Modulation of extracellular glutamate concentration in rat brain slices by cystine‐glutamate exchange

Abstract

The role of cystine‐glutamate exchange in controlling the extracellular glutamate concentration in the central nervous system was examined by whole‐cell clamping neurons in rat brain slices, and using their glutamate receptors as sensors of extracellular glutamate concentration. Applying cystine to cerebellar slices generated a membrane current in Purkinje cells which was abolished by glutamate receptor blockers. Similar cystine‐evoked currents were seen in pyramidal cells of frontal cortex slices. Control experiments on non‐N‐methyl‐D‐aspartate (non‐NMDA) receptors in enzymatically isolated Purkinje cells showed that cystine did not produce a current in slice Purkinje cells by directly activating glutamate receptors, nor by potentiating the action of background levels of glutamate on receptors. Experiments on isolated salamander Müller cells showed that cystine did not block Na+‐dependent GLAST glutamate transporters (homologous to the transporters in the Bergmann glia ensheathing the Purkinje cells), nor did it block the current produced by EAAT4 and EAAC1 glutamate transporters in Purkinje cells. Thus the cystine‐evoked current in Purkinje cells is not due to a rise in extracellular glutamate concentration caused by block of Na+‐dependent uptake. The dependence of cystine‐evoked current on cystine concentration in slice Purkinje cells could be fitted by a Michaelis‐Menten relation with a Km of 250 μM. The Km predicted from this for cystine activating glutamate efflux is less than 140 μM, because of the non‐linear dependence on glutamate concentration of the Purkinje cell current. The current evoked by 1 mM cystine was little affected by removal of extracellular chloride or addition of 1 mM furosemide (frusemide), but was potentiated by 1 mM 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS). These data suggest that external cystine generates a current in slice Purkinje cells by activating cystine‐glutamate exchange in cells of the slice, releasing glutamate which activates non‐NMDA receptors in the Purkinje cell membrane.

Impairment in Consolidation of Learned Place Preference Following Dopaminergic Neurotoxicity in Mice is Ameliorated by N-acetylcysteine but not D1 and D2 Dopamine Receptor Agonists

“…For instance, systemic administration of NAC to cocaine-withdrawn rats increased extracellular levels of glutamate in the nucleus accumbens (Baker et al, 2003) via the cystine-glutamate exchange system (Warr et al, 1999). In addition, evidence suggests the co-activation of ionotropic glutamate receptors through direct interaction with GSH (Oja et al, 2000;Varga et al, 1997).…”

Expression of the cystine-glutamate exchanger (xc −) in retinal ganglion cells and regulation by nitric oxide and oxidative stress

“…This issue has been raised in studies of system x c -in brain slices [36]. While a system that provides necessary constituents for synthesis of glutathione would arguably be beneficial, it must be kept in mind also that when system x cis functioning in vivo, it is exchanging glutamate for cystine.…”

“…This has been demonstrated in many neuronal cell types [32][33][34] including retinal neurons [33,35]. In retina, a low concentration of NO plays a protective role in glutamate neurotoxicity by closing the NMDA receptor gated ion channel, but elevated levels of NO, interacting with oxygen radicals, become toxic and mediate glutamate-induced neurotoxicity [35,36].…”

Context-specific Enhancement of Glutamate Transmission by Cocaine

“…However, there is no clear mechanism by which different environmental stimuli can modulate glial function. Alternatively, cysteine-glutamate exchange makes a contribution towards maintaining extracellular glutamate levels in the brain (Warr et al 1999;Jabaudon et al 1999), and it is possible that the environment may modify this process.…”