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Neural Engineering

Neural Engineering - Poster Session C

Poster K6 - Investigating How Astrocytes Respond to Intracortical Microstimulation (ICMS) and How This Response is Affected by Alertness

Friday, October 25, 2024
10:00 AM - 11:00 AM EST
Location: Exhibit Hall E, F & G

Introduction:  Electrical stimulation is a valuable tool for localizing and examining particular brain functions. For example, stimulation in the cerebral cortex can evoke a variety of immediate responses such as visual or auditory precepts, depending on the region. Intracortical microstimulation (ICMS) is commonly used with neural prosthetics to provide patients with sensory feedback, and with further developments has the potential for sensory restoration. It remains unclear what cell types are activated during stimulation and how to selectively activate particular cell types responsible for sensory activity. During sensory processing, astrocytes, a type of glial cell, integrate the activity of many neurons to provide modulatory feedback in a calcium-dependent manner. For example, astrocytes react to heightened levels of alertness by releasing so-called “gliotransmitters”, and this signal is linked to a feedback-driven mechanism that reduces the arousal state. However, little is known about how astrocytes respond to different ICMS parameters on a network scale and if animal alertness can influence stimulation-induced calcium activity. I hypothesize that a heightened level of alertness during stimulation could result in increased astrocyte calcium activity. To test this hypothesis, we simultaneously imaged mesoscale astrocyte calcium activity and pupil diameter (i.e. alertness) in mice chronically implanted with an electrode in the visual cortex during stimulation trials with different patterns of stimulation.

Materials and

Methods:  To determine how astrocytes differentially respond to different ICMS patterns, we implanted a single shank Michigan-style electrode into mice (n=2) selectively co-expressing the calcium sensor GCaMP6f and tdTomato (salsa6f) in astrocytes (aldh1l1-CRE ERT). Mesoscale calcium activity was imaged at 10 Hz during stimulation trials over the course of 12 weeks following electrode implantation. Changes in pupil diameter have been shown to be a strong reporter of animal alertness. Therefore, we trained a DEEPLABCUT model on 1000 images of a mouse’s eye to track the pupil diameter during trials. Stimulation trials consisted of a 30s baseline period followed by 30s of stimulation at 4 different stimulation patterns followed by 60s post-stimulation. Stimulation consisted of 10 Hz uniform stimulation, 100 Hz uniform stimulation, or 2 different bursting patterns with an average frequency of 10 Hz [10 Hz-burst and theta-burst stimulation (TBS)] or a no stimulation condition (baseline).

Astrocyte calcium activity was quantified within 100 µm of the electrode to determine how astrocytes respond to different stimulation patterns. To determine how changes in alertness affect stimulation-induced calcium activity, we first identified pupil dilation events and extracted the calcium activity in a window of 5s before to 15s after the dilation event. A dilation event was characterized as the derivative of pupil diameter exceeding 4 standard deviations above the mean of the derivative of pupil diameter over the stimulation trial.

Results, Conclusions, and Discussions: The stimulation-induced calcium activity was the largest for 100 Hz stimulation consisting of a mean response of 0.074 ± 0.036 over the stimulation period compared to 10 Hz: 0.0032 ± 0.0077, 10 Hz-burst: 0.0085 ± 0.0066, and TBS: 0.0039 ± 0.0070. This is consistent with previous results that higher frequency stimulation induced larger astrocyte calcium events, indicating that astrocytes may differentially influence neural activity during different stimulation parameters (Fig. 1). Initially, the calcium response dropped in the first few days but stabilized by day 7 or 14 (Fig. 2). This initial destabilization might be the result of an inflammatory response from astrocytes or trauma from the insertion of the electrode. Therefore, analyses correlating arousal with calcium activity were pooled across weeks 2-12. Consistent with previous work, astrocyte calcium activity tends to increase after a pupil dilation where the maximum calcium activity occurs 0.71 ± 0.65 seconds after the start/peak of the pupil dilation event during prestimulus. This was present even during stimulation where a dilation event during the stimulation period resulted in a transient increase in calcium activity that was not present in other trials without a dilation event (Fig. 3). The brief increase in calcium activity following the dilation event implies alertness may influence how astrocytes respond to stimulation and that their modulation of neural activity could also depend on the alertness.

Altogether the data is consistent with previous work that astrocyte calcium activity increases in response to heightened levels of alertness (i.e. pupil dilation) and supports the hypothesis that increased alertness may increase astrocyte calcium activity during stimulation. Furthermore, the results provide insight on how astrocytes respond to different ICMS parameters over the chronic implantation period. This is a necessary step to understanding how astrocytes modulate the neural network’s responses, offering valuable information on the therapeutic mechanisms of ICMS that lead to sensory restoration.

Acknowledgements (Optional):  I would like to express my immense gratitude to Dr. Kozai, the Swanson School of Engineering, and the Office of the Provost at the University of Pittsburgh for funding this research.