Slow wave activity (SWA) is a characteristic brain oscillation in sleep and quiet
wakefulness. Although the cell types contributing to SWA genesis are not yet identified,
the principal role of neurons in the emergence of this essential cognitive mechanism
has not been questioned. To address the possibility of astrocytic involvement in SWA,
we used a transgenic rat line expressing a calcium sensitive fluorescent protein in
both astrocytes and interneurons and simultaneously imaged astrocytic and neuronal
activity in vivo. Here we demonstrate, for the first time, that the astrocyte network
display synchronized recurrent activity in vivo coupled to UP states measured by field
recording and neuronal calcium imaging. Furthermore, we present evidence that extensive
synchronization of the astrocytic network precedes the spatial build-up of neuronal
synchronization. The earlier extensive recruitment of astrocytes in the synchronized
activity is reinforced by the observation that neurons surrounded by active astrocytes
are more likely to join SWA, suggesting causality. Further supporting this notion,
we demonstrate that blockade of astrocytic gap junctional communication or inhibition
of astrocytic Ca2+ transients reduces the ratio of both astrocytes and neurons involved
in SWA. These in vivo findings conclusively suggest a causal role of the astrocytic
syncytium in SWA generation.
