Microdialysis of interstitial amino acids during spreading depression and anoxic depolarization in rat neocortex

Effect of systemic kynurenine on cortical spreading depression and its modulation by sex hormones in rat

BACKGROUND

The aura symptoms in migraine are most likely due to cortical spreading depression (CSD). CSD is favored by NMDA receptor activation and increased cortical excitability. The latter probably explains why migraine with aura may appear when estrogen levels are high, like during pregnancy. Kynurenic acid, a derivative of tryptophan metabolism, is an endogenous NMDA receptor antagonist whose cerebral concentrations can be augmented by systemic administration of its precursor L-kynurenine.

OBJECTIVE

To determine if exogenous administration of L-kynurenine is able to influence KCl-induced CSD in rat, if the effect is sex-dependent and if it differs in females between the phases of the estrous cycle.

METHODS

Adult Sprague-Dawley rats (n=8/group) received intraperitoneal (i.p.) injections of L-kynurenine (L-KYN, 300 mg/kg), L-KYN combined with probenecid (L-KYN+PROB) that increases cortical concentration of KYNA by blocking its excretion from the central nervous system, probenecid alone (PROB, 200 mg/kg) or NaCl. Cortical kynurenic acid concentrations were determined by HPLC (n=7). Thirty minutes after the injections, CSDs were elicited by application of 1M KCl over the occipital cortex and recorded by DC electrocorticogram. In NaCl and L-KYN groups, supplementary females were added and CSD frequency was analyzed respective to the phases of the estrous cycle determined by vaginal smears.

RESULTS

In both sexes, PROB, L-KYN and L-KYN+PROB increased cortical kynurenic acid level. PROB, L-KYN and L-KYN+PROB with increasing potency decreased CSD frequency in female rats, while in males such an effect was significant only for L-KYN+PROB. The inhibitory effect of L-KYN on CSD frequency in females was most potent in diestrus.

CONCLUSION

L-Kynurenine administration suppresses CSD, most likely by increasing kynurenic acid levels in the cortex. Females are more sensitive to this suppressive effect of L-kynurenine than males. These results emphasize the role of sex hormones in migraine and open interesting novel perspectives for its preventive treatment.

Synaptic release and extracellular actions of Zn2+ limit propagation of spreading depression and related events in vitro and in vivo

Cortical spreading depression (CSD) is a consequence of a slowly propagating wave of neuronal and glial depolarization (spreading depolarization; SD). Massive release of glutamate contributes to SD propagation, and it was recently shown that Zn(2+) is also released from synaptic vesicles during SD. The present study examined consequences of extracellular Zn(2+) accumulation on the propagation of SD. SD mechanisms were studied first in murine brain slices, using focal KCl applications as stimuli and making electrical and optical recordings in hippocampal area CA1. Elevating extracellular Zn(2+) concentrations with exogenous ZnCl(2) reduced SD propagation rates. Selective chelation of endogenous Zn(2+) (using TPEN or CaEDTA) increased SD propagation rates, and these effects appeared due to chelation of Zn(2+) derived from synaptic vesicles. Thus, in tissues where synaptic Zn(2+) release was absent [knockout (KO) of vesicular Zn(2+) transporter ZnT-3], SD propagation rates were increased, and no additional increase was observed following chelation of endogenous Zn(2+) in these tissues. The role of synaptic Zn(2+) was then examined on CSD in vivo. ZnT-3 KO animals had higher susceptibility to CSD than wild-type controls as evidenced by significantly higher propagation rates and frequencies. Studies of candidate mechanisms excluded changes in neuronal excitability, presynaptic release, and GABA receptors but left open a possible contribution of N-methyl-d-aspartate (NMDA) receptor inhibition. These results suggest the extracellular accumulation of synaptically released Zn(2+) can serve as an intrinsic inhibitor to limit SD events. The inhibitory action of extracellular Zn(2+) on SD may counteract to some extent the neurotoxic effects of intracellular Zn(2+) accumulation in acute brain injury models.

Kynurenine metabolites and migraine: experimental studies and therapeutic perspectives

Migraine is one of the commonest neurological disorders. Despite intensive research, its exact pathomechanism is still not fully understood and effective therapy is not always available. One of the key molecules involved in migraine is glutamate, whose receptors are found on the first-, second- and third-order trigeminal neurones and are also present in the migraine generators, including the dorsal raphe nucleus, nucleus raphe magnus, locus coeruleus and periaqueductal grey matter. Glutamate receptors are important in cortical spreading depression, which may be the electrophysiological correlate of migraine aura. The kynurenine metabolites, endogenous tryptophan metabolites, include kynurenic acid (KYNA), which exerts a blocking effect on ionotropic glutamate and α7-nicotinic acetylcholine receptors. Thus, KYNA and its derivatives may act as modulators at various levels of the pathomechanism of migraine. They can give rise to antinociceptive effects at the periphery, in the trigeminal nucleus caudalis, and may also act on migraine generators and cortical spreading depression. The experimental data suggest that KYNA or its derivatives might offer a novel approach to migraine therapy.

Activation of central trigeminovascular neurons by cortical spreading depression

OBJECTIVE

Cortical spreading depression (CSD) has long been implicated in migraine attacks that begin with visual aura. Having shown that a wave of CSD can trigger long-lasting activation of meningeal nociceptors--the first-order neurons of the trigeminovascular pathway thought to underlie migraine headache--we now report that CSD can activate central trigeminovascular neurons in the spinal trigeminal nucleus (C1-2).

METHODS

Stimulation of the cortex with pinprick or KCl granule was used to induce CSD in anesthetized rats. Neuronal activity was monitored in C1-2 using single-unit recording.

RESULTS

In 25 trigeminovascular neurons activated by CSD, mean firing rate (spikes/s) increased from 3.6 ± 1.2 before CSD (baseline) to 6.1 ± 1.8 after CSD (p < 0.0001) for a period >13 minutes. Neuronal activity returned to baseline level after 30.0 ± 3.1 minutes in 14 units, and remained elevated for 66.0 ± 8.3 (22-108) minutes through the entire recording period in the other 11 units. Neuronal activation began within 0.9 ± 0.4 (0-2.5) minutes after CSD in 7 neurons located in laminae I-II, or after a latency of 25.1 ± 4.0 (7-75) minutes in 9 neurons located in laminae I-II, and 9 neurons located in laminae III-V. In 27 trigeminovascular neurons not activated by CSD, mean firing rate was 2.0 ± 0.7 at baseline and 1.8 ± 0.7 after CSD.

INTERPRETATION

We propose that CSD constitutes a nociceptive stimulus capable of activating peripheral and central trigeminovascular neurons that underlie the headache of migraine with aura.

Spreading depression and related events are significant sources of neuronal Zn2+ release and accumulation

Spreading depression (SD) involves coordinated depolarizations of neurons and glia that propagate through the brain tissue. Repetitive SD-like events are common following human ischemic strokes, and are believed to contribute to the enlargement of infarct volume. Accumulation of Zn(2+) is also implicated in ischemic neuronal injury. Synaptic glutamate release contributes to SD propagation, and because Zn(2+) is costored with glutamate in some synaptic vesicles, we examined whether Zn(2+) is released by SD and may therefore provide a significant source of Zn(2+) in the postischemic period. Spreading depression-like events were generated in acutely prepared murine hippocampal slices by deprivation of oxygen and glucose (OGD), and Zn(2+) release was detected extracellularly by a Zn(2+)-selective indicator FluoZin-3. Deprivation of oxygen and glucose-SD produced large FluoZin-3 increases that propagated with the event, and signals were abolished in tissues from ZnT3 knockout animals lacking synaptic Zn(2+). Synaptic Zn(2+) release was also maintained with repetitive SDs generated by microinjections of KCl under normoxic conditions. Intracellular Zn(2+) accumulation in CA1 neurons, assessed using microinjection of FluoZin-3, showed significant increases following SD that was attributed to synaptic Zn(2+) release. These results suggest that Zn(2+) is released during SDs and could provide a significant source of Zn(2+) that contributes to neurodegeneration in the postischemic period.

Clinical relevance of cortical spreading depression in neurological disorders: migraine, malignant stroke, subarachnoid and intracranial hemorrhage, and traumatic brain injury

Cortical spreading depression (CSD) and depolarization waves are associated with dramatic failure of brain ion homeostasis, efflux of excitatory amino acids from nerve cells, increased energy metabolism and changes in cerebral blood flow (CBF). There is strong clinical and experimental evidence to suggest that CSD is involved in the mechanism of migraine, stroke, subarachnoid hemorrhage and traumatic brain injury. The implications of these findings are widespread and suggest that intrinsic brain mechanisms have the potential to worsen the outcome of cerebrovascular episodes or brain trauma. The consequences of these intrinsic mechanisms are intimately linked to the composition of the brain extracellular microenvironment and to the level of brain perfusion and in consequence brain energy supply. This paper summarizes the evidence provided by novel invasive techniques, which implicates CSD as a pathophysiological mechanism for this group of acute neurological disorders. The findings have implications for monitoring and treatment of patients with acute brain disorders in the intensive care unit. Drawing on the large body of experimental findings from animal studies of CSD obtained during decades we suggest treatment strategies, which may be used to prevent or attenuate secondary neuronal damage in acutely injured human brain cortex caused by depolarization waves.

Migraine is a neuronal disease

Migraine is a common, paroxysmal, highly disabling primary headache disorder with a genetic background. The primary cause and the origin of migraine attacks are enigmatic. Numerous clinical and experimental results suggest that activation of the trigeminal system (TS) is crucial in its pathogenesis, but the primary cause of this activation is not fully understood. Since activation of the peripheral and central arms of the TS might be related to cortical spreading depression and to the activity of distinct brainstem nuclei (e.g. the periaqueductal grey), we conclude that migraine can be explained as an altered function of the neuronal elements of the TS, the brainstem, and the cortex, the centre of this process comprising activation of the TS. In light of our findings and the literature data, therefore, we can assume that migraine is mainly a neuronal disease.

Changes in membrane potential and the intracellular calcium concentration during CSD and OGD in layer V and layer II/III mouse cortical neurons

Cortical spreading depression (CSD) is an episode of electrical silence following intense neuronal activity that propagates across the cortex at ∼3-6 mm/min and is associated with transient neuronal depolarization. CSD is benign in normally perfused brain tissue, but there is evidence suggesting that repetitive CSD contributes to infarct growth following focal ischemia. Studies to date have assumed that the cellular responses to CSD are uniform across neuronal types because there are no data to the contrary. In this study, we investigated the effect of CSD on membrane potential and the intracellular calcium concentration ([Ca(2+)](i)) of mouse layer V and layer II/III pyramidal neurons in brain slices. To place the data in context, we made similar measurements during anoxic depolarization induced by oxygen and glucose deprivation (OGD). The [Ca(2+)](i) was quantified using the low-affinity ratiometric indicator Fura-4F. During both CSD- and OGD-induced depolarization, the membrane potential approached 0 mV in all neurons. In layer V pyramids OGD resulted in an increase in [Ca(2+)](i) to a maximum of 3.69 ± 0.73 (SD) μM (n = 12), significantly greater than the increase to 1.81 ± 0.70 μM in CSD (n = 34; P < 0.0001). Membrane potential and [Ca(2+)](i) returned to nearly basal levels following CSD but not OGD. Layer II/III neurons responded to CSD with a greater peak increase in [Ca(2+)](i) than layer V neurons (2.88 ± 0.6 μM; n = 9; P < 0.01). We conclude there is a laminar difference in the response of pyramidal neurons to CSD; possible explanations are discussed.

Activation of meningeal nociceptors by cortical spreading depression: implications for migraine with aura

Attacks of migraine with aura represent a phenomenon in which abnormal neuronal activity in the cortex produces sensory disturbances (aura) some 20-40 min before the onset of headache. The purpose of this study was to determine whether cortical spreading depression (CSD)--an event believed to underlie visual aura--can give rise to activation of nociceptors that innervate the meninges--an event believed to set off migraine headache. CSD was induced in anesthetized male rats by stimulation of the visual cortex with electrical pulses, pin prick, or KCl; single-unit activity of meningeal nociceptors was monitored in vivo in the rat before and after CSD. Regardless of the method of cortical stimulation, induction of CSD was recorded in 64 trials. In 31 of those trials, CSD induced a twofold increase in meningeal nociceptor firing rate that persisted for 37.0 +/- 4.6 min in trials in which activity returned to baseline, or >68 min in trials in which activity remained heightened at the time recording was interrupted. In two-thirds of the trials, onset of long-lasting neuronal activation began approximately 14 min after the wave of CSD. The findings demonstrates for the first time that induction of CSD by focal stimulation of the rat visual cortex can lead to long-lasting activation of nociceptors that innervate the meninges. We suggest that migraine with aura is initiated by waves of CSD that lead up to delayed activation of the trigeminovascular pathway.

Cortical spreading depression-new insights and persistent questions

Since its original extensive description by Leao in 1944, thousands of publications have characterized the phenomenon of cortical spreading depression (CSD). Despite the attention that CSD has received over more than six decades, however, many fundamental questions regarding its initiation, propagation, functional consequences, and relationship to migraine and other human disorders remain unanswered. Advances in genetics and cellular imaging have led to important insights into the basic mechanisms of CSD, with increasing attention focused on specific neuronal ion channels, neurotransmitters and neuromodulators. In addition, there is growing recognition that astrocytes and the vasculature may play an active, rather than simply a passive or reactive role in CSD. Several recent descriptions of CSD in humans in the setting of brain injury provide definitive evidence that this phenomenon can occur and have important functional consequences in the human brain. Although the exact role of CSD in migraine has yet to be conclusively established, there is strong evidence that the investigation of CSD in animal models can provide meaningful information about migraine that can be translated into the clinical setting. This review will briefly address the extensive work that has been done on CSD over more than half a century, but focus primarily on more recent studies with a particular emphasis on relevance to migraine.

The human brain utilizes lactate via the tricarboxylic acid cycle: a 13C-labelled microdialysis and high-resolution nuclear magnetic resonance study

Energy metabolism in the human brain is not fully understood. Classically, glucose is regarded as the major energy substrate. However, lactate (conventionally a product of anaerobic metabolism) has been proposed to act as an energy source, yet whether this occurs in man is not known. Here we show that the human brain can indeed utilize lactate as an energy source via the tricarboxylic acid cycle. We used a novel combination of (13)C-labelled cerebral microdialysis both to deliver (13)C substrates into the brain and recover (13)C metabolites from the brain, and high-resolution (13)C nuclear magnetic resonance. Microdialysis catheters were placed in the vicinity of focal lesions and in relatively less injured regions of brain, in patients with traumatic brain injury. Infusion with 2-(13)C-acetate or 3-(13)C-lactate produced (13)C signals for glutamine C4, C3 and C2, indicating tricarboxylic acid cycle operation followed by conversion of glutamate to glutamine. This is the first direct demonstration of brain utilization of lactate as an energy source in humans.

Ischemia independent lesion evolution during focal stroke in rats

Lesion evolution during focal cerebral ischemia may depend on flow restrictions or on accumulation of toxic mediators within the infarct and expansion of these factors to the periinfarct region. So far, the precise contribution of flow dependent versus spreading-mediated impairment of viable periinfarct tissue has not been determined. Therefore, we measured lesion expansion, flow restrictions and glutamate distribution on serial brain sections at different time points after experimental focal ischemia. Permanent focal ischemia was induced by occlusion of the right middle cerebral artery in male rats and the flow reduction was subsequently measured at 1, 12 and 24 h using iodo[14C]antipyrine autoradiography. Additionally, the necrotic volume was determined on serial brain sections and the glutamate content was measured in tissue samples from adjacent microdissections. Twelve hours after focal ischemia no noteworthy viable areas with blood flow restrictions of 20-40 ml 100 g(-1) min(-1) existed but at 24 h the necrotic tissue exceeded the hemodynamically compromised region by 40 +/- 21 mm3 (24%). Furthermore, at 12 and 24 h the glutamate content was elevated in areas surrounding the infarct. Relevant flow restrictions are detectable only during early stages of infarct maturation, whereas the propagation of secondary factors may be the predominant mechanism for delayed infarct evolution.

Mechanisms involved in the cerebrovascular dilator effects of cortical spreading depression

Cortical spreading depression (CSD) leads to dramatic changes in cerebral hemodynamics. However, mechanisms involved in promoting and counteracting cerebral vasodilator responses are unclear. Here we review the development and current status of this important field of research especially with respect to the role of perivascular nerves and nitric oxide (NO). It appears that neurotransmitters released from the sensory and the parasympathetic nerves associated with cerebral arteries, and NO released from perivascular nerves and/or parenchyma, promote cerebral hyperemia during CSD. However, the relative contributions of each of these factors vary according to species studied. Related to CSD, axonal and reflex responses involving trigeminal afferents on the pial surface lead to increased blood flow and inflammation of the overlying dura mater. Counteracting the cerebral vascular dilation is the production and release of constrictor prostaglandins, at least in some species, and other possibly yet unknown agents from the vascular wall. The cerebral blood flow response in healthy human cortex has not been determined, and thus it is unclear whether the cerebral oligemia associated with migraines represents the normal physiological response to a CSD-like event or represents a pathological response. In addition to promoting cerebral hyperemia, NO produced during CSD appears to initiate signaling events which lead to protection of the brain against subsequent ischemic insults. In summary, the cerebrovascular response to CSD involves multiple dilator and constrictor factors produced and released by diverse cells within the neurovascular unit, with the contribution of each of these factors varying according to the species examined.

Induction of neurogenesis in the adult dentate gyrus by cortical spreading depression

BACKGROUND AND PURPOSE

Spreading depression (SD) is an epiphenomenon of neurological disorders, like stroke or traumatic brain injury. These diseases have been associated with an increased neurogenesis in the adult rodent dentate gyrus. Such proliferative activity can also be induced by conditions that--like SD--coincide with a disturbed neuronal excitability, eg, epilepsy. Thus we hypothesized that SD might likewise influence hippocampal neurogenesis and potentially act as mediator of injury-induced neurogenesis.

METHODS

Repetitive cortical SD were induced by epidural application of 3 mol/L KCl. At different time points thereafter dentate gyrus neurogenesis was investigated by means of intraperitoneal bromodeoxyuridine injections and immunocytochemistry. Spatial learning and memory was tested in a Morris water maze.

RESULTS

Cortical SD significantly increased proliferative activity in the ipsilateral subgranular zone on days 2 and 4. We detected about 280% more newborn cells in the dentate gyrus of rats that received bromodeoxyuridine during the first week after SD and were allowed to recover for 6 weeks. Most of these cells expressed the mature neuronal marker NeuN. The mitogenic action of SD was suppressed by systemic administration of the NMDA receptor antagonist MK-801. Behavioral performance of SD animals in the Morris water maze did not improve significantly.

CONCLUSIONS

From our data we postulate that the increased dentate gyrus neurogenesis observed after brain injury may at least partly be mediated by SD-like epiphenomena. Furthermore they indicate that even a strongly enhanced dentate gyrus neurogenesis may occur without significant improvements in hippocampus-dependent spatial learning and memory.

Métabolisme énergétique et agression cérébrale

Brain energy metabolism and signal transduction are intimely intricated. At the cellular level this is reflected by the interdependent metabolism of glutamate and glucose and the energetic compartmentalization between astrocytic glycolysis and neuronal metabolism. Astrocytes appear to have a particular importance in brain metabolism by regulating microcirculation and the repartition of energetic substrates in function of synaptic activity. The high level of O(2) consumption compared to the mass of tissue confers a particular vulnerability of brain to oxidative stress. The synthesis of glutathione, the main anti-oxidant of brain, appears to be dependent of the regulation of synaptic glutamate concentration by astrocytes. Deficiencies of astrocytes functions appear to play a key role in the physiopathology of brain injury.

Role of glutamate transporters in the clearance and release of glutamate during ischemia and its relation to neuronal death

Glutamate neurotransmitter action on postsynaptic receptors is terminated by its clearance from the synaptic cleft by transporter proteins located in neurons and glial cells. Failure of glutamate removal can lead to neuronal death due to its well-known neurotoxic properties. Glutamate transporters are dependent on external Na+, and thus on the activity of Na+/K+ ATPases, which maintain the Na+ concentration gradient. When the energy brain requirements are not fulfilled by the appropriate blood supply of glucose and oxygen, the Na+ gradient collapses leading to impaired glutamate and aspartate removal, or even to the release of these amino acids through the reverse operation of their transporters. Such a scenario would be associated with brain ischemia and hypoglycemia due to the prompt decline in ATP levels. In addition, some evidence suggests that downregulation of glutamate transporters after the ischemic period, or the dysfunction induced by oxidation, contributes to the accumulation of extracellular glutamate and neuronal death. Neuronal damage is associated with excitotoxicity, a type of cell death triggered by the overactivation of glutamate receptors and the loss of calcium homeostasis. Throughout this review we will discuss recent evidence suggesting that failure of glutamate transport during ischemia contributes to the elevation of extracellular glutamate and to the induction of excitotoxicity. We will also discuss the contribution of glial vs. neuronal glutamate transporters in ischemic damage, and the involvement of the different glutamate transporter subtypes. We will focus on experimental data from rodent models, because many of the studies on glutamate transport and ischemic damage have been performed in these animal species.

A role for glutamate and glia in the fast network oscillations preceding spreading depression

The mechanism of the propagation of spreading depression is unclear. Classical theories proposed a self-maintained cycle fed by elevated potassium and/or glutamate in the extracellular space. Earlier we found in vivo a characteristic oscillatory field activity that is synchronous in a strip of tissue ahead of the oncoming wave of neuron depolarization and that occurs before the extracellular potassium level begins to rise [Herreras O, Largo C, Ibarz JM, Somjen GG, Marrín del Río R (1994) Role of neuronal synchronizing mechanisms in the propagation of spreading depression in the in vivo hippocampus. J Neurosci 14:7087-7098]. We investigated here the possible participation of glutamate and the role of glia in the prodromal field oscillations using extra and intracellular recordings and pharmacological manipulations in rat hippocampal slices. As earlier shown in vivo, field oscillations propagated ahead of the negative potential shift covering distances of up to 1 mm. The oscillatory prodromals were initially subthreshold but then each wave became crowned by a population spike. The frequency of the oscillatory prodromals was variable among slices (80-115 Hz), but constant in individual slices. The blockade of ionotropic glutamate receptors decreased the frequency of prodromal oscillations, retarded spreading depression propagation, and shortened the duration of depolarization. Blocking the glutamate membrane transport increased the oscillatory frequency. The selective metabolic poisoning of astrocytes led to gradual disorganization of prodromal oscillations whose frequency first increased and then decreased. Also, the amplitude of the population spikes within the burst diminished as individual cells fired fewer action potentials, although still phase-locked with population spikes. The effects of glial metabolic impairment were observed within the period when neuron electrical properties were still normal, and were blocked by glutamate receptor antagonists. These data suggest that glutamate released from glial cells and possibly also from neurons has a role in the generation of oscillations and neuron firing synchronization that precede the spreading depression-related depolarization, but additional mechanisms are required to fully explain the onset and propagation of spreading depression.

Effects of delayed intrathecal infusion of an NMDA receptor antagonist on ischemic injury and peri-infarct depolarizations

The potent NMDA receptor antagonist, Conantokin-G (CGX-1007), a snail peptide, has an 8-h therapeutic window in rat focal cerebral ischemia. We hypothesized that the mechanism of neuroprotection is the inhibition of 'secondary phase' peri-infarct depolarizations (PIDs), recently shown to recur 6--24 h post-reperfusion. Rats were implanted with intrathecal (i.t.) catheters for drug delivery and DC-compatible electrodes for continuous PID monitoring and subjected to transient (2 h) middle cerebral artery occlusion. Four groups were studied. In two groups (C(40)C and C(20)C), continuous infusion of CGX--1007 was administered over 8--24 h post-occlusion at 0.1 microg/h (0.04 nmol/h) following either a 40- or 20-nmol bolus dose at 8 h. Another group (C(40)S) received the 40-nmol bolus followed by saline infusion, and a control group received saline. Intrathecal drug treatment reduced infarct volumes relative to controls by 61%, 31%, and 10% in C(40)C, C(40)S, and C(20)C groups, respectively, but also induced dose-dependent paralysis and elevated mortality. All rats had PID rates similar to the control group prior to treatment, but following treatment secondary phase PIDs were reduced by 47--57% in each drug group compared to controls. Because several animals exhibited PID inhibition but no neuroprotection, there was no significant correlation between these endpoints across groups. However, drug-treated animals that did not exhibit secondary phase PIDs prior to treatment had significantly smaller infarcts and reduced subsequent PID activity than corresponding control rats. Results suggest that post-reperfusion PIDs play a substantial, though still undefined pathogenic role in delayed maturation of cerebral infarction and NMDA receptor-targeted neuroprotection.

Interstitial concentrations of amino acids in the rat striatum during global forebrain ischemia and potassium-evoked spreading depression

The early detection and appropriate treatment of brain ischemia is of paramount importance. The interstitial concentrations of neurotransmitter amino acids are often used as an index of neuronal injury. However, monitoring of non-neurotransmitter amino acids may be equally important. We have studied the behavior of 10 amino acids during K(+)-induced spreading depression (application of 70 mM KCl during 40 min) and global forebrain ischemia (two-vessel occlusion with hypotension during 20 min). The concentrations of glutamate, aspartate, taurine, GABA, glycine, and alanine, measured in the rat striatum by microdialysis, increased during both ischemia and spreading depression, whereas glutamine concentrations decreased in both cases. Only ischemia, but not spreading depression, led to enhanced release of serine, threonine, and asparagine. We thus conclude that an elevation in the interstitial concentrations of non-neurotransmitter amino acids is specific to deep ischemic injury to nervous tissue. We propose the monitoring of serine, asparagine, and threonine, together with excitatory amino acids, as an index of the degree of ischemic brain injury.

Compromised metabolic recovery following spontaneous spreading depression in the penumbra

Spreading depression (SD) has been demonstrated following focal ischemia, and the additional workload imposed by SD on a tissue already compromised by a marked reduction in blood flow may contribute to the evolution of irreversible damage in the ischemic penumbra. SD was elicited in one group of rats by injecting KCl directly into a frontal craniectomy and the wave of depolarization was recorded in two craniectomies 3 and 6 mm posterior to the first one. In a second group, the middle cerebral artery was occluded using the monofilament technique and a recording electrode was placed 5 mm lateral to the midline and 0.2 mm posterior to bregma. To determine the metabolic response in the penumbral region of the cortex ipsilateral to the occlusion, brains from both groups were frozen in situ when the deflection of the SD was maximal. The spatial metabolic response of SD in the ischemic cortex was compared to that in the non-ischemic cortex. Coronal sections of the brains were lyophilized, pieces of the dorsolateral cortex were dissected and weighed, and analyzed for ATP, P-creatine, inorganic phosphate (Pi), glucose, glycogen and lactate at varying distances anterior and posterior to the recording electrode. ATP and P-creatine levels were significantly decreased at the wavefront in both groups and the levels recovered after passage of the wavefront in the normal brain, but not in the ischemic brain. Glucose and glycogen levels were significantly decreased and lactate levels significantly increased in the tissue after the passage of the wavefront. While the changes in the glucose-related metabolites persisted during recovery even in anterior portions of the cortex in both groups in the aftermath of the SD, the magnitude of the changes was greater in the penumbra than in the normal cortex. SD appears to impose an equivalent increase in energy demands in control and ischemic brain, but the ability of the penumbra to recover from the insult is compromised. Thus, increasing the energy imbalance in the penumbra after multiple SDs may hasten the deterioration of the energy status of the tissue and eventually contribute to terminal depolarization and cell death, particularly in the penumbra.

Slow death in the leopard frogRana pipiens: neurotransmitters and anoxia tolerance

While frogs such as Rana temporaria are known to withstand 4-5 h anoxia at room temperature, little is known about the neurological adaptations that permit this. Previous research has shown that changes in neuroactive compounds such as glutamate and dopamine in anoxia-sensitive (mammalian)brains follow a strikingly different pattern than is observed in truly anoxia-tolerant vertebrates such as the freshwater turtle. The present study measured changes in the levels of whole brain and extracellular amino acids,and extracellular dopamine, in the normoxic and 3-4 h anoxic frog Rana pipiens, in order to determine whether their neurotransmitter responses resemble the anoxia-vulnerable or anoxia-tolerant response. Increases in whole brain serine, glycine, alanine and GABA levels were similar to those seen in anoxia-tolerant species, although the levels of glutamine, taurine and glutamate did not increase as occurs in true facultative anaerobes. Extracellular levels of aspartate, taurine and GABA also increased significantly, while glutamate levels decreased. The maintenance of low extracellular glutamate was the most significant difference between the frog and the anoxic/ischemic mammalian brain, although aspartate did increase 215%over a 4 h period of anoxia. A 12-fold increase in extracellular dopamine levels during anoxia was the biggest contrast between anoxia-tolerant vertebrates and R. pipiens. The frog could thus be an interesting model in which to examine the mechanisms of dopamine failure in early anoxia,which occurs rapidly in the mammal but over a period of hours in the `slow death' of the anoxic frog brain.

Uptake of glutamate is impaired in the cortical penumbra of the rat following middle cerebral artery occlusion: an in vivo microdialysis extraction study

By using microdialysis extraction of (3)H-D-aspartate and concomitant recordings of extracellular direct current (DC) potentials, the effect of middle cerebral artery occlusion (MCAO) was studied continuously over a period of 100 min in the cerebral cortex of rats. From analysis of the DC potentials, rats subjected to MCAO could be divided into three groups, one in which the dialysis probe was located in the ischemic core, one in which the probe was in the penumbra, and one in which the probe was in nonischemic tissue. In general, extraction of (3)H-D-aspartate was positively correlated with the DC potential; i.e., changes in the extraction were concurrent with changes in the DC potential. Comparing the different animal groups by integration of all extraction values obtained during MCAO over time, (3)H-D-aspartate extraction was reduced by 40% in the penumbra, and by 58% in the ischemic core, compared with the sham-operated controls. No changes was found in the nonischemic group. In the penumbra group, extraction of (3)H-D-aspartate was reduced initially upon institution of MCAO but recovered to control-like levels over a period of 15-40 min, despite ongoing MCAO. In addition, extraction was reduced transiently during periinfarct depolarizations. A mean of all extraction values obtained during MCAO in the penumbra group was reduced by 47% compared with a mean of values obtained before institution of MCAO. Induction of death resulted in a reduction of (3)H-D-aspartate extraction by 86%. The present results provide direct evidence that uptake of Glu is reduced both in the ischemic core and in the penumbra of the cerebral cortex following MCAO in rats, possibly contributing to the initiation and spread of infarction. The results further indicate that uptake of Glu in the penumbra recovers to control-like levels, despite ongoing MCAO, providing evidence that Glu uptake by the Glu transporter proteins is reinstituted and/or up-regulated.

Cortical spreading depression in the feline brain following sustained and transient stimuli studied using diffusion-weighted imaging

Cortical spreading depression (CSD) was induced by transient (10 min) applications of KCl in agar upon the cortical surface of alpha-chloralose anaesthetised cats. Its features were compared with CSD resulting from sustained applications of crystalline KCl through a mapping of the apparent diffusion coefficient (ADC) using diffusion-weighted echo planar imaging (DWI) over a poststimulus period of 60-100 min. Individual CSD events were computationally detected with the aid of Savitzky-Golay smoothing applied to critically sampled data derived from regions of interest (ROIs) made up of 2 x 2 pixel matrices. The latter were consistently placed at three selected sites on the suprasylvian gyrus (SG) and six sites on the marginal gyrus (MG). The CSD events thus detected were then quantitatively characterised for each ROI using the original time series. Both stimuli consistently elicited similar spreading patterns of initial, primary CSD events that propagated over the SG and marginal MG and were restricted to the hemispheres on which the stimuli were applied. There followed secondary events over smaller extents of cortical surface. Sustained stimuli elicited primary and secondary CSD events with similar amplitudes of ADC deflection that were distributed around a single mean. The ADC deflections were also conserved in peak amplitude throughout the course of their propagation. The initial primary event showed a poststimulus latency of 1.1 +/- 0.1 min. Successive secondary events followed at longer, but uniform, time intervals of around 10 min. Primary and secondary CSDs showed significantly different velocities of conduction (3.32 +/- 0.43 mm min(-1) vs. 2.11 +/- 0.21 mm min(-1), respectively; n = 5) across the cerebral hemisphere. In contrast, transient stimuli produced significantly fewer numbers of CSD events (3.8 +/- 0.5 events per animal, n = 5) than did sustained stimuli (7.4 +/- 0.5 events per animal, mean +/- S.E.M., n = 5, P = 0.002). The peak ADC deflection of their primary CSD events declined by approximately 30 % as they propagated from their initiation site to the interhemispheric boundary. The primary CSD event following a transient stimulus showed a latency of 1.4 +/- 0.1 min. It was followed by successive and smaller secondary ADC deflections that were separated by progressively longer time intervals. Conduction velocities of secondary events were similar to those of primary events. Conduction velocities of both primary and secondary events were slower than their counterparts following a sustained stimulus. ADC changes associated with CSD thus persist at times well after stimulus withdrawal and vary markedly with the nature of the initiating stimulus even in brain regions remote from the stimulus site.

Cortical spreading depression transiently activates MAP kinases

Cortical spreading depression (CSD) has been shown to have neuroprotective effects when administered in advance of cerebral ischemia. The mechanism by which CSD induces its neuroprotective effect however remains to be elucidated. Since MAP kinases have been shown to impart neuroprotection in ischemic preconditioning paradigms, we attempted to determine the role CSD may have in the activation of MAPK. We show that CSD is capable of increasing the phosphorylation of ERK in a MEK-dependent manner. This phosphorylation is, however, transient, as phosphorylated ERK levels return to control levels 45 min after 2 h of CSD elicitation. Immunohistochemical analysis reveals that the phosphorylated form of ERK is located ubiquitously in cells of the CSD-treated cortex while CSD-elicited MEK phosphorylation resides solely in the nuclei. These data suggest that CSD may act via the MAP kinase pathways to mediate preconditioning.

Stroke induces widespread changes of gene expression for glial cell line-derived neurotrophic factor family receptors in the adult rat brain

Gene expression for glial cell line-derived neurotrophic factor (GDNF) family ligands and receptors was analyzed with in situ hybridization after two focal ischemic insults of different severities. Focal ischemia was induced in rats by either 30 min or 2 h of middle cerebral artery occlusion (MCAO), causing damage to the striatum only, or involving also the parietal cortex, respectively. We found modest, transient elevation of GDNF mRNA in the dentate granule cell layer. In addition, the number of GDNF mRNA-expressing cells increased in the cortex and striatum after 2 h or 30 min of MCAO, respectively. No changes of neurturin or persephin mRNA expression were detected. Both c-Ret and GFRalpha1 mRNA levels were markedly increased in the ipsilateral cortex outside the ischemic lesion at 6-24 h after the 2-h insult, whereas GFRalpha2 expression was decreased in cortical areas both within and outside the lesion. Similar increases of c-Ret and GFRalpha1 mRNA levels were detected in the striatum, and to a lesser extent, in the cortex following 30 min of MCAO. The 2-h insult also gave rise to transient increases of c-Ret and GFRalpha1 mRNA in hippocampal subregions. Thirty minutes and 2 h of MCAO lead to elevated c-Ret, and GFRalpha1 or GFRalpha2 mRNA expression, respectively, in the ipsilateral ventroposterolateral thalamic nucleus. Both insults induced increased levels of GFRalpha1 mRNA in the subventricular zone of the lateral ventricle. Our data indicate major changes of GDNF family signaling in the forebrain, regulated mainly through altered receptor levels, in the post-ischemic phase. These changes could enhance neuroprotective and neuroregenerative responses both to endogenous and exogenous GDNF ligands.

Local uptake of (14)C-labeled acetate and butyrate in rat brain in vivo during spreading cortical depression

Spreading depression severely disrupts ion homeostasis, causes sensory neglect and motor impairment, and is associated with stroke and migraine. Glucose utilization (CMR(glc)) and lactate production rise during spreading depression, but the metabolic changes in different brain cell types are unknown. Uptake of (14)C-labeled compounds known to be preferentially metabolized by the glial tricarboxylic acid cycle was, therefore, examined during unilateral KCl-induced spreading cortical depression in conscious, normoxic rats. [(14)C]Metabolites derived from [(14)C]butyrate in K+ -treated tissue rose 21% compared to that of untreated contralateral control cortex, whereas incorporation of H(14)CO(3) into metabolites in K+ -treated tissue was reduced to 86% of control. Autoradiographic analysis showed that laminar labeling of cerebral cortex by both (14)C-labeled acetate and butyrate was elevated heterogeneously throughout cortex by an average of 23%; the increase was greatest (approximately 40%) in tissue adjacent to the K+ application site. Local uptake of acetate, butyrate, and deoxyglucose showed similar patterns, and monocarboxylic acid uptake was highest in the structures in which apparent loss of labeled metabolites of [6-(14)C]glucose was greatest. Enhancement of net uptake of acetate and butyrate in cerebral cortex during spreading depression is tentatively ascribed to increased astrocyte metabolism.

Spreading depression in human neocortical slices

Cortical spreading depression (CSD) occurrence has been suggested to be associated with seizures, migraine aura, head injury and brain ischemia-infarction. Only few studies identified CSD in human neocortical slices and no comprehensive study so far evaluated this phenomenon in human. Using the neocortical tissue excised for treatment of intractable epilepsy, we aimed to investigate CSD in human. CSD was induced by KCl injection and by modulating T-type Ca(2+) currents in incubated human neocortical tissues in an interphase mode. The DC-fluctuations were recorded by inserting microelectrodes into different cortical layers. Local injection of KCl triggered single CSD that propagated at 3.1+/-0.1 mm/min. Repetitive CSD also occurred spontaneously during long lasting application (5 h) of the T-type Ca(2+) channel blockers amiloride (50 microM) or NiCl(2) (10 microM) which was concomitant with a reversible extracellular potassium increase up to 50 mM. CSD could be blocked by the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid in all cases. The results demonstrate that modulation of the Ca(2+) dynamics conditioned human neocortical slices and increased their susceptibility to generate CSD. Furthermore, these data indicate that glutamatergic pathway plays a role in CSD phenomenon in human.

Mechanisms of spreading depression and hypoxic spreading depression-like depolarization

Spreading depression (SD) and the related hypoxic SD-like depolarization (HSD) are characterized by rapid and nearly complete depolarization of a sizable population of brain cells with massive redistribution of ions between intracellular and extracellular compartments, that evolves as a regenerative, "all-or-none" type process, and propagates slowly as a wave in brain tissue. This article reviews the characteristics of SD and HSD and the main hypotheses that have been proposed to explain them. Both SD and HSD are composites of concurrent processes. Antagonists of N-methyl-D-aspartate (NMDA) channels or voltage-gated Na(+) or certain types of Ca(2+) channels can postpone or mitigate SD or HSD, but it takes a combination of drugs blocking all known major inward currents to effectively prevent HSD. Recent computer simulation confirmed that SD can be produced by positive feedback achieved by increase of extracellular K(+) concentration that activates persistent inward currents which then activate K(+) channels and release more K(+). Any slowly inactivating voltage and/or K(+)-dependent inward current could generate SD-like depolarization, but ordinarily, it is brought about by the cooperative action of the persistent Na(+) current I(Na,P) plus NMDA receptor-controlled current. SD is ignited when the sum of persistent inward currents exceeds persistent outward currents so that total membrane current turns inward. The degree of depolarization is not determined by the number of channels available, but by the feedback that governs the SD process. Short bouts of SD and HSD are well tolerated, but prolonged depolarization results in lasting loss of neuron function. Irreversible damage can, however, be avoided if Ca(2+) influx into neurons is prevented.

ATP modulates Na+ channel gating and induces a non-selective cation current in a neuronal hippocampal cell line

Extracellular ATP evoked two excitatory responses in hippocampal neuroblastoma cells (HN2). The first, an opening of a receptor-operated non-selective cation channel and the second was a leftward shift in Na+ channel activation. Both ATP (5-1000 microM) and 2',3'-(4-benzoyl)-benzoyl-ATP (Bb-ATP, 50 microM) activated a non-selective cation current reversing near 0 mV and shifted the Na+ activation and inactivation curves to the left. Based on a comparison of a series of agonists and antagonists, the inward current appeared to be partially mediated by activation of a P2X7 receptor, although hybrid channels cannot be ruled out. The shift in Na+ channel gating could be separated from the opening of the cation channel, as application of the P2Y antagonist Reactive Blue-2 and GTP shifted the Na+ current activation to the left but failed to elicit the inward cation current. Both responses to ATP and Bb-ATP were insensitive to block by the P2X antagonist suramin (300 microM) but were prevented by incubation in oxidized ATP (200 microM); a putative P2X7 receptor antagonist. Prior screening of the surface negative charge of the membrane with a high concentration of divalent cations prevented both responses. We suggest that ATP4- activates a P2X receptor and becomes trapped on a site, on or near the Na+ channel. Activation of the P2X receptor leads to the opening of a non-specific cation channel, while the binding of ATP4- leads to a modified charge sensed by the Na+ channel, similar to what occurs in the presence of charged amphiphiles as well as a number of beta-scorpion toxins.

BTS 72664-- a novel CNS drug with potential anticonvulsant, neuroprotective, and antimigraine properties

BTS 72664, (R)-7-[1-(4-chlorophenoxy)]ethyl]-1,2,4-triazolo(1,5-alpha)pyrimidine, was identified as a drug development candidate from a research program designed to discover novel, broad-spectrum, non-sedative anticonvulsant drugs. BTS 72664 antagonized bicuculline (BIC)- and maximal electroshock (MES)-induced convulsions with ED(50) values of 1.9 and 47.5 mg/kg p.o., respectively. In rodents, it has a wide spectrum of activity preventing seizures induced by picrotoxin, pentylenetetrazol, i.c.v. 4-aminopyridine or NMDA, and audiogenic seizures in DBA-2 mice and GEPR-9 rats. BTS 72664 was also effective in preventing convulsions in amygdala-kindled rats The lack of sedative potential was predicted on the basis of wide separation between ED(50) in anticonvulsant models and TD(50) for motor impairment in mice in rotating rod and inverted horizontal grid tests. BTS 72664 is likely to produce its anticonvulsant effect by enhancing chloride currents through picrotoxin-sensitive chloride channels, and by weak inhibition of Na(+) and NMDA channels. It does not act, however, at the benzodiazepine binding site. In addition to its potential use in the treatment of epilepsy BTS 72664 may be useful in the treatment of stroke. At 50 mg/kg p.o. x 4, given to rats at 12 hourly intervals, starting at 15 min after permanent occlusion of middle cerebral artery (MCA), it reduced cerebral infarct size by 31% (measured at 2 days after insult) and accelerated recovery in a functional behavioral model. BTS 72664 prevented increases in extraneuronal concentrations of glutamate, glycine and serine brain levels induced by a cortical insult to rats (cf. cortical spreading depression). It may, therefore, have also antimigraine activity.

Real-time measurement of glutamate release from the ischemic penumbra of the rat cerebral cortex using a focal middle cerebral artery occlusion model

Following permanent middle cerebral artery occlusion, extracellular penumbral glutamate levels, measured by a real-time glutamate electrode, increased in two different patterns. In 7/11 rats, glutamate increased from baseline levels of 19+/-4 (mean+/-SEM) to 208+/-29 microM and then declined towards baseline levels. Blood flow in the penumbral area declined to 30% of pre-ischemic levels with recovery to 60 and 70% of baseline values by 3 and 6 h, respectively. Four of 11 rats in the study also exhibited late peaks of glutamate release (120+/-40 microM ) 2 h after the onset of ischemia. There were no changes in the EEG recordings or cerebral blood flow during these late glutamate peaks.

Modulation of spreading depression by changes in extracellular pH

Spreading depression (SD) and related phenomena have been implicated in hypoxic-ischemic injury. In such settings, SD occurs in the presence of marked extracellular acidosis. SD itself can also generate changes in extracellular pH (pH(o)), including a pronounced early alkaline shift. In a hippocampal slice model, we investigated the effect of interstitial acidosis on the generation and propagation of SD in the CA1 stratum radiatum. In addition, a carbonic anhydrase inhibitor (benzolamide) was used to decrease buffering of the alkaline shift to investigate its role in the modulation of SD. pH(o) was lowered by a decrease in saline HCO(3)(-) (from 26 to 13 to 6.5 mM at 5% CO(2)), or by an increase in the CO(2) content (from 5 to 15% in 26 mM HCO(3)(-)). Recordings with pH microelectrodes revealed respective pHo values of 7.23 +/- 0. 13, 6.95 +/- 0.10, 6.67 +/- 0.09, and 6.97 +/- 0.12. The overall effect of acidosis was an increase in the threshold for SD induction, a decrease in velocity, and a shortened SD duration. This inhibition was most pronounced at the lowest pH(o) (in 6.5 mM HCO(3)(-)) where SD was often blocked. The effects of acidosis were reversible on return to control saline. Benzolamide (10 microM) caused an approximate doubling of the early alkaline shift to an amplitude of 0.3-0.4 U pH. The amplified alkalosis was associated with an increased duration and/or increased velocity of the wave. These effects were most pronounced in acidic media (13 mM HCO(3)(-)/5% CO(2)) where benzolamide increased the SD duration by 55 +/- 32%. The initial velocity (including time for induction) and propagation velocity (measured between distal electrodes) were enhanced by 35 +/- 25 and 26 +/- 16%, respectively. Measurements of [Ca(2+)](o) demonstrated an increase in duration of the Ca(2+) transient when the alkaline shift was amplified by benzolamide. The augmentation of SD caused by benzolamide was blocked in media containing the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid. These data indicate that the induction and propagation of SD is inhibited by a fall in baseline pH characteristic of ischemic conditions and that the early alkaline shift can remove this inhibition by relieving the proton block on NMDA receptors. Under ischemic conditions, the intrinsic alkalosis may therefore enable SD and thereby contribute to NMDA receptor-mediated injury.

High-performance liquid chromatographic analysis of muscular interstitial arginine and norepinephrine kinetics. A microdialysis study in rats

Complex interactions between the L-arginine/nitric oxide synthase (NOS) pathway and the sympathetic nervous system have been reported. Methods capable of measuring L-arginine and norepinephrine (NE) have mainly been reported for plasma. We report the use of the microdialysis technique combined with high-performance liquid chromatography (HPLC) for measurement of both L-arginine and NE within the same tissue microdialysis sample. The microdialysis probe consisted of linear flexible probes (membrane length: 10 mm, outside diameter: 290 microm, molecular weight cut-off 50 kDa). The method used for L-arginine measurement was HPLC with fluorescence detection, giving a within-run and a between-day coefficient of variation of 2.9 and 12.8%, respectively. The detection limit was 0.5 pM/20 microl injected for L-/D-arginine. The method used for NE measurement was HPLC with electrochemical detection. The coefficients of variation were 4% for within-assay precision and 7.5% for between-assay precision. The detection limit for NE was 1 fmol/20 microl injected. The microdialysis technique coupled with HPLC system was validated in vivo to measure muscular interstitial concentrations of both arginine and NE under baseline conditions and after intravenous infusion of 500 mg/kg of L-arginine or D-arginine. In conclusion, the microdialysis technique coupled to HPLC allows the simultaneous measurements of both L-arginine and NE within the same tissue microenvironment and will enable the study of the complex interactions between the L-arginine/NO pathway and sympathetic nervous system within the interstitial space of different organs.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

Rapid efflux of lactate from cerebral cortex during K+ -induced spreading cortical depression

Rapid transport of lactate from activated brain regions to blood, perhaps reflecting enhanced metabolite trafficking, would prevent local trapping of labeled metabolites of [6-14C]glucose and cause underestimation of calculated CMRglc. Because the identities of glucose metabolites lost from activated structures and major routes of their removal are not known, arteriovenous differences across brains of conscious normoxic rats for derivatives of [6-14C]glucose were determined under steady-state conditions in blood during K+ -induced spreading cortical depression. Lactate was identified as the major labeled product lost from brain. Its entry to blood was detected within 2 minutes after a pulse of [6-14C]glucose, and it accounted for 96% of the 14C lost from brain within approximately 8 minutes. Lactate efflux corresponded to 20% of glucose influx, but accounted for only half the magnitude of underestimation of CMRglc when [14C]glucose is the tracer, suggesting extensive [14C]lactate trafficking within brain. [14C]Lactate spreading within brain is consistent with (1) relatively uniform pattern labeling of K+ -treated cerebral cortex by [6-14C]glucose contrasting heterogeneous labeling by [14C]deoxyglucose, and (2) transport of 14C-labeled lactate and inulin up to 1.5 and 2.4 mm, respectively, within 10 minutes. Thus, newly synthesized lactate exported from activated cells rapidly flows to blood and probably other brain structures.

Amino acid release during spreading depression in a flow-compromised cortical area

The glutamate concentrations in dialysate samples obtained from microdialysis probe implanted in the cortex were assayed during artificially induced spreading depression (SD) and SD with hypoperfusion. The glutamate concentrations did not differ even after SD induction with hypoperfusion (all p>0.05 cf. control), whereas anoxic depolarization caused significantly high glutamate release. Prolonged SD in hypoperfused area did not expose cerebral neurons to high glutamate concentrations.

Attenuated neurotransmitter release and spreading depression-like depolarizations after focal ischemia in mutant mice with disrupted type I nitric oxide synthase gene

Nitric oxide (NO) plays a complex role in the pathophysiology of cerebral ischemia. In this study, mutant mice with disrupted type I (neuronal) NO synthase (nNOS) were compared with wild-type littermates after permanent focal ischemia. Cerebral blood flow in the central and peripheral zones of the ischemic distribution were measured with laser doppler flowmetry. Simultaneously, microdialysis electrodes were used to measure extracellular amino acid concentrations and DC potential in these same locations. Blood flow was reduced to <25 and 60% of baseline levels in the central and peripheral zones, respectively; there were no differences in nNOS mutants versus wild-type mice. Within the central ischemic zone, DC potentials rapidly shifted to -20 mV in all mice. In the ischemic periphery, spreading depression (SD)-like waves of depolarization were observed. SD-like events were significantly fewer in the nNOS mutant mice. Concurrent with these hemodynamic and electrophysiological perturbations, extracellular elevations in amino acids occurred after ischemia. There were no detectable differences between wild-type and mutant mice in the ischemic periphery. However, in the central zone of ischemia, elevations in glutamate and GABA were significantly lower in the nNOS mutants. Twenty-four hour infarct volumes in the nNOS mutant mice were significantly smaller than in their wild-type littermates. Overall, the number of SD-like depolarizations and the integrated efflux of glutamate were significantly correlated with infarct size. These results suggest that NO derived from the nNOS isoform contributes to tissue damage after focal ischemia by amplifying excitotoxic amino acid release in the core and deleterious waves of SD-like depolarizations in the periphery.

Relationships between glutamate release, blood flow and spreading depression: real-time monitoring using an electroenzymatic dialysis electrode

Spreading depression (SD) in a flow-restricted area of the brain may be prolonged and may become potentially harmful by releasing glutamate. We induced SD in an oligemia model and examined the subsequent glutamate release. In 18 anesthetized male Fischer rats, a laser Doppler flowmeter, an electroenzymatic electrode for continuous measurement of glutamate, and a calomel electrode for measuring DC potential were placed through a cranial window positioned 3 mm away from a second window where KCl-soaked cotton was placed to initiate SD. The left carotid artery or both the common carotid arteries were ligated to suppress reactive hyperemia of SD. SD produced an increase in glutamate from 24.8 +/- 13.8 to 33.5 +/- 25.3 microM (peak value) (P < 0.0001). After ligation of both carotid arteries, the duration of SD increased from 1.5 +/- 0.6 min (before ligation) to 6.4 +/- 5.1 min (P < 0.05). Glutamate reached a peak level of 63.9 +/- 72.3 microM, then quickly returned to the control value. However, there was no positive correlation between the duration of SD and glutamate concentration. It is concluded that prolonged SD is not accompanied by a progressive increase in glutamate. Therefore, glutamate release induced by SD may not exert harmful effects on neurons.

Cortical spreading depression activates trophic factor expression in neurons and astrocytes and protects against subsequent focal brain ischemia

We recently reported that cortical spreading depression (CSD), used to precondition rat brain, reduced cortical infarction volume resulting from focal cerebral ischemia by middle cerebral artery occlusion (MCAO) 3 days later. The mechanisms underlying this protective effect by CSD remains to be explored. In this study, we confirm that CSD is neuroprotective when KCl is applied epidurally rather than intracortically. Neocortical infarct volume was 101.3+/-48.5 mm3 and 45.3+/-44.1 mm3 in the sham and CSD group, respectively (p<0.05). Using image analysis, we identified the cortical region spared from infarction by the prior CSD. We then determined the distribution of brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (bFGF) mRNA and the time course of their expression in groups of animals treated with CSD and their controls. We also examined the response of astrocytes to CSD using glial fibrillary acidic protein (GFAP) as a marker. In situ hybridization (done at 0, 3, 12, 24, 72 or 168 h after CSD) showed significant elevation of BDNF mRNA in the cortex immediately after CSD in a distribution surrounding the spared cortex, while bFGF mRNA rose 12 h after CSD and appeared more within the core of the ischemic region. Immunohistochemistry (done at 1, 3 or 7 days after CSD) demonstrated GFAP in the neocortex, with a peak at 3 days after CSD. Heat shock protein 72 (HSP72) expression was not affected by CSD. We concluded that upregulation of trophic factors and activation of glial cells may contribute to the neuroprotection induced by CSD.

Distribution of N-acetylaspartylglutamate immunoreactivity in human brain and its alteration in neurodegenerative disease

The dipeptide N-acetylaspartylglutamate (NAAG) may be involved in the process of glutamatergic signaling by both acting at glutamate receptors and as a glutamate protransmitter. In the present study we determined the cellular localization and distribution of NAAG-like immunoreactivity (NAAG-LI) in normal human brain and in neurodegenerative disorders to ascertain the degree of NAAG's colocalization to putative glutamatergic pathways. Immunohistochemistry with an antibody against NAAG was performed on control, Huntington's disease (HD) and Alzheimer's disease (AD) human autopsy and biopsy brain sections from the cerebral cortex, hippocampus, amygdala, neostriatum, brainstem and spinal cord. In normal human brain, NAAG-LI was widespread localized to putative glutamatergic pyramidal neurons of the cerebral cortex and hippocampus. Punctate NAAG-LI was present in areas known to receive neuronal glutamatergic input, such as layer IV of the cerebral cortex, striatal neuropil, and the outer portion of the molecular layer of the hippocampal dentate gyrus. In the two pathologic brain regions examined, the HD neostriatum and the AD temporal cortex, we observed a widespread loss of NAAG-LI neurons. In addition NAAG-LI reactive microglia surrounding plaques were seen in AD temporal cortex but not in the HD striatum. Our results suggest that NAAG is substantially localized to putative glutamatergic pathways in human brain and that NAAG-LI neurons are vulnerable to the neurodegenerative process in HD and AD.

Cortical spreading depression induces long-term alterations of BDNF levels in cortex and hippocampus distinct from lesion effects: implications for ischemic tolerance

Cortical spreading depression (CSD) protects hippocampal and cortical neurons from an otherwise lethal ischemic insult delivered days later. The present study was undertaken to evaluate changes in the expression of BDNF following CSD, distinct from lesion effects and its possible involvement in delayed ischemic tolerance. CSD was elicited by KCl application and a cortical lesion was made by hyperosmolar NaCl application. BDNF mRNA was examined by in situ hybridization and Northern blot up to 7 days post-CSD. BDNF protein content was measured by ELISA. In the cortex, BDNF protein was mildly elevated despite minimal increases of mRNA in the NaCl lesion group. CSD specifically up-regulated BDNF mRNA at 4 h, followed by a delayed secondary increase at 2-3 days. BDNF protein exhibited smaller biphasic increases at 24 h and 3-7 days post-CSD which were significantly higher than the NaCl lesion group. In the hippocampus, BDNF protein levels showed a delayed decrease in both groups independent of mRNA changes, but CSD specifically delayed this decrease. Thus, CSD can alter BDNF levels independent of lesion effects. The increased BDNF following CSD in the cortex is consistent with the involvement of BDNF in cortical ischemic tolerance. BDNF could not, however, be directly related to ischemic tolerance in the hippocampus.

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.

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.

Depolarization with high-K+ causes Ca(2+)-independent but partially Cl(-)-dependent glutamate release in rat hippocampal slice cultures

We studied the neurotoxic glutamate release induced by high-K+ depolarization in rat hippocampal slice cultures. Depolarization with 90 mM K+ for 30 min caused a significant, three-fold increase in glutamate release. This release was not inhibited by removing extracellular Ca2+, but was significantly inhibited by replacement of extracellular Cl- with SO4(2-). These findings suggest that glutamate is released by mechanisms other than conventional vesicular release under the high-K+ condition.

Effects of hypothermia on the rate of excitatory amino acid release after ischemic depolarization

BACKGROUND AND PURPOSE

Hypothermia slows the increase in extracellular excitatory amino acid (EAA) concentrations during temporary cerebral ischemia. However, it is unclear whether hypothermia slows the rate of EAA release or just delays the time until the first sharp increase (which occurs coincident with terminal depolarization).

METHODS

Pericranial temperatures were adjusted to 38 degrees C, 34 degrees C, 31 degrees C, or 25 degrees C in halothane-anesthetized rats. The cortical DC voltage was recorded from a glass microelectrode while the cortical concentrations of glutamate, aspartate, glycine, and gamma-aminobutyric acid (GABA) were measured by microdialysis. A cardiac arrest was induced with intravenous KCl, and the times until electroencephalograph isoelectricity and terminal depolarization were recorded. Dialysate concentrations of the four compounds were measured at 10, 20, and 30 minutes after depolarization.

RESULTS

The times to isoelectricity and depolarization varied inversely with temperature; depolarization time increased from 70 +/- 9 seconds at 38 degrees C (mean +/- SD) to 294 +/- 34 seconds at 25 degrees C. The dialysate concentrations of all four compounds increased during ischemia, and the rate of increase was inhibited by cooling. After 30 minutes of ischemia, glutamate concentration in 38 degrees C animals was 58.4 +/- 31.8 mumol/L; this decreased to 15.9 +/- 8.4 mumol/L at 25 degrees C. The magnitude of the effects of temperature on amino acid release differed with the compound measured. For glutamate, the calculated Q10 was 3.63. Corresponding values for aspartate and glycine were 3.68 and 1.95, respectively. By contrast, Q10 for GABA release was 6.31, indicating greater sensitivity to cooling.

CONCLUSIONS

These results suggest that effects of hypothermia on EAA concentrations during cerebral ischemia may be the result of both a delay until initial EAA release as well as a direct effect of temperature on the rate of amino acid release. The observed temperature effects are more consistent with carrier-mediated processes controlling EAA release.

Induction of spreading depression in the ischemic hemisphere following experimental middle cerebral artery occlusion: effect on infarct morphology

This study was undertaken to test whether transient depolarizations occurring in periinfarct regions are important in contributing to infarct spread and maturation. Following middle cerebral artery (MCA) occlusion we stimulated the ischemic penumbra with recurrent waves of spreading depression (SD) and correlated the histopathological changes with the electrophysiological recordings. Halothane-anesthetized, artificially ventilated Sprague-Dawley rats underwent repetitive stimulation of SD in intact brain (Group 1; n = 8) or photothrombotic MCA occlusion coupled with ipsilateral common carotid artery occlusion (Groups 2 and 3, n = 9 each). The electroencephalogram and direct current (DC) potential were recorded for 3 h in the parietal cortex, which represented the periinffarct border zone in ischemic rats. In Group 2, only spontaneously occurring negative DC shifts occurred; in Group 3, the (nonischemic) frontal pole of the ischemic hemisphere was electrically stimulated to increase the frequency of periinfarct DC shifts. Animals underwent perfusion-fixation 24 h later, and volumes of complete infarction and scattered neuronal injury ("incomplete infarction") were assessed on stained coronal sections by quantitative planimetry. Electrical induction of SD in Group 1 did not cause morphological injury. During the initial 3 h following MCA occlusion, the number of spontaneous periinfarct depolarization in Group 2 (7.0 +/- 1.5 DC shifts) was doubled in Group 3 by frontal current application (13.4 +/- 2.7 DC shifts; p < 0.001). The duration as well as the integrated negative amplitude of DC shifts over time were significantly greater in Group 3 than in Group 2 rats (duration, 5.7 +/- 3.8 vs. 4.1 +/- 2.5 min; p < 0.05). Histopathological examination disclosed well-defined areas of pannecrosis surrounded by a cortical rim exhibiting selectively damaged acidophilic neurons and astrocytic swelling in otherwise normal-appearing brain. Induction of SD in the ischemic hemisphere led to a significant increase in the volume of incomplete infarction (19.0 +/- 6.1 mm3 in Group 3 vs. 10.3 +/- 5.1 mm3 in Group 2; p < 0.01) and of total ischemic injury (100.7 +/- 41.0 mm3 in Group 3 vs. 66.5 +/- 24.7 mm3 in Group 2; p < 0.05). The integrated magnitude of DC negativity per experiment correlated significantly with the volume of total ischemic injury (r = 0.780, p < 0.0001). Thus, induction of SD in the ischemic hemisphere accentuated the development of scattered neuronal injury and increased the volume of total ischemic injury. This observation may be explained by the fact that with limited perfusion reserve, periinfarct depolarization are associated with episodic energy failure in the acute ischemic penumbra.