3,5,3'-triiodothyronine (T3) availability, or to the thyroid hormone receptor β antagonist methylsulfonylnitrobenzoate (C1). Both treatments accelerated optic tectum (OT) reinnervation. At 7days post injury (7dpi) there was a clear increase in the biocytin labeled area in the OT following anterograde tracing as well as an increased immunostaining of Gap43, a protein expressed in outgrowing axons. This effect was attenuated by T3 supplementation to IOP-treated fish. ON crush induced very limited cell death and proliferation at the level of the retina in control, IOP- and C1-treated fish. The treatments also had no effect on the mRNA upregulation of the regeneration markers gap43, tub1a, and socs3b at the level of the retina at 4 and 7dpi. We did, however, find a correlation between the accelerated OT reinnervation and a more rapid resolution of microglia/macrophages in the ON and the OT of IOP-treated fish. Taken together these data indicate that lowering T3 signaling accelerates OT reinnervation following ON crush in zebrafish and that this is accompanied by a more rapid resolution of the inflammatory response.

Changes in thyroid hormone receptors after permanent cerebral ischemia in male rats

Thyroid hormones (TH) and receptors (TRs) may play an important role in the pathophysiology of acute cerebral ischemia. In the present study, we sought to determine whether serum triodothyronine (T3)/thyroxine (T4) and brain TRs (TRα1, TRβ1) might change after experimental stroke. Male adult Wistar rats were subjected to permanent middle cerebral artery occlusion (group P) and compared to sham-operated controls (group S). Animals were followed clinically for 14 days until brain collection for Western blot (WB) or neuropathological analysis of TRs in three different brain areas (infarcted tissue, E1; noninfarcted ipsilateral hemisphere, E2; and contralateral hemisphere, E3). Analysis of serum TH levels showed a reduction of T4 in group P (p = 0.002) at days 2 to 14, while half of the animals also displayed "low T3" values (p = 0.012) on day 14. This T4 reduction was inversely correlated to the clinical severity of stroke and the concomitant body weight loss (p < 0.005). WB analysis of TRα1 and TRβ1 protein expression showed heterogenic responses at day 14: total and nuclear TRα1 were similar between the two groups, while total TRβ1 decreased 7.5-fold within E1 (p ≤ 0.001) with a concomitant 1.8-fold increase of nuclear TRβ1 in E2 area (p = 0.03); TRβ1 expression did not differ in E3. Neuropathological analysis revealed that activated macrophages/microglia exclusively expressed nuclear TRα1 within the infarct core. Astrocytes mildly expressed nuclear TRα1 in and around the infarct, along with a prominent TRβ nuclear signal restricted in the astrocytic scar. Neurons around the infarct expressed mainly TRα1 and, to a milder degree, TRβ. Surprisingly enough, we detected for the first time a TRβ expression in the paranodal region of Ranvier nodes, of unknown significance so far. Our data support that cerebral ischemia induces a low TH response, associated with significant and heterogenic changes in brain TR expression. These findings could imply an important role of TH signaling in cerebral ischemia.

Role of thyroid hormones in different aspects of nervous system regeneration in vertebrates

Spontaneous functional recovery from injury in the adult human nervous system is rare and trying to improve recovery remains a clinical challenge. Nervous system regeneration is a complicated sequence of events involving cell death or survival, cell proliferation, axon extension and remyelination, and finally reinnervation and functional recovery. Successful recovery depends on the cell-specific and time-dependent activation and repression of a wide variety of growth factors and guidance molecules. Thyroid hormones (THs), well known for their regulatory role in neurodevelopment, have recently emerged as important modulators of neuroregeneration. This review focuses on the endogenous changes in the proteins regulating TH availability and action in different cell types of the adult mammalian nervous system during regeneration as well as the impact of TH supplementation on the consecutive steps in this process. It also addresses possible differences in TH involvement between different vertebrate classes, early or late developmental stages and peripheral or central nervous system. The available data show that THs are able to stimulate many signaling pathways necessary for successful neurogeneration. They however also suggest that supplementation with T4 and/or T3 may have beneficial or detrimental influences depending on the dose and more importantly on the specific phase of the regeneration process.

Current view on the functional regulation of the neuronal K(+)-Cl(-) cotransporter KCC2

In the mammalian central nervous system (CNS), the inhibitory strength of chloride (Cl(-))-permeable GABAA and glycine receptors (GABAAR and GlyR) depends on the intracellular Cl(-) concentration ([Cl(-)]i). Lowering [Cl(-)]i enhances inhibition, whereas raising [Cl(-)]i facilitates neuronal activity. A neuron's basal level of [Cl(-)]i, as well as its Cl(-) extrusion capacity, is critically dependent on the activity of the electroneutral K(+)-Cl(-) cotransporter KCC2, a member of the SLC12 cation-Cl(-) cotransporter (CCC) family. KCC2 deficiency compromises neuronal migration, formation and the maturation of GABAergic and glutamatergic synaptic connections, and results in network hyperexcitability and seizure activity. Several neurological disorders including multiple epilepsy subtypes, neuropathic pain, and schizophrenia, as well as various insults such as trauma and ischemia, are associated with significant decreases in the Cl(-) extrusion capacity of KCC2 that result in increases of [Cl(-)]i and the subsequent hyperexcitability of neuronal networks. Accordingly, identifying the key upstream molecular mediators governing the functional regulation of KCC2, and modifying these signaling pathways with small molecules, might constitute a novel neurotherapeutic strategy for multiple diseases. Here, we discuss recent advances in the understanding of the mechanisms regulating KCC2 activity, and of the role these mechanisms play in neuronal Cl(-) homeostasis and GABAergic neurotransmission. As KCC2 mediates electroneutral transport, the experimental recording of its activity constitutes an important research challenge; we therefore also, provide an overview of the different methodological approaches utilized to monitor function of KCC2 in both physiological and pathological conditions.

Thyroid hormone-dependent development of the GABAergic pre- and post-synaptic components in the rat hippocampus

Thyroid hormone (TH) plays essential roles in normal brain development mainly by regulating gene expression through binding to specific nuclear receptors which serve as transcription factors. Previous studies showed that perinatal deficiency of TH or impairment of its signaling severely affect brain development, especially the development of the γ-aminobutyric acid (GABA) system, but cellular and molecular targets of the hormone are only partly uncovered. In the present study, we focused on the developing rat hippocampus which was confirmed to be one of the regions highly sensitive to TH status, and found two new targets of the hormone among the pre- and post-synaptic components of the GABAergic system. One was glutamic acid decarboxylase 65 (GAD65), the protein level of which was reduced to less than 50% of control in the hippocampus of hypothyroid rats (obtained by administering 0.025% methimazole in drinking water to pregnant dams from gestational day 15 until 4 weeks postpartum) and recovered to control levels by daily thyroxine-replacement after birth. Reduction in GAD65 protein was correlated immunohistochemically with a 37% reduction in the number of GAD65-positive cells as well as a reduction in GAD65-positive processes. In contrast, the other GAD isotype, GAD67, was not affected by TH status. A subpopulation of GABAergic neurons containing parvalbumin was also confirmed to be highly dependent on TH status. The second target of thyroid hormone was neuron-specific K(+)/Cl(-) co-transporter, KCC2, which is responsible for switching of GABA action from excitatory to inhibitory. In the euthyroid hippocampus, a sharp rise of kcc2 expression was observed at postnatal day (PND)10 which was followed by a large increase in KCC2 protein at PND15. This transient rise in kcc2 expression was completely suppressed by hypothyroidism, resulting in nearly 80% reduction in KCC2 protein at PND15. These results indicate that the development of GABAergic terminals and the excitatory to inhibitory maturation of GABA signaling are strongly dependent on TH.