Interaction of selective cellular elements in white matter injury: the role of astrocytes

White matter injury is common in neurological conditions including stroke and cerebral palsy. However, how different cellular elements of white matter interact during injury remains largely unknown. This study examined the effects of astrocyte damage on neighbouring axons and oligodendrocytes. Astrocytes were metabolically inactivated by an astrocyte specific toxin, fluorocitric acid in rat optic nerves from three different developmental ages, and the effects assessed by immunohistochemical staining. Subsequently, functionality of the nerves was assessed using electrophysiological recording of compound action potentials, and the role of glutamate was examined using glutamate receptor antagonists. Ultrastructural analysis of different cellular elements of white matter was performed using electron microscopy. Lastly, nerves were stimulated at different frequencies; compound action potential measurements and electron microscopic analysis were performed. The nerves exhibited stable compound action potentials and normal ultrastructural features in the presence of glucose or lactate showing that lactate can be as effective as glucose in supporting axonal functions. Fluorocitric acid treatment produced age-dependent structural injury in astrocytes, axons and oligodendrocytes, with compound action potentials decline in all ages. Adult nerves were most sensitive to fluorocitric acid-induced injury and perinatal (P0) nerves behaved uniquely. Although the axons stained normally, they did not conduct action potentials. Glutamate receptor blockers only partially recovered compound action potentials. These results show that rat optic nerves provide a stable model to study white matter physiology and pharmacology over several hours. Although axons stained normally, they did not conduct action potentials: therefore, immunohistochemical staining alone can be misleading in assessing cell viability and functionality. Metabolic inhibition of astrocytes led to age-dependant injury in all ages, with adult cells most sensitive to injury. Adult myelin sheaths showed features of injury common in aging suggesting that astrocyte malfunction produces a microenvironment around axons similar to that in normally aging brains. Lastly, higher frequency activation of nerves improved the neurological outcome when astrocytes were damaged, perhaps reflecting better utilization of the lactate by the stimulated nerves.