Nerve terminal damage in cerebral ischemia: protective effect of alpha-methyl-para-tyrosine

Pharmacological Approach for Neuroprotection After Cardiac Arrest-A Narrative Review of Current Therapies and Future Neuroprotective Cocktail

Cardiac arrest (CA) results in global ischemia-reperfusion injury damaging tissues in the whole body. The landscape of therapeutic interventions in resuscitation medicine has evolved from focusing solely on achieving return of circulation to now exploring options to mitigate brain injury and preserve brain function after CA. CA pathology includes mitochondrial damage and endoplasmic reticulum stress response, increased generation of reactive oxygen species, neuroinflammation, and neuronal excitotoxic death. Current non-pharmacologic therapies, such as therapeutic hypothermia and extracorporeal cardiopulmonary resuscitation, have shown benefits in protecting against ischemic brain injury and improving neurological outcomes post-CA, yet their application is difficult to institute ubiquitously. The current preclinical pharmacopeia to address CA and the resulting brain injury utilizes drugs that often target singular pathways and have been difficult to translate from the bench to the clinic. Furthermore, the limited combination therapies that have been attempted have shown mixed effects in conferring neuroprotection and improving survival post-CA. The global scale of CA damage and its resultant brain injury necessitates the future of CA interventions to simultaneously target multiple pathways and alleviate the hemodynamic, mitochondrial, metabolic, oxidative, and inflammatory processes in the brain. This narrative review seeks to highlight the current field of post-CA neuroprotective pharmaceutical therapies, both singular and combination, and discuss the use of an extensive multi-drug cocktail therapy as a novel approach to treat CA-mediated dysregulation of multiple pathways, enhancing survival, and neuroprotection.

Neurogenic orthostatic hypotension and supine hypertension in Parkinson's disease and related synucleinopathies: prioritisation of treatment targets

Neurogenic orthostatic hypotension and supine hypertension are common manifestations of cardiovascular dysautonomia in Parkinson's disease and related synucleinopathies. Because these disorders are haemodynamic opposites, improvement in one might be achieved at the expense of worsening of the other. Thus, management decisions necessitate assessment of the individual risks for patients with coexistent neurogenic orthostatic hypotension and supine hypertension. Whereas neurogenic orthostatic hypotension poses risks for falls and can be associated with cognitive impairment in the short term, chronic supine hypertension can be associated with stroke and myocardial infarction in the long term. Because few clinical trial data exist for outcomes in patients with coexistent neurogenic orthostatic hypotension and supine hypertension, clinicians need to balance, on the basis of comorbidities and disease staging, the potential immediate benefits of treatment for neurogenic orthostatic hypotension and the long-term risks of supine hypertension treatment in each patient. Future research needs to focus on ascertaining a safe degree of supine hypertension when treating neurogenic orthostatic hypotension; the effectiveness of nocturnal antihypertensive therapy in patients with coexistent neurogenic orthostatic hypotension and supine hypertension; and the prevalence, scope, and therapeutic requirements for managing neurogenic orthostatic hypotension that manifests with falls or cognitive impairment, but without postural lightheadedness or near syncope.

Omega-3 Fatty Acids: Possible Neuroprotective Mechanisms in the Model of Global Ischemia in Rats

Background. Omega-3 (ω3) administration was shown to protect against hypoxic-ischemic injury. The objectives were to study the neuroprotective effects of ω3, in a model of global ischemia. Methods. Male Wistar rats were subjected to carotid occlusion (30 min), followed by reperfusion. The groups were SO, untreated ischemic and ischemic treated rats with ω3 (5 and 10 mg/kg, 7 days). The SO and untreated ischemic animals were orally treated with 1% cremophor and, 1 h after the last administration, they were behaviorally tested and euthanized for neurochemical (DA, DOPAC, and NE determinations), histological (Fluoro jade staining), and immunohistochemical (TNF-alpha, COX-2 and iNOS) evaluations. The data were analyzed by ANOVA and Newman-Keuls as the post hoc test. Results. Ischemia increased the locomotor activity and rearing behavior that were partly reversed by ω3. Ischemia decreased striatal DA and DOPAC contents and increased NE contents, effects reversed by ω3. This drug protected hippocampal neuron degeneration, as observed by Fluoro-Jade staining, and the increased immunostainings for TNF-alpha, COX-2, and iNOS were partly or totally blocked by ω3. Conclusion. This study showed a neuroprotective effect of ω3, in great part due to its anti-inflammatory properties, stimulating translational studies focusing on its use in clinic for stroke managing.

Canadian Association of Neurosciences Review: The Role of Dopamine Receptor Function in Neurodegenerative Diseases

Dopamine (DA) receptors, which are heavily expressed in the caudate/putamen of the brain, represent the molecular target of several drugs used in the treatment of various neurological disorders, such as Parkinson's disease. Although most of the drugs are very effective in alleviating the symptoms associated with these conditions, their long-term utilization could lead to the development of severe side-effects. In addition to uncovering novel mediators of physiological DA receptor functions, recent research advances are suggesting a role of these receptors in toxic effects on neurons. For instance, accumulating evidence indicates that DA receptors, particularly D1 receptors, are central in the neuronal toxicity induced by elevated synaptic levels of DA. In this review, we will discuss recent findings on DA receptors as regulators of long term neuronal dysfunction and neurodegenerative processes.

Pathoetiological model of delirium: a comprehensive understanding of the neurobiology of delirium and an evidence-based approach to prevention and treatment

Delirium is the most common complication found in the general hospital setting. Yet, we know relatively little about its actual pathophysiology. This article contains a summary of what we know to date and how different proposed intrinsic and external factors may work together or by themselves to elicit the cascade of neurochemical events that leads to the development delirium. Given how devastating delirium can be, it is imperative that we better understand the causes and underlying pathophysiology. Elaborating a pathoetiology-based cohesive model to better grasp the basic mechanisms that mediate this syndrome will serve clinicians well in aspiring to find ways to correct these cascades, instituting rational treatment modalities, and developing effective preventive techniques.

Resveratrol protects rat striatal slices against anoxia-induced dopamine release

Incubation of rat striatal slices in anoxic medium caused significant alterations in dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) outputs; while DA release increased several times, 50% decline in DOPAC output was observed under this condition. Tissue ATP level, on the other hand, was decreased 40% by anoxia. Presence of resveratrol in the medium decreased anoxia-induced DA release in a concentration-dependent manner. Enhanced DA output, however, was declined slightly by epicatechine and catechine, and not altered significantly by morin hydrate and quercetin dehydrate which are other penolic compounds present in the red wine. In contrary to DA output, anoxia-induced decline in tissue ATP level was not ameliorated by resveratrol. In addition to anoxia, resveratrol, as observed with DA uptake blocker nomifensine, also reduced DA release stimulated by ouabain. Efficiencies of both resveratrol and nomifensine to attenuate ouabain-induced DA output, however, were closely dependent on ouabain concentration in the medium. These results indicate that some phenolic compounds, particularly resveratrol decrease anoxia-induced DA output and appear promising agents to improve the alterations occurred under anoxic-ischemic conditions.

Proteomic analysis of hypoxia/ischemia-induced alteration of cortical development and dopamine neurotransmission in neonatal rat

Perinatal hypoxia/ischemia (HI) is a common cause of neurological deficits in children. Our goal was to elucidate the underlying mechanisms that contribute to the neurological sequelae of HI-induced brain injury. HI was induced by permanent ligation of the left carotid artery followed by 90 min of hypoxia (7.8% O2) in female P7 rats. A two-dimensional differential proteome analysis was used to assess changes in protein expression in cortex 2 h after HI. In total, 17 proteins reflecting a 2-fold or higher perturbation of expression after HI as compared to sham-treated pups were identified by mass spectrometry. Of the altered proteins, 14-3-3epsilon and TUC-2, both playing an important role in the development of the central nervous system, decrease after HI, consistent with an early disturbance of cortical development. Also affected, DARPP-32 and alpha-synuclein, two proteins important for dopamine neurotransmission, increased more than 2-fold 2 h after HI injury. The differential expression of these proteins was validated by individual Western blot assays. The expression of several metabolic enzymes and translational factors was also perturbed early after HI brain injury. These findings provide initial insights into the mechanisms underlying neurodegenerative events after HI and may allow for the rational design of therapeutic strategies that enhance neuronal adaptation and compensation after HI.

Mitochondrial stress-induced dopamine efflux and neuronal damage by malonate involves the dopamine transporter

Endogenous striatal dopamine (DA) overflow has been associated with neuropathological conditions resulting from ischemia, psychostimulants, and metabolic inhibition. Malonate, a reversible inhibitor of succinate dehydrogenase, models the effects of energy impairment in neurodegenerative disorders. We have previously reported that the striatal DA efflux and damage to DA nerve terminals resulting from intrastriatal malonate infusions is prevented by prior DA depletion, suggesting that DA plays a role in the neuronal damage. We presently report that the malonate-induced DA efflux is partially mediated by reverse transport of DA from the cytosol to the extracellular space via the DA transporter (DAT). Pharmacological blockade of the DAT with a series of structurally different inhibitors [cocaine, mazindol, 1-(2-(bis(4-fluophenyl methoxy) ethyl)-4-(3-(4-fluorophenyl)-propyl)piperazine) dimethane sulfonate (GBR 13098) and methyl(-)-3beta-(p-fluorophenyl)-1alphaH,5alphaH-tropane-2beta-carboxylate1,5-naphthalene (Win 35,428)] attenuated malonate-induced DA overflow in vivo and protected mice against subsequent damage to DA nerve terminals. Consistent with these findings, the DAT inhibitors prevented malonate-induced damage to DA neurons in mesencephalic cultures and also protected against the loss of GABA neurons in this system. The DAT inhibitors did not modify malonate-induced formation of reactive oxygen species or lactate production, indicating that the DAT inhibitors neither exert antioxidant effects nor interfere with the actions of malonate. Taken together, these findings provide direct evidence that mitochondrial impairment and metabolic stress cause striatal DA efflux via the DAT and suggest that disruptions in DA homeostasis resulting from energy impairment may contribute to the pathogenesis of neurodegenerative diseases.

Evolving therapeutic approaches to treating acute ischemic stroke

Stroke contributes significantly to death, disability, and healthcare costs; however, recombinant tissue plasminogen activator (rt-PA) is the only approved thrombolytic therapy for acute ischemic stroke. One area of development for new ischemic stroke treatment options is focused on neuroprotection of viable tissue in the ischemic vascular bed. The ischemic penumbra is recognizable on MRI by decreased perfusion, in contrast to the core area of ischemia, which includes diffusion and perfusion abnormalities. Understanding the mechanisms of neuronal death, including the role of excitotoxic neurotransmitters, free radical production, and apoptotic pathways, is important in developing new therapies for stroke. This article reviews these causes and results of stroke, as well as current and future neuroprotective treatment options. Several compounds have shown neuroprotective effects in animal studies, but have failed to be effective in human clinical trials. Several promising therapeutic areas include targeting of free radicals, modulation of glutamatergic transmission, and membrane stabilization via ion channels.

Brain oxygenation and metabolism during repetitive apnea with resuscitation of 21% and 100% oxygen in newborn piglets

The oxygen distribution in the microcirculation of the piglet's brain and striatal extracellular dopamine were determined during repetitive apnea and resuscitation with 21% or 100% oxygen. Pre-apnea cortical oxygen was 49.5+/-10.4 mm Hg and during each apnea decreased to 8+/-0.9 mm Hg. After ten apneic episodes followed by resuscitation with 21% or 100% oxygen, 7.48+/-1.6% or 2.6+/-0.5% of the tissue volume was below 10 mm Hg, respectively. Extracellular dopamine increased progressively with an increase in the number of apneas with resuscitation of 21% oxygen and at the end of ten apneic episodes it was up to 59,500+/-11,320% of control. There was no increase in extracellular dopamine during apnea resuscitated with 100% oxygen. Repetitive apnea caused progressive increase in fraction of hypoxic brain tissue in newborn. The magnitude of the increase is dependent on whether the animals were resuscitated with room air or 100% oxygen.

Mechanisms of Brain Injury after Global Cerebral Ischemia

Cerebral ischemia results in a rapid depletion of energy stores that triggers a complex cascade of cellular events such as cellular depolarization and Ca2+ influx, resulting in excitotoxic cell death. The critical determinant of severity of brain injury is the duration and severity of the ischemic insult and early restoration of CBF. Induced therapeutic hypothermia following CA is the only strategy that has demonstrated improvement in outcomes in prospective, randomized clinical trials. Although pharmacologic neuro-protection has been disappointing thus far in a variety of experimental animal models, further research efforts are directed at using some agents that demonstrate marginal or moderate efficacy in combination with hypothermia. Although the signal transduction pathways and intracellular molecular events during cerebral ischemia and reperfusion are complex, potential therapeutic neuroprotective strategies hold promise for the future.

Mechanisms of ischemic brain damage

Neurologic complications from cerebral ischemia occur frequently following cardiac arrest, as well as in the perioperative period in cardiac surgery. The cellular and molecular mechanisms of cerebral ischemia are complex. This article discusses several important cell death and salvage pathways that are important in experimental cerebral ischemia that may be critical to outcome in clinical brain injury.

Potent sigma 1-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine provides ischemic neuroprotection without altering dopamine accumulation in vivo in rats

The in vivo signaling of ischemic neuroprotection provided by sigma-receptor ligands remains unclear. Catecholamines have been implicated in the propagation of ischemic neuronal injury, and previous in vitro studies suggest that sigma ligands modulate dopaminergic neurotransmission. In this study, we tested the hypothesis that the potent sigma(1)-receptor ligand 4-phenyl-1-(4-phenylbutyl) piperidine (PPBP) attenuates the increase of extracellular dopamine in ischemic striatum. Under controlled physiological conditions, a microdialysis probe was implanted in right caudoputamen (CP) complex of adult male Wistar rats. Rats were subjected to 2 h of transient middle cerebral artery occlusion (MCAO) by the intraluminal suture technique. In a blinded, randomized fashion, rats were divided into five treatment groups: Group 1 (n = 8; saline-saline) continuous i.v. infusion of saline vehicle 30 min before MCAO followed by saline at reperfusion until the end of the experiment; Group 2 (n = 8; PPBP-PPBP) i.v. PPBP 30 min before MCAO followed by 1 micromol x kg(-1) x h(-1) of PPBP; Group 3 (n = 8; saline-PPBP) i.v. saline before MCAO followed by PPBP; Group 4 (n = 4) surgical shams (saline-saline); and Group 5 (n = 4) surgical shams (PPBP-PPBP). Infarction volume at 22 h of reperfusion in the CP complex (percentage of ipsilateral structure) was significantly attenuated in rats treated with PPBP-PPBP (27.3% +/- 9.1%) and saline-PPBP (27.8% +/- 12.7%) compared with saline-saline (59.3% +/- 7.3%) treatment. There was a three- to fourfold increase in dopamine concentrations in the microdialysates within 40 min of the onset of MCAO. Dopamine and its metabolites dihydroxy phenylacetic acid and homovallinic acid levels were similar among the three groups subjected to MCAO. Therefore, PPBP provides significant ischemic neuroprotection in the CP complex without altering the acute accumulation of dopamine in vivo during transient focal ischemia in the rat.

Asymmetrical changes of excitatory synaptic transmission in dopamine-denervated striatum after transient forebrain ischemia

Spiny neurons in the neostriatum are highly vulnerable to cerebral ischemia. Recent studies have shown that the postischemic cell death in the right striatum was reduced after ipsilateral dopamine denervation whereas no protection was observed in the left striatum after dopamine denervation in the left side. In order to reveal the mechanisms of such asymmetrical protection, electrophysiological changes of dopamine-denervated striatal neurons were compared after ischemia between the left and right striatum using intracellular recording and staining techniques in vivo. No difference in cortically evoked initial excitatory postsynaptic potentials was found between the left and right striatum in intact animals after ipsilateral dopamine denervation. The initial excitatory postsynaptic potentials in the dopamine-denervated right striatum were suppressed after transient forebrain ischemia while no significant changes were found in the dopamine-denervated left striatum. Paired-pulse tests suggested that these changes involved presynaptic mechanisms. Although the incidence of a late depolarizing postsynaptic potential elicited by cortical stimulation increased after ischemia in both sides, the increase was greater in the left side. The analysis of current-voltage relationship of spiny neurons indicated that inward rectification in the left striatum transiently disappeared shortly after ischemia whereas that in the right side remained unchanged. The intrinsic excitability of spiny neurons in both sides were suppressed after ischemia, however, the suppression in the right side was stronger than in the left side. The above results demonstrate that after ipsilateral dopamine denervation, the depression of excitatory synaptic transmission and neuronal excitability in the right striatum is more severe than that in the left striatum following ischemia. The depression of excitatory synaptic transmission and neuronal excitability, therefore, might play an important role in neural protection after ischemic insult.

Cysteine and mercapturate conjugates of oxidized dopamine are in human striatum but only the cysteine conjugate impedes dopamine trafficking in vitro and in vivo

Recent results have suggested that some products of mercapturic acid pathway (MAP) metabolism of oxidized dopamine (DA) may contribute to mesostriatal dopaminergic neurodegeneration, and that at least one product, 5-S-cysteinyldopamine (Cys-DA), is elevated in patients with advanced Parkinson's disease (PD) who have been treated with L-DOPA. Here we investigated MAP enzymes and products in the midbrain and striatum of control individuals and patients with dementia with Lewy bodies (DLB) who had less severe dopaminergic degeneration than PD patients and who were not treated with L-DOPA. We also determined the biological activity of MAP metabolites of oxidized DA using primary rat mesencephalic cultures, rat cerebral synaptosomes, and rat striatum in vivo microdialysis. Our results showed that the human mesostriatal dopaminergic pathway generates Cys-DA but has limited enzymatic capacity for mercapturate formation, that striatal levels of MAP products of oxidized DA are not elevated in DLB patients compared with controls, and that Cys-DA interferes with trafficking of DA in vitro and in vivo. These results indicate that while Cys-DA is not increased in striatum of patients with mild dopaminergic neurodegeneration, it may interfere with uptake of DA in patients with advanced PD.

Differential changes of synaptic transmission in spiny neurons of rat neostriatum following transient forebrain ischemia

Spiny neurons in neostriatum are vulnerable to cerebral ischemia. To reveal the mechanisms underlying the postischemic neuronal damage, the spontaneous activities, evoked postsynaptic potentials and membrane properties of spiny neurons in rat neostriatum were compared before and after transient forebrain ischemia using intracellular recording and staining techniques in vivo. In control animals the membrane properties of spiny neurons were about the same between the left and right neostriatum but the inhibitory synaptic transmission was stronger in the left striatum. After severe ischemia, the spontaneous firing and membrane potential fluctuation of spiny neurons dramatically reduced. The cortically evoked initial excitatory postsynaptic potentials were suppressed after ischemia indicated by the increase of stimulus threshold and the rise time of these components. The paired-pulse facilitation test indicated that such suppression might involve presynaptic mechanisms. The inhibitory postsynaptic potentials in spiny neurons were completely abolished after ischemia and never returned to the control levels. A late depolarizing postsynaptic potential that was elicited from approximately 5% of the control neurons by cortical stimulation could be evoked from approximately 30% of the neurons in the left striatum and approximately 50% in the right striatum after ischemia. The late depolarizing postsynaptic potential could not be induced after acute thalamic transection. The intrinsic excitability of spiny neurons was suppressed after ischemia evidenced by the significant increase of spike threshold and rheobase as well as the decrease of repetitive firing rate following ischemia. The membrane input resistance and time constant increased within 6 h following ischemia and the amplitude of fast afterhyperpolarization significantly increased after ischemia. These results indicate the depression of excitatory monosynaptic transmission, inhibitory synaptic transmission and excitability of spiny neurons after transient forebrain ischemia whereas the excitatory polysynaptic transmission in neostriatum was potentiated. The facilitation of excitatory polysynaptic transmission is stronger in the right neostriatum than in the left neostriatum after ischemia. The suppression of inhibitory component and the facilitation of excitatory polysynaptic transmission may contribute to the pathogenesis of neuronal injury in neostriatum after transient cerebral ischemia.

Inhibition by neuroprotective drug NS-7 of nicotine-induced 22Na(+) influx, 45Ca(2+) influx and catecholamine secretion in adrenal chromaffin cells

In cultured bovine adrenal chromaffin cells, NS-7 [4-(4-fluorophenyl)-2-methyl-6-(5-piperidinopentyloxy) pyrimidine hydrochloride], a newly-synthesized neuroprotective drug, inhibited nicotine-induced 22Na(+) influx via nicotinic receptors (IC(50)=15.5 microM); the suppression by NS-7 was observed in the presence of ouabain, an inhibitor of Na(+),K(+)-ATPase, and was not attenuated upon the washout of NS-7. NS-7 decreased nicotine-induced maximum influx of 22Na(+) without altering the EC(50) value of nicotine. Also, NS-7 diminished nicotine-induced 45Ca(2+) influx via nicotinic receptors and voltage-dependent Ca(2+) channels (IC(50)=14.1 microM) and catecholamine secretion (IC(50)=19.5 microM). These results suggest that NS-7 produces noncompetitive and long-lasting inhibitory effects on neuronal nicotinic receptors in adrenal chromaffin cells, and interferes with the stimulus-secretion coupling.

Role for dopamine in malonate-induced damage in vivo in striatum and in vitro in mesencephalic cultures

Defects in mitochondrial energy metabolism have been implicated in the pathology of several neurodegenerative disorders. In addition, the reactive metabolites generated from the metabolism and oxidation of the neurotransmitter dopamine (DA) are thought to contribute to the damage to neurons of the basal ganglia. We have previously demonstrated that infusions of the metabolic inhibitor malonate into the striata of mice or rats produce degeneration of DA nerve terminals. In the present studies, we demonstrate that an intrastriatal infusion of malonate induces a substantial increase in DA efflux in awake, behaving mice as measured by in vivo microdialysis. Furthermore, pretreatment of mice with tetrabenazine (TBZ) or the TBZ analogue Ro 4-1284 (Ro-4), compounds that reversibly inhibit the vesicular storage of DA, attenuates the malonate-induced DA efflux as well as the damage to DA nerve terminals. Consistent with these findings, the damage to both DA and GABA neurons in mesencephalic cultures by malonate exposure was attenuated by pretreatment with TBZ or Ro-4. Treatment with these compounds did not affect the formation of free radicals or the inhibition of oxidative phosphorylation resulting from malonate exposure alone. Our data suggest that DA plays an important role in the neurotoxicity produced by malonate. These findings provide direct evidence that inhibition of succinate dehydrogenase causes an increase in extracellular DA levels and indicate that bioenergetic defects may contribute to the pathogenesis of chronic neurodegenerative diseases through a mechanism involving DA.

Cerebral oligaemia episode triggers free radical formation and late cognitive deficiencies

Sixty minutes of cerebral oligaemic hypoxia, induced by bilateral clamping of the carotid arteries (BCCA) in pentobarbital-anaesthetized normotensive rats, induces a late progressive cognitive decline when compared with sham-operated controls. Analysis at BCCA of hippocampal metabolism using microdialysis showed increased release of glutamate, aspartate and gamma-aminobutyric acid, followed by a progressive rise in the formation of hydroxyl free radicals measured as 2,3-dihydroxybenzoic acid (2,3-DHBA), their reaction product with salicylate, though only in the re-perfusion phase. In the striatum increased dopamine release occurred during BCCA, whereas glutamate and aspartate showed an increase only during the late re-perfusion phase. gamma-Aminobutyric acid (GABA) concentration increased during BCCA and early re-perfusion. An increase in 2,3-DHBA was seen during BCCA, and persisted over 2 h of re-perfusion. Six and 13 months after surgery, though not as early as 3 months, BCCA-treated rats perform worse than sham-operated controls in a water-maze, where decreased swimming speed reveals striatal dysfunction, while hippocampal dysfunction manifested as diminished spatial bias. These results show that cerebral oligaemia, similarly to cerebral ischaemia, leads to increased extracellular dopamine, aspartate and glutamate, and the production of hydroxyl radicals in structures associated with learning and memory processes. Unlike cerebral ischaemia, in cerebral oligaemia the appearance of spatial memory deficits is delayed.

Ascorbic acid is neuroprotective against global ischaemia in striatum but not hippocampus: histological and voltammetric data

Following reports that ascorbic acid (AA) blocks NMDA receptors, we examined its possible neuroprotective properties in vivo (gerbil bilateral carotid artery occlusion model: BCAO) and in vitro (ischaemia-induced dopamine (DA) release in brain slices). Five minutes of BCAO caused substantial cell loss of 90-95% and 40-50% in gerbil CA1 hippocampus and striatum, respectively, measured in haematoxylin and eosin-stained sections, 5 days post-insult. AA (500 mg kg(-1) day(-1) i.p. for 312 days, first dose 1 h before occlusion) significantly (P<0.05) reduced striatal cell loss (from 40 to 13%) while only reducing CA1 cell loss from 95 to 88%. A lower dose (250 mg kg(-1) day(-1) i.p. for 312 days) was ineffective in either region. AA (750 mg kg(-1) day(-1) i.p. for 312 days) caused significant striatal protection (cell loss reduced from 49 to 20%) if treatment was initiated 1 h before occlusion. Initiation of treatment immediately post occlusion did not cause significant protection. Neither treatment regime protected CA1 hippocampus. In separate experiments we examined the effect of AA on DA release, monitored by voltammetry, in an in vitro model of striatal ischaemia. Four DA release variables were measured: T(on)--time from initiation of ischaemia to the onset of DA release, T(pk)--the time from onset of DA release to maximum, deltaDA/deltat--the mean rate of DA release and [DA](max)-- the maximum extracellular DA concentration. Control values in drug-naive slices were: T(on)=193+/-8 s, T(pk) = 24 +/- 4 s, [DA](max) = 69 +/- 6 microM and deltaDA/deltat = 4.2 +/- 0.7 microM s(-1) (means+/-S.E.M., n=15). 212 h pretreatment with AA (0.4 to 10 mM) did not affect T(on) or [DA](max) but increased T(pk) and decreased deltaDA/deltat (P<0.05) with an EC50 of 1.66 mM. NMDA (100 microM) shortened T(on). N-ethylmaleimide (20 microM) had no effect on the response to AA but potentiated the action of NMDA on T(on). AA (2 or 10 mM) had no effect on the response to NMDA. We conclude that AA is neuroprotective against global ischaemia in the striatum and that some of this action may be due to attenuation of ischaemia-induced DA release. This action is mediated neither by blockade of the NMDA receptor nor modulation of its redox status.

Anoxia-induced dopamine release from rat striatal slices: involvement of reverse transport mechanism

Incubation of rat striatal slices in the absence of oxygen (anoxia), glucose (aglycemia), or oxygen plus glucose (ischemia) caused significant increases in dopamine (DA) release. Whereas anoxia decreased extracellular 3,4-dihydroxyphenylacetic acid levels by 50%, aglycemia doubled it, and ischemia returned this aglycemia-induced enhancement to its control level. Although nomifensine, a DA uptake blocker, completely protected the slices against anoxia-induced DA depletion, aglycemia- and ischemia-induced increases were not altered. Moreover, hypothermia differentially affected DA release stimulated by anoxia, aglycemia, and ischemia. Involvement of glutamate in DA release induced by each experimental condition was tested by using MK-801 and also by comparing the glutamate-induced DA release with that during anoxia, aglycemia, or ischemia. MK-801 decreased the anoxia-induced DA depletion in a dose-dependent manner. This treatment, however, showed a partial protection in aglycemic conditions but failed to improve ischemia-induced DA depletion. Like anoxia, DA release induced by exogenous glutamate was also sensitive to nomifensine and hypothermia. These results indicate that anoxia enhances DA release by a mechanism involving both the reversed DA transporter and endogenous glutamate. Partial or complete lack of effect of nomifensine, hypothermia, or MK-801 in the absence of glucose or oxygen plus glucose also suggests that experimental conditions, such as the degree of anoxia/ischemia, may alter the mechanism(s) involved in DA depletion.

Effects of norepinephrine on NMDA-induced neurotoxicity in cerebellar granular cell culture of rat pups

In this study, norepinephrine was tested in 0.1, 1, 10, 25 and 50 microM doses in 100 microM NMDA toxicity on cerebellar granular cell culture of rats. NMDA in 100 microM concentration induced cell death significantly with respect to controls. Death cell population was 1.08 +/- 0.44% in control and 22.15 +/- 2.46% in 100 microM NMDA (P < 0.0001). None of the norepinephrine concentrations administrated 15 min prior to NMDA was able to reduce death cell scores to control levels. Results were 8.75 +/- 0.83% in 0.1 microM, 7.0 +/- 1.01% in 1 microM, 17.25 +/- 1.31% in 10 microM, 35.5 +/- 1.38% in 25 microM and 17.9 +/- 1.72% in 50 microM norepinephrine plus 100 microM NMDA administrated groups (P < 0.0001 for all with respect to control). Labetalol, as an alpha and beta blocker in 0.5 microM concentration which was given 15 min prior to norepinephrine was able to block the effects of it. In comparison with 100 microM NMDA administered group, only low doses of norepinephrine reduced the death cell scores significantly (for 0.1 and 1 microM norepinephrine plus NMDA groups; P < 0.0001). For 10 and 50 microM norepinephrine plus NMDA groups, death cell scores were found statistically insignificant from the NMDA-administered group (P > 0.05 for both) while for the 25 microM norepinephrine plus NMDA group, the death cell score was found to be statistically increased (P < 0.0001).

Characteristics of the NMDA receptor modulating hypoxia/hypoglycaemia-induced rat striatal dopamine release in vitro

We investigated the functional characteristics of the NMDA receptor that modulates hypoxia/hypoglycaemia-induced striatal dopamine release. Dopamine release was detected by fast cyclic voltammetry in rat neostriatal slices. Four variables were measured: T(on) -- time from initiation of hypoxia/hypoglycaemia to the onset of dopamine release, Tpk -- time from onset to maximum, deltaDA/delta(t) -- rate of dopamine release and DAmax -- maximum extracellular dopamine concentration. In controls, T(on) = 164.9 +/- 1.7 s, Tpk = 20.9 +/- 0.9 s, deltaDA/delta(t) = 5.31 +/- 0.44 microM/s and DAmax = 79.1 +/- 2.5 microM (means +/- S.E.M., n = 203). Cis-4-(phosphonomethyl)piperidine-2-carboxylic acid (CGS 19755, 20 microM) lengthened, while N-methyl-D-aspartate (NMDA) (100 microM) shortened T(on). (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,1 0-imine hydrogen maleate (MK 801, 1 and 10 microM) and dextromethorphan (10 and 100 microM) increased Tpk and decreased DAmax. Neither glycine (100 microM), 7-chlorokynurenic acid (50 microM) nor 5-nitro-6,7-dichloro-1,4-dihydroquinoxaline-2,3-dione (ACEA 1021, 100 microM) had any effect although 7-chlorokynurenic acid blocked the effect of NMDA. Increasing [Mg2+] from 1.3 to 3.7 mM, increased Tpk and decreased deltaDA/delta(t). Dithiothreitol (1 mM) accelerated T(on) while 5.5-dithio-bis-(2-nitrobenzoic acid) (1 mM) delayed T(on). Neither drug affected Tpk, DAmax or deltaDA/delta(t). Neither spermidine (100 microM) nor arcaine (100 microM) affected T(on), Tpk or deltaDA/delta(t) although arcaine decreased DAmax. In conclusion, hypoxia/hypoglycaemia-induced dopamine release was influenced by an NMDA receptor although modulation of the glycine recognition site of the receptor was ineffective, as were agents acting at polyamine modulatory zones. These findings highlight differences between recombinant and native NMDA receptors and suggest caution in extrapolating molecular biology to functional studies.

Sodium channel blockade unmasks two temporally distinct mechanisms of striatal dopamine release during hypoxia/hypoglycaemia in vitro

Massive striatal dopamine release during cerebral ischaemia has been implicated in the resulting neuronal damage. Sodium influx is an early event in the biochemical cascade during ischaemia and blockade of sodium channels may increase resistance to ischaemia by reducing energy demand involved in compensation for sodium and potassium fluxes. In this study, we have determined the effects of opening and blockade of voltage-gated sodium channels on hypoxia/hypoglycaemia-induced dopamine release. Slices of rat caudate nucleus were maintained in a slice chamber superfused by an oxygenated artificial cerebrospinal fluid containing 4 mM glucose. Ischaemia (hypoxia/hypoglycaemia) was mimicked by a switch to a deoxygenated artificial cerebrospinal fluid containing 2 mM glucose and dopamine release was measured using fast cyclic voltammetry. In drug-free (control) slices, there was a 2-3 min delay after the onset of hypoxia/hypoglycaemia followed by a rapid dopamine release event which was associated with anoxic depolarization. In slices treated with the Na+ channel opener, veratridine (1 microM), the time to onset of dopamine release was shortened (101 +/- 20 s, compared with 171 +/- 8 s in controls, P < 0.05). Conversely, phenytoin (100 microM), lignocaine (200 microM) and the highly selective sodium channel blocker, tetrodotoxin (1 microM) markedly delayed and slowed dopamine release vs paired controls. In the majority of cases, dopamine release was biphasic after sodium channel blockade: a slow phase preceded a more rapid dopamine release event. The latter was associated with anoxic depolarization. Neither the fast nor the slow release events were affected by pretreatment with the selective dopamine uptake blocker GBR 12935 (0.2 microM), suggesting that uptake carrier reversal did not contribute to these events. In conclusion, sodium channel antagonism delays and slows hypoxia/hypoglycaemia-induced dopamine release in vitro. Furthermore, sodium channel blockade delays anoxic depolarization and its associated neurotransmitter release, revealing an earlier dopamine release event that does not result from reversal of the uptake carrier.

Modulation of ischemic excitatory neurotransmitter and gamma-aminobutyric acid release during global temporary cerebral ischemia by selective neuronal nitric oxide synthase inhibition

Nitric oxide release during cerebral ischemia is the result of both neuronal and endothelial subclasses of nitric oxide synthase (NOS). In this study, we examined the role of specific neuronal NOS inhibition (nNOSI) on excitatory neurotransmitter and gamma-aminobutyric acid (GABA) release during global cerebral ischemia. Microdialysis probes were placed into the striatum of 24 rats. After probe stabilization, rats were randomized to receive 7-nitroindazole (7-NI), a selective nNOSI, in doses of 0, 5, 10, or 20 mg/kg. Temporary global forebrain ischemia was induced for 15 min, followed by 60 min of reperfusion. nNOSI administration did not produce detectable changes in neurotransmitter recovery prior to ischemia. There were significant increases in aspartate (ASP), glutamate (GLU), glycine (GLY), and GABA recovery during ischemia in the absence of nNOSI. 7-NI resulted in an attenuation in GLU, GLY, and GABA recovery during ischemia and reperfusion. No differences in ASP recovery were detected with nNOSI. Differences between the present study and other studies that examine the role of nonspecific constitutive NOSI during cerebral ischemia demonstrate the contribution of neuronal NOS on the modulation of ischemic excitatory neurotransmitter and GABA release.

Involvement of N- and P/Q- but not L- or T-type voltage-gated calcium channels in ischaemia-induced striatal dopamine release in vitro

Calcium influx and transmitter efflux are central events in the neuropathological cascade that occurs during and following cerebral ischaemia. This study explored the role of voltage-gated calcium channels (VGCCs) in ischaemia-induced striatal dopamine (DA) release in vitro. Slices (350 microm thickness) of rat neostriatum were superfused (400 ml/h) with an artificial cerebrospinal fluid (aCSF) at 34 degrees C and subjected to episodes of 'ischaemia' by reduction of the glucose concentration from 4 to 2 mM and gassing with 95% N2/5% CO2. DA release was monitored with fast cyclic voltammetry at implanted carbon fibre microelectrodes. The time to onset, time to peak, rate and magnitude of DA release were measured. Non-selective blockade of VGCCs with a high concentration of Ni2+ (2.5 mM), markedly delayed (P < 0.01) and slowed (P < 0.05) DA release but preferential blockade of T-type VGCCs with a lower concentration (200 microM) had no effect. DA release was also unaffected by selective antagonism of L-type VGCCs with nimodipine and nicardipine (10 microM each). Selective blockade of N-type VGCCs with omega-conotoxin GVIA (100 nM) delayed DA release (P < 0.05) but did not affect its rate or magnitude. Blockade of P- and possibly Q-type VGCCs with omega-agatoxin IVA (up to 200 nM) both delayed (P < 0.05) and slowed (P < 0.05) DA release. Preferential blockade of P- type VGCCs with neomycin (500 microM) also delayed (P < 0.05) and slowed (P < 0.05) DA release. These findings suggest that N-, P- and possibly Q- but not L- or T-type VGCCs mediate ischaemia-induced DA release. Although it is not possible to say, on the basis of these results, that the effects are directly upon the dopamine terminals, these calcium channels nevertheless constitute promising targets for therapeutic intervention.

Loss of dopaminergic neurons in parkinsonism: possible role of reactive dopamine metabolites

Parkinson's disease affects one out of every 100 people above the age of 55. Its cause is unknown and although the symptoms can be treated, there is no cure. The disease is associated with the selective loss of neurons that contain biogenic amines, and among these it is the dopamine (DA) neurons of the nigrostraital projection that are the most consistently and severely affected (Bernheimer et al., 1973). In this review we discuss the possibility that DA may act as an endogenous neurotoxin, causing the degeneration of the very neurons that release it. We further suggest that although treatments which increase the synthesis and release of DA reduce the symptoms, they also may serve to exacerbate the neurodegenerative process. We propose that the treatments which increase the antioxidant capacity of brain may be protective.

The effect of hypoxia and catecholamines on regional expression of heat-shock protein-72 mRNA in neonatal piglet brain

The present study has shown that hypoxia leads to expression of heat-shock protein in the brain of newborn piglets and this process is almost completely abolished by depletion of catecholamines prior to the hypoxic episode. The piglets were anesthetized and mechanically ventilated. One hour of hypoxia was generated by decreasing the oxygen fraction in the inspired gas (FiO2) from 22% to 6%-10%. FiO2 was then returned to the control value for a period of 2 h. Following the 2 h of reoxygenation, regional expression of the 72-kDa heat-shock protein (hsp72) mRNA was determined using in situ hybridization and autoradiography. The hypoxic insult (cortical pO2 = 3-10 mmHg) induced expression of hsp72 mRNA in regions of both white and gray matter, with strong expression occurring in the cerebral cortex of individual animals. Depleting the brain of catecholamines prior to hypoxia, by treating the animals with alpha-methyl-p-tyrosine (AMT), resulted in a major change in the hsp72 mRNA expression. In the catecholamine depleted group of animals, the intensity of hsp72 mRNA expression was greatly decreased or almost completely abolished relative to the nondepleted hypoxic group. These results suggest that the catecholamines play a significant role in the expression of the hsp72 gene in response to hypoxic insult in neonatal brain.

Role of oxidation in the neurotoxic effects of intrastriatal dopamine injections

We have examined the biochemical and histological effects of high concentrations of dopamine (0.05-1.0 micromol) injected into the rat striatum. Twenty-four hours after such injections, the oxidation products of dopamine and dihydroxyphenylacetic acid were detected as both free and protein-bound cysteinyl dopamine and cysteinyl dihydroxyphenylacetic acid. Protein-bound cysteinyl catechols were increased 7- to 20-fold above control tissue levels. By 7 days postinjection, the protein-bound cysteinyl catechols were still detectable, although reduced in concentration, whereas the free forms could no longer be measured. Histological examination of striatum at 7 days revealed a central core of nonspecific damage including neuronal loss and gliosis. This core was surrounded by a region containing a marked reduction in tyrosine hydroxylase immunoreactivity but no apparent loss of serotonin or synaptophysin immunoreactivity. When dopamine was injected with an equimolar concentration of either ascorbic acid or glutathione, the formation of protein-bound cysteinyl catechols was greatly reduced. Moreover, the specific loss of tyrosine hydroxylase immunoreactivity associated with dopamine injections was no longer detectable, although the nonspecific changes in cytoarchitecture were still apparent. Thus, following its oxidation, dopamine in high concentrations binds to protein in the striatum, an event that is correlated with the specific loss of dopaminergic terminals. We suggest that the selective degeneration of dopamine neurons in Parkinson's disease may be caused by an imbalance between the oxidation of dopamine and the availability of antioxidant defenses.

Nitric oxide modulates dopamine release during global temporary cerebral ischemia

Dopamine (DA) is released in large quantities from the striatum during cerebral ischemia. Along with excitatory neurotransmitters, DA plays a role in cellular neuronal ischemic injury. In this study we examined the role of nitric oxide (NO) in the ischemia-induced release of DA. A microdialysis probe was stereotactically placed into the corpus striatum of 16 Sprague-Dawley rats for DA, 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanillic acid (HVA) level determinations. After probe stabilization, the animals received either NG-nitro-L-arginine-methyl ester (L-NAME), a NO synthase inhibitor, or vehicle through the microdialysis probe. Temporary global forebrain ischemia was induced using bilateral carotid artery ligature tightening and controlled hemorrhagic hypotension for 15 min. L-NAME administration caused a reduction in ischemic estimated extraneuronal DA concentration by 60% (P < 0.005) compared with control. There was an increase in both DOPAC and HVA concentrations during the recovery period compared to baseline values in the control group (P < 0.05). L-NAME also caused a reduction in HVA concentration compared to vehicle administration during the latter part of recovery (P < 0.05). These data support the concept that ischemic dopamine release may be mediated by NO. This NO-modulated DA release may contribute to the previously reported deleterious neurotoxic effects of NO during ischemia.

Involvement of D1 dopamine receptor mechanism in ischemia-induced impairment of CA1 presynaptic fiber spikes in rat hippocampal slices

The effect of dopamine (DA) receptor agonists and antagonists on hypoxia/hypoglycemia (ischemia)-induced decrease in CA1 presynaptic fiber spikes elicited by the stimulation of Schaffer collateral were investigated using hippocampal slices. Treatment with D1 dopamine receptor antagonist, SCH23390 produced a concentration-dependent attenuation of the ischemia-induced decrease of presynaptic potentials. The magnitude of recovery of the CA1 presynaptic potential in SCH233390-treated slices at 10 and 100 microM was 28 and 54%, respectively. Whereas, treatment with D1 dopamine receptor agonist, SKF38393 exacerbated the ischemia-induced decrease in the CA1 presynaptic potential. The decrease of CA1 presynaptic potential by ischemia was affected by neither D2 dopamine receptor agonist, bromocriptin and quinpirole nor D2 dopamine receptor antagonist, sulpiride. The neuroprotective effect of SCH23390 was completely blocked by cotreatment with SKF38393. The present results demonstrated that the blockade of D1 dopamine receptor function played a neuroprotective role in ischemic damage, suggesting a facilitatory role of D1 dopamine receptor-operated function in ischemia-induced neuronal deficits.

Relationship of extracellular dopamine in striatum of newborn piglets to cortical oxygen pressure

The present studies describes the relationship between extracellular dopamine in striatum of newborn piglets and cortical oxygen pressure. The extracellular level of dopamine was measured by in vivo microdialysis and the oxygen pressure in the cortex was measured by phosphorescence lifetime of oxygen probe in the blood. Controlled, graded levels of hypoxic insult to the brain of animals were generated by decreasing of the oxygen fraction in the inspired gas (FiO2) from 21% to 14%, 11%, and 9%. This resulted in decrease in the cortical oxygen pressure from 31-35 Torr to about 24 Torr, 15 Torr and 4 Torr, respectively. The changes in extracellular level of dopamine, DOPAC and HVA were dependent on changes in cortical oxygen pressure. Stepwise decrease in the cortical oxygen pressure (see above) caused increases in extracellular dopamine of about 80%, 200% and 550%, respectively. The levels of DOPAC and HVA progressively decreased and when cortical oxygen decreased to 4-6 Torr were about 50% and 70% of control, respectively. After return of FiO2 to control (21%), the cortical oxygen pressure rapidly increased to above normal, then returned to control values. The extracellular levels of dopamine, DOPAC, and HVA recovered more slowly, attaining control values in about 30 minutes. The data show that extracellular levels of dopamine increase with even very small decreases in oxygen pressure. Thus, there is no "oxygen reserve" which protects dopamine release and metabolism from decrease in oxygen pressure.

Changes in concentrations of dopamine, serotonin, and their metabolites induced by carbon monoxide (CO) in the rat striatum as determined by in vivo microdialysis

Striatal microdialysis was performed in rats exposed to carbon monoxide (CO). Extracellular changes of dopamine, serotonin, and their metabolites were monitored before and after CO exposure at 15-min intervals by HPLC analysis. After CO exposure, extracellular dopamine increased (3.8 times that of baseline), whereas 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) decreased (by 20-25% of baseline). The decrease in HVA at individual time points, however, was not significant. After a transient increment of the dopamine, it was cleared from the extracellular fluid within 45 min and reached a stable level. Serotonin and 5-hydroxyindoleacetic acid (5-HIAA) showed a pattern different to that of dopamine and its acid metabolites, i.e., the changes in extracellular levels were small. Pretreatment with dizocilpine (MK-801), a noncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist, 45 min before CO exposure antagonized the changes in the extracellular concentration of DOPAC. However, the change in dopamine levels was not antagonized by pretreatment with MK-801. MK-801 itself had no effect on the levels of monoamines. Therefore, NMDA receptors may not have an important role for regulating striatal dopamine neurons in hypoxic condition.

Ischemia-induced changes in catecholamine release and their mechanisms: a study using cultured bovine adrenal chromaffin cells

Ischemia-induced changes in neurotransmitter release and their mechanisms were examined using cultured bovine adrenal chromaffin cells. When the cells were incubated in glucose-free media equilibrated with 0% O2/100% N2 (ischemia), ATP content decreased and reached the minimum level within 40 min. Control incubation was done in media equilibrated with 21% O2 in N2. After 10-min incubation under ischemic conditions, basal catecholamine (CA) release was elevated and the elevation persisted up to 90 min. High K(+)-evoked CA release was transiently enhanced at 10 min, but after that, it decreased to reach the minimum level at 60 min. At 10 min, cytosolic free Ca2+ concentration ([Ca2+]i) and 45Ca2+ uptake of the resting cells (basal values) and high K(+)-evoked increases in these two parameters were unchanged, but CA release from permeabilized cells in response to Ca2+ in media was augmented. After 60-min incubation under ischemic conditions, basal [Ca2+]i was elevated: the elevation was observed even in the absence of extracellular Ca2+. In contrast, high K(+)-evoked increases in [Ca2+]i and in 45Ca2+ uptake were suppressed, but basal 45Ca2+ uptake into intact cells and CA release from permeabilized cells were unchanged. These results suggest that in an early phase (10 min) of ischemia, both basal and stimulation-evoked CA release are augmented because of increased sensitivity of exocytotic machinery to Ca2+. In the late phase (60 min), basal CA release is augmented because of an increase in basal [Ca2+]i, which is due to accumulation of Ca2+ derived from intracellular Ca2+ pools: stimulation-evoked CA release is suppressed because of inhibition of stimulation-evoked increase in [Ca2+]i, which is due to functional disturbance of voltage-dependent Ca2+ channels.

Effects of ischaemia on neurotransmitter release from the isolated retina

The effects of "ischaemia" (glucose-free Krebs-bicarbonate medium gassed with N2/CO2) on the release of glutamate and other major neurotransmitters in the retina were examined using the isolated rat and rabbit retina. Amino acid transmitters, acetylcholine, and dopamine were measured by HPLC. The release of glutamate, aspartate, GABA, and glycine from ischaemic retinas was more than doubled after 30 min, and after 90 min of ischaemia the release of amino acids was approximately 15-20-fold that of control values. Ischaemia also produced large increases in the release of dopamine from both the rat and especially the rabbit retina. In contrast, the release of acetylcholine from the rat retina was significantly decreased by ischaemia, although the release of choline was increased. Because the ischaemia-induced release of glutamate, aspartate, and GABA from the rat retina was completely Ca independent, and exposure of the retina to high K (50 mM) did not stimulate amino acid release, it is concluded that the mechanisms underlying the ischaemia-induced release do not involve an initial release of K or an influx of calcium.

Selective effects of retinal dopamine depletion on partial ischemia-induced electroretinographic hyperresponses in rabbits

The interaction of retinal dopamine depletion and partial ischemia on the a- and b-wave amplitudes and implicit times of the electroretinogram was examined in adult pigmented rabbits. Seven days after 6-hydroxydopamine treatment, which resulted in a depletion of the amine, partial retinal ischemia was induced by raising the intraocular pressure. As expected, moderate elevation of intraocular pressure produced increases in both a- and b-wave amplitudes. Amplitude hyperresponses were significantly reduced in dopamine-depleted retinas. These reductions were more prominent with relatively lower intensities. However, response delays were not shortened but lengthened by 6-hydroxydopamine pretreatment. Together, these results point to a selective role of dopamine in partial retinal ischemia induced by moderate elevation of intraocular pressure in rabbits.

Striatal extracellular dopamine levels are not increased by hyperglycemic exacerbation of ischemic brain damage in rats

We tested the hypothesis that hyperglycemic exacerbation of incomplete forebrain ischemia is mediated by increased extracellular dopamine levels. Normoglycemic and hyperglycemic Sprague-Dawley rats (eight each) with previously placed coaxial striatal microdialysis probes underwent 12 min of forebrain ischemia produced by bilateral carotid artery occlusion and trimethaphan-induced hypotension. Microdialysis was performed before, during, and for 6 h after ischemia, then perfusion-fixation was performed. Hyperglycemic rats had more severe postischemic damage in the caudate-putamen, neocortex, and hippocampus. Extracellular striatal dopamine levels were increased by ischemia, but were unaffected by hyperglycemia. These data show that hyperglycemic exacerbation of ischemic striatal damage does not depend on elevated extracellular dopamine levels.

A facilitatory role of vasopressin in hypoxia/hypoglycemia-induced impairment of dopamine release from rat striatal slices

The excitatory amino acid, glutamate plays a crucial role in the pathogenesis of brain damage caused by anoxia and/or hypoglycemia. Although vasopressin (VP) also acts as an excitatory transmitter in the CNS, little is known about its effect on hypoxic and/or ischemic brain damage. In this study, we investigated the effect of arginine vasopressin (AVP) on hypoxia/hypoglycemia-induced impairment of dopamine release from striatal slices. Striatal slices were incubated in hypoxia-/hypoglycemia-inducing medium with or without AVP (0.01-1.0 microM) for 20 min. After 1-3 h of washout in normal medium, high K(+)-evoked dopamine release from the slices were examined. Hypoxia/hypoglycemia-induced decrease of striatal dopamine release was reversed by the removal of Ca2+ in the medium, but not by VP1- or VP2-receptor antagonist. In contrast, AVP potentiated the hypoxia/hypoglycemia-induced decrease of dopamine release in the striatum. This AVP-induced deterioration of the striatal response was antagonized by VP2 receptor antagonist, but not by VP1 receptor antagonist. The present results suggest that AVP may play a facilitatory role in hypoxia/hypoglycemia-induced dopamine release deficit mediated through the activation of VP2 receptor.