Administration of excitatory amino acid antagonists via microdialysis attenuates the increase in glucose utilization seen following concussive brain injury

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.

Effects of muscarinic receptor antagonism on the phosphatidylinositol bisphosphate signal transduction pathway after experimental brain injury

Hippocampal levels of fatty acids extracted from phosphatidylinositol 4,5-bisphosphate (PIP2), free fatty acids (FFA), and lactate were measured after central fluid percussion traumatic brain injury (TBI) in rats. At 5 min after injury, there was a decrease in fatty acids extracted from PIP2 suggesting a decrease in PIP2. At the same time point, total FFA increased in saline-treated TBI rats. Levels of arachidonic acid were significantly decreased in PIP2, while at the same time arachidonic and stearic acids increased in FFA in saline-treated TBI rats. No significant alterations in PIP2-derived fatty acids or FFA were observed at 20 min after TBI. Hippocampal concentrations of lactate were significantly elevated at 5 and 20 min after injury in saline-treated rats. In general, these alterations were blunted by preinjury administration of the muscarinic antagonist, scopolamine. These results suggest that the PIP2 signal transduction pathway is activated in the hippocampus at the onset of central fluid percussion TBI and that the enhanced phospholipase C-catalyzed phosphodiestric breakdown of PIP2 is a major mechanism of liberation of FFA in these sites immediately after such injury. The blunting of PIP2 and FFA alterations in animals treated with scopolamine suggests that activation of muscarinic receptors significantly contributes to the phospholipase C (PLC) signal transduction pathophysiology in TBI. The attenuation of lactate accumulation in scopolamine-treated rats suggests that TBI-induced muscarinic receptor activation also contributes to increased glycolytic metabolism and/or ionic imbalances.

Hyperemia following traumatic brain injury: relationship to intracranial hypertension and outcome

The role of posttraumatic hyperemia in the development of raised intracranial pressure (ICP) has important pathophysiological and therapeutic implications. To determine the relationship between hyperemia (cerebral blood flow (CBF) > 55 ml/100 g/minute), intracranial hypertension (ICP > 20 mm Hg), and neurological outcome, 193 simultaneous measurements of ICP and CBF (xenon-133 method) were obtained in 59 patients with moderate and severe head injury. Hyperemia was associated with an increased incidence of simultaneous intracranial hypertension compared to nonhyperemic CBF measurements (32.2% vs. 21.6%, respectively; p < 0.059). However, in 78% of blood flow studies in which ICP was greater than 20 mm Hg, CBF was less than or equal to 55 ml/100 g/minute. At least one episode of hyperemia was documented in 34% of patients, all of whom had a Glasgow Coma Scale (GCS) score of 9 or below. In 12 individuals with hyperemia without simultaneous intracranial hypertension, ICP was greater than 20 mm Hg for an average of 11 +/- 16 hours and favorable outcomes were seen in 75% of patients. In contrast, in eight individuals with hyperemia and at least one episode of hyperemia-associated intracranial hypertension, ICP was greater than 20 mm Hg for an average of 148 +/- 84 hours (p < 0.001), and a favorable outcome was seen in only one patient (p < 0.001). Compared to the remainder of the cohort, patients with hyperemia-associated intracranial hypertension were distinctive in being the youngest, exhibiting the lowest GCS scores (all < or = 6), and having the highest incidence of effaced basilar cisterns and intractable intracranial hypertension. In the majority of individuals with hyperemia-associated intracranial hypertension, their clinical profile suggests the occurrence of a severe initial insult with resultant gross impairment of metabolic vasoreactivity and pressure autoregulation. In a minority of these patients, however, high CBF may be coupled to a hypermetabolic state, given their responsiveness to metabolic suppressive therapy. In patients with hyperemia but without intracranial hypertension, elevated CBF is also likely to be a manifestation of appropriate coupling to increased metabolic demand consistent with a generally favorable outcome. This study supports the concept that there are multiple etiologies of both elevated blood flow and intracranial hypertension after head injury.

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.

Fluid percussion brain injury in the developing and adult rat: a comparative study of mortality, morphology, intracranial pressure and mean arterial blood pressure

Changes in intracranial pressure (ICP) and mean arterial blood pressure (MABP) were measured for 30 min following an experimental fluid percussion traumatic brain injury in postnatal day 17 (P17), P28 and adult rats. Under enflurane anesthesia the left femoral artery was cannulated for MABP measurements and a 20 gauge needle was stereotaxically positioned into the right lateral ventricle for ICP measurements. Three different injury severities (mild: 1.35-1.45 atm, moderate: 2.65-2.75 atm, severe: 3.65-3.75 atm) were delivered over the left parietal cortex to each of the age groups. The biomechanical/physiological results indicated that fluid percussion generated reproducible traumatic brain injuries in the developing rat. Furthermore, with increasing injury severity the physiological responses (in terms of ICP and MABP) became more pronounced, resulting in a corresponding increase in mortality (mild, moderate, severe, respectively, P17: 27%, 36%, 100%; P28: 33%, 30%, 75%; adult: 0%, 20%, 55%). Compared to adult animals, developing rats exhibited pronounced hypotension in response to closed head injury, which most likely explains the greater percent mortality among the younger animals. The utilization of this model will allow for future studies addressing the consequences of traumatic brain injury when it is sustained early in development.

Glut1 glucose transporter activity in human brain injury

The principal glucose transporter at the blood-brain barrier (BBB) is the Glut1 isoform, and transporter density is believed to be an index of cerebral metabolic rate. In the present study, glucose transporter expression was studied in tissue resected 7-8 h after acute traumatic brain injuries in 2 patients. Light microscopic immunochemistry indicated a zone of complete loss of the Glut1 glucose transporter isoform in microvessel endothelial cells adjacent to sites of small vessel injury, concentrically surrounded by a narrow zone of variable Glut1, and distally surrounded by capillaries with typically immunoreactive endothelia in nondisrupted parenchyma. Variably reactive capillaries displayed alternating sectors of greatly reduced and highly reactive Glut1 density, suggesting a high density and low density of transporter activity in contiguous endothelial cells. Quantitative electron microscopic immunogold analyses demonstrated that the transporter was predominantly localized to the luminal and abluminal endothelial membranes, with lesser reactivity in cytoplasm; pericyte Glut1 was minimally above background levels. In endothelial sectors with reduced Glut1 transporter immunoreactivity, the luminal:abluminal ratio of Glut1 epitòpes was less than unity; while it is greater than unity in highly reactive endothelial cells. The number of Glut1-immunoreactive sites per micrometer of capillary membrane was not significantly different from previous reported Glut1 density in seizure resections, and about 2- to 3-fold higher than in human red cells. In the same tissue samples, qualitative immunogold electron microscopy of human serum albumin indicated leakage of this protein (MW 65,000) from the vascular space into pericapillary regions. Thus the high Glut1 density observed in capillaries from acutely injured brain occurs concomitantly with compromised barrier function.

Hyperglycemia results in an increase in myocardial interstitial glucose and glucose uptake during ischemia

The purpose of this investigation was to assess the effects of hyperglycemia, in the absence of changes in plasma insulin and arterial free fatty acid (FFA) levels, on interstitial glucose levels and glucose uptake across the left ventricular wall during ischemia in domestic swine. Insulin secretion was suppressed with a continuous infusion of somatostatin. Arterial FFA levels remained stable due to the suppression of insulin. Microdialysis probes were used to estimate changes in interstitial glucose and lactate, and were placed in the subepicardium and the subendocardium of the left anterior descending ([LAD] ischemic) coronary artery perfusion bed and in the midmyocardium of the circumflex ([CFX] nonischemic) perfusion bed. The LAD coronary artery was cannulated and perfused with blood from the femoral artery through an extracorporal perfusion circuit. Ischemia was induced in the LAD perfusion bed by reducing the flow of the LAD perfusion pump by 60% for 50 minutes, and was followed by 30 minutes of reperfusion. Twenty minutes into the ischemic period, seven animals were given a bolus injection of 50% glucose (200 mg/kg) followed by a glucose infusion (10 mg/kg/min), resulting in an increase in arterial glucose levels from 5 to 13 mmol/L in the hyperglycemic group. Hyperglycemia resulted in a marked increase in dialysate glucose during ischemia and a greater than twofold increase in glucose extraction and uptake. Dialysate glucose correlated with plasma glucose in all three perfusion beds. In conclusion, hyperglycemia, in the absence of an increase in insulin and a decrease in arterial FFA, resulted in a doubling of glucose extraction, delivery, and uptake, which corresponded to the twofold elevation in interstitial glucose during ischemia.

Calcium movements in traumatic brain injury: the role of glutamate receptor-operated ion channels

Ion-selective microelectrodes were used to study acute effects of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy- 5-methyl-4-isoxazole (AMPA) receptor blockade on posttraumatic calcium disturbances. An autoradiographic technique with 45 Ca2+ was used to study calcium disturbances at 8, 24, and 72 h. Compression contusion trauma of the cerebral cortex was produced by a 21-g weight dropped from a height of 35 cm onto a piston that compressed the brain 2 mm. Pre- and posttrauma interstitial [Ca2+] ([Ca2+]e) concentrations were measured in the perimeter, i.e., the shear stress zone (SSZ) and in the central region (CR) of the trauma site. For the [Ca2+]e studies the animals were divided into controls and groups pretreated with dizocilipine maleate (MK-801) or with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[F]quinoxaline (NBQX). In all groups, [Ca2+]e decreased from pretrauma values of approximately 1 mM to posttraumatic values of 0.1 mM in both the CR and the SSZ. This was followed by a slow restitution toward pretraumatic levels during the 2-h observation period. There was no significant difference in recovery pattern between controls and pretreated animals. Accumulation of 45Ca2+ and serum proteins was seen in the entire SSZ, while neuronal necrosis was confined to a narrow band within the SSZ. The CR was unaffected apart from occasional eosinophilic neurons and showed no accumulation of 45Ca2+. Posttraumatic treatment with MK-801 or NBQX had no obvious effect on neuronal injury in the SSZ. We conclude that (a) acute [Ca2+]e disturbances in compression contusion brain trauma are not affected by blockade of NMDA or AMPA receptors, (b) 45Ca2+ accumulation in the SSZ reflects mainly protein accumulation due to blood-brain barrier breakdown rather than cell death, and (c) acute cellular Ca2+ over-load per se does not seem to be a major determinant of cell death after cerebral trauma in our model.

Regional concentrations of cyclic nucleotides after experimental brain injury

Regional concentrations of lactate, glucose, cAMP, and cGMP were measured after lateral fluid percussion brain injury in rats. At 5 min after injury, while tissue concentrations of lactate were elevated in the cortices and hippocampi of both the ipsilateral and contralateral hemispheres, those of glucose were decreased in these brain regions. By 20 min after injury, increases of lactate concentrations and decreases of glucose concentrations were observed only in the cortices and in the hippocampus of the ipsilateral hemisphere. Whereas the cAMP concentrations were unchanged in the cortices and hippocampi of the ipsilateral and contralateral hemispheres at 5 min after injury, decreases were found in the injured cortex and ipsilateral hippocampus at 20 min after injury. The tissue concentrations of cGMP were found to be elevated only in the ipsilateral hippocampus at 5 min after injury. The present observation that tissue glucose decreases in the injured cortex and the ipsilateral hippocampus are consistent with the published findings of increased hyperglycolysis and oxidative metabolism in brain immediately after injury. The present findings that the concentrations of cAMP and cGMP change in the cortex and hippocampus provide biochemical evidence for the neurotransmitter's surge after brain injury.

Role of excitatory amino acid-mediated ionic fluxes in traumatic brain injury

One major event taking place at the moment of traumatic brain injury in neuronal cells is the occurrence of massive ionic fluxes across the plasma membrane, which can be referred to as traumatic depolarization (TD). Unlike spreading depression, TD can occur over wide brain areas simultaneously. Furthermore, recovery from TD often takes far longer than recovery from ionic perturbation elicited by the passage of a single wave of spreading depression. Neuronal cell damage caused by ischemic brain injury is also initiated by massive ionic fluxes, termed anoxic depolarization. The occurrence of similar ionic events in these two forms of brain injury may account for the genesis of diffuse ischemia-like damage without actual episodes of hypoxia or ischemia in traumatic brain injury. We review the data indicating that excitatory amino acids (EAA) may play a vital role in producing TD, and that such EAA-mediated ionic perturbation is responsible for a number of posttraumatic events including subcellular metabolic dysfunction and cellular responses such as microglial activation and astrocytic transformation. TD may represent one of the most important mechanisms of diffuse neuronal cell dysfunction and damage associated with traumatic brain injury.

The rationale for glutamate antagonists in the treatment of traumatic brain injury

The recent development of potent antagonists for the most widespread neurotransmitter in the mammalian brain has opened up possibilities for many forms of therapy. The excitotoxic hypothesis implicates excessive release of excitatory amino acids (EAAs) as an important cause of brain damage, especially in acute ischemia, and chronic neurodegeneration. Focal ischemic damage and diffuse axonal injury are the major causes of brain damage after traumatic human brain injury. Evidence from animal models has shown that excitatory amino acid-induced events maybe responsible for a proportion of the posttraumatic sequelae and that these effects can be blocked by EAA antagonists. This evidence is reviewed, and the implications for human pathophysiology and treatment are discussed.

Chronic depression of glucose metabolism in postischemic rat brains

BACKGROUND AND PURPOSE

The present investigation aimed to quantify functional activity in rat brains after long-term recovery from transient forebrain ischemia.

METHODS

With the use of the [14C]2-deoxyglucose method, local cerebral glucose utilization was measured in 62 cortical and subcortical brain regions in postischemic rat brains. Transient forebrain ischemia of 10 minutes' duration was induced by clamping the common carotid arteries and simultaneously lowering blood pressure to 40 mm Hg. Rats survived the insults for 1 week, 2 weeks, 3 weeks, or 3 months.

RESULTS

Reductions predominated in the majority of gray matter structures at all time points investigated (P < .05). Except for a few areas, recoveries of local cerebral glucose utilization to preischemic levels did not occur.

CONCLUSIONS

The data illustrate that widespread alterations of functional activity prevail in postischemic brains beyond the selectively vulnerable regions. The present functional data are in line with previous stereological results of reduced fresh volumes in the majority of postischemic brain structures. The data suggest that chronic alterations of ischemic brains are not confined to the selectively vulnerable regions.

Lactate accumulation following concussive brain injury: the role of ionic fluxes induced by excitatory amino acids

During the first few minutes following traumatic brain injury, cells are exposed to an indiscriminate release of glutamate from nerve terminals resulting in a massive ionic flux (e.g., K+ efflux) via stimulation of excitatory amino acid (EAA)-coupled ion channels. The present study was undertaken to elucidate the causal relationship between these ionic shifts and lactate accumulation in the injured brain, by examining the effects of ouabain (an inhibitor of Na+/K+-ATPase), Ba2+ (an inhibitor or non-energy-dependent glial K+ uptake) and kynurenic acid (KYN; a broad-spectrum EAA antagonist) on lactate accumulation. Two microdialysis probes were placed bilaterally in the rat parietal cortex. One was perfused with a test drug (1.0 mM ouabain, 2.0 mM Ba2+ or 10 mM KYN) and the other with Ringer's solution (control) for 30 min prior to injury. Following a 2.2-2.7 atm fluid-percussion injury, lactate levels in the dialysate increased (up to 116.6% above baseline) for the first 16 min and returned to baseline levels within 20 min after injury. This lactate accumulation was attenuated by preinjury administration of ouabain and KYN and was prolonged by Ba2+ administration. These findings indicate that lactate accumulations following concussive brain injury is a result of increased glycolysis which supports ion-pumping mechanisms, thereby, restoring the ionic balance which was disrupted by stimulation of EAA-coupled ion channels.

Inhibition of phospholipase C with neomycin improves metabolic and neurologic outcome following traumatic brain injury

Activation of phospholipase C has been implicated as a factor in the development of irreversible tissue damage following injury to the central nervous system. We have used phosphorus magnetic resonance spectroscopy and a battery of postinjury motor function tests to characterize the role that phospholipase C activity may play in determining biochemical and neurologic outcome following traumatic brain injury in rats. Moderate (2.7 atmospheres) fluid percussion induced lateral brain injury caused a decline in free magnesium concentration, phosphorylation potential, and increased mitochondrial rate of oxidative phosphorylation. Neurologic motor score at 24 h and 1 week posttrauma in these animals was consistent with moderate injury. In contrast, treatment with the phospholipase C inhibitor neomycin B (15 mg/kg i.v.) immediately prior to injury significantly improved free magnesium status, bioenergetic state and neurological outcome (P < 0.01) after injury. We propose that phospholipase C activated second messenger pathways affecting magnesium homeostasis are involved in determining outcome after brain injury.

Widespread metabolic depression and reduced somatosensory circuit activation following traumatic brain injury in rats

The effects of fluid percussion brain injury on the basal metabolic state and responsiveness of a somatosensory circuit to physiologic activation were investigated with [14C]2-deoxyglucose autoradiography. Under controlled physiologic conditions and normothermic brain temperature (37 degrees C), rats were injured with a moderate fluid percussion pulse ranging from 1.7 to 2.1 atm. At 4 or 24 h after traumatic brain injury (TBI), unilateral vibrissae stimulation was carried out, resulting in the metabolic activation of the whisker-barrel circuit. In sham-operated control animals, whisker stimulation resulted in the metabolic activation of the ipsilateral trigeminal medullary complex (177% of control), contralateral ventrobasal thalamus (143% control), and primary somatosensory cortex (153% control). At 4 h after injury, local cerebral metabolic rates of glucose (ICMRglu) were significantly depressed throughout the traumatized hemisphere. Although depressed ICMRglu was most pronounced in cortical regions adjacent to the evolving contusion (53% of control), significant decreases were also seen in more remote areas, including the frontal cortex (75% of control), hippocampus (79% control), and lateral thalamus (68% of control). At 24 h following TBI, ICMRglu remained significantly reduced at the impact site, within the ipsilateral somatosensory cortex and lateral thalamus. Stimulus-evoked increases in ICMRglu were depressed within all three relay stations of the vibrissae-barrel-field circuit at 4 and 24 h after TBI. These results demonstrate both focal and diffuse metabolic depression after moderate TBI. Although the most severe and longer lasting metabolic consequences occurred in cortical and thalamic regions destined to exhibit histopathologic damage, milder abnormalities, most prominent in the early posttraumatic period, were also seen in noninjured areas. The inability to activate the somatosensory circuit metabolically indicates that circuit dysfunction is an acute consequence of TBI. Widespread circuit or synaptic dysfunction would be expected to participate in the functional and behavioral consequences of TBI.

Combined fluid percussion brain injury and entorhinal cortical lesion: a model for assessing the interaction between neuroexcitation and deafferentation

Laboratory studies suggest that excessive neuroexcitation and deafferentation contribute to long-term morbidity following human head injury. Because no current animal model of traumatic brain injury (TBI) has been shown to combine excessive neuroexcitation and significant levels of deafferentation, we developed a rat model combining the neuroexcitation of fluid percussion TBI with subsequent entorhinal cortical (EC) deafferentation. In this paradigm, moderate fluid percussion TBI was induced in each rat, followed 24 h later by bilateral EC lesion (BEC). Six conditions were examined: (1) fluid percussion TBI followed 24 h later by bilateral EC lesion (TBEC), (2) fluid percussion TBI (TBI), (3) bilateral EC lesion (BEC), (4) sham fluid percussion TBI (SHAM), (5) TBI followed 24 h later by unilateral EC lesion (TUEC), and (6) unilateral EC lesion (UEC). The first four groups were assessed for motor (with beam-balance and beam-walk testing) and cognitive deficits (with the Morris water maze) and hippocampal morphology (with immunocytochemistry and electron microscopy). The TUEC and UEC groups were assessed for cognitive deficits alone. Motor deficits were greater in the TBEC injury than in TBI or sham alone; however, no significant difference was observed between the TBEC and BEC conditions in motor performance. Cognitive deficits were of a greater magnitude in the combined TBEC injury model relative to each individual insult. These cognitive deficits appeared to be additive for the two experimental injuries, BEC deafferentation producing deficits intermediate between TBI and TBEC insults. Morphologic analysis of the dentate gyrus molecular layer at 15 days after TBEC showed that the distribution of synaptophysin-positive presynaptic terminals was distinct from that observed after either TBI or BEC alone. Specifically, the laminar pattern of presynaptic rearrangement induced by BEC lesion did not occur after TBEC injury. The present results show that axonal injury and its attendant deafferentation, when coupled with traumatically induced neuroexcitation, produce an enhancement of the morbidity associated with TBI. Moreover, they indicate that this model can effectively be used to study the interaction between neuroexcitation and synaptic plasticity.

N-methyl-D-aspartate receptor-mediated, prolonged afterdischarges of CA1 pyramidal cells following transient cerebral ischemia in the rat hippocampus in vivo

We previously reported the post-ischemic potentiation (PIP) of synaptic efficacy in hippocampal Schaffer collateral/CA1 responses of the rat beginning at 6-8 h following 12 min transient cerebral ischemia in vivo. The present study demonstrated that repetitive stimulation with a relatively low frequency (5 Hz, 6 s), which produced short-lasting afterdischarges (ADs; duration, 4.49 +/- 4.26 s; n = 7) in sham-controls, resulted in prolonged ADs (duration, 26.33 +/- 12.63 s; n = 6; P < 0.001) at the same period after ischemia. The PIP was not affected by 2-amino-5-phosphonovalerate (APV) administered via microdialysis at 7 h post-ischemia. The prolonged ADs in response to repetitive stimulation were, however, reversed to short-lasting ADs (duration, 7.13 +/- 1.44 s; n = 4; P < 0.02) by the same procedure, leaving the response to single stimulation unaffected. These findings suggest that, during the reperfusion period, Ca2+ influx into the CA1 pyramidal cells can be greatly increased through N-methyl-D-aspartate (NMDA) receptor-coupled ion channels if appropriately timed multiple synaptic inputs bombard these cells. Such Ca2+ influx may contribute to delayed death of CA1 pyramidal cells after transient cerebral ischemia if synaptic activity is maintained at relatively high levels during the reperfusion period.

Decreased interstitial glucose and transmural gradient in lactate during ischemia

The purpose of this investigation was to assess the effects of ischemia and reperfusion on the transmural levels of glucose and lactate in the interstitium in 11 open-chest swine. Microdialysis probes were used to estimate changes in interstitial metabolities across the ventricular wall. Probes were placed in the subepicardium and the subendocardium of the left anterior descending (LAD) coronary artery perfusion bed and in the midmyocardium of the circumflex (CFX) perfusion bed. The LAD coronary artery was cannulated and perfused with blood from the femoral artery through an extracorporal perfusion circuit. Ischemia was induced in the LAD perfusion bed by reducing the flow of the LAD perfusion pump by 60% for 50 min, and was followed by 30 min of reperfusion. Regional myocardial blood flow was assessed with fluorescent microspheres. Ischemia resulted in a transmural gradient in blood flow, with the most severe reduction in flow occurring in the subendocardium (p < 0.05). We found a significant reduction in interstitial glucose in both the LAD subepicardium (1.26 +/- 0.24 mM) (p = 0.0009) and subendocardium (0.89 +/- 0.21 mM) (p = 0.0001) during ischemia compared to the aerobic (non-ischemic) period (1.97 +/- 0.25 mM, 2.03 +/- 0.29 mM for the subepicardium and subendocardium, respectively). This coincided with a significant reduction in glucose delivery (LAD pump flow * arterial glucose) to the LAD perfusion bed during ischemia (54.5 +/- 8.5 mumol/min) compared to aerobic values (182.1 +/- 25.3 mumol/min) (p < 0.05). Interstitial lactate levels were significantly increased during ischemia in the LAD subendocardium (3.39 +/- 0.46 mM) compared to the aerobic values (1.73 +/- 0.46 mM) (p < 0.0029). A transmural gradient in interstitial lactate levels was observed during ischemia: this gradient was not seen during the aerobic period and was negated upon reperfusion. In conclusion, ischemia resulted in a decrease in interstitial glucose in both the LAD subepicardium and subendocardium, and an increase in interstitial lactate in the LAD subendocardium. Further, a transmural gradient in interstitial lactate levels was observed during ischemia, with the highest lactate values appearing in the subendocardium.

Bioenergetic analysis of oxidative metabolism following traumatic brain injury in rats

Studies of fluid percussion-induced traumatic brain injury have shown that moderate trauma results in ionic imbalances, with resultant increases in energy demand to restore these ion gradients. Because there are also increased rates of glucose metabolism during periods of focal decline in blood flow, it has been suggested that the mitochondria may be incapable of sufficient oxidative metabolism to cope with this increased energy demand after injury and that ATP derived from substrate level phosphorylation must meet this demand. In the present study, we used phosphorus magnetic resonance spectroscopy to determine the mitochondrial capacity for oxidative phosphorylation after moderate brain trauma. Before injury, mean oxidative capacity was 54% +/- 1%. After injury, mean capacity increased significantly (p < 0.001) to a maximum of 61% +/- 1%, indicating that mitochondrial oxidative metabolism was enhanced after trauma. Increased oxidative capacity was accompanied by increases in ADP, AMP, and inorganic phosphate concentrations and was correlated to decreases in cytosolic phosphorylation ratio. We conclude that moderate brain trauma increases mitochondrial rate of ATP synthesis over the first 4 h posttrauma, and that during this time of increased ATP turnover, positive feedback regulation of glycolysis by increased concentrations of ADP, AMP, and inorganic phosphate contributes to maintenance of metabolic steady state.

Early microvascular and neuronal consequences of traumatic brain injury: a light and electron microscopic study in rats

The purpose of this study was to document the early morphologic consequences of moderate traumatic brain injury (TBI) in anesthetized Sprague-Dawley rats. Normothermic rats (37 degrees C) were injured with a fluid percussion pulse (1.7-2.1 atm) administered by an injury cannula positioned parasagittally over the right cerebral cortex (n = 7). At 45 min following TBI, rats were injected with the protein tracer horseradish peroxidase (HRP) and perfusion fixed or immersion fixed 15 min later for light and electron microscopic analysis. Blood-brain barrier (BBB) breakdown to HRP was present overlying the pial surface and superficial cortical layers of the injured hemisphere. A focal area of severe HRP leakage was also present at the gray-white interface of the lateral cortex. Light microscopic examination of this site revealed petechial hemorrhages associated with small venules. Dark shrunken neurons and swollen astrocytes were detected within cortical areas overlying the evolving contusion, CA3 and CA4 hippocampal subsectors, and lateral thalamus. Ultrastructural studies obtained evidence for irreversible neuronal injury and mechanical damage to vessel walls at this early posttraumatic period. In nonperfused traumatized rats, luminal platelet aggregates were also detected at sites of hemorrhage. In this model of TBI, a consistent pattern of microvascular and neuronal abnormalities can be documented in the early posttraumatic period. Pathomechanisms underlying these early changes are discussed in terms of primary and secondary injury processes.

Post-traumatic brain hypothermia reduces histopathological damage following concussive brain injury in the rat

The purposes of this study were (1) to document the histopathological consequences of moderate traumatic brain injury (TBI) in anesthetized Sprague-Dawley rats, and (2) to determine whether post-traumatic brain hypothermia (30 degrees C) would protect histopathologically. Twenty-four hours prior to TBI, the fluid percussion interface was positioned over the right cerebral cortex. On the 2nd day, fasted rats were anesthetized with 70% nitrous oxide, 1% halothane, and 30% oxygen. Under controlled physiological conditions and normothermic brain temperature (37.5 degrees C), rats were injured with a fluid percussion pulse ranging from 1.7 to 2.2 atmospheres. In one group, brain temperature was maintained at normothermic levels for 3 h after injury. In a second group, brain temperature was reduced to 30 degrees C at 5 min post-trauma and maintained for 3 h. Three days after TBI, brains were perfusion-fixed for routine histopathological analysis. In the normothermic group, damage at the site of impact was seen in only one of nine rats. In contrast, all normothermic animals displayed necrotic neurons within ipsilateral cortical regions lateral and remote from the impact site. Intracerebral hemorrhagic contusions were present in all rats at the gray-white interface underlying the injured cortical areas. Selective neuronal necrosis was also present within the CA3 and CA4 hippocampal subsectors and thalamus. Post-traumatic brain hypothermia significantly reduced the overall sum of necrotic cortical neurons (519 +/- 122 vs 952 +/- 130, mean +/- SE, P = 0.03, Kruskal-Wallis test) as well as contusion volume (0.50 +/- 0.14 vs 2.14 +/- 0.71 mm3, P = 0.004).(ABSTRACT TRUNCATED AT 250 WORDS)

Epileptic seizure activity in the acute phase following cortical impact trauma in rat

The aim of this investigation was to determine the incidence of seizure activity in the acute phase following traumatic brain injury. Compression contusion trauma was produced in the right parietal cortex in 19 artificially ventilated rats. Electroencephalographic recordings were carried out in 17 of the animals for 2 h following the impact. The extracellular levels of neuroactive amino acids were simultaneously monitored in 9 of the experiments using microdialysis. In 14 of the 17 animals a generalized seizure activity with an average duration of 59 s (range 30-101 s) was recorded. The mean time lag between trauma and seizure onset was 67 s (range 26-90 s). The seizure activity was consistently followed by post-ictal depression. The trauma was accompanied by a transient increase of aspartate, taurine, glutamate and glycine, in decreasing rank order. The seizure activity occurred when the levels of these neuroactive amino acids were elevated. It is concluded that the high incidence of seizure activity observed may be an important factor contributing to secondary ischemia after traumatic brain injury. Aspartate and glutamate, potentiated by glycine, may play a role in post-traumatic seizure activity.

Endogenous neuroprotection factors and traumatic brain injury: mechanisms of action and implications for therapy

Throughout evolution the brain has acquired elegant strategies to protect itself against a variety of environmental insults. Prominent among these are signals released from injured cells that are capable of initiating a cascade of events in neurons and glia designed to prevent further damage. Recent research has identified a remarkably large number of neuroprotection factors (NPFs), whose expression is increased in response to brain injury. Examples include the neurotrophins (NGF, NT-3, NT-5, and BDNF), bFGF, IGFs, TGFs, TNFs and secreted forms of the beta-amyloid precursor protein. Animal and cell culture studies have shown that NPFs can attenuate neuronal injury initiated by insults believed to be relevant to the pathophysiology of traumatic brain injury (TBI) including excitotoxins, ischemia, and free radicals. Studies of the mechanism of action of these NPFs indicate that they enhance cellular systems involved in maintenance of Ca2+ homeostasis and free radical metabolism. Recent work has identified several low-molecular-weight lipophilic compounds that appear to mimic the action of NPFs by activating signal transduction cascades involving tyrosine phosphorylation. Such compounds, alone or in combination with antioxidants and calcium-stabilizing agents, have proved beneficial in animal studies of ischemic brain injury and provide opportunities for development of preventative/therapeutic approaches for TBI.

Metabolic changes following cortical contusion: relationships to edema and morphological changes

Rats with contusion injury to the right cortex exhibited significant formation of edema 6 and 24 hours after injury which resolved by 8 days and was replaced by cavitation necrosis. The contusions produced hyperglycolysis and ischemia in the impacted cortical tissue and underlying hippocampus immediately through 30 minutes post-injury. Glucose utilization was depressed throughout the contused cortex and in ipsilateral subcortical regions, as was blood flow, at chronic (1 and 10 days) periods after injury.

Excitatory amino acid release from contused brain tissue into surrounding brain areas

The EAA release from contused brain tissue and its effect on the extracellular EAA levels in brain areas surrounding the contusion were investigated with microdialysis technique in the rat. A significant increase in extracellular EAA levels was observed in the contused brain tissue. The EAA increase was significantly greater in the contused brain tissue than in the isolated but non-contused brain tissue. It was further demonstrated that EAAs were released from non-contused brain areas 1-2 mm distant from contused brain tissue. No such EAA release from surrounding brain areas was demonstrated when the cavity was filled with isolated but non-contused brain tissue. The increase in EAAs was attenuated by KYN administered through microdialysis, suggesting that the EAA release from the surrounding brain areas appears to be a consequence that is secondary to the EAA release from the contused brain tissue. Such a diffusion-reaction process is probably mediated by the neurotransmitter actions of EAAs. The results of the present study are of clinical importance, since surgical removal of contused brain tissue and administration of EAA antagonists may serve to protect the surrounding brain areas from EAA neurotoxicity.

The nootropic compound BMY-21502 improves spatial learning ability in brain injured rats

Although long-lasting cognitive dysfunction often follows clinical traumatic brain injury (TBI), few pharmacologic regimens have been developed to treat post-traumatic cognitive deficits. We have previously shown that, in the rat, experimental lateral fluid-percussion (FP) brain injury induces a profound impairment in retrograde memory. In the present study, we characterized alterations in the ability of rats to learn a novel task following lateral FP brain injury and examined the potential modulatory effects of the nootropic cognitive enhancer BMY-21502 on post-injury learning. Male Sprague-Dawley rats were subjected to lateral (parasagittal) FP brain injury of moderate severity (2.4 atm) or sham surgery (no injury). On days 7 and 8 post-injury, animals were tested in a Morris water maze for their ability to learn to navigate to a submerged, invisible platform using external visual cues. BMY-21502 (10 mg/kg) or vehicle was administered 30 min prior to the first trial on both days. A highly significant (P < 0.001) impairment in post-injury learning was observed in vehicle-treated brain-injured animals compared with vehicle-treated sham animals. Injured animals treated with BMY-21502 at one week post-injury showed significantly (P < 0.05) improvement in post-injury learning ability compared to injured animals treated with vehicle. Paradoxically, in uninjured control animals BMY-21502 treatment appeared to worsen learning scores. The results of this study indicate that BMY-21502 may be useful for attenuating the dysfunction in learning ability that occurs following TBI.

Concussive brain injury is associated with a prolonged accumulation of calcium: a 45Ca autoradiographic study

In order to determine the extent and duration of calcium (Ca2+) flux following a lateral fluid percussion brain injury in the rat, 45Ca autoradiography was used to study animals immediately, 6, 24 and 96 h after the insult. In addition, cell suspension studies were conducted to determine the extent of cellular flux of 45Ca. Optical density and/or scintillation counting was utilized to provide a relative measure of 45Ca accumulation within 20 different structures. The results indicated that in animals who exhibited no gross morphological damage, 45Ca accumulation following injury was exhibited primarily within the ipsilateral cerebral cortex, dorsal hippocampus and striatum. This accumulation continued for several days returning to control levels by the 4th day after injury. In animals who sustained morphological damage, the contusion site exhibited a marked accumulation of 45Ca which did not resolve spontaneously over the course of 4 days. We conclude from this work that Ca2+ flux is a major component of this experimental model of traumatic injury. Furthermore, that depending on the extent of cell damage, the accumulation of Ca2+ is regionally different. Finally, that even in an injury which by itself does not produce gross morphological tissue damage, accumulation of Ca2+ can continue for at least 48 h.

Novel pharmacologic therapies in the treatment of experimental traumatic brain injury: a review

Delayed or secondary neuronal damage following traumatic injury to the central nervous system (CNS) may result from pathologic changes in the brain's endogenous neurochemical systems. Although the precise mechanisms mediating secondary damage are poorly understood, posttraumatic neurochemical changes may include overactivation of neurotransmitter release or re-uptake, changes in presynaptic or postsynaptic receptor binding, or the pathologic release or synthesis of endogenous "autodestructive" factors. The identification and characterization of these factors and the timing of the neurochemical cascade after CNS injury provides a window of opportunity for treatment with pharmacologic agents that modify synthesis, release, receptor binding, or physiologic activity with subsequent attenuation of neuronal damage and improvement in outcome. Over the past decade, a number of studies have suggested that modification of postinjury events through pharmacologic intervention can promote functional recovery in both a variety of animal models and clinical CNS injury. This article summarizes recent work suggesting that pharmacologic manipulation of endogenous systems by such diverse pharmacologic agents as anticholinergics, excitatory amino acid antagonists, endogenous opioid antagonists, catecholamines, serotonin antagonists, modulators of arachidonic acid, antioxidants and free radical scavengers, steroid and lipid peroxidation inhibitors, platelet activating factor antagonists, anion exchange inhibitors, magnesium, gangliosides, and calcium channel antagonists may improve functional outcome after brain injury.

Introducing NMDA antagonists into clinical practice: why head injury trials?

1. Head injury is the major cause of death and severe disability in young adults. Evidence from clinical studies shows ischaemic brain damage to be the major determinant of bad outcome, and that a proportion of this (perhaps up to 40%) is delayed, thus offering an opportunity for 'prophylactic' therapy. 2. Laboratory studies in several relevant animal models of human head injury (fluid percussion, subdural haematoma, and focal ischaemia by middle cerebral occlusion) indicate that excitatory amino acids are important mediators of brain damage. Pretreatment with NMDA antagonists has shown that the extent of ischaemic damage may be dramatically reduced in these models (68% reduction in the cat MCA occlusion model, 54% in the rat subdural haematoma model). 3. Trials of NMDA antagonists in human head injury are therefore strongly indicated.

Hippocampal CA3 lesion prevents postconcussive metabolic dysfunction in CA1

Immediately following fluid-percussion (F-P) brain injury, the hippocampus exhibits a marked increase in its local CMRglc (LCMRglc; mumol/100 g/min) as determined using [14C]2-deoxy-D-glucose autoradiography. This injury-induced increase in metabolism is followed in 6 h by a subsequent decrease in LCMRglc. These two postinjury metabolic states may be the result of ionic disruptions following trauma via stimulation of glutamate-gated ion channels. To determine if endogenous glutamate innervation to the CA1 region of the hippocampus can provide an anatomical basis for this proposed mechanism, it was removed by kainic-acid-induced destruction of CA3, and the effect on CA1 metabolism following concussive injury was studied. Five days before a lateral F-P injury (3.5-4.5 atm), kainic acid (0.5 microgram) or vehicle was stereotaxically injected into the left ventricle of 65 rats. Histological inspection indicated that kainic acid produced severe cell loss primarily in the CA3 region of the hippocampus ipsilateral to the injection. The metabolic results indicated that immediately following injury, animals with an intact hippocampus exhibited an increase in LCMRglc to 84.6 +/- 5 within the CA1 region, representing a 81.5% increase over controls. However, in the CA3-lesioned animals, CA1 showed no evidence of an injury-induced hypermetabolism, with LCMRglc remaining at control levels (51.4 +/- 3.9). At 6 h postinjury, the intact hippocampus exhibited a reduction of LCMRglc to rates of 40.7 +/- 4.7 within the CA1 region, representing a 17.9% reduction compared with controls. In contrast, CA3-lesioned animals exhibited less of an injury-induced decrease in LCMRglc within the CA1 region, exhibiting a mean rate of 43.4 +/- 4.5, representing only a 12.5% reduction compared with controls. These results indicate that the removal of the CA3 projection to CA1 protects the CA1 cells from the metabolic dysfunction typically seen following injury. This supports our previous work indicating the important role glutamate plays in the ionic flux and subsequent metabolic changes that follow traumatic brain injury.