Bringing astrocytes into the spotlight of electrical brain stimulation

Astrocytes, primarily viewed only as supportive units, are now emerging as active players in the information processing of the brain. Accumulated evidence supports that the bidirectional communication between astrocytes and neurons maintains complex animal behaviours such as memory formation and decision-making. The lack of characterisation of astrocytic electrophysiology is, in our opinion, associated with the early idea of a passive electrical nature of astrocytes, in opposition to the electrically active neurons. A better understanding of the effect of electrical stimulation on astrocytes physiology and activity will greatly strengthen the current knowledge in neural biology. Here, we assessed if astrocytes may have a role in therapies based on electrical brain stimulation by being able to respond to the same electrical stimulus used to modulate neuronal activity. To do so, we took advantage of microelectrode arrays (MEAs) capability to simultaneously record and deliver extracellular electrical signals. Additionally, we synchronized the recording of electrophysiological data with the recording of calcium activity, a hallmark of astrocytic activity. Here, we show that astrocytes respond to electrical stimulation with the generation of strong membrane voltage oscillations and simultaneous production of calcium waves, demonstrating, unequivocally, that astrocytes respond to electrical stimulation in the same range as neurons do. Importantly, these responses are dependent on the stimuli amplitude. Furthermore, membrane voltage oscillations are significantly reduced in the absence of extracellular calcium, but not abolished, while calcium activity is not detected.