CALCIUM SIGNALING OF GLIAL-CELLS ALONG MAMMALIAN AXONS

Glial [Ca2+], signaling was examined in a mammalian white matter lacki ng neuronal cell bodies and synapses. Rat optic nerves (postnatal days 2 and 7) were stained with calcium indicator dyes and confocal images of [Ca2+]i were recorded at approximately 25-degrees-C or approximate ly 37-degrees-C. Glial cell bodies showed spiking or sustained [Ca2+]i response to bath-applied glutamate (50-500 muM). The metabotropic glu tamate agonist trans-ACPD elicited transient, sometimes spiking, [Ca2]i responses, whereas ionotropic agonists kainate and AMPA elicited a 6,7-dinitroquinoxaline-2,3-dione-sensitive, mostly sustained [Ca2+]i r esponse. Transient and spiking glial [Ca2+]i, responses also were elic ited by adenosine and ATP (0.1-100 muM). Repetitive nerve stimulation (10-20 Hz) elicited [Ca2+]i spiking in 15-25% of glial cells in postna tal day 7 nerves, with spiking typically occurring 15-60 sec after ons et of nerve stimulation. At 37-degrees-C, the frequency of glial [Ca2]i spikes increased from approximately 0.06 Hz to approximately 0.11 H z when axonal stimulation was increased from 10 to 20 Hz. This activit y-dependent glial spiking was inhibited by TTX, could not be mimicked by increasing the bath K+ by 20 mm, and occurred when nerves were stim ulated in the absence of bath calcium. Activity-dependent and glutamat e-induced glial spiking could be mimicked by altering ionic gradients known to favor release of glutamate via glutamate transporters, includ ing elevation of intracellular Na+ by veratridine concurrent with exte rnal K+ elevation. We suggest that glial [Ca2+]i spiking observed duri ng electrical activity resulted from activation of glial receptors (e. g., metabotropic glutamate receptor, adenosine receptor) by substances (e.g., glutamate, adenosine) released from the optic nerve in a nonve sicular fashion, possibly through a reversal of sodium-coupled transpo rters when Na+ and K+ gradients are altered by prolonged nerve activit y.