Mechanisms of ATP‐ and glutamate‐mediated calcium signaling in white matter astrocytes

Abstract

Neurotransmitters released at synapses mediate Ca2+ signaling in astrocytes in CNS grey matter. Here, we show that ATP and glutamate evoke these Ca2+ signals in white matter astrocytes of the mouse optic nerve, a tract that contains neither neuronal cell bodies nor synapses. We further demonstrate that action potentials along white matter axons trigger the release of ATP and the intercellular propagation of astroglial Ca2+ signals. These mechanisms were studied in astrocytes in intact optic nerves isolated from transgenic mice expressing enhanced green fluorescent protein (EGFP) under control of the human glial fibrillary acidic protein promoter (GFAP) by Fura-2 ratiometric Ca2+ imaging. ATP evoked astroglial Ca2+ signals predominantly via metabotropic P2Y1 and ionotropic P2X7 purinoceptors. Glutamate acted on both AMPA- and NMDA-type receptors, as well as on group I mGlu receptors to induce an increase in astroglial [Ca2+]i. The direct Ca2+ signal evoked by glutamate was small, and the main action of glutamate was to trigger the release of the "gliotransmitter" ATP by a mechanism involving P2X7 receptors; propagation of the glutamate-mediated Ca2+ signal was significantly reduced in P2X7 knock-out mice. Furthermore, axonal action potentials and mechanical stimulation of astrocytes both induced the release of ATP, to propagate Ca2+ signals in astrocytes and neighboring EGFP-negative glia. Our data provide a model of multiphase axon-glial signaling in the optic nerve as follows: action potentials trigger axonal release of ATP, which evokes further release of ATP from astrocytes, and this acts by amplifying the initiating signal and by transmitting an intercellular Ca2+ wave to neighboring glia.

Alcohol Abrogates Intracellular Ca2+ Elevation by Angiotensin II and ATP in Cultured Rat Astrocytes

“…In comparison, purinergic neuromodulator ATP caused a brief and steeper Ca 2+ peak with a much faster recovery phase (i.e., lack of prolonged plateau elevation of [Ca 2+ ]i. These results confirmed our previous work and were consistent with the data published by other investigators [2,10,11]. The commercially available primary culture of glial fibrillary acidic protein (GFAP)-positive astrocytes is ]i, the Ca 2+ transient signals elicited by these two stimuli are distinguishable by their peak amplitude and duration (Figure 1).…”

Neurotransmitter signaling in white matter

“…In addition, electrophysiological criteria have been used to distinguish glial cell types, allowing the first descriptions of functional glutamate, GABA-A and glycine receptors in identified astrocytes, oligodendrocytes, glioblasts and OPCs in situ (Berger et al, 1992;Butt and Jennings 1994;Pastor et al, 1995 ] does not duplicate the effects of GABA upon excitability leading to the conclusion that receptor expression is axonal (Sakatani et al, 1994). In general, it may be assumed that the effects of neurotransmitters on glial Ca 21 and membrane properties appear to be mediated largely by glial expression of neurotransmitter receptors (Butt and Jennings, 1994;Hamilton et al, 2008), whereas effects upon axonal excitability appear to be mediated largely by axolemma expression, rather than glial responses that subsequently modify excitability (Nikolaeva et al, 2009;Sun and Chiu, 1999;Zhang et al, 2004). …”

Neuroprotective roles of the P2Y2 receptor

“…Extracellular ATP release significantly increases process extension toward an injury site for resting or activated microglia [148]. Astrocytes under pathological conditions also can release ATP to activate P2Rs in neighboring cells [18,188,189], and inflammation in vivo can elevate extracellular ATP levels sufficiently to activate P2 receptors [18].…”