Kainic acid-induced changes of extracellular amino acid levels, evoked potentials and EEG activity in the rabbit olfactory bulb

Significance of Taurine in the Brain

Two main functions of taurine in the brain are here discussed: the role of taurine in cell volume regulation and the neuromodulatory actions of taurine liberated by depolarization. Taurine takes part in cell volume regulation with other small-molecular compounds. Extracellular taurine inhibits neuronal firing through GABA and glycine receptors. However, the existence of specific taurine receptors is still not excluded.

Reduction of brain taurine: Effects on neurotoxic and metabolic actions of kainate

The effects of chronic administration of 2-guanidinoethane sulfonic acid on the levels of intra- and extracellular amino acids in the rat hippocampus were studied. The tissue content of taurine was selectively reduced by almost one third after 9 days of peroral administration of 1% 2-guanidinoethane sulfonate. Extracellular levels of amino acids were monitored with the brain microdialysis method. The taurine concentration in the extracellular fluid was depressed in relation to the decrease in intracellular taurine. Unexpectedly, extracellular (but not intracellular) glutamate was doubled in 2-guanidinoethane sulfonate treated animals. The kainic acid evoked release of taurine was suppressed in the 2-guanidinoethane sulfonate group, whereas the kainate stimulated efflux of glutamate was elevated after 2-guanidinoethane sulfonate administration. The acute metabolic effects of kainate were studied by measuring the efflux of the adenosine triphosphate breakdown products hypoxanthine, xanthine, inosine and adenosine. No differences were found between control and 2-guanidinoethane sulfonate treated rats with respect to basal or kainic acid evoked release of purine catabolites. Also, the neuronal loss caused by kainate injection into the hippocampus was not modified by 2-guanidinoethane sulfonate treatment, suggesting that endogenous taurine does not affect these responses. We conclude that chronic administration of 2-guanidinoethane sulfonate does not sensitize central neurons to the metabolic and toxic actions of kainate.

Content and traffic of taurine in hippocampal reactive astrocytes

Taurine is one of the most abundant free amino acids in the mammalian central nervous system, where it is crucial to proper development. Moreover, taurine acts as a neuroprotectant in various diseases; in epilepsy, for example, it has the capacity to reduce or abolish seizures. In the present study, taurine levels has been determine in mice treated with Kainic Acid (KA) and results showed an increase of this amino acid in hippocampus but not in whole brain after 3 and 7 days of KA treatment. This increase occurs when gliosis was observed. Moreover, taurine transporter (TAUT) was found in astrocytes 3 and 7 days after KA treatment, together with an increase in cysteine sulfinic acid decarboxylase (csd) mRNA, that codifies for the rate-limiting enzyme of taurine synthesis, in the hippocampus at the same times after KA treatment. Glial cultures enriched in astrocytes were developed to demonstrate that these cells are responsible for changes in taurine levels after an injury to the brain. The cultures were treated with proinflammatory cytokines to reproduce gliosis. In this experimental model, an increase in the immunoreactivity of GFAP was observed, together with an increase in CSD and taurine levels. Moreover, an alteration in the taurine uptake-release kinetics was detected in glial cells treated with cytokine. All data obtained indicate that astrocytes could play a key role in taurine level changes induced by neuronal damage. More studies are, therefore, needed to clarify the role taurine has in relation to neuronal death and repair.

Taurine treatment inhibits CaMKII activity and modulates the presence of calbindin D28k, calretinin, and parvalbumin in the brain

Taurine, 2-aminoethanesulfonic acid, is present at high concentrations in many invertebrate and vertebrate systems and has several biological functions. In addition, it has been related to a neuroprotective role against several diseases such as epilepsy. In the present work, we treated mice with taurine and examined its effects on the expression of proteins in the hippocampus associated with calcium regulation. Taurine treatment alters the presence of calbindin-D28k, calretinin, and parvalbumin in the brain, mainly in the hippocampus. It also reduced CaMKII activity, indicating that taurine could alter calcium signaling pathways. However, the activity of calpain, a protease related to apoptosis induced by calcium signalling, did not change. The concentration of taurine in the hippocampus was also unaffected by the treatment. These results provide new insight into the role of taurine in calcium homeostasis.

Synthesis, uptake and release of taurine in astrocytes treated with 8-Br-cAMP

Taurine is one of the most abundant free amino acids in the mammalian central nervous system, where it is crucial for proper development. Moreover, taurine has been related with epilepsy, as it can reduce or prevent seizures. It is also a neuroprotectant in other experimental conditions. Glial cultures were analysed to determine the changes in taurine synthesis and traffic that occur in a more differentiated state of these cells. The cultures were treated with 8-Br-cAMP, an analogue of cAMP that induces differentiation in astrocytes. We observed an increase in immunoreactivity for GFAP, as well as an alteration in uptake-release kinetics in these cells. Moreover, we noted an increase in taurine levels and in cysteine sulfinic decarboxylase, which is the rate-limiting enzyme in taurine synthesis. The data indicate that taurine synthesis and traffic kinetics vary according to the differentiation state of the astrocytes. Thus, our results highlight the importance of astrocytes in modulating taurine levels in the brain.

Prevention of epilepsy by taurine treatments in mice experimental model

An experimental model based on kainic acid (KA) injections replicates many phenomenological features of human temporal lobe epilepsy, the most common type of epilepsy in adults. Taurine, 2-aminoethanesulfonic acid, present in high concentrations in many invertebrate and vertebrate systems, is believed to serve several important biological functions. In addition, it is believed to have a neuroprotective role against several diseases. In the present study, an experimental mouse model based on taurine pretreatment prior to KA administration has been improved to study whether taurine has a neuroprotective effect against KA-induced behavior and cell damage. Under different treatments tested, taurine's most neuroprotective effects were observed with intraperitoneal taurine injection (150 mg/kg dosage) 12 hr before KA administration. Thus, a reduction in or total absence of seizures, together with a reduction in or even disappearance of cellular and molecular KA-derived effects, was detected in mice pretreated with taurine compared with those treated only with KA. Moreover, the use of tritiated taurine revealed taurine entry into the brain, suggesting possible changes in intracellular:extracellular taurine ratios and the triggering of pathways related to neuroprotective effects.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Extracellular taurine in the substantia nigra: Taurine-glutamate interaction

Taurine has been proposed as an inhibitory transmitter in the substantia nigra (SN), but the mechanisms involved in its release and uptake remain practically unexplored. We studied the extracellular pool of taurine in the rat's SN by using microdialysis methods, paying particular attention to the taurine-glutamate (GLU) interaction. Extracellular taurine increased after cell depolarization with high-K(+) in a Ca(2+)-dependent manner, being modified by the local perfusion of GLU, GLU receptor agonists, and zinc. Nigral administration of taurine increased the extracellular concentration of gamma-aminobutyric acid (GABA) and GLU, the transmitters of the two main inputs of the SN. The modification of the glial metabolism with fluocitrate and L-methionine sulfoximine also changed the extracellular concentration of taurine. The complex regulation of the extracellular pool of taurine, its interaction with GABA and GLU, and the involvement of glial cells in its regulation suggest a volume transmission role for taurine in the SN.

Purines and neuroprotection

The activation of adenosine A1, A2 andA3 receptors can protect neurones against damage generated by mechanical or hypoxic/ischaemic insults as well as excitotoxins. A1 receptors are probably effective by suppressing transmitter release and producing neuronal hyperpolarisation. They are less likely to be of therapeutic importance due to the plethora of side effects resulting from A1 agonism, although the existence of receptor subtypes and recent synthetic chemistry efforts to increase ligand selectivity, may yet yield clinically viable compounds. Activation of A2A receptors can protect neurons, although there is much uncertainty as to whether agonists are acting centrally or via a peripheral mechanism such as altering blood flow or immune cell function. Selective antagonists at the A2A receptor, such as 4-(2-[7-amino-2-(2-furyl)(1,2,4)triazolo(2,3-a)(1,3,5)triazin-5-yl-amino]ethyl)phenol (ZM 241385) and 7-(2-phenylethyl)-5-amino-2-(2-furyl)-pyrazolo-[4,3e]-1,2,4-triazolo[1,5-c]pyrimidine (SCH 58261), can also protect against neuronal death produced by ischaemia or excitotoxicity. In addition, A2A receptor antagonists can reduce damage produced by combinations of subthreshold doses of the endogenous excitotoxin quinolinic acid and free radicals. Since the A2A receptors do not seem to be activated by normal endogenous levels of adenosine, their blockade should not generate significant side effects, so that A2A receptor antagonists appear to be promising candidates as new drugs for the prevention of neuronal damage. Adenosine A3 receptors have received less attention to date, but agonists are clearly able to afford protection against damage when administered chronically. Given the disappointing lack of success of NMDA receptor antagonists in human stroke patients, despite their early promise in animal models, it is possible that A2A receptor antagonists could have a far greater clinical utility.

Regional differences in feeding and other behaviors elicited by N-methyl-D-aspartic acid in the rodent hypothalamus: a reverse microdialysis mapping study

Regional differences in the feeding stimulatory actions of hypothalamically delivered N-methyl-D-aspartate (NMDA) were investigated. NMDA (660 microM intraprobe) delivered by reverse microdialysis into the tuberal lateral hypothalamus (tLH) reliably elicited feeding in satiated rats. The average food intake was 8.6 g in 50 min, and during the infusion rats spent 26% of the time eating, compared to less than 1% before NMDA treatment. In contrast, NMDA did not affect feeding when reverse dialyzed into the anterior LH (aLH), posterior LH (pLH) or the medial hypothalamus (MH). NMDA had no apparent behavioral effect in the aLH; in contrast, it significantly decreased the time spent resting/sleeping when infused into each of the other three areas tested. Additionally, in the medial hypothalamus, NMDA infusions increased time spent grooming; while in the pLH only alertness was significantly increased. These data underscore the functional and anatomical heterogeneity of the hypothalamus, and implicate glutamate and NMDA receptors in different portions of the hypothalamus in the control of eating, grooming and arousal.

Reverse microdialysis of N-methyl-D-aspartic acid into the lateral hypothalamus of rats: effects on feeding and other behaviors

The effects of reverse microdialysis of N-methyl-D-aspartic acid (NMDA) into the lateral hypothalamus (LH) on feeding and other behaviors were examined. Consistent with similar studies utilizing central microinjections, NMDA reverse microdialysed into the rat LH rapidly elicited a strong concentration-dependent stimulation of feeding. The minimum perfusate concentration of NMDA needed to elicit feeding with reverse microdialysis was 660 microM, a concentration 1/50 of that needed with pressure injections. Further, eating responses could be consistently elicited in sequential tests separated by 2-4 h in the same subject, and the magnitude of the eating in the first and second tests was highly correlated (r=0.87). Behavioral analysis revealed that the main response to NMDA consisted of eating without the concomitant hyperactivity produced by central microinjections of this agonist. The other behaviors exhibited during NMDA administration were those that normally occur during spontaneous feeding in rats. Also, rats precisely compensated for the increased food intake elicited by NMDA by reducing spontaneous feeding during the subsequent nocturnal phase, so as to maintain normal daily intakes. In contrast, N-methyl-L-aspartate (NMLA) reverse microdialysed in to the LH (660 microM) did not elicit feeding nor affect any other behaviors we examined. These data support a role for LH glutamate and NMDA receptors in the control of feeding.

Methodological issues in microdialysis sampling for pharmacokinetic studies

Microdialysis is an in vivo technique that permits monitoring of local concentrations of drugs and metabolites at specific sites in the body. Microdialysis has several characteristics, which makes it an attractive tool for pharmacokinetic research. About a decade ago the microdialysis technique entered the field of pharmacokinetic research, in the brain, and later also in peripheral tissues and blood. Within this period much has been learned on the proper use of this technique. Today, it has outgrown its child diseases and its potentials and limitations have become more or less well defined. As microdialysis is a delicate technique for which experimental factors appear to be critical with respect to the validity of the experimental outcomes, several factors should be considered. These include the probe; the perfusion solution; post-surgery interval in relation to surgical trauma, tissue integrity and repeated experiments; the analysis of microdialysate samples; and the quantification of microdialysate data. Provided that experimental conditions are optimized to give valid and quantitative results, microdialysis can provide numerous data points from a relatively small number of individual animals to determine detailed pharmacokinetic information. An example of one of the added values of this technique compared with other in vivo pharmacokinetic techniques, is that microdialysis reflects free concentrations in tissues and plasma. This gives the opportunity to assess information on drug transport equilibration across membranes such as the blood-brain barrier, which already has provided new insights. With the progress of analytical methodology, especially with respect to low volume/low concentration measurements and simultaneous measurement of multiple compounds, the applications and importance of the microdialysis technique in pharmacokinetic research will continue to increase.

Protection by an adenosine analogue against kainate-induced extrahippocampal neuropathology

1. The glutamate analogue kainic acid produces neuronal damage in the central nervous system. We have reported that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA) can, at doses as low as 10 microg/kg IP, prevent the hippocampal damage that follows the systemic administration of kainate. The present work was designed to examine purine protection against kainate in extrahippocampal regions by using histological methods. 2. The results show that R-PIA, at a dose of 25 microg/kg IP in rats, can protect against the neuronal damage caused by kainate in the basolateral amygdaloid nuclei, the pyriform cortex and around the rhinal fissure. This protection could be prevented by the simultaneous administration of the A1 adenosine receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine, confirming that the protection involved adenosine A1 receptors. No protection was observed in the posterior amygdaloid nuclei or the entorhinal cortex, suggesting the absence of relevant adenosine receptors or a different mechanism of excitotoxicity.

Brain microdialysis of GABA and glutamate: what does it signify?

Microdialysis has become a frequently used method to study extracellular levels of GABA and glutamate in the central nervous system. However, the fact that the major part of GABA and glutamate as measured by microdialysis does not fulfill the classical criteria for exocytotic release questions the vesicular origin of the amino acids in dialysates. Glial metabolism or reversal of the (re)uptake sites has been suggested to be responsible for the pool of nonexocytotically released amino-acid transmitters that seem to predominate over the neuronal exocytotic pool. The origin of extracellular GABA and glutamate levels and, as a consequence, the implications of changes in these levels upon manipulations are therefore obscure. This review critically analyzes what microdialysis data signify, i.e., whether amino-acid neurotransmitters sampled by microdialysis represent synaptic release, carrier-mediated release, or glial metabolism. The basal levels of GABA and glutamate are virtually tetrodotoxin- and calcium-independent. Given the fact that evidence for nonexocytotic release mediated by reversal of the uptake sites as a release mechanism relevant for normal neurotransmission is so far limited to conditions of "excessive stimulation," basal levels most likely reflect a nonneuronal pool of amino acids. Extracellular GABA and glutamate concentrations can be enhanced by a wide variety of pharmacological and physiological manipulations. However, it is presently impossible to ascertain that the stimulated GABA and glutamate in dialysates are of neuronal origin. On the other hand, under certain stimulatory conditions, increases in amino-acid transmitters can be obtained in the presence of tetrodotoxin, again suggesting that aspecific factors not directly related to neurotransmission underlie these changes in extracellular levels. It is concluded that synaptic transmission of GABA and glutamate is strictly compartmentalized and as a result, these amino acids can hardly leak out of the synaptic cleft and reach the extracellular space where the dialysis probe samples.

Methodological considerations of intracerebral microdialysis in pharmacokinetic studies on drug transport across the blood-brain barrier

For the study of the pharmacokinetics of drugs in the brain a number of in vivo techniques is available, including autoradiography, imaging techniques, cerebrospinal fluid sampling and in vivo voltammetry, which all have their specific advantages and limitations. Intracerebral microdialysis is a relatively new in vivo technique. It permits monitoring of local concentrations of drugs and metabolites at specific sites in the brain which makes it an attractive tool for pharmacokinetic research. In the use of this technique a number of factors should be considered. These include: type of probe, surgical trauma, post-surgery interval, perfusion flow rate, as well as composition and temperature of the perfusion medium. In particular in studies on drug transport across the blood-brain barrier (BBB), effects of insertion of the probe on BBB functionality is important. It appears that BBB functionality is not significantly affected if surgical and experimental conditions are well-controlled. The relationship between dialysate concentrations and those in the extracellular fluid of the periprobe tissue, the recovery of the drug, depends on periprobe processes governing the actual concentration of the drug at that site. These include extracellular-microvascular exchange, metabolism, and diffusion of the drug. Several methods have been proposed to determine recovery values. In particular the no net flux method and the extended no net flux method are useful in practice. Several microdialysis studies on BBB transport of drugs are presented showing that intracerebral microdialysis is capable to assess local BBB transport profiles. Compared with other in vivo techniques, intracerebral microdialysis is the only (affordable) technique that offers the possibility to monitor local BBB transport of drugs in unanaesthetized animals, under physiological and pathological conditions.

Prevention by a purine analogue of kainate-induced neuropathology in rat hippocampus

Systemic injection of kainic acid produces a characteristic regional and cellular pattern of neuronal loss in the central nervous system by mechanisms which may be relevant to an understanding of neurodegenerative disorders. It has previously been found, by measuring the binding of a glial marker ligand, that analogues of adenosine, such as R-N6-phenylisopropyladenosine (R-PIA), can prevent kainate-induced damage of the hippocampus at doses as low as 10 micrograms/kg, i.p. The use of gliotic markers, however, is open to misinterpretation, and the present work was designed to re-examine purine protection against kainate using histological methods. The results show that R-PIA, at a dose of 25 micrograms/kg i.p. in rats, can protect against the neuronal damage caused by kainate and that this protection could be completely prevented by the simultaneous administration of 1,3-dipropyl-8-cyclopentylxanthine, indicating the involvement of adenosine A1 receptors in the protection.

Cerebrospinal fluid concentrations of glutamate and GABA during perinatal cerebral hypoxia-ischemia and seizures

Cerebrospinal fluid (CSF) concentrations of glutamate and gamma- aminobutyric acid (GABA), as estimates of levels in the extracellular compartment of brain, were determined in 7-day postnatal rats at the terminus of hypoxia-ischemia and during status epilepticus, induced with bicuculline, at 2 and 24 h of recovery. Hypoxia-ischemia was associated with increased CSF glutamate, which was not increased further during status epilepticus. In contrast, CSF GABA was increased by hypoxia-ischemia as well as by status epilepticus during recovery. CSF glutamate/GABA ratios in rat pups subjected to status epilepticus with or without prior hypoxia-ischemia were lower than control animals during recovery. The lack of any significant increase in glutamate or in the glutamate/GABA ratio during status epilepticus would preclude any neuronal injury from occurring in those immature rats sustaining seizures alone or any accentuation of brain damage in those animals subjected to prior cerebral hypoxia-ischemia.

Changes in extracellular levels of glutamate and aspartate in rat substantia nigra induced by dopamine receptor ligands: in vivo microdialysis studies

The microdialysis technique was utilized to study the local effects of D1 and D2 family type dopamine (DA) receptor (R) ligands on the in vivo release of endogenous glutamate (GLU) and aspartate (ASP) from rat substantia nigra (SN). Addition to the dialysis perfusion solution of either D1-R and D2-R agonists, such as SKF-38393 (50 and 100 microM) and Quinpirole (5 and 10 microM), resulted in dose-dependent increases in extracellular concentrations of GLU and ASP, respectively. The SKF-38393 and Quinpirole-induced effects were reduced by SCH-23390 (0.5 microM), a D1-R antagonist, and by Spiperone (1.0 microM), a D2-R antagonist, respectively. However, SCH-23390 and Spiperone did increase GLU and ASP extracellular concentrations. Local infusion with Tetrodotoxin (TTX) (1.0 microM), a blocker of voltage-dependent Na+ channels, increased basal extracellular levels of GLU. In addition, co-infusion of TTX and SKF-38393 evoked increases in extracellular GLU levels higher than those observed after SKF-38393 alone. Finally, chemical lesions of nigral DA cells with 6-OH-DA increased the basal extracellular levels of GLU. It is proposed that the release of GLU and ASP from SN may be regulated by D1- and D2-receptors present in this basal ganglia structure. In addition, part of the D1 receptors present in SN might be located presynaptically on GLU-containing nerve endings.

Release of endogenous catecholamines from the striatum and bed nucleus of stria terminalis evoked by potassium and N-methyl-D-aspartate: in vitro microdialysis studies

Induced release of endogenous dopamine and noradrenaline from coronal slices containing the striatum and the bed nucleus of the stria terminalis, respectively, was studied by means of in vitro microdialysis. A Ca(+2)-dependent and reserpine-sensitive K(+)-induced release of catecholamines was detected in both nuclei. We confirmed that N-methyl-D-aspartate (2.5 and 5.0 mM in the dialysis perfusion solution) induces the release of dopamine from the striatum, and this effect was blocked by prior dialysis perfusion with 500 microM MK-801, a noncompetitive N-methyl-D-aspartate receptor antagonist. Infusion of N-methyl-D-aspartate (1-10 mM) or glutamate through the dialysis probe did not produce any detectable modification in the extracellular levels of noradrenaline in the bed nucleus of the stria terminalis. In addition, perfusion with D-serine (100 microM) alone or in the presence of desipramine (10 microM), resulted in a slight increase in extracellular noradrenaline in the bed nucleus of the stria terminalis. However, N-methyl-D-aspartate in the presence of D-serine and desipramine produced a marked increase in extracellular noradrenaline from the bed nucleus of the stria terminalis. These results indicate that N-methyl-D-aspartate receptors might regulate the release of noradrenaline from the bed nucleus of the stria terminalis as is the case of dopamine release in the striatum. The in vitro microdialysis seems to be a suitable complement to the in vivo microdialysis for the study of catecholamine release in discrete regions of the central nervous system and its local regulation by excitatory amino acid receptors.

Effect of systemic administration of N-methyl-D-aspartic acid on extracellular taurine level measured by microdialysis in the hippocampal CA1 field and striatum of rats

The extracellular concentrations of amino acids in the hippocampal CA1 field and striatum of conscious freely moving rats were monitored simultaneously by in vivo brain microdialysis using HPLC with electrochemical detection. Under basal conditions, aspartate, glutamate, glutamine, glycine, taurine, and alanine were detected, but gamma-aminobutyric acid was undetectable in both regions. Intraperitoneal injection of N-methyl-D-aspartic acid (NMDA; 10 mg/kg) caused a significant increase (three- to fivefold) in the taurine concentration in the dialysate obtained from both the hippocampal CA1 and striatum, whereas other amino acids (aspartate, glutamate, and alanine) did not show significant changes. Local application of NMDA (300 microM) to both regions via the dialysis probes also caused a similar increase (three- to fivefold) in both regions. Under infusion of hypertonic Ringer's solution containing 150 mM sucrose, the effect of NMDA on the level of taurine in both the regional dialysates was not affected. The effect of NMDA was totally reduced by intraperitoneal administration of MK-801 (0.3-1.0 mg/kg), a noncompetitive antagonist of NMDA receptors. Continuous infusion of DL-2-amino-5-phosphonovaleric acid (1.0 mM), a competitive antagonist of NMDA receptors, via the dialysis probes completely inhibited the effect of NMDA. These findings suggest that systemic administration of NMDA is effective as well as local administration into the brain and that NMDA receptors might be involved in the regulation of the extracellular taurine level in the brain without dependence on cell swelling.

Glutamate-induced inhibition of paired pulse facilitation of monosynaptic excitatory post-synaptic potentials in frog spinal motoneurons

To evaluate actions of glutamate on excitatory synaptic transmission in the central nervous system, we examined glutamate-induced changes in the paired pulse facilitation of monosynaptic excitatory post-synaptic potentials evoked by stimulation of the lateral column fibers (LC-EPSPs) on lumbar motoneurons in the frog spinal cord. Glutamate (1 mM) depolarized motoneurons both in the presence and absence of Mg2+. In most cells perfused with Mg(2+)-free or high Ca(2+)-Mg2+ solutions, the glutamate potential was accompanied by a reduction in peak amplitude of EPSPs, although the degree of change varied with the cells. Glutamate enhanced the EPSP amplitude in a few cells with Mg(2+)-free and high Ca(2+)-Mg2+ solutions, and in most cells with high Mg2+ medium. In 3/5 cells tested, the paired pulse facilitation of EPSPs was reduced by glutamate when the EPSP amplitude either increased or decreased. NMDA (50 microM), kainate (50-100 microM), quisqualate (5-50 microM) and L-2-amino-4-phosphonobutyrate (L-AP4, 1 mM) also decreased the facilitation in about half of the cells tested. The glutamate-induced decrease in the facilitation was observed in both the presence and absence of Mg2+ and was not affected by the concomitant application of glutamate and antagonists for non-NMDA or NMDA receptors, such as 6-cyano-7-nitro-quinoxalinediones (CNQX, 60 microM) or 2-amino-5-phosphonovalerate (APV, 250 microM). Glutamate reduced the facilitation of excitatory post-synaptic currents (EPSCs) recorded at a constant membrane potential under voltage clamp, when the EPSC amplitude either increased or decreased and when the input conductance either increased or decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of excitatory amino acid release by N-methyl-d-aspartate receptors in rat striatum: in vivo microdialysis studies

The microdialysis technique was utilized to study the effects of N-methyl-D-aspartate (NMDA) receptor ligands on the in vivo release of endogenous glutamate (Glu) and aspartate (Asp) from the rat striatum. Addition of NMDA (250 and 500 microM) to the dialysis perfusion solution resulted in a striking dose-dependent increase in extracellular concentrations of Glu and Asp in the striatum. The NMDA-induced effects were reduced in a dose-related way by prior perfusion with 75 microM dizocilpine (MK-801), a non-competitive NMDA receptor antagonist. MK-801, at 75 microM, produced no changes on basal levels of Glu and Asp. However, 100 microM MK-801 did increase Glu and Asp extracellular concentrations. Local infusion with 500 microM D-serine, an agonist at the glycine site associated to the NMDA receptor, significantly increased basal level of Glu, but not Asp. Such D-serine-induced effects were reduced by 7-Cl-kynurenic acid (200 microM), a selective blocker of the glycine site present in the NMDA receptor. It is proposed that activation of NMDA receptors by endogenous Glu and Asp enhances the subsequent release of these excitatory amino acids in the striatum. Part of these NMDA receptors might be located presynaptically on cortico-striatal nerve endings. In addition, postsynaptic NMDA receptors present in the striatum may also indirectly modulate the release of Glu and Asp, through trans-synaptic mechanism.

N-methyl-D-aspartate releases excitatory amino acids in rat corpus striatum in vivo

There is a considerable amount of conflicting evidence from several studies as to the action of applied N-methyl-D-aspartate (NMDA) on the release of glutamate and aspartate in the brain. In the present study the effect of NMDA on extracellular levels of endogenous amino acids was investigated in conscious, unrestrained rats using intracerebral microdialysis. NMDA caused dose-related increases in extracellular levels of glutamate and aspartate; threonine and glutamine were unaffected. The NMDA-evoked release of glutamate and aspartate was significantly decreased by the specific NMDA receptor antagonist 3-[(+-)-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid. In addition, increasing the perfusate concentration (and therefore the extracellular concentration) of Ca2+ significantly enhanced the NMDA-evoked release of glutamate and aspartate, whereas removal of Ca2+ and addition of a high Mg2+ concentration to the perfusate caused a significant reduction in their NMDA-evoked release. Moreover, the NMDA-evoked release of glutamate and aspartate was reduced in decorticate animals. These results demonstrate that, in the striatum in vivo, NMDA causes selective release of endogenous glutamate and aspartate from neurone terminals and that this action occurs through an NMDA receptor-mediated mechanism. The ability of NMDA receptor activation to induce release of glutamate and aspartate, perhaps by a positive feedback mechanism, may be relevant to the pathologies underlying epilepsy and ischaemic and hypoglycaemic brain damage.

Sensory modulation of hippocampal transmission. II. Evidence for a cholinergic locus of inhibition in the Schaffer-CA1 synapse

The present work studied the neurotransmitter mediating the depressive effect of sensory stimulation on the Schaffer-CA1 transmission. Field responses of the CA1 region evoked by ipsilateral CA3 stimuli were recorded in paralyzed, locally anesthetized rats following the same experimental paradigm as in the previous work. The tissue zone under recording was perfused in vivo by an implanted hollow fiber (brain dialysis device) with either Krebs-Ringer bicarbonate (KRB), or KRB with penicillin, atropine, acetylcholine or eserine. Results were the following: (1) atropine increased the field excitatory postsynaptic potential (EPSP) amplitude in a dose-dependent manner and totally abolished the modulatory action of sensory stimulation; (2) both the field EPSP and the modulatory action of sensory stimulation remained unaltered during the blockade of GABAergic activity by penicillin; (3) acetylcholine as well as eserine induced a great diminution of both field EPSP and population spike amplitudes, without altering the effect of sensory stimulation; (4) penicillin and atropine induced multiple population spikes, reversing the effect of sensory stimulation and increasing the cell excitability. These results demonstrate that the sensory modulation of information transfer through the Schaffer-CA1 synapse is mediated by a muscarinic cholinergic mechanism. The dose-dependent increase in the field EPSP by muscarinic blockade is evidence for the existence of a cholinergic presynaptic inhibition on the Schaffer collateral terminals.

Quinolinic acid effects on amino acid release from the rat cerebral cortex in vitro and in vivo

1. The effect of quinolinic acid, N-methyl-D,L-aspartate (NMDLA) and kainate on the release of endogenous and exogenous amino acids from the rat cerebral cortex in vitro and in vivo was studied. 2. Neither quinolinic acid nor NMDLA had any effect on the basal or potassium-evoked release of [3H]-D-aspartate from slices of rat cerebral cortex either in the presence or absence of magnesium. Kainic acid failed to modify the basal efflux of [3H]-D-aspartate but significantly inhibited (by 34.4% +/- 0.04%, P less than 0.05) the potassium-evoked release. 3. Neither quinolinate nor NMDLA had any effect on the basal efflux of endogenous amino acids from rat cortical slices either in the presence or absence of magnesium ions at concentrations between 10 microM and 5 mM. 4. Both NMDLA (1 mM) and quinolinate (5 mM) produced an efflux of endogenous aspartate (371.4% +/- 11.6%; 389.3% +/- 12.1%) and glutamate (405.4% +/- 13.6%; 430.1 +/- 8.7%) respectively from the rat cerebral cortex in vivo (P less than 0.01). The quinolinic acid-evoked efflux was abolished by the NMDLA antagonist, 2-amino-5-phosphonovaleric acid (200 microM). 5. Kainic acid also caused an efflux of endogenous amino acids from the rat cerebral cortex in vivo. However, the profile of this release was different from that produced by quinolinate and NMDLA. 6. The results add further support to the suggestion that quinolinic acid acts at the NMDLA-preferring receptor and may also explain the requirement for intact afferent projections for the neurotoxic effects of quinolinate to be manifested.

Potentiation by kainate of excitatory amino acid release in striatum: complementary in vivo and in vitro experiments

The effect of kainate on extracellular levels of amino acids in corpus striatum was investigated in vitro and in vivo, to elucidate the mechanism underlying its neurotoxicity. Kainate increased extracellular glutamate and aspartate in both striatal slices in vitro and intact striatum in vivo, as previously reported. Both in vitro and in vivo, DL-threo-3-hydroxyaspartate increased extracellular glutamate and aspartate levels (to between 150 and 200% of basal), and also enhanced their kainate-evoked release. The action of kainate in vivo was reduced by prior frontal decortication, whereas in vitro the kainate-evoked responses were only slightly reduced by tetrodotoxin, and remained above control values. These results confirm that kainate increases extracellular glutamate and aspartate, and provide evidence that this is due to synaptic release evoked by an action on receptors on glutamatergic neurone terminals. These findings may be relevant to the understanding of epilepsy.

Pentylenetetrazol seizure threshold and extracellular levels of cortical amino acids in taurine-deficient kittens

Directly after weaning, kittens were raised on a semisynthetic diet supplemented with 0.4% taurine or devoid of this amino acid. Eight to twelve weeks later the blood plasma concentration of taurine was decreased by 98-99% in kittens fed a taurine-free regimen. Parietal cortex dialysis, performed in anaesthetized kittens, revealed a selective, but less marked, reduction of extracellular taurine. When kainic acid was included in the dialysis buffer, taurine was doubled in taurine-supplemented kittens, but it was only slightly affected in deficient animals. The animals were also used for determination of the threshold of pentylenetetrazol-induced epilepsy 3 days after the dialysis experiment. This was not significantly different between the groups. The present work shows that taurine deficiency in its own right does not elevate interstitial glutamate, an effect previously observed in the 2-guanidino-ethane sulphonic acid model for taurine deficiency. The results further suggest that taurine is better retained in neural cells in the taurine-deficient state. Moreover, the findings argue against a role for endogenous taurine in the control of epileptiform discharge initiation and/or spread.

In vivo microdialysis studies on the effects of decortication and excitotoxic lesions on kainic acid-induced calcium fluxes, and endogenous amino acid release, in the rat striatum

The in vivo effects of kainate (1 mM) on fluxes of 45Ca2+, and endogenous amino acids, were examined in the rat striatum using the brain microdialysis technique. Kainate evoked a rapid decrease in dialysate 45Ca2+, and an increase in the concentration of amino acids in dialysates in Ca2+-free dialysates. Taurine was elevated six- to 10-fold, glutamate two- to threefold, and aspartate 1.5- to twofold. There was also a delayed increase in phosphoethanolamine, whereas nonneuroactive amino acids were increased only slightly. The kainic acid-evoked reduction in dialysate 45Ca2+ activity was attenuated in striata lesioned previously with kainate, suggesting the involvement of intrinsic striatal neurons in this response. The increase in taurine concentration induced by kainate was slightly smaller under these conditions. Decortication did not affect the kainate-evoked alterations in either dialysate 45Ca2+ or amino acids. These data suggest that kainate does not release acidic amino acids from their transmitter pools located in corticostriatal terminals.

Effects of dihydrokainic acid on extracellular amino acids and neuronal excitability in the in vivo rat hippocampus

The effect of inhibition of the high-affinity uptake of glutamate on the extracellular concentration of amino acids and on neuronal excitability was studied in vivo in the hippocampus of the rat. The dentate gyrus or CA1 field were perfused through a dialytrode with Krebs-Ringer-bicarbonate or dihydrokainic acid solutions. The spontaneous electrical activity and evoked field potentials were recorded concomitantly at dendritic or somatic levels. The results showed that with dihydrokainic acid: the extracellular concentrations of both glutamate and taurine were markedly increased in both areas of the hippocampus, the response of taurine being greater in CA1, while other amino acids were unaffected; in the dentate gyrus, the field excitatory postsynaptic potential was decreased while the population spikes were augmented, indicating an increased excitability of the neuronal population. In CA1, both the excitatory postsynaptic potential and spikes were reduced in amplitude. These results indicate that changes in the extracellular concentration of endogenous glutamate influences excitability of the tissue and that inhibition of the uptake processes for putative amino acid neurotransmitters increases the postsynaptic action of synaptically-released endogenous amino acids.

An analysis of the effects of excitatory amino acid receptor antagonists on evoked field potentials in the olfactory bulb

The effects of excitatory amino acid antagonists on extracellular field potentials in the olfactory bulb produced by lateral olfactory tract stimulation were analysed in vivo. The compounds tested D-2-amino-5-phosphonovalerate, L-(+)2-amino-4-phosphonobutyrate, gamma-D-glutamylglycine, L-glutamic acid diethylester and cis-2,3-piperidine dicarboxylic acid, were administered by brain dialysis. Of the compounds tested, only cis-2,3 piperidine-dicarboxylic acid and gamma-D-glutamylglycine were able to suppress the synaptic excitation of granule cells. This pharmacological profile suggests the involvement of non-N-methyl-D-aspartate receptors. However, the suppression was accompanied by a reduction in the amplitude of the presynaptic volley. A second finding was that D-2-amino-5-phosphono-valerate and gamma-D-glutamyl glycine attenuated granule cell mediated inhibition of mitral cells, suggesting the involvement of N-methyl-D-aspartate-sensitive receptors. The possibility that mitral cells and that either centrifugal fibres, or an intrinsic olfactory bulb feedback loop might use an excitatory amino acid as its neurotransmitter is therefore discussed.

N-Methylaspartate-evoked liberation of taurine and phosphoethanolamine in vivo: site of release

The effect of N-methyl-D,L-aspartic acid (NMA) on extracellular amino acids was studied in the rabbit hippocampus with the brain dialysis technique. Administration of 0.5 or 5 mM NMA caused a concentration-dependent liberation of taurine and phosphoethanolamine (PEA). Taurine increased by 1,200% and PEA by 2,400% during perfusion with 5 mM NMA whereas most other amino acids rose by 20-100%. The effect of NMA appeared to be receptor-mediated, as coperfusion with D-2-amino-5-phosphonovaleric acid curtailed the NMA response by some 90%. The NMA-stimulated release of taurine and PEA was suppressed when Ca2+ was omitted and further inhibited when Co2+ was included in the perfusion medium. The effect of NMA was mimicked by the endogenous NMA agonist quinolinic acid and the partial NMA agonist D,L-cis-2,3-piperidine dicarboxylic acid. Although the NMA-evoked release of taurine and PEA was Ca2+-dependent in vivo, NMA had no effect on Ca2+ accumulation in hippocampal synaptosomes. The previously reported NMA-induced activation of dendritic Ca2+ spikes and the lack of effect on synaptosomal Ca2+ uptake suggest that taurine and PEA are released from sites other than nerve terminals, possibly from dendrosomatic sites. This notion was strengthened by the absence of an effect of NMA on the efflux of radiolabelled taurine from hippocampal synaptosomes. In contrast, high K+ stimulated synaptosomal uptake of Ca2+ and release of taurine.