Studies on excitatory amino acid receptor-linked brain disorders in rat and man using in vivo microdialysis

Approaches to Monitor Circuit Disruption after Traumatic Brain Injury: Frontiers in Preclinical Research

Mild traumatic brain injury (TBI) often results in pathophysiological damage that can manifest as both acute and chronic neurological deficits. In an attempt to repair and reconnect disrupted circuits to compensate for loss of afferent and efferent connections, maladaptive circuitry is created and contributes to neurological deficits, including post-concussive symptoms. The TBI-induced pathology physically and metabolically changes the structure and function of neurons associated with behaviorally relevant circuit function. Complex neurological processing is governed, in part, by circuitry mediated by primary and modulatory neurotransmitter systems, where signaling is disrupted acutely and chronically after injury, and therefore serves as a primary target for treatment. Monitoring of neurotransmitter signaling in experimental models with technology empowered with improved temporal and spatial resolution is capable of recording in vivo extracellular neurotransmitter signaling in behaviorally relevant circuits. Here, we review preclinical evidence in TBI literature that implicates the role of neurotransmitter changes mediating circuit function that contributes to neurological deficits in the post-acute and chronic phases and methods developed for in vivo neurochemical monitoring. Coupling TBI models demonstrating chronic behavioral deficits with in vivo technologies capable of real-time monitoring of neurotransmitters provides an innovative approach to directly quantify and characterize neurotransmitter signaling as a universal consequence of TBI and the direct influence of pharmacological approaches on both behavior and signaling.

A Precision Medicine Approach to Cerebral Edema and Intracranial Hypertension after Severe Traumatic Brain Injury: Quo Vadis?

PURPOSE OF REVIEW

Standard clinical protocols for treating cerebral edema and intracranial hypertension after severe TBI have remained remarkably similar over decades. Cerebral edema and intracranial hypertension are treated interchangeably when in fact intracranial pressure (ICP) is a proxy for cerebral edema but also other processes such as extent of mass lesions, hydrocephalus, or cerebral blood volume. A complex interplay of multiple molecular mechanisms results in cerebral edema after severe TBI, and these are not measured or targeted by current clinically available tools. Addressing these underpinnings may be key to preventing or treating cerebral edema and improving outcome after severe TBI.

RECENT FINDINGS

This review begins by outlining basic principles underlying the relationship between edema and ICP including the Monro-Kellie doctrine and concepts of intracranial compliance/elastance. There is a subsequent brief discussion of current guidelines for ICP monitoring/management. We then focus most of the review on an evolving precision medicine approach towards cerebral edema and intracranial hypertension after TBI. Personalization of invasive neuromonitoring parameters including ICP waveform analysis, pulse amplitude, pressure reactivity, and longitudinal trajectories are presented. This is followed by a discussion of cerebral edema subtypes (continuum of ionic/cytotoxic/vasogenic edema and progressive secondary hemorrhage). Mechanisms of potential molecular contributors to cerebral edema after TBI are reviewed. For each target, we present findings from preclinical models, and evaluate their clinical utility as biomarkers and therapeutic targets for cerebral edema reduction. This selection represents promising candidates with evidence from different research groups, overlap/inter-relatedness with other pathways, and clinical/translational potential. We outline an evolving precision medicine and translational approach towards cerebral edema and intracranial hypertension after severe TBI.

Effect of Perfusion Fluids on Recovery of Inflammatory Mediators in Microdialysis

Microdialysis is an excellent tool to assess tissue inflammation in patients, but in vitro systems to evaluate recovery of inflammatory mediators have not been standardized. We aimed to develop a reference plasma preparation and evaluate different perfusion fluids with respect to recovery of metabolic and inflammatory markers. The reference preparation was produced by incubation of human blood with lipopolysaccharide and cobra venom factor to generate cytokines and activate complement, respectively. Microdialysis with 100 kDa catheters was performed using different colloid and crystalloid perfusion fluids (hydroxyethyl starch (HES) 130/0.4, HES 200/0.5, hyperosmolar HES 200/0.5, albumin 200 g/l, T1 perfusion fluid and Ringer's acetate) compared to today's recommended dextran 60 solution. Recovery of glucose, glycerol and pyruvate was not significantly different between the perfusion fluids, whereas lactate had lower recovery in HES 200/0.5 and albumin perfusion fluids. Recovery rates for the inflammatory proteins in comparison with the concentration in the reference preparation differed substantially: IL-6 = 9%, IL-1β = 18%, TNF = 0.3%, MCP-1 = 45%, IL-8 = 48%, MIG = 48%, IP-10 = 25%, C3a = 53% and C5a = 12%. IL-10 was not detectable in microdialysis dialysate. HES 130/0.4 and HES 200/0.5 yielded a recovery not significantly different from dextran 60. Hyperosmolar HES 200/0.5 and albumin showed significantly different pattern of recovery with increased concentration of MIG, IP-10, C3a and C5a and decreased concentration of IL-1β, TNF, MCP-1 and IL-8 in comparison with dextran 60. In conclusion, microdialysis perfusion fluid dextran 60 can be replaced by the commonly used HES 130/0.4, whereas albumin might be used if specific immunological variables are in focus. The present reference plasma preparation is suitable for in vitro evaluation of microdialysis systems.

The role of PSD-95 and cypin in morphological changes in dendrites following sublethal NMDA exposure

Focal swelling or varicosity formation in dendrites and loss of dendritic spines are the earliest indications of glutamate-induced excitotoxicity. Although it is known that microtubule dynamics play a role in varicosity formation, very little is known about the proteins that directly impact microtubules during focal swelling and dendritic spine loss. Our laboratory has recently reported that the postsynaptic protein PSD-95 and its cytosolic interactor (cypin) regulate the patterning of dendrites in hippocampal neurons. Cypin promotes microtubule assembly, and PSD-95 disrupts microtubule organization. Thus, we hypothesized that cypin and PSD-95 may play a role in altering dendrite morphology and spine number in response to sublethal NMDA-induced excitotoxicity. Using an in vitro model of glutamate-induced toxicity in rat hippocampal cultures, we found that cypin overexpression or PSD-95 knockdown increases the percentage of neurons with varicosities and the number of varicosities along dendrites, decreases the size of varicosities after sublethal NMDA exposure, and protects neurons from NMDA-induced death. In contrast, cypin knockdown or PSD-95 overexpression results in opposite effects. We further show that cypin regulates the density of spines/filopodia: cypin overexpression decreases the number of protrusions per micrometer of dendrite while cypin knockdown results in an opposite effect. Cypin overexpression and PSD-95 knockdown attenuate NMDA-promoted decreases in protrusion density. Thus, we have identified a novel pathway by which the microtubule cytoskeleton is regulated during sublethal changes to dendrites.

Alterations in cerebral oxidative metabolism following traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) generates regional alterations in cerebral metabolism, leading to the potential evolution of persistent metabolic dysfunction. In the case of penetrating, firearm-related TBI, the pathophysiological mechanisms underlying these acute-phase metabolic derangements are not entirely understood-hindering the potential effectiveness of therapeutic intervention. The use of cerebral microdialysis to monitor biochemical alterations that occur, post-TBI, provides critical insight into the events that perpetuate neurological deterioration.

METHODS

Cerebral microdialysis was used to monitor alterations in the brain tissue chemistry of a 22-year-old female patient who sustained a penetrating gunshot wound to the head. Extracellular glucose, lactate, pyruvate, and lactate pyruvate ratio (LPR) were monitored over the course of the first-week post-injury.

RESULTS

Analysis of the microdialysate revealed sustained elevations in LPR with peaks in excess of those seen in patients who have sustained permanent ischemic injury. This interval of persistently elevated LPR was followed by a spontaneous reduction of values, to levels below the defined threshold for metabolic crisis, over a period of several days.

CONCLUSIONS

Microdialysis studies may significantly improve the understanding of the metabolic alterations that occur in patients who sustain a variety of forms of neurotrauma. Ultimately, monitoring these variations in brain tissue chemistry will improve the insight into the neuropathological mechanisms underlying penetrating traumatic brain injury, and enhance the therapeutic approach of these patients.

Role of extracellular glutamate measured by cerebral microdialysis in severe traumatic brain injury

OBJECT

Authors of several studies have implied a key role of glutamate, an excitatory amino acid, in the pathophysiology of traumatic brain injury (TBI). However, the place of glutamate measurement in clinical practice and its impact on the management of TBI has yet to be elucidated. The authors' objective in the present study was to evaluate glutamate levels in TBI, analyzing the factors affecting them and determining their prognostic value.

METHODS

A prospective study of patients with severe TBI was conducted with an inclusion criterion of a Glasgow Coma Scale score < or = 8 within 48 hours of injury. Invasive monitoring included intracranial pressure measurements, brain tissue PO(2), jugular venous O(2) saturation, and cerebral microdialysis. Patients received standard care including mass evacuation when indicated and treatment of elevated intracranial pressure values. Demographic data, CT findings, and outcome at 6 months of follow-up were recorded.

RESULTS

One hundred sixty-five patients were included in the study. Initially high glutamate values were predictive of a poor outcome. The mortality rate was 30.3% among patients with glutamate levels > 20 micromol/L, compared with 18% among those with levels < or = 20 micromol/L. Two general patterns were recognized: Pattern 1, glutamate levels tended to normalize over the monitoring period (120 hours); and Pattern 2, glutamate levels tended to increase with time or remain abnormally elevated. Patients showing Pattern 1 had a lower mortality rate (17.1 vs 39.6%) and a better 6-month functional outcome among survivors (41.2 vs 20.7%).

CONCLUSIONS

Glutamate levels measured by microdialysis appear to have an important role in TBI. Data in this study suggest that glutamate levels are correlated with the mortality rate and 6-month functional outcome.

Feasibility study using surface-enhanced Raman spectroscopy for the quantitative detection of excitatory amino acids

The release of excitatory amino acids (EAAs) from injured neurons has been associated with secondary injury following head trauma. The development of a rapid and sensitive method for the quantification of EAAs may provide a means for clinical management of patients affected by head trauma. We explore the potential application of surface-enhanced Raman spectroscopy (SERS) for rapid quantification of the concentration of EAAs in aqueous silver colloids. The EAAs glutamate (Glu) and aspartate (Asp) are released following head injury and have been observed to exhibit SERS spectra that should enable them to be distinguished in a complex aqueous media. Of the two EAAs, the concentration of Glu has been shown to be more indicative of injury to the central nervous system. Using 30-s scans and a 50-mW argon laser, aqueous Glu is quantifiable from 0.4 to 5 micromol/L and is spectrally distinguishable from Asp. In addition, initial in vivo microdialysis experiments suggest that this SERS system is capable of measuring chemical changes following head trauma in the rat brain. Compared with current high-performance liquid chromatography (HPLC) techniques for amino acid detection, the short scanning and processing time associated with the SERS approach enables measurement on a near-real-time basis, providing clinical information in anticipation of pharmaceutical intervention.

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.

Changes of extracellular levels of amino acids after graded compression trauma to the spinal cord: an experimental study in the rat using microdialysis

We evaluated in rats, the time course of changes in extracellular levels of amino acids, lactate and pyruvate, which ensued spinal cord compression of mild, moderate, and severe degrees. The neurochemical findings measured by HPLC were compared with known outcome measures of this model. A laminectomy of vertebrae Th7 and Th8 was made and a microdialysis probe was inserted in one dorsal horn. Fluid samples were collected at intervals of 10 min. Dialysate lactate and lactate/pyruvate ratios increased in proportion to the severity of injury, suggesting a progressive derangement of energy metabolism. Mild trauma, with no neurologic deficits, did not induce any remarkable change of amino acids, but taurine values were temporarily slightly elevated. Moderate trauma, leading to transient paraparesis, resulted in a transient rise of glutamate and taurine. Severe trauma resulting in paraplegia of the hind limbs induced profound changes of extracellular amino acids. Glutamate and aspartate rose 5-6 times above basal level. There were marked elevations of taurine, glycine, arginine, alanine, asparagine, histidine, serine, threonine, and tyrosine after this degree of trauma. Glutamate, aspartate, and taurine returned to the basal level within 50 min, whereas most of the other amino acids remained elevated throughout the experiment. Thus, we found profound disturbances of extracellular amino acids and energy metabolites. The elevations of glutamate and aspartate correlated with previously recorded data on neurological outcome. The composition of the early extracellular edema showed marked temporal changes related to the severity of impact. Future studies regarding treatment of traumatic edema should focus on its chemical composition as well as its volume since such edema is not uniform in composition.

Massive persistent release of excitatory amino acids following human occlusive stroke

BACKGROUND

Animal stroke models demonstrate excitatory amino acid (EAA) release in ischemic tissue, as measured by microdialysis. Currently glutamate antagonist drugs are being developed to protect brain tissue after ischemic events. However, the role of EAAs in human occlusive stroke is not well known. We therefore measured glutamate and aspartate release in a patient after occlusive stroke.

CASE DESCRIPTION

We describe a case of occlusive stroke in a 50-year-old man. A partial temporal lobectomy was done to remove infarcted tissue and to prevent brain stem compression as well as uncal herniation. A microdialysis probe was placed into the cortex to measure EAAs. Massively increased levels of glutamate and aspartate were detected in the extracellular fluid in this patient (> 300 times normal levels 6 days after infarction).

CONCLUSIONS

These findings indicate that EAAs are tremendously increased in brain tissue after occlusive stroke. The time course of the release of EAAs is much longer than animal studies have suggested previously. Administration of EAA antagonists to patients with ischemic stroke may therefore be beneficial.