Modulation of human GABAρ1 receptors by taurine

Functional Role of Taurine in Aging and Cardiovascular Health: An Updated Overview

Taurine, a naturally occurring sulfur-containing amino acid, has attracted significant attention in recent years due to its potential health benefits. Found in various foods and often used in energy drinks and supplements, taurine has been studied extensively to understand its impact on human physiology. Determining its exact functional roles represents a complex and multifaceted topic. We provide an overview of the scientific literature and present an analysis of the effects of taurine on various aspects of human health, focusing on aging and cardiovascular pathophysiology, but also including athletic performance, metabolic regulation, and neurological function. Additionally, our report summarizes the current recommendations for taurine intake and addresses potential safety concerns. Evidence from both human and animal studies indicates that taurine may have beneficial cardiovascular effects, including blood pressure regulation, improved cardiac fitness, and enhanced vascular health. Its mechanisms of action and antioxidant properties make it also an intriguing candidate for potential anti-aging strategies.

Roles of hydrophilic residues in GABA binding site of GABA-ρ1 receptor explain the addition/inhibition effects of competitive ligands

The orthosteric binding site of GABA-gated ion channels has been widely explored. Many residues in the binding site of GABA were studied. The interactions due to the binding of GABA into the binding site drive channel activation and determine the potency and efficacy of GABA response. The combined effect of a competitive ligand and GABA on GABA-ρ1 receptors has been poorly studied. Here, we used point mutations, molecular modeling, and electrophysiological studies to explore the role of two hydrophilic residues (Serine 168 and Serine 243) of the GABA-ρ1 receptors in response to the binding of GABA and other studied ligands. Our results suggested that Ser168 residue stabilizes either closed state or open conformation depending on the other determinant interactions of each state. On the other hand, Ser243 residue is predicted to form different inter-subunit interactions with residues in the adjacent subunit at different states of the channel. Our current findings enlighten us to reasonably explain the additive/inhibitive effects of applying a competitive ligand with GABA simultaneously. Understanding the mixed effect of potentiation and inhibition would facilitate the discovery of new drugs to work as a direct GABA's activity modulators with more selectivity at various subunits forming GABA-gated ion channels.

The Role of Amino Acids in Neurotransmission and Fluorescent Tools for Their Detection

Neurotransmission between neurons, which can occur over the span of a few milliseconds, relies on the controlled release of small molecule neurotransmitters, many of which are amino acids. Fluorescence imaging provides the necessary speed to follow these events and has emerged as a powerful technique for investigating neurotransmission. In this review, we highlight some of the roles of the 20 canonical amino acids, GABA and β-alanine in neurotransmission. We also discuss available fluorescence-based probes for amino acids that have been shown to be compatible for live cell imaging, namely those based on synthetic dyes, nanostructures (quantum dots and nanotubes), and genetically encoded components. We aim to provide tool developers with information that may guide future engineering efforts and tool users with information regarding existing indicators to facilitate studies of amino acid dynamics.

Safety and efficacy of taurine as an add-on treatment for tics in youngsters

BACKGROUND AND PURPOSE

The pathophysiological model of tics generally describes disruption of γ-aminobutyric acid transmission, and taurine is found to be an agonist of γ-aminobutyric acid receptors. The study aimed to evaluate the safety and efficacy of taurine as an add-on treatment for tics.

METHODS

Four hundred and four youngsters with tic disorders were randomly assigned to 12 weeks of either oral taurine or placebo. The Yale Global Tic Severity Scale was used to measure tic severity. The primary outcome measure was global severity scores reduced by more than 60% compared with baseline scores.

RESULTS

Three hundred and eighty-two patients were successfully treated. At week 4, no significant differences were found in the treatment effect and the total occurrence of adverse drug reactions between the taurine and placebo groups. At week 12, the proportion of significant improvement in tics was significantly higher in the taurine group than the placebo group (53.4% with taurine versus 34.5% without taurine; relative risk 1.546; P < 0.001), and no group differences were found in the total occurrence of adverse drug reactions.

CONCLUSIONS

Taurine is safe and effective for tics.

Taurine potentiates the anticonvulsive effect of the GABAA agonist muscimol and pentobarbital in the immature mouse hippocampus

OBJECTIVE

The high incidence of epileptic seizures in neonates and their frequent refractoriness to pharmacologic therapies require identification of new therapeutical options. Therefore, we investigated whether the modulatory effect of taurine on γ-aminobutyric acid (GABA)_A receptors can enhance the anticonvulsive potential of the GABA_A receptor agonist muscimol and of the barbiturate pentobarbital.

METHODS

We performed field potential recordings in in toto hippocampus preparations of immature (postnatal days 4-7) C57Bl/6 mouse pups. Spontaneous epileptiform activity was induced by the continuous presence of the potassium channel blocker 4-aminopyridine and the glycinergic antagonist strychnine in Mg²⁺ -free solutions.

RESULTS

Bath application of 0.1 μmol/L muscimol increases the occurrence of recurrent epileptiform discharges, whereas they are significantly attenuated in a dose-dependent manner by muscimol in concentrations between 0.5 and 5 μmol/L. Taurine at concentrations between 0.1 and 0.5 mmol/L induces a proconvulsive effect, but upon coapplication, it significantly augments the anticonvulsive effect of moderate muscimol doses (0.5-1 μmol/L). In addition, the anticonvulsive effect of 100 and 200 μmol/L pentobarbital is increased significantly in the presence of 0.5 μmol/L taurine.

SIGNIFICANCE

These observations demonstrate that taurine can indeed enhance the anticonvulsive effects of muscimol and pentobarbital, suggesting that taurine may act as a positive modulator on GABA_A receptors. Thus, interfering with the modulatory taurine binding site of GABA_A receptors or the interstitial taurine concentration may provide new therapeutical options for anticonvulsive therapies in neonates.

Differential modulation of human GABAC-ρ1 receptor by sulfur-containing compounds structurally related to taurine

BACKGROUND

The amino acid taurine (2-Aminoethanesulfonic acid) modulates inhibitory neurotransmitter receptors. This study aimed to determine if the dual action of taurine on GABA_C-ρ1R relates to its structure. To address this, we tested the ability of the structurally related compounds homotaurine, hypotaurine, and isethionic acid to modulate GABA_C-ρ1R.

RESULTS

In Xenopus laevis oocytes, hypotaurine and homotaurine partially activate heterologously expressed GABA_C-ρ1R, showing an increment in its deactivation time with no changes in channel permeability, whereas isethionic acid showed no effect. Competitive assays suggest that hypotaurine and homotaurine compete for the GABA-binding site. In addition, their effects were blocked by the ion-channel blockers picrotixin and Methyl(1,2,5,6-tetrahydropyridine-4-yl) phosphinic acid. In contrast to taurine, co-application of GABA with hypotaurine or homotaurine revealed that the dual effect is present separately for each compound: hypotaurine modulates positively the GABA current, while homotaurine shows a negative modulation, both in a dose-dependent manner. Interestingly, homotaurine diminished hypotaurine-induced currents. Thus, these results strongly suggest a competitive interaction between GABA and homotaurine or hypotaurine for the same binding site. "In silico" modeling confirms these observations, but it also shows a second binding site for homotaurine, which could explain the negative effect of this compound on the current generated by GABA or hypotaurine, during co-application protocols.

CONCLUSIONS

The sulfur-containing compounds structurally related to taurine are partial agonists of GABA_C-ρ1R that occupy the agonist binding site. The dual effect is unique to taurine, whereas in the case of hypotaurine and homotaurine it presents separately; hypotaurine increases and homotaurine decreases the GABA current.

Taurine as an Essential Neuromodulator during Perinatal Cortical Development

A variety of experimental studies demonstrated that neurotransmitters are an important factor for the development of the central nervous system, affecting neurodevelopmental events like neurogenesis, neuronal migration, programmed cell death, and differentiation. While the role of the classical neurotransmitters glutamate and gamma-aminobutyric acid (GABA) on neuronal development is well established, the aminosulfonic acid taurine has also been considered as possible neuromodulator during early neuronal development. The purpose of the present review article is to summarize the properties of taurine as neuromodulator in detail, focusing on the direct involvement of taurine on various neurodevelopmental events and the regulation of neuronal activity during early developmental epochs. The current knowledge is that taurine lacks a synaptic release mechanism but is released by volume-sensitive organic anion channels and/or a reversal of the taurine transporter. Extracellular taurine affects neurons and neuronal progenitor cells mainly via glycine, GABA(A), and GABA(B) receptors with considerable receptor and subtype-specific affinities. Taurine has been shown to directly influence neurogenesis in vitro as well as neuronal migration in vitro and in vivo. It provides a depolarizing signal for a variety of neuronal population in the immature central nervous system, thereby directly influencing neuronal activity. While in the neocortex, taurine probably enhance neuronal activity, in the immature hippocampus, a tonic taurinergic tone might be necessary to attenuate activity. In summary, taurine must be considered as an essential modulator of neurodevelopmental events, and possible adverse consequences on fetal and/or early postnatal development should be evaluated for pharmacological therapies affecting taurinergic functions.

Investigating the Role of Loop C Hydrophilic Residue 'T244' in the Binding Site of ρ1 GABAC Receptors via Site Mutation and Partial Agonism

The loop C hydrophilic residue, threonine 244 lines the orthosteric binding site of ρ1 GABAC receptors was studied by point mutation into serine, alanine and cysteine, and tested with GABA, some representative partial agonists and antagonists. Thr244 has a hydroxyl group essential for GABA activity that is constrained by the threonine methyl group, orienting it toward the binding site. Significant decreases in activation effects of the studied ligands at ρ1 T244S mutant receptors, suggests a critical role for this residue. Results of aliphatic and heteroaromatic partial agonists demonstrate different pharmacological effects at ρ1 T244S mutant receptors when co-applied with GABA EC50 responses. ρ1 T244A and ρ1 T244C mutant receptors have minimal sensitivity to GABA at high mM concentrations, whereas, the ρ1 WT partial agonists, β-alanine and MTSEA demonstrate more efficacy and potency, respectively, than GABA at these mutant receptors. This study explores the role of Thr244 in the binding of agonists as an initial step during channel gating by moving loop C towards the ligand.

Persistent GABAA/C responses to gabazine, taurine and beta-alanine in rat hypoglossal motoneurons

In hypoglossal motoneurons, a sustained anionic current, sensitive to a blocker of ρ-containing GABA receptors, (1,2,5,6-tetrahydropyridin-4-yl)methylphosphinic acid (TPMPA) and insensitive to bicuculline, was previously shown to be activated by gabazine. In order to better characterize the receptors involved, the sensitivity of this atypical response to pentobarbital (30μM), allopregnanolone (0.3μM) and midazolam (0.5μM) was first investigated. Pentobarbital potentiated the response, whereas the steroid and the benzodiazepine were ineffective. The results indicate the involvement of hybrid heteromeric receptors, including at least a GABA receptor ρ subunit and a γ subunit, accounting for the pentobarbital-sensitivity. The effects of the endogenous β amino acids, taurine and β-alanine, which are released under various pathological conditions and show neuroprotective properties, were then studied. In the presence of the glycine receptor blocker strychnine (1μM), both taurine (0.3-1mM) and β-alanine (0.3mM) activated sustained anionic currents, which were partly blocked by TPMPA (100μM). Thus, both β amino acids activated ρ-containing GABA receptors in hypoglossal motoneurons. Bicuculline (20μM) reduced responses to taurine and β-alanine, but small sustained responses persisted in the presence of both strychnine and bicuculline. Responses to β-alanine were slightly increased by allopregnanolone, indicating a contribution of the bicuculline- and neurosteroid-sensitive GABAA receptors underlying tonic inhibition in these motoneurons. Since sustained activation of anionic channels inhibits most mature principal neurons, the ρ-containing GABA receptors permanently activated by taurine and β-alanine might contribute to some of their neuroprotective properties under damaging overexcitatory situations.

Metabolomics study on the effects of Buchang Naoxintong capsules for treating cerebral ischemia in rats using UPLC-Q/TOF-MS

ETHNOPHARMACOLOGICAL RELEVANCE

Buchang Naoxintong Capsules (BNC) are widely prescribed in Chinese medicine for the treatment of cerebrovascular and cardiovascular diseases. However, the therapeutic effects and mechanisms are not yet well understood.

MATERIALS AND METHODS

In this study, a UPLC/TOF-MS-based metabolomic study was conducted to explore potential biomarkers that will increase our understanding of cerebral ischemia and to assess the integral efficacy of BNC in a middle cerebral artery occlusion (MCAO) rat model. Plasma metabolic profiles were analyzed and metabolic biomarkers were identified through multivariate data analysis.

RESULTS

Clear separations were observed between the sham, MCAO and BNC-treated groups. We identified 28 biomarkers in the MCAO rats using variable importance for the projections (VIP) values (VIP>1) and a t-test (P<0.05). The identified biomarkers were mainly related to disturbances in monoamine neurotransmitter metabolism, amino acid metabolism, energy metabolism and lipid metabolism. Moreover, a correlation network diagram of the plasma biomarkers perturbed by MCAO was constructed. Some biomarkers, such as glutamine, PE (17:0), LysoPE (20:1), LysoPE (24:0), and the ratios of LysoPE (24:1) to LysoPE (24:0), LysoPE (24:2) to LysoPE (24:0), showed obvious changes and a tendency for returning to baseline values in BNC-treated MCAO rats. In addition, MCAO rats receiving BNC treatment had improved neurological deficits and reduced cerebral infarct size demonstrating the therapeutic potential of BNC for treating cerebral ischemia.

CONCLUSION

This study provides a useful approach for exploring the mechanism of MCAO-induced cerebral ischemia and evaluating the efficacy of BNC.

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.

GABAA receptors containing ρ1 subunits contribute to in vivo effects of ethanol in mice

GABA_A receptors consisting of ρ1, ρ2, or ρ3 subunits in homo- or hetero-pentamers have been studied mainly in retina but are detected in many brain regions. Receptors formed from ρ1 are inhibited by low ethanol concentrations, and family-based association analyses have linked ρ subunit genes with alcohol dependence. We determined if genetic deletion of ρ1 in mice altered in vivo ethanol effects. Null mutant male mice showed reduced ethanol consumption and preference in a two-bottle choice test with no differences in preference for saccharin or quinine. Null mutant mice of both sexes demonstrated longer duration of ethanol-induced loss of righting reflex (LORR), and males were more sensitive to ethanol-induced motor sedation. In contrast, ρ1 null mice showed faster recovery from acute motor incoordination produced by ethanol. Null mutant females were less sensitive to ethanol-induced development of conditioned taste aversion. Measurement of mRNA levels in cerebellum showed that deletion of ρ1 did not change expression of ρ2, α2, or α6 GABA_A receptor subunits. (S)-4-amino-cyclopent-1-enyl butylphosphinic acid (“ρ1” antagonist), when administered to wild type mice, mimicked the changes that ethanol induced in ρ1 null mice (LORR and rotarod tests), but the ρ1 antagonist did not produce these effects in ρ1 null mice. In contrast, (R)-4-amino-cyclopent-1-enyl butylphosphinic acid (“ρ2” antagonist) did not change ethanol actions in wild type but produced effects in mice lacking ρ1 that were opposite of the effects of deleting (or inhibiting) ρ1. These results suggest that ρ1 has a predominant role in two in vivo effects of ethanol, and a role for ρ2 may be revealed when ρ1 is deleted. We also found that ethanol produces similar inhibition of function of recombinant ρ1 and ρ2 receptors. These data indicate that ethanol action on GABA_A receptors containing ρ1/ρ2 subunits may be important for specific effects of ethanol in vivo.

Characterization of an outward rectifying chloride current of Xenopus tropicalis oocytes

Here, we describe an outward rectifying current in Xenopus tropicalis oocytes that we have called xtClC-or. The current has two components; the major component is voltage activated and independent of intracellular or extracellular Ca(2+), whereas the second is a smaller component that is Ca(2+) dependent. The properties of the Ca(2+)-independent current, such as voltage dependence and outward rectification, resemble those of ClC anion channels/transporters. This current is sensitive to NPPB and NFA, insensitive to 9AC and DIDS, and showed a whole-cell conductance sequence of SCN(-)>I(-)>Br(-)>CI(-). RT-PCR revealed the expression in oocytes of ClC-2 to ClC-7, and major reductions of current amplitudes were observed when a ClC-5 antisense oligonucleotide was injected into oocytes. The Ca(2+)-dependent component was abated after injection of 10mM BAPTA or EGTA, whereas 10mMMg(2+) inhibited the current to 26±3.1%. This component was blocked by 9-AC, NFA, and NPPB, whereas DIDS did not elicit any evident effect. The ion sequence selectivity was SCN=I(-)>Br(-)>Cl(-). To try to determine the molecular identity that gives rise to this component we assessed by RT-PCR the expression of the Ca(2+)-dependent Cl(-) channel TMEM16A, which was found to be present in the oocytes. However, injection of antisense TMEM16A oligonucleotides did not inhibit the transient outward current. This result fits well with the electrophysiological data. Together, these results suggest that ClC-5 is a major, but not the sole channel responsible for this outwardly rectifying Cl(-) current.

Dopamine and serotonin modulate human GABAρ1 receptors expressed in Xenopus laevis oocytes

GABAρ1 receptors are highly expressed in bipolar neurons of the retina and to a lesser extent in several areas of the central nervous system (CNS), and dopamine and serotonin are also involved in the modulation of retinal neural transmission. Whether these biogenic amines have a direct effect on ionotropic GABA receptors was not known. Here, we report that GABAρ1 receptors, expressed in X. laevis oocytes, were negatively modulated by dopamine and serotonin and less so by octopamine and tyramine. Interestingly, these molecules did not have effects on GABA(A) receptors. 5-Carboxamido-tryptamine and apomorphine did not exert evident effects on any of the receptors. Schild plot analyses of the inhibitory actions of dopamine and serotonin on currents elicited by GABA showed slopes of 2.7 ± 0.3 and 6.1 ± 1.8, respectively, indicating a noncompetitive mechanism of inhibition. The inhibition of GABAρ1 currents was independent of the membrane potential and was insensitive to picrotoxin, a GABA receptor channel blocker and to the GABAρ-specific antagonist (1,2,5,6-tetrahydropyridine-4-yl)methyl phosphinic acid (TPMPA). Dopamine and serotonin changed the sensitivity of GABAρ1 receptors to the inhibitory actions of Zn(2+). In contrast, La(3+) potentiated the amplitude of the GABA currents generated during negative modulation by dopamine (EC(50) 146 μM) and serotonin (EC(50) 196 μM). The functional role of the direct modulation of GABAρ receptors by dopamine and serotonin remains to be elucidated; however, it may represent an important modulatory pathway in the retina, where GABAρ receptors are highly expressed and where these biogenic amines are abundant.

Neurochemicals for the investigation of GABA(C) receptors

GABA(C) receptors are being investigated for their role in many aspects of nervous system function including memory, myopia, pain and sleep. There is evidence for functional GABA(C) receptors in many tissues such as retina, hippocampus, spinal cord, superior colliculus, pituitary and the gut. This review describes a variety of neurochemicals that have been shown to be useful in distinguishing GABA(C) receptors from other receptors for the major inhibitory neurotransmitter GABA. Some selective agonists (including (+)-CAMP and 5-methyl-IAA), competitive antagonists (such as TPMPA, (±)-cis-3-ACPBPA and aza-THIP), positive (allopregnanolone) and negative modulators (epipregnanolone, loreclezole) are described. Neurochemicals that may assist in distinguishing between homomeric ρ1 and ρ2 GABA(C) receptors (2-methyl-TACA and cyclothiazide) are also covered. Given their less widespread distribution, lower abundance and relative structural simplicity compared to GABA(A) and GABA(B) receptors, GABA(C) receptors are attractive drug targets.

Taurine regulation of short term synaptic plasticity in fragile X mice

BACKGROUND

Fragile X Syndrome is the most common known genetic cause of autism. The Fmr1-KO mouse, lacks the fragile X mental retardation protein (FMRP), and is used as a model of the syndrome. The core behavioral deficits of autism may be conceptualized either as excessive adherence to patterns as seen in repetitive actions and aberrant language, or as insensitivity to subtle but socially important changes in patterns. The hippocampus receives information from the entorhinal cortex and plays a crucial role in the processing of patterned information. To gain more insight into the physiological function of FMRP and the neuronal mechanisms underlying fragile X syndrome, we examined the electrophysiological response of the hippocampus to pair pulse stimulation as a measure of patterned information processing and how it is affected in the Fmr1-KO mouse.

METHODS

In this study, we used paired-pulse stimulation of the afferent perforant path and recorded from the CA1 region of the hippocampus. Two-month-old FVB/NJ male mice and age-matched Fmr1-KO mice were used in this study. Hippocampal slices were prepared, equilibrated in artificial cerebrospinal fluid (aCSF), and excitatory post synaptic potentials (EPSPs) measured by stimulating the perforant path of the dentate gyrus (DG) while recording from the molecular layer of CA1. Stimulation occurred by setting current and pulse width to evoke a fixed percentage of maximal EPSP amplitude. This stimulation paradigm allowed us to examine the processing capabilities of the hippocampus as a function of increasing interstimulus intervals (ISI) and how taurine, a GABAA receptor agonist, affects such information processing.

RESULTS

We found that hippocampal slices from wild type (WT) showed pair-pulse facilitation at ISI of 100-300 ms whereas slices from Fmr1-KO brains showed a consistent pair-pulse depression at a comparable ISI. Addition of 10 muM taurine to WT slices resulted in a drastic decrease of the peak response to the second stimulus, resulting in an initial depression at 100 ms ISI followed by potentiation at higher ISI (150 ms and above). In the presence of taurine, the amplitude of the second response remained significantly lower than in its absence. Fmr1-KO mice however, were completely insensitive to taurine application and pair-pulse stimulation always resulted in a depression of the response to the second stimulus.

CONCLUSIONS

Previously we reported that Fmr1-KO mice have reduced beta subunits of the GABAA receptors. We also showed as well as others that taurine acts as an agonist or a modulator for GABAA receptors. Therefore, the insensitivity of Fmr1-KO slices to taurine application could be due to the reduced binding sites on the GABAA receptors in the Fmr1-KO mice.

Functional impact of serial deletions at the C-terminus of the human GABArho1 receptor

GABArho1 receptors are formed by homopentameric assemblies that gate a chloride ion-channel upon activation by the neurotransmitter. Very little is known about the structural and functional roles played by the different domains that form each subunit; but one of them, the fourth transmembrane segment (TM4), is known to form a hydrophobic bundle together with three other TM segments that are necessary to stabilize the structure of the receptor. In this study we progressively removed amino acid residues from the C-terminus of the human GABArho1 and studied the functional properties of the receptor mutants expressed in X. laevis oocytes. We found that deletions of up to the last four residues gave rise to receptors that were still functional, generating currents of 3.92 microA for the wt, 5.75 microA for S479X, 1.82 microA for F478X, 0.52 microA for I477X and 0.27 microA for S476X when exposed to 5 microM GABA; surprisingly, the mutant with one residue removed resulted more sensitive to the agonists. Further deletions, up to residue W475, resulted in receptors that did not gate an ion-channel. In addition, deleting the signal sequence, from R2-A15, in the N-terminus produced non-functional receptors. This study reveals that GABArho1 can tolerate removal of several residues that form the fourth transmembrane segment up to a critical point, signaled by W475, beyond which the mutant protein is translated but does not form functional receptors. A comparative study is presented of some electrophysiological and pharmacological properties of the deletion mutants that were able to generate GABA currents.

Effect of taurine and caffeine on sleep-wake activity in Drosophila melanogaster

Caffeine and taurine are two major neuromodulators present in large quantities in many popular energy drinks. We investigated their effects on sleep-wake control in constant darkness using the fruit fly Drosophila as a model system. It has been shown that caffeine, as the most widely used psychostimulant, can boost arousal through the dopamine pathway in the mushroom bodies of flies. Taurine is a GABA receptor agonist, which is inhibitory to neuronal firing. We show here that flies receiving a low dose of caffeine (0.01%) increase locomotor activity by 25%, and decrease total sleep by 15%. Treatment with taurine at 0.1% to 1.5% reduces locomotor activity by 28% to 86%, and shifts it from diurnal to nocturnal. At 0.75%, taurine also increases total sleep by 50%. Our results show that taurine increases sleep, while caffeine, as previously reported, attenuates sleep. Flies treated with both caffeine and taurine exhibit two differential effects which depend upon the ratio of taurine to caffeine. A high taurine:caffeine ratio promotes sleep, while a low ratio of taurine:caffeine inhibits sleep to a greater extent than the equivalent amount of caffeine alone. This intriguing enhancement of caffeine action by low doses of taurine may account for the presence of both compounds in energy-promoting drinks such as Red Bull® and Monster®.

Review of recent advances in analytical techniques for the determination of neurotransmitters

Methods and advances for monitoring neurotransmitters in vivo or for tissue analysis of neurotransmitters over the last five years are reviewed. The review is organized primarily by neurotransmitter type. Transmitter and related compounds may be monitored by either in vivo sampling coupled to analytical methods or implanted sensors. Sampling is primarily performed using microdialysis, but low-flow push-pull perfusion may offer advantages of spatial resolution while minimizing the tissue disruption associated with higher flow rates. Analytical techniques coupled to these sampling methods include liquid chromatography, capillary electrophoresis, enzyme assays, sensors, and mass spectrometry. Methods for the detection of amino acid, monoamine, neuropeptide, acetylcholine, nucleoside, and soluble gas neurotransmitters have been developed and improved upon. Advances in the speed and sensitivity of these methods have enabled improvements in temporal resolution and increased the number of compounds detectable. Similar advances have enabled improved detection at tissue samples, with a substantial emphasis on single cell and other small samples. Sensors provide excellent temporal and spatial resolution for in vivo monitoring. Advances in application to catecholamines, indoleamines, and amino acids have been prominent. Improvements in stability, sensitivity, and selectivity of the sensors have been of paramount interest.

Activation of the tonic GABAC receptor current in retinal bipolar cell terminals by nonvesicular GABA release

Within the second synaptic layer of the retina, bipolar cell (BC) output to ganglion cells is regulated by inhibitory input to BC axon terminals. GABA(A) receptors (GABA(A)Rs) mediate rapid synaptic currents in BC terminals, whereas GABA(C) receptors (GABA(C)Rs) mediate slow evoked currents and a tonic current, which is strongly regulated by GAT-1 GABA transporters. We have used voltage-clamp recordings from BC terminals in goldfish retinal slices to determine the source of GABA for activation of these currents. Inhibition of vesicular release with concanamycin A or tetanus toxin significantly inhibited GABA(A)R inhibitory postsynaptic currents and glutamate-evoked GABA(A)R and GABA(C)R currents but did not reduce the tonic GABA(C)R current, which was also not dependent on extracellular Ca(2+). The tonic current was strongly potentiated by inhibition of GABA transaminase, under both normal and Ca(2+)-free conditions, and was activated by exogenous taurine; however inhibition of taurine transport had little effect. The tonic current was unaffected by GAT-2/3 inhibition and was potentiated by GAT-1 inhibition even in the absence of vesicular release, indicating that it is unlikely to be evoked by reversal of GABA transporters or by ambient GABA. In addition, GABA release does not appear to occur via hemichannels or P2X(7) receptors. BC terminals therefore exhibit two forms of GABA(C)R-mediated inhibition, activated by vesicular and by nonvesicular GABA release, which are likely to have distinct functions in visual signal processing. The tonic GABA(C)R current in BC terminals exhibits similar properties to tonic GABA(A)R and glutamate receptor currents in the brain.