A high-sensitive nano-modified biosensor for dynamic monitoring of glutamate and neural spike covariation from rat cortex to hippocampal sub-regions

Real-Time Monitoring of Neurotransmitters in the Brain of Living Animals

Neurotransmitters, as important chemical small molecules, perform the function of neural signal transmission from cell to cell. Excess concentrations of neurotransmitters are often closely associated with brain diseases, such as Alzheimer's disease, depression, schizophrenia, and Parkinson's disease. On the other hand, the release of neurotransmitters under the induced stimulation indicates the occurrence of reward-related behaviors, including food and drug addiction. Therefore, to understand the physiological and pathological functions of neurotransmitters, especially in complex environments of the living brain, it is urgent to develop effective tools to monitor their dynamics with high sensitivity and specificity. Over the past 30 years, significant advances in electrochemical sensors and optical probes have brought new possibilities for studying neurons and neural circuits by monitoring the changes in neurotransmitters. This Review focuses on the progress in the construction of sensors for in vivo analysis of neurotransmitters in the brain and summarizes current attempts to address key issues in the development of sensors with high selectivity, sensitivity, and stability. Combined with the latest advances in technologies and methods, several strategies for sensor construction are provided for recording chemical signal changes in the complex environment of the brain.

Carbon-Based Electrochemical Sensors for In Vivo and In Vitro Neurotransmitter Detection

As essential neurological chemical messengers, neurotransmitters play an integral role in the maintenance of normal mammalian physiology. Aberrant neurotransmitter activity is associated with a range of neurological conditions including Parkinson's disease, Alzheimer's disease, and Huntington's disease. Many studies to date have tested different approaches to detecting neurotransmitters, yet the detection of these materials within the brain, due to the complex environment of the brain and the rapid metabolism of neurotransmitters, remains challenging and an area of active research. There is a clear need for the development of novel neurotransmitter sensing technologies capable of rapidly and sensitively monitoring specific analytes within the brain without adversely impacting the local microenvironment in which they are implanted. Owing to their excellent sensitivity, portability, ease-of-use, amenability to microprocessing, and low cost, electrochemical sensors methods have been widely studied in the context of neurotransmitter monitoring. The present review, thus, surveys current progress in this research field, discussing developed electrochemical neurotransmitter sensors capable of detecting dopamine (DA), serotonin (5-HT), acetylcholine (Ach), glutamate (Glu), nitric oxide (NO), adenosine (ADO), and so on. Of these technologies, those based on carbon nanostructures-modified electrodes including carbon nanotubes (CNTs), graphene (GR), gaphdiyne (GDY), carbon nanofibers (CNFs), and derivatives thereof hold particular promise owing to their excellent biocompatibility and electrocatalytic performance. The continued development of these and related technologies is, thus, likely to lead to major advances in the clinical diagnosis of neurological diseases and the detection of novel biomarkers thereof.

High frequency, real-time neurochemical and neuropharmacological measurements in situ in the living body

The beautiful and complex brain machinery is perfectly synchronized, and our bodies have evolved to protect it against a myriad of potential threats. Shielded physically by the skull and chemically by the blood brain barrier, the brain processes internal and external information so that we can efficiently relate to the world that surrounds us while simultaneously and unconsciously controlling our vital functions. When coupled with the brittle nature of its internal chemical and electric signals, the brain's "armor" render accessing it a challenging and delicate endeavor that has historically limited our understanding of its structural and neurochemical intricacies. In this review, we briefly summarize the advancements made over the past 10 years to decode the brain's neurochemistry and neuropharmacology in situ, at the site of interest in the brain, with special focus on what we consider game-changing emerging technologies (eg, genetically encoded indicators and electrochemical aptamer-based sensors) and the challenges these must overcome before chronic, in situ chemosensing measurements become routine.

Microelectrode Arrays Modified with Nanocomposites for Monitoring Dopamine and Spike Firings under Deep Brain Stimulation in Rat Models of Parkinson's Disease

Little is known about the efficacy of deep brain stimulation (DBS) as an effective treatment for Parkinson's Disease (PD) because of the lack of multichannel neural electrical and chemical detection techniques at the cellular level. In this study, a 7-mm-long and 250-μm-wide microelectrode array (MEA) was fabricated to provide real-time monitoring of dopamine (DA) concentration and neural spike firings in the caudate putamen (CPU) of rats with PD. Platinumn nanoparticles and reduced graphene oxide nanocomposites (Pt/rGO) were modified onto the sensitive microelectrode sites. The detection limit (50 nM) and sensitivity (8.251 pA/μM) met the specific requirements for DA detection in vivo. A single neural spike was isolated due to the high signal-to-noise ratio of the MEA. DBS was applied in the affected side of the globus pallidus internal (GPi) in PD rats. After DBS, the concentration of DA in the bilateral CPU increased markedly. The mean increment of the ipsilateral DA was 7.33 μM (increasing from 0.54 μM to 7.87 μM), which was 2.2-fold higher than the increment in the contralateral side. The mean amplitude of neural spikes in the bilateral CPU decreased more than 10%, and was more obvious in the ipsilateral side where the spike amplitude changed from 169 μV to 134 μV. Spike firing rate decreased by 65% (ipsilateral side) and 51% (contralateral side). The power of the local field potential decreased to 940 μW (ipsilateral side) and 530 μW (contralateral side) in 0-30 Hz. Collectively, our data show that the GPi-DBS plays a significant regulatory role in the bilateral CPU in terms of DA concentration, spike firing, and power; furthermore, the ipsilateral variations of the dual mode signals were more significant than those in the contralateral side. These results provide new detection and stimulation technology for understanding the mechanisms underlying Parkinson's disease and should, therefore, represent a useful resource for the design of future treatments.

Enabling biodegradable functional biomaterials for the management of neurological disorders

An increasing number of patients are being diagnosed with neurological diseases, but are rarely cured because of the lack of curative therapeutic approaches. This situation creates an urgent clinical need to develop effective diagnosis and treatment strategies for repair and regeneration of injured or diseased neural tissues. In this regard, biodegradable functional biomaterials provide promising solutions to meet this demand owing to their unique responsiveness to external stimulation fields, which enable neuro-imaging, neuro-sensing, specific targeting, hyperthermia treatment, controlled drug delivery, and nerve regeneration. This review discusses recent progress in the research and development of biodegradable functional biomaterials including electroactive biomaterials, magnetic materials and photoactive biomaterials for the management of neurological disorders with emphasis on their applications in bioimaging (photoacoustic imaging, MRI and fluorescence imaging), biosensing (electrochemical sensing, magnetic sensing and opical sensing), and therapy strategies (drug delivery, hyperthermia treatment, and tissue engineering). It is expected that this review will provide an insightful discussion on the roles of biodegradable functional biomaterials in the diagnosis and treatment of neurological diseases, and lead to innovations for the design and development of the next generation biodegradable functional biomaterials.

Electrophysiological Detection of Cortical Neurons under Gamma-Aminobutyric Acid and Glutamate Modulation Based on Implantable Microelectrode Array Combined with Microinjection

Understanding the relationships between different cortical neurons under excitatory or inhibitory modulation is important for researches of many neurological disorders. In order to monitor the neural activities in cortex under gamma-aminobutyric acid (GABA) and glutamate (Glu) modulation, an implantable microelectrode array (MEA) was combined with microinjection capillary. The neurons in motor cortex of rat were modulated by GABA and Glu injection, and multichannel neural electrophysiological signals were simultaneously recorded. Spike analysis showed that the interneurons recorded by the MEA were inhibited after GABA injection and excited after Glu injection, but one pyramidal neuron was found to be not sensitive to the drug. The local field potentials (LFP) were most affected in the frequency band of 5~10 Hz after Glu injection, which greatly increased the amplitudes of the spindle-like waveforms. The MEA combined with microinjection provided a low-cost and effective tool for neurological drug modulation and evaluation in brain tissue.

Bio-electrochemical microelectrode arrays for glutamate and electrophysiology detection in hippocampus of temporal lobe epileptic rats

Temporal Lobe Epilepsy (TLE) is a chronic neurological disorder, characterized by sudden, repeated and transient central nervous system dysfunction. For better understanding of TLE, bio-nanomodified microelectrode arrays (MEA) are designed, for the achievement of high-quality simultaneous detection of glutamate signals (Glu) and multi-channel electrophysiological signals including action potentials (spikes) and local field potentials (LFPs). The MEA was fabricated by Micro-Electro-Mechanical System fabrication technology and all recording sites were modified with platinum black nano-particles, the average impedance decreased by nearly 90 times. Additionally, glutamate oxidase was also modified for the detection of Glu. The average sensitivity of the electrode in Glu solution was 1.999 ± 0.032 × 10^-2pA/μM·μm²(n = 3) and linearity was R = 0.9986, with a good selectivity of 97.82% for glutamate and effective blocking of other interferents. In the in-vivo experiments, the MEA was subjected in hippocampus to electrophysiology and Glu concentration detection. During seizures, the fire rate of spikes increases, and the interspike interval is concentrated within 30 ms. The amplitude of LFPs increases by 3 times and the power increases. The Glu level (4.22 μM, n = 4) was obviously higher than normal rats (2.24 μM, n = 4). The MEA probe provides an advanced tool for the detection of dual-mode signals in the research of neurological diseases.

An integrated system for synchronous detection of neuron spikes and dopamine activities in the striatum of Parkinson monkey brain

BACKGROUND

Synchronous detecting neuron spikes and dopamine (DA) activities in the non-human primate brain play an important role in understanding of Parkinson's disease (PD). At present, most experiments are carried out by combing of electrodes and commercial instruments, which are inconvenient, time-consuming and inefficient.

NEW METHOD

Herein, this study describes a novel integrated system for monitoring neuron spikes and DA activities in non-human primate brain synchronously. This system integrates an implantable sensor, a dual-function head-stage and a low noise detection instrument.

METHODS

The system was developed efficiently by using the key technologies of noise reduction, interference protection and differential amplification. To demonstrate the utility of this system, synchronous recordings of electrophysiological signals and DA were in vivo performed in a monkey before and after treated as a Parkinson model monkey.

RESULTS

The system typically exhibited input-referred noise levels of only ∼ 3 μV_RMS, input impedance levels of up to 5.1 GΩ, and a sensitivity of 14.075 pA/μM for DA and could detect electrophysiological signals and DA without mutual interference. In monkey experiments, lower DA concentrations in the striatum and more intensive spikes of the Parkinson model monkey than the normal one were synchronously recorded efficiently.

COMPARISON WITH EXISTING METHODS

This integrated system will not only significantly simplify the experimental operation and improve the experimental efficiency, but also improve the signal quality and synchronization performance.

CONCLUSIONS

This integrated system, which is practical, efficient and convenient, can be widely used for the study of PD and other neurological disorders.

Recent Advances in the Detection of Neurotransmitters

Neurotransmitters are chemicals that act as messengers in the synaptic transmission process. They are essential for human health and any imbalance in their activities can cause serious mental disorders such as Parkinson’s disease, schizophrenia, and Alzheimer’s disease. Hence, monitoring the concentrations of various neurotransmitters is of great importance in studying and diagnosing such mental illnesses. Recently, many researchers have explored the use of unique materials for developing biosensors for both in vivo and ex vivo neurotransmitter detection. A combination of nanomaterials, polymers, and biomolecules were incorporated to implement such sensor devices. For in vivo detection, electrochemical sensing has been commonly applied, with fast-scan cyclic voltammetry being the most promising technique to date, due to the advantages such as easy miniaturization, simple device architecture, and high sensitivity. However, the main challenges for in vivo electrochemical neurotransmitter sensors are limited target selectivity, large background signal and noise, and device fouling and degradation over time. Therefore, achieving simultaneous detection of multiple neurotransmitters in real time with long-term stability remains the focus of research. The purpose of this review paper is to summarize the recently developed sensing techniques with the focus on neurotransmitters as the target analyte, and to discuss the outlook of simultaneous detection of multiple neurotransmitter species. This paper is organized as follows: firstly, the common materials used for developing neurotransmitter sensors are discussed. Secondly, several sensor surface modification approaches to enhance sensing performance are reviewed. Finally, we discuss recent developments in the simultaneous detection capability of multiple neurotransmitters.