S100β-mediated astroglial control of firing and input processing in layer 5 pyramidal neurons of the mouse visual cortex.

KEY POINTS Inputs impinging on layer 5 pyramidal neurons perform essential operations as these cells represent one of the most important output carrier of the cerebral cortex. However, the contribution of astrocytes, a type of glial cell to these operations is poorly documented. Here we found that optogenetic activation of astrocytes in the vicinity of layer 5 in the mouse primary visual cortex induces spiking in local pyramidal neurons through Nav1.6 ion channels and prolongs the responses elicited in these neurons by stimulation of their distal inputs in cortical layer 1. This effect partially involved glutamatergic signalling but relied mostly on the astrocytic calcium-binding protein S100β, which regulates the concentration of calcium in the extracellular space around neurons. These findings show that astrocytes contribute to the fundamental computational operations of the cortex by acting on the ionic environment of neurons. ABSTRACT The most complex cerebral functions are performed by the cortex which most important output is carried out by its layer 5 pyramidal neurons. Their firing reflects integration of sensory and contextual information that they receive. There is evidence that astrocytes influence cortical neurons firing through the release of gliotransmitters such as ATP, glutamate or GABA. These effects were described at the network and at the synaptic levels, but it is still unclear how astrocytes influence neurons input-output transfer function at the cellular level. Here, we used optogenetic tools coupled with electrophysiological, imaging and anatomical approaches to test whether and how astrocytic activation affected processing of distal inputs to layer 5 pyramidal neurons (L5PN). We show that optogenetic activation of astrocytes near L5PN cell body prolonged firing induced by distal inputs to L5PN and potentiated their ability to trigger spikes. The observed astrocytic effects on L5PN firing involved glutamatergic transmission to some extent but relied mostly on release of S100β, an astrocytic Ca2+-binding protein that decreases extracellular Ca2+ once released. This astrocyte-evoked decrease of extracellular Ca2+ elicited firing mediated by activation of Nav1.6 channels. Our findings suggest that astrocytes contribute to the cortical fundamental computational operations by controlling the extracellular ionic environment. This article is protected by copyright. All rights reserved.