Microthalamotomy effect during deep brain stimulation: potential involvement of adenosine and glutamate efflux

Transient Adenosine Modulates Serotonin Release Indirectly in the Dorsal Raphe Nuclei

Rapid adenosine transiently regulates dopamine and glutamate via A1 receptors, but other neurotransmitters, such as serotonin, have not been studied. In this study, we examined the rapid modulatory effect of adenosine on serotonin release in the dorsal raphe nuclei (DRN) of mouse brain slices by using fast-scan cyclic voltammetry. To mimic adenosine release during damage, a rapid microinjection of adenosine at 50 pmol was applied before electrical stimulation of serotonin release. Transient adenosine significantly reduced electrically evoked serotonin release in the first 20 s after application, but serotonin release recovered to baseline as adenosine was cleared from the slice. The continuous perfusion of adenosine did not change the evoked serotonin release. Surprisingly, the modulatory effects of adenosine were not regulated by A1 receptors as adenosine still inhibited serotonin release in A1KO mice and also after perfusion of an A1 antagonist (8-cyclopentyl-1,3-dipropyl xanthine). The inhibition was also not regulated by A3 receptors as perfusion of the A3 antagonist (MRS 1220) in A1KO brain slices did not eliminate the inhibitory effects of transient adenosine. In addition, adenosine also inhibited serotonin release in A2AKO mice, showing that A2A did not modulate serotonin. However, perfusion of a selective 5HT1A autoreceptor antagonist drug [(S)-WAY 100135 dihydrochloride] abolished the inhibitory effect of transient adenosine on serotonin release. Thus, the transient neuromodulatory effect of adenosine on DRN serotonin release is regulated by serotonin autoreceptors and not by adenosine receptors. Rapid, transient adenosine modulation of neurotransmitters such as serotonin may have important implications for diseases such as depression and brain injury.

A longitudinal study on deep brain stimulation of the medial forebrain bundle for treatment-resistant depression

Deep brain stimulation (DBS) to the superolateral branch of the medial forebrain bundle (MFB) has been reported to lead to rapid antidepressant effects. In this longitudinal study, we expand upon the initial results we reported at 26 weeks (Fenoy et al., 2016), showing sustained antidepressant effects of MFB DBS on six patients with treatment-resistant depression (TRD) over 1 year. The Montgomery-Åsberg Depression Rating Scale (MADRS) was used as the primary assessment tool. Deterministic fiber tracking was used to individually map the target area; analysis was performed to compare modulated fiber tracts between patients. Intraoperatively, upon stimulation at target, responders reported immediate increases in energy and motivation. An insertional effect was seen during the 4-week sham stimulation phase from baseline (28% mean MADRS reduction, p = 0.02). However, after 1 week of initiating stimulation, three of six patients had a > 50% decrease in MADRS scores relative to baseline (43% mean MADRS reduction, p = 0.005). One patient withdrew from study participation. At 52 weeks, four of remaining five patients have > 70% decrease in MADRS scores relative to baseline (73% mean MADRS reduction, p = 0.007). Evaluation of modulated fiber tracts reveals significant common orbitofrontal connectivity to the target region in all responders. Neuropsychological testing and ¹⁸F-fluoro-deoxyglucose-positron emission tomography cerebral metabolism evaluations performed at baseline and at 52 weeks showed minimal changes and verified safety. This longitudinal evaluation of MFB DBS demonstrated rapid antidepressant effects, as initially reported by Schlaepfer et al. (2013), and supports the use of DBS for TRD.

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.

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.

Clearance of rapid adenosine release is regulated by nucleoside transporters and metabolism

Adenosine is a neuromodulator that regulates neurotransmission in the brain and central nervous system. Recently, spontaneous adenosine release that is cleared in 3-4 sec was discovered in mouse spinal cord slices and anesthetized rat brains. Here, we examined the clearance of spontaneous adenosine in the rat caudate-putamen and exogenously applied adenosine in caudate brain slices. The V max for clearance of exogenously applied adenosine in brain slices was 1.4 ± 0.1 μmol/L/sec. In vivo, the equilibrative nucleoside transport 1 (ENT1) inhibitor, S-(4-nitrobenzyl)-6-thioinosine (NBTI) (1 mg/kg, i.p.) significantly increased the duration of adenosine, while the ENT1/2 inhibitor, dipyridamole (10 mg/kg, i.p.), did not affect duration. 5-(3-Bromophenyl)-7-[6-(4-morpholinyl)-3-pyrido[2,3-d]byrimidin-4-amine dihydrochloride (ABT-702), an adenosine kinase inhibitor (5 mg/kg, i.p.), increased the duration of spontaneous adenosine release. The adenosine deaminase inhibitor, erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA) (10 mg/kg, i.p.), also increased the duration in vivo. Similarly, NBTI (10 μmol/L), ABT-702 (100 nmol/L), or EHNA (20 μmol/L) also decreased the clearance rate of exogenously applied adenosine in brain slices. The increases in duration for blocking ENT1, adenosine kinase, or adenosine deaminase individually were similar, about 0.4 sec in vivo; thus, the removal of adenosine on a rapid time scale occurs through three mechanisms that have comparable effects. A cocktail of ABT-702, NBTI, and EHNA significantly increased the duration by 0.7 sec, so the mechanisms are not additive and there may be additional mechanisms clearing adenosine on a rapid time scale. The presence of multiple mechanisms for adenosine clearance on a time scale of seconds demonstrates that adenosine is tightly regulated in the extracellular space.

Chemically functionalized single-walled carbon nanotubes enhance the glutamate uptake characteristics of mouse cortical astrocytes

Using a radioactive glutamate uptake assay and immunolabeling, we report that single-walled carbon nanotubes, chemically functionalized with polyethylene glycol (SWCNT-PEG), delivered as a colloidal solute, cause an increase in the uptake of extracellular glutamate by astrocytes and an increase in the immunoreactivity of the glutamate transporter GLAST on their cell surface, which is likely a consequence of an increase in the immunoreactivity of glial fibrillary acidic protein. Additional corollary is that astrocytes exposed to SWCNT-PEG became larger and stellate, morphological characteristics of maturation and heightened activity of these glial cells. These results imply that SWCNT-PEG could potentially be used as a viable candidate for neural prosthesis applications, perhaps to alleviate the death toll of neurons due to glutamate excitotoxicity, a pathological process observed in brain and spinal cord injuries.

Fast-scan Cyclic Voltammetry for the Characterization of Rapid Adenosine Release

Adenosine is a signaling molecule and downstream product of ATP that acts as a neuromodulator. Adenosine regulates physiological processes, such as neurotransmission and blood flow, on a time scale of minutes to hours. Recent developments in electrochemical techniques, including fast-scan cyclic voltammetry (FSCV), have allowed direct detection of adenosine with sub-second temporal resolution. FSCV studies have revealed a novel mode of rapid signaling that lasts only a few seconds. This rapid release of adenosine can be evoked by electrical or mechanical stimulations or it can be observed spontaneously without stimulation. Adenosine signaling on this time scale is activity dependent; however, the mode of release is not fully understood. Rapid adenosine release modulates oxygen levels and evoked dopamine release, indicating that adenosine may have a rapid modulatory role. In this review, we outline how FSCV can be used to detect adenosine release, compare FSCV with other techniques used to measure adenosine, and present an overview of adenosine signaling that has been characterized using FSCV. These studies point to a rapid mode of adenosine modulation, whose mechanism and function will continue to be characterized in the future.

A neurochemical closed-loop controller for deep brain stimulation: toward individualized smart neuromodulation therapies

Current strategies for optimizing deep brain stimulation (DBS) therapy involve multiple postoperative visits. During each visit, stimulation parameters are adjusted until desired therapeutic effects are achieved and adverse effects are minimized. However, the efficacy of these therapeutic parameters may decline with time due at least in part to disease progression, interactions between the host environment and the electrode, and lead migration. As such, development of closed-loop control systems that can respond to changing neurochemical environments, tailoring DBS therapy to individual patients, is paramount for improving the therapeutic efficacy of DBS. Evidence obtained using electrophysiology and imaging techniques in both animals and humans suggests that DBS works by modulating neural network activity. Recently, animal studies have shown that stimulation-evoked changes in neurotransmitter release that mirror normal physiology are associated with the therapeutic benefits of DBS. Therefore, to fully understand the neurophysiology of DBS and optimize its efficacy, it may be necessary to look beyond conventional electrophysiological analyses and characterize the neurochemical effects of therapeutic and non-therapeutic stimulation. By combining electrochemical monitoring and mathematical modeling techniques, we can potentially replace the trial-and-error process used in clinical programming with deterministic approaches that help attain optimal and stable neurochemical profiles. In this manuscript, we summarize the current understanding of electrophysiological and electrochemical processing for control of neuromodulation therapies. Additionally, we describe a proof-of-principle closed-loop controller that characterizes DBS-evoked dopamine changes to adjust stimulation parameters in a rodent model of DBS. The work described herein represents the initial steps toward achieving a “smart” neuroprosthetic system for treatment of neurologic and psychiatric disorders.

Deep brain stimulation: are astrocytes a key driver behind the scene?

Despite its widespread use, the underlying mechanism of deep brain stimulation (DBS) remains unknown. Once thought to impart a "functional inactivation", there is now increasing evidence showing that DBS actually can both inhibit neurons and activate axons, generating a wide range of effects. This implies that the mechanisms that underlie DBS work not only locally but also at the network level. Therefore, not only may DBS induce membrane or synaptic plastic changes in neurons over a wide network, but it may also trigger cellular and molecular changes in other cells, especially astrocytes, where, together, the glial-neuronal interactions may explain effects that are not clearly rationalized by simple activation/inhibition theories alone. Recent studies suggest that (1) high-frequency stimulation (HFS) activates astrocytes and leads to the release of gliotransmitters that can regulate surrounding neurons at the synapse; (2) activated astrocytes modulate synaptic activity and increase axonal activation; (3) activated astrocytes can signal further astrocytes across large networks, contributing to observed network effects induced by DBS; (4) activated astrocytes can help explain the disparate effects of activation and inhibition induced by HFS at different sites; (5) astrocytes contribute to synaptic plasticity through long-term potentiation (LTP) and depression (LTD), possibly helping to mediate the long-term effects of DBS; and (6) DBS may increase delta-opioid receptor activity in astrcoytes to confer neuroprotection. Together, the plastic changes in these glial-neuronal interactions network-wide likely underlie the range of effects seen, from the variable temporal latencies to observed effect to global activation patterns. This article reviews recent research progress in the literature on how astrocytes play a key role in DBS efficacy.

Change in plasma levels of amino acid neurotransmitters and its correlation with clinical heterogeneity in early Parkinson's disease patients

BACKGROUND

The correlation between plasma amino acid (AA) neurotransmitters and clinical heterogeneity in early patients with Parkinson's disease (PD) is still poorly understood.

AIMS

To examine the plasma levels of AA neurotransmitters in early patients with PD and to evaluate their correlation with PD subtypes.

METHODS

Based on the predominant symptoms, fifty-one patients with PD were enrolled and divided into four subgroups: (1) akinetic-rigid type (ART), (2) tremor-dominant type (TDT), (3) postural instability/gait difficulty type (PIGD), and (4) mixed type (MT). Plasma levels of AA were measured by HPLC-RF, and their potential diagnostic practicality and their association with PD subtypes were evaluated by the receiver operating characteristic (ROC) and correlation analysis, respectively.

RESULTS

Patients with PD exhibited markedly lower levels of Asp, Glu, Tau, L-ser, and lower values of Glu/GABA ratio than healthy controls. The ROC analysis revealed their high sensitivity (77.1-87.5%) and specificity (58.8-88.2%). Furthermore, the glutamic acid (Glu), γ-aminobutyric acid (GABA) level in the PIGD subtype was increased as compared with other subtypes and was negatively correlated with the ART/PIGD ratio.

CONCLUSION

The decrease in plasma Asp, Glu, Tau, L-ser levels, and the value of Glu/GABA ratio may be helpful for early PD diagnosis. The elevated GABA level may be the biochemical basis for the specific symptoms of PIGD PD.

Wireless neurochemical monitoring in humans

Electrochemical techniques have long been utilized to investigate chemical changes in the neuronal microenvironment. Preclinical models have demonstrated the successful monitoring of changes in various neurotransmitter systems in vivo with high temporal and spatial resolution. The expansion of electrochemical recording to humans is a critical yet challenging goal to elucidate various aspects of human neurophysiology and to create future therapies. We have designed a novel device named the WINCS (Wireless Instantaneous Neurotransmitter Concentration Sensing) system that combines rapid scan voltammetry with wireless telemetry for highly resolved electrochemical recording and analysis. WINCS utilizes fast-scan cyclic voltammetry and fixed potential amperometry for in vivo recording and has demonstrated high temporal and spatial resolution in detecting changes in extracellular levels of a wide range of analytes including dopamine, adenosine, glutamate, serotonin, and histamine. Neurochemical monitoring in humans represents a new approach to understanding the neurophysiology of the central nervous system, the neurobiology of numerous diseases, and the underlying mechanism of various neurosurgical therapies. This article addresses the current understanding of electrochemistry, its application in humans, and future directions.

Neuromodulation: selected approaches and challenges

The brain operates through complex interactions in the flow of information and signal processing within neural networks. The 'wiring' of such networks, being neuronal or glial, can physically and/or functionally go rogue in various pathological states. Neuromodulation, as a multidisciplinary venture, attempts to correct such faulty nets. In this review, selected approaches and challenges in neuromodulation are discussed. The use of water-dispersible carbon nanotubes has been proven effective in the modulation of neurite outgrowth in culture and in aiding regeneration after spinal cord injury in vivo. Studying neural circuits using computational biology and analytical engineering approaches brings to light geometrical mapping of dynamics within neural networks, much needed information for stimulation interventions in medical practice. Indeed, sophisticated desynchronization approaches used for brain stimulation have been successful in coaxing 'misfiring' neuronal circuits to resume productive firing patterns in various human disorders. Devices have been developed for the real-time measurement of various neurotransmitters as well as electrical activity in the human brain during electrical deep brain stimulation. Such devices can establish the dynamics of electrochemical changes in the brain during stimulation. With increasing application of nanomaterials in devices for electrical and chemical recording and stimulating in the brain, the era of cellular, and even intracellular, precision neuromodulation will soon be upon us.

Selective GABA release as a mechanistic basis of high-frequency stimulation used for the treatment of neuropsychiatric diseases

Electrical high-frequency stimulation (HFS) is applied in many brain areas to treat various clinical syndromes. The nearly identical constellation of stimulation parameters raises the question of a unique mechanism of action of this therapeutic option. The identification of a single HFS mechanism may help to optimize the HFS technology by targeting this single mechanism. Experimentally, only axonal membranes are targets of HFS, but not other membranes of neurons or glial cells. Within all HFS target regions, axons of excitatory glutamatergic and inhibitory GABAergic neurons are present and play roles in all clinical syndromes treated successfully with HFS. Therefore, glutamatergic or GABAergic fibres are likely candidates as mediators of a unique HFS mode of action. The selective involvement of another neuronal fibre type (e.g. monoaminergic, cholinergic, etc.) in the HFS mode of action is highly unlikely since the regional and syndromal dissimilarity of the clinical HFS applications precludes the assumption of such a fibre type as primary HFS site of action. Our recent experimental finding that HFS of human neocortical slices induces the action potential-mediated release of GABA, but not of glutamate, simplifies the possibilities to explain the HFS mode of action, as the explanation now may concentrate on GABAergic axons only. Thus, we are analysing, on the basis of the pathophysiological grounds of the various syndromes treated with deep brain stimulation, whether a selective GABA release is a collective explanation of the mode of action of HFS. We suggest that selective GABA release indeed may needfully and sufficiently explain efficacy and side effects of HFS.

Development of intraoperative electrochemical detection: wireless instantaneous neurochemical concentration sensor for deep brain stimulation feedback

Deep brain stimulation (DBS) is effective when there appears to be a distortion in the complex neurochemical circuitry of the brain. Currently, the mechanism of DBS is incompletely understood; however, it has been hypothesized that DBS evokes release of neurochemicals. Well-established chemical detection systems such as microdialysis and mass spectrometry are impractical if one is assessing changes that are happening on a second-to-second time scale or for chronically used implanted recordings, as would be required for DBS feedback. Electrochemical detection techniques such as fast-scan cyclic voltammetry (FSCV) and amperometry have until recently remained in the realm of basic science; however, it is enticing to apply these powerful recording technologies to clinical and translational applications. The Wireless Instantaneous Neurochemical Concentration Sensor (WINCS) currently is a research device designed for human use capable of in vivo FSCV and amperometry, sampling at subsecond time resolution. In this paper, the authors review recent advances in this electrochemical application to DBS technologies. The WINCS can detect dopamine, adenosine, and serotonin by FSCV. For example, FSCV is capable of detecting dopamine in the caudate evoked by stimulation of the subthalamic nucleus/substantia nigra in pig and rat models of DBS. It is further capable of detecting dopamine by amperometry and, when used with enzyme linked sensors, both glutamate and adenosine. In conclusion, WINCS is a highly versatile instrument that allows near real-time (millisecond) detection of neurochemicals important to DBS research. In the future, the neurochemical changes detected using WINCS may be important as surrogate markers for proper DBS placement as well as the sensor component for a "smart" DBS system with electrochemical feedback that allows automatic modulation of stimulation parameters. Current work is under way to establish WINCS use in humans.