pHi regulation in astrocytes has been examined primarily

pHi regulation in astrocytes has been examined primarily in cell culture rather than in situ. Two HCO3−-dependent transporters, NBC and Na+-driven Cl-HCO3 exchange (NDCBE), have been described [10,16], in addition to Cl-HCO3 anion exchange (AE) [96]. Culture studies indicate that NHE is the principal HCO3−-independent H+-extrusion protein in astrocytes, coupling H+ extrusion to the inward Na+ gradient [10,12,14,16,81]. Cultured astrocytes also express a V-type H-ATPase that can be selectively blocked by bafilomycin, although the involvement of this pump in pH maintenance and recovery following an 20(S)-Hydroxycholesterol load is limited in culture [78,79]. Very little data is available on the mechanisms used by WM astrocytes to regulate pH. Using the post-natal day 2 (P2) rodent optic nerve (RON), we recently examined the involvement of V-ATPase and NHE in the maintenance of steady-state pHi and pHi recovery following an acid load in perinatal WM astrocytes in situ [49]. In contrast to cultured astrocytes, V-ATPase is the dominant HCO3−-independent H+-extrusion mechanism in this in situ WM astrocyte population [49]. V-ATPase is sensitive to Cl− and high V-ATPase expression endows these astrocytes with a relatively basic pHi that is highly Cl−-dependent; features not found in cultured cortical astrocytes. These observations raise the possibility that WM astrocytes differ significantly from the established model for astrocyte pH regulation in the brain, which is likely to significantly influence how GluR in WM respond to ischemic conditions.
Oligodendrocytes: the focus of GluR-mediated injury in WM? GluR-mediated oligodendrocyte death is typically caused by excitotoxicity and it is clearly involved in ischemic damage to GM and WM [63,84]. Thus, over-activation of AMPA and kainate receptors causes oligodendrocyte death and primary and/or secondary myelin destruction as a consequence of massive influx of Ca2+ that causes mitochondrial depolarization, increased production of radical oxygen species, and the release of pro-apoptotic factors, which in turn activate caspase-dependent and -independent oligodendrocyte death [93]. Oligodendrocyte excitotoxicity is favoured by perturbed glutamate homeostasis during neuroinflammation as glutamate transporter function is altered as to increase the extracellular concentration of this transmitter (reviewed in [13]). In turn, excessive activation of internodal axonal glutamate receptors may induce the release of substantial amounts of Ca2+ from axoplasmic ER and activate calcium-dependent enzymes that ultimately ignite the collapse of the axon [102]. Oligodendrocytes are the major cell type in mature WM, and damage of WM is a major cause of functional disability in cerebrovascular disease including hypoxic-ischemic injury periventricular leukomalacia in neonates, stroke and cardiac arrest in adults, as well as in vascular dementia in the aging brain. Ischemic insults typically result in transmembrane ion gradient breakdown and membrane depolarization, leading ultimately to toxic intracellular Ca2+ overload. The final stage is the activation of Ca2+-dependent enzymes (e.g. calpains, phospholipases, and other enzymes), resulting in irreversible damage of WM glia and axons [48,104,112]. Immature and less so mature oligodendrocytes are very sensitive to transient oxygen and glucose deprivation [34]. Thus, simulated ischemia in young animals induces an inward current in oligodendrocytes that is partly mediated by NMDA and AMPA/kainate receptors [57], and which is directly toxic to the cell processes [92]. In addition, Ca2+ levels also increase in myelin itself during ischemia (an effect that is abolished by broad-spectrum NMDA receptor antagonists), causing ultrastructural damage to the myelin sheath [68]. Adult WM becomes intrinsically more vulnerable to ischemia with age and the mechanisms of glutamate-mediated damage change [7]. Thus, ischemic WM injury in older mice is predominately mediated by glutamate release through reverse glutamate transport (probably from astrocytes) and the ensuing activation of AMPA/kainate-type glutamate receptors [7]. Intriguingly, blockade of NMDA receptors aggravates the outcome of ischemia in older animals [7].