Intracerebral microdialysis of extracellular amino acids in the human epileptic focus

Elevated extracellular levels of glutamate, aspartate and gamma-aminobutyric acid within the intraoperative, spontaneously epileptiform human hippocampus

We report preliminary results from four patients subjected to hippocampal electrocorticography and microdialysis during temporal lobe epilepsy surgery. In two cases, spontaneously vigorous hippocampal epileptiform activity (EA) was identified; basal dialysate levels for hippocampal glutamate, aspartate, and gamma-aminobutyric acid ranged from approximately 23- to 84-fold, 19- to 33-fold and 10- to 34-fold higher, respectively, compared to the two cases of minimal hippocampal EA. These findings represent the first intraoperative evidence of elevated extracellular levels of neuroactive amino acids within the spontaneously epileptiform human hippocampus.

Changes in extracellular levels of amygdala amino acids in genetically fast and slow kindling rat strains

A neurochemical basis for many of the epilepsies has long been suspected to result from an imbalance between excitatory and inhibitory neurotransmitter mechanisms. Data supporting changes in extrasynaptic amino acid levels during epileptogenesis, however, remain controversial. In the present study, we used in vivo microdialysis to measure the levels of extracellular GABA (gamma-aminobutyric acid) and glutamate during seizure development in rats with a genetic predisposition for (Fast), or against (Slow), amygdala kindling. Dialysates were collected from both amygdalae before, during, and up to 12 min after a threshold-triggered amygdala afterdischarge (AD). One hour later, samples were again collected from both amygdalae in response to a hippocampal threshold AD. Daily amygdala kindling commenced the next day but without dialysis. After the rats were fully kindled, the same protocol was again employed. Amino acid levels were not consistently increased above baseline with triggered seizures in either strain. Instead, before kindling, a focal seizure in the Slow rats was associated with a large decrease in GABA in the non-stimulated amygdala, while amino acid levels in the Fast rats remained near baseline in both amygdalae. Similar results were seen after kindling. By contrast, before and after kindling, hippocampal stimulation caused large decreases in all amino acid levels in both amygdalae in both strains. These data suggest that, in response to direct stimulation, extracellular amino acid concentrations remain stable in tissues associated with either greater natural (Fast) or induced (kindled Fast/Slow) excitability, but are lowered with indirect stimulation (hippocampus) and/or low excitability.

The neurophysiology of concussion

Cerebral concussion is both the most common and most puzzling type of traumatic brain injury (TBI). It is normally produced by acceleration (or deceleration) of the head and is characterized by a sudden brief impairment of consciousness, paralysis of reflex activity and loss of memory. It has long been acknowledged that one of the most worthwhile techniques for studying the acute pathophysiology of concussion is by the recording of neurophysiological activity such as the electroencephalogram (EEG) and sensory evoked potentials (EPs) from experimental animals. In the first parts of this review, the majority of such studies conducted during the past half century are critically reviewed. When potential methodological flaws and limitations such as anesthetic protocols, infliction of multiple blows and delay in onset of recordings were taken into account, two general principles could be adduced. First, the immediate post-concussive EEG was excitatory or epileptiform in nature. Second, the cortical EP waveform was totally lost during this period. In the second parts of this review, five theories of concussion which have been prominent during the past century are summarized and supportive evidence assessed. These are the vascular, reticular, centripetal, pontine cholinergic and convulsive hypotheses. It is concluded that only the convulsive theory is readily compatible with the neurophysiological data and can provide a totally viable explanation for concussion. The chief tenet of the convulsive theory is that since the symptoms of concussion bear a strong resemblance to those of a generalized epileptic seizure, then it is a reasonable assumption that similar pathobiological processes underlie them both. Further, it is demonstrated that EPs and EEGs recorded acutely following concussive trauma are indeed the same or similar to those obtained following the induction of a state of generalized seizure activity (GSA). According to the present incarnation of the convulsive theory, the energy imparted to the brain by the sudden mechanical loading of the head may generate turbulent rotatory and other movements of the cerebral hemispheres and so increase the chances of a tissue-deforming collision or impact between the cortex and the boney walls of the skull. In this conception, loss of consciousness is not orchestrated by disruption or interference with the function of the brainstem reticular activating system. Rather, it is due to functional deafferentation of the cortex as a consequence of diffuse mechanically-induced depolarization and synchronized discharge of cortical neurons. A convulsive theory can also explain traumatic amnesia, autonomic disturbances and the miscellaneous collection of symptoms of the post-concussion syndrome more adequately than any of its rivals. In addition, the symptoms of minor concussion (a.k.a. being stunned, dinged, or dazed) are often strikingly similar to minor epilepsy such as petit mal. The relevance of the convulsive theory to a number of associated problems is also discussed. These include the relationship between concussion and more serious types of closed head injury, the utility of animal models of severe brain trauma, the etiology of the cognitive deficits which may linger long after a concussive injury, the use of concussive (captive bolt) techniques to stun farm animals prior to slaughter and the question of why some animals (such as the woodpecker) can tolerate massive accelerative forces without being knocked out.

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.

Electrophysiological characteristics of reactive astrocytes in experimental cortical dysplasia

Neocortical freeze lesions have been widely used to study neuronal mechanisms underlying hyperexcitability in dysplastic cortex. Comparatively little attention has been given to biophysical changes in the surrounding astrocytes that show profound morphological and biochemical alterations, often referred to as reactive gliosis. Astrocytes are thought to aid normal neuronal function by buffering extracellular K(+). Compromised astrocytic K(+) buffering has been proposed to contribute to neuronal dysfunction. Astrocytic K(+) buffering is mediated, partially, by the activity of inwardly rectifying K(+) channels (K(IR)) and may involve intracellular redistribution of K(+) through gap-junctions. We characterized K(+) channel expression and gap-junction coupling between astrocytes in freeze-lesion-induced dysplastic neocortex. Whole cell patch-clamp recordings were obtained from astrocytes in slices from postnatal day (P) 16--P24 rats that had received a freeze-lesion on P1. A marked increase in glial fibrillary acidic protein immunoreactivity was observed along the entire length of the freeze lesion. Clusters of proliferative (bromo-deoxyuridine nuclear staining, BrdU+) astrocytes were seen near the depth of the microsulcus. Astrocytes in cortical layer I surrounding the lesion were characterized by a significant reduction in K(IR). BrdU-positive astrocytes near the depth of the microsulcus showed essentially no expression of K(IR) channels but markedly enhanced expression of delayed rectifier K(+) (K(DR)) channels. These proliferative cells showed virtually no dye coupling, whereas astrocytes in the hyperexcitable zone adjacent to the microsulcus displayed prominent dye-coupling as well as large K(IR) and outward K(+) currents. These findings suggest that reactive gliosis is accompanied by a loss of K(IR) currents and reduced gap junction coupling, which in turn suggests a compromised K(+) buffering capacity.

A new model for diffuse brain injury by rotational acceleration: II. Effects on extracellular glutamate, intracranial pressure, and neuronal apoptosis

The aim of this study is to monitor excitatory amino acids (EAAs) in the extracellular fluids of the brain and to characterize regional neuronal damage in a new experimental model for brain injury, in which rabbits were exposed to 180-260 krad/s2 rotational head acceleration. This loading causes extensive subarachnoid hemorrhage, focal tissue bleeding, reactive astrocytosis, and axonal damage. Animals were monitored for intracranial pressure (ICP) and for amino acids in the extracellular fluids. Immunohistochemistry was used to study expression of the gene c-Jun and apoptosis with the terminal deoxynucleotidyl transferase nick-end labeling (TUNEL) technique. Extracellular glutamate, glycine, and taurine increased significantly in the hippocampus within a few hours and remained high after 24 h. Neuronal nuclei in the granule layers of the hippocampus and cerebellum were positive for c-Jun after 24 h. Little immunoreactivity was detected in the cerebral cortex. c-Jun-positive neuronal perikarya and processes were found in granule and pyramidal CA4 layers of the hippocampus and among the Purkinje cells of the cerebellum. Also some microglial cells stained positively for c-Jun. TUNEL reactivity was most intense at 10 days after trauma and was extensive in neurons of the cerebral cortex, hippocampus, and cerebellum. The initial response of the brain after rotational head injury involves brain edema after 24 h and an excitotoxic neuronal microenvironment in the first hour, which leads to extensive delayed neuronal cell death by apoptosis necrosis in the cerebral cortex, hippocampus and cerebellum.

Intracerebral microdialysis in neurointensive care: the use of urea as an endogenous reference compound

OBJECT

When evaluating the results of intracerebral microdialysis, the in vivo performance of the microdialysis probe must be considered, because this determines the fraction of the interstitial concentration obtained in the microdialysis samples. The in vivo performance is dependent on several factors, for example, the interstitial compartment's diffusion characteristics, which may vary during the course of the acute brain injury process. In the present study the authors investigated the method of controlling the in vivo performance by using urea, which is evenly distributed in all body fluid compartments, as an endogenous reference compound and by comparing the urea levels in three compartments: the brain (CNS), abdominal subcutaneous tissue (SC), and blood serum (BS).

METHODS

Sixty-nine patients with traumatic brain injury or cerebrovascular disease were included in the study. In 63 of these patients a CNS probe was used, an SC probe was used in 40, and both were used in 34. Urea was measured by enzymatic methods, at bedside for the microdialysis samples and in routine clinical laboratory studies for the BS samples, with the probe calibrated to give identical results. The correlation coefficient for CNS/SC urea was 0.88 (2414 samples), for CNS/BS urea it was 0.89 (180 samples), and for SC/BS urea it was 0.98 (112 samples).

CONCLUSIONS

Urea levels in the CNS, SC, and BS were highly correlated, which supports the assumption that urea is evenly distributed. The CNS/SC urea ratio can therefore be used for monitoring the CNS probe's in vivo performance. Fluctuations in other substances measured with microdialysis are probably caused by biological changes in the brain, as long as the CNS/SC urea ratio remains constant.

Extracellular amino acid levels measured with intracerebral microdialysis in the model of posttraumatic epilepsy induced by intracortical iron injection

An iron induced model of posttraumatic chronic focal epilepsy in rats was studied with respect to extracellular amino acids, electrophysiology, and morphology, approx. 6 months after intracortical injection of ferrous chloride. Twenty-six of the twenty-eight (93%) rats developed spontaneous epileptiform EEG-activity and electrical cortical stimulation done in eight animals evoked seizure activity in five animals (62.5%). Epileptic brain tissue displayed significantly higher extracellular interictal levels of aspartate (ASP), compared to normal brain, measured with intracerebral microdialysis. The interictal levels of serine (SER) were significantly higher at the lesion side compared to the contralateral cortex in epileptic animals. Spontaneous elevations of ASP and glutamate (GLU) levels up to 8 times the basal level were found in 4/5 (80%). There was no consistent amino acid pattern following the electrically induced seizures, but in association with more intense seizure activity ASP and GLU were elevated. Histopathologically, the necrotic lesions in the cortex contained small vessels and iron pigment loaded astrocytes. Scattered eosinophilic neurons were found in the hippocampus, bilaterally in 37% of the animals. The results show that a focal epileptiform activity developed in a high percentage of animals that received an intracortical iron injection. The observed amino acid changes in epileptic animals may be involved in the development of seizures in this model of posttraumatic epilepsy.

Microdialysis: intraoperative and posttraumatic applications in neurosurgery

Microdialysis offers a unique opportunity to study the extracellular human brain environment. Our aim was to improve the ability to detect neuronal injury in patients undergoing complicated neurovascular procedures and to detect secondary brain lesions after severe head injury with the use of in vivo microdialytic monitoring. We employed intraoperative microdialysis monitoring in patients with a variety of neurovascular procedures including high-flow and low-flow bypass surgery and aneurysm clipping. In the second group there were patients after severe brain injury where in vivo microdialysis was performed as a bedside monitoring in the intensive care unit. In this review we present our results for these various groups of patients, in which a variety of neurochemical parameters which are known to change in ischemia were studied. The advantages and disadvantages of the technique are discussed. Our results show that microdialysis is a useful tool for detecting ischemic changes intraoperatively and in severe head trauma. However, further studies are needed to get more data and to better refine the online microdialysis technique before we can recommend it for routine clinical use.

Multiple interstitial substances measured by microdialysis in patients with subarachnoid hemorrhage

OBJECTIVE

Intracerebral microdialysis is a tool to monitor metabolic disturbances in the brains of patients with severe head injuries or subarachnoid hemorrhage (SAH). In the search for putative indicators of primary and secondary brain damage, we measured multiple metabolites in the dialysates of patients with SAH, to elucidate their significance for the outcomes of the patients as well as their temporal profiles of liberation after the insult.

METHODS

Microdialysis probes were placed, with a ventriculostomy catheter for drainage of cerebrospinal fluid, into a frontal lobe of 10 patients with aneurysmal SAH, for 4.6 +/- 0.5 days. Amino acids, metabolites of glycolysis, purines, catecholamines, and nitric oxide oxidation byproducts were measured by high-performance liquid chromatography. Spearman's correlation coefficient and Student's t test were used to compare the levels of the metabolites with the outcomes of the patients, as assessed using the Glasgow Outcome Scale, 3 months after the ictus.

RESULTS

For patients with unfavorable outcomes (Glasgow Outcome Scale scores of 1-3), which were primarily associated with the development of large infarctions, dialysate levels of excitatory amino acids increased up to 30-fold, those of lactate up to 10-fold, and those of nitrite up to 5-fold, compared with normal levels observed for patients with favorable outcomes (Glasgow Outcome Scale scores of 4 or 5). When average peak concentrations in the dialysates of patients with favorable and unfavorable outcomes were compared, significantly higher levels of excitatory amino acids, taurine, lactate, and nitrite, but not of purines and catecholamines, were observed for those with poor outcomes (P < 0.05). With respect to the temporal profiles of the average metabolite concentrations, the significantly increased levels of amino acids observed for patients with poor outcomes followed a biphasic course, with maximal concentrations on the first and second days or the seventh day after the insult (P < 0.01).

CONCLUSION

These data confirm the usefulness of excitatory amino acids and lactate as major parameters for neurochemical monitoring for patients threatened by acute cerebral disorders. Other substances, such as taurine and nitrite, were also demonstrated to be potentially predictive. Release of these substances into the extracellular fluid of the brain might be particularly relevant for the development of secondary brain damage after SAH, e.g., infarction or brain swelling.

Decreased cortical levels of astrocytic glutamate transport protein GLT-1 in a rat model of posttraumatic epilepsy

The extracellular homeostasis of glutamate in the brain is maintained by the efficient uptake into astroglial cells. The high extracellular glutamate levels seen during seizures are therefore probably a result of both an increased synaptic release and a deranged glutamate uptake. In this study we used immuno-blotting technique to measure the cortical levels of the astrocytic glutamate transport protein (GLT-1) and of the glutamate and aspartate transporting protein (GLAST) in an epilepsy model induced by ferrous chloride injection in the cortex of rats. The levels of GLT-1 were lower in epileptic rats than in controls, day 1 and 5 after induction, but not at 3 months. Glial fibrillary protein (GFAP) levels increased with time in the epileptic model, whereas GLAST and beta-tubulin III remained unchanged compared to controls. The results suggest that the transient decrease of GLT-1 could play a role in epileptogenesis, while recurrent seizure activity may be maintained by other mechanisms.

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.

Glutamate and gamma-aminobutyric acid content and release of synaptosomes from temporal lobe epilepsy patients

During surgical intervention in medically refractory temporal lobe epilepsy (TLE) patients, diagnosed with either mesial temporal lobe sclerosis (MTS)- or tumor (T)-associated TLE, biopsies were taken from the anterior temporal neocortex and the hippocampal region. Synaptosomes, isolated from these biopsies were used to study intrasynaptosomal Ca(2+) levels ([Ca(2+)](i)), and glutamate and gamma-aminobutyric acid (GABA) contents and release. All synaptosomal preparations demonstrated a basal [Ca(2+)](i) of about 200 nM, except neocortical synaptosomes from MTS-associated TLE patients (420 nM). K(+)-induced depolarization resulted in a robust increase of the basal [Ca(2+)](i) in all preparations. Neocortical synaptosomes from TLE patients contained 22.9 +/- 3.0 nmol glutamate and 4.6 +/- 0.5 nmol GABA per milligram synaptosomal protein, whereas rat cortical synaptosomes contained twice as much glutamate and four times as much GABA. Hippocampal synaptosomes from MTS-associated TLE patients, unlike those from T-associated TLE patients, contained about 70% less glutamate and 55% less GABA than neocortical synaptosomes. Expressed as percentage of total synaptosomal content, synaptosomes from MTS-associated TLE patients exhibited an increased basal and a reduced K(+)-induced glutamate and GABA release compared to rat cortical synaptosomes. In MTS-associated TLE patients, only GABA release from neocortical synaptosomes was partially Ca(2+)-dependent. Control experiments in rat synaptosomes demonstrated that at least part of the reduction in K(+)-induced release can be ascribed to resection-induced hypoxia in biopsies. Thus, synaptosomes from MTS-associated TLE patients exhibit a significant K(+)-induced increase in [Ca(2+)](i), but the consequent release of glutamate and GABA is severely impaired. Our data show that at least part of the differences in glutamate and GABA content and release between human biopsy material and fresh rat tissue is due to the resection time.

Evidence for coupling between glucose metabolism and glutamate cycling using FDG PET and 1H magnetic resonance spectroscopy in patients with epilepsy

The purpose of this study was to examine the relation between glucose metabolism and glutamate concentration in the human brain, in both the normal and diseased state. Regional values of glucose metabolism measured with 2-deoxy-2[F-18]fluoro-D-glucose positron emission tomography (FDG PET) studies and single-voxel proton magnetic resonance spectroscopy (1H MRS) measurements of the glutamate/ glutamine/gamma-aminobutyric acid (Glx) tissue concentration were determined in multiple brain regions in 11 patients (5 girls and 6 boys, mean age 7.5 years) with medically intractable partial epilepsy. FDG PET and 1H MRS studies were performed in the interictal state in seven patients and in the ictal/periictal state in four patients. Regions of interest were identified in epileptic cortex (determined by intracranial and/or scalp electroencephalography) and in contralateral normal brain regions. Lower glucose metabolism and lower Glx concentrations were found in the epileptic focus than in the contralateral normal cortex in all seven patients examined in the interictal state, whereas higher glucose metabolism and higher Glx concentrations were observed in the epileptic focus in the four patients who had ictal/periictal studies. Significant correlations were found between the values of cerebral glucose utilization and Glx concentration in epileptic brain region, in nonepileptic brain regions, and in epileptic and nonepileptic regions combined. These results demonstrate a significant relation between glucose metabolism and glutamate/glutamine concentration in normal and epileptic cerebral cortex. This relation is maintained in both the interictal and ictal states.

Glutamate and epilepsy

Epileptic syndromes have very diverse primary causes, which may be genetic, developmental or acquired. In rodent models, altering glutamate receptor or glutamate transporter expression by knockout or knockdown procedures can induce or suppress epileptic seizures. Regardless of the primary cause, synaptically released glutamate acting on ionotropic and metabotropic receptors appears to play a major role in the initiation and spread of seizure activity. In rodent models of acquired epilepsy and in human temporal lobe epilepsy, there is evidence for enhanced functional efficacy of ionotropic N-methyl-D-aspartate (NMDA) and metabotropic (Group I) receptors. In animal models of epilepsy, antagonists acting at NMDA receptors or at Group I metabotropic receptors have potent anticonvulsant actions.

IV glycine and oral D-cycloserine effects on plasma and CSF amino acids in healthy humans

BACKGROUND

The amino acid glycine, modulates neurotransmission via actions at GLY-A receptor and GLY-B receptor. The latter are coagonist sites associated with N-Methyl-D-Aspartate (NMDA) glutamate receptors. The central bioavailability of peripherally administered glycine has not been adequately characterized in humans.

METHODS

Healthy human subjects were administered either oral D-cycloserine (50 mg or placebo) and intravenous glycine (saline, 100 mg/kg or 200 mg/kg) in random order over 4 test days under double-blind conditions. Cerebrospinal fluid was collected by lumbar puncture performed on the first test day was analyzed to determine amino acid levels. The acoustic startle response was measured on the second test day.

RESULTS

Intravenous glycine dose-dependently increased both serum and CSF glycine and serine levels. Neither glycine nor DCS produced any significant effects on behavior, cognition or the acoustic startle response. Neither IV glycine nor DCS were associated with any toxicity.

CONCLUSIONS

Thus, peripheral glycine administration raised CSF glycine levels without producing any clear central nervous system effects. Glycine and D-cycloserine did not worsen cognitive test performance and did not induce behavioral symptoms on their own. The possibility that glycine and D-cycloserine enhanced cognitive test performance cannot be excluded given the psychometric limitations of the test battery.

Theory and practice of microdialysis--prospect for future clinical use

The application of microdialysis for neurochemical monitoring in neurosurgery and neurointensive care is rapidly expanding in a number of clinical centers around the world. In order for microdialysis to become a future routine method in these clinical settings a number of problems, outlined in this communication, must be solved by the clinical researchers and the commercial companies. Regardless of the future success as a routine method, it is already obvious that microdialysis will be an important clinical research tool for years to come, providing new important insights into the pathophysiology of acute human brain injury.

Bedside detection of brain ischemia using intracerebral microdialysis: subarachnoid hemorrhage and delayed ischemic deterioration

OBJECTIVE

Intracerebral microdialysis has been demonstrated to be a useful method for detection of brain ischemia in experimental models and in patients. We have applied new mobile microdialysate analysis equipment that allows a bedside comparison of changes in neurochemistry with the neurological status of the patient. Ten patients with severe aneurysmal subarachnoid hemorrhage (that is, with a high risk of vasospasm and a high risk of subsequent ischemic deficits) were selected.

METHODS

Microdialysis catheters were inserted into the temporal and subfrontal cortex at the end of aneurysm surgery. Samples, collected hourly for 4 to 11 days, were analyzed immediately at the bedside for glucose, lactate, and glycerol and later for pyruvate and glutamate. The patients' neurological status was monitored constantly, and daily recordings of blood flow velocities were performed using transcranial Doppler sonography.

RESULTS

Concentrations of the measured substances varied widely. Individual analyses revealed that patients with uneventful clinical courses generally demonstrated low and stable levels of the different metabolites, and those with signs of cerebral ischemia demonstrated various patterns of neurochemical changes. Lactate and glutamate seemed to be sensitive markers of impending ischemia, and increased glycerol levels were associated with severe ischemic deficits. Obtaining the microdialysis data directly at the bedside seemed to be of great advantage when relating the values to other clinical findings.

CONCLUSION

Bedside intracerebral microdialysis monitoring of patients with subarachnoid hemorrhage and signs of delayed ischemia revealed dramatic changes in extracellular concentrations of glucose, lactate, and glycerol that could be directly correlated to the clinical status of the patients. These findings emphasize the potential of microdialysis in neurosurgical intensive care patients.

Neuroactive amino acids in focally epileptic human brain: a review

Studies of neuroactive amino acids and their regulatory enzymes in surgically excised focally epileptic human brain are reviewed. Concentrations of glutamate, aspartate and glycine are significantly increased in epileptogenic cerebral cortex. The activities of the enzymes, glutamate dehydrogenase and aspartate aminotransferase, involved in glutamate and aspartate metabolism are also increased. Polyamine synthesis is enhanced in epileptogenic cortex and may contribute to the activation of N-methyl-D-aspartate (NMDA) receptors. Nuclear magnetic resonance spectroscopy (NMRS) reveals that patients with poorly controlled complex partial seizures have a significant diminution in occipital lobe gamma aminobutyric acid (GABA) concentration. The activity of the enzyme GABA-aminotransaminase (GABA-T) which catalyzes GABA degradation is not altered in epileptogenic cortex. NMRS studies show that vigabatrin, a GABA-T inhibitor and effective antiepileptic, significantly increases brain GABA. Glutamate decarboxylase (GAD), responsible for GABA synthesis, is diminished in interneurons in discrete regions of epileptogenic cortex and hippocampus. In vivo microdialysis performed in epilepsy surgery patients provides measurements of extracellular amino acid levels during spontaneous seizures. Glutamate concentrations are higher in epileptic hippocampi and increase before seizure onset reaching potentially excitotoxic levels. Frontal or temporal cortical epileptogenic foci also release aspartate, glutamate and serine particularly during intense seizures or status epilepticus. GABA in contrast, exhibits a delayed and feeble rise in the epileptic hippocampus possibly due to a reduction in the number and/or efficiency of GABA transporters.

Cerebral microdialysis combined with single-neuron and electroencephalographic recording in neurosurgical patients. Technical note

Monitoring physiological changes in the brain parenchyma has important applications in the care of neurosurgical patients. A technique is described for measuring extracellular neurochemicals by cerebral microdialysis with simultaneous recording of electroencephalographic (EEG) and single-unit (neuron) activity in selected targets in the human brain. Forty-two patients with medically intractable epilepsy underwent stereotactically guided implantation of a total of 423 intracranial depth electrodes to delineate potentially resectable seizure foci. The electrodes had platinum alloy contacts for EEG recordings and four to nine 40-microm microwires for recording single-unit neuron activity. Eighty-six electrodes also included microdialysis probes introduced via the electrode lumens. During monitoring on the neurosurgical ward, electrophysiological recording and cerebral microdialysis sampling were performed during seizures, cognitive tasks, and sleep-waking cycles. The technique described here could be used in developing novel approaches for evaluation and treatment in a variety of neurological conditions such as head injury, subarachnoid hemorrhage, epilepsy, and movement disorders.

Glutamate receptors and transporters in genetic and acquired models of epilepsy

Glutamate, the principal excitatory neurotransmitter in the brain, acts on three families of ionotropic receptor--AMPA (alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid), kainate and NMDA (N-methyl-D-aspartate) receptors and three families of metabotropic receptor (Group I: mGlu1 and mGlu5; Group II: mGlu2 and mGlu3; Group III: mGlu4, mGlu6, mGlu7 and mGlu8). Glutamate is removed from the synaptic cleft and the extracellular space by Na+-dependent transporters (GLAST/EAAT1, GLT/EAAT2, EAAC/EAAT3, EAAT4, EAAT5). In rodents, genetic manipulations relating to the expression or function of glutamate receptor proteins can induce epilepsy syndromes or raise seizure threshold. Decreased expression of glutamate transporters (EAAC knockdown, GLT knockout) can lead to seizures. In acquired epilepsy syndromes, a wide variety of changes in receptors and transporters have been described. Electrically-induced kindling in the rat is associated with functional potentiation of NMDA receptor-mediated responses at various limbic sites. Group I metabotropic responses are enhanced in the amygdala. To date, no genetic epilepsy in man has been identified in which the primary genetic defect involves glutamate receptors or transporters. Changes are found in some acquired syndromes, including enhanced NMDA receptor responses in dentate granule cells in patients with hippocampal sclerosis.

Characterization of neocortical and hippocampal synaptosomes from temporal lobe epilepsy patients

To investigate epilepsy-associated changes in the presynaptic terminal, we isolated and characterized synaptosomes from biopsies resected during surgical treatment of drug-resistant temporal lobe epilepsy (TLE) patients. Our main findings are: (1) The yield of synaptosomal protein from biopsies of epilepsy patients was about 25% of that from rat brain. Synaptosomal preparations were essentially free of glial contaminations. (2) Synaptosomes from TLE patients and naive rat brain, quickly responded to K(+)-depolarization with a 70% increase in intrasynaptosomal Ca(2+) ([Ca(2+)](i)), and a 40% increase in B-50/GAP-43 phosphorylation. (3) Neocortical and hippocampal synaptosomes from TLE patients contained 20-50% of the glutamate and gamma-aminobutyric acid (GABA) contents of rat cortical synaptosomes. (4) Although the absolute amount of glutamate and GABA released under basal conditions from neocortical synaptosomes of TLE patients was lower than from rat synaptosomes, basal release expressed as percentage of total content was higher (16.4% and 17.3%, respectively) than in rat (11.5% and 9. 9%, respectively). (5) Depolarization-induced glutamate and GABA release from neocortical synaptosomes from TLE patients was smaller than from rat synaptosomes (3.9% and 13.0% vs. 21.9% and 25.0%, respectively). (6) Analysis of breakdown of glial fibrillary acid protein (GFAP) indicates that resection time (anoxic period during the operation) is a critical parameter for the quality of the synaptosomes. We conclude that highly pure and viable synaptosomes can be isolated even from highly sclerotic human epileptic tissue. Our data show that in studies on human synaptosomes it is of critical importance to distinguish methodological (i.e., resection time) from pathology-related abnormalities.

Glutamate receptors in epilepsy

Glutamatergic synapses play a critical role in all epileptic phenomena. Broadly enhanced activation of post-synaptic glutamate receptors (ionotropic and metabotropic) is proconvulsant. Antagonists of NMDA receptors and AMPA receptors are powerful anticonvulsants in many animal models of epilepsy. A clinical application of pure specific glutamate antagonists has not yet been established. Many different alterations in glutamate receptors or transporters can potentially contribute to epileptogenesis. Several genetic alterations have been shown to be epileptogenic in animal models but no specific mutation relating to glutamatergic function has yet been linked to a human epilepsy syndrome. There is clear evidence for altered NMDA receptor function in acquired epilepsy in animal models and in man. Changes in metabotropic receptor function may also play a key role in epileptogenesis.

Increase in extracellular glutamate caused by reduced cerebral perfusion pressure and seizures after human traumatic brain injury: a microdialysis study

OBJECT

To determine the extent and duration of change in extracellular glutamate levels after human traumatic brain injury (TBI), 17 severely brain injured adults underwent implantation of a cerebral microdialysis probe and systematic sampling was conducted for 1 to 9 days postinjury.

METHODS

A total of 772 hourly microdialysis samples were obtained in 17 patients (median Glasgow Coma Scale score 5+/-2.5, mean age 39.4+/-20.4 years). The mean (+/-standard deviation) glutamate levels in the dialysate were evaluated for 9 days, during which the mean peak concentration reached 25.4+/-13.7 microM on postinjury Day 3. In each patient transient elevations in glutamate were seen each day. However, these elevations were most commonly seen on Day 3. In all patients there was a mean of 4.5+/-2.5 transient elevations in glutamate lasting a mean duration of 4.4+/-4.9 hours. These increases were seen in conjunction with seizure activity. However, in many seizure-free patients the increase in extracellular glutamate occurred when cerebral perfusion pressure was less than 70 mm Hg (p < 0.001). Given the potential injury-induced uncoupling of cerebral blood flow and metabolism after TBI, these increases in extracellular glutamate may reflect a degree of enhanced cellular crisis, which in severe head injury in humans appears to last up to 9 days.

CONCLUSIONS

Extracellular neurochemical measurements of excitatory amino acids may provide a marker for secondary insults that can compound human TBI.

Factors affecting excitatory amino acid release following severe human head injury

OBJECT

Recent animal studies demonstrate that excitatory amino acids (EAAs) play a major role in neuronal damage after brain trauma and ischemia. However, the role of EAAs in patients who have suffered severe head injury is not understood. Excess quantities of glutamate in the extracellular space may lead to uncontrolled shifts of sodium, potassium, and calcium, disrupting ionic homeostasis, which may lead to severe cell swelling and cell death. The authors evaluated the role of EEAs in human traumatic brain injury.

METHODS

In 80 consecutive severely head injured patients, a microdialysis probe was placed into the gray matter along with a ventriculostomy catheter or an intracranial pressure (ICP) monitor for 4 days. Levels of EAAs and structural amino acids were analyzed using high-performance liquid chromatography. Multifactorial analysis of the amino acid pattern was performed and its correlations with clinical parameters and outcome were tested. The levels of EAAs were increased up to 50 times normal in 30% of the patients and were significantly correlated to levels of structural amino acids both in each patient and across the whole group (p < 0.01). Secondary ischemic brain injury and focal contusions were most strongly associated with high EAA levels (27+/-22 micromol/L). Sustained high ICP and poor outcome were significantly correlated to high levels of EAAs (glutamate > 20 micromol/L; p < 0.01).

CONCLUSIONS

The release of EAAs is closely linked to the release of structural amino acids and may thus reflect nonspecific development of membrane micropores, rather than presynaptic neuronal vesicular exocytosis. The magnitude of EAA release in patients with focal contusions and ischemic events may be sufficient to exacerbate neuronal damage, and these patients may be the best candidates for treatment with glutamate antagonists in the future.

Hippocampal AMPA and NMDA mRNA levels and subunit immunoreactivity in human temporal lobe epilepsy patients and a rodent model of chronic mesial limbic epilepsy

This study compared temporal lobe epilepsy patients, along with kindled animals and self sustained limbic status epilepticus (SSLSE) rats for parallels in hippocampal AMPA and NMDA receptor subunit expression. Hippocampal sclerosis patients (HS), non-HS cases, and autopsies were studied for: hippocampal AMPA GluR1-3 and NMDAR1&2b mRNA levels using in situ hybridization: GluR1, GluR2/3, NMDAR1, and NMDAR2(a&b) immunoreactivity (IR); and neuron densities. Similarly, spontaneously seizing rats after SSLSE, kindled rats, and control animals were studied for: fascia dentata neuron densities: GluR1 and NMDAR2(a&b) IR; and neo-Timm's staining. In HS and non-HS cases, the mRNA hybridization densities per granule cell, as well as molecular layer IR, showed increased GluR1 (relative to GluR2/3) and increased NMDAR2b (relative to NMDAR1) compared to autopsies. Likewise, the molecular layer of SSLSE rats with spontaneous seizures demonstrated more neo-Timm's staining, and higher levels of GluR1 and NMDAR2(a&b) IR compared to kindled animals and controls. These results indicate that hippocampal AMPA and NMDA receptor subunit mRNAs and their proteins are differentially increased in association with spontaneous, but not kindled, seizures. Furthermore, there appears to be parallels in fascia dentata AMPA and NMDA receptor subunit expression between HS (and non-HS) epileptic patients and SSLSE rats. This finding supports the hypothesis that spontaneous seizures in humans and SSLSE rats involve differential alterations in hippocampal ionotrophic glutamate receptor subunits. Moreover, non-HS hippocampi were more like HS cases than hippocampi from kindled animals with respect to glutamate receptors; therefore, hippocampi from kindled rats do not accurately model human non-HS cases, despite some similarities in neuron densities and mossy fiber axon sprouting.

Factors affecting excitatory amino acid release following severe human head injury

Recent animal studies demonstrate that excitatory amino acids (EAAs) play a major role in neuronal damage after brain trauma and ischemia. However, the role of EAAs in patients who have suffered severe head injury is not understood. Excess quantities of glutamate in the extracellular space may lead to uncontrolled shifts of sodium, potassium, and calcium, disrupting ionic homeostasis, which may lead to severe cell swelling and cell death. The authors evaluated the role of EEAs in human traumatic brain injury.

In 80 consecutive severely head injured patients, a microdialysis probe was placed into the gray matter along with a ventriculostomy catheter or an intracranial pressure (ICP) monitor for 4 days. Levels of EAAs and structural amino acids were analyzed using high-performance liquid chromatography. Multifactorial analysis of the amino acid pattern was performed and its correlations with clinical parameters and outcome were tested. The levels of EAAs were increased up to 50 times normal in 30% of the patients and were significantly correlated to levels of structural amino acids both in each patient and across the whole group (p < 0.01). Secondary ischemic brain injury and focal contusions were most strongly associated with high EAA levels (27 ± 22 μmol/L). Sustained high ICP and poor outcome were significantly correlated to high levels of EAAs (glutamate > 20 μmol/L; p < 0.01).

The release of EAAs is closely linked to the release of structural amino acids and may thus reflect nonspecific development of membrane micropores, rather than presynaptic neuronal vesicular exocytosis. The magnitude of EAA release in patients with focal contusions and ischemic events may be sufficient to exacerbate neuronal damage, and these patients may be the best candidates for treatment with glutamate antagonists in the future.

Interaction between Ca2+, K+, carbamazepine and zonisamide on hippocampal extracellular glutamate monitored with a microdialysis electrode

1. Multiple components of hippocampal glutamate release were examined by study of Ca2+- and K+-evoked hippocampal extracellular glutamate release using an in vivo microdialysis glutamate biosensor in urethane-anaesthetized rats. In addition, the effects of the antiepileptic drugs, carbamazepine (CBZ) and zonisamide (ZNS) perfused through the probe on glutamate release were assessed. 2. Basal glutamate levels were below detection limits (approximately 0.1 microM). An increase in extracellular KCl (from 2.7 to 50 and 100 mM) increased extracellular hippocampal glutamate levels to 9.2+/-1.4 and 20.0+/-2.6 microM, respectively, calculated from the area under curve (AUC) for 60 min. 3. This KCl-evoked glutamate release consisted of three components: an initial transient rise, a late gentle rise, and late multiple phasic transient rises. 4. An increase in or removal of extracellular CaCl2 levels respectively enhanced and reduced the 50 mM KCl-evoked hippocampal glutamate release (AUC for 60 min) from 9.2+/-1.4 to 12.4+/-2.1 and 5.8+/-0.9 microM. 5. Perfusion with 100 microM CBZ or 1 mM ZNS inhibited both the 50 mM KCl-evoked hippocampal glutamate release (AUC for 60 min) from 9.2+/-1.4 to 5.5+/-1.1 and to 5.8+/-1.3 microM, respectively, as well as the stimulatory effects of Ca2+ on KCl-evoked hippocampal glutamate release. 6. These results suggest that both CBZ and ZNS may reduce epileptiform events by inhibiting excitatory glutamatergic transmission.

Interstitial glycerol as a marker for membrane phospholipid degradation in the acutely injured human brain

OBJECTIVE

Brain interstitial glycerol was studied as a potential marker for membrane phospholipid degradation in acute human brain injury.

METHODS

Glycerol was measured in microdialysis samples from the frontal lobe cortex in four patients in the neurointensive care unit, during the acute phase after severe aneurysmal subarachnoid haemorrhage. Microdialysis probes were inserted in conjunction with a ventriculostomy used for routine intracranial pressure monitoring. Clinical events involving hypoxia/ischaemia were diagnosed by neurological signs, neuroimaging (CT and PET), and neurochemical changes of the dialysate-for example, lactate/pyruvate ratios and hypoxanthine concentrations.

RESULTS

Altogether 1554 chemical analyses on 518 microdialysis samples were performed. Clinical events involving secondary hypoxia/ischaemia were generally associated with pronounced increases (up to 15-fold) of the dialysate glycerol concentration. In a patient with a stable condition and no signs of secondary hypoxia/ischaemia the glycerol concentration remained low. Simultaneous determination of glycerol in arterial plasma samples showed that the changes in brain interstitial glycerol could not be attributed to systemic changes and an injured blood brain barrier.

CONCLUSIONS

This study suggests that membrane phospholipid degradation occurs in human cerebral ischaemia. Interstitial glycerol harvested by microdialysis seems to be a promising tool for monitoring of membrane lipolysis in acute brain injury. The marker may be useful for studies on cell membrane injury mechanisms mediated by for example, Ca2+ disturbances, excitatory amino acids, and reactive oxygen species; and in the evaluation of new neuroprotective therapeutic strategies.

Weak anticonvulsant effects of two novel glycineB receptor antagonists in the amygdala-kindling model in rats

In the present work we evaluated the anticonvulsant effects of two novel antagonists of the glycine co-agonist site (glycineB receptor) within the N-methyl-D-aspartate (NMDA) receptor complex, MRZ 2/576 (a tricyclic pyrido-phtalazin dione derivative) and L-701,324 (7-chloro-4-hydroxy-3-(3-phenoxy)phenyl-2(H)quinoline). As a model of epilepsy we used amygdala-kindled rats, which are considered as a model to study the efficacy of drugs against human complex partial seizures. MRZ 2/576 (2.5-10 mg/kg i.p. 15 min before testing) did not influence afterdischarge threshold, which is believed to be the most subtle indicator of efficacy against kindled seizures, nor did it affect other measures of seizure activity such as seizure severity, seizure duration and afterdischarge duration. However, MRZ 2/576 produced dose-dependent ataxia as measured in the open field and rotarod test. The highest dose tested (10 mg/kg) also markedly reduced rectal temperature (by about 1.5 degrees C). L-701,324 (2.5-10 mg/kg i.p. 30 min before testing) dose dependently and significantly increased afterdischarge threshold, but other seizure parameters remained unchanged. The ataxia produced by lower doses of L-701,324 (2.5 and 5 mg/kg) was more pronounced than that caused by MRZ 2/576. However, the ataxia observed following the higher dose of L-701,324 (10 mg/kg) was less intense than that elicited by MRZ 2/576. The behavioral alterations produced by the two drugs did not resemble those characteristic for classical competitive and uncompetitive NMDA receptor antagonists. In conclusion, our data indicate that glycineB receptor antagonists are not promising candidates for the treatment of complex partial seizures in humans, at least as monotherapy.

Co-variation of free amino acids in brain interstitial fluid during pentylenetetrazole-induced convulsive status epilepticus

Effects of pentylenetetrazole (PTZ)-induced convulsive status epilepticus on free amino acids changes in venous blood, CSF and interstitial fluid (IF) of the brain were examined in dogs. A volume of brain IF sufficient for analysis was obtained by chronically implanted tissue cages. The onset of PTZ-induced convulsive seizures seemed mainly related to a marked increase of glutamate, aspartate, taurine, glycine and phosphoserine while, the maintenance and frequency of seizures seemed related to a marked increase of serine and glycine, in combination with a moderate rise of glutamate. L-alpha-Aminoadipate was recovered in moderate amount in epileptic brain IF, while, in controls, this compound was present in minimal amount. The observed complex temporal variation of the amino acidic pattern may play a role in PTZ-induced seizures and, possibly, in pharmacological kindling and brain structural alterations induced by PTZ.

Hippocampal and cerebellar extracellular amino acids during pilocarpine-induced seizures in freely moving rats

Limbic seizures were provoked in freely moving rats by intrahippocampal administration of the muscarinic receptor agonist pilocarpine via a microdialysis probe (10 mM for 40 min at 2 microliters/min). Changes in extracellular hippocampal and cerebellar glutamate, aspartate and gamma-aminobutyric acid (GABA) levels were monitored during and after pilocarpine administration. Effects of systemic or local administration of anticonvulsants on the seizures and concomitant changes in amino-acid concentrations, were investigated. Pilocarpine-induced seizures were completely abolished after intraperitoneal premedication for 7 days with phenobarbital (15 mg/kg per day) and after intrahippocampal administration of 10 mM phenobarbital and 1 mM carbamazepine (180 min at 2 microliters/min). Rats premedicated with carbamazepine (5 mg/kg per day) still developed seizures. The changes in extracellular hippocampal amino-acid levels suggest that glutamate, aspartate and GABA are not involved in seizure onset, but may play a role in seizure maintenance and/or spread in the pilocarpine animal model of epilepsy. The increases in extracellular amino acids in ipsi- and contralateral cerebellum following limbic seizures provoked in the hippocampus, probably play a role in the 'reversed' diaschisis phenomenon.

Altered glutamatergic transmission in neurological disorders: from high extracellular glutamate to excessive synaptic efficacy

This review is a critical appraisal of the widespread assumption that high extracellular glutamate, resulting from enhanced pre-synaptic release superimposed on deficient uptake and/or cytosolic efflux, is the key to excessive glutamate-mediated excitation in neurological disorders. Indeed, high extracellular glutamate levels do not consistently correlate with, nor necessarily produce, neuronal dysfunction and death in vivo. Furthermore, we exemplify with spreading depression that the sensitivity of an experimental or pathological event to glutamate receptor antagonists does not imply involvement of high extracellular glutamate levels in the genesis of this event. We propose an extension to the current, oversimplified concept of excitotoxicity associated with neurological disorders, to include alternative abnormalities of glutamatergic transmission which may contribute to the pathology, and lead to excitotoxic injury. These may include the following: (i) increased density of glutamate receptors; (ii) altered ionic selectivity of ionotropic glutamate receptors; (iii) abnormalities in their sensitivity and modulation; (iv) enhancement of glutamate-mediated synaptic efficacy (i.e. a pathological form of long-term potentiation); (v) phenomena such as spreading depression which require activation of glutamate receptors and can be detrimental to the survival of neurons. Such an extension would take into account the diversity of glutamate-receptor-mediated processes, match the complexity of neurological disorders pathogenesis and pathophysiology, and ultimately provide a more elaborate scientific basis for the development of innovative treatments.

Comparison of seizure related amino acid release in human epileptic hippocampus versus a chronic, kainate rat model of hippocampal epilepsy

Recent microdialysis studies of excitatory and inhibitory amino acid release associated with paroxysmal hippocampal activity have found significant increases in the hippocampus of epileptic patients, but minimal or variable increases in animal models. One possible reason for the difference is that the animal models employed in these studies have not adequately reflected the pathophysiology of human epilepsy. The present study sought to verify the amino acid release reported in human epileptic hippocampus and then employs animal studies using a chronic rat model of epilepsy, in which rats exhibit spontaneous seizure activity 3 to 4 months after injection of kainic acid into the hippocampus. In agreement with earlier reports, we found increases in glutamate, aspartate and GABA during seizures in human hippocampus. In addition we found increases in taurine which have not previously been reported. The chronic rat model shows increases in the same amino acids as in the human epileptic hippocampus, both during spontaneous seizures and stimulation evoked after-discharges (ADs). In contrast, minimal increases are elicited by hippocampal stimulation in control (non-kainate injected) animals. These results correlate with the degree of mossy fiber reorganization found in the dentate gyrus of kainate rats or epileptic humans.

In vivo elevation of extracellular potassium in the rat amygdala increases extracellular glutamate and aspartate and damages neurons

It is well known that high potassium (K+) solutions introduced by microdialysis into normal brain increase the extracellular concentration of the excitatory amino acid glutamate, and in vitro studies suggest that a high exogenously applied glutamate concentration can produce excitotoxic neuronal death. However, only recently were in vivo studies undertaken to determine whether high-K+ exposure damages neurons. We implanted microdialysis probes into rat amygdalae bilaterally, and after a 2-h baseline period exposed one side to a modified Krebs-Ringer-bicarbonate solution containing 100 mmol/l KCl for 30,50 and 70 min, followed by a 2-h recovery period, and 70 min and 3 h without a recovery period. Of 100.9 +/- 2.0 mmol/l KCl, 12.0 +/- 1.0% was extracted by amygdalar tissue in vivo. Election of the extracellular K+ concentration in the amygdala for 70 min or longer without a recovery period produced extensive neuronal damage and edematous-appearing neuropil in the tissue dialysed, as well as loss of normal neurons. Histological evidence of edema subsided in the groups with a 2-h recovery period. Although the number of damaged neurons was not significantly higher in the group with a 70 min high-K+ exposure and 2-h recovery period, the number of normal neurons was reduced, suggesting cell loss. During 70-min high-K+ exposure, the extracellular glutamate concentration increased to 242-377% of baseline during the first 60 min, and extracellular aspartate rose to 162-213% during the first 50 min; extracellular taurine rose even higher, to 316-567% of baseline, and glutamine fell to 14-27% of baseline. Extracellular serine was decreased at 20, 50 and 70 min of high-K+ exposure; extracellular glycine was unchanged. The elevated extracellular glutamate and aspartate concentrations suggest that exposure of the amygdala to high extracellular K+ may produce cell death through an excitotoxic process, and point the way to future studies to define the specific mechanisms involved.

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.

New technologies for amperometric biosensors

Amperomeric-based detectors have successfully been used as personal monitors for blood glucose levels. However, there is a desire to increase the number of compounds measured in a small blood sample, the speed of detection and enhance the reliability of the measurement. Furthermore, with the increasing use of microdialysis as a clinical sampling method in metabolic medicine, paediatric medicine and neurointensive care, there is a need for rapid on-line detection of analytes such as lactate, glucose and glutamate in low microlitre volume samples. Two approaches to these problems are described. The first uses enzymes immobilized in a packed bed with electrochemical detection of a ferrocene mediator as a flow-injection assay for use with microdialysis. Results from microdialysis of the brain of freely moving rats are described. In the second approach, thin-film techniques are used to fabricate arrays of microdisk and micro line electrodes. The properties of these arrays in free solution and in a flow cell are presented together with an example using multiple arrays to identify an analyte by oxidation potential. Finally, different enzymes are entrapped onto the surface of two arrays by electrochemical polymerization of o-phenylenediamine. The resulting device detects glucose and lactate in real-time.

Normal and abnormal calcium homeostasis in neurons: a basis for the pathophysiology of traumatic and ischemic central nervous system injury

Clinical recovery after central nervous system (CNS) trauma or ischemia may be limited by a neural injury process that is triggered and perpetuated at the cellular level, rather than by a lesion amenable to surgical repair. It is widely thought that one such process, a fundamental pathological mechanism initiated by CNS injury, is a disruption of cellular Ca2+ homeostasis. Because of the critical role of Ca2+ ions in regulating innumerable cellular functions, this major homeostatic disturbance is thought to trigger neuronal and axonal degeneration and produce clinical disability. We review those aspects of normal and pathological Ca2+ homeostasis in neurons that relate to neurodegeneration and to the application of neuroprotective strategies for the treatment of CNS injury. In particular, we examine the contribution of Ca(2+)-permeable ionic channels, Ca2+ pumps, intracellular Ca2+ stores, intracellular Ca2+ buffering systems, and the roles of secondary, Ca(2+)-dependent processes in neurodegeneration. A number of hypotheses linking Ca2+ ions and Ca2+ permeable channels to neurotoxicity are discussed with an emphasis on strategies for lessening Ca(2+)-related damage. A number of these strategies may have a future role in the treatment of traumatic and ischemic CNS injury.

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.

Clinical microdialysis: the role of on-line measurement and quantitative microdialysis

The use of microdialysis in the clinic is examined in the light of lessons learnt from microdialysis in freely moving rats. Changes in concentrations of metabolites are an important index of the state of health of tissues. For effective therapeutic intervention rapid assays are essential Enzyme-based on-line assays for glucose and lactate are described. By combining two of these assays simultaneous measurements of glucose and lactate, sampled at 2 min intervals can be obtained. The relation between dialysate concentrations and the true extracellular concentration of an analyte is dependent on conditions in the tissue sampled and cannot be calculated from in vitro probe recoveries. Furthermore, with acute implantation of the probe and possibly rapidly changing tissue conditions, there will be changes in probe recovery in vivo. Quantitative microdialysis allows the measurement of the true extracellular concentration and the probe recovery in vivo. The clinical applicability of a number of quantitative microdialysis methods is discussed, and three approaches highlighted. By increasing membrane length and reducing flow rate recovery in vivo can be increased to 100%. In this case dialysate concentrations equal extracellular ones. By perfusing an inert exogenous compound an index of changes to extracellular volume and hence tissue oedema can be obtained. In the zero net flux method the infusion of a few concentrations of the analyte under study allows the direct determination of both the ECF concentration and the in vivo recovery. The latter can provide valuable information about changes in the physical as well as chemical state of the tissue. This can guide rapid effective therapeutic intervention.

The effect of the desglycinyl metabolite of remacemide hydrochloride (FPL 12495AA) and dizocilpine (MK-801) on endogenous amino acid release from mouse cortex

1. In this study the effect of FPL 12495AA, the desglycinyl metabolite of remacemide hydrochloride and dizocilpine (MK-801), on potassium- and veratridine-stimulated release of neurotransmitter amino acids from mouse cortical slices was investigated. 2. Veratridine (20 microM) and potassium (60 mM) produced a preferential release of glutamate and aspartate. Potassium-stimulated release was calcium-dependent, while veratridine-stimulated release was only partially affected by removal of calcium from the medium. 3. FPL 12495AA significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate. Lower concentrations of FPL 12495AA were needed to inhibit veratridine-stimulated release of glutamate (12.5 microM) than potassium-stimulated release (100 microM). 4. Dizocilpine significantly inhibited veratridine- and potassium-stimulated release of glutamate and aspartate at concentrations of 100 microM and above. 5. FPL 12495AA and dizocilpine both have an affinity for the ion channel subsite of the N-methyl-D-aspartate (NMDA) receptor. The reduction of potassium-stimulated release of glutamate and aspartate by FPL 12495AA and dizocilpine is probably due to NMDA receptor blockade. 6. FPL 12495AA inhibited veratridine-stimulated release at a concentration of 12.5 microM while dizocilpine was effective only at a concentration of 100 microM. This difference in efficacy is probably due to the higher affinity of FPL 12495AA compared to dizocilpine at the veratridine-binding site on the sodium channel.

A comparison between the stimulated and paroxysmal release of endogenous amino acids from rat cerebellar, striatal and hippocampal slices: a manifestation of spreading depression?

Spreading depression, which can be evoked by a variety of stimuli both in vitro and in vivo, is associated with profound changes in extracellular ion concentrations and enhanced release of neurotransmitter amino acids. We have observed a transient spontaneous release of amino acids in slice preparations obtained from rat cerebellum, striatum and hippocampus; this phenomenon has similar properties to stimulus-evoked spreading depression. Aspartate, glutamate, glutamine, serine, glycine and gamma-aminobutyric acid (GABA) release were potentiated during these episodes in all three brain regions, with a variable effect upon taurine release. When compared to glutamate release, a consistently high release of aspartate, glycine and serine was observed. Amino acid release, evoked by whole slice depolarization using veratridine (10-25 microM) or elevated potassium (35-60 mM) consistently enhanced glutamate release, and to a lesser extent aspartate release, but had negligible effect upon the other amino acids. Thus, the release profiles for spontaneous and depolarization-evoked release are markedly different. We suggest that the spontaneous release observed in brain slices represents a spreading depression-like phenomenon; the putative roles of the amino acids are discussed.

Neurochemical and morphological changes associated with human epilepsy

To date a multitude of studies into the morphology and neurochemistry of human epilepsy have been undertaken with variable, and often inconsistent, results. This review summarises these studies on a range of neurotransmitters, neuromodulators, neuropeptides and their receptors. In addition to this, novel changes in cell viability and sprouting have been identified and are discussed. Whether the alterations observed are a result of the seizures or are a contributory factor is unclear. However, it may be that following an initial insult (such as febrile convulsions, status epilepticus or head injury) secondary processes occur both of an anticonvulsant nature in an attempt to compensate for seizure activity, and in a kindling type of fashion, resulting in an increased susceptibility to seizures, leading to future seizures. Many of the alterations documented in this study probably represent one or both of these processes. Clearly no single chemical abnormality or morphological alteration is going to explain the clinically diverse disorder of epilepsy. However, by drawing together the neurochemistry and morphology of epilepsy, we may begin to understand the mechanisms involved in seizure disorders.

Mesial temporal sclerosis: pathogenesis and significance

Mesial temporal sclerosis (MTS) is a common pathologic finding in patients with temporal lobe epilepsy. Rarely MTS can be detected in children during the first decade of life, but is not commonly found until adolescence. Although the etiology of MTS remains controversial, there is now a considerable amount of evidence demonstrating that MTS is both a result and a cause of seizures. Clinical studies suggest that prolonged seizures or complicated febrile seizures may result in MTS. A variety of epileptogenic agents administered to adult animals have resulted in MTS and spontaneous recurrent seizures. The mechanism of the lesions is due to excessive excitability secondary to release of excitatory amino acids, primarily glutamate. Glutamate, acting at a number of subreceptors on the postsynaptic membrane, leads to prolonged depolarization of neurons and results in the entry of cytotoxic amounts of calcium. Interestingly, the same agents that produce MTS in adult animals do not produce MTS in immature animals. Clinical and experimental evidence suggests that although prolonged seizures or complicated febrile seizures can place a child at risk for MTS, a period of time is required for the lesions to develop fully.