Electrically evoked calcium waves and membrane oscillations reveal astrocytic excitability in the neurostimulation range

OBJECTIVE: Electrical brain stimulation (EBS) has emerged as a valuable therapeutic tool for neurological and psychiatric disorders, but its underlying mechanisms remain poorly understood. Namely, the direct impact of electrical stimulation on astrocytic activity has been overlooked, as neurons have been historically the primary focus of EBS research. This study aimed to improve the characterization of astrocytes' response to extracellular electrical stimulation.
Approach. Primary astrocyte cultures (rat) were cultured on microelectrode arrays to assess direct astrocytic responses, eliminating neuronal contributions. Astrocytic responsiveness to electrical stimulation was assessed by simultaneously monitoring electrophysiological activity and intracellular calcium dynamics, the hallmark of astrocytic signalling. This in vitro system provided ideal control conditions for high spatiotemporal analysis of astrocytic dynamics.
Main results. Electrical stimulation elicited both membrane voltage oscillations and radial calcium waves in astrocytes. Calcium responses were significantly larger than spontaneous activity and displayed a stimulus-dependent refractory period. Responses were partially dependent on extracellular calcium and modulated by N-type voltage-gated calcium channels, ATP-mediated signalling via P2X receptors, and gap junction communication. Calcium waves were abolished upon substitution with nominally calcium-free HBSS, while electrophysiological responses persisted, indicating parallel and partially independent signalling mechanisms. Importantly, astrocytes exhibited reliable activation exactly in the same stimulation amplitudes as those used for neuronal activation.
Significance. Altogether, this study demonstrates that astrocytes respond to electrical stimulation within physiologically relevant ranges, highlighting their potential as key contributors, and direct therapeutic targets (astrocyte-optimized electrical stimulation), in neurostimulation. These results urge a reconsideration of astrocytic roles in neurostimulation protocols and may guide the design of more effective brain stimulation therapies.