Acute decrease in net glutamate uptake during energy deprivation

Abstract

(2, 3). Whereas some studies suggest that the source of accumulating glutamate is vesicular (synaptic) (4-6), others provide evidence for nonvesicular mechanisms (7-11).Previous investigations usually have used biochemical (e.g., microdialysis) or radiolabeling techniques to measure excitatory amino acid concentrations at predefined time points (3, 7-9). These methods, however, provide low spatial and͞or temporal resolution and offer little information on glutamate levels at the synaptic cleft, where the receptors mediating excitotoxicity are located. To detect changes in ambient glutamate levels during energy deprivation (ED) in real time, we have adopted an alternative approach in which the NMDARs of neurons undergoing ED are used as high-affinity glutamate sensors (12). This technique has allowed us to determine the relative contribution of vesicular and nonvesicular release to the increase in [glu] o during ED. Materials and MethodsOrganotypic Hippocampal Slice Preparation. Experiments were performed on rat organotypic hippocampal slice cultures. Tissue slices of 400-m thickness were prepared from 6-day-old rats and cultured by means of the roller-tube technique as described previously (13).Experimental Conditions. After 12-25 days in vitro, cultures were transferred to a recording chamber mounted onto the stage of an upright microscope (Axioscop FS 2; Zeiss), and then they were superfused with external solution (137 mM Na ϩ ͞2.7 mM K ϩ ͞146.2 mM Cl Ϫ ͞2.8 mM Ca 2ϩ ͞0.5 mM Mg 2ϩ ͞11.6 mM HCO 3 Ϫ ͞0.4 mM H 2 PO 4 Ϫ ͞5.6 mM D-glucose) at 29-31°C, pH 7.4. To induce ED, glucose was replaced with 2 mM 2-deoxyglucose, and 5 mM sodium cyanide (NaCN) was added (14).

Effects of Peroxisome Proliferator-Activated Receptor Gamma Agonists on Brain Glucose and Glutamate Transporters after Stress in Rats

“…Indeed, stress has been shown to increase extracellular glutamate concentrations in many brain areas (Lowy et al, 1993;Moghaddam et al, 1994;SteinBehrens et al, 1994), an effect that is proposed to result from compromised activity of the energy-dependent excitatory amino acid transporters. On the other hand, it has recently been shown that glutamate release is mainly due to reversed operation of neuronal glutamate transporters in processes such as brain ischemia and others (Warner et al, 1996;Jabaudon et al, 2000;Rossi et al, 2000). Thus, in stress, this is one among the various mechanisms, which, alone or combined, may be responsible for glutamate release (Lawrence and Sapolsky, 1994) although we have not seen modifications in glutamate uptake in neurons (EAAT-3).…”

The neuron-astrocyte-microglia triad in CA3 after chronic cerebral hypoperfusion in the rat: Protective effect of dipyridamole

“…At early stages after an ischemic insult the pyramidal neurons from CA1 and CA3 are exposed to similar triggering events such as the increase of extracellular glutamate (Mitani et al 1992). Energy deprivation reduces intracellular ATP modifying the ionic gradients and inverting glutamate transport both in pyramidal CA1 (Rossi et al 2000) and CA3 neurons (Jabaudon et al 2000). At later times, NMDAmediated responses increase in CA1 pyramidal neurons, which become more sensitive, while are transiently depressed in CA3 pyramidal neurons, that become more resistant.…”

Reversed Actrocytic GLT-1 during Ischemia is Crucial to Excitotoxic Death of Neurons, but Contributes to the Survival of Astrocytes themselves

“…During ischemia, the activity of Na + ,K + -ATPase is markedly suppressed, resulting in the accumulation of intracellular sodium ions (8). Under such conditions, the operation of GLT-1 is reversed, and three sodium ions are thus co-transported with a glutamate ion out of the cell (9). It has been postulated that the excessive influx of Na + into astroglial cells causes significant astroglial cell death (10) due to Ca 2+ overload probably via a reversed Na + /Ca 2+ exchanger.…”