Glutamine—a Major Substrate for Nerve Endings

Summary:We have recently demonstrated the age related vulnerability of hippocampal neurons to 20-min forebrain ischemia in spontaneously hypertensive rats (SHR). In the present study, we investigated the effect of aging on the release of amino acids in the hippocampus during transient cerebral ischemia for 20 min. Concentra tions of extracellular amino acids and cerebral blood flow in the CA 1 subfield were examined by an in vivo brain dialysis technique and a hydrogen clearance method, re spectively, in adult (5-7 month) and aged (19-23 month) female SHR. During cerebral ischemia by bilateral ca rotid artery occlusion, cerebral blood flow to the hippo campus decreased to 20% of the resting values in both groups. After recirculation, both groups showed delayed hypoperfusion which was more prominent in the aged Aging and hypertension are well-known risk fac tors for cerebrovascular diseases and vascular de mentia (Ueda et aI. , 1992). Aged animals with ex perimental cerebral ischemia have been claimed as clinically relevant (Millikan, 1992), although such studies on the effects of aging on neuronal ischemic damage are limited (Futrell et aI. , 1991). Recently, we reported that the hippocampus in aged (18-22 month) spontaneously hypertensive rats (SHR) is more susceptible to transient cerebral ischemia than in the adult (�8 month) SHR (Yao et aI., 1991). Reduction of perfusion pressure was identical be tween adult and aged SHR, and thus, factors other 227SHR. In the adult rats, concentrations of both aspartate and glutamate increased to -8-fold of the resting values during ischemia. The elevation of these excitatory amino acids in the adult SHR was not significantly different from that in the aged rats. In contrast, the concentration of taurine increased 26-fold in the adult SHR but only 16-fold in the aged rats. Changes in other amimo acids were not different between the two groups. These results indi cate that an imbalance of excitatory and inhibitory amino acids, e.g., smaller release of taurine, during ischemia may, at least in part, contribute to the age-related vulner ability of hippocampal neurons to transient cerebral isch emia in SHR.

Section: Discussionsupporting

confidence: 91%

Ischemia-Induced Release of Amino Acids in the Hippocampus of Aged Hypertensive Rats

Ooboshi

Sadoshima²,

Yao³

et al. 1995

“…In particular, MSO inhibition of glu tamine synthetase blocks glutamine production in astrocytes, an effect which might affect neuronal survival under pathological conditions (Swanson et aI., 1990b). In brain, MSO lowers extracellular glu tamine (Tews and Stone, 1964), a major precursor for neurotransmitter glutamate (Bradford et al, 1978;Somers and Beckstead, 1990). In the present study, however, glutamine was excluded from the media of all cultures during glucose deprivation, and de novo synthesis of glutamine was limited by the absence of glucose and glutamate in the medium CONTROL INSULIN MSO (Waniewski and Martin, 1986;Yudkoff et aI., 1988;Zielke et aI., 1989).…”

Section: Discussionmentioning

confidence: 56%

Glial Glycogen Stores Affect Neuronal Survival during Glucose Deprivation in vitro

Swanson

Choi

1993

225

155

Summary: Glia perform several energy-dependent func tions that may aid neuronal survival under pathological conditions. Glycogen is the major energy reserve in brain, and it is localized almost exclusively to astrocytes. Using murine cortical cell cultures containing both glia and neu rons, we examined the effect of altered glial glycogen stores on neuronal survival following glucose depriva tion. As previously reported, cultures exposed for several hours to media lacking glucose developed widespread neuronal degeneration without glial degeneration. If glial astrocyte glycogen content was increased to 2-3 times Astrocytes perform multiple functions essential for the normal activity of neurons in the brain. In addition to providing mechanical support and trophic factors (Bunge and Waksman, 1985), astro cytes maintain homeostasis of the brain extracellu lar fluid (ECF). Astrocytes are largely responsible for the uptake and catabolism of excitatory amino acids and other neurotransmitters (Ericinska et aI., 1986;Hertz, 1979;Varon and Somjen, 1979), and for the uptake and redistribution of K + (Hertz, 198 1; Jendelova and Sykova, 199 1). Astrocytic am monia fixation provides the only mechanism other than diffusion for clearing brain ammonia (Cooper and Lai, 1987). Astrocytes may also play an impor-

“…A release of cy toplasmic glutamate (through reversal of uptake) would be compatible with the decreased immunola beling for glutamine. A fall in the cytoplasmic level of glutamate would relieve the strong feedback in hibition of glutaminase (Bradford et al, 1978), and increase glutamine consumption. The vesicular pool of glutamate is spatially segregated from the glutaminase and would not be expected to interfere with the activity of this enzyme (see, for example, Kvamme and Lenda, 1981).…”

Section: Redistribution Of Glutamatementioning

confidence: 99%

Redistribution of Glutamate and Glutamine in Slices of Human Neocortex Exposed to Combined Hypoxia and Glucose Deprivation in vitro

Aas

Berg-Johnsen

Hegstad

et al. 1993

Summary: This study was undertaken to elucidate the roles of neurons and glial cells in the handling of gluta mate and glutamine, a glutamate precursor, during cere bral ischemia. Slices (400--6 00 fLm) from human neocortex obtained during surgery for epilepsy or brain tumors were incubated in artificial cerebrospinal fluid and subjected to 30 min of combined hypoxia and glucose deprivation (an in vitro model of brain ischemia). These slices, and con trol slices that had not been subjected to "ischemic" con ditions, were then fixed and embedded. Ultrathin sec tions were processed according to a postembedding im munocytochemical method with polyclonal antibodies raised against glutamate or glutamine, followed by colloi dal gold-labeled secondary antibodies. The gold particle densities over various tissue profiles were calculated from electron micrographs using a specially designed computer program. Combined hypoxia and glucose dep rivation caused a reduced glutamate immunolabeling in neuronal somata, while that of glial processes increased. Following 1 h of recovery, the glutamate labeling of neu ronal somata declined further to very low values, com pared to control slices. The glutamate labeling of glial cells returned to normal levels following recovery. InThe pathogenesis of ischemic brain damage is not fully understood, and a number of neurotoxic sub stances has been implicated (Krause et aI. , 1988; Siesjo, 1988;Ginsberg, 1990; Siesjo et aI. , 1990 503axon terminals, no consistent change in the level of glu tamate immunolabeling was observed. Immunolabeling of glutamine was low in both nerve terminals and neuronal somata in normal slices and was reduced to nondetectable levels in nerve terminals upon hypoxia and glucose dep rivation. This treatment was also associated with a re duced glutamine immunolabeling in glial cells. Reversed glutamate uptake due to perturbations of the transmem brane ion concentrations and membrane potential proba bly contributes to the loss of neuronal glutamate under "ischemic" conditions. The increased glutamate labeling of glial cells under the same conditions can best be ex plained by assuming that glial cells resist a reversal of glutamate uptake, and that their ability to convert gluta mate into glutamine is compromised due to the energy failure. The persistence of a nerve terminal pool of glu tamate is compatible with recent biochemical data indi cating that the exocytotic glutamate release is contingent on an adequate energy supply and therefore impeded dur ing ischemia.