In vivo electrochemical monitoring of the change of cochlear perilymph ascorbate during salicylate-induced tinnitus

Local Delivery of Therapeutics to the Cochlea Using Nanoparticles and Other Biomaterials

Hearing loss negatively impacts the well-being of millions of people worldwide. Systemic delivery of ototherapeutics has limited efficacy due to severe systemic side effects and the presence of the blood-labyrinth barrier that selectively limits or enables transfer of molecules between plasma and inner ear tissues and fluids. Local drug delivery into the middle and inner ear would be preferable for many newly emerging classes of drugs. Although the cochlea is a challenging target for drug delivery, recent technologies could provide a safe and efficacious delivery of ototherapeutics. Local drug delivery routes include topical delivery via the external auditory meatus, retroauricular, transtympanic, and intracochlear delivery. Many new drug delivery systems specifically for the inner ear are under development or undergoing clinical studies. Future studies into these systems may provide a means for extended delivery of drugs to preserve or restore hearing in patients with hearing disorders. This review outlines the anatomy of the (inner) ear, describes the various local delivery systems and routes, and various quantification methodologies to determine the pharmacokinetics of the drugs in the inner ear.

Echem methods and electrode types of the current in vivo electrochemical sensing

For a long time, people have been eager to realize continuous real-time online monitoring of biological compounds. Fortunately, in vivo electrochemical biosensor technology has greatly promoted the development of biological compound detection. This article summarizes the existing in vivo electrochemical detection technologies into two categories: microdialysis (MD) and microelectrode (ME). Then we summarized and discussed the electrode surface time, pollution resistance, linearity and the number of instances of simultaneous detection and analysis, the composition and characteristics of the sensor, and finally, we also predicted and prospected the development of electrochemical technology and sensors in vivo.

Standard cochlear implants as electrochemical sensors: Intracochlear oxygen measurements in vivo

Cochlear implants are the most successful neural prostheses worldwide and routinely restore sensorineural hearing loss by direct electrical stimulation of the auditory nerve. Enhancing this standard implant by chemical sensor functionality opens up new possibilities, ranging from access to the biochemical microenvironment of the implanted electrode array to the long-term study of the electrode status. We developed an electrochemical method to turn the platinum stimulation microelectrodes of cochlear implants into electrochemical sensors. The electrodes showed excellent and stable chemical sensor properties, as demonstrated by in vitro characterizations with combined amperometric and active potentiometric dissolved oxygen and hydrogen peroxide measurements. Linear, stable and highly reproducible sensor responses within the relevant concentration ranges with negligible offset were shown. This approach was successfully applied in vivo in an animal model. Intracochlear oxygen dynamics in rats upon breathing pure oxygen were reproducibly and precisely measured in real-time from the perilymph. At the same time, correct implant placement and its functionality was verified by measurements of electrically evoked auditory brainstem responses with clearly distinguishable peaks. Acute measurements indicated no adverse influence of electrical stimulation on electrochemical measurements and vice versa. Our work is ground-breaking towards advanced implant functionality, future implant lifetime monitoring, and implant-life-long in situ investigation of electrode degradation in cochlear implant patients.

A proposed implantable voltammetric carbon fiber-based microsensor for corticosteroid monitoring by cochlear implants

A novel carbon fiber microsensor (CFMS) with the capability of being inserted in the cochlear implant structure is introduced for in situ measurement of corticosteroid concentration. The microsensor structure is composed of a carbon microfiber, an Ag wire, and a Pt wire acting respectively as a working electrode, a reference electrode, and a counter electrode. In addition, a silicone septum is used for isolation purposes in place of the epoxy resin. The septum-insulated microsensor is capable of monitoring the concentration of the corticosteroids in the perilymph fluid without a need for sampling from the inner ear fluid and the consequent ex vivo analysis. The electrochemical determination of the corticosteroids was investigated on the carbon fiber electrode surface by differential pulse voltammetry. During the reduction of dexamethasone (DEX), a cathodic peak with a peak potential of -1.3 V appeared at the CFMS. Using the CFMS under optimized conditions, a calibration plot of the dexamethasone (DEX) in the artificial perilymph solution exhibited two linear ranges from 10 nM to 2 μM and 2 to 40 μM (sensitivity equal to 16.55 μA μM-1 cm-2; LOD = 4 nM) conforming with the DEX concentration range inside the inner ear after the insertion of a drug-eluting cochlear implant electrode (CIE). Furthermore, the interferences occurring in the hearing functions of the CIE after the presence and function of the CFMS were simulated numerically using the finite element method. According to our results, decreasing the size of the microsensor introduces lower interferences with the auditory function of the cochlear implant electrode.

Analysis of Pharmacokinetics in the Cochlea of the Inner Ear

Hearing loss affects >5% of the global population and therefore, has a great social and clinical impact. Sensorineural hearing loss, which can be caused by different factors, such as acoustic trauma, aging, and administration of certain classes of drugs, stems primarily from a dysfunction of the cochlea in the inner ear. Few therapeutic strategies against sensorineural hearing loss are available. To develop effective treatments for this disease, it is crucial to precisely determine the behavior of ototoxic and therapeutic agents in the microenvironment of the cochlea in live animals. Since the 1980s, a number of studies have addressed this issue by different methodologies. However, there is much less information on pharmacokinetics in the cochlea than that in other organs; the delay in ontological pharmacology is likely due to technical difficulties with accessing the cochlea, a tiny organ that is encased with a bony wall and has a fine and complicated internal structure. In this review, we not only summarize the observations and insights obtained in classic and recent studies on pharmacokinetics in the cochlea but also describe relevant analytical techniques, with their strengths, limitations, and prospects.

Neuroprotective effects of MK-801 on auditory cortex in salicylate-induced tinnitus: Involvement of neural activity, glutamate and ascorbate

Tinnitus may cause anxiety, depression, insomnia, which impair the quality of life of millions worldwide. However, the mechanism of tinnitus remains to be understood, it has been previously hypothesized that the activation of N-methyl-D-aspartate (NMDA) receptor is involved in the tinnitus processes and blockade of the NMDA receptor is regarded as a therapeutic strategy for tinnitus treatment even if the rescue treatment is still proved invalid in some cases. To demonstrate the therapeutic effect of the NMDA receptor blocker on tinnitus, we examined here the spontaneous firing rate (SFR) and the neurochemical dynamics in the auditory cortex (AC) of rats after sodium salicylate (SS) injection, which is a widely used model for tinnitus research. We also recorded their responses to MK-801 treatment. Electrophysiological studies showed that MK-801 significantly suppresses SFR in AC of rats with SS-induced tinnitus. In addition, by using a technique that combining in vivo microdialysis with an online electrochemical system (OECS) and a high-performance liquid chromatography (HPLC), we found that the levels of both glutamate and ascorbate in AC dramatically increased after SS injection and that MK-801 administration attenuated those response. Further studies found that MK-801 given at a time point of 30 min pre- or post-injection of SS were more effective than that given at a time point of 60 min post-SS injection, indicating that the time point of MK-801 intervention has a critical impact on the therapeutic effect. These findings suggest that MK-801 plays a neuroprotective role against hyperactivity during tinnitus induced by SS and that the therapeutic effect depends on the time point of MK-801 intervention, which would advance the studies on understanding of the therapeutic potential of NMDA receptor antagonist in tinnitus therapy.

A microsensing system for the in vivo real-time detection of local drug kinetics

Real-time recording of the kinetics of systemically administered drugs in in vivo microenvironments may accelerate the development of effective medical therapies. However, conventional methods require considerable analyte quantities, have low sampling rates and do not address how drug kinetics correlate with target function over time. Here, we describe the development and application of a drug-sensing system consisting of a glass microelectrode and a microsensor composed of boron-doped diamond with a tip of around 40 μm in diameter. We show that, in the guinea pig cochlea, the system can measure-simultaneously and in real time-changes in the concentration of bumetanide (a diuretic that is ototoxic but applicable to epilepsy treatment) and the endocochlear potential underlying hearing. In the rat brain, we tracked the kinetics of the drug and the local field potentials representing neuronal activity. We also show that the actions of the antiepileptic drug lamotrigine and the anticancer reagent doxorubicin can be monitored in vivo. Our microsensing system offers the potential to detect pharmacological and physiological responses that might otherwise remain undetected.

Online electrochemical system as an in vivo method to study dynamic changes of ascorbate in rat brain during 3-methylindole-induced olfactory dysfunction

This study demonstrates the application of an OECS as an in vivo method to investigate the dynamic change of ascorbate in the olfactory bulb of rats during the acute period of olfactory dysfunction.

In vivo fluorescence sensing of the salicylate-induced change of zinc ion concentration in the auditory cortex of rat brain

This study demonstrates a fluorescence method for in vivo sensing of the dynamic change of Zn(2+) concentration in auditory cortex microdialysates induced by salicylate with N'-(7-nitro-2,1,3-benzoxadiazole-4-yl)-N,N,N'-tris(pyridine-2-ylmethyl) ethane-1,2-diamine (NBD-TPEA) as a probe. The excellent properties of the NBD-TPEA probe make it possible to achieve a high selectivity for Zn(2+) sensing with the co-existence of amino acids and other metal ions as well as the species commonly existing in the cerebral system. To validate the method for in vivo fluorescence sensing of Zn(2+) in the rat brain, we pre-mix the microdialysates in vivo sampled from the auditory cortex with the NBD-TPEA probe and then perfuse the mixtures into a fluorescent cuvette for continuous-flow fluorescence detection. The method demonstrated here shows a linear relationship between the signal output and Zn(2+) concentration within the concentration range from 0.5 μM to 4 μM, with a detection limit of 156 nM (S/N = 3). The basal level of extracellular Zn(2+) in auditory cortex microdialysates is determined to be 0.52 ± 0.082 μM (n = 4). This value is increased by the injection of 100 mg mL(-1) of salicylate (1 μL min(-1), 5 min, i.p.), reaches a peak at the time point of 90 min, and levels off with time. Such an increase is attenuated by the injection of MK-801, a potent and specific NMDA receptor antagonist, after the pre-injection of 100 mg mL(-1) salicylate for 5 min. This study offers a fluorescence method for in vivo sensing of Zn(2+) in the rat brain that could be useful for the investigations of chemical processes involved in brain functions.

Hybridization of bioelectrochemically functional infinite coordination polymer nanoparticles with carbon nanotubes for highly sensitive and selective in vivo electrochemical monitoring

This study demonstrates the formation of a three-dimensional conducting framework through hybridization of bioelectrochemically active infinite coordination polymer (ICP) nanoparticles with single-walled carbon nanotubes (SWNTs) for highly sensitive and selective in vivo electrochemical monitoring with combination with in vivo microdialysis. The bioelectrochemically active ICP nanoparticles are synthesized through the self-assembly process of NAD(+) and Tb(3+), in which all biosensing elements including an electrocatalyst (i.e., methylene green, MG), cofactor (i.e., β-nicotinamide adenine dinucleotide, NAD(+)), and enzyme (i.e., glucose dehydrogenase, GDH) are adaptively encapsulated. The ICP/SWNT-based biosensors are simply prepared by drop-coating the as-formed ICP/SWNT nanocomposite onto a glassy carbon substrate. Electrochemical studies demonstrate that the simply prepared ICP/SWNT-based biosensors exhibit excellent biosensing properties with a higher sensitivity and stability than the ICP-based biosensors prepared only with ICP nanoparticles (i.e., without hybridization of SWNTs). By using a GDH-based electrochemical biosensor as an example, we demonstrate a technically simple yet effective online electroanalytical platform for continuously monitoring glucose in the brain of guinea pigs with the ICP/SWNT-based biosensor as an online detector in a continuous-flow system combined with in vivo microdialysis. Under the experimental conditions employed here, the dynamic linear range for glucose with the ICP/SWNT-biosensor is from 50 to 1000 μM. Moreover, in vivo selectivity investigations with the biosensors prepared by the GDH-free ICPs reveal that ICP/SWNT-based biosensors are very selective for the measurement of glucose in the cerebral system. The basal level of glucose in the microdialysates from the striatum of guinea pigs is determined to be 0.31 ± 0.03 mM (n = 3). The study offers a simple route to the preparation of electrochemical biosensors, which is envisaged to be particularly useful for probing the chemical events involved in some physiological and pathological processes.