An electrochemical investigation into the effects of local and systemic administrations of sodium nitroprusside in brain extracellular fluid of mice.

Sodium nitroprusside (SNP) is a nitric oxide (NO)-donor drug used clinically to treat severe hypertension, however, there are limitations associated with its mechanism of action that prevent widespread adoption. In particular, its impact on cerebral hemodynamics is controversial and direct evidence on its effects are lacking. Electrochemical methods provide an attractive option to undertake real time neurochemical measurements in situ using selective microsensors. Herein, we report the novel application of an existing platinum (Pt)-Nafion® sensor to measure the release of NO from SNP under in vitro and in vivo conditions. Initially, the temporal release of NO was measured and the effect of the reducing agent, ascorbic acid (AA), was elucidated in vitro. A combined microdialysis/NO sensor construct was implanted into the striatum of anaesthetised mice and the local perfusion of 10 mM SNP with/without AA resulted in increased NO concentration detected using the Pt-Nafion® sensor. Subsequently, the NO sensor, coupled with carbon paste electrodes (CPEs) for the electrochemical measurement of O2, were applied to investigate SNP effects in freely moving mice. A complex mechanism of action was identified that infers NO inhibition and biphasic O2 dynamics. The preliminary findings within support a strong cerebrovascular effect of systemic SNP administration that warrants careful consideration for clinical use.

A platinum oxide-based microvoltammetric pH electrode suitable for physiological investigations.

Attempts to develop miniaturised pH electrodes for in vivo monitoring have received much attention in recent years. Continuous real-time pH measurements may be predictive of potentially dangerous deviations in metabolic events that could improve patient prognosis. Herein, we report the in vitro investigation of a physiologically relevant, Pt oxide-based microvoltammetric pH electrode. Cycling through the potential window range -0.65 V to +0.8 V vs. SCE, gave rise to well-established monolayer oxide (MO) and hydrogen (H2) adsorption redox peaks in aqueous solution. The H2 desorption and MO reduction peaks demonstrated pH dependent, linear responses (49 ± 11 mV pH-1 and 76 ± 4 mV pH-1 respectively), following pre-activation of the electrode surface in HCl. Since in vivo monitoring is at the core of this design, the effect of incorporating a miniaturised pseudo reference electrode (PRE) was determined. The Ag/AgCl PRE demonstrated near Nernstian behaviour for the MO reduction peak (58 ± 5 mV pH-1) and sub-Nernstian behaviour (43 ± 6 mV pH-1) for its H2 desorption counterpart. Finally, a preliminary in vivo recording performed in the striatum of a freely moving mouse confirmed that the MO reduction peak was maintained under physiological conditions. These findings support the ability of the Pt oxide-based pH electrode to perform continuous, stable recordings in vivo and warrants further characterisation.

In vitro development and in vivo application of a platinum-based electrochemical device for continuous measurements of peripheral tissue oxygen.

Acute limb ischaemia is caused by compromised tissue perfusion and requires immediate attention to reduce the occurrence of secondary complications that could lead to amputation or death. To address this, we have developed a novel platinum (Pt)-based electrochemical oxygen (O2) device for future applications in clinical monitoring of peripheral tissue ischaemia. The effect of integrating a Pt pseudo-reference electrode into the O2 device was investigated in vitro with an optimum reduction potential of -0.80V. A non-significant (p=0.11) decrease in sensitivity was recorded when compared against an established Pt-based O2 sensor operating at -0.65V. Furthermore, a biocompatible clinical sensor (ClinOX) was designed, demonstrating excellent linearity (R2=0.99) and sensitivity (1.41±0.02nAµM-1) for O2 detection. Significant rapid decreases in the O2 current during in vivo ischaemic insults in rodent limbs were reported for Pt-Pt (p<0.001) and ClinOX (p<0.01) and for ClinOX (p<0.001) in porcine limbs. Ex vivo sensocompatibility investigations identified no significant difference (p=0.08) in sensitivity values over 14days of exposure to tissue homogenate. The Pt-Pt based O2 design demonstrated high sensitivity for tissue ischaemia detection and thus warrants future clinical investigation.

Real-Time Amperometric Recording of Extracellular H2O2 in the Brain of Immunocompromised Mice: An In Vitro, Ex Vivo and In Vivo Characterisation Study.

We detail an extensive characterisation study on a previously described dual amperometric H2O2 biosensor consisting of H2O2 detection (blank) and degradation (catalase) electrodes. In vitro investigations demonstrated excellent H2O2 sensitivity and selectivity against the interferent, ascorbic acid. Ex vivo studies were performed to mimic physiological conditions prior to in vivo deployment. Exposure to brain tissue homogenate identified reliable sensitivity and selectivity recordings up to seven days for both blank and catalase electrodes. Furthermore, there was no compromise in pre- and post-implanted catalase electrode sensitivity in ex vivo mouse brain. In vivo investigations performed in anaesthetised mice confirmed the ability of the H2O2 biosensor to detect increases in amperometric current following locally perfused/infused H2O2 and antioxidant inhibitors mercaptosuccinic acid and sodium azide. Subsequent recordings in freely moving mice identified negligible effects of control saline and sodium ascorbate interference injections on amperometric H2O2 current. Furthermore, the stability of the amperometric current was confirmed over a five-day period and analysis of 24-h signal recordings identified the absence of diurnal variations in amperometric current. Collectively, these findings confirm the biosensor current responds in vivo to increasing exogenous and endogenous H2O2 and tentatively supports measurement of H2O2 dynamics in freely moving NOD SCID mice.

Long Term Amperometric Recordings in the Brain Extracellular Fluid of Freely Moving Immunocompromised NOD SCID Mice.

We describe the in vivo characterization of microamperometric sensors for the real-time monitoring of nitric oxide (NO) and oxygen (O2) in the striatum of immunocompromised NOD SCID mice. The latter strain has been utilized routinely in the establishment of humanized models of disease e.g., Parkinson's disease. NOD SCID mice were implanted with highly sensitive and selective NO and O2 sensors that have been previously characterized both in vitro and in freely moving rats. Animals were systemically administered compounds that perturbed the amperometric current and confirmed sensor performance. Furthermore, the stability of the amperometric current was investigated and 24 h recordings examined. Saline injections caused transient changes in both currents that were not significant from baseline. l-NAME caused significant decreases in NO (p < 0.05) and O2 (p < 0.001) currents compared to saline. l-Arginine produced a significant increase (p < 0.001) in NO current, and chloral hydrate and Diamox (acetazolamide) caused significant increases in O2 signal (p < 0.01) compared against saline. The stability of both currents were confirmed over an eight-day period and analysis of 24-h recordings identified diurnal variations in both signals. These findings confirm the efficacy of the amperometric sensors to perform continuous and reliable recordings in immunocompromised mice.

Continuous real-time in vivo measurement of cerebral nitric oxide supports theoretical predictions of an irreversible switching in cerebral ROS after sufficient exposure to external toxins.

BACKGROUND: Mathematical models of the interactions between alphasynuclein (αS) and reactive oxygen species (ROS) predict a systematic and irreversible switching to damagingly high levels of ROS after sufficient exposure to risk factors associated with Parkinson's disease (PD). OBJECTIVES: We tested this prediction by continuously monitoring real-time changes in neurochemical levels over periods of several days in animals exposed to a toxin known to cause Parkinsonian symptoms. METHODS: Nitric oxide (NO) sensors were implanted in the brains of freely moving rats and the NO levels continuously recorded while the animals were exposed to paraquat (PQ) injections of various amounts and frequencies. RESULTS: Long-term, real-time measurement of NO in a cohort of animals showed systematic switching in levels when PQ injections of sufficient size and frequency were administered. The experimental observations of changes in NO imply a corresponding switching in endogenous ROS levels and support theoretical predictions of an irreversible change to damagingly high levels of endogenous ROS when PD risks are sufficiently large. CONCLUSIONS: Our current results only consider one form of PD risk, however, we are sufficiently confident in them to conclude that: (i) continuous long-term measurement of neurochemical dynamics provide a novel way to measure the temporal change and system dynamics which determine Parkinsonian damage, and (ii) the bistable feedback switching predicted by mathematical modelling seems to exist and that a deeper analysis of its characteristics would provide a way of understanding the pathogenic mechanisms that initiate Parkinsonian cell damage.

An investigation of hypofrontality in an animal model of schizophrenia using real-time microelectrochemical sensors for glucose, oxygen, and nitric oxide.

Glucose, O2, and nitric oxide (NO) were monitored in real time in the prefrontal cortex of freely moving animals using microelectrochemical sensors following phencyclidine (PCP) administration. Injection of saline controls produced a decrease in glucose and increases in both O2 and NO. These changes were short-lived and typical of injection stress, lasting ca. 30 s for glucose and between 2 and 6 min for O2 and NO, respectively. Subchronic PCP (10 mg/kg) resulted in increased motor activity and increases in all three analytes lasting several hours: O2 and glucose were uncoupled with O2 increasing rapidly following injection reaching a maximum of 70% (ca. 62 µM) after ca. 15 min and then slowly returning to baseline over a period of ca. 3 h. The time course of changes in glucose and NO were similar; both signals increased gradually over the first hour post injection reaching maxima of 55% (ca. 982 µM) and 8% (ca. 31 nM), respectively, and remaining elevated to within 1 h of returning to baseline levels (after ca. 5 and 7 h, respectively). While supporting increased utilization of glucose and O2 and suggesting overcompensating supply mechanisms, this neurochemical data indicates a hyperfrontal effect following acute PCP administration which is potentially mediated by NO. It also confirms that long-term in vivo electrochemical sensors and data offer a real-time biochemical perspective of the underlying mechanisms.

Brain nitric oxide: regional characterisation of a real-time microelectrochemical sensor.

A reliable method of directly measuring endogenously generated nitric oxide (NO) in real-time and in various brain regions is presented. An extensive characterisation of a previously described amperometric sensor has been carried out in the prefrontal cortex and nucleus accumbens of freely moving rats. Systemic administration of saline caused a transient increase in signal from baseline levels in both the prefrontal cortex (13 ± 3pA, n=17) and nucleus accumbens (12 ± 3pA, n=8). NO levels in the prefrontal cortex were significantly increased by 43 ± 9pA (n=9) following administration of l-arginine. A similar trend was observed in the nucleus accumbens, where an increase of 44 ± 9pA (n=8) was observed when compared against baseline levels. Systemic injections of the non-selective NOS inhibitor l-NAME produced a significant decrease in current recorded in the prefrontal cortex (24 ± 6pA, n=5) and nucleus accumbens (17 ± 3pA, n=6). Finally it was necessary to validate the sensors functionality in vivo by investigating the effect of the interferent ascorbate on the oxidation current. The current showed no variation in both regions over the selected time interval of 60 min, indicating no deterioration of the polymer membrane. A detailed comparison identified significantly greater affects of administrations on NO sensors implanted in the striatum than those inserted in the prefrontal cortex and the nucleus accumbens.

Nitric oxide monitoring in brain extracellular fluid: characterisation of Nafion-modified Pt electrodes in vitro and in vivo.

A Nafion(5 pre-coats/2 dip-coats)-modified Pt sensor developed for real-time neurochemical monitoring has now been characterised in vitro for the sensitive and selective detection of nitric oxide (NO). A potentiodynamic profile at bare Pt established +0.9 V (vs. SCE) to be the most appropriate applied potential for NO oxidation. The latter was confirmed using oxyhaemoglobin and N(2), both of which reduced the NO signal to baseline levels. Results indicated enhanced NO sensitivity at the Nafion(5/2) sensor (1.67 +/- 0.08 nA microM(-1)) compared to bare Pt (1.08 +/- 0.20 nA microM(-1)) and negligible interference from a wide range of endogenous electroactive interferents such as ascorbic acid, dopamine and its metabolites, NO(2)(-) and H(2)O(2). The response time of 33.7 +/- 2.7 s was found to improve (19.0 +/- 3.4 s) when the number of Nafion layers was reduced to 2/1 and an insulating outer layer of poly(o-phenylenediamine) added. When tested under physiological conditions of 37 degrees C the response time of the Nafion(5/2) sensor improved to 14.00 +/- 2.52 s. In addition, the NO response was not affected by physiological concentrations of O(2) despite the high reactivity of the two species for each other. The limit of detection (LOD) was estimated to be 5 nM while stability tests in lipid (phosphatidylethanolamine; PEA) and protein (bovine serum albumin; BSA) solutions (10%) found an initial ca. 38% drop in sensitivity in the first 24 h which remained constant thereafter. Preliminary in vivo experiments involving systemic administration of NO and L-arginine produced increases in the signals recorded at the Nafion(5/2) sensor implanted in the striatum of freely-moving rats, thus supporting reliable in vivo recording of NO.

Calibration of NO sensors for in-vivo voltammetry: laboratory synthesis of NO and the use of UV-visible spectroscopy for determining stock concentrations.

The increasing scientific interest in nitric oxide (NO) necessitates the development of novel and simple methods of synthesising NO on a laboratory scale. In this study we have refined and developed a method of NO synthesis, using the neutral Griess reagent, which is inexpensive, simple to perform, and provides a reliable method of generating NO gas for in-vivo sensor calibration. The concentration of the generated NO stock solution was determined using UV-visible spectroscopy to be 0.28+/-0.01 mmol L(-1). The level of NO(2) (-) contaminant, also determined using spectroscopy, was found to be 0.67+/-0.21 mmol L(-1). However, this is not sufficient to cause any considerable increase in oxidation current when the NO stock solution is used for electrochemical sensor calibration over physiologically relevant concentrations; the NO sensitivity of bare Pt-disk electrodes operating at +900 mV (vs. SCE) was 1.08 nA micromol(-1) L, while that for NO(2) (-) was 5.9 x 10(-3) nA micromol(-1) L. The stability of the NO stock solution was also monitored for up to 2 h after synthesis and 30 min was found to be the time limit within which calibrations should be performed.