2. Evidence for A m i n o Acids as T ransmi t t e r s . . . . . . . . . . . . . . . . . . . . 99 2.1. Synthesis and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . 100 2.2. Synapt ic Release . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 2.3. Postsynapt ic Act ion . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 2.4. Postsynapt ic An tagon i s t s . . . . . . . . . . . . . . . . . . . . . . . . . 103 2.5. Inac t iva t ion and R e m o v a l . . . . . . . . . . . . . . . . . . . . . . . . . 104

Amino acid transmitters in the mammalian central nervous system

The importance of A TP as the main source of chemical energy in living matter and its involve­ ment in cellular processes has long been recog­ nized. The primary mechanism whereby higher or­ ganisms, including humans, generate ATP is through mitochondrial oxidative phosphorylation. For the majority of organs, the main metabolic fuel is glucose, which in the presence of oxygen under­ goes complete combustion to CO2 and H20: C6H1206 + 602 � 602 + 6H20 + energy ( 1) The free energy (.:lG) liberated in this exergonic re­ action is partially trapped as ATP in two consecu­ tive processes: glycolysis (cytosol) and oxidative phosphorylation (mitochondria). The first produces 2 mol of ATP per mol of glucose, and the second 36 mol of ATP per mol of glucose. In the latter case, 6 mol of ATP are contributed from the oxidation of 2 mol of NADH generated in the cytosol during gly­ colysis and transferred into the mitochondria indi­ rectly through various \"shuttle\" systems. (In the a-glycerophosphate shuttle, the yield of ATP per NADH is reduced from 3 to 2 because the relevant mitochondrial dehydrogenase is a flavoprotein­ linked enzyme). Thus, oxidative phosphorylation yields 1718 times as much useful energy in the form of ATP as can be obtained from the same amount of glucose by glycolysis alone. It is there­ fore not surprising that limitation of O2 supply pro­ duces very damaging effects on cellular function. The brain is one of the organs that is particularly sensitive to lack of oxygen and in humans at rest is responsible for 20% of total O2 consumption al­ though it accounts for only 2% of the body weight. The role of energy in the maintenance of central

https://journals.sagepub.com/doi/pdf/10.1038/jcbfm.1989.2

ATP and brain function

N-Acetyl-L-Aspartic acid: A literature review of a compound prominent in 1H-NMR spectroscopic studies of brain

Brain ischemia was induced for 10 or 30 min by clamping the common carotid arteries in rabbits whose vertebral arteries had previously been electrocauterized. EEG and tissue content of high energy phosphates were used to verify the ischemic state and to evaluate the degree of postischemic recovery. Extracellular levels and total contents of amino acids were followed in the hippocampus during ischemia and 4 h of recirculation. At the end of a 30-min ischemic period, GABA had increased 250 times, glutamate 160 times, and aspartate and taurine 30 times in the extracellular phase. The levels returned to normal within 30 min of reflow. A delayed increase of extracellular phosphoethanolamine and ethanolamine peaked after 1–2 h of reflow. Ten minutes of ischemia elicited considerably smaller but similar effects. With respect to total amino acids in the hippocampus, glutamate and aspartate decreased to 30–50% of control while GABA appeared unaffected after 4 h of reflow. Alanine, valine, phenylalanine, leucine, a...

https://journals.sagepub.com/doi/pdf/10.1038/jcbfm.1985.56

Ischemia-Induced Shift of Inhibitory and Excitatory Amino Acids from Intra- to Extracellular Compartments

Metabolism and role of glutamate in mammalian brain.

PREVIOUS studies in this laboratory have been concerned with chemical changes in the brain associated with convulsive activity (STONE et al., 1960) as revealed by analysis of brain tissue after in vivo fixation with liquid air. The development by MOORE and STEIN (1954) of ion exchange procedures for the separation of amino acids has made it feasible to extend the study of cerebral constituents during convulsions to include many free amino acids and related substances. These compounds are of particular interest in view of recent investigations by GEIGER and his coworkers (1960 a, 6) indicating that protein turnover in the brain is accelerated during convulsive activity induced by pentylenetetrazol. Accordingly, the cerebral levels of a large number of nitrogenous constituents have been determined in control experiments and during seizures induced by picrotoxin or by pentylenetetrazol. In an attempt to classify different types of seizures on the basis of specific chemical patterns (STONE et al., 1960), the seizures induced by these two agents have been tentatively classed together as representing a type in which distinctive metabolic changes have not yet been observed. As an aid in the interpretation of any changes which might be observed during seizures or other conditions, it was considered essential to study also post-mortem changes and the effects of anoxia and of ammonium chloride infusion.

Free amino acids and related compounds in dog brain: post-mortem and anoxic changes, effects of ammonium chloride infusion, and levels during seizures induced by picrotoxin and by pentylenetetrazol.

EARLY studies of insulin shock, particularly those by HIMWICH and his collaborators, showed that profound hypoglycaemia is accompanied by a decreased cerebral uptake of glucose and of oxygen from the blood stream. HIMWICH (1951) reviewed these studies and cited much evidence indicating that the behavioural and electrographic manifestations of hypoglycaemia are related to the reduced supply of glucose to the brain and the consequent decrease in available oxidative energy. More recently, GEIGER (1958) and his coworkers found that the perfused brain of the cat is able to function for more than 1 hr without exogenous glucose if a high perfusion rate is maintained ; this finding suggests that the brain can utilize endogenous non-carbohydrate substrates and that hypoglycaemic symptoms are due in part to the accumulation of toxic breakdown products of these substrates. ABOOD and GEIGER (1955) found decreases in the protein, lipid and nucleic acid contents of the perfused cat brain deprived of glucose and an associated accumulation in the perfusing blood of small quantities of nitrogenous substances, especially GABA, glutathione, glutamate and creatine. On the other hand, SAMSON, DAHL, DAHL and HIMWICH (1959) found that insulin shock did not significantly decrease the cerebral proteins in cats, and DAWSON (1950) made the same observation in rats. A number of other cerebral constituents are known to undergo quantitative changes during hypoglycaemia. These include glycogen, acid-soluble phosphates, and certain of the free amino acids and related compounds. Some of these substances represent possible sources of limited amounts of energy, either as oxidative substrates or as phosphorylated intermediates, while others may occur as breakdown products of proteins, lipids, or other substrates. Pertinent reports are cited in subsequent sections of this paper; in general, these chemical changes have not been studied in temporal relation to the development of electrographic abnormality. We have attempted to follow simultaneously the progression of the electrographic and a number of chemical changes in insulin shock, and their reversal on termination of the hypoglycaemia. E X P E R I M E N T A L The experiments were done on adult male dogs weighing 8-23 kg. Food was withheld for a period of 18-24 hr, after which the cranium was exposed and opened and most of the calvarium was removed, the dura mater remaining intact. Preparation was made for electrographic recording from three cortical areas and for freezing the brain in situ with liquid air. Blood pressure was measured from a cannulated femoral artery in the final stages of the experiment. The details of these procedures have been described (TEWS, CARTER, ROA and STONE, 1963).

Chemical changes in the brain during insulin hypoglycaemia and recovery

THE first reliable method for the measurement of glycogen in brain tissue was published by KERR (1936). He and his coworkers found that a profound insulin hypoglycemia reduced the brain glycogen level in dogs, cats, and rabbits (KERR and GHANTUS, 1936; KERR, HAMPEL and GHANTUS, 1937). This change was confirmed in dogs by CHESLER and HIMWICH (1944), in cats by OLSEN and KLEIN (1947~7, b), and recently in rats by SCHILLER (1958). KERR and ANTAKI (1937) studied effects of picrotoxin, pentylenetetrazol, and strychnine in rabbits. These convulsants did not induce measurable changes in the brain glycogen. In these experiments, however, the convulsive activity was suppressed by administration of an anaesthetic before the brain was frozen. LEPAGE (1946) reported that intense and prolonged pentylenetetrazol seizures reduced the brain glycogen in rats. KLEIN and OLSEN (1947) tested a series of convulsants on cats paralysed with dihydro-B-erythroidine and found a decrease in brain glycogen associated with cerebral stimulation. In a smd1 series of similar experiments on dogs under morphine, GURDJIAN, WEBSTER and STONE (1 947) found that pentylenetetrazol did not induce statistically significant changes in the glycogen level. CHANCE and YAXLEY (1950) reported findings in the mouse which are in sharp contrast with all of those quoted above. These workers studied the effects of insulin and a number of other convulsing agents, all of which appeared to increase the brain glycogen above the normal level. The conclusion was stated that an increase of glycogen in the mouse brain is specifically related to the convulsive discharge. Further data in support of this thesis were presented by CHANCE (1951, 1953) and by CHANCE and WALKER (1953-54). This discrepancy has never been resolved. Since the possibility of a species difference is apparent, further studies of brain glycogen in the mouse seemed desirable. The determination of glycogen has been improved and simplified by the application, after hydrolysis, of a specific enzymic method for glucose. The convulsants used were pentylenetetrazol, picrotoxin, and insulin. Pentobarbital was also tested in order to compare the effect of an anaesthetic with those of the excitatory agents.

Samuel H. Carter

Papers

Free amino acids and related compounds in dog brain: post-mortem and anoxic changes, effects of ammonium chloride infusion, and levels during seizures induced by picrotoxin and by pentylenetetrazol.

Chemical changes in the brain during insulin hypoglycaemia and recovery

Effect of convulsants on brain glycogen in the mouse.