[show abstract][hide abstract] ABSTRACT: Traumatic brain injury features deficits are often ameliorated by dopamine (DA) agonists. We have previously shown deficits in striatal DA neurotransmission using fast scan cyclic voltammetry after controlled cortical impact (CCI) injury that are reversed after daily treatment with the DA uptake inhibitor methylphenidate (MPH). The goal of this study was to determine how a single dose of MPH (5 mg/kg) induces changes in basal DA and metabolite levels and with electrically evoked overflow (EO) DA in the striatum of CCI rats. MPH-induced changes in EO DA after a 2-week daily pre-treatment regime with MPH was also assessed. There were no baseline differences in basal DA or metabolite levels. MPH injection significantly increased basal [DA] output in dialysates for control but not injured rats. Also, MPH injection increased striatal peak EO [DA] to a lesser degree in CCI (176% of baseline) versus control rats (233% of baseline). However, daily pre-treatment with MPH resulted in CCI rats having a comparable increase in EO [DA] after MPH injection when compared with controls. The findings further support the concept that daily MPH therapy restores striatal DA neurotransmission after CCI.
Journal of Neurochemistry 06/2009; 110(3):801-10. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Traumatic brain injury (TBI) results in functional deficits that often are effectively treated clinically with the neurostimulant, methylphenidate (MPH). We hypothesized that daily MPH administration would reverse striatal neurotransmission deficits observed in the controlled cortical impact (CCI) model of TBI. CCI or naïve rats received daily injections of MPH (5 mg/kg) or saline for 14 days and were assessed on day 15 using fast scan cyclic voltammetry. Dopamine (DA) transporter (DAT) localization, DA-related proteins, and transcription factor (c-fos) expression were also assessed. CCI resulted in reduced electrically evoked overflow of DA and maximal velocity of DA clearance (V(max)). In contrast, CCI was associated with a decrease in the apparent K(M) of DAT. Daily dose of MPH after CCI resulted in robust increases in evoked DA overflow and V(max) as well as increased apparent K(M). Reductions in total striatal DAT expression occurred after CCI and were not further affected by MPH. In contrast, membrane-bound striatal DAT levels were increased in both CCI groups. MPH post-CCI significantly increased striatal c-fos levels compared with saline. These results support the hypothesis that daily MPH improves striatal DA neurotransmission after CCI. DAT expression and transcriptional changes affecting DA protein function may underlie the injury and MPH-induced alterations in neurotransmission observed.
Journal of Neurochemistry 01/2009; 108(4):986-97. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: The therapeutic benefits of dopamine (DA) agonists after traumatic brain injury (TBI) imply a role for DA systems in mediating functional deficits post-TBI. We investigated how experimental TBI affects striatal dopamine systems using fast scan cyclic voltammetry (FSCV), western blot, and d-amphetamine-induced rotational behavior. Adult male Sprague-Dawley rats were injured by a controlled cortical impact (CCI) delivered unilaterally to the parietal cortex, or were naïve controls. Amphetamine-induced rotational behavior was assessed 10 days post-CCI. Fourteen days post-CCI, animals were anesthetized and underwent FSCV with bilateral striatal carbon fiber microelectrode placement and stimulating electrode placement in the medial forebrain bundle (MFB). Evoked DA overflow was assessed in the striatum as the MFB was electrically stimulated at 60 Hz for 10 s. In 23% of injured animals, but no naïve animals, rotation was observed with amphetamine administration. Compared with naïves, striatal evoked DA overflow was lower for injured animals in the striatum ipsilateral to injury (p < 0.05). Injured animals exhibited a decrease in V(max) (52% of naïve, p < 0.05) for DA clearance in the hemisphere ipsilateral to injury compared with naïves. Dopamine transporter (DAT) expression was proportionally decreased in the striatum ipsilateral to injury compared with naïve animals (60% of naïve, p < 0.05), despite no injury-related changes in vesicular monoamine transporter or D2 receptor expression (DRD2) in this region. Collectively, these data appear to confirm that the clinical efficacy of dopamine agonists in the treatment of TBI may be related to disruptions in the activity of subcortical dopamine systems.
Journal of Neurochemistry 10/2005; 95(2):457-65. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Resting and evoked extracellular dopamine levels in the striatum of the anesthetized rat were measured by fast-scan cyclic voltammetry in conjunction with carbon fiber microelectrodes. Identification of the substance detected in vivo was achieved by inspection of background-subtracted voltammograms. Intrastriatal microinfusion of kynurenate, a broad-spectrum antagonist of ionotropic glutamate receptors, caused a decrease in the resting extracellular level of dopamine. The kynurenate-induced decrease was unaffected by systemic pretreatment with pargyline, an inhibitor of monoamine oxidase, but was significantly attenuated by systemic pretreatment with alpha-methyl-p-tyrosine, an inhibitor of tyrosine hydroxylase. Although glutamate by itself did not affect resting extracellular dopamine levels, glutamate did attenuate the kynurenate-induced decrease. Kynurenate decreased dopamine release in response to electrical stimulation of the medial forebrain bundle, an effect that was also attenuated by glutamate. These results suggest that both spontaneous and evoked dopamine release in the rat striatum are under the local tonic excitatory influence of glutamate. Interactions between central dopamine and glutamate systems that have been implicated in the etiologies of Parkinson's disease, schizophrenia, stress, and substance abuse. The precise nature of those interactions, however, remains a matter of some controversy.
[show abstract][hide abstract] ABSTRACT: Carbon fiber microelectrodes that support enzyme-containing redox polymer gels permit the amperometric detection of glutamate, choline, and glucose. These devices are of interest for in vivo neurochemical monitoring because their small dimensions may permit highly localized measurements within small brain nuclei. In vitro calibration procedures confirm that the sensors respond in a selective fashion towards their respective target analyte. In the current work, the selectivity of the in vivo response of the microsensors during pharmacological manipulations is considered. The response of choline and glucose microsensors during the local infusion of tetrodotoxin and neostigmine in rat striatum is reported. The results of this study support the conclusion that these microsensors respond selectively to their respective targets under in vivo conditions.
Journal of Neuroscience Methods 02/2001; 104(2):183-9. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: A random-walk simulation of microdialysis is used to examine how a reaction that consumes analyte in the medium external to the probe affects the extraction and recovery processes. The simulations suggest that such a reaction can promote the extraction process while simultaneously inhibiting the recovery process, which appears to be consistent with recent experimental evidence of asymmetry in the extraction and recovery of the neurotransmitter, dopamine, during brain microdialysis. This suggests that quantitative microdialysis strategies that rely on the extraction fraction as a measure of the probe recovery value, such as the no-net-flux method, will produce an underestimate of the analyte concentration in the external medium when that analyte is consumed by a reaction in the external medium. Furthermore, if experimental conditions arise under which the kinetics of the reaction are changed, then changes in the extraction and recovery processes are likely to occur as well. The implications of these theoretical findings for the quantitative interpretation of in vivo microdialysis results obtained for the neurotransmitter dopamine are examined.
[show abstract][hide abstract] ABSTRACT: The objective of this study was to examine whether the limited diffusion distance of dopamine in rat striatum produces spatial heterogeneity in the extracellular dopamine concentration on a dimensional scale of a few micrometers. Such heterogeneity would be significant because it would imply that the concentration of dopamine at a given receptor depends on the receptor's ultrastructural location. Spatially resolved measurements of extracellular dopamine were performed in the striatum of chloral hydrate-anesthetized rats with carbon fiber microdisk electrodes. Dopamine was monitored during electrical stimulation of the nigrostriatal pathway before and after administration of drugs that selectively affect the kinetics of evoked dopamine release and dopamine uptake. The effects of nomifensine (20 mg/kg), L-DOPA (250 mg/kg), and alpha-methyl-p-tyrosine (250 mg/kg) on the amplitude of the stimulation responses were examined. The outcome of these experiments was compared with predictions derived from a mathematical model that combines diffusion with the kinetics of release and uptake. The results demonstrate that the extracellular dopamine concentration is spatially heterogeneous on a micrometer scale and that changing the kinetics of dopamine release and uptake has different effects on this spatial distribution. The impact of these results on brain neurochemistry is considered.
Journal of Neurochemistry 05/2000; 74(4):1563-73. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Certain aspects of schizophrenia and Parkinson's disease suggest that glutamate might have an inhibitory effect on dopamine release. Several studies have reported that the excitatory actions of ionotropic glutamate agonists on extracellular dopamine levels in striatum are resistant to tetrodotoxin, which suggests that glutamate excites an impulse-independent mechanism of dopamine release. We tested the hypothesis that an inhibitory action of glutamate on dopamine terminals in the striatum specifically involves an impulse-dependent mechanism of dopamine release. We used voltammetry to monitor electrically-evoked dopamine release in striatal slices, which is completely tetrodotoxin- and Ca(2+)-sensitive and so provides a model of impulse-dependent dopamine release. Agonists of the ionotropic glutamate receptors significantly decreased the amplitude of the response, while antagonists significantly increased the amplitude of the response, by as much as approximately 60% in the case of kynurenic acid. These results support the hypothesis that ionotropic glutamate receptors can inhibit impulse-dependent dopamine release by a mechanism that acts locally within the striatum. This finding contrasts with previous reports that glutamate can excite impulse-independent dopamine release. This extends earlier findings that glutamate may both excite and inhibit subcortical dopamine systems by suggesting that the excitatory and inhibitory actions of striatal ionotropic glutamate receptors are specifically associated with impulse-independent and impulse-dependent dopamine release, respectively.
[show abstract][hide abstract] ABSTRACT: This paper describes electrochemical microsensors for the in vivo measurement of glutamate and ascorbate in the extracellular space of brain tissue. To prepare glutamate microsensors, carbon fiber microelectrodes (10 microns in diameter and 300-400 microns long) were modified with a cross-linked redox polymer film containing enzymes. The microsensors were coated with a thin Nafion film before use. The glutamate microsensors were both selective and sensitive toward glutamate, with detection limits in the low micromolar range. Physiologically relevant concentrations of several electroactive compounds found in brain tissue produced no response at the glutamate microsensors and also did not affect their glutamate response, the only exception being glutamine, for which a small response was observed in the absence, but not in the presence, of glutamate. The ascorbate microsensors were used in conjunction with cyclic voltammetry. They were sensitive and selective toward ascorbate, but did exhibit a small sensitivity toward the dopamine metabolite, dihydroxyphenylacetic acid. The in vivo measurements performed establish the ability of the glutamate microsensors to monitor the component of the basal extracellular glutamate level that is derived from the neuronal activity of brain tissue.
[show abstract][hide abstract] ABSTRACT: An approach to capillary electrophoresis with electrochemical detection (CE-EC) suitable for determination of dopamine in 1-min brain microdialysate samples is described. The CE-EC system includes an electrochemical detection cell that permits easy, precise, and permanent alignment of a carbon fiber microelectrode with a separation capillary (30-micron i.d., 75-cm length). Amperometric detection was performed at a constant applied potential of 600 mV with respect to a Ag/AgCl reference electrode. Decoupling of the electrophoretic current from the amperometric detector was accomplished with an integrated end-column decoupler prepared by etching the capillary outlet with HF. The decoupler produces baseline noise of 50 fA, or less, in the presence of 10-20-muA current in the separation capillary. The low baseline noise affords low mass (attomoles) and low concentration (nanomolar) detection limits for dopamine and 4-methylcatechol. A peak attributable to dopamine was identified in electropherograms of brain microdialysate samples obtained from anesthetized rats. Identification of the dopamine peak was confirmed by pharmacological methods. Dopamine was readily detected in 1-min brain microdialysate samples. The dopamine concentration in 1-min brain microdialysis samples was significantly altered by drug treatments and by brief electrical stimulation of dopaminergic axons.
[show abstract][hide abstract] ABSTRACT: Voltammetric microelectrodes and microdialysis probes were used simultaneously to monitor extracellular dopamine in rat striatum during electrical stimulation of the medial forebrain bundle. Microelectrodes were placed far away (1 mm) from, immediately adjacent to, and at the outlet of microdialysis probes. In drug-naive rats, electrical stimulation (45 Hz, 25 s) evoked a robust response at microelectrodes far away from the probes, but there was no response at microelectrodes adjacent to and at the outlet of the probes. After nomifensine administration (20 mg/kg i.p.), stimulation evoked robust responses at all three microelectrode placements. These results demonstrate first that evoked release in tissue adjacent to microdialysis probes is suppressed in comparison with evoked release in tissue far away from the probes and second that equilibration of the dopamine concentration in the extracellular fluid adjacent to and far away from the probes is prevented by the high-affinity dopamine transporter. Hence, models of microdialysis, which assume the properties of tissue to be spatially uniform, require modification to account for the distance that separates viable sites of evoked dopamine release from the probe. We introduce new mass transfer resistance parameters that qualitatively explain the observed effects of uptake inhibition on stimulation responses recorded with microdialysis and voltammetry.
Journal of Neurochemistry 09/1998; 71(2):684-92. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Numerical modeling was used as a means to examine the relationship between the outcome of in vivo voltammetry and microdialysis experiments and dopamine concentrations in the extracellular fluid of rat striatum. In the case of microdialysis, quantitative interpretation of results demands knowledge of the in vivo values for the extraction and recovery ratios of the probes toward dopamine. Equality of the extraction and recovery ratios is a necessary condition for the direct application of the no-net-flux method as a quantitative technique. Recent results have suggested that the extraction and recovery ratios are not equal, and this interpretation is now supported by theory. A new relationship between extraction and recovery is proposed.
Journal of Neurochemistry 03/1998; 70(2):594-603. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Carbon fiber microelectrodes either were implanted directly into striatal tissue or were mounted into the outlet of microdialysis probes that were implanted into striatal tissue. This allowed voltammetry and microdialysis to be used under identical in vivo experimental conditions to monitor extracellular dopamine levels during electrical stimulation of the medial forebrain bundle both before and after uptake inhibition with nomifensine. The marked differences between the results obtained with each technique cannot be explained on the basis of their inherent analytical attributes (sensitivity, temporal response, etc.). The results demonstrate that the microdialysis recovery factor for endogenous dopamine increases after uptake inhibition, an observation that stands in contradiction to the existing theory and practice of the microdialysis technique. The observations led to the development of a numerical model that rationalizes the observations reported herein and that allows in vivo voltammetry and in vivo microdialysis results to be interpreted within a single theoretical framework.
Journal of Neurochemistry 03/1998; 70(2):584-93. · 3.97 Impact Factor
[show abstract][hide abstract] ABSTRACT: Selective amperometric enzyme microsensors for monitoring low micromolar concentrations of choline in extracellular fluid of rat brain have been developed. Preparation of the choline microsensors involved the modification of carbon fiber microcylinder electrodes (10 microns diameter, 300-400 microns long) with a cross-linked redox-active gel containing horseradish peroxidase and choline oxidase. Rejection of the noise recorded from the choline microsensors implanted in living brain tissue improved the in vivo detection capabilities of the sensors. The microsensors and a differential detection scheme were used to estimate the basal concentration of choline in striatal tissue at 6.6 +/- 2.9 microM and to measure changes in choline concentrations of 6.1 +/- 2.7 microM in vivo. The microsensors were also used to monitor choline produced following the injections of acetylcholine in vivo. Coinjections of neostigmine and acetylcholine significantly lowered the choline response recorded with the microsensors, confirming that the response following the injections of acetylcholine alone was due to the activity of endogenous acetylcholinesterase. Comparison of the maximal rate of decrease in choline concentration following the injections of 1 mM choline and 1 mM acetylcholine was used to estimate the rate of acetylcholine clearance from extracellular fluid through cholinesterase activity at approx. 2.5 microM/min.
Journal of Neuroscience Methods 01/1997; 70(1):73-82. · 2.11 Impact Factor
[show abstract][hide abstract] ABSTRACT: Amperometric microsensors for the detection of choline in the extracellular fluid of brain tissue have been prepared by immobilizing horseradish peroxidase and choline oxidase onto carbon fiber microcylinder electrodes with a cross-linkable redox polymer. The microcylinders have diameters of 7 or 10 microns and lengths of 200-400 microns. To detect choline, the microsensors are operated at an applied potential of -0.1 V vs SCE. At this potential, ascorbate and other easily oxidizable interferant molecules present in brain tissue are not detected by the electrode. Ascorbate, however, can interfere with the response to choline by acting as a reducing agent in the enzyme-containing polymer film. So, a Nafion overlayer is required in order to reliably detect choline in the presence of physiologically relevant concentrations of ascorbate (approximately 200 microM). The Nafion-coated microsensors have a detection limit of approximately 5 microM choline and give a linear response beyond 100 microM when calibrated in vitro at 37 degrees C. Exposure of the microsensors to brain tissue for several hours causes less than a 10% loss in redox polymer surface coverage and less than a 25% loss in sensitivity to choline. To assess the ability of the microsensors to monitor choline levels in brain tissue, small volumes of a choline solution were injected into brain tissue at a site about 1 mm away from a microsensor. The current arising at the microsensor was converted to choline concentration by calibrating the sensor following the in vivo experiment. The resultant choline concentrations were in excellent agreement with those predicted by appropriate diffusion equations.
[show abstract][hide abstract] ABSTRACT: Amperometric sensors have been developed for hydrogen peroxide, choline, and acetylcholine by immobilization of horseradish peroxidase, (HRP), choline oxidase, and acetylcholinesterase in a cross-linked redox polymer deposited on glassy carbon electrodes. Peroxide sensors, prepared by immobilization of HRP alone, gave detection limits of 10 nM and a linear response up to ca. 1 mM. Coimmobilization of HRP and glucose oxidase was used to establish the feasibility of highly efficient bienzyme sensors at low substrate levels. Replacing glucose oxidase with choline oxidase produced sensors with submicromolar detection limits and a linear response up to 0.8 mM. Addition of acetylcholinesterase to the sensors generated a relatively small response to acetylcholine that demonstrates the feasibility of trienzyme sensors. At low substrate concentrations, no loss in sensitivity during a 1-day experiment was observed. The response times of these sensors are all less than 30 s with 2-s response times achieved in some cases.
[show abstract][hide abstract] ABSTRACT: Fast-scan cyclic voltammetry in the striatum of anesthetized rats has been used to monitor extracellular dopamine during forced electrical stimulation of the media forebrain bundle using parameters that mimic intracranial self-stimulation. The temporal resolution provided by microelectrodes positioned very near sites of dopamine release allows resolution of the response to individual 500-ms stimulation trains separated by 500-ms intervals. Uptake inhibition by Nomifensine alters the resolution obtained at short times after initiation of stimulation.
Brain Research 02/1993; 600(2):305-7. · 2.88 Impact Factor
[show abstract][hide abstract] ABSTRACT: Cyclic voltammetry at microdisk electrodes (i.e. whose radii are less than ≈20 μm) imbedded in infinite coplanar insulators has been simulated using a two-dimensional finite differences method. Prior to solving the diffusion equation, a change of variables is used to allow the diffusion and reaction layers surrounding the microdisk to be efficiently divided into discrete elements. The Hopscotch algorithm is used to solve the diffusion equation in the new, conformally mapped spatial coordinates. Simulations have been carried out for reversible, irreversible and quasi-reversible charge transfer reactions and the results are shown to agree with available analytical solutions. The simulation can also be applied to conditions for which analytical solutions are not available presently. The effect of unequal diffusion coefficients of the oxidized and reduced form of a redox couple has been investigated. To demonstrate the application to coupled chemical steps, the EC mechanism has also been simulated.
Journal of Electroanalytical Chemistry - J ELECTROANAL CHEM. 01/1989; 267:33-45.
[show abstract][hide abstract] ABSTRACT: A method is described for developing and evaluating models of neurochemical processes. Computer simulation and simplex optimization are used to examine a model of the dopaminergic nerve terminal of the rat striatum. In the model, synthesis, storage, release, uptake, and metabolism are described by a set of non-linear differential equations. Parameters of the model are optimized with respect to diverse experimental data. These data include steady state passage of radioactivity, decline in total dopamine after synthesis inhibition, and change in extracellular dopamine concentration during electrical stimulation.
Journal of Neuroscience Methods 02/1988; 22(3):239-52. · 2.11 Impact Factor