Time course of changes in extracellular lactate evoked by transient K(+)-induced depolarisation in the rat striatum

Excitotoxicity and Metabolic Crisis Are Associated with Spreading Depolarizations in Severe Traumatic Brain Injury Patients

Cerebral microdialysis has enabled the clinical characterization of excitotoxicity (glutamate >10 μM) and non-ischemic metabolic crisis (lactate/pyruvate ratio [LPR] >40) as important components of secondary damage in severe traumatic brain injury (TBI). Spreading depolarizations (SD) are pathological waves that occur in many patients in the days following TBI and, in animal models, cause elevations in extracellular glutamate, increased anaerobic metabolism, and energy substrate depletion. Here, we examined the association of SD with changes in cerebral neurochemistry by placing a microdialysis probe alongside a subdural electrode strip in peri-lesional cortex of 16 TBI patients requiring neurosurgery. In 107 h (median; range: 76-117 h) of monitoring, 135 SDs were recorded in six patients. Glutamate (50 μmol/L) and lactate (3.7 mmol/L) were significantly elevated on day 0 in patients with SD compared with subsequent days and with patients without SD, whereas pyruvate was decreased in the latter group on days 0 and 1 (two-way analysis of variance [ANOVA], p values <0.05). In patients with SD, both glutamate and LPR increased in a dose-dependent manner with the number of SDs in the microdialysis sampling period (0, 1, ≥2 SD) [glutamate: 2.1→7.0→52.3 μmol/L; LPR: 27.8→29.9→45.0, p values <0.05]. In these patients, there was a 10% probability of SD occurring when glutamate and LPR were in normal ranges, but a 60% probability when both variables were abnormal (>10 μmol/L and >40 μmol/L, respectively). Taken together with previous studies, these preliminary clinical results suggest SDs are a key pathophysiological process of secondary brain injury associated with non-ischemic glutamate excitotoxicity and severe metabolic crisis in severe TBI patients.

Methylphenidate increases glucose uptake in the brain of young and adult rats

BACKGROUND

Methylphenidate (MPH) is the drug of choice for pharmacological treatment of attention deficit hyperactivity disorder. Studies have pointed to the role of glucose and lactate as well as in the action mechanisms of drugs used to treat these neuropsychiatric diseases. Thus, this study aims to evaluate the effects of MPH administration on lactate release and glucose uptake in the brains of young and adult rats.

METHODS

MPH (1.0, 2.0 and 10.0mg/kg) or saline was injected in young and adult Wistar male rats either acutely (once) or chronically (once daily for 28 days). Then, the levels of lactate release and glucose uptake were assessed in the prefrontal cortex, hippocampus, striatum, cerebellum and cerebral cortex.

RESULTS

Chronic MPH treatment increased glucose uptake at the dose of 10.0mg/kg in the prefrontal cortex and striatum, and at the dose of 2.0mg/kg in the cerebral cortex of young rats. In adult rats, an increase in glucose uptake was observed after acute administration of MPH at the dose of 10.0mg/kg in the prefrontal cortex. After chronic treatment, there was an increase in glucose uptake with MPH doses of 2.0 and 10.0mg/kg in the prefrontal cortex, and at an MPH dose of 2.0mg/kg in the striatum of adult rats. The lactate release did not change with either acute or chronic treatments in young or adult rats.

CONCLUSIONS

These findings indicate that MPH increases glucose consumption in the brain, and that these changes are dependent on age and posology.

Dynamic metabolic response to multiple spreading depolarizations in patients with acute brain injury: an online microdialysis study

Spreading depolarizations (SDs) occur spontaneously with high incidence in patients with acute brain injury. They can be detected by subdural electrocorticographic recordings. We here characterize the dynamic metabolic response to these events. A microdialysis catheter was inserted into perilesional cortical tissue adjacent to a strip for electrocorticography following craniotomy in 10 patients. The microdialysis catheter was connected to an online microdialysis assay measuring glucose and lactate concentrations every 30 to 60 secs. Spontaneously occurring SDs systematically caused a reduction in dialysate glucose by -32.0 micromol/L (range: -92.3 to -18.4 micromol/L, n=90) and increase in lactate by +23.1 micromol/L (range: +5.5 to +93.6 micromol/L, n=49). The changes were sustained at 20 mins after the SD events and highly significant using an area under the curve analysis (P<0.0001). Multiple and frequent SDs led to a progressive stepwise depletion of brain glucose. Hence, SD events cause a massive energy imbalance and their frequent occurrence leads to a local insufficiency of glucose supply. Such a failure would compromise cellular repolarization and hence tissue viability. The findings offer a new mechanism to account for otherwise unexplained instances of depletion of brain microdialysate glucose.

Intercellular metabolic compartmentation in the brain: past, present and future

The first indication of 'metabolic compartmentation' in brain was the demonstration that glutamine after intracisternal [14C]glutamate administration is formed from a compartment of the glutamate pool that comprises at most one-fifth of the total glutamate content in the brain. This pool, which was designated 'the small compartment,' is now known to be made up predominantly or exclusively of astrocytes, which accumulate glutamate avidly and express glutamine synthetase activity, whereas this enzyme is absent from neurons, which eventually were established to constitute 'the large compartment.' During the following decades, the metabolic compartment concept was refined, aided by emerging studies of energy metabolism and glutamate uptake in cellularly homogenous preparations and by the histochemical observations that the two key enzymes glutamine synthetase and pyruvate carboxylase are active in astrocytes but absent in neurons. It is, however, only during the last few years that nuclear magnetic resonance (NMR) spectroscopy, assisted by previously obtained knowledge of metabolic pathways, has allowed accurate determination in the human brain in situ of actual metabolic fluxes through the neuronal tricarboxylic acid (TCA) cycle, the glial, presumably mainly astrocytic, TCA cycle, pyruvate carboxylation, and the 'glutamate-glutamine cycle,' connecting neuronal and astrocytic metabolism. Astrocytes account for 20% of oxidative metabolism of glucose in the human brain cortex and accumulate the bulk of neuronally released transmitter glutamate, part of which is rapidly converted to glutamine and returned to neurons in the glutamate-glutamine cycle. However, one-third of released transmitter glutamate is replaced by de novo synthesis of glutamate from glucose in astrocytes, suggesting that at steady state a corresponding amount of glutamate is oxidatively degraded. Net degradation of glutamate may not always equal its net production from glucose and enhanced glutamatergic activity, occurring during different types of cerebral stimulation, including the establishment of memory, may be associated with increased de novo synthesis of glutamate. This process may contribute to a larger increase in glucose utilization rate than in rate of oxygen consumption during brain activation. The energy yield in astrocytes from glutamate formation is strongly dependent upon the fate of the generated glutamate.

Effects of the nitric oxide donor, DEA/NO on cortical spreading depression

Cortical spreading depression (CSD) is a transient disruption of local ionic homeostasis that may promote migraine attacks and the progression of stroke lesions. We reported previously that the local inhibition of nitric oxide (NO) synthesis with Nomega-nitro-L-arginine methyl ester (L-NAME) delayed markedly the initiation of the recovery of ionic homeostasis from CSD. Here we describe a novel method for selective, controlled generation of exogenous NO in a functioning brain region. It is based on microdialysis perfusion of the NO donor, 2-(N,N-diethylamino)-diazenolate-2-oxide (DEA/NO). As DEA/NO does not generate NO at alkaline pH, and as the brain has a strong acid-base buffering capacity, DEA/NO was perfused in a medium adjusted at alkaline (but unbuffered) pH. Without DEA/NO, such a microdialysis perfusion medium did not alter CSD. DEA/NO (1, 10 and 100 microM) had little effect on CSD by itself, but it reversed in a concentration-dependent manner the effects of NOS inhibition by 1 mM L-NAME. These data demonstrate that increased formation of endogenous NO associated with CSD is critical for subsequent, rapid recovery of cellular ionic homeostasis. In this case, the molecular targets for NO may be located either on brain cells to suppress mechanisms directly involved in CSD genesis, or on local blood vessels to couple flow to the increased energy demand associated with CSD.

Nitric oxide formation during cortical spreading depression is critical for rapid subsequent recovery of ionic homeostasis

Cortical spreading depression (CSD) is a temporary disruption of local ionic homeostasis that propagates slowly across the cerebral cortex. Cortical spreading depression promotes lesion progression in experimental stroke, and may contribute to the initiation of migraine attacks. The purpose of this study was to investigate the roles of the marked increase of nitric oxide (NO) formation that occurs with CSD. Microdialysis electrodes were implanted in the cortex of anesthetized rats to perform the following operations within the same region: (1) elicitation of CSD by perfusion of high K+ medium; (2) recording of CSD elicitation; (3) application of the NO synthase inhibitor, NG-nitro-l-arginine methyl ester (l-NAME); and (4) recording of dialysate pH changes. The primary effect of l-NAME (0.3 to 3.0 mmol/L in the perfusion medium) was a marked widening of individual CSD wave, resulting essentially from a delayed initiation of the repolarization phase. This change was due to NO synthase inhibition because it was not observed with the inactive isomer d-NAME, and was reversed by l-arginine. This effect did not appear to be linked to the suppression of a sustained, NO-mediated vascular change associated with the superposition of NO synthase inhibition on high levels of extracellular K+. The delayed initiation of repolarization with local NO synthase inhibition may reflect the suppression of NO-mediated negative feedback mechanisms acting on neuronal or glial processes involved in CSD genesis. However, the possible abrogation of a very brief, NO-mediated vascular change associated with the early phase of CSD cannot be ruled out.

Glucose and lactate metabolism during brain activation

The dependence of brain function on blood glucose as a fuel does not exclude the possibility that lactate within the brain might be transferred between different cell types and serve as an energy source. It has been recently suggested that 1) about 85% of glucose consumption during brain activation is initiated by aerobic glycolysis in astrocytes, triggered by demand for glycolytically derived energy for Na+ -dependent accumulation of transmitter glutamate and its amidation to glutamine, and 2) the generated lactate is quantitatively transferred to neurons for oxidative degradation. However, astrocytic glutamate uptake can be fueled by either glycolytically or oxidatively derived energy, and the extent to which "metabolic trafficking" of lactate might occur during brain function is unknown. In this review, the potential for an astrocytic-neuronal lactate flux has been estimated by comparing rates of glucose utilization in brain and in cultured neurons and astrocytes with those for lactate release and uptake. Working brain tissue and isolated brain cells release large amounts of lactate. Cellular lactate uptake occurs by carrier-mediated facilitated diffusion and is normally limited by its dependence on metabolism of accumulated lactate to maintain a concentration gradient. The rate of this process is similar in cultured astrocytes and glutamatergic neurons, and, at physiologically occurring lactate concentrations, lactate uptake corresponds at most to 25% of the rate of glucose oxidation, which accordingly is the upper limit for "metabolic trafficking" of lactate. Because of a larger local release than uptake of lactate and the necessity for rapid lactate clearance to maintain the intracellular redox state to support lactate production in the presence of normal oxygen levels, brain activation in vivo is probably, in many cases, accompanied by a substantial overflow of glycolytically generated lactate, both to different brain areas and under some conditions (spreading depression, hyperammonemia) to circulating blood.

Evaluation of CA1 damage using single-voxel 1H-MRS and un-biased stereology: Can non-invasive measures of N-acetyl-asparate following global ischemia be used as a reliable measure of neuronal damage?

Global brain ischemia provoked by transient occlusion of the carotid arteries (2VO) in gerbils results in a severe loss of neurons in the hippocampal CA1 region. We measured the concentration of the neuron specific N-acetyl-aspartate, [NAA], in the gerbil dorsal hippocampus by proton MR spectroscopy (1H-MRS) in situ, and HPLC, 4 days after global ischemia. The [NAA] was correlated with graded hippocampus damage scoring and stereologically determined neuronal density. A basal hippocampal [NAA] of 8.37+/-0.10 and 9.81+/-0.44 mmol/l were found from HPLC and 1H-MRS, respectively. HPLC measurements of [NAA] obtained from hippocampus 4 days after 2VO showed a 20% reduction in the [NAA] following 4 min of ischemia (P<0.001). 1H-MRS measurements on gerbils subjected to 4 or 8 min of ischemia showed a similar 24% decline in the [NAA] (P<0.05). Thus, there was correlation between the HPLC and 1H-MRS determined NAA decline. There was also a significant correlation between 1H-MRS [NAA] and the corresponding reduction in CA1 neuronal density (P<0.004). In summary our findings show that single voxel 1H-MRS can be used as a supplement to histological evaluation of neuronal injury in studies after global brain ischemia. Accordingly, volume selective spectroscopy has a potential for assessment of neuroprotective therapeutic compounds/strategies with respect to neuronal rescue for delayed ischemic brain damage.

Microdialysis coupled to online enzymatic assays

In vivo sampling of interstitial fluid by using microdialysis fibers has become a standard and accepted procedure. This sampling method is generally coupled to offline analysis of consecutive dialysate samples by high-performance liquid chromatography or capillary electrophoresis, but this combination is not the best approach for some applications, especially those which require high temporal resolution and rapid data collection. The purpose of this review is to provide information on enzyme-based online assays, i.e., continuous analysis of the dialysate as it emerges from the outlet of the sampling device. We have focused on methods developed specifically for the analysis of solutions perfused at a very slow flow rate, i.e., a feature of microdialysis and ultrafiltration techniques. These methods include flow enzyme-fluorescence assays, flow enzyme-amperometric assays, and sequential enzyme-amperometric detection. Each type of assay is discussed in terms of principle, applications, advantages, and limitations. We also comment on implantable biosensors, an obvious next step forward for in vivo monitoring of molecules in neuroscience.

Excitotoxicity in neurological disorders--the glutamate paradox

Beneficial effects of glutamate-receptor antagonists in models of neurological disorders are often used to support the notion that endogenous excitotoxicity (i.e. resulting from extracellular accumulation of endogenous glutamate) is a major contributor to neuronal death associated with these conditions. However, this interpretation conflicts with a number of robust and important experimental evidence. Here, emphasis is placed on two key elements: (i) very high extracellular levels of glutamate must be reached to initiate neuronal death, far above those measured in models of neurological disorders; and (ii) changes in extracellular glutamate as measured by microdialysis are not related to changes in the synaptic cleft, i.e. the compartment where neurotransmitter glutamate interacts with its receptors. It has become clear that the diversity and complexity of glutamate-mediated processes allow for a wide range of potential abnormalities (e.g. loss of selectivity of glutamate-operated ion channels, abnormal modulation of glutamate receptors). In addition, as neuronal death subsequent to ischemia and other insults is likely to result from multifactorial processes that may be inter-related, inhibition of glutamate-mediated synaptic transmission may be neuroprotective by increasing the resistance of neurons to other deleterious mechanisms (e.g. inadequate energy supply) that are not directly related to glutamatergic transmission.

High extracellular glutamate and neuronal death in neurological disorders. Cause, contribution or consequence?

In models of neurological disorders, increased extracellular glutamate and beneficial effects produced by glutamate-receptor antagonists are consistently taken as supporting evidence of excitotoxicity. This systematic interpretation is over-simplified and potentially misleading. High extracellular glutamate is not a reliable indicator of endogenous excitotoxicity, i.e., the intrinsic, potential neurotoxicity of endogenous glutamate whenever it accumulates extracellularly. Firstly, because the extracellular levels of glutamate necessary to produce depolarization and death in vivo, are far above those measured in models of neurological disorders. Secondly, because changes in the concentration of glutamate in the synaptic cleft (i.e., the relevant compartment for endogenous excitotoxicity) are not reflected extracellularly. Protection by glutamate-receptor antagonists does not necessarily imply inhibition of excitotoxic abnormalities. Indeed, neuronal death initiated by insults such as ischemia results from multifactorial processes that may be interrelated. Therefore, beneficial effects resulting from an interaction with glutamate-mediated transmission may actually render the cell more resistant to other deleterious mechanisms (e.g., mitochondrial injury, oxidative stress).

Hypoosmolarity induces an increase of extracellular N-acetylaspartate concentration in the rat striatum

We previously showed that extracellular levels of N-acetylaspartate (NAA) increase when a medium with reduced NaCl concentration is perfused through a microdialysis probe, and proposed that NAA may be released during hypoosmotic swelling. Here, we demonstrate that this effect is due to hypoosmolarity of the perfusion medium, and not to low NaCl. NAA changes in the dialysate were compared with those of taurine as the osmoregulatory role of this amino acid is established. Reduction of the NaCl concentration in the perfusion medium increased the dialysate levels of NAA and taurine, but this effect was abolished when NaCl was replaced by sucrose to maintain isosmolarity. The NAA response to hypoosmolarity was smaller than that of taurine, but it may still be important to neurons as NAA is predominantly neuronal in the mammalian CNS.

Effects of probenecid on the elicitation of spreading depression in the rat striatum

Spreading depression (SD) is a wave of cellular depolarization which contributes to neuronal damage in experimental focal ischaemia, and may also underlie the migraine aura. The purpose of this study was to examine the effects of probenecid, an inhibitor of organic anion transport, on K+-evoked SD in vivo. Microdialysis electrodes were implanted in the rat striatum, and recurrent SD elicited by perfusion of artificial cerebrospinal fluid containing 160 mM K+ for 20 min. Probenecid was administered either directly through the microdialysis probe, starting 50 min before application of high K+, or intravenously. SD was markedly reduced by perfusion of 5 mM probenecid through the microdialysis probe. In contrast, a high intravenous dose of probenecid (250 mg/kg) only slightly inhibited SD elicitation 90 min after treatment, despite clear changes in the amplitude and spectrum of the electroencephalogram, as early as 10 min after drug administration, confirming that probenecid readily penetrated the central nervous system. As SD is inhibited by hypercapnia, we have examined the possibility that probenecid may inhibit SD through extracellular acidification subsequent to blockade of lactate transport. Perfusion of 1-20 mM probenecid increased dose-dependently the dialysate levels of lactate, but without extracellular acidosis since the dialysate pH was not significantly reduced. How probenecid inhibits SD deserves further investigation because it may help identify novel strategies to suppress this phenomenon, now recognized deleterious to neuronal function and survival.

Effect of acidotic challenges on local depolarizations evoked by N-methyl-D-aspartate in the rat striatum

We have examined how various challenges to brain acid-base homeostasis, resulting in extracellular acidosis, alter N-methyl-D-aspartate (NMDA)-evoked depolarizations in vivo. Repeated stimuli were produced by perfusion of 200 microM NMDA for 2 min through a microdialysis probe implanted into the striatum of halothane anesthetized rats. Hypercapnia reduced NMDA-evoked responses in a concentration-dependent manner, with 7.5 and 15 % CO2 in the breathing mixture reducing the depolarization amplitude to 74 % and 64 % of that of the initial stimuli, respectively. Application of 50 mM NH4+ progressively reduced dialysate pH, and a further acidification was observed when NH4+ was discontinued. Perfusion of NMDA after NH4+ application evoked smaller depolarizations (56 % of the corresponding control, 5 min after NH4+ removal), and this effect persisted for over 1 h. Perfusion of acidic ACSF did not alter the amplitude of NMDA-evoked depolarization, despite changes in dialysate pH confirming that exchange/buffering of acid equivalents took place between the perfusion medium and the surrounding tissue. This negative result probably reflected the remarkable capacity of the brain to buffer H+. Together, these results demonstrate that extracellular acidosis, such as that associated with excessive neuronal activation or ischemia, inhibits NMDA-evoked responses in vivo.

Changes in extracellular acid-base homeostasis in cerebral ischemia

The purpose of this study was to examine the changes in extracellular CO32- and lactate concentration produced by ischemia, especially in relation to the occurrence of anoxic depolarization, and how some of these changes are altered by the inhibition of organic acid transport systems with probenecid. These data demonstrate that (i) the transmembrane mechanisms contributing to intracellular acid-base regulation (Na+/H+ and HCO3-/Cl- exchanges, and lactate/H+ cotransport) are markedly activated during ischemia; (ii) the efficacy of these mechanisms is abolished as the cellular membrane permeability to ions, including H+ and pH-changing anions, suddenly increases with anoxic depolarization; and (iii) efflux of intracellular lactate during ischemia, and its reuptake with reperfusion, mainly occur via a transporter. These findings imply that residual cellular acid-base homeostasis persists as long as cell depolarization does not occur, and strengthen the concept that anoxic depolarization is a critical event for cell survival during ischemia.

Neurochemical monitoring using intracerebral microdialysis in patients with subarachnoid hemorrhage

The authors have developed a method for routine monitoring of disturbances in brain energy metabolism and extracellular levels of excitatory amino acids using intracerebral microdialysis in 10 patients with subarachnoid hemorrhage. Microdialysis was conducted for periods ranging from 6 to 11 days after ictus. Altogether, 16,054 chemical analyses from 1647 dialysate samples were performed. Concentrations of the energy-related substances lactate, pyruvate, glucose, and hypoxanthine were measured, and the lactate/pyruvate ratio was calculated. The excitatory amino acids glutamate and aspartate were measured. The microdialysis data were matched with computerized tomography findings, clinical course, and outcome. The results support the concepts that microdialysis is a promising tool for chemical monitoring of the human brain and that extracellular fluid levels of lactate, lactate/pyruvate ratio, glucose, hypoxanthine, and glutamate are useful markers of disturbances in brain energy metabolism in neurointensive care patients. These results have generated a working hypothesis that the pattern of these extracellular markers may help differentiate between various causes of energy perturbations, such as hypoxia and different degrees of ischemia. The correlation between the dialysate levels of excitatory amino acids and outcome supports the concept of glutamate receptor overactivation in acute human brain injury.

Inhibition of cortical spreading depression by L-701,324, a novel antagonist at the glycine site of the N-methyl-D-aspartate receptor complex

1. Spreading depression (SD) is a propagating transient suppression of electrical activity, associated with cellular depolarization, which probably underlies the migraine aura and may contribute to neuronal damage in focal ischaemia. The purpose of this study was to examine whether L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2-(1H)-quinolone), a high affinity antagonist at the glycine site of the N-methyl-D-aspartate (NMDA) receptor complex, inhibits the initiation and propagation of K(+)-induced SD in the rat cerebral cortex in vivo. 2. Microdialysis probes incorporating a recording electrode were implanted in the cerebral cortex of anaesthetized rats and perfused with artificial cerebrospinal fluid (ACSF). Five episodes of repetitive SD were elicited by switching to a medium containing 130 mM K+ for 20 min, each separated by 40 min of recovery (i.e. perfusion with normal ACSF). The brief negative shifts of the extracellular direct current (d.c.) potential, characteristic of SD elicitation, were recorded with the microdialysis electrode and a reference electrode placed under the scalp. Propagation of SD was examined using glass capillary electrodes inserted about 3 mm posterior to the microdialysis electrode. L-701,324 (5 or 10 mg kg-1) or its vehicle were administered i.v. 10 min after the end of the second K(+)-stimulus. The effects of L-701,324 were compared to those of dizocilpine (MK-801; 1 mg kg-1 i.v.), a NMDA-channel blocker known to potently block SD elicitation. 3. Potassium-induced SD initiation was inhibited by 10 mg kg-1 (but not by 5 mg kg-1) of L-701,324. Thirty minutes after administration of 10 mg kg-1 L-701,324, the cumulative area of SD peaks elicited during 20 min was 15.3 +/- 2.1 mV min, versus 23.2 +/- 1.1 mV min in animals which received only the drug vehicle (P < 0.02; n = 6). The delay between application of 130 mM K+ and occurrence of the first SD was also significantly increased. It was approximately doubled in animals treated with 10 mg kg-1 of L-701,324. 4. SD propagation was more sensitive than SD elicitation to L-701,324, as both 5 and 10 mg kg-1 produced an effective inhibition. Even at the lower dose of 5 mg kg-1, L-701,324 completely blocked the propagation of SD elicited 30 min after drug administration. This differential sensitivity of SD elicitation and propagation is not specific to L-701,324 since it was previously observed with other drugs. At doses effective against SD, L-701,324 did not produce any marked alterations of the electroencephalogram. 5. L-701,324 (10 mg kg-1) and MK-801 (1 mg kg-1) had identical effects on the d.c. potential when administered during the recovery which followed the second K+ stimulus. Both drugs produced a positive shift of around 4.5 mV within 10 min of i.v. drug administration, indicating rapid drug penetration into the CNS. Paradoxically, L-701,324 (10 mg kg-1) was markedly less effective than MK-801 (1 mg kg-1) in blocking SD, since this dose of MK-801 was sufficient virtually to abolish SD initiation and completely block its propagation. The higher potency of MK-801 against SD may reflect its use-dependency, i.e. binding of MK-801 and channel blockade are enhanced when the NMDA-receptor ionophore is open. 6. Taken together, these data demonstrate that L-701,324 has an inhibitory effect on both SD initiation and propagation. This action may be beneficial in focal ischaemia, and possibly also against migraine, especially as this drug was shown to be active when administered orally.

Quantitative in vivo measurements using microdialysis on-line with capillary zone electrophoresis

A system which couples microdialysis with capillary zone electrophoresis (CZE) on-line is used to monitor ascorbate and lactate in the caudate nucleus of rat brain. On-line interface of microdialysis probe and electrophoresis capillary, along with the high mass sensitivity of CZE, allows the probe to be operated at flow rates as low as 40 nl/min. Under these conditions, the relative recovery is nearly 100% and quantitative monitoring is possible. The microscale system also facilitates calibration by the low flow rate method. In spite of the low flow rate, temporal resolution in the 45-125 s range is possible for these compounds. The system is demonstrated by observing changes in ascorbate due to infusions of elevated K+ through the dialysis probe and systemic injections of amphetamine and an anesthetic (ketamine/xylazine/acepromazine mixture). Lactate is monitored in response to elevated K+ infusions.