Taurine-induced synaptic potentiation and the late phase of long-term potentiation are related mechanistically

Mitigation of aging-related plasticity decline through taurine supplementation and environmental enrichment

Aging-related biochemical changes in nerve cells lead to dysfunctional synapses and disrupted neuronal circuits, ultimately affecting vital processes such as brain plasticity, learning, and memory. The imbalance between excitation and inhibition in synaptic function during aging contributes to cognitive impairment, emphasizing the importance of compensatory mechanisms. Fear conditioning-related plasticity of the somatosensory barrel cortex, relying on the proper functioning and extensive up regulation of the GABAergic system, in particular interneurons containing somatostatin, is compromised in aging (one-year-old) mice. The present research explores two potential interventions, taurine supplementation, and environmental enrichment, revealing their effectiveness in supporting learning-induced plasticity in the aging mouse brain. They do not act through a mechanism normalizing the Glutamate/GABA balance that is disrupted in aging. Still, they allow for increased somatostatin levels, an effect observed in young animals after learning. These findings highlight the potential of lifestyle interventions and diet supplementation to mitigate age-related cognitive decline by promoting experience-dependent plasticity.

Tributyrin reverses the deleterious effect of saturated fat on working memory and synaptic plasticity in juvenile mice: differential effects in males and females

Short-chain fatty acids, such as butyric acid, derived from the intestinal fermentation of dietary fiber, have been proposed as a treatment for certain pathologies of the central nervous system. Our research group has shown that tributyrin (TB), a butyric acid prodrug, reverses deficits in spatial memory and modulates hippocampal synaptic plasticity. In the present work, diets enriched in either saturated (SOLF; Saturated OiL-enriched Food) or unsaturated (UOLF; Unsaturated OiL-enriched Food) fat were supplied during either 2 h or 8 weeks to 5-week-old male and female mice undergoing a treatment schedule with TB. After the dietary treatment, spatial learning and memory (SLM) was assessed in both the Y-maze and the eight-arm radial maze (RAM). Hippocampal expression of genes involved in glutamatergic transmission as well as synaptic plasticity (long-term potentiation -LTP- and long-term depression -LTD-) were also analyzed. Our results show that 2 h of SOLF intake impaired LTP as well as the performance in the Y-Maze in juvenile male mice whereas no effect was found in females. Moreover, TB reversed both effects in SLM and LTP in males. In the case of chronic intake, both SOLF and UOLF deteriorated SLM measured in the RAM in both sexes whereas TB only reversed LTP impairment induced by SOLF in male mice. These results suggest that TB may have a potentially beneficial influence on learning and memory processes, contingent upon the type of diet and the sex of the individuals.

Saturated and unsaturated fat diets impair hippocampal glutamatergic transmission in adolescent mice

Consumption of high-fat diets (HFD) has been associated with neuronal plasticity deficits and cognitive disorders linked to the alteration of glutamatergic disorders in the hippocampus. As young individuals are especially vulnerable to the effects of nutrients and xenobiotics on cognition, we studied the effect of chronic consumption of saturated (SOLF) and unsaturated oil-enriched foods (UOLF) on: i) spatial memory; ii) hippocampal synaptic transmission and plasticity; and iii) gene expression of glutamatergic receptors and hormone receptors in the hippocampus of adolescent and adult mice. Our results show that both SOLF and UOLF impair spatial short-term memory. Accordingly, hippocampal synaptic plasticity mechanisms underlying memory, and gene expression of NMDA receptor subunits are modulated by both diets. On the other hand, PPARγ gene expression is specifically down-regulated in adolescent SOLF individuals and up-regulated in adult UOLF mice.

Intermittent-Excessive and Chronic-Moderate Ethanol Intake during Adolescence Impair Spatial Learning, Memory and Cognitive Flexibility in the Adulthood

Intermittent and excessive ethanol consumption over very short periods of time, known as binge drinking, is common in the adolescence, considered a vulnerable period to the effects of alcohol in terms of cognitive performance. One of the brain functions most drastically affected by ethanol in adolescent individuals seems to be spatial learning and memory dependent on the hippocampus. In the current study we have focused on the long-lasting effects on spatial learning and memory of intermittent and excessive alcohol consumption compared to chronic and moderate alcohol exposure during adolescence. Five-week old male Wistar rats consumed ethanol for 24 days following two different self-administration protocols that differed in the intake pattern. Spatial learning and memory were evaluated in the radial arm maze. Hippocampal synaptic plasticity was assessed by measuring field excitatory postsynaptic potentials. Hippocampal expression of AMPA and NMDA receptor subunits as well as levels of phosphorylated Ser⁹-GSK3β (the inactive form of GSK3β) were also quantified. Our results show that both patterns of ethanol intake during adolescence impair spatial learning, memory and cognitive flexibility in the adulthood in a dose-dependent way. Nevertheless, changes in synaptic plasticity, gene expression and levels of inactive GSK3β depended on the pattern of ethanol intake.

Taurine protects dopaminergic neurons in a mouse Parkinson's disease model through inhibition of microglial M1 polarization

Microglia-mediated neuroinflammation is implicated in multiple neurodegenerative disorders, including Parkinson’s disease (PD). Hence, the modulatioein of sustained microglial activation may have therapeutic potential. This study is designed to test the neuroprotective efficacy of taurine, a major intracellular free β-amino acid in mammalian tissues, by using paraquat and maneb-induced PD model. Results showed that mice intoxicated with paraquat and maneb displayed progressive dopaminergic neurodegeneration and motor deficits, which was significantly ameliorated by taurine. Taurine also attenuated the aggregation of α-synuclein in paraquat and maneb-intoxicated mice. Mechanistically, taurine suppressed paraquat and maneb-induced microglial activation. Moreover, depletion of microglia abrogated the dopaminergic neuroprotective effects of taurine, revealing the role of microglial activation in taurine-afforded neuroprotection. Subsequently, we found that taurine suppressed paraquat and maneb-induced microglial M1 polarization and gene expression levels of proinflammatory factors. Furthermore, taurine was shown to be able to inhibit the activation of NADPH oxidase (NOX2) by interfering with membrane translocation of cytosolic subunit, p47^phox and nuclear factor-kappa B (NF-κB) pathway, two key factors for the initiation and maintenance of M1 microglial inflammatory response. Altogether, our results showed that taurine exerted dopaminergic neuroprotection through inactivation of microglia-mediated neuroinflammation, providing a promising avenue and candidate for the potential therapy for patients suffering from PD.

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.

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.

Cocaine facilitates protein synthesis-dependent LTP: the role of metabotropic glutamate receptors

Cocaine addiction alters synaptic plasticity in many brain areas involved in learning and memory processes, including the hippocampus. Long-term potentiation (LTP) is one of the best studied examples of hippocampal synaptic plasticity and it is considered as one of the molecular basis of learning and memory. We previously demonstrated that in the presence of cocaine, a long lasting form of hippocampal LTP is induced by a single pulse of high frequency stimulation, which in normal conditions evokes only an early form of LTP. In this study, we further explore the molecular basis of this modulation of synaptic plasticity by cocaine. By performing pharmacological experiments on hippocampal slices, we were able to show that cocaine converts early LTP to a form of LTP dependent on protein synthesis, probably through the cAMP-dependent protein kinase and extracellular signal-regulated kinase signaling cascades. We also found that metabotropic glutamate receptors are involved in this phenomenon. These studies further clarify the molecular machinery used by cocaine to alter synaptic plasticity and modulate learning and memory processes.

Intracerebroventricular administration of taurine impairs learning and memory in rats

OBJECTIVES

Taurine is a semi-essential amino acid widely distributed in the body and we take in it from a wide range of nutritive-tonic drinks to improve health. To date, we have elucidated that oral supplementation of taurine does not affect learning and memory in the rat. However, there are few studies concerning the direct effects of taurine in the brain at the behavior level. In this study, we intracerebroventricularly administered taurine to rats and aimed to elucidate the acute effects on learning and memory using the Morris water maze method.

METHODS

Escape latency, swim distance, and distance to zone, which is the integral of the distance between the rats and the platform for every 0.16 seconds, were adopted as parameters of the ability of learning and memory. We also tried to evaluate the effect of intraperitoneal taurine administration.

RESULTS

Escape latency, swim distance, and distance to zone were significantly longer in the intracerebroventricularly taurine-administered rats than in the saline-administered rats. Mean swimming velocity was comparable between these two groups, although the physical performance was improved by taurine administration. Probe trials showed that the manner of the rats in finding the platform was comparable. In contrast, no significant differences were found between the intraperitoneally taurine-administered rats and the saline-administered rats.

DISCUSSION

These results indicate that taurine administered directly into the brain ventricle suppresses and delays the ability of learning and memory in rats. In contrast, it is implied that taurine administered peripherally was not involved in learning and memory.

Role of glycine receptors in glycine-induced LTD in hippocampal CA1 pyramidal neurons

Glycine in the hippocampus can exert its effect on both synaptic NMDA receptors (NMDARs) and extrasynaptic functional glycine receptors (GlyRs) via distinct binding sites. Previous studies have reported that glycine induces long-term potentiation (LTP) through the activation of synaptic NMDARs. However, little is known about the potential role of the activated GlyRs that are largely located in extrasynaptic regions. We report here that relatively high levels of glycine achieved either by exogenous glycine application or by the elevation of endogenous glycine accumulation with an antagonist of the glycine transporter induced long-term depression (LTD) of excitatory postsynaptic currents (EPSCs) in hippocampal CA1 pyramidal neurons. The co-application of glycine with the selective GlyR antagonist strychnine changed glycine-induced LTD (Gly-LTD) to LTP. Blocking the postsynaptic GlyR-gated net chloride flux by manipulating intracellular chloride concentrations failed to elicit any changes in EPSCs. These results suggest that GlyRs are involved in Gly-LTD. Furthermore, this new form of chemical LTD was accompanied by the internalization of postsynaptic AMPA receptors and required the activation of NMDARs. Therefore, our present findings reveal an important function of GlyR activation and modulation in gating the direction of synaptic plasticity.

Chronic periadolescent cannabinoid treatment enhances adult hippocampal PSA-NCAM expression in male Wistar rats but only has marginal effects on anxiety, learning and memory

Pubertal and adolescent exposure to cannabinoids is associated with enduring alterations in anxiety and memory. However, periadolescence virtually remains unexplored. Here, we measured anxiety in the Elevated Plus Maze (EPM) in adult Wistar rats treated at periadolescence (P28-P38) with the cannabinoid agonist CP 55,940 (CP) (0.4 mg/kg; 2 ml/kg i.p., 1 daily injection), and we also defined their recognition memory in the novel object paradigm and spatial learning and memory in the water maze. Additionally, we measured the expression of hippocampal PSA-NCAM (Polysialic Acid-Neural Cell Adhesion Molecule) and long-term potentiation (LTP) as well as, given their role in mnemonic processing, the levels of plasma corticosterone and estradiol. We found that CP had no robust effects on anxiety or in recognition memory. In the water maze, only a slight decreased percentage of failed trials in the reference memory task and an improvement in an indirect index of attention were observed. However, we detected an up-regulation of hippocampal PSA-NCAM expression, only in CP-males, although this effect was not related to changes in LTP. No hormonal alterations were evident. Based on our data, minimal long-term effects on anxiety, learning and memory appear to result from cannabinoid exposure during the periadolescent period.

Domoic acid induces a long-lasting enhancement of CA1 field responses and impairs tetanus-induced long-term potentiation in rat hippocampal slices

Domoic acid (DOM) is known to cause hippocampal neuronal damage and produces amnesic effects. We examined synaptic plasticity changes induced by DOM exposure in rat hippocampal CA1 region. Brief bath application of DOM to hippocampal slices produces a chemical form of long-term potentiation (LTP) of CA1 field synaptic potentials. The potentiation cannot be blocked by NMDA receptor antagonist MK-801 but can be blocked by the calcium-calmodulin-dependent protein kinase II (CaMKII) inhibitor KN-62 or cAMP-dependent protein kinase (PKA) inhibitor H-89. DOM-potentiated slices show decreased autophosphorylated CaMKII (p-Thr286), an effect that is also dependent on the activity of CaMKII and PKA. Increased phosphorylation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor subunit GluR1 (p-Ser831) was seen in DOM-potentiated slices. Therefore, aberrant regulation of CaMKII and GluR1 phosphorylation occurs after DOM application. In addition, tetanus-induced LTP as well as the increase of phosphorylation of CaMKII (p-Thr286) were reduced in DOM-potentiated slices. Compared with brief exposure, slices recovering from prolonged exposure did not show potentiation or altered levels of CaMKII (p-Thr286) or GluR (p-Ser831). However, decreased phosphorylation of GluR1 at Ser845 was seen. These results describe a new chemical form of LTP and uncover novel molecular changes induced by DOM. The observed impairment of tetanus LTP and misregulation of CaMKII and GluR1 phosphorylation may partially account for DOM neurotoxicity and underlie the molecular basis for DOM-induced memory deficit.

Pleiotrophin inhibits hippocampal long-term potentiation: a role of pleiotrophin in learning and memory

Pleiotrophin (PTN) is a growth factor that has been shown to be involved in hippocampal synaptic plasticity and learning. To further understand the involvement of PTN in memory processes, we performed in vitro electrophysiological studies in PTN-stimulated CA1 from rat hippocampal slices combined with the behavioural testing of PTN deficient (PTN - / - ) mice. We found that PTN inhibited hippocampal long-term potentiation (LTP) induced by high-frequency stimulation (HFS) consisted in three trains of 100 Hz separated by 20 s. To test the possibility that PTN might be involved in behavioural memory processes, we tested the learning behaviour of PTN - / - mice using the Y-maze test. We did not observe significant differences in recognition memory between PTN - / - and Wild Type (WT) mice when a 30 min-interval intertrial (ITI) was used in the Y-maze test. However, whereas WT mice showed disruption of recognition memory using a 60 min-ITI, PTN - / - mice maintained the recognition memory. The data demonstrate that PTN inhibits hippocampal LTP in vitro and might play a role in memory processes in vivo.

Glycine-induced long-term synaptic potentiation is mediated by the glycine transporter GLYT1

The negative symptoms of schizophrenia are reverted by treatment with glycine or other agonists of the glycine-B site which facilitate NMDA receptor function. On the other hand, there are experimental observations showing that exogenous application of glycine (0.5-10mM) results in a long-lasting potentiation of glutamatergic synaptic transmission (LTP-GLY). The characterization of the mechanisms underlying LTP-GLY could be useful to develop new therapies for schizophrenia. The main goal of this work is to deepen the understanding of this potentiation phenomenon. The present study demonstrates in rat hippocampal slices that superfusion of glycine 1mM during 30 min produces a potentiation of excitatory postsynaptic potentials in CA3-CA1 pathway lasting at least 1h. Glycine application does not modify neither presynaptic fiber volley nor paired-pulse facilitation of synaptic potentials. This LTP-GLY is independent of both strychnine-sensitive glycine receptors and nifedipine-sensitive calcium channels. Interestingly, LTP-GLY is not inhibited but strengthened by NMDA receptors antagonists such as AP-5 or MK-801. In contrast, LTP-GLY is partially or totally blocked with the antagonists of glycine transporter GLYT1, sarcosine or ALX-5407, respectively. These results indicate that LTP-GLY requires the activation of GLYT1, a glycine transporter co-localized and associated to NMDA receptors. In addition, the fact that NMDA receptor inhibition increases LTP-GLY magnitude, opens the possibility that these receptors could have a negative control on GLYT1 activity.

Toluene induces rapid and reversible rise of hippocampal glutamate and taurine neurotransmitter levels in mice

Toluene, a widely used aromatic organic solvent, has been well characterized as a neurotoxic chemical. Although the neurobehavioral effects of toluene have been studied substantially, the mechanisms involved are not clearly understood. Hippocampus, which is one of the limbic areas of brain associated with neuronal plasticity, and learning and memory functions, may be a principal target of toluene. In the present study, to establish a mouse model for investigating the effects of acute toluene exposure on the amino acid neurotransmitter levels in the hippocampus, in vivo microdialysis study was performed in freely moving mice after a single intraperitoneal administration of toluene (150 and 300 mg/kg). Amino acid neurotransmitters in microdialysates were measured by a high performance liquid chromatography system. The extracellular levels of glutamate and taurine were rapidly and reversibly increased within 30 min after the toluene administration in a dose-dependent manner and returned to the basal level by 1h. Conversely, the extracellular level of glycine and GABA were stable, and no significant change was observed after the toluene administration. To further investigate the brain toluene level in the hippocampus of toluene-administered mice, we used a solid-phase microextraction (SPME) method and examined the time course changes of toluene in the hippocampus of living mice. The brain toluene level reached the peak at 30 min after injection and returned to the basal level after 2h. In the present study, we observed the relationship between brain toluene levels and amino acid neurotransmitter glutamate and taurine levels in the hippocampus. Therefore, we suggest that toluene may mediate its action through the glutamatergic and taurinergic neurotransmission in the hippocampus of freely moving mice.

Enhancement of hippocampal long-term potentiation induced by cocaine self-administration is maintained during the extinction of this behavior

Drug addiction may involve learning and memory processes requiring the participation of hippocampal formation. One of the best studied examples of hippocampal synaptic plasticity is the long-term potentiation (LTP) which usually occurs when hippocampal synapses are stimulated with high-frequency stimulation. The aim of this work has been to study the effect of extinction of cocaine self-administration behavior on synaptic plasticity in rat hippocampal slices. LTP was induced using a tetanization paradigm consisting of a single train of high-frequency (100 Hz) stimulation for one second. This tetanization protocol evoked a greater and more perdurable LTP in slices obtained after 10 days of extinction of cocaine self-administration (1 mg/kg/injection) than that elicited in slices from saline self-administering (0.9% NaCl) animals. In addition, this LTP facilitation in animals which have followed the cocaine self-administration extinction protocol was very similar to that obtained in slices from cocaine self-administering animals. These results suggest that chronic cocaine self-administration induces enduring neuroadaptive changes in hippocampal synaptic plasticity which last even after the extinction of this behavior and that they may be involved in cocaine dependence.

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA(A) and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.

Hippocampal Synaptic Plasticity and Water Maze Learning in Cocaine Self-Administered Rats

Previously, we have shown that long-term potentiation (LTP) in hippocampus of Lewis rats was significantly modulated by cocaine self-administration. Using a single train of high-frequency stimulation of 100 Hz for 1s (HFS), we found an enhancement of LTP after cocaine self-administration that was maintained even during the extinction of this behavior. However, the effects of cocaine self-administration on a hippocampal-dependent spatial learning task were unknown. Therefore, in the present study our first objective was to analyze if cocaine self-administration might affect the performance in a hippocampus-dependent task, such as the Morris water maze test. Male adult Lewis (LEW) rats self-administered cocaine (1 mg/kg/injection) or saline (0.9% NaCl) for 3 weeks. Three hours after finishing the last self-administration session, animals were submitted to Morris water maze training for 3 consecutives days. A memory test was carried out 24 h after the last training session. No significant differences were found in escape latencies and time spent in the quadrant where the platform was located during training. Given that we did not find any cocaine effect on this spatial learning task, our second objective was to estimate indirectly if brain cocaine levels have failed to modulate LTP in animals that were performing the water maze trials. To this end, we tested if cocaine application to hippocampal slices of naïve subjects was able to evoke LTP. The results indicated that cocaine produced an enhanced LTP in these hippocampal slices. Taking together, the results of the present study suggest that hippocampal LTP-like processes generated after cocaine self-administration are not related to spatial learning hippocampal-dependent tasks, such as the water maze test.

Mechanisms of long-lasting enhancement of corticostriatal neurotransmission by taurine

The long-lasting enhancement of corticostriatal neurotransmission by taurine, LLE-TAU represents a complex phenomenon requiring concurrent activation of glycine, DA and Ach receptors as well as taurine uptake. The data on the mechanisms of corticostriatal LLE-TAU can be integrated in the following scheme. Taurine interaction with glycine and GABAA receptors causes depolarization of striatal medium spiny cells (Chepkova et al., 2002) which is enhanced by taurine electrogenic uptake by TauT (Sarkar et al., 2003). This depolarization leads to Ca2+ entry via low voltage gated Ca2+ channels. Muscarinic M1 receptors are expressed in medium spiny neurons (Yan et al., 2001) and regulate their excitability mostly via phospholipase C (PLC)/PKC cascade (Lin et al., 2004). Concurrent activation of M1 and PLC-coupled D1 receptors (O'Sullivan et al., 2004) can amplify Ca2+ signal via IP3- stimulated Ca2+ release from intracellular stores and stimulate PKC.

Protection by a taurine supplemented diet from lead-induced deficits of long-term potentiation/depotentiation in dentate gyrus of rats in vivo

Previous studies have demonstrated that synaptic plasticity, which includes long-term potentiation (LTP) and depotentiation (DP) in hippocampus, is important for learning and memory. The purpose of this study is to evaluate the effect of taurine via drinking water on the lead-induced impairments of LTP and DP in rat dentate gyrus (DG) in vivo. The experiments were carried out in four groups of rats (control, lead-exposed, control and lead-exposed with a taurine-supplement diet, respectively). The input-output (I/O) function, excitatory postsynaptic potential (EPSP) and population spike (PS) amplitude were measured in the DG area of adult rats (60-90 days) in response to stimulation applied to the lateral perforant path. The results show that: 1. chronic lead exposure impaired LTP/DP measured on both EPSP slope and PS amplitude in DG area of the hippocampus; 2. in control rats, taurine had no effect on LTP/DP; 3. the amplitudes of LTP/DP of lead-exposed group were significantly increased by applying taurine. These results suggest that dietary taurine supplement could protect rats from the lead-induced impairments of synaptic plasticity and might be a preventive medicine to cure the cognitive deficits induced by lead.

What turns CREB on?

The transactivation domain of the cAMP response element-binding protein (CREB) consists of two major domains. The glutamine-rich Q2 domain, which interacts with the general transcription factor TAFII130/135, is sufficient for the recruitment of a functional RNA polymerase II complex and allows basal transcriptional activity. The kinase-inducible domain, however, mediates signal-induced activation of CREB-mediated transcription. It is generally believed that recruitment of the coactivators CREB-binding protein (CBP) and p300 after signal-induced phosphorylation of this domain at serine-133 strongly enhances CREB-dependent transcription. Transcriptional activity of CREB can also be potentiated by phosphoserine-133-independent mechanisms, and not all stimuli that provoke phosphorylation of serine-133 stimulate CREB-dependent transcription. This review presents an overview of the diversity of stimuli that induce CREB phosphorylation at Ser-133, focuses on phosphoserine-133-dependent and -independent mechanisms that affect CREB-mediated transcription, and discusses different models that may explain the discrepancy between CREB Ser-133 phosphorylation and activation of CREB-mediated transcription.

Role of taurine uptake on the induction of long-term synaptic potentiation

Taurine application in the CA1 area of rat hippocampal slices induces a long-lasting potentiation of excitatory synaptic transmission that has some mechanistic similitude with the late phase of long-term potentiation (L-LTP). Previous indirect evidence such as temperature and sodium dependence indicated that taurine uptake is one of the primary steps leading to the taurine-induced synaptic potentiation. We show that taurine-induced potentiation is not related to the intracellular accumulation of taurine and is not impaired by 2-guanidinoethanesulphonic acid, a taurine transport inhibitor that is a substrate of taurine transporter. We have found that taurine uptake in hippocampal synaptosomes was inhibited by SKF 89976A, a GABA uptake blocker that is not transportable by GABA transporters. SKF 89976A prevents the induction of synaptic potentiation by taurine application. This effect is neither mimicked by nipecotic acid, a broad inhibitor of GABA transporters that does not affect taurine uptake, nor by NO-711, a specific and potent inhibitor of GABA transporter GAT-1. In addition, L-LTP induced by trains of high-frequency stimulation is also inhibited by SKF 89976A, and taurine, at a concentration that does not change basal synaptic transmission, overcomes such inhibition. We conclude that taurine induces synaptic potentiation through the activation of a system transporting taurine and that taurine uptake is required for the induction of synaptic plasticity phenomena such as L-LTP.

Amino acid neurotransmitter release and learning: a study of visual imprinting

The intermediate and medial part of the hyperstriatum ventrale (IMHV) is an area of the domestic chick forebrain that stores information acquired through the learning process of imprinting. The effects of visual imprinting on the release of the amino acids aspartate, arginine, citrulline, gamma-aminobutyric acid (GABA), glutamate, glycine and taurine from the left and right IMHVs in vitro were measured at 3.5, 10 and 24 h after training. Chicks were exposed to an imprinting stimulus for 1 h, their preferences measured 10 min afterward and a preference score calculated as a measure of the strength of learning. Potassium stimulation was used to evoke amino acid release from the IMHVs of trained and untrained chicks in the presence and absence of extracellular Ca2+. Ca2+-dependent, K+-evoked release of glutamate was significantly (34.4%) higher in trained than in untrained chicks. This effect was not influenced by time after training or by side (left or right IMHV). Training influenced the evoked release of GABA and taurine from the left IMHV at both 3.5 and 10 h. The training effects at the two times were statistically homogeneous so data (< or = 10 h group) were combined for each amino acid respectively. For this < or = 10 h group, evoked release increased significantly with preference score. In contrast, for the 24 h group, evoked release of GABA and taurine was not significantly correlated with preference score. There were no significant correlations between preference score and GABA or taurine release in the right IMHV at any time, nor in the absence of extracellular calcium. No significant effects of training condition, time or side were observed for any other amino acid in the study. The present findings suggest that soon after chicks have been exposed to an imprinting stimulus glutamatergic excitatory transmission in IMHV is enhanced, and remains enhanced for at least 24 h. In contrast, the learning-related elevations in taurine and GABA release are not sustained over this period. The change in GABA release may reflect a transient increase in inhibitory transmission in the left IMHV.

Behavioral reinforcement of long-term potentiation in rat dentate gyrus in vivo is protein synthesis-dependent

A transient, protein synthesis-independent long-term potentiation (early-LTP, <4 h) can be reinforced into a maintained protein synthesis-dependent late-LTP (>4 h) by specific electrical stimulation of limbic structures (J. Neurosci. 21 (2001) 3697). Similarly, LTP-modulation can be obtained by behavioral stimuli with strong motivational content. However, the requirement of protein synthesis during behavioral reinforcement has not been shown so far. Thus, we have studied here this specific question using a behavioral reinforcement protocol, i.e. allowing water-deprived animals to drink 15 min after induction of early-LTP. This procedure transformed early-LTP into late-LTP. Anisomycin, a reversible protein synthesis inhibitor, abolished behavioral LTP-reinforcement. These results demonstrate that behavioral reinforcement depends on protein synthesis.