Astrocytic modulation of information processing by layer 5 pyramidal neurons of the mouse visual cortex

Abstract

<jats:title>A<jats:sc>bstract</jats:sc></jats:title><jats:p>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 and integration 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 on release of S100β, an astrocytic Ca<jats:sup>2+</jats:sup>-binding protein that decreases extracellular Ca<jats:sup>2+</jats:sup> once released. This astrocyte-evoked decrease of extracellular Ca<jats:sup>2+</jats:sup> 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.</jats:p><jats:sec><jats:title>K<jats:sc>ey</jats:sc> P<jats:sc>oints</jats:sc> S<jats:sc>ummary</jats:sc></jats:title><jats:list list-type="simple"><jats:list-item><jats:p>Integration of inputs along the dendritic tree of layer 5 pyramidal neurons is an essential operation as these cells represent 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.</jats:p></jats:list-item><jats:list-item><jats:p>Here we found that optogenetic activation of astrocytes in the vicinity of layer 5 in the mouse primary visual cortex induce 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.</jats:p></jats:list-item><jats:list-item><jats:p>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.</jats:p></jats:list-item><jats:list-item><jats:p>These findings show that astrocytes contribute to the fundamental computational operations of the cortex by acting on the ionic environment of neurons.</jats:p></jats:list-item></jats:list></jats:sec>

Rab6A as a Pan-Astrocytic Marker in Mouse and Human Brain, and Comparison with Other Glial Markers (GFAP, GS, Aldh1L1, SOX9)

“…Ongoing studies aim at testing whether Rab6A + TGN-derived vesicles are exocytosed in astrocytes. If so, this might shed a light on the identity of glial organelles related to release of transmitters and proteins involved in glia-neuronal communication [ 10 , 15 , 16 , 17 , 18 , 19 ], mechanisms known to regulate vital functions [ 56 ] and behavior [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ].…”

“…A key mechanism in signalling from the glial to the neuronal domain is vesicular exocytosis, mostly involving small molecule “gliotransmitters” such as glutamate and d -serine [ 9 ]. Moreover, astrocyte-derived proteins, such as S100β, are released to the extracellular space to modulate neuronal firing [ 10 ]. Although exocytotic organelles have in fact been observed in astrocytes in situ [ 11 , 12 , 13 , 14 ], their identity has remained a matter of debate [ 15 , 16 , 17 , 18 , 19 ].…”

A common role for astrocytes in rhythmic behaviours?

“…Within brainstem circuits that control mastication, astrocytes release S100β to facilitate extracellular Ca 2+ buffering, which modulates rhythmic neuronal bursting (Morquette et al, 2015). Although it remains to be determined how widely such mechanisms are employed, recent work has also implicated astrocytic release of S100β in the control of cortical neuron firing (Ryczko et al, 2020). Therefore, astrocytes may regulate neuronal activity and rhythmicity through the control of the extracellular environment as opposed to more directly purposed gliotransmission mechanism acting via neurotransmitters, for example.…”

Freely behaving mice can brake and turn during optogenetic stimulation of the Mesencephalic Locomotor Region

“…Coronal brainstem slices were obtained from 15-23-day old mice as previously described [2]. Briefly, mice were anesthetized with isoflurane (0.5-1 mL of isoflurane in a 1.5 L induction chamber) and decapitated with a guillotine.…”