Neurotransmitter signaling in white matter

Abstract

White matter (WM) tracts are bundles of myelinated axons that provide for rapid communication throughout the CNS and integration in grey matter (GM). The main cells in myelinated tracts are oligodendrocytes and astrocytes, with small populations of microglia and oligodendrocyte precursor cells. The prominence of neurotransmitter signaling in WM, which largely exclude neuronal cell bodies, indicates it must have physiological functions other than neuron-to-neuron communication. A surprising aspect is the diversity of neurotransmitter signaling in WM, with evidence for glutamatergic, purinergic (ATP and adenosine), GABAergic, glycinergic, adrenergic, cholinergic, dopaminergic and serotonergic signaling, acting via a wide range of ionotropic and metabotropic receptors. Both axons and glia are potential sources of neurotransmitters and may express the respective receptors. The physiological functions of neurotransmitter signaling in WM are subject to debate, but glutamate and ATP-mediated signaling have been shown to evoke Ca 21 signals in glia and modulate axonal conduction. Experimental findings support a model of neurotransmitters being released from axons during action potential propagation acting on glial receptors to regulate the homeostatic functions of astrocytes and myelination by oligodendrocytes. Astrocytes also release neurotransmitters, which act on axonal receptors to strengthen action potential propagation, maintaining signaling along potentially long axon tracts. The co-existence of multiple neurotransmitters in WM tracts suggests they may have diverse functions that are important for information processing. Furthermore, the neurotransmitter signaling phenomena described in WM most likely apply to myelinated axons of the cerebral cortex and GM areas, where they are doubtless important for higher cognitive function. GLIA 2014;62:1762-1779 Key words: glia, axon, astrocyte, oligodendrocyte, glutamate, ATP Introduction W hite matter (WM) is defined as a tract of myelinated axons-WM appears opaque or dense due to the fatty myelin in anatomical sections and in brain scans. Notwithstanding this, myelination is not restricted to WM and is also critical to rapid communication and integration in grey matter (GM) areas, such as in axons in the cortical GM and hippocampus. Hence, many aspects of neurotransmitter signaling to be covered in this review have resonance in GM and higher cognitive function. Indeed, in the human brain WM is a prominent feature of the cerebral cortex, the seat of higher intelligence. Myelination of GM is also evident in rodents, but discrete WM tracts are not pronounced in the cerebral cortex. As a general concept, WM are simply concentrations of myelinated axons bundled together into tracts that interconnect areas of GM. The molecular physiology of myelinated axons will be very similar in WM and GM, and they will be subject to the same neurotransmitter signaling phenomena that is the subject of this review. The main cells associated with myelinated axons are the myelinating oligodendro...

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“…In addition to glutamate transporters, white matter macroglia express a large repertoire of receptors for different transmitters [26,27]. Their activation upon action potential (AP) propagation triggers calcium signals in astrocytes in the optic nerve and corpus callosum, involving both glutamate and purine receptors [19,[28][29][30].…”

“…While it is thus established that glial cells in white matter can undergo calcium signalling in response to axonal AP firing [26,27], it is not known, if axonal activity does induce detectable increases in the sodium concentration of surrounding macroglial cells. In the present study, we addressed this question by performing quantitative ratiometric imaging with the fluorescent sodium indicator sodium-binding benzofurane isophthalate (SBFI) in acute tissue slices of the juvenile mouse corpus callosum.…”

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