Propagation and Coordination of Intercellular Astrocyte Ca²⁺ Waves by Glutamatergic Signaling

Abstract

Brain functions are highly dependent on a fine regulation of cerebral blood flow by a mechanism known as neurovascular coupling. This mechanism is mediated by astrocytes, which are located between neurons and parenchymal arterioles. Neurotransmitters released during an increase in synaptic activity activate receptors located on astrocytes, which initiates Ca2+ waves that are propagated to the astrocytic endfeet and neighboring astrocytes through gap junctions and activation of a purinergic signaling by ATP release via connexin (Cx)‐formed hemichannels. However, Cx hemichannels may also participate in the astrocyte signaling by providing the pathway for the release of D‐serine and glutamate. However, the relevance of this process in the astrocytic Ca2+ signaling has not been determined. We evaluated if activation of metabotropic glutamate receptors (mGluR) and NMDA receptors (NMDAR) contribute to the propagation of astrocytic Ca2+ signals in primary cultures of rat cortex astrocytes loaded with the Ca2+ indicator Fluo‐4. Ca2+ waves were initiated in a single astrocyte by applying a controlled mechanical stimulation (MS) using a micropipette. Stimulation with glutamate or t‐ACPD, a mGluR agonist, induced a rapid increase in astrocyte [Ca2+]i, which was abolished by MPEP, a mGluR5 antagonist, but, in addition, this response was attenuated by DL‐AP5, a NMDAR antagonist; phenazine methosulfate (Phen‐Meth), a serine racemase inhibitor; or D‐amino acid oxidase (DAOX), an enzyme that deaminates D‐serine. As expected, stimulation with NMDA plus D‐serine evoked an increase in astrocyte [Ca2+]i, but, interestingly, this NMDAR‐mediated Ca2+ signal was abolished by MPEP. On the other hand, blockade of connexin‐based hemichannels with the Cx blocking peptide 37,43Gap27 also attenuated the glutamate‐elicited increase in [Ca2+]i. Consistent with the involvement of hemichannels in the astrocyte communication, the intercellular propagation of the Ca2+ signaling initiated by MS was inhibited by MPEP, DL‐AP5 or Phen‐Meth. These results suggest that glutamate and D‐serine release through hemichannels coordinates the Ca2+ signal initiated by NMDAR and mGluR, which contributes to enhance the propagation of astrocytic Ca2+ waves.Support or Funding InformationGrants: FONDECYT 1150530 and CONICYT 21160646.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.