Astrocyte Ca(2+)-evoked ATP release regulates myelinated axon excitability and conduction speed*

INTRODUCTION: Astrocytes support neuronal function throughout the central nervous system. In the grey matter they regulate synapse number during development, remove synaptically-released neurotransmitters to terminate their action and prevent excitotoxicity, control the extracellular potassium concentration to prevent hyperexcitability, regulate blood flow to ensure an adequate energy supply, provide lactate to neurons for energy, and respond to rises of intracellular calcium concentration ([Ca(2+)](i)) by releasing ATP and other gliotransmitters that act on neuronal receptors to modulate information processing. However, their role is unclear in the white matter, which transmits information rapidly between grey matter areas using axons wrapped with capacitance-reducing myelin, although they have been suggested to regulate myelination during development and during normal function. RATIONALE: Recently it has been suggested that learning and memory may reflect, not only changes of synaptic function in the grey matter, but also changes of white matter function. In particular, neural circuit function might be regulated by changes in the conduction speed of myelinated axons that result in an altered arrival time of action potentials at a distant neuron. These speed changes might be brought about by alterations of the properties of the passively conducting myelinated internodes, or of the intervening excitable nodes of Ranvier where the action potential is generated. We applied immunohistochemistry to assess how astrocytes interact with myelinated axons, used neuronal stimulation and light evoked calcium uncaging in astrocytes to evoke Ca(2+)-dependent release of gliotransmitters, and employed electrophysiology and pharmacology to characterise how astrocyte-released substances might affect the axon initial segment (AIS) and nodes of Ranvier of myelinated neurons. Measurements of conduction velocity and computer modelling allowed us to interpret the results. RESULTS: Astrocytes closely approach the axons of myelinated neurons in layer V of the cerebral cortex. Uncaging Ca(2+) within astrocytes, or stimulating spike trains in neurons, evoked a rise of astrocyte [Ca(2+)](i) that triggered the release of ATP-containing vesicles from these cells. This evoked an inward current in the AIS and nodes of Ranvier of the pyramidal neurons. Pharmacology showed that this was mediated by the activation of G(s)-linked adenosine A(2a) receptors, implying that the released ATP was converted to adenosine by extracellular enzymes. The A(2a) receptors raise the intracellular concentration of cyclic AMP, which activates HCN channels mediating the inward hyperpolarization-activated current I(h), and thus depolarizes the cell. In the AIS the activation of A(2a) receptors alters excitability and hence action potential generation, while in the nodes of Ranvier it decreases the conduction speed of the action potential along the axon. CONCLUSION: As in the grey matter, astrocyte [Ca(2+)](i) regulates the release of ATP into the extracellular space in the white matter. After conversion to adenosine, this regulates the excitability and conduction speed of myelinated axons. The changes in excitability at the AIS will lead to changes in the relationship between the synaptic input and action potential output of the cell. The altered conduction speed of the myelinated axon may change neural circuit function by changing the action potential arrival time at the cell’s output synapses, thus altering the integration of signals in postsynaptic neurons. Variations in astrocyte-derived adenosine level can occur between wake and sleep states, and the extracellular adenosine concentration rises during energy deprivation conditions. These changes of adenosine level could thus control white matter information flow and neural circuit function. [Figure: see text] In the brain’s grey matter, astrocytes regulate synapse properties, but their role is unclear for the white matter where myelinated axons rapidly transmit information between grey matter areas. We found that, in rodents, neuronal activity raised [Ca(2+)](i) in astrocyte processes located near action potential generating sites in the axon initial segment (AIS) and nodes of Ranvier of myelinated axons. This released ATP which was converted extracellularly to adenosine and thus, via A(2a) receptors, activated HCN2-containing cation channels that regulate two aspects of myelinated axon function: excitability of the AIS and speed of action potential propagation. Variations in astrocyte-derived adenosine level, between wake and sleep states or during energy deprivation, could thus control white matter information flow and neural circuit function.