GABAERGIC SIGNALING EVOKES CA2+ OSCILLATIONS AND GLUTAMATE RELEASE IN CORTICAL ASTROCYTES

During the last decade, there has been an increasing interest in the reciprocal signalling between principal excitatory neurons and astrocytes. Conversely, the signalling between inhibitory interneurons and astrocytes remains largely unexplored. The main goal of our study was to identify and characterize the astrocytic response to GABAergic signals. We used slice preparations from the somatosensory cortex of P14-P22 mice loaded with the Ca2+ indicator Fluo-4 AM and the astrocytic marker SR101-AM. We found that i) about 60% of layer V astrocytes showed large amplitude somatic Ca2+ increases in response to GABA or baclofen (Bac), a GABAB receptor agonist; ii) blocking Gi/o proteins by pertussis toxin prevented Bac-mediated Ca2+ transients in astrocytes; iii) both GABA and Bac failed to induce Ca2+ events in mice lacking the inositol-1,4,5-trisphosphate (IP3) receptor type 2 in astrocytes. These results reveal an involvement of the Gq/IP3 cascade and suggest possible Gi/o-Gq protein interactions in the astrocyte response to GABA signals. Optogenetic stimulation of ChR2-expressing parvalbumin fast-spiking (Pv-FS) interneurons also evoked astrocytic Ca2+ events, and current pulse depolarization of single Pv-FS or a somatostatin (Som) interneurons increased Ca2+ peaks in nearby astrocytes from 0.41 ± 0.04 to 0.65 ± 0.08 (events/minute; p<0.05) and from 0.10 ± 0.31 to 1.09 ± 0.16 (p<0.001), respectively. Patch-clamp recordings in the presence of TTX showed that GABAB activation triggered glutamate release in astrocytes and NMDAR-mediated slow inward currents (SICs) in nearby neurons. The frequency of SICs was strongly increased both in Pv-FS interneurons (from 0.15 ± 0.06 to 0.46 ± 0.04 event/min) and pyramidal neurons (from 0.30 ± 0.07 to 0.79 ± 0.17 event/min). The increase in SIC frequency lasted for about three minutes, outlasting the time of GABA agonist applications. Due to the intrinsic membrane properties, GABAB mediated SICs were more effective in inducing action potential firing in pyramidal neurons than in Pv-FS interneurons, suggesting that in local circuits astrocytes activated by GABAergic interneurons convert a transient inhibition into a delayed excitation. The activation of astrocytes by two of the major GABAergic interneurons in the brain (Pv and Som) and the consequent gliotransmitter release represent a new form of homeostatic control of local network excitability.