The role of GABA, glycine, nicotinic, and adrenergic receptors in developing central white matter ischaemic injury and the features of NG2 expressing cells in this tissue

Prolonged perinatal ischaemia is an important factor in the development of periventricular leukomalacia (PVL), which is the most common white matter pathology associated with cerebral palsy (CP): approximately 1.5 to 2.5 per 1,000 live births per year suffer from CP. During central nervous system (CNS) ischaemia, there is an excessive accumulation of extracellular neurotransmitters. Ischaemia affects axons and glial cells of white matter, which occupy approximately 50% of the human brain. The mechanisms of ischaemic-induced injury in astrocytes vary at different ages. Ischaemic injury in postnatal day 0 (P0) of rat optic nerve (RON) astrocytes is Ca[superscript 2+]-dependent and mediated by voltage-gated Ca[superscript 2+] channels (VGCCs). Astrocytes are replete with neurotransmitter receptors. Using an ex vivo model of ischaemia (oxygen-glucose deprivation: OGD) and Ca[superscript 2+] imaging, I investigated the role of gamma-aminobutyric acid (GABA), glycine, nicotine and norepinephrine in P0 RON during ischaemia. OGD produced a rapid and significant increase in cell death. A GABA antagonist (picrotoxin), a glycine antagonist (strychnine) and their combination protected P0 RON astrocytes from ischaemic injury. They reduced the total percentage of cell death and postponed the initial cell death. Immunohistochemistry revealed that GABA and glycine receptors are expressed by astrocytes and axons in P0 RON. On the other hand, blocking nicotinic acetylcholine receptors (nAChRs: mecamylamine or α-bungarotoxin) or blocking adrenergic receptors (combination of propranolol and phentolamine) had no effect. Using a glial injury scoring system, ultrastructural studies confirmed the protective action of picrotoxin and strychnine against ischaemia, which cannot be achieved by mecamylamine or a combination of propranolol and phentolamine. Chondroitin sulphate proteoglycans (NG2) (+) cells are a distinct type of glial cell which, over time, have come to be known as the fourth type of glia. These cells are distributed throughout the developing and adult CNS, and are known to be mitotically active even in the adult CNS. The astrocyte fate of NG2 (+) cells is a matter of debate. In the current study, the focus was on studying the morphological features of NG2 (+) cells at the ultrastructural level and under confocal microscope, and more precisely their relation to astrocytes in nRON following post-embedding immunolabelling. Immuno-electron microscope (I-EM) revealed NG2 immunoreactivity in nRON astrocytes. Double immunolabelling showed an overlap between NG2 (+) and (glial fibrillary acidic protein) GFAP (+) populations in nRON, adult RON and cortical grey matter.