Microdialysis sampling of the neuronal environment in basic and clinical research

Determination of free concentration of endocannabinoids in brain tissue

The release of metabolites from their bound to free forms is the main regulatory path in living species. Therefore, the ability to determine the free concentrations of small molecules is highly critical in many biological samples. The main challenges in achieving this task are the interferences inherent to complex matrices and the ability to distinguish between the free and total concentrations. This paper presents a non-invasive microextraction method that enables the determination of endocannabinoids in brain tissue. The proposed method is based on two key principles: the availability of the free concentration of endocannabinoids for partitioning to the solid-phase microextraction (SPME) fiber; and negligible depletion enabled by the small volume of extraction phase on the fiber. These features allow the presented SPME method to provide information about the free concentration of analytes without disturbing the binding equilibrium between the analytes and the matrix. The determination of spiked samples with known concentrations enables the percentage of analyte bound to the tissue to be calculated, which can then be applied to calculate the total concentration from the determined free concentration. This manuscript focuses on the determination of the free concentration and tissue binding percentages of endocannabinoids in brain tissue. Significantly, SPME's small size and potential for non-invasive sampling enable its application in live animal subjects with minimal tissue damage.

Evaluation of cefuroxime concentration in the intrathecal and extrathecal compartments of the lumbar spine-an experimental study in pigs

BACKGROUND AND PURPOSE

Optimal antibiotic prophylaxis is crucial to prevent postoperative infection in spinal surgery. Sufficient time above the minimal inhibitory concentration (fT > MIC) for relevant bacteria in target tissues is required for cefuroxime. We assessed cefuroxime concentrations and fT > MIC of 4 μg·ml-1 for Staphylococcus aureus in the intrathecal (spinal cord and cerebrospinal fluid, CSF) and extrathecal (epidural space) compartments of the lumbar spine.

EXPERIMENTAL APPROACH

Eight female pigs were anaesthetized and laminectomized at L3-L4. Microdialysis catheters were placed for sampling in the spinal cord, CSF, and epidural space. A single dose of 1500 mg cefuroxime was administered intravenously over 10 min. Microdialysates and plasma were obtained continuously during 8 h. Cefuroxime concentrations were determined by ultra-high-performance liquid chromatography.

KEY RESULTS

Mean fT > MIC (4 μg·ml-1 ) was 58 min in the spinal cord, 0 min in the CSF, 115 min in the epidural space, and 123 min in plasma. Tissue penetration was 32% in the spinal cord, 7% in the CSF, and 63% in the epidural space.

CONCLUSION AND IMPLICATIONS

fT > MIC (4 μg·ml-1 ) and tissue penetration for cefuroxime were lower in the intrathecal compartments (spinal cord and CSF) than in the extrathecal compartment (epidural space) and plasma, suggesting a significant effect of the blood-brain barrier. In terms of fT > MIC, a single dose of 1500 mg cefuroxime seems inadequate to prevent intrathecal infections related to spinal surgery for bacteria presenting with a MIC target of 4 μg· ml-1 or above.

Development of a Region-Specific Physiologically Based Pharmacokinetic Brain Model to Assess Hippocampus and Frontal Cortex Pharmacokinetics

Central nervous system drug discovery and development is hindered by the impermeable nature of the blood-brain barrier. Pharmacokinetic modeling can provide a novel approach to estimate CNS drug exposure; however, existing models do not predict temporal drug concentrations in distinct brain regions. A rat CNS physiologically based pharmacokinetic (PBPK) model was developed, incorporating brain compartments for the frontal cortex (FC), hippocampus (HC), "rest-of-brain" (ROB), and cerebrospinal fluid (CSF). Model predictions of FC and HC C_max, t_max and AUC were within 2-fold of that reported for carbamazepine and phenytoin. The inclusion of a 30% coefficient of variation on regional brain tissue volumes, to assess the uncertainty of regional brain compartments volumes on predicted concentrations, resulted in a minimal level of sensitivity of model predictions. This model was subsequently extended to predict human brain morphine concentrations, and predicted a ROB C_max of 21.7 ± 6.41 ng/mL when compared to "better" (10.1 ng/mL) or "worse" (29.8 ng/mL) brain tissue regions with a FC C_max of 62.12 ± 17.32 ng/mL and a HC C_max of 182.2 ± 51.2 ng/mL. These results indicate that this simplified regional brain PBPK model is useful for forward prediction approaches in humans for estimating regional brain drug concentrations.

Proteomics of brain extracellular fluid (ECF) and cerebrospinal fluid (CSF)

Mass spectrometry has become the gold standard for the identification of proteins in proteomics. In this review, I will discuss the available literature on proteomic experiments that analyze human cerebrospinal fluid (CSF) and brain extracellular fluid (ECF), mostly obtained by cerebral microdialysis. Both materials are of high diagnostic value in clinical neurology, for example, in cerebrovascular disorders like stroke, neurodegenerative diseases like Alzheimer's Disease, Parkinson's Disease, amyotrophic lateral sclerosis (ALS), traumatic brain injury and cerebral infectious and inflammatory disease, such as multiple sclerosis. Moreover, there are standard procedures for sampling. In a number of studies in recent years, biomarkers have been proposed in CSF and ECF for improved diagnosis or to control therapy, based on proteomics and mass spectrometry. I will also discuss the needs for a transition of research-based experimental screening with mass spectrometry to fast and reliable diagnostic instrumentation for clinical use.

Proteomics of human cerebral microdialysate: From detection of biomarkers to clinical application

Cerebral microdialysis is applied in clinical neurology and neurosurgery as monitoring tool in patients to evaluate the progression of severe diseases, such as stroke or trauma. Besides small molecules, e.g. metabolites and neurotransmitters, also the macromolecules, such as proteins and larger chemical compounds cross the dialysis membrane of the catheters implanted into the human brain parenchyma. Microdialysis can be used to extract molecules from the extracellular space of the brain in vivo, but additionally to deliver drugs, since the exchange is dependent on concentration gradients. Cerebral microdialysis may also be useful in the prediction of the clinical onset of symptoms, based on changes in the composition of pre-symptomatic microdialysate. For example, symptomatic vasospasm, which is a complication after subarachnoid hemorrhage, may be predicted by the combination of cerebral microdialysis and a proteomics approach. We will introduce the basic concepts of cerebral microdialysis, discuss possible clinical applications, and evaluate the application of proteomic approaches. With regard to technological aspects, we describe two-dimensional gel electrophoresis, high-pressure liquid chromatography, and mass spectrometry. With regard to clinical aspects, we discuss ethics, feasibility, time-course, and therapeutic options. In conclusion, proteomics of cerebral microdialysate may be used for diagnosis, disease monitoring, and therapeutic intervention of neurological patients.

Translational neurochemical research in acute human brain injury: the current status and potential future for cerebral microdialysis

Microdialysis (MD) was introduced as an intracerebral sampling method for clinical neurosurgery by Hillered et al. and Meyerson et al. in 1990. Since then MD has been embraced as a research tool to measure the neurochemistry of acute human brain injury and epilepsy. In general investigators have focused their attention to relative chemical changes during neurointensive care, operative procedures, and epileptic seizure activity. This initial excitement surrounding this technology has subsided over the years due to concerns about the amount of tissue sampled and the complicated issues related to quantification. The interpretation of mild to moderate MD fluctuations in general remains an issue relating to dynamic changes of the architecture and size of the interstitial space, blood-brain barrier (BBB) function, and analytical imprecision, calling for additional validation studies and new methods to control for in vivo recovery variations. Consequently, the use of this methodology to influence clinical decisions regarding the care of patients has been restricted to a few institutions. Clinical studies have provided ample evidence that intracerebral MD monitoring is useful for the detection of overt adverse neurochemical conditions involving hypoxia/ischemia and seizure activity in subarachnoid hemorrhage (SAH), traumatic brain injury (TBI), thromboembolic stroke, and epilepsy. There is some data strongly suggesting that MD changes precede the onset of secondary neurological deterioration following SAH, hemispheric stroke, and surges of increased ICP in fulminant hepatic failure. These promising investigations have relied on MD-markers for disturbed glucose metabolism (glucose, lactate, and pyruvate) and amino acids. Others have focused on trying to capture other important neurochemical events, such as excitotoxicity, cell membrane degradation, reactive oxygen species (ROS) and nitric oxide (NO) formation, cellular edema, and BBB dysfunction. However, these other applications need additional validation. Although these cerebral events and their corresponding changes in neurochemistry are important, other promising MD applications, as yet less explored, comprise local neurochemical provocations, drug penetration to the human brain, MD as a tool in clinical drug trials, and for studying the proteomics of acute human brain injury. Nevertheless, MD has provided new important insights into the neurochemistry of acute human brain injury. It remains one of very few methods for neurochemical measurements in the interstitial compartment of the human brain and will continue to be a valuable translational research tool for the future. Therefore, this technology has the potential of becoming an established part of multimodality neuro-ICU monitoring, contributing unique information about the acute brain injury process. However, in order to reach this stage, several issues related to quantification and bedside presentation of MD data, implantation strategies, and quality assurance need to be resolved. The future success of MD as a diagnostic tool in clinical neurosurgery depends heavily on the choice of biomarkers, their sensitivity, specificity, and predictive value for secondary neurochemical events, and the availability of practical bedside methods for chemical analysis of the individual markers. The purpose of this review was to summarize the results of clinical studies using cerebral MD in neurosurgical patients and to discuss the current status of MD as a potential method for use in clinical decision-making. The approach was to focus on adverse neurochemical conditions in the injured human brain and the MD biomarkers used to study those events. Methodological issues that appeared critical for the future success of MD as a routine intracerebral sampling method were addressed.

Hyperventilation impairs brain function in acute cerebral air embolism in pigs

OBJECTIVE

To evaluate, in a model of cerebral air embolism (CAE), the effects of ventilation-induced hypocapnia and hyperoxemia on intracranial pressure (ICP), cerebral perfusion pressure (CPP), brain oxygen (PbrO(2)), brain carbon dioxide (PbrCO(2)), brain pH (brpH) and levels of brain glucose and lactate.

DESIGN AND SETTING

Prospective animal study in a university medical center.

SUBJECTS

Fifteen Landrace/Yorkshire pigs.

INTERVENTIONS

In 15 anesthetized pigs ICP, PbrO(2), PbrCO(2) and brpH were measured with multi-parameter sensors, and brain glucose and lactate by microdialysis. All these parameters were recorded for 2 h after injection of air into the internal carotid artery. Nine animals were hyperventilated (PaCO(2 )+/-25 mmHg) and hyperoxygenated (FiO(2) 1.0) and six animals were normoventilated (PaCO(2)()+/-40 mmHg with an FiO(2) 0.4) and served as controls. RESULTS. In the treatment group the ICP rose from 8+/-1 to 52+/-6 mmHg, which was similar to that in the control group (12+/-1 to 57+/-8 mmHg). At the end of the 2-h study period, there were no significant differences in PbrO(2), PbrCO(2) and brpH between the two groups. The decreased brain glucose and increased brain lactate reached severe pathological values in both groups by the end of the 2-h study period.

CONCLUSIONS

Hypocapnia and hyperoxemia in acute CAE did not improve pathological functional brain parameters compared with normoventilated controls. Similarly, the pathological changes in brain glucose/lactate could also not be improved by hypocapnia and hyperoxemia.

Effects of cerebral air embolism on brain metabolism in pigs

OBJECTIVES

Cerebral air embolism was induced in pigs and changes in intracranial pressure (ICP), brain oxygen (PbrO2), brain carbon dioxide (PbrCO2), brain pH (brpH) and glucose, lactate and pyruvate levels were used to characterize this model.

METHODS

In seven anesthetized pigs, ICP, PbrO2, PbrCO2 and brpH were measured continuously with multiparameter sensors and brain glucose metabolism by microdialysis. After injection of air into the internal carotid artery, these parameters were recorded for 2 h.

RESULTS

ICP increased (433%) from 12 +/- 1 to 52 +/- 8 mmHg (P < 0.05). PbrO2 decreased from 25.7 +/- 6.2 to 11.9 +/- 5.2 mmHg. PbrCO2 increased (109%) from 57.7 +/- 2.7 to 120.4 +/- 21.5 mmHg (P < 0.05). Brain glucose decreased (38%) from 3.05 +/- 0.91 to 1.91 +/- 0.55 mmol, while brain lactate increased (384%) from 1.36 +/- 0.15 to 5.22 +/- 0.53 mmol/l (P < 0.05).

CONCLUSIONS

Cerebral air embolism has a deleterious effect on ICP and brain metabolism. Therefore, this model may be suitable for testing therapeutic regimens in cerebral air embolism.

Astrocytic amino acid metabolism under control conditions and during oxygen and/or glucose deprivation

Amino acid contents were measured in 1- and 3-week-old primary cultures of astrocytes and in their incubation media, an amino acid-free salt solution with or without glucose, during 3-h incubation under normoxic or anoxic conditions. Most essential amino acids were rapidly released to the medium during the beginning of the incubation. A subsequent slow medium increase reflected proteolysis. Glutamate and aspartate were absent from the media during all conditions, indicating fueling of their uptake by either glycolytically or oxidatively derived energy. The total content of glutamine increased, except during incubation in glucose-deprived media, when it declined or remained constant. Changes in aspartate were negligible, suggesting oxidative degradation of aspartate-derived oxaloacetate during normoxia and its reduction to succinate during anoxia, driving regeneration of NAD+ from NADH. An increase of alanine was reduced in glucose-free media, whereas serine showed especially large increase during isolated glucose deprivation, suggesting its production from glutamine via 3-phosphoglycerate.

How can we measure substrate, metabolite and neurotransmitter concentrations in the human brain?

Cerebral injury and disease is associated with fundamental derangements in metabolism, with changes in the concentration of important substrates (e.g. glucose), metabolites (e.g. lactate) and neurotransmitters (e.g. glutamate and y-aminobutyric acid) in addition to changes in oxygen utilization. The ability to measure these substances in the human brain is increasing our understanding of the pathophysiology of trauma, stroke, epilepsy and tumours. There are several techniques in clinical practice already in use and new methods are under evaluation. Such techniques include the use of cerebral probes (e.g. microdialysis. voltammetry and spectrophotometry) and functional imaging (e.g. positron emission tomography and magnetic resonance spectroscopy). This review describes these techniques in terms of their principles and clinical applications.

Glucose monitoring with long-term subcutaneous microdialysis in neonates

BACKGROUND

Microdialysis is a new approach for continuous monitoring of small molecules in the extracellular space, and hypoglycemia is a common problem in neonatal intensive care. The objective of this study was to evaluate subcutaneous microdialysis for long-term glucose monitoring in neonatal intensive care. We determined the relative recovery of the microdialysis system in vitro and in vivo, the stability of the relative recovery in vivo during long-term microdialysis, and the correlation between blood and dialysate concentrations of glucose and urea. Furthermore, we evaluated the sensitivity and specificy of subcutaneous microdialysis for the diagnosis of hypoglycemia.

PATIENT AND METHODS

Thirteen infants (10 neonates) with gestational ages of 30.2 to 45.6 weeks were investigated by microdialysis of subcutaneous adipose tissue and blood sampling. Subcutaneous microdialysis was performed for a median (range) duration of 9 (4-16) days.

RESULTS

The application was safe, even in extremely low birth weight infants (<1000 g) with scanty subcutaneous adipose tissue. The mean +/- standard deviation of the relative recovery in vitro was 101 +/- 3% for glucose and 100 +/- 2% for urea. Using urea as the internal standard, the mean relative recovery in vivo was 96.4 +/- 12.7% at the beginning and remained constant up to 16 days. The correlation between microdialysate and blood was significant for glucose (r = 0.88) and urea (r = 0.98). Subcutaneous microdialysis allowed the detection of asymptomatic hypoglycemias. The diagnostic sensitivity of a dialysate glucose </=2.9 mM to predict a blood glucose level </=2.8 mM was 92.3%, with 88.1% specificy. The positive predictive value with a 13.4% prevalence of a blood glucose </=2.8 mM was 54.5%, with a negative predictive value of 98.7% and an accuracy of 88.7%.

CONCLUSIONS

Subcutaneous microdialysis is a safe method, well suited for long-term glucose monitoring in neonates during intensive care. Subcutaneous microdialysis can be used to reduce blood loss and painful stress resulting from diagnostic blood sampling in high-risk neonates.

Clinical cerebral microdialysis: a methodological study

OBJECT

Clinical microdialysis enables monitoring of the cerebral extracellular chemistry of neurosurgical patients. Introduction of the technique into different hospitals' neurosurgical units has resulted in variations in the method of application. There are several variables to be considered, including length of the catheter membrane, type of perfusion fluid, flow rate of perfusion fluid, and on-line compared with delayed analysis of samples. The objects of this study were as follows: 1) to determine the effects of varying catheter characteristics on substance concentration; 2) to determine the relative recovery and true extracellular concentration by varying the flow rate and extrapolating to zero flow; and 3) to compare substance concentration obtained using a bedside enzyme analyzer with that of off-line high-performance liquid chromatography (HPLC).

METHODS

A specially designed bolt was used to conduct two adjacent microdialysis catheters into the frontal cortex of patients with head injury or poor-grade subarachnoid hemorrhage who were receiving ventilation. One reference catheter (10-mm membrane, perfused with Ringer's solution at 0.3 microl/minute) was constant for all studies. The other catheter was varied in terms of membrane length (10 mm or 30 mm), perfusion fluid (Ringer's solution or normal saline), and flow rate (0.1-1.5 microl/minute). The effect of freezing the samples on substance concentration was established by on-line analysis and then repeated analysis after storage at -70 degrees C for 3 months. Samples assayed with the bedside enzyme analyzer were reassessed using HPLC for the determination of glutamate concentrations.

CONCLUSIONS

Two adjacent microdialysis catheters that were identical in membrane length, perfusion fluid, and flow rate showed equivalent results. Variations in perfusion fluid and freezing and thawing of samples did not result in differences in substance concentration. Catheter length had a significant impact on substance recovery. Variations in flow rate enabled the relative recovery to be calculated using a modification of the extrapolation-to-zero-flow method. The recovery was approximately 70% at 0.3 microl/minute and 30% at 1 microl/minute (10-mm membrane) for all analytes. Glutamate results obtained with the enzyme analyzer showed good correlation with those from HPLC.

Microdialysis of skeletal muscle at rest

Techniques in human skeletal muscle research are by necessity predominantly 'descriptive'. Microdialysis has raised high expectations that it could meet the demand for a method that allows 'mechanistic' investigations to be performed in human skeletal muscle. In the present review, some views are given on how well the initial expectations on the use of the microdialysis technique in skeletal muscle have been fulfilled, and the areas in which additional work is needed in order to validate microdialysis as an important metabolic technique in this tissue. The microdialysis catheter has been equated to an artificial blood vessel, which is introduced into the tissue. By means of this 'vessel' the concentrations of compounds in the interstitial space can be monitored. The concentration of substances in the collected samples is dependent on the rate of perfusate flow. When perfusate flow is slow enough to allow complete equilibration between interstitial and perfusate fluids, the concentration in the perfusate is maximal and identical to the interstitial concentration. Microdialysis data may be influenced by changes in blood flow, especially in instances where the tissue diffusivity limits the recovery in vivo, i.e. when recovery in vitro is 100%, whereas the recovery in vivo is less than 100%. Microdialysis data indicate that a significant arterial-interstitial glucose concentration gradient exists in skeletal muscle but not in adipose tissue at rest. While the concentrations of glucose and lactate in the dialysate from skeletal muscle are close to the expected values, the glycerol values obtained for muscle are still puzzling. Ethanol added to the perfusate will be cleared by the tissue at a rate that is determined by the nutritive blood flow (the microdialysis ethanol technique). It is concluded that microdialysis of skeletal muscle has become an important technique for mechanistic studies in human metabolism and nutrition.

In vivo drug-response measurements in target tissues by microdialysis

To study the suitability of the microdialysis technique for the measurement of target tissue pharmacodynamics in humans, the model compounds theophylline, milrinone, and compound 48/80 were administered locally by means of reversed microdialysis to the interstitial space of skeletal muscle or skin in 24 healthy volunteers. Simultaneously, interstitial concentrations of cyclic adenosine monophosphate (cAMP; as an indicator of phosphodiesterase activity) were measured in skeletal muscle, and interstitial concentrations of histamine (as an indicator of mast cell release) were measured in skin. In muscle, reversed microdialysis with milrinone led to a dose-dependent increase in interstitial cAMP concentrations (n = 8), whereas no significant effect on cAMP was observed for theophylline versus placebo (1.63 +/- 0.53 nmol/L; n = 6), even at local concentrations exceeding those attained after therapeutic doses. In skin, reversed microdialysis with compound 48/80 increased interstitial histamine concentration dose dependently versus placebo (5.99 +/- 2.74 nmol/L; n = 10). From our experiments in human skeletal muscle and skin, we concluded that microdialysis was a suitable technique for the characterization of in vivo drug response at the relevant target site. Extension of these measurements to several other human tissues is readily feasible.

Quantitative measurement of extracellular histamine concentrations in intact human skin in vivo by the microdialysis technique: methodological aspects

Calculation of recovery is needed in microdialysis studied to calculate absolute concentrations of compounds in the extracellular water space. The purposes of this study were to determine the extracellular concentration of histamine in intact human skin in vivo and to study the validity of absolute histamine measurements during allergic skin reactions. A skin microdialysis technique and two calibration techniques, the no net flux method and the flow rate method, were used to quantify histamine concentrations in resting skin. To validate these techniques, skin glucose concentrations were analysed as well. In addition, the influence of vasodilation and plasma extravasation on recovery was followed after intradermal injection of codeine, a mast-cell secretagogue. As expected, both calibration methods estimated skin glucose concentrations to be identical with venous blood glucose concentrations. However, skin histamine levels could not be calculated by the no net method, because the data did not meet the theoretic assumptions of this method. In contrast, histamine data fitted theoretically with the flow rate method, and skin histamine concentrations of 18.8 +/- 2.8 nM were found to be significantly greater than plasma histamine concentrations of 4.3 +/- 0.7 nM. Within minutes after intradermal injection of codeine, recovery increased significantly in a dose-dependent fashion. Vasodilation per se did not influence recovery. In conclusion, absolute assessment of skin histamine concentrations can be made by microdialysis by the flow rate method. The validity of such an estimate and the theoretic prerequisites for the calculations are discussed. Quantitative measurement of skin histamine levels during allergic reactions cannot be performed since recovery is altered by plasma extravasation after skin challenge.

Effect of a non-steroidal anti-inflammatory drug on tissue levels of immunoreactive prostaglandin E2, immunoreactive leukotriene, and pain after periodontal surgery

The aim of this study was to measure tissue levels of immunoreactive prostaglandin E2 (iPGE2), immunoreactive leukotriene B4 (iLTB4), and pain after periodontal surgery and to evaluate the effect of the non-steroidal anti-inflammatory drug (NSAID), ibuprofen, on these levels. Two contralateral quadrants in each of nine patients were selected to undergo separate surgical procedures, one with ibuprofen (800 mg 1 hour presurgery and 400 mg postsurgery) and one with a placebo. Intra-operatively, a custom-made microdialysis probe, with a 3,000 dalton molecular weight cut-off, was inserted beneath the soft tissue flap and a dialysate collected every 20 minutes for 4 hours after surgery. Pain perception was measured at the same time intervals using two pain scales. Dialysate samples were assayed using two enzyme immunoassays. Mean tissue levels of iPGE2 in the placebo group increased from 74 nM at 40 minutes to a peak of 261 nM at 200 minutes. Mean tissue levels of iLTB4 in the placebo group fluctuated between 0.2 and 0.6 nM. Pain levels in this group increased continuously with time, peaking at 4 hours. Mean tissue levels of iPGE2 in the ibuprofen group were significantly suppressed, exhibiting more than a 95% reduction. This was accompanied by a significant reduction in pain. Ibuprofen had no detectable effect on tissue levels of iLTB4. These data indicate that iPGE2 and iLTB4 are present at relatively high concentrations in the periodontal tissues after surgery. Since these concentrations exceed the Kd values for binding to their respective receptors, PGE2 and LTB4 may be associated with the development of postsurgical pain and inflammation. These data also indicate that ibuprofen can successfully inhibit iPGE2 production in the periodontal tissues and in this way may help reduce postoperative pain and inflammation.

Lactate and excitatory amino acids measured by microdialysis are decreased by pentobarbital coma in head-injured patients

Primary traumatic brain injury and secondary ischemic/hypoxic injury are being increasingly characterized at the neurochemical level. Neurochemical monitoring using microdialysis has shown that these forms of tissue damage share many common features. In particular, anaerobic glycolysis with increased lactate production and release of excitatory amino acids into the extracellular space are seen in both conditions. Clinical microdialysis studies have heretofore focused on methodological issues, establishment of basal analyte values, and clinico-neurochemical correlation. Here we report the neurochemical consequences of therapeutic intervention in head injury. Specifically, induction of thiopental coma to manage severe increased intracranial pressure in seven patients was associated with a 37% reduction of lactate, 59% reduction of glutamate, and 66% reduction in aspartate in the extracellular space of the brain.

Intravenous microdialysis in the mouse and the rat: development and pharmacokinetic application of a new probe

PURPOSE

A flexible microdialysis probe was designed for intravenous sampling in small laboratory animals.

METHODS

Surgical techniques were developed to implant this probe via the femoral vein in the vena cava of the mouse and the rat. The in- and outlet of the probe were exteriorized above the tail of the animal and were directly connected to the microsyringe pump for perfusate delivery and to the injection valve for on-line HPLC analysis of the microdialysate samples.

RESULTS

The in vitro recoveries of flurbiprofen and naproxen for these probes were 68.2 +/- 6.9% (mean +/- S.D., n = 12) and 66.5 +/- 7.3%, respectively. The relatively loss by in vivo retrodialysis, measured the day after the implantation of the probes, was 66.1 +/- 8.8% for flurbiprofen and 60.9 +/- 9.9% for naproxen. The pharmacokinetics of unbound flurbiprofen were studied following i.v. bolus administration of flurbiprofen to the mouse (n = 4) and the rat (n = 6) with on-line HPLC analysis of microdialysates to unbound concentrations using the in vivo loss of flurbiprofen by retrodialysis carried out just before the start of the pharmacokinetic experiment. The integrity of the probe throughout the experiment was monitored by continuous retrodialysis of naproxen.

CONCLUSIONS

The developed techniques can be used to carry out routine pharmacokinetic studies in the mouse and the rat illustrated by our experiments with flurbiprofen, a compound with very high plasma protein binding.

Lactic acid and amino acid fluctuations measured using microdialysis reflect physiological derangements in head injury

We examined the extracellular neurochemical milieu in 34 head injured patients using microdialysis while simultaneously monitoring intracranial pressure, cerebral perfusion pressure, and jugular venous oxygen saturation. Derangements of anaerobic metabolism reflected by increased lactate and lactate/pyruvate ratios, and release of amino acids were seen at the same time as physiological deterioration in the majority of instances. Clinical microdialysis may provide insights into the neurochemistry of head injury, and such information may lead to new methods of monitoring and treating head injured patients.

Glucose modulates rat substantia nigra GABA release in vivo via ATP-sensitive potassium channels

Glucose modulates beta cell insulin secretion via effects on ATP-sensitive potassium (KATP) channels. To test the hypothesis that glucose exerts a similar effect on neuronal function, local glucose availability was varied in awake rats using microdialysis in the substantia nigra, the brain region with the highest density of KATP channels. 10 mM glucose perfusion increased GABA release by 111 +/- 42%, whereas the sulfonylurea, glipizide, increased GABA release by 84 +/- 20%. In contrast, perfusion of the KATP channel activator, lemakalim, or depletion of ATP by perfusion of 2-deoxyglucose with oligomycin inhibited GABA release by 44 +/- 8 and 45 +/- 11%, respectively. Moreover, the inhibition of GABA release by 2-deoxyglucose and oligomycin was blocked by glipizide. During systemic insulin-induced hypoglycemia (1.8 +/- 0.3 mM), nigral dialysate GABA concentrations decreased by 49 +/- 4% whereas levels of dopamine in striatal dialysates increased by 119 +/- 18%. We conclude that both local and systemic glucose availability influences nigral GABA release via an effect on KATP channels and that inhibition of GABA release may in part mediate the hyperexcitability associated with hypoglycemia. These data support the hypothesis that glucose acts as a signaling molecule, and not simply as an energy-yielding fuel, for neurons.

Microdialysis in cutaneous pharmacology: kinetic analysis of transdermally delivered nicotine

Direct measurements of cutaneous drug levels and kinetics have long been hampered by lack of appropriate methods. Recently, studies have indicated that microdialysis, a method of continuous in vivo sampling of extracellular fluid, may also be performed in human skin. The present study was designed to evaluate this technique for kinetic analyses of cutaneous drug levels. Using a transdermal nicotine delivery system with 35 mg of nicotine as a model, nicotine levels were determined in the dialysate of human skin by means of high performance liquid chromatography. In vitro studies demonstrated that nicotine levels in the dialysate strictly correlated with nicotine concentrations in the dialyzed medium. In nine healthy male volunteers receiving nicotine by transdermal delivery, nicotine was detectable within 90-180 min, and peak levels of approximately 1000 ng/ml were detected within 240-360 min of patch application. Correlation analyses of the individual data from our subjects revealed that nicotine kinetics were independent of skin barrier function, as assessed by transepidermal water loss, but indicated that the detectable maximum nicotine levels may depend on the location of the probe. In summary, the present study demonstrates that microdialysis may be a novel, powerful tool to study cutaneous pharmacology in vivo.

Validation of the 'internal reference technique' for calibrating microdialysis catheters in situ

In vivo calibration of microdialysis catheters with [3H]glucose as internal reference was done in rat (n = 17) and human (n = 12) subcutaneous tissue. The estimated interstitial glucose level was compared with the glucose concentration in venous plasma which, in turn, has been shown to be identical to the interstitial glucose concentration. In subcutaneous tissue of anaesthetized male Sprague-Dawley rats, interstitial glucose was significantly overestimated (43%, P < 0.005, n = 8, and 19%, P < 0.005, n = 9, in normoglycaemic and hyperglycaemic animals, respectively). Furthermore, fractional outflux of [3H]glucose decreased continuously during prolonged perfusion of the microdialysis catheter. In contrast, in human subcutaneous tissue microdialysed with two catheters, correct measurements of interstitial glucose could be achieved and the precision was comparable to that obtained with equilibration calibration in vivo. The average relative error of the mean result of two catheters was 8.9% at a perfusate flow rate of 1 microL min-1. It may be suggested that calibration in vivo of microdialysis catheters with internal references may be used in human subcutaneous tissue. However, it is necessary to validate the calibration technique in each different tissue under reproducible experimental conditions since accumulation of the reference substance in the tissue may create artefactual results.