Intracranial microdialysis of salicylic acid to detect hydroxyl radical generation by monoamine oxidase inhibitor in the rat

Pharmacology profile of F17464, a dopamine D3 receptor preferential antagonist

F17464 (N-(3-{4-[4-(8-Oxo-8H-[1,3]-dioxolo-[4,5-g]-chromen-7-yl)-butyl]-piperazin-1-yl}-phenyl)-methanesulfonamide, hydrochloride) is a new potential antipsychotic with a unique profile. The compound exhibits high affinity for the human dopamine receptor subtype 3 (hD3) (Ki = 0.17 nM) and the serotonin receptor subtype 1a (5-HT1a) (Ki = 0.16 nM) and a >50 fold lower affinity for the human dopamine receptor subtype 2 short and long form (hD2s/l) (Ki = 8.9 and 12.1 nM, respectively). [14C]F17464 dynamic studies show a slower dissociation rate from hD3 receptor (t1/2 = 110 min) than from hD2s receptor (t1/2 = 1.4 min) and functional studies demonstrate that F17464 is a D3 receptor antagonist, 5-HT1a receptor partial agonist. In human dopaminergic neurons F17464 blocks ketamine induced morphological changes, an effect D3 receptor mediated. In vivo F17464 target engagement of both D2 and 5-HT1a receptors is demonstrated in displacement studies in the mouse brain. F17464 increases dopamine release in the rat prefrontal cortex and mouse lateral forebrain - dorsal striatum and seems to reduce the effect of MK801 on % c-fos mRNA medium expressing neurons in cortical and subcortical regions. F17464 also rescues valproate induced impairment in a rat social interaction model of autism. All the neurochemistry and behavioural effects of F17464 are observed in the dose range 0.32-2.5 mg/kg i.p. in both rats and mice. The in vitro - in vivo pharmacology profile of F17464 in preclinical models is discussed in support of a therapeutic use of the compound in schizophrenia and autism.

Oxidative stress and ischemic injuries in heat stroke

When rats were exposed to high environmental temperature (e.g., 42 or 43 degrees C), hyperthermia, hypotension, and cerebral ischemia and damage occurred during heat stroke were associated with increased production of free radicals (specifically hydroxyl radicals and superoxide anions), higher lipid peroxidation, lower enzymatic antioxidant defenses, and higher enzymatic pro-oxidants in the brain of heat stroke-affected rats. Pretreatment with conventional hydroxyl radical scavengers (e.g., mannitol or alpha-tocopherol) prevented increased production of hydroxyl radicals, increased levels of lipid peroxidation, and ischemic neuronal damage in different brain structures attenuated with heat stroke and increased subsequent survival time. Heat shock preconditioning (a mild sublethal heat exposure for 15min) or regular, daily exercise for at least 3 weeks, in addition to inducing overproduction of heat shock protein 72 in multiple organs including brain, significantly attenuated the heat stroke-induced hyperthermia, hypotension, cerebral ischemia and damage, and overproduction of hydroxyl radicals and lipid peroxidation. The precise function of heat shock protein 72 are unknown, but there is considerable evidence that these proteins are essential for survival at both normal and elevated temperatures. They also play a critical role in the development of thermotolerance and protection from oxidative damage associated with cerebral ischemia and energy depletion during heat stroke. In addition, Shengmai San or magnolol (Chinese herbal medicines) or hypervolemic hemodilution (produced by intravenous infusion of 10% human albumin) is effective for prevention and repair of ischemic and oxidative damage in the brain during heat stroke. Thus, it appears that heat shock protein 72 preconditioning induced by prior heat shock or regular exercise training, as well as pretreatment with Shengmai San or magnolol is able to prevent the oxidative damage during heat stroke. On the other hand, hypervolemic hemodilution, Shengmai San, or magnolol is able to treat the oxidative damage after heat stroke onset.

Heat shock pretreatment may protect against heatstroke-induced circulatory shock and cerebral ischemia by reducing oxidative stress and energy depletion

The mechanisms underlying the protective effects of heat shock pretreatment on heatstroke remain unclear. Here we attempted to ascertain whether the possible occurrence of oxidative stress and energy depletion exhibited during heatstroke can be reduced by heat shock preconditioning. In the present study, colonic temperature, mean arterial pressure, heart rate, striatal levels of heat shock protein 72 (HSP72), local Po2, brain temperature, cerebral blood flow, cellular ischemia and damage markers, dihydroxybenzoic acid (DHBA), lipid peroxidation, glutathione, glutathione peroxidase and reductase activities, and ATP were assayed in normothermic control rats and in heatstroke rats with or without preconditioning 16 or 96 h before initiation of heatstroke. Heatstroke was induced by exposing the anesthetized rats to a high ambient temperature (Ta = 43 degrees C) until the moment at which MAP decreased from its peak level. Sublethal heat shock pretreatment 16 h before initiation of heatstroke, in addition to increasing striatal HSP72 levels, conferred significant protection against heatstroke-induced arterial hypotension, striatal ischemia and damage, increment of hydroxyl radical formation, lipid peroxidation, glutathione oxidation, and decrement of glutathione peroxidase activity and ATP. However, at 96 h after heat shock, when striatal HSP72 expression returned to basal levels, the above responses that occurred during onset of heatstroke were indistinguishable between the two groups. These results suggest that heat shock pretreatment induces HSP72 overexpression in striatum and confers protection against heatstroke-induced striatal ischemia and damage by reducing oxidative stress and energy depletion.

Chinese Herbal Medicine, Shengmai San, Is Effective for Improving Circulatory Shock and Oxidative Damage in the Brain During Heatstroke

The aim of this study was to investigate the effect of Shengmai San (SMS), a traditional Chinese herbal medicine, on heatstroke-induced circulatory shock and oxidative damage in the brain in rats. Anesthetized rats were exposed to a high ambient temperature (43 degrees C) to induce heatstroke. After the onset of heatstroke, the values of mean arterial pressure, cerebral perfusion pressure, cerebral blood flow, and brain partial pressure of O(2) were all significantly lower than those in normothermic controls. However, the values of intracranial pressure, brain and colonic temperatures, and brain levels of free radicals, lipid peroxidation, and cellular ischemia and damage markers were all greater in heatstroke rats compared with those of normothermic controls. Pretreatment or post-treatment with SMS significantly reduced the hypotension, intracranial hypertension, cerebral hypoperfusion and hypoxia and increased levels of ischemia and damage markers in the brain during heatstroke. The protective effects exerted by SMS pretreatment is superior to those of SMS post-treatment. The results demonstrate that SMS is effective for prevention and repair of circulatory shock and ischemic and oxidative damage in the brain during heatstroke.

Protective effects of alpha-tocopherol and mannitol in both circulatory shock and cerebral ischaemia injury in rat heatstroke

1. There is evidence that hydroxyl radicals are accumulated and oxidative stress is produced in multiple organs, including the brain, of rats with heat stroke. Herein, we investigated the effect on heat stroke-induced circulatory shock and cerebral ischaemic injury of two free radical scavengers, namely mannitol and alpha-tocopherol. 2. Urethane-anaesthetized rats were exposed to heat stress (ambient temperature 42 degrees C) to induce heat stroke. Control rats were exposed to 24 degrees C. Mean arterial pressure and cerebral blood flow after the onset of heat stroke were significantly lower in heat stroke rats than in control rats. However, cerebral free radicals, lipid peroxidation and the neuronal damage score were greater in heat stroke rats compared with control rats. Similarly, plasma cytokines, including tumour necrosis factor-alpha, interleukin (IL)-1beta and IL-6, were significantly higher in heat stroke rats compared with their normothermic controls. 3. Pretreatment with alpha-tocopherol (20 mg/kg, i.v.) or mannitol (10%, i.v.) 30 min before the onset of heat exposure significantly attenuated heat stroke-induced arterial hypotension, cerebral ischaemia and neuronal damage, the increased free radical formation and lipid peroxidation in the brain and the increased plasma levels of cytokines. Pretreatment with alpha-tocopherol or mannitol resulted in a prolongation of survival time in heat stroke. 4. These results demonstrate that although pretreatment with alpha-tocopherol and mannitol does not prevent the heat stroke syndrome entirely, an attenuation of the syndrome is observed.

Magnolol protects against cerebral ischaemic injury of rat heatstroke

1. Free radicals mediate cerebral ischaemic injury associated with heatstroke. Magnolol, an active component of Magnolia officinalis, is 1000-fold more potent than alpha-tocopherol in inhibiting lipid peroxidation in rat mitochondria. The aim of the present study was to ascertain whether magnolol attenuated cerebral ischaemic injury and free radical formation associated with heatstroke. 2. Urethane-anaesthetized rats were exposed to heat stress (ambient temperature 42 degrees C) to induce heatstroke. Controlled rats were exposed to 24 degrees C. Mean arterial pressure, cerebral perfusion pressure and cerebral blood flow after the onset of heatstroke were all significantly lower than in control rats. However, colonic temperature, intracranial pressure, heart rate, cerebral free radicals, lipid peroxidation and the neuronal damage score were greater after the onset of heatstroke. 3. Magnolol (20 or 40 mg/kg, i.v.) significantly attenuated the heatstroke-induced hyperthermia, arterial hypotension, intracranial hypertension, cerebral ischaemia and neuronal damage and increased free radical formation and lipid peroxidation in the brain. The extracellular concentrations of ischaemic (e.g. glutamate and lactate/pyruvate ratio) and damage (e.g. glycerol) markers in the corpus striatum were increased after the onset of heatstroke. Magnolol significantly attenuated the increase in striatal ischaemia and damage markers associated with heatstroke. 4. Thus, it appears that magnolol has impressive effects against heatstroke reactions.

Role of hydroxyl radical formation in neurotoxicity as revealed by in vivo free radical trapping

Reactive oxygen species have been implicated in dopaminergic toxicity caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and iron. Although MPTP produces a parkinsonian syndrome after its conversion to 1-methyl-4-phenylpyridine (MPP(+)) by type B monoamine oxidase (MAO-B) in the brain, the etiology of this disease remains obscure. MPP(+) is a highly potent dopaminbergic-releasing agents and dopamine (DA) autoxidation catalyzed by iron and oxidative stress may be involved in the pathogenesis of Parkinson's disease. Neuromelanine synthesis from DA produce highly reactive free radicals. Although the controversy possible neurotoxin and/or neuroprotective roles of nitric oxide (NO) was discussed, NO contributes to oxidative injury to brain neurons in vivo. An environmental estrogen-like chemical also related to MPP(+)-induced *OH generation. This review describes actual mechanism of the free radicals formation by dialysis studies of in vivo free radical trapping in the pathogenesis of neurodegenerative disorders, including in the Parkinson's disease, Alzheimer disease and traumatic brain injuries.

Oxidative stress in rats with heatstroke-induced cerebral ischemia

BACKGROUND AND PURPOSE

Heatstroke is associated with cerebral ischemia as well as increased levels of interleukin-1beta, dopamine, and glutamate in the brain. These factors are known to increase free radical production. This study attempted to ascertain whether an excessive accumulation of cytotoxic free radicals in the brain and oxidative stress can occur during heatstroke.

METHODS

Urethane-anesthetized rats underwent instrumentation for the measurement of mean arterial pressure, cerebral blood flow, neuronal damage score, and colonic temperature. Rats were exposed to heat stress (ambient temperature, 42 degrees C) until mean arterial pressure and cerebral blood flow began to decrease from their peak levels, which was arbitrarily defined as the onset of heatstroke. Controlled rats were exposed to 24 degrees C. Concentrations of dihydroxybenzoic acid, lipid peroxidation, rate of O2*- generation, superoxide dismutase, and catalase activity of the brain or other vital organs were assessed during heatstroke.

RESULTS

The values of mean arterial pressure and cerebral blood flow after heatstroke onset were all significantly lower than those in control rats. However, the values of colonic temperature, dihydroxybenzoic acid levels in the striatum, and neuronal damage score were greater. The extent of lipid peroxidation in the brain and the rate of O2*- generation in the brain, liver, and heart were all greater in rats after heatstroke onset. In contrast, the values of total superoxide dismutase in the brain, liver, and heart and the catalase activity in the brain were lower.

CONCLUSIONS

Taken together, these results indicate that hydroxyl radicals mediate cerebral ischemic injury associated with heatstroke.

Measurement of free radical production by in vivo microdialysis during ischemia/reperfusion injury to skeletal muscle

Microdialysis techniques have been used to detect hydroxyl radical and superoxide release into the interstitial space of anaesthetized rat anterior tibialis muscles during a period of prolonged (4 h) limb ischemia and subsequent reperfusion. Data indicate that reperfusion of the ischemic skeletal muscle was associated with a large increase in hydroxyl radical activity in the interstitial space, which may contribute to the significant oxidation of muscle glutathione, protein thiols, and lipids also seen in this model. No evidence for release of superoxide into the interstitial space was found during reperfusion, although this was observed during electrically stimulated contractile activity of the rat limb muscle. These data imply that therapeutic approaches aimed at reduction of hydroxyl radical generation in the interstitial fluid are more likely to be beneficial in reduction of skeletal muscle reperfusion injury than approaches designed to scavenge superoxide radicals.

Effects of nitric oxide on reactive oxygen species production and infarction size after brain reperfusion injury

OBJECTIVE

Deleterious effects of strokes may be ameliorated when thrombolysis (i.e., with recombinant tissue plasminogen activator) restores circulation. However, reperfusion injury, mediated by oxygen free radicals (reactive oxygen species [ROS]), may limit the benefits of recombinant tissue plasminogen activator treatment. We hypothesized that, during reperfusion, exogenous nitric oxide (NO) would reduce stroke size by quenching ROS.

METHODS

To investigate this hypothesis, we used two in vivo ischemia-reperfusion models, i.e., autologous cerebral embolism in rabbits and filament middle cerebral artery occlusion in rats. Using these models, we measured ROS levels (rabbit model) and stroke volumes (rat model) in response to transient ischemia, with and without intracarotid administration of ultrafast NO donor proline NO (proliNO).

RESULTS

In the rabbit cerebral embolism model, intracarotid administration of proliNO (10(-6) mol/L) (n = 6) during reperfusion decreased free radical levels from 538 +/- 86 nmol/L in the vehicle-treated group (n = 7) to 186 +/- 31 nmol/L (2,3'-dihydroxybenzoic acid; P < 0.001) and from 521 +/- 86 nmol/L (n = 7) to 201 +/- 39 nmol/L (2,5'-dihydroxybenzoic acid; P < 0.002). In the rat middle cerebral artery occlusion model, intracarotid administration of proliNO (10(-5) mol/L) (n = 10) during reperfusion reduced the brain infarction volume from 256 +/- 48 mm3 in the vehicle-treated group (n = 8) to 187 +/- 41 mm3 (P < 0.005). In both experimental groups, intracarotid infusion of proliNO did not affect regional cerebral blood flow, mean arterial blood pressure, or brain and body temperatures.

CONCLUSION

The beneficial effects of early restoration of cerebral circulation after cerebral ischemia were enhanced by intracarotid infusion of proliNO, most likely because of ROS scavenging by NO. These findings suggest the possibility of preventive treatment of reperfusion injury using NO donors.

[Parkinsonism induced by MPTP and free radical generation]

Oxygen free radical formation has been implicated in dopaminergic toxicity caused by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and iron. Although MPTP produces a parkinsonian syndrome after its conversion to 1-methyl-4-phenylpyridine (MPP+) by type B monoamine oxidase (MAO-B) in the brain, the etiology of this disease remains obscure. MPP+ is one of the most potent dopamine (DA)-releasing agents. Iron-catalyzed DA autoxidation and oxidative stress may be involved in the pathogenesis of Parkinson's disease. If indeed the effect of MPP+ on hydroxyl radical (.OH) formation is due to DA release, reserpine-induced DA depletion may reduce MPP(+)-induced .OH formation. Imidapril, an angiotensin converting enzyme (ACE) inhibitor, can resist MPP(+)-induced .OH formation via suppression of release of DA by angiotensin. Histidine, a singlet oxygen (1O2) scavenger, protects MPP(+)-induced .OH formation. Fluvastatin, an inhibitor of low-density lipoprotein (LDL) oxidation, can resist MPP(+)-induced .OH formation. The inhibitory effect on the susceptibility of LDL oxidation can reduce .OH generation. These drugs may be applied as antiparkinsonian agents. Further clinical investigation is necessary in the future.

Detection of salicylate and its hydroxylated adduct 2,3-dihydroxybenzoic acid in glutamate neurotoxicity and the effects of verapamil and ryanodine in rats

It has been suggested that salicylate hydroxylation can be used to detect hydroxyl radical formation in vivo. In the present study we investigated the effects of verapamil and or ryanodine on salicylate (SA) and its hydroxylated adduct; 2,3-dihydroxybenzoic acid (2,3-DHBA) levels in glutamate induced neurotoxicity of whole rat brains. To detect SA and 2,3-DHBA, an HPLC-EC/UV method was used. Retention time was found to be 3.9 min for 2,3-DHBA and 12.0 min for SA. Verapamil at 10(-5) and 10(-7) and ryanodine at 10(-5) M concentrations were found to have a significant decreasing effect on this degradation induced by glutamate. This was the highest dose for ryanodine tested. As an L-type voltage dependent calcium channel blocker, verapamil was found ineffective at 10(-4), 10(-6) and 10(-8) M concentrations. Surprisingly, none of the combined application groups (verapamil + ryanodine) was found effective on SA hydroxylation. As a result, ryanodine was effective only at the highest dose, while verapamil exerts its effect in a dose dependent fashion as reported before in the literature.

Determination of hydroxyl free radical formation in human platelets using high-performance liquid chromatography with electrochemical detection

The formation of the hydroxyl free radical (HFR) can be quantified indirectly, by measuring two products of the hydroxylation of salicylic acid, 2,3-dihydroxybenzoate (2,3-DHB) and 2,5-dihydroxybenzoate (2,5-DHB). In this study, we used reversed-phase high-performance liquid chromatography with electrochemical (coulometric) detection to measure 2,3-and 2,5-DHB levels in human platelets. The limits of detection of the method were 10 and 5 fmol on column for 2,3-DHB and 2,5-DHB, respectively. We tested the technique by measuring increases in dihydroxybenzoate levels after exposure of platelets to experimentally induced oxidative stress. Then, we measured platelet levels of 2,3- and 2,5-DHB in patients with Parkinson's disease, under therapy with L-DOPA, and in normal subjects. We also measured platelet concentrations of L-DOPA and its major metabolite, 3-O-methyldopa (3-OMD). Parkinsonian patients showed increased levels of both 2,3- and 2,5-DHB. Platelet levels of 2,3-DHB were positively correlated with platelet levels of L-DOPA and 3-OMD. The technique we describe proved simple and extremely sensitive and may represent a useful tool for the study of oxidative stress in humans.

Peripheral markers of oxidative stress in Parkinson's disease. The role of L-DOPA

Oxidative stress plays a central role in the pathogenesis of Parkinson's disease (PD). L-DOPA, the gold standard in PD therapy, may paradoxically contribute to the progression of the disease because of its pro-oxidant properties. The issue, however, is controversial. In this study, we evaluated peripheral markers of oxidative stress in normal subjects, untreated PD patients and PD patients treated only with L-DOPA. We also measured platelet and plasma levels of L-DOPA, 3-O-methyldopa (the long-lasting metabolite of the drug), and dopamine. We found that isolated platelets of treated PD patients form higher amounts of 2,3-dihydroxybenzoate, an index of hydroxyl radical generation, than platelets of controls or untreated patients. In treated patients, platelet levels of 2,3-dihydroxybenzoate were positively correlated with platelet levels of L-DOPA, 3-O-methyldopa, and with the score of disease severity. Disease severity was correlated with platelet and plasma levels of L-DOPA, as well as with the daily intake of the drug. No significant differences in platelet levels of cytosolic and mitochondrial isoforms of the antioxidant enzyme superoxide dismutase were found between PD patients, either treated or untreated, and controls. Our findings lend further support to the hypothesis that L-DOPA might promote free radical formation in PD patients.

Reserpine prevents hydroxyl radical formation by MPP+ in rat striatum

The present study investigated the blockage of dopamine (DA) oxidation by reserpine. 1-Methyl-4 phenylpyridinium ion (MPP+) increased the release of DA and the formation of hydroxyl radical ( r22. OH). The r22. OH generated by DA when captured as the hydroxylated derivative of salicylic acid was measured by the high-performance liquid chromatographic-electrochemical (HPLC-EC) procedure. MPP+ concentration for half-maximal effect of DA producing release (EC50) was 5.2 mM. The maximum attainable concentration of dialysate DA (Emax) by MPP+ was 7.7 microM. However, the EC50 and Emax values with reserpinized animal were 5.2 mM and 1.2 microM, respectively. When high concentration of pargyline (10 mM) were infused in MPP+ (5 mM)-pretreated animals, a marked elevation of DA and r22. OH formation was observed. The level of DA and 2, 3-DHBA formations was drastically reduced, as compared with the MPP+-only treated group. Although the levels of MPP+-induced DA and 2,3-DHBA formation after pargyline treatment increased, pargyline failed to increase either the level of MPP+-induced DA or 2,3-DHBA in the reserpinized group. When DA was administered to the MPP+-pretreatment group, a marked elevation was observed, showing a positive linear correlation DA and r22. OH formation trapped as 2,3-DHBA (R2=0.978) in the dialysate. When corresponding experiment were performed with iron (II), the same results were obtained: a positive linear correlation between the release of DA and 2,3-DHBA (R2=0.989) in the dialysate. These results indicated that reserpine-induced DA depletion may reduce MPP+-induced r22. OH formation.

Synthesis of [11C]salicylic acid and related compounds and their biodistribution in mice

For in vivo measurement of the hydroxyl radical (.OH), we synthesized [11C]salicylic acid, [11C]O-acetylsalicylic acid and [11C]2-methoxybenzoic acid by carboxylation of 2-bromomagnesiumanisol using [11C]CO2. The radiochemical yield of [11C]salicylic acid, [11C]O-acetylsalicylic acid and [11C]2-methoxybenzoic acid calculated from trapped [11C]CO2 in a liquid argon cooled stainless tube was 7.3 +/- 1.6, 5.2 and 10.2 +/- 1.7% (decay corrected), respectively. The uptake of 11C tracers by mouse brain was 0.46, 0.32, and 0.46% dose/g tissue, respectively, at 10 min post injection and presented washout patterns thereafter.

Effect of hemorrhagic hypotension on hydroxyl radicals in cat brain

This study investigated the relationships between blood pressure, cortical oxygen pressure and hydroxyl radicals in the brain of adult cats during hemorrhagic hypotension and retransfusion. Oxygen pressure in the blood of the cortex was measured optically by the oxygen dependent quenching of phosphorescence and hydroxyl radicals by in vivo microdialysis. Following a 2 h stabilization period after implantation of the microdialysis probe in the striatum, the mean arterial blood pressure (MAP) was decreased in a stepwise manner from 132 +/- 2 Torr (control) to 90 +/- 1 Torr, 70 +/- 3 Torr and 50 +/- 3 Torr, holding the pressure at each level for 15 min. The whole blood was then retransfused and measurements were continued for 90 min. Cortical oxygen pressure progressively decreased with decrease in MAP, decreasing from 50 +/- 2 Torr (control) to 42 +/- 1 Torr, 31 +/- 2 Torr and 22 +/- 2 Torr, respectively. The level of hydroxyl radical increased by 20-25% following first 15 min of bleeding and stay on this level during the remaining period of hypotension. Maximal increase (by 78%) in level of hydroxyl radicals was observed after 15 min of retransfusion. The present study demonstrated that during hypotension and retransfusion there was an increase in the level of hydroxyl radicals in striatum. These can be important mediators of postischemic injury to the striatum.

The effects of aging and oxidative stress on psychomotor and cognitive behavior

Decrements in motor and cognitive function occur in aging, possibly due to oxidative stress-induced damage to the brain. Declines in antioxidant defense mechanisms have been postulated as a causative factor in these age-related decrements, however a clear link between oxidative stress (OS) and behavioral changes in aging has yet to be established. This review shows that age-validated psychomotor and cognitive tests are sensitive to behavioral deficits under different models of OS, including: 1) decreasing OS protection by depleting glutathione and then increasing the OS with dopamine; 2) 100% oxygen exposure; and 3) radiation. Furthermore, interventions that reduce OS result in concurrent improvements in age-associated behavioral deficits. Therefore, age-related changes in behavior may result from an inability to cope with OS that occurs throughout the life-span.

Glibenclamide, an antagonist of ATP sensitive K+ channels, blocks free radical generation in the rat myocardium

The present study examined the effect of glibenclamide, an ATP-sensitive K+ (K(ATP)) channels antagonist, on the potassium chloride (KCl)-induced hydroxyl free radical (.OH) generation. Sodium salicylate in Ringer's solution (0.5 nmol/microl per min) was infused directly through a microdialysis probe to detect the generation of .OH as reflected by the formation of dihydroxybenzoic acid (DHBA) in the myocardium of anesthetized rat. The high concentration of KCl (70 mM) significantly increased the level of 2,3- and 2,5-DHBA by the action of depolarization by KCl. However, in the presence of glibenclamide (10 microM), KCl failed to increase the 2,3- and 2,5-DHBA formation. Moreover, when allopurinol (10 mg/kg), a xanthine oxidase inhibitor, was administered by i.v. injection, the elevation of DHBA was not observed. These results suggest that openings of cardiac K(ATP) channel by depolarization evokes .OH generation via xanthine oxidase reaction.

7-Nitroindazole prevents dopamine depletion caused by low concentrations of MPP+ in rat striatal slices

A significant loss of dopamine was found in rat striatal slices incubated with 1-methyl-4-phenylpyridinium ion (MPP+) at a concentration of 2 microM or higher. The addition of 7-nitroindazole, a specific inhibitor of neuronal nitric oxide synthase (nNOS), prevented this effect on dopamine when the concentration of MPP+ was between 2-5 microM, but not at higher concentrations. This protection was reproduced with other less specific NOS-inhibitors, such as nitro-arginine and nitro-arginine methylester. 7-nitroindazole did not protect against the dopamine depletion caused by the non-specific mitochondrial chain blocker rotenone. Neither MPP- nor rotenone significantly increased the nitrite concentration in striatal slices, measured as an index of nitric oxide production. The basal production of nitric oxide may be enough to trigger the dopamine depletion at very low concentrations of MPP+, probably acting synergistically with cytosolic calcium increase. Higher concentrations of MPP+ are toxic by themselves without the mediation of nitric oxide. The inhibition of nNOS may protect against dopamine loss at early stages of a neurodegenerative process, and it could then be considered in the treatment or prevention of neurodegenerative human processes such as Parkinson's disease.

Spatial learning and memory deficits induced by dopamine administration with decreased glutathione

Administration of buthionine sulfoximine (BSO) selectively inhibits glutathione (GSH) biosynthesis and induces a GSH deficiency. Decreased GSH levels in the brain may result in less oxidative stress (OS) protection, because GSH contributes substantially to intracellular antioxidant defense. Under these conditions, administration of the pro-oxidant, dopamine (DA), which rapidly oxidizes to form reactive oxygen species, may increase OS. To test the cognitive behavioral consequences of decreased GSH, BSO (3.2 mg in 30 microliters, intracerebroventricularly) was administered to male Fischer 344 rats every other day for 4 days. In addition, DA (15 microliters of 500 microM) was administered every day [either 1 h after BSO (BSO + DA group) or 1 h before BSO (DA + BSO group), when given on the same day as BSO] and spatial learning and memory assessed (Morris water maze, six trials/day). BSO + DA rats, but not DA + BSO rats, demonstrated cognitive impairment compared to a vehicle group, as evidenced by increased latencies to find the hidden platform, particularly on the first trial each day. Also, the BSO + DA group utilized non-spatial strategies during the probe trials (swim with no platform): i.e., less time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent in the platform quadrant, fewer crossings and longer latencies to the previous platform location, and more time spent around the edge of the pool rather than in the platform zone. Therefore, the cognitive behavioral consequences of decreasing GSH brain levels with BSO in conjunction with DA administration depends on the order of administration. These findings are similar to those seen previously on rod and plank walking performance, as well as to those seen in aged rats, suggesting that the oxidation of DA coupled with a reduced capacity to respond to oxidative stress may be responsible for the induction of age-related cognitive deficits.

Adenosine deaminase activity in rat intestine: assay with a microdialysis technique

Using microdialysis, we measured adenosine deaminase activity in rat intestine by detecting inosine, a breakdown product of adenosine. The dialysis probe consisted of a 3 x 0.22 mm dialysis fiber with a 50,000 mol wt cut off. When the probe was perfused at 1 microl/min in vitro, the average relative recovery rate of inosine was 22.1+/-0.9%). The dialysis probe was implanted in the intestinal mucosa and perfused with Tyrode solution containing adenosine at 1 microl/min. The dialysate samples were analyzed for inosine by high-performance liquid chromatography with ultraviolet (HPLC-UV) detection at 260 nm. When adenosine (100-1000 microM) was perfused, the level of inosine increased dose-dependently and was saturatable at about 1 mM adenosine. The ED50 of adenosine was 192.6 microM, with a maximum attainable inosine concentration of 59.7 microM. In the presence of aminoguanidine, a adenosine deaminase inhibitor (10 mM or 10 n mol/microl/min), the elevation of inosine was not observed. The dialysis technique makes it possible to measure adenosine deaminase activity in intestinal mucosa.

In vivo evidence for free radical involvement in the degeneration of rat brain 5-HT following administration of MDMA ('ecstasy') and p-chloroamphetamine but not the degeneration following fenfluramine

1. Administration of 3,4-methylenedioxymethamphetamine (MDMA or 'ecstasy') to several species results in a long lasting neurotoxic degeneration of 5-hydroxytryptaminergic neurones in several regions of the brain. We have now investigated whether this degeneration is likely to be the result of free radical-induced damage. 2. Free radical formation can be assessed by measuring the formation of 2,3- and 2,5-dihydroxybenzoic acid (2,3-DHBA and 2,5-DHBA) from salicylic acid. An existing method involving implantation of a probe into the hippocampus and in vivo microdialysis was modified and validated. 3. Administration of MDMA (15 mg kg-1, i.p.) to Dark Agouti (DA) rats increased the formation of 2,3-DHBA (but not 2,5-DHBA) for at least 6 h. Seven days after this dose of MDMA, the concentration of 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) was reduced by over 50% in hippocampus, cortex and striatum, reflecting neurotoxic damage. There was no change in the concentration of dopamine or 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. 4. p-Chloroamphetamine (PCA), another compound which produces a neurotoxic loss of cerebral 5-HT content, when given at a dose of 5 mg kg-1 also significantly increased the formation of 2.3-DHBA (but not 2,5-DHBA) in the dialysate for over 4.5 h. post-injection starting 2 h after treatment. 5. In contrast, fenfluramine administration (15 mg kg-1, i.p.) failed to increase the 2,3-DHBA or 2,5-DHBA concentration in the dialysate. A single fenfluramine injection nevertheless also markedly decreased the concentration of 5-HT and 5-HIAA in the hippocampus, cortex and striatum seven days later. 6. When rats pretreated with fenfluramine (15 mg kg-1, i.p.) seven days earlier were given MDMA (15 mg kg-1, i.p.) no increase in 2,3-DHBA was seen in the dialysate from the hippocampal probe. This indicates that the increase in free radical formation following MDMA is occurring in 5-HT neurones which have been damaged by the prior fenfluramine injection. 7. Administration of the free radical scavenging agent alpha-phenyl-N-tert-butyl nitrone (PBN; 120 mg kg-1, i.p.) 10 min before and 120 min after an MDMA (15 mg kg-1, i.p.) injection prevented the acute rise in the 2,3-DHBA concentration in the dialysate and attenuated by 30% the long term damage to hippocampal 5-HT neurones (as indicated by a smaller MDMA-induced decrease in both the concentration of 5-HT and 5-HIAA and also the binding of [3H]-paroxetine). 8. These data indicate that a major mechanism by which MDMA and PCA induce damage to 5-hydroxytryptaminergic neurones in rat brain is by increasing the formation of free radicals. These probably result from the degradation of catechol and quinone metabolites of these substituted amphetamines. In contrast, fenfluramine induces damage by another mechanism not involving free radicals; a proposal supported by some of our earlier indirect studies. 9. We suggest that these different modes of action render untenable the recent suggestion that MDMA will not be neurotoxic in humans because fenfluramine appears safe at clinical doses.

Use of microdialysis for in-vivo monitoring of hydroxyl free-radical generation in the rat

Free-radicals are reported to cause the tissue-damage associated with some toxins and diseases, yet there is no suitable method for routine in-vivo monitoring of these species. This paper introduces an in-vivo microdialysis technique in which the hydroxyl radical reacts with salicylate to generate dihydroxybenzoic acids (DHBA) which are measured by HPLC with electrochemical detection. When pargyline, a monoamine oxidase inhibitor, was infused into rat brain, the levels of DHBA increased markedly. When noradrenaline was administered to animals pre-treated with pargyline, DHBA levels increased markedly compared with the group treated with noradrenaline only. When the heart was subjected to 15-min regional ischaemia by occlusion of the left anterior descending coronary artery, levels of DHBA in heart dialysate were unchanged. Electrical stimulation of the stellate ganglion resulted in marked elevation of levels of DHBA the myocardial dialysate. Infusion of Fe2+ into rat liver resulted in increased formation of DHBA. When the intestine was rendered ischaemic for 10, 20 and 30 min, the highest DHBA level was obtained after 10-min ischaemia and the lowest after 30 min. These results confirm that free-radical production might make a major contribution at certain stages in the progression of the injury.

Cardiac microdialysis of salicylic acid .OH generation on nonenzymatic oxidation by norepinephrine in rat heart

The effect of pargyline, a monoamine oxidase inhibitor, on the generation of hydroxyl free radicals (.OH) was investigated using cardiac microdialysis. Salicylic acid in Ringer's solution (0.5 nmol x microL(-1) x min(-1)) was infused directly through a microdialysis probe to detect the generation of .OH as reflected by the formation of dihydroxybenzoic acid (DHBA) in the myocardium of anesthetized rats. When pargyline (100 nmol x microL(-1) x min(-1)) was infused in rat heart, the level of norepinephrine (NE) gradually increased in a time-dependent manner and an increase of DHBA was also observed. When NE was administered to the pargyline pretreated animals, a marked elevation in the levels of 2,3- and 2,5-DHBA formation was obtained, as compared to the group treated with NE only, showing a positive linear correlation between NE and .OH formation trapped as 2,3-DHBA (R2 = 0.981) or 2,5-DHBA (R2 = 0.984) in the dialysate. NE clearly produced an increase in .OH formation. These results indicate that accumulation of NE in the extracellular fluid elicited by pargyline can be auto-oxidized, which in turn, leads (possibly by an indirect mechanism) to the formation of cytotoxic .OH free radicals.

Protective effect of carbidopa on hydroxyl radical generation in the rat striatum by dopamine

The effect of L-3,4-dihydroxyphenylalanine (L-DOPA) on the generation of hydroxyl free radicals (.OH) was investigated using striatal microdialysis technique. Salicylic acid in Ringer's solution (0.5 nmol.microliters-1.min-1) was infused directly through a microdialysis probe to detect the generation of .OH as reflected by the formation of dihydroxybenzoic acids (DHBA) in the striatum. When L-DOPA (0.1 mM; 1 microliter.min-1) was infused in the rat brain, the level of 3,4-dihydroxyphenylacetic acid (DOPAC) gradually increased in a time-dependent manner. In addition, a marked elevation of DHBA was observed. However, in the presence of carbidopa, a decarboxylase inhibitor, the elevation in DHBA formation was not observed. These results suggest that carbidopa may suppress the .OH generation by dopamine (DA).

Cardiac microdialysis of salicylic acid to detect hydroxyl radical generation associated with sympathetic nerve stimulation

We examined the relationship between norepinephrine (NE) and hydroxyl free radical (OH) generation on cardiac nerve stimulation. Salicylic acid in Ringer's solution (0.5 nmol microliter-1 min-1) was infused directly through a microdialysis probe to detect the generation of hydroxyl radicals (OH) as reflected by the formation of dihydroxybenzoic acid (DHBA) in the myocardium of anesthetized rats. Sympathetic nerve stimulation increased the release of NE and the formation of DHBA. A positive linear correlation between the release of NE and the formation of 2,3-DHBA (R2 = 0.982) or 2,5-DHBA (R2 = 0.976) was observed. These data indicate that the sustained elevation of NE in the extracellular fluid can be auto-oxidized, which in turn leads (possibly by an indirect mechanism) to the formation of cytotoxic OH free radicals.