Glutamic Acid and Cerebral Function

Corrosion behavior of nitinol in blood serum and PBS containing amino acids

The susceptibility to localized corrosion of metallic, implant medical devices is typically evaluated in a simulated physiological solution, such as phosphate buffered saline (PBS). For implant devices in contact with blood, the underlying premise is that proteins and other components in blood do not significantly change the corrosion susceptibility. This study examined the corrosion behavior of nitinol in bovine serum and PBS containing amino acids (cysteine, glutamine, and tryptophan). Tests were performed on mechanically polished (MP) and black oxide (BO) nitinol wire using cyclic potentiodynamic polarization and electrochemical impedance spectroscopy. They showed that the susceptibility to pitting corrosion of MP nitinol in PBS can be influenced by the presence of amino acids, depending on the type of amino acid. However, the pitting susceptibility of MP and BO nitinol did not differ significantly (based on a t-test, with p < 0.05 being statistically significant) between serum and PBS, suggesting that the combination of proteins and amino acids in serum had little effect. The impedance spectra showed near-capacitive behavior in serum and in PBS alone and with amino acids, and the data could be fitted by a parallel resistance-capacitance (as a constant phase element) circuit associated with the passive oxide film. The capacitance indicated that serum proteins were adsorbed on nitinol but that little, if any, adsorption of the three amino acids under study occurred at the corrosion potential. There did not appear to be a correlation between breakdown and the adsorption of organic compounds in serum.

Blocked gap junctional coupling increases glutamate-induced neurotoxicity in neuron-astrocyte co-cultures

Gap junctional communication is likely one means by which neurons can endure glutamate cytotoxicity associated with CNS insults (i.e. ischemia). To examine this neuroprotective role of gap junctions, we employed gap junctional blockers to neuronal and astrocytic co-cultures during exposure to a high concentration of extracellular glutamate. Co-cultures were treated with the blocking agents carbenoxolone (CBX; 25 microM), 18alpha-glycyrrhetinic acid (AGA; 10 microM), vehicle or the inactive blocking analogue glycyrrhizic acid (GZA; 25 microM). Twenty-four hours following the insult, cell mortality was analyzed and quantified by the release of lactate dehydrogenase (LDH) into the media, the cells' inability to exclude propidium iodide, and terminal dUTP nick end labeling (TUNEL). Measurement of LDH release revealed that the glutamate insult was detrimental to the co-cultures when gap junctions were blocked with CBX and AGA. Based on propidium iodide and TUNEL labeling, the glutamate insult caused significant cell death compared to sham vehicle and mortality was amplified in the presence of CBX and AGA. Since blockers were not themselves toxic and did not affect astrocytic uptake of glutamate, it is likely that blocked gap junctions lead to the increased glutamate cytotoxicity. These findings support the hypothesis that gap junctions play a neuroprotective role against glutamate cytotoxicity.

Forty years of amino acid transmission in the brain

This article is concerned with the discovery that amino acids, particularly L-glutamate and gamma-aminobutyrate (GABA), are central neurotransmitters. The crucial observations that lead to the conclusion that these two amino acids produce most of the synaptic excitation and inhibition in the central nervous system, were made in late 1950's. The combination of neurochemical knowledge and improved electrophysiological techniques was paramount in making these discoveries possible. In particular, the use of specific antagonists in microiontophoretic experiments provided the most decisive evidence. The relationship is also explored between these early findings and those of the present era characterised by extensive use of techniques of molecular biology and the development of drugs against targets identified 30 to 40 years ago.

Glutamate and high-dose glucose-insulin-potassium (GIK) in the treatment of severe cardiac failure after cardiac operations

Postischemic derangement of myocardial metabolism that is further aggravated by the systemic neuroendocrine response to surgical trauma may explain reversible myocardial dysfunction after cardiac surgical procedures. Since 1991, all patients with signs of cardiac failure after operation for ischemic heart disease (45/515 patients) were treated according to our metabolic strategy. Sixteen patients in whom we previously would have considered use of an intraaortic balloon pump were treated by prolonged unloading of the heart with cardiopulmonary bypass, by glutamate infusion, and by high-dose glucose-insulin-potassium. Rapid improvement in hemodynamic performance was seen in the first hour and almost full recovery within 6 hours in the surviving patients (12/16). None of the 3 patients requiring mechanical assist survived. Our early clinical experience suggests that metabolic support with glutamate and high-dose glucose-insulin-potassium is a safe treatment with a high success rate in reversible cardiac failure.

Patients receiving glutamine-supplemented intravenous feedings report an improvement in mood

Nutritional effects have traditionally focused on outcomes, such as nitrogen balance, wound healing, or muscle strength. Little emphasis has been placed on how biochemical or physical improvements translate into functional changes as perceived by the patient. Because glutamine (GLN)-supplemented nutrition promotes protein synthesis and improves nitrogen balance, we assessed the mood of individuals participating in a randomized controlled blinded trial receiving GLN solutions. Patients (n = 23) undergoing marrow transplantation were randomized by the research pharmacist to receive either standard total parenteral nutrition (TPN) (control) or GLN-containing TPN (40 g of glutamine total). The solutions were isocaloric and isonitrogenous and were administered until the patient was eating 50% of estimated requirements. Before TPN and on admission to the hospital, the patient completed the Profile of Mood States questionnaire, a standardized test quantifying the degree of tension, depression, anger, vigor, fatigue, and confusion. The patient completed the questionnaire again at the end of TPN near discharge. The tests were scored and the change from baseline for each mood for both groups of patients was calculated at the completion of TPN. The scores for vigor in the control group (delta scores) decreased over the course of hospitalization as would be expected with a serious illness. The group receiving glutamine TPN, however, essentially showed little change in vigor from baseline and the delta score was significantly different from the control group (delta vigor score -0.85 +/- 2.1 in the glutamine group vs. -5.90 +/- 1.7 in the control group; p = .07).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization and mechanism of glutamate neurotoxicity in primary striatal cultures

Excitatory amino acids may play a role in the pathogenesis of cell death in neurodegenerative diseases, including Huntington's disease (HD). In an attempt to develop a tissue culture model for HD, the toxicity of glutamate was examined in primary striatal cultures derived from newborn rats. Morphological criteria were used to determine the toxic effects of glutamate in 6-, 12-, and 18-day-old cultures which were examined before and after 1-3 h of exposure to glutamate. Although younger cultures demonstrated little susceptibility to glutamate relative to controls, the number of neurons in older cultures was significantly depleted in the presence of glutamate. Glutamate toxicity was dose-dependent, with an ED50 of approximately 300 microns glutamate, and a maximal effect was observed within 3 h of initial exposure. Affected neurons demonstrated somal swelling within 1 h of glutamate exposure and disruption of neuritic processes and somal integrity within 3 h. Cell death was significantly increased by raising the extracellular calcium concentration and could be decreased by the addition of magnesium to the incubation medium. Moreover, the N-methyl-D-aspartate (NMDA) receptor agonist, quinolinic acid, showed a toxicity profile similar to that of glutamate. The NMDA receptor competitive antagonist, 2-amino-5-phosphonovalerate (APV) significantly reduced toxicity, albeit incompletely. An additional component of glutamate mediated toxicity in striatal cultures could be explained by activation of non-NMDA receptor subtypes. These in vitro studies indicate that glutamate is toxic to a subset of mature striatal neurons in the absence of a glutamatergic afferent input, and that this toxicity is mediated partially by the NMDA receptor, with an additional component due to non-NMDA receptors.

N-methyl-D-aspartate antagonists: ready for clinical trial in brain ischemia?

Antagonists of the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors may offer a new approach for the treatment of ischemic brain injury. This strategy is supported by a well-developed scientific foundation and encouraging results in a variety of in vivo and in vitro experimental models. Several specific antagonists, including MK-801, dextrorphan, dextromethorphan, and ketamine, have already been used at low doses in humans for other indications and are potential candidates for Phase I clinical trials.

Ketamine protects cultured neocortical neurons from hypoxic injury

The general anesthetic ketamine, which has recently been reported to block the excitation of cortical neurons by N-methyl-D-aspartate (NMDA), was found to markedly reduce neuronal loss in murine neocortical cell cultures exposed to a hypoxic atmosphere or to cyanide. These observations may be relevant to attempts to find pharmacological means of minimizing hypoxic brain damage in the clinical setting.

Glutamine synthetase activity in subdivisions of brain of the shark, Squalus acanthias

Specific activity of glutamine synthetase in Squalus acanthias (spiny dogfish) central nervous system regions was highest in the cerebellum and lowest in the spinal cord. The levels of activity may relate to the excitability of each region by regulating the glutamate pool.

Pre- and post-synaptic inhibition

We now know a great deal about central inhibitory mechanisms: how they are organized in various neuronal circuits ("feed-forward" and "feed-back" inhibitions, inhibition of inhibitory cells giving "disinhibition" which releases neuronal activity in a finely graded and particularly safe manner); how they exercise a preponderant control over much of central neurl activity (9, 13, 14, 57); and of course, how inhibition operates at the cell membrane, by increasing Cl- permeability; this has a stabilizing action post-synaptically, but presynaptically, in afferent fibers, it depresses transmitter release from afferent terminals. In addition, there is some evidence that GABA transport may be electrogenic and therefore may significantly modulate membrane excitability, and that GABA may selectively depress Ca2+ influx in afferent terminals and thus inhibit transmitter release particularly effectively. However, it is by no means certain that we are fully aware of all the possible ways in which GABA affects neuronal excitability and synaptic transmission, and there may well be further exciting surprises in store.

AMINO ACID TRANSPORT IN BRAIN CORTEX SLICES: III. THE UTILIZATION OF ENERGY FOR TRANSPORT

When rat brain cortex slices are incubated with amino acids in the presence of various sugars, the ratio of amino acid transported to the steady-state level of ATP (termed the transport quotient) is constant. The actual amount of ATP maintained is greatest with glucose and mannose but somewhat less with fructose and galactose. Certain α-keto acids (pyruvate, oxalacetate, α-ketoglutarate, or glutamate which can give rise to α-ketoglutarate) also support glycine transport but with a lesser efficiency. Fumarate, malate, and succinate have little ability to support amino acid transport.The transport supported by pyruvate is particularly resistant to the action of agents which markedly reduce the transport supported by glucose as substrate. These agents include dinitrophenol, digoxin, and uranyl ions. Inhibitory effects are obtained with high concentrations of potassium ions whether the substrate is glucose or pyruvate.These results are interpreted in terms of pyruvate dismutation and acetate thiokinase leading to ATP production and in terms of intracellular localization of ATP.

Free amino acids in the human aqueous humour

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