High potassium induces taurine release by osmosensitive and osmoresistant mechanisms in the rat hippocampus in vivo

Unbalanced redox status network as an early pathological event in congenital cataracts

The lens proteome undergoes dramatic composition changes during development and maturation. A defective developmental process leads to congenital cataracts that account for about 30% of cases of childhood blindness. Gene mutations are associated with approximately 50% of early-onset forms of lens opacity, with the remainder being of unknown etiology. To gain a better understanding of cataractogenesis, we utilized a transgenic mouse model expressing a mutant ubiquitin protein in the lens (K6W-Ub) that recapitulates most of the early pathological changes seen in human congenital cataracts. We performed mass spectrometry-based tandem-mass-tag quantitative proteomics in E15, P1, and P30 control or K6W-Ub lenses. Our analysis identified targets that are required for early normal differentiation steps and altered in cataractous lenses, particularly metabolic pathways involving glutathione and amino acids. Computational molecular phenotyping revealed that glutathione and taurine were spatially altered in the K6W-Ub cataractous lens. High-performance liquid chromatography revealed that both taurine and the ratio of reduced glutathione to oxidized glutathione, two indicators of redox status, were differentially compromised in lens biology. In sum, our research documents that dynamic proteome changes in a mouse model of congenital cataracts impact redox biology in lens. Our findings shed light on the molecular mechanisms associated with congenital cataracts and point out that unbalanced redox status due to reduced levels of taurine and glutathione, metabolites already linked to age-related cataract, could be a major underlying mechanism behind lens opacities that appear early in life.

Attenuation of initial pilocarpine-induced electrographic seizures by methionine sulfoximine pretreatment tightly correlates with the reduction of extracellular taurine in the hippocampus

OBJECTIVE

Initiation and development of early seizures by chemical stimuli is associated with brain cell swelling resulting in edema of seizure-vulnerable brain regions. We previously reported that pretreatment with a nonconvulsive dose of glutamine (Gln) synthetase inhibitor methionine sulfoximine (MSO) mitigates the intensity of initial pilocarpine (Pilo)-induced seizures in juvenile rats. We hypothesized that MSO exerts its protective effect by preventing the seizure-initiating and seizure-propagating increase of cell volume. Taurine (Tau) is an osmosensitive amino acid, whose release reflects increased cell volume. Therefore, we tested whether the poststimulus rise of amplitude of Pilo-induced electrographic seizures and their attenuation by MSO are correlated with the release of Tau from seizure-affected hippocampus.

METHODS

Lithium-pretreated animals were administered MSO (75 mg/kg ip) 2.5 h before the induction of convulsions by Pilo (40 mg/kg ip). Electroencephalographic (EEG) power was analyzed during 60 min post-Pilo, at 5-min intervals. Extracellular accumulation of Tau (eTau) served as a marker of cell swelling. eTau, extracellular Gln (eGln), and extracellular glutamate (eGlu) were assayed in the microdialysates of the ventral hippocampal CA1 region collected at 15-min intervals during the whole 3.5-h observation period.

RESULTS

The first EEG signal became apparent at ~10 min post-Pilo. The EEG amplitude across most frequency bands peaked at ~40 min post-Pilo, and showed strong (r ~ .72-.96) temporal correlation with eTau, but no correlation with eGln or eGlu. MSO pretreatment delayed the first EEG signal in Pilo-treated rats by ~10 min, and depressed the EEG amplitude across most frequency bands, to values that remained strongly correlated with eTau (r > .92) and moderately correlated (r ~ -.59) with eGln, but not with eGlu.

SIGNIFICANCE

Strong correlation between attenuation of Pilo-induced seizures and Tau release indicates that the beneficial effect of MSO is due to the prevention of cell volume increase concurrent with the onset of seizures.

Maternal Immune Activation Affects Hippocampal Excitatory and Inhibitory Synaptic Transmission in Offspring From an Early Developmental Period to Adulthood

One of the risk factors for schizophrenia is maternal infection. We have previously shown that Polyriboinosinic-polyribocytidylic acid (poly I:C) induced maternal immune activation in mice caused histological changes in the hippocampal CA1 area of offspring during the developmental period and impaired sensorimotor gating in offspring during adulthood, resulting in behavioral changes. However, it remains unclear how maternal immune activation functionally impacts the hippocampal neuronal activity of offspring. We studied the effect of prenatal poly I:C treatment on synaptic transmission of hippocampal CA1 pyramidal cells in postnatal and adult offspring. Treatment with poly I:C diminished excitatory and enhanced inhibitory (GABAergic) synaptic transmission on pyramidal cells in adult offspring. During the early developmental period, we still observed that treatment with poly I:C decreased excitatory synaptic transmission and potentially increased GABAergic synaptic transmission, which was uncovered under a condition of high extracellular potassium-activated neurons. In conclusion, we demonstrate that maternal immune activation decreased excitatory and increased inhibitory synaptic transmission on hippocampal pyramidal cells from an early developmental period to adulthood, which could result in net inhibition in conjunction with poor functional organization and integration of hippocampal circuits.

Pathway-Specific Control of Striatal Neuron Vulnerability by Corticostriatal Cannabinoid CB1 Receptors

The vast majority of neurons within the striatum are GABAergic medium spiny neurons (MSNs), which receive glutamatergic input from the cortex and thalamus, and form two major efferent pathways: the direct pathway, expressing dopamine D1 receptor (D1R-MSNs), and the indirect pathway, expressing dopamine D2 receptor (D2R-MSNs). While molecular mechanisms of MSN degeneration have been identified in animal models of striatal damage, the molecular factors that dictate a selective vulnerability of D1R-MSNs or D2R-MSNs remain unknown. Here, we combined genetic, chemogenetic, and pharmacological strategies with behavioral and neurochemical analyses, and show that the pool of cannabinoid CB1 receptor (CB1R) located on corticostriatal terminals efficiently safeguards D1R-MSNs, but not D2R-MSNs, from different insults. This cell-specific response relies on the regulation of glutamatergic signaling, and is independent from the CB1R-dependent control of astroglial activity in the striatum. These findings define cortical CB1R as a pivotal synaptic player in dictating a differential vulnerability of D1R-MSNs versus D2R-MSNs, and increase our understanding of the role of coordinated cannabinergic-glutamatergic signaling in establishing corticostriatal circuits and its dysregulation in neurodegenerative diseases.

Deep Brain Stimulation

High-frequency deep brain stimulation (DBS) is an effective treatment for some movement disorders. Though mechanisms underlying DBS are still unclear, commonly accepted theories include a “functional inhibition” of neuronal cell bodies and the excitation of axonal projections near the electrodes. It is becoming clear, however, that the paradoxical dissociation “local inhibition” and “distant excitation” is far more complex than initially thought. Despite an initial increase in neuronal activity following stimulation, cells are often unable to maintain normal ionic concentrations, particularly those of sodium and potassium. Based on currently available evidence, we proposed an alternative hypothesis. Increased extracellular concentrations of potassium during DBS may change the dynamics of both cells and axons, contributing not only to the intermittent excitation and inhibition of these elements but also to interrupt abnormal pathological activity. In this article, we review mechanisms through which high extracellular potassium may mediate some of the effects of DBS.

Neuronal serine racemase regulates extracellular D-serine levels in the adult mouse hippocampus

In the hippocampus of mice lacking the gene for serine racemase (SR), a D-serine synthesizing enzyme, in the CaMKIIα-expressing neurons, we observed a significant decrease in the extracellular concentration of D-serine, a coagonist for the N-methyl-D-aspartate type glutamate receptor (NMDAR), and NMDAR hypofunction as revealed by diminished extracellular taurine concentrations after an intra-hippocampal NMDA infusion when compared to the wild type controls. Therefore, the neuronal SR could regulate the extracellular D-serine signaling responsible for NMDAR activation in the hippocampus.

Optimal excitation and emission wavelengths to analyze amino acids and optimize neurotransmitters quantification using precolumn OPA-derivatization by HPLC

We describe an analytical methodology to obtain high sensitivity and better resolution through the study of fluorometric excitation (λex) and emission (λem) spectrum wavelengths of OPA-amino acids. The spectrum emission study revealed a maximum signal peak at 450 nm for aspartate and glutamine. For glycine, taurine, and GABA, the maximum signal peak was at 448 and for glutamate at 452 nm. The remaining amino acids analyzed showed a maximum emission around 450 nm. The best signal obtained within the spectrum excitation experiments was using 229- to 450-nm λex-λem. The drawbacks observed at these wavelengths were a baseline drift and negative peaks occurrence. Thus, the excitation wavelength of 240 nm was chosen (240- to 450-nm λex-λem) as a compromise between a very good signal response and a baseline stability to resolve the 18 amino acids studied. Furthermore, this protocol was properly validated. On the other hand, the elution gradient program used for neuroactive amino acids (aspartate, glutamate, glycine, taurine and GABA) showed separation to the baseline, in a 15-min run in all of them. Other amino acids, up to 18, also exhibited a very good separation in a 25-min run. In conclusion, we propose the use of 240- to 450-nm λex-λem wavelengths, in OPA-amino acids analysis, as the most suitable protocol to obtain the best signal response, maintaining an optimum chromatographic resolution.

Modulation of GABA-A receptors of astrocytes and STC-1 cells by taurine structural analogs

Taurine activates and modulates GABA receptors in vivo as well as those expressed in heterologous systems. This study aimed to determine whether the structural analogs of taurine: homotaurine and hypotaurine, have the ability to activate GABA-A receptors that include GABAρ subunits. The expression of GABA-A receptors containing GABAρ has been reported in the STC-1 cells and astrocytes. In both cell types, taurine, homo-, and hypotaurine gated with low efficiency a picrotoxin-sensitive GABA-A receptor. The known bimodal modulatory effect of taurine on GABAρ receptors was not observed; however, differences between the activation and deactivation rates were detected when they were perfused together with GABA. In silico docking simulations suggested that taurine, hypo-, and homotaurine do not form a cation-π interaction such as that generated by GABA in the agonist-binding site of GABAρ. This observation complements the electrophysiological data suggesting that taurine and its analogs act as partial agonists of GABA-A receptors. All the observations above suggest that the structural analogs of taurine are partial agonists of GABA-A receptors that occupy the agonist-binding site, but their structures do not allow the proper interaction with the receptor to fully gate its Cl(-) channel.

Physiological concentrations of zinc reduce taurine-activated GlyR responses to drugs of abuse

Taurine is an endogenous ligand acting on glycine receptors in many brain regions, including the hippocampus, prefrontal cortex, and nucleus accumbens (nAcc). These areas also contain low concentrations of zinc, which is known to potentiate glycine receptor responses. Despite an increasing awareness of the role of the glycine receptor in the rewarding properties of drugs of abuse, the possible interactions of these compounds with zinc has not been thoroughly addressed. Two-electrode voltage-clamp electrophysiological experiments were performed on α1, α2 α1β and α2β glycine receptors expressed in Xenopus laevis oocytes. The effects of zinc alone, and zinc in combination with other positive modulators on the glycine receptor, were investigated when activated by the full agonist glycine versus the partial agonist taurine. Low concentrations of zinc enhanced responses of maximally-effective concentrations of taurine but not glycine. Likewise, chelation of zinc from buffers decreased responses of taurine- but not glycine-mediated currents. Potentiating concentrations of zinc decreased ethanol, isoflurane, and toluene enhancement of maximal taurine currents with no effects on maximal glycine currents. Our findings suggest that the concurrence of high concentrations of taurine and low concentrations of zinc attenuate the effects of additional modulators on the glycine receptor, and that these conditions are more representative of in vivo functioning than effects seen when these modulators are applied in isolation.

Behavioral effects of taurine pretreatment in zebrafish acutely exposed to ethanol

Taurine (TAU) is an amino sulfonic acid that plays protective roles against neurochemical impairments induced by ethanol (EtOH). Mounting evidence shows the applicability of zebrafish for evaluating locomotor parameters and anxiety-like behavioral phenotypes after EtOH exposure in a large scale manner. In this study, we assess the effects of TAU pretreatment on the behavior of zebrafish in the open tank after acute 1% EtOH (v/v) exposure (20 and 60 min of duration) and on brain alcohol contents. The exposure for 20 min exerted significant anxiolytic effects, which were prevented by 42, 150, and 400 mg/L TAU. Conversely, the 60-min condition induced depressant/sedative effects, in which the changes on vertical activity were associated to modifications on the exploratory profile. Although all TAU concentrations kept locomotor parameters at basal levels, 150 mg/L TAU, did not prevent the impairment on vertical activity of EtOH[60]. Despite the higher brain EtOH content detected in the 60-min exposure, 42, 150, and 400 mg/L TAU attenuated the increase of alcohol content in EtOH[60] group. In conclusion, our data suggest that both protocols of acute EtOH exposure induce significant changes in the spatio-temporal behavior of zebrafish and that TAU may exert a preventive role by antagonizing the effects induced by EtOH possibly due to its neuromodulatory role and also by decreasing brain EtOH levels. The hormetic dose-response of TAU on vertical exploration suggests a complex interaction between TAU and EtOH in the central nervous system.

Positive allosteric modulators differentially affect full versus partial agonist activation of the glycine receptor

Taurine acts as a partial agonist at the glycine receptor (GlyR) in some brain regions such as the hippocampus, striatum, and nucleus accumbens. Ethanol, volatile anesthetics, and inhaled drugs of abuse are all known positive allosteric modulators of GlyRs, but their effects on taurine-activated GlyRs remain poorly understood, especially their effects on the high concentrations of taurine likely to be found after synaptic release. Two-electrode voltage-clamp electrophysiology in Xenopus laevis oocytes was used to compare the enhancing effects of ethanol, anesthetics, and inhalants on human homomeric α1-GlyR activated by saturating concentrations of glycine versus taurine. Allosteric modulators had negligible effects on glycine-activated GlyR while potentiating taurine-activated currents. In addition, inhaled anesthetics markedly enhanced desensitization rates of taurine- but not glycine-activated receptors. Our findings suggest that ethanol, volatile anesthetics, and inhalants differentially affect the time courses of synaptic events at GlyR, depending on whether the receptor is activated by a full or partial agonist.

GABA(A) receptor and glycine receptor activation by paracrine/autocrine release of endogenous agonists: more than a simple communication pathway

It is a common and widely accepted assumption that glycine and GABA are the main inhibitory transmitters in the central nervous system (CNS). But, in the past 20 years, several studies have clearly demonstrated that these amino acids can also be excitatory in the immature central nervous system. In addition, it is now established that both GABA receptors (GABARs) and glycine receptors (GlyRs) can be located extrasynaptically and can be activated by paracrine release of endogenous agonists, such as GABA, glycine, and taurine. Recently, non-synaptic release of GABA, glycine, and taurine gained further attention with increasing evidence suggesting a developmental role of these neurotransmitters in neuronal network formation before and during synaptogenesis. This review summarizes recent knowledge about the non-synaptic activation of GABA(A)Rs and GlyRs, both in developing and adult CNS. We first present studies that reveal the functional specialization of both non-synaptic GABA(A)Rs and GlyRs and we discuss the neuronal versus non-neuronal origin of the paracrine release of GABA(A)R and GlyR agonists. We then discuss the proposed non-synaptic release mechanisms and/or pathways for GABA, glycine, and taurine. Finally, we summarize recent data about the various roles of non-synaptic GABAergic and glycinergic systems during the development of neuronal networks and in the adult.

Perinatal taurine exposure alters renal potassium excretion mechanisms in adult conscious rats

Perinatal taurine exposure has long-term effects on the arterial pressure and renal function. This study tests its influence on renal potassium excretion in young adult, conscious rats. Female Sprague-Dawley rats were fed normal rat chow and given water alone (C), 3% beta-alanine in water (taurine depletion, TD) or 3% taurine in water (taurine supplementation, TS), either from conception until delivery (fetal period; TDF or TSF) or from delivery until weaning (lactation period; TDL or TSL). In Experiment 1, male offspring were fed normal rat chow and tap water, while in Experiment 2, beta-alanine and taurine were treated from conception until weaning and then female pups were fed normal rat chow and 5% glucose in drinking water (CG, TDG or TSG) or water alone (CW, TDW or TSW). At 7-8 weeks of age, renal potassium excretion was measured at rest and after an acute saline load (5% of body weight) in conscious, restrained rats. Although all male groups displayed similar renal potassium excretion, TSF rats slightly increased fractional potassium excretion at rest but not in response to saline load, whereas TDF did the opposite. Plasma potassium concentration was only slightly altered by the diet manipulations. In female offspring, none of the perinatal treatments significantly altered renal potassium excretion at rest or after saline load. High sugar intake slightly decreased potassium excretion at rest in TDG and TSG, but only the TDG group displayed a decreased response to saline load. The present data indicates that perinatal taurine exposure only mildly influences renal potassium excretion in adult male and female rats.