Effects of acidic dipeptides on aminoacidergic neurotransmission in the brain

Distribution of N-acetylaspartylglutamate immunoreactivity in human brain and its alteration in neurodegenerative disease

The dipeptide N-acetylaspartylglutamate (NAAG) may be involved in the process of glutamatergic signaling by both acting at glutamate receptors and as a glutamate protransmitter. In the present study we determined the cellular localization and distribution of NAAG-like immunoreactivity (NAAG-LI) in normal human brain and in neurodegenerative disorders to ascertain the degree of NAAG's colocalization to putative glutamatergic pathways. Immunohistochemistry with an antibody against NAAG was performed on control, Huntington's disease (HD) and Alzheimer's disease (AD) human autopsy and biopsy brain sections from the cerebral cortex, hippocampus, amygdala, neostriatum, brainstem and spinal cord. In normal human brain, NAAG-LI was widespread localized to putative glutamatergic pyramidal neurons of the cerebral cortex and hippocampus. Punctate NAAG-LI was present in areas known to receive neuronal glutamatergic input, such as layer IV of the cerebral cortex, striatal neuropil, and the outer portion of the molecular layer of the hippocampal dentate gyrus. In the two pathologic brain regions examined, the HD neostriatum and the AD temporal cortex, we observed a widespread loss of NAAG-LI neurons. In addition NAAG-LI reactive microglia surrounding plaques were seen in AD temporal cortex but not in the HD striatum. Our results suggest that NAAG is substantially localized to putative glutamatergic pathways in human brain and that NAAG-LI neurons are vulnerable to the neurodegenerative process in HD and AD.

Interactions of gamma-L-glutamyltaurine with excitatory aminoacidergic neurotransmission

The in vitro effects of gamma-L-glutamyltaurine on different stages of excitatory aminoacidergic neurotransmission were tested with gamma-D-glutamyltaurine as reference. gamma-L-Glutamyltaurine enhanced the K(+)-stimulated release of [3H]glutamate from cerebral cortical slices (25% at 0.1 mM) and slightly inhibited the uptake by crude brain synaptosomal preparations (about 10% at 1 mM). gamma-L-Glutamyltaurine was also a weak displacer of glutamate and its agonists from their binding sites in brain synaptic membrane preparations, being, however, less selective to quisqualate (QA) sites than gamma-D-glutamyltaurine. The basal influx of Ca2+ into cultured cerebellar granular cells was not affected by 1 mM gamma-L-glutamyltaurine, but the glutamate- and its agonist-activated influx was significantly inhibited in low-Mg2+ (0.1 mM) and Mg(2+)-free media. The glutamate-evoked increase in free intracellular Ca2+ and the kainate-activated formation of cGMP in cerebellar slices were both markedly inhibited by 0.1 mM gamma-L-glutamyltaurine. We propose that gamma-L-glutamyltaurine may act as endogenous modulator in excitatory aminoacidergic neurotransmission.

Phencyclidine treatment in mice: effects on phencyclidine binding sites and glutamate uptake in cerebral cortex preparations

The effects of a psychotomimetic drug, phencyclidine (PCP), on glutamatergic neurotransmission were studied in mice. The binding of tritiated N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) to cerebral cortical membranes and the uptake of [3H]glutamate by cortical synaptosomal preparations were assessed after PCP treatment (1 mg/d/mouse for 3 days) with implanted minipumps. The binding capacity Bmax of TCP significantly increased but the binding constant KD remained the same after PCP exposure, indicating that more binding sites became available. The basic properties of the binding remained unaltered but the actions of glutamate, glutamate receptor agonists and glycine were potentiated in PCP-treated mice. The uptake of glutamate was saturable, consisting of both high- and low-affinity transport components. After PCP exposure the transport constant Km of the high-affinity component increased and that of the low-affinity component was not changed. The maximal velocity V of the high-affinity component increased while that of the low-affinity transport decreased. Moreover, inhibition by structural analogues was potentiated, suggesting modification of the glutamate transporter. The results show that chronic PCP treatment, used as a model of psychosis, markedly affects the studied glutamatergic parameters.

Taurine induces bicuculline/strychnine-insensitive dose-dependent inhibition of cortical visual evoked responses

Modulatory influences of taurine on cortical visual evoked responses and their susceptibility to GABAergic and glycinergic antagonists were studied in adult rats. Taurine topically administered to the visual cortex produced dose-dependent inhibition of the positive-negative fast component of the cortical visual evoked responses. Neither bicuculline nor strychnine antagonized the taurine effect, as revealed by absence of a shift to right, a change in slope or in the taurine IC50 value in the dose-response curve. Results suggest that taurine-induced depression of cortical responses evoked in the visual cortex are mediated by receptors other than the GABAA and glycine receptors.

Distribution of taurine in the rat cerebellum and insect brain: application of a new antiserum against carbodiimide-conjugated taurine

The production, specificity and application of an antiserum against taurine conjugated to succinylated ovalbumin by means of 1-ethyl-3(3-dimethylaminopropyl)-carbodiimide is reported. The antiserum was produced in rabbits. The carbodiimide was used also as a tissue fixative. The development of the antibody titre was followed by dot-blot tests on nitrocellulose filters using different amino acid conjugates and with immunohistochemical reaction in the rat and insect brain. Blocking controls were also used. Taurine antiserum, sufficiently specific and sensitive, developed after the fourth booster injection, after which the antiserum was characterized. In the insect brain, intense taurine-like immunoreactivity was observed in the photoreceptors, in the Kenyon cells and the neuropile of the mushroom bodies, in the lower part of the central body and in the antennal lobes. In the rat carebellum, intense taurine-like immunoreactivity was seen in the Purkinje cells. Immunoreaction was seen also in small cells most probably corresponding to the basket cells. The use of the carbodiimide in the production of antisera against taurine provides a parallel method for comparison of the distribution of taurine-like immunoreactivity obtained with antisera made against conjugates prepared with aldehydes.

Extracellular acidic sulfur-containing amino acids and gamma-glutamyl peptides in global ischemia: postischemic recovery of neuronal activity is paralleled by a tetrodotoxin-sensitive increase in cysteine sulfinate in the CA1 of the rat hippocampus

An excessive activation of the excitatory amino acid system has been proposed as one possible mediator of the ischemia-induced delayed death of CA1 pyramidal cells in the hippocampus. Using dialytrodes in the CA1 of the rat, we have investigated multiple-unit activity and extracellular changes in acidic sulfur-containing amino acids and gamma-glutamyl peptides during ischemia (20-min, four-vessel occlusion) and during 8 h of reflow. Multiple-unit activity was abolished during ischemia and for the following 1 h, but then recovered, gradually reaching preischemic levels after 8 h of reflow. Extracellular cysteate, cysteine sulfinate, and gamma-glutamyltaurine increased (1.5- to threefold) during ischemia, and extracellular glutathione and gamma-glutamylaspartate plus gamma-glutamylglutamine increased during early reflow (two- to threefold). The recovery of neuronal activity at 4-8 h was paralleled by an increase in extracellular cysteine sulfinate (2.5-fold at 8 h of reflow). Perfusion with 10 microM tetrodotoxin at 8 h of reflow abolished the multiple-unit activity and reduced extracellular cysteine sulfinate. Considering the glutamate-like properties of cysteine sulfinate, the observed postischemic increase may be involved in the development of the delayed neuronal death.

Automated determination of neuroactive acidic sulphur-containing amino acids and gamma-glutamyl peptides using liquid chromatography with fluorescence and electrochemical detection

A column liquid chromatographic method is presented for the determination of trace levels of acidic sulphur-containing amino acids and gamma-glutamyl di- and tripeptides in microdialysates sampled from rat brain in vivo. Automated precolumn derivatization was performed with o-phthaldialdehyde-beta-mercaptoethanol. The derivatives were separated by reversed-phase liquid chromatography with electrochemical and fluorescence detection. The mean relative standard deviation (n = 10) was 1.03 and 4.59% for retention times and peak heights, respectively. The mean correlation coefficient of linearity (r) was 0.9982 in the range 4.5-450 pmol (n = 15), and the lowest detectable amount was 200 fmol for the homocysteinesulphinic acid derivative, (k' = 5.4, at a signal-to-noise ratio of 3). A microcolumn electrochemical detection method, developed for volume-limited samples, produced a fifteen-fold increase in mass sensitivity. Neurochemical applications using microdialysis in vivo are presented.

Displacement of excitatory amino acid receptor ligands by acidic oligopeptides

A number of L-glutamyl and L-aspartyl dipeptides, glutathione, gamma-D-glutamylglycine and gamma-D-glutamyltaurine, were tested for their efficacy to displace ligands specific for different subtypes of excitatory amino acid receptors from rat brain synaptic membranes. In general, the L enanthiomorphs of gamma-glutamyl peptides were more potent displacers than gamma-D-glutamylglycine and -taurine but the latter were more specific for the quisqualate type of receptors. gamma-L-glutamyl-L-glutamate was the most effective dipeptide in displacing the binding of glutamate, 2-amino-3-hydroxy-5-methylisoxazole-4-proprionate (AMPA) and 2-amino-5-phosphonoheptanoate (APH), whereas gamma-L-glutamyl-L-aspartate was the most effective in the binding of kainate. Both oxidized and reduced glutathione were inhibitory, being most potent in the binding of AMPA. gamma-L-Glutamylaminomethylsulphonate was most effective in the binding of APH. The most potent gamma-L-glutamyl peptides (glutathione, gamma-L-glutamyl-L-glutamate, -L-aspartate, and -glycine) may act as endogenous modulators of excitatory aminoacidergic neurotransmission.

Modulation of GABAergic neurotransmission in the brain by dipeptides

The effects of endogenous and synthetic peptides containing GABA or its analogues on the GABA/benzodiazepine/chloride ionophore complex. GABAB receptor, Cl fluxes, GABA release and GABA uptake were studied using synaptic membranes, crude synaptoneurosomal preparations and slices prepared from the rat and mouse brain. The sodium-independent binding of GABA was strongly inhibited by GABA-histidine, followed by gamma-glutamyl-homotaurine, GABA-glycine and gamma-glutamyl-GABA. The binding of diazepam was slightly enhanced by the same peptides. The peptides alone had no effect on the chloride fluxes, but GABA-histidine, gamma-glutamyl-GABA and GABA-glycine enhanced while gamma-glutamyl-homotaurine and GABA-taurine inhibited GABA-stimulated chloride uptake. GABA-histidine was the most effective displacer of baclofen binding, but gamma-glutamylhomotaurine was entirely ineffective. The uptake of GABA was markedly inhibited in synaptosomal preparations by GABA-histidine, while all other peptides were less effective. gamma-Glutamyl-taurine attenuated but gamma-glutamyl-homotaurine and GABA-glycine enhanced the potassium-stimulated release of GABA. The present actions of GABA-histidine in vitro may be of significance for GABAergic neurotransmission in vivo.