Somatostatin interneurons inhibit excitatory transmission mediated by astrocytic GABA and presynaptic GABA and adenosine A receptors in the hippocampus.

GABAergic network activity has been established to be involved in numerous physiological processes and pathological conditions. Extensive studies have corroborated that GABAergic network activity regulates excitatory synaptic networks by activating presynaptic GABA receptors (GABA Rs). It is well documented that astrocytes express GABA Rs and respond to GABAergic network activity. However, little is known about whether astrocytic GABA Rs regulate excitatory synaptic transmission mediated by GABAergic network activity. To address this issue, we combined whole-cell recordings, optogenetics, calcium imaging, and pharmacological approaches to specifically activate hippocampal somatostatin-expressing interneurons (SOM-INs), a type of interneuron that targets pyramidal cell dendrites, while monitoring excitatory synaptic transmission in CA1 pyramidal cells. We found that optogenetic stimulation of SOM-INs increases astrocyte Ca signaling via the activation of astrocytic GABA Rs and GAT-3. SOM-INs depress excitatory neurotransmission by activating presynaptic GABA Rs and astrocytic GABA Rs, the latter inducing the release of ATP/adenosine. In turn, adenosine inhibits excitatory synaptic transmission by activating presynaptic adenosine A receptors (A Rs). Overall, our results reveal a novel mechanism that SOM-INs activation-induced synaptic depression is partially mediated by the activation of astrocytic GABA Rs.