Structural and functional changes of deep layer pyramidal neurons surrounding microelectrode arrays implanted in rat motor cortex.

Devices capable of recording or stimulating neuronal signals have created new opportunities to understand normal physiology and treat sources of pathology in the brain. However, it is possible that the tissue response to implanted electrodes may influence the nature of the signals detected or stimulated. In this study, we characterized structural and functional changes in deep layer pyramidal neurons surrounding silicon or polyimide-based electrodes implanted in the motor cortex of rats. Devices were captured in 300 µm-thick tissue slices collected at the 1 or 6 week time point post-implantation, and individual neurons were assessed using a combination of whole-cell electrophysiology and 2-photon imaging. We observed disrupted dendritic arbors and a significant reduction in spine densities in neurons surrounding devices. These effects were accompanied by a decrease in the frequency of spontaneous excitatory post-synaptic currents, a reduction in sag amplitude, an increase in spike frequency adaptation, and an increase in filopodia density. We hypothesize that the effects observed in this study may contribute to the signal loss and instability that often accompany chronically implanted electrodes. STATEMENT OF SIGNIFICANCE: Implanted electrodes in the brain can be used to treat sources of pathology and understand normal physiology by recording or stimulating electrical signals generated by local neurons. However, a foreign body response following implantation undermines the performance of these devices. While several studies have investigated the biological mechanisms of device-tissue interactions through histology, transcriptomics, and imaging, our study is the first to directly interrogate effects on the function of neurons surrounding electrodes using single-cell electrophysiology. Additionally, we provide new, detailed assessments of the impacts of electrodes on the dendritic structure and spine morphology of neurons, and we assess effects for both traditional (silicon) and newer polymer electrode materials. These results reveal new potential mechanisms of electrode-tissue interactions.

Genetic Modulation at the Neural Microelectrode Interface: Methods and Applications.

The use of implanted microelectrode arrays (MEAs), in the brain, has enabled a greater understanding of neural function, and new treatments for neurodegenerative diseases and psychiatric disorders. Glial encapsulation of the device and the loss of neurons at the device-tissue interface are widely believed to reduce recording quality and limit the functional device-lifetime. The integration of microfluidic channels within MEAs enables the perturbation of the cellular pathways, through defined vector delivery. This provides new approaches to shed light on the underlying mechanisms of the reactive response and its contribution to device performance. In chronic settings, however, tissue ingrowth and biofouling can obstruct or damage the channel, preventing vector delivery. In this study, we describe methods of delivering vectors through chronically implanted, single-shank, "Michigan"-style microfluidic devices, 1⁻3 weeks, post-implantation. We explored and validated three different approaches for modifying gene expression at the device-tissue interface: viral-mediated overexpression, siRNA-enabled knockdown, and cre-dependent conditional expression. We observed a successful delivery of the vectors along the length of the MEA, where the observed expression varied, depending on the depth of the injury. The methods described are intended to enable vector delivery through microfluidic devices for a variety of potential applications; likewise, future design considerations are suggested for further improvements on the approach.

Multi-scale, multi-modal analysis uncovers complex relationship at the brain tissue-implant neural interface: new emphasis on the biological interface.

OBJECTIVE: Implantable neural electrode devices are important tools for neuroscience research and have an increasing range of clinical applications. However, the intricacies of the biological response after implantation, and their ultimate impact on recording performance, remain challenging to elucidate. Establishing a relationship between the neurobiology and chronic recording performance is confounded by technical challenges related to traditional electrophysiological, material, and histological limitations. This can greatly impact the interpretations of results pertaining to device performance and tissue health surrounding the implant. APPROACH: In this work, electrophysiological activity and immunohistological analysis are compared after controlling for motion artifacts, quiescent neuronal activity, and material failure of devices in order to better understand the relationship between histology and electrophysiological outcomes. MAIN RESULTS: Even after carefully accounting for these factors, the presence of viable neurons and lack of glial scarring does not convey single unit recording performance. SIGNIFICANCE: To better understand the biological factors influencing neural activity, detailed cellular and molecular tissue responses were examined. Decreases in neural activity and blood oxygenation in the tissue surrounding the implant, shift in expression levels of vesicular transporter proteins and ion channels, axon and myelin injury, and interrupted blood flow in nearby capillaries can impact neural activity around implanted neural interfaces. Combined, these tissue changes highlight the need for more comprehensive, basic science research to elucidate the relationship between biology and chronic electrophysiology performance in order to advance neural technologies.

Publisher Correction: Glial responses to implanted electrodes in the brain.

In the version of this Review Article originally published, in Fig. 4b, the label 'Glutamate' was mistakenly duplicated and an arrow between a purinergic P2 receptor and a glutamate transporter was missing. The figure has now been updated in all versions of the Review Article.

Functional remodeling of subtype-specific markers surrounding implanted neuroprostheses.

Microelectrode arrays implanted in the brain are increasingly used for the research and treatment of intractable neurological disease. However, local neuronal loss and glial encapsulation are known to interfere with effective integration and communication between implanted devices and brain tissue, where these observations are typically based on assessments of broad neuronal and astroglial markers. However, both neurons and astrocytes comprise heterogeneous cellular populations that can be further divided into subclasses based on unique functional and morphological characteristics. In this study, we investigated whether or not device insertion causes alterations in specific subtypes of these cells. We assessed the expression of both excitatory and inhibitory markers of neurotransmission (vesicular glutamate and GABA transporters, VGLUT1 and VGAT, respectively) surrounding single-shank Michigan-style microelectrode arrays implanted in the motor cortex of adult rats by use of quantitative immunohistochemistry. We found a pronounced shift from significantly elevated VGLUT1 within the initial days following implantation to relatively heightened VGAT by the end of the 4-wk observation period. Unexpectedly, we observed VGAT positivity in a subset of reactive astrocytes during the first week of implantation, indicating heterogeneity in early-responding encapsulating glial cells. We coupled our VGLUT1 data with the evaluation of a second marker of excitatory neurons (CamKiiα); the results closely paralleled each other and underscored a progression from initially heightened to subsequently weakened excitatory tone in the neural tissue proximal to the implanted electrode interface (within 40 µm). Our results provide new evidence for subtype-specific remodeling surrounding brain implants that inform observations of suboptimal integration and performance.NEW & NOTEWORTHY We report novel changes in the local expression of excitatory and inhibitory synaptic markers surrounding microelectrode arrays implanted in the motor cortex of rats, where a progressive shift toward increased inhibitory tone was observed over the 4-wk observation period. The result was driven by declining glutamate transporter expression (VGLUT1) in parallel with increasing GABA transporter expression (VGAT) over time, where a reactive VGAT+ astroglial subtype made an unexpected contribution to our findings.

Glial responses to implanted electrodes in the brain.

The use of implants that can electrically stimulate or record electrophysiological or neurochemical activity in nervous tissue is rapidly expanding. Despite remarkable results in clinical studies and increasing market approvals, the mechanisms underlying the therapeutic effects of neuroprosthetic and neuromodulation devices, as well as their side effects and reasons for their failure, remain poorly understood. A major assumption has been that the signal-generating neurons are the only important target cells of neural-interface technologies. However, recent evidence indicates that the supporting glial cells remodel the structure and function of neuronal networks and are an effector of stimulation-based therapy. Here, we reframe the traditional view of glia as a passive barrier, and discuss their role as an active determinant of the outcomes of device implantation. We also discuss the implications that this has on the development of bioelectronic medical devices.