Conduction block and glial injury induced in developing central white matter by glycine, GABA, noradrenalin or nicotine, studied in isolated neonatal rat optic nerve

The damaging effects of excessive glutamate receptor activation have been highlighted recently during injury in developing central white matter. We have examined the effects of acute exposure to four other neurotransmitters that have known actions on white matter. 80 min of Glycine or GABA-A receptor activation produced a significant fall in the compound action potential recorded from isolated post-natal day 10 rat optic nerve. This effect was largely reversed upon washout. Nicotinic acetylcholine receptor (nAChR) or adrenoreceptor activation with noradrenalin resulted in an ~35% block of the action potential that did not reverse during a 30 min washout period. While the effect of nAChR activation was blocked by a nAChR antagonist, the effect of noradrenalin was not ablated by α- or β-adrenoreceptor blockers applied alone or in combination. In the absence of noradrenalin, co-perfusion with α- or β-adrenoreceptor blockers resulted in non-reversible nerve failure indicating that tonic adrenoreceptor activation is required for nerve viability, while over activation of these receptors is also damaging. Nerves exposed to nAChR + adrenoreceptor activation showed no axon pathology but had extensive glial injury revealed by ultrastructural analysis. Oligodendroglia exhibited regions of membrane vacuolization while profound changes were evident in astrocytes and included the presence of swollen and expanded mitochondria, vacuolization, cell processes disintegration and membrane breakdown. Blinded assessment revealed higher levels of astrocyte injury than oligodendroglial injury. The findings show that over-activation of neurotransmitter receptors other than those for glutamate can produce extensive injury to developing white matter, a phenomenon that may be clinically significant.