Distribution of spin-trapping compounds in rat blood and brain: in vivo microdialysis determination

Oxidative stress in rat brain during experimental status epilepticus: effect of antioxidants

Antioxidants have been proposed as a treatment for diseases of the central nervous system. However, few studies actually studied their effects in the brain. To test central actions of antioxidants, we used the lithium-pilocarpine (Li-Pilo) model of status epilepticus (SE) in the rat in which seizures are accompanied by significant oxidative stress. We used in vivo microdialysis to determine isoprostane levels during SE in real time and brain homogenates for other measures of oxidative stress. Six different antioxidants were tested in acute and preventive experiments (vitamin C, vitamin E, ebselen, resveratrol, n-tert-butyl-α-phenylnitrone and coenzyme Q10). None of the antioxidants had an effect when given acutely during SE. In contrast, when antioxidants were given for 3 days prior to seizure induction, vitamins C and E reduced isoprostane formation by 58% and 65%, respectively. Pretreatment with the other antioxidants was ineffective. In brain homogenates prepared after 90 min of seizures, SE decreased the ratio of reduced vs. oxidized glutathione (GSH/GSSG ratio) from 60.8 to 7.50 and caused a twofold increase of 8-hydroxy-2'-deoxyguanosine (8-OHdG) levels and protein carbonyls. Pretreatment with vitamin C or vitamin E mitigated these effects and increased the GSH/GSSG ratio to 23.9 and 28.3, respectively. Again, the other antioxidants were not effective. We conclude that preventive treatment with vitamin C or vitamin E ameliorates seizure-induced oxidative damage in the brain. Several well-studied antioxidants were inactive, possibly due to limited brain permeability or a lack of chain-breaking antioxidant activity in hydrophilic compounds.

Pharmacokinetics of doxorubicin in glioblastoma multiforme following ultrasound-Induced blood-brain barrier disruption as determined by microdialysis

The goal of this study was to investigate the in vivo extracellular kinetics of doxorubicin (Dox) in glioblastoma multiforme (GBM)-bearing mice following focused ultrasound (FUS)-induced blood-brain barrier (BBB) disruption using microdialysis. An intracranial brain tumor model in NOD-scid mice using human brain GBM 8401 cells was used in this study. Prior to each sonication, simultaneous intravenous administration of Dox and microbubbles, and the Dox concentration in the brains was quantified by high performance liquid chromatography (HPLC). Drug administration with sonication elevated the tumor-to-normal brain Dox ratio of the target tumors by about 2.35-fold compared with the control tumors. The mean peak concentration of Dox in the sonicated GBM dialysate was 10 times greater than without sonication, and the area under the concentration-time curve was 3.3 times greater. This study demonstrates that intracerebral microdialysis is an effective means of evaluating real-time target BBB transport profiles and offers the possibility of investigating the pharmacokinetics of drug delivery in the sonicated brain.

The free radical scavenger S-PBN significantly prolongs DSG-mediated graft survival in experimental xenotransplantation

BACKGROUND

Nitrones such as 2-sulfo-phenyl-N-tert-butyl nitrone (S-PBN) are known to trap and stabilize free radicals and to reduce inflammation. Recently, S-PBN was shown to reduce infiltration of T lymphocytes and the expression of adhesion molecules on the endothelium in experimental traumatic brain injury. We hypothesized that S-PBN could reduce infiltration of T lymphocytes during cell-mediated xenograft rejection and thereby increase graft survival. The concordant mouse-to-rat heart transplantation model was used to test the hypothesis. In this model, grafts undergo acute humoral xenograft rejection (AHXR) almost invariably on day 3 and succumb to cell-mediated rejection on approximately day 8 if AHXR is inhibited by treatment with 15-deoxyspergualin (DSG).

MATERIAL AND METHODS

Hearts from Naval Medical Research Institute (NMRI) mice were transplanted to the neck vessels of Lewis rats. Recipients were treated with S-PBN (n=9), DSG (n=9), S-PBN and DSG in combination (n=10) or left untreated (n=9) for survival studies. S-PBN was given daily intraperitoneally at a dose of 150 mg/kg body weight (BW) on day -1 to 30, and DSG was given daily intraperitoneally at a dose of 10 mg/kg BW on day -1 to 4 and 5 mg/kg BW on day 5 to 21. Nine additional recipients were given S-PBN only on days -1 and 0 in combination with continuous DSG treatment. Grafts were monitored until they stopped beating. Additional recipients were treated with S-PBN (n=5), DSG (n=5), S-PBN and DSG in combination (n=6) or left untreated (n=5) for morphological, immunohistochemical and flow cytometry analyses on days 2 and 6 after transplantation.

RESULTS

S-PBN treatment in combination with DSG resulted in increased median graft survival compared to DSG treatment alone (14 vs. 7 days; P=0.019). Lower number of T lymphocytes on day 6 (P=0.019) was observed by ex vivo propagation and flow cytometry when combining S-PBN with DSG, whereas immunohistochemical analyses demonstrated a significant reduction in the number of infiltrated CD4+, but not TCR+, cells. S-PBN treatment alone had no impact on graft survival compared to untreated rats (3 vs. 3 days). No differences were seen in ICAM-1 and VCAM-1 expression or in morphology between the groups.

CONCLUSION

The combination of S-PBN and DSG treatment increases xenograft survival. The main effect of S-PBN appears to be in direct connection with the transplantation. Because of its low toxicity, S-PBN could become useful in combination with other immunosuppressants to reduce cell-mediated xenograft rejection.

The nitrone free radical scavenger NXY-059 is neuroprotective when administered after traumatic brain injury in the rat

Reactive oxygen species (ROS) are important contributors to the secondary injury cascade following traumatic brain injury (TBI), and ROS inhibition has consistently been shown to be neuroprotective following experimental TBI. NXY-059, a nitrone free radical trapping compound, has been shown to be neuroprotective in models of ischemic stroke but has not been evaluated in experimental TBI. In the present study, a continuous 24-h intravenous infusion of NXY-059 or vehicle was initiated 30 min following a severe lateral fluid percussion brain injury (FPI) in adult rats (n=22), and histological and behavioral outcomes were evaluated. Sham-injured animals (n=22) receiving identical drug infusion were used as controls. Visuospatial learning was evaluated in the Morris water maze at post-injury days 11-14, followed by a probe trial (memory test) at day 18. The animals were sacrificed at day 18, and loss of hemispheric brain tissue was measured in microtubule-associated protein (MAP)-2 stained sections. Brain-injured, NXY-059-treated animals showed a significant reduction of visuospatial learning deficits when compared to the brain-injured, vehicle-treated control animals (p < 0.05). NXY-059-treated animals significantly reduced the loss of hemispheric tissue compared to brain-injured controls (43.0 +/- 11 mm3 versus 74.4 +/- 19 mm3, respectively; p < 0.01). The results show that post-injury treatment with NXY-059 significantly attenuated the loss of injured brain tissue and improved cognitive outcome, suggesting a major role for ROS in the pathophysiology of TBI.

Fine-tuning the amphiphilicity: a crucial parameter in the design of potent alpha-phenyl-N-tert-butylnitrone analogues

A new series of hydrophilic, lipophilic, and amphiphilic alpha-phenyl-N-tert-butylnitrone (PBN) derivatives were synthesized to explore the relationship between their hydrophilic-lipophilic properties and antioxidant potency. Very potent protective effects of amphiphilic lactobionamide and tris(hydroxymethyl)aminomethane PBN derivatives were observed in mitochondrial preparations, in cell cultures, and in rotifers exposed to unspecific and mitochondria targeted oxidotoxins.

Antioxidants attenuate MK-801-induced cortical neurotoxicity in the rat

Oxidative stress has been implicated in the pathogenesis of several neurodegenerative diseases and may result from excessive free radical production due to increased local metabolism. Non-competitive N-methyl-D-aspartate (NMDA) antagonists (MK-801 and phencyclidine) increase glucose metabolism in many brain areas and induce cytoplasmic vacuoles, heat shock protein and necrotic cell death in neurones of the rodent posterior cingulate and retrosplenial cortex. We have investigated the effect of several antioxidants with differing properties on MK-801-induced neuronal loss. Free radical scavengers (dimethyl sulfoxide (DMSO) and alpha-tocopherol) and spin traps (N-tert-butyl-alpha-(2-sulfophenyl)-nitrone (S-PBN) and 5-(diethoxyphosphoryl)-5-methyl-1-pyrrole N-oxide (DEPMPO)), produced marked attenuation of MK-801-induced neuronal necrosis in the rat posterior cingulate and retrosplenial cortex. Further, administration of DMSO could be delayed by up to 4 h after MK-801 dosing and still achieve between 80 and 86% reduction in neuronal loss. We also show that MK-801 administration rapidly induced a four-fold and prolonged increase in cerebral blood flow in the posterior cingulate. This elevated regional blood flow was only transiently reduced by DMSO administration. The anterior cingulate, a region which undergoes no neuronal loss, showed only a two-fold increase in regional blood flow following MK-801 administration. These results support a hypothesis that oxidative stress plays a role in MK-801-induced neuronal necrosis since pathological changes can be attenuated by several antioxidants.

In vivo detection of free radicals induced by diethylnitrosamine in rat liver tissue

Diethylnitrosamine (DEN) is a well-known carcinogenic substance that requires microsomal activation before it can react with DNA to cause mutations and cancer. The aim of this study was to use in vivo spin trapping and spin probe techniques to investigate whether free radicals are generated in rat liver tissue during DEN activation. We used alpha-phenyl-n-tert-butylnitrone (PBN) as the spin trapping agent, which was delivered through an intraperitoneal injection before DEN administration. One hour after DEN administration, multicomponent PBN adducts in the bile were detected, and the intensities were diminished by the cytochrome P450 inhibitor SKF-525A. A computer simulation of the ESR signals revealed the presence of a lipid-derived radical. Using the in vivo spin probe/ESR technique, the signal decay rate of methoxycarbonyl-PROXYL was significantly increased in the DEN-treated group compared with the rate in the vehicle group. The enhanced signal decay rate was restored with PBN and/or SKF-525A pretreatment. These results suggested that lipid-derived free radicals were generated in the liver within 1 h after DEN administration.

Use of reversed-phase liquid chromatography for determining the lipophilicity of alpha-aryl-N-cyclopropylnitrones

The relationship between a reversed-phase high-performance liquid chromatography (RP-HPLC) retention parameter and various calculated log P-values of our previously synthesized alpha-aryl-N-cyclopropyl-nitrone derivatives was investigated. The RP-HPLC experiments were carried out with acetonitrile-water and methanol-water mixtures as mobile phases and with two kinds of stationary phases of different polarity. The retention parameter, log k(w) was obtained by linear extrapolation of the log k retention to pure water as the mobile phase. The calculated log P-values were C log P, ACD/log P, R log P, A log P, LogKow, X log P and M log P. Statistically, highly significant correlations were found between log k(w) and the calculated log P-values with squared correlation coefficients ranging from 0.771 (with A log P) to 0.956 (with C log P). In addition, the comparative molecular similarity indices analysis (CoMSIA) method was also applied to correlate the log k(w) retention parameter of the compounds with their molecular fields. Statistically significant CoMSIA models were obtained between log k(w) and the hydrophobic and steric molecular fields of our compounds. The CoMSIA models describe how the structure of the nitrone derivatives influences (through hydrophobic and steric interactions with the stationary phase) the chromatographic retention of the compounds.

Stilbazulenyl nitrone, a second-generation azulenyl nitrone antioxidant, confers enduring neuroprotection in experimental focal cerebral ischemia in the rat: neurobehavior, histopathology, and pharmacokinetics

Stilbazulenyl nitrone (STAZN) is a potent lipophilic second-generation azulenyl nitrone antioxidant, which is highly neuroprotective in rodent models of cerebral ischemia and trauma. This study was conducted to establish whether the neuroprotection induced by STAZN persists with chronic survival and to characterize STAZN's pharmacokinetics. Physiologically regulated rats received a 2-h middle cerebral artery occlusion by intraluminal suture and were treated with either STAZN [four 0.6 mg/kg doses i.p. administered at 2 (i.e., onset of recirculation), 4, 24, and 48 h; n = 16] or dimethyl sulfoxide vehicle (n = 11). They received sequential neurobehavioral examinations followed by quantitative neuropathology at 30 days. STAZN improved neurological deficits compared with vehicle controls, beginning within <2 h of the first dose and persisting throughout a 30-day survival. Large cystic necrotic infarcts were common in vehicle-treated rats but infrequent in STAZN-treated rats, and noninfarcted forebrain tissue was increased on average by 15%. In normal rats administered 5 mg/kg STAZN i.v. in Solutol HS 15/ethanol/saline vehicle, STAZN blood levels exhibited a biexponential decline, with an initial half-life of 28 min and a subsequent slow decay with half-life of approximately 7 h. STAZN tissue levels at 2 to 3 h were, on average, 2.5% of blood levels in forebrain, 56% in myocardium, and 41% in kidney. STAZN was concentrated in liver with initial concentrations averaging 5.2-fold above blood levels and a subsequent linear decline of 40% between 24 and 72 h. These results establish that STAZN confers enduring ischemic neuroprotection, has a long circulating half-life, and penetrates well into brain and other organs-characteristics favoring its potential therapeutic utility.

Time course of oxidative stress, lesion and edema after intrastriatal injection of malonate in rat: effect of alpha-phenyl-N-tert-butylnitrone

The aim of this study was to characterize the model of oxidative stress consisting in the infection of malonate (3 mumol), an inhibitor of mitochondrial complex II, in the rat striatum. The striatal concentrations of both the reduced and oxidized forms of glutathione (the major endogenous antioxidant) were determined at various times after malonate injection (1-4 h) in order to evaluate the evolution of oxidative stress. The progression of lesion size and edema was also determined up to 24 h after malonate administration. Finally, the effect of alpha-phenyl-N-tert-butylnitrone (PBN), an antioxidant nitrone, was studied. The levels of reduced glutathione (GSH) progressively decreased after malonate injection up to 40% of those of sham animals at 4 h. An increase in the concentrations of oxidized glutathione (GSSG) was also observed as early as 1 h after malonate administration which was maintained up to 4 h. The size of the lesion was maximal within 2 h of malonate injection, whereas edema continued to increase between 2 and 24 h. Injection of PBN at 100 mg/kg i.p. 30 min before and 2 h after malonate administration abolished the GSSG increase caused by malonate but did not modify the drop in GSH. This moderate antioxidant effect of PBN was associated with a slight decrease of the lesion area at two levels (10.7 and 10.2 mm anterior to the interaural line), but the lesion volume remained unchanged. By contrast, PBN reduced edema by 30%. Taken together, these results show that malonate induced a severe oxidative stress leading to the rapid development of the lesion. PBN demonstrates anti-edematous properties that are not sufficient to reduce the lesion.

Electron spin resonance spectroscopy reveals alpha-phenyl-N-tert-butylnitrone spin-traps free radicals in rat striatum and prevents haloperidol-induced vacuous chewing movements in the rat model of human tardive dyskinesia

The typical antipsychotic drug haloperidol causes vacuous chewing movements (VCM) in rats, which are representative of early-Parkinsonian symptoms or later-onset extrapyramidal side effects of tardive dyskinesia (TD) in humans. Haloperidol (HP) has been hypothesized to potentiate increases in oxidative stress or free radical-mediated levels of toxic metabolites in rat striatum while simultaneous upregulating dopamine (DA)-D2 receptors leading to presumed DA supersensitivity. Alpha(alpha)-Phenyl-N-tert-butylnitrone (PBN) is an antioxidant used to combat oxidative stress and measure increases in PBN spin-adduct activity. Thus, the aim of this study was to investigate whether VCMs are related to upregulation of DA-D2 receptors or to increased levels of free radicals produced during oxidative stress, and whether PBN had any protective effects. Rats received daily chronic (28 day) i.p. injections of saline, haloperidol (2 mg/kg), PBN (150 mg/kg), or haloperidol + PBN. The VCM model was used to measure extrapyramidal side effects of drug treatments. Electron spin resonance (ESR) spectroscopy was performed to compare concentrations of free radical species in rats receiving injections of HP + PBN. To examine the upregulation of DA-D2 receptors, binding assays were carried out to assess the increase in DA-D(2) receptor numbers with respect to VCMs following treatment of rats injected with HP, PBN, and HP + PBN. Results of these experiments show that HP-induced VCMs in rats results from increases in oxidative cellular events and may not be related to increases in striatal DA-D(2) receptors.

Nitrone-related therapeutics: potential of NXY-059 for the treatment of acute ischaemic stroke

At present, none of the neurodegenerative diseases such as Parkinson's disease, Alzheimer's disease and stroke are treatable with compounds that slow or halt neuronal cell death. However, the prototype nitrone radical trap alpha-phenyl-tert-butylnitrone (PBN) has been shown to be an effective neuroprotective agent in various models of neurodegeneration. Some of these data are briefly reviewed as an introduction to an examination of the effect of the novel nitrone radical trapping agent disodium 2,4-disulfophenyl-N-tert-butylnitrone (NXY-059) in various animal models of stroke. NXY-059 has been shown to be an effective neuroprotective agent in both transient (reperfusion) and permanent focal ischaemia models in rats. In both types of model, NXY-059 has a large window of opportunity, providing effective neuroprotection when given up to 5 hours after the start of the occlusion in transient ischaemia and 4 hours after the start of permanent ischaemia. The compound is also effective in a marmoset permanent ischaemia model when administered up to 4 hours after the start of the occlusion. In this model it has been found to attenuate the problem of spatial neglect and maintain function to the paretic arm. NXY-059 administration also improves motor function in a rat haemorrhagic stroke model and has a neuroprotective effect in a rabbit thromboembolic stroke model. The compound is also well tolerated in stroke patients at plasma levels shown to provide a maximum neuroprotective effect in animal models of stroke.NXY-059, like PBN, is a nitrone with free radical trapping properties and this may be the basis of its neuroprotective action. However, experiments with PBN and NXY-059 suggest the possibility of other mechanisms being involved and these are also reviewed. Further experiments are required to fully elucidate the mechanism of action of these very effective neuroprotective agents.

Effects of alpha-phenyl-N-tert-butyl nitrone (PBN)on brain cell membrane function and energy metabolism during transient global cerebral hypoxia-ischemia and reoxygenation-reperfusion in newborn piglets

We sought to know whether a free radical spin trap agent, alpha-phenyl-N-tert-butyl nitrone (PBN) influences brain cell membrane function and energy metabolism during and after transient global hypoxia-ischemia (HI) in the newborn piglets. Cerebral HI was induced by temporary complete occlusion of bilateral common carotid arteries and simultaneous breathing with 8% oxygen for 30 min, followed by release of carotid occlusion and normoxic ventilation for 1 hr (reoxygenation-reperfusion,RR). PBN (100 mg/kg) or vehicle was administered intravenously just before the induction of HI or RR. Brain cortex was harvested for the biochemical analyses at the end of HI or RR. The level of conjugated dienes significantly increased and the activity of Na+, K+ -ATPase significantly decreased during HI,and they did not recover during RR. The levels of ATP and phosphocreatine (PCr)significantly decreased during HI, and recovered during RR. PBN significantly decreased the level of conjugated dienes both during HI and RR, but did not influence the activity of Na+, K+ -ATPase and the levels of ATP and PCr. We demonstrated that PBN effectively reduced brain cell membrane lipid peroxidation, but did not reverse ongoing brain cell membrane dysfunction nor did restore brain cellular energy depletion, in our piglet model of global hypoxic-ischemic brain injury.

Protein oxidation and the degradation of oxidized proteins in the rat oligodendrocyte cell line OLN 93-antioxidative effect of the intracellular spin trapping agent PBN

Oligodendrocytes are the myelin-producing cells in the central nervous system. It was proposed that these cells are much more prone to oxidative damage than to other cells of the central nervous system. This fact seems to be due to their high iron store and low antioxidative defense mechanisms. Consequently, free radical induced damage should lead to an enhanced damage of oligodendrocytes. Thus, we chose the oligodendrocyte cell line OLN 93 to measure the stability of the protein pool after oxidation and the possibilities of protecting proteins by alpha-phenyl-N-tert-butylnitrone (PBN). We were able to demonstrate for the first time that OLN 93 cells are able to respond with an increase in overall proteolysis when exposed to various oxidants. This increase was the consequence of an enhanced protein oxidation. The activity of the 20S proteasome, which is thought to be involved in the removal of oxidized proteins, was not effected by moderate concentrations of the oxidants. The spin-trap PBN was used as an antioxidant and was able to prevent protein oxidation in OLN 93 cells effectively. Consequently, we proved that PBN is also able to prevent the increase in overall protein oxidation. We were able to demonstrate that OLN 93 oligodendrocytes react to oxidative stress with an increase in the protein turnover directed towards the removal of oxidized proteins. The intracellular spin-trap PBN is able to prevent protein oxidation in OLN 93 cells.

Nitrones as neuroprotective agents in cerebral ischemia, with particular reference to NXY-059

Stroke is a major clinical problem, and acute pharmacological intervention with neuroprotective agents has so far been unsuccessful. Recently, there has been considerable interest in the potential therapeutic benefit of nitrone-derived free radical trapping agents as neuroprotective agents. Nitrone compounds have been shown to be beneficial in animal models of various diseases, and the prototypic compound alpha-phenyl-N-tert-butylnitrone (PBN) has been extensively demonstrated to be neuroprotective in rat models of transient and permanent focal ischemia. The nitrone radical trapping agent disodium 2,4-disulfophenyl-N-tert-butylnitrone (NXY-059) has also been shown to be neuroprotective in these models. Furthermore, it has recently been shown to improve neurological function and reduce infarct volume in a primate model of permanent focal ischemia even when given 4 hr postocclusion. While radical trapping activity is demonstrable with NXY-059 and other nitrone compounds such as PBN, this activity is weak. Arguments for and against ascribing radical trapping as the therapeutic mechanism of action are discussed. This compound is well tolerated in human stroke patients and can be administered to produce plasma concentrations exceeding those effective in animal models; crucially, at the same time, it has also been shown to be effective in animal models. NXY-059 may thus be the first compound to be examined in stroke patients using drug exposure and time to treatment that have been shown to be effective in animal models of stroke.

The effects of PBN (phenyl-butyl-nitrone) on GLT-1 levels and on the extracellular levels of amino acids and energy metabolites in a model of iron-induced posttraumatic epilepsy

This study investigates astrocytic glutamate uptake in the iron-induced animal model of posttraumatic epilepsy. Since formation of free radicals may be involved in epileptogenesis after brain trauma and hemorrhage the effects of the nitrone radical scavenger alpha-phenyl-tert-N-butyl nitrone (PBN) were also studied. Animals received an intracortical iron injection, or were sham-operated. They were given PBN intraperitoneally or saline as control. Twenty-four hours after lesion, brain tissue was collected and the level of glial glutamate transporter (GLT-1) was analyzed using immunoblotting. The extracellular concentrations of amino acids and energy metabolites were measured using microdialysis. The results showed significantly decreased levels of GLT-1 (70 kDa), higher basal levels of glutamate, and lower levels of glutamine as well as low arginine/citrulline ratios at the lesion compared to controls. PBN significantly attenuated the decrease of 70 kDa GLT-1 in the lesioned animals and attenuated the alterations in amino acid levels but not to a significant level. PBN also increased the arginine/citrulline ratios indicating reduced nitric oxide synthase activity. Our results suggest that astrocytic uptake of glutamate is oxidatively impaired in iron-induced epileptogenesis and that the administration of a radical scavenger can attenuate this process.

Differences in cerebral reperfusion and oxidative injury after cardiac arrest in pigs

BACKGROUND

An investigation of the free radical scavenger sodium 2-sulfophenyl-N-tert-butyl nitrone (S-PBN) and the weak vasodilatator Tris buffer mixture (TBM) on cerebral cortical blood flow (CCBF) and the jugular bulb concentration of two eicosanoids, indicators of oxidative stress and inflammation, was undertaken in 30 anaesthetized piglets during cardiopulmonary resuscitation (CPR) and after restoration of spontaneous circulation (ROSC).

METHODS

Thirty animals were subjected to 8 min of untreated circulatory arrest followed by 8 min of closed-chest CPR. During CPR, the animals were randomized to receive 60 mg/kg S-PBN, 1 mmol/kg TBM or 2 ml/kg normal saline (n = 10 in each group). Systemic haemodynamic variables, CCBF and jugular bulb plasma concentrations of 8-iso-PGF2alpha and 15-keto-dihydro-PGF2alpha were measured.

RESULTS

The CCBF during reperfusion after ROSC was greater in the TBM group than in the S-PBN group, the regression coefficient between CCBF and mean arterial blood pressure being lower in the S-PBN group than in the TBM group. The jugular bulb plasma concentration of 8-iso-PGF2alpha during the first 30 min after ROSC was greater in the TBM group than in the S-PBN group. Administration of TBM after vasopressin did not attenuate the pressor effect of vasopressin.

CONCLUSION

Administration of S-PBN during CPR results in less cerebral oxidative stress, possibly by promoting normal distribution of cerebral blood flow.

In vitro blood-brain barrier permeability and cerebral endothelial cell uptake of the neuroprotective nitrone compound NXY-059 in normoxic, hypoxic and ischemic conditions

The free radical trapping nitrone compounds alpha-phenyl-N-tert-butylnitrone (PBN), 2-sulfophenyl-N-tert-butylnitrone (S-PBN) and disodium 2,4-disulfophenyl-N-tert-butyl nitrone (NXY-059) are effective neuroprotective agents in experimental models of both transient and permanent focal ischemia. A recent in vivo study suggested that NXY-059 had poor brain uptake in a transient ischemia model. We have now examined its blood-brain barrier permeability and cerebral endothelial uptake during hypoxic and ischemic conditions using an in vitro model of the blood-brain barrier. The in vitro blood-brain barrier permeability and cerebral endothelial uptake of NXY-059 and S-PBN were low during normoxic conditions. In contrast, PBN had very high blood-brain barrier penetration in vitro which confirmed earlier in vivo results. The permeability of [14C]NXY-059 increased 3.5 times after 9 h of hypoxia or 3 h of ischemia. There was, respectively, a 5-fold and more than 10-fold increase, after 6 and 9 h of ischemia. The control molecule [3H]inulin (M(r) approximately 5000) showed a similar increase in permeability under the same experimental conditions indicating a major change in the transport properties of the endothelium. There was a 60% reduction in the ATP levels of astrocytes after 3 h of ischemia and a 90% reduction after 9 h. The reduction in ATP levels in endothelial cells was somewhat lower. The uptake of NXY-059 in cerebral endothelial cells under normoxic, hypoxic or 9 h of ischemic conditions was negligible. NXY-059, S-PBN and PBN showed no effects on vesicular transport or the integrity of the blood-brain barrier in normoxic or ischemic conditions, nor did the compounds induce any change in the ATP levels of the cells. In conclusion, it is possible that the increase in blood-brain barrier permeability of [14C]NXY-059 which occurs during prolonged ischemia in vitro reflects a change which may be of importance to the neuroprotective effects of this nitrone free radical trapping agent.

ESR characterization of a novel spin-trapping agent, 15N-labeled N-tert-butyl-alpha-phenylnitrone, as a nitric oxide donor

We previously found that one of the pharmacological effects of N-tert-butyl-alpha-phenylnitrone (PBN) is the release of nitric oxide (NO) under oxidative conditions. However, to confirm this hypothesis in vivo, NO released from PBN must be distinguished from NO produced in biological systems, and therefore we undertook the synthesis of PBN using labeled 15N to identify its corresponding 15NO in vivo. The properties were examined with an ESR spectrometer. To synthesize 15N-PBN, the starting material, ammonium-15N chloride, was converted to 2-amino-15N-2-methylpropane, oxidized to 2-methyl-2-nitropropane-15N, and finally reacted with benzaldehyde to give 15N-PBN. The final product was purified by repeated sublimation. With ferrous sulfate-methyl glucamine dithiocarbamate complex, Fe (MGD)2, as a trapping agent to measure the NO levels of 15N-PBN or 14N-PBN in vitro, the peak intensity of 15NO[Fe(MGD)2] was over 50% stronger than that of 14NO[Fe(MGD)2], and that 15NO and 14NO had the corresponding two-and three line hyperfine structures due to their nuclear spin quantum numbers. Subsequently, the ESR spectrum of 15NO derived from 15N-PBN was significantly different than that of lipopolysaccharide (LPS)-induced NO, which was derived from biological cells, and therefore we have demonstrated the possibility to distinguish 15NO from PBN and 14NO generated from cells. These results suggested that 15N-PBN is a useful molecule, not only as a spin-trapping agent, but also as an NO donor to explore the pharmacological mechanisms of PBN in vivo.

Effect of traumatic brain injury and nitrone radical scavengers on relative changes in regional cerebral blood flow and glucose uptake in rats

Changes in regional cerebral blood flow (rCBF) and glucose metabolism are commonly associated with traumatic brain injury (TBI). Reactive oxygen species (ROS) have been implicated as key contributors to the secondary injury process after TBI. Here, pretreatment with the nitrone radical scavengers (alpha-phenyl-N-tert-butyl nitrone (PBN) or its sulfonated analogue sodium 2-sulfophenyl-N-tert-butyl nitrone (S-PBN) were used as tools to study the effects of ROS on rCBF and glucose metabolism after moderate (2.4-2.6 atm) lateral fluid percussion injury (FPI) in rats. S-PBN has a half-life in plasma of 9 min and does not penetrate the blood-brain barrier (BBB). In contrast, PBN has a half-life of 3 h and readily penetrates the BBB. Regional cerebral blood flow (rCBF) and glucose metabolism was estimated by using (99m)Tc-HMPAO and [(18)F]Fluoro-2-deoxyglucose (FDG) autoradiography, respectively, at 42 min (n = 37) and 12 h (n = 34) after the injury. Regions of interest were the parietal cortex and hippocampus bilaterally. As expected, FPI produced an early (42-min) hypoperfusion in ipsilateral cortex and an increase in glucose metabolism in both cortex and hippocampus, giving way to a state of hypoperfusion and decreased glucose metabolism at 12 h postinjury. On the contralateral side, a hypoperfusion in the cortex and hippocampus was seen at 12 h only, but no significant changes in glucose metabolism. Both S-PBN and PBN attenuated the trauma-induced changes in rCBF and glucose metabolism. Thus, the early improvement in rCBF and glucose metabolism correlates with and may partly mediate the improved functional and morphological outcome after TBI in nitrone-treated rats.

Region-selective effects of neuroinflammation and antioxidant treatment on peripheral benzodiazepine receptors and NMDA receptors in the rat brain

Following induction of acute neuroinflammation by intracisternal injection of endotoxin (lipopolysaccharide) in rats, quantitative autoradiography was used to assess the regional level of microglial activation and glutamate (NMDA) receptor binding. The possible protective action of the antioxidant phenyl-tert-butyl nitrone in this model was tested by administering the drug in the drinking water for 6 days starting 24 hafter endotoxin injection. Animals were killed 7 days post-injection and consecutive cryostat brain sections labeled with [3H]PK11195 as a marker of activated microglia and [125I]iodoMK801 as a marker of the open-channel, activated state of NMDA receptors. Lipopolysaccharide increased [3H]PK11195 binding in the brain, with the largest increases (two- to threefold) in temporal and entorhinal cortex, hippocampus, and substantia innominata. A significant (> 50%) decrease in [125I]iodoMK801 binding was found in the same brain regions. Phenyl-tert-butyl nitrone treatment resulted in a partial inhibition (approx. 25% decrease) of the lipopolysaccharide-induced increase in [3H]PK11195 binding but completely reversed the lipopolysaccharide-induced decrease in [125I]iodoMK80 binding in the entorhinal cortex, hippocampus, and substantia innominata. Loss of NMDA receptor function in cortical and hippocampal regions may contribute to the cognitive deficits observed in diseases with a neuroinflammatory component, such as meningitis or Alzheimer's disease.

Nitrones as neuroprotectants and antiaging drugs

Specific nitrones have been used for more than 30 years in analytical chemistry and biochemistry to trap and stabilize free radicals for the purpose of their identification and characterization. PBN (alpha-phenyl-tert-butyl nitrone), one of the more widely used nitrones for this purpose, has been shown to have potent pharmacologic activities in models of a number of aging-related diseases, most notably the neurodegenerative diseases of stroke and Alzheimer's disease. Studies in cell and animal models strongly suggest that PBN has potent antiaging activity. A novel nitrone, CPI-1429, has been shown to extend the life span of mice when administration was started in older animals. It has also shown efficacy in the prevention of memory dysfunction associated with normal aging in a mouse model. Mechanistic studies have shown that the neuroprotective activity of nitrones is not due to mass-action free radical-trapping activity, but due to cessation of enhanced signal transduction processes associated with neuroinflammatory processes known to be enhanced in several neurodegenerative conditions. Enhanced neuroinflammatory processes produce higher levels of neurotoxins, which cause death or dysfunction of neurons. Therefore, quelling of these processes is considered to have a beneficial effect allowing proper neuronal functioning. The possible antiaging activity of nitrones may reside in their ability to quell enhanced production of reactive oxygen species associated with age-related conditions. On the basis of novel ideas about the action of secretory products formed by senescent cells on bystander cells, it is postulated that nitrones will mitigate these processes and that this may be the mechanism of their antiaging activity.

Oxidative stress in brain during experimental bacterial meningitis: differential effects of alpha-phenyl-tert-butyl nitrone and N-acetylcysteine treatment

Antioxidant treatment has previously been shown to be neuroprotective in experimental bacterial meningitis. To obtain quantitative evidence for oxidative stress in this disease, we measured the major brain antioxidants ascorbate and reduced glutathione, and the lipid peroxidation endproduct malondialdehyde in the cortex of infant rats infected with Streptococcus pneumoniae. Cortical levels of the two antioxidants were markedly decreased 22 h after infection, when animals were severely ill. Total pyridine nucleotide levels in the cortex were unaltered, suggesting that the loss of the two antioxidants was not due to cell necrosis. Bacterial meningitis was accompanied by a moderate, significant increase in cortical malondialdehyde. While treatment with either of the antioxidants alpha-phenyl-tert-butyl nitrone or N-acetylcysteine significantly inhibited this increase, only the former attenuated the loss of endogenous antioxidants. Cerebrospinal fluid bacterial titer, nitrite and nitrate levels, and myeloperoxidase activity at 18 h after infection were unaffected by antioxidant treatment, suggesting that they acted by mechanisms other than modulation of inflammation. The results demonstrate that bacterial meningitis is accompanied by oxidative stress in the brain parenchyma. Furthermore, increased cortical lipid peroxidation does not appear to be the result of parenchymal oxidative stress, because it was prevented by NAC, which had no effect on the loss of brain antioxidants.

N-tert-butyl-alpha-phenylnitrone, a free radical scavenger with anticholinesterase activity does not improve the cognitive performance of scopolamine-challenged rats

Reversible inhibitors of acetylcholinesterase improve spatial learning and memory in animal models of cognitive impairment. Here we investigate if the beneficial effects of free radical scavenger N-tert-butyl-alpha-phenylnitrone (PBN) on cognitive performance could be explained by its recently discovered anticholinesterase activity. Morris water maze experiment was performed to examine the effect of PBN on the impairment of spatial learning and memory induced by the antagonist of cholinergic muscarinic transmission scopolamine. In situ hybridization histochemistry experiment was performed to study its effects on the induction of immediate early gene expression (c-fos, c-jun) by dopamine D1 receptor agonist SKF-82958 and on the augmentation of the SKF-82958-induced expression of these genes by scopolamine. In both experiments, the effects of PBN were compared to the effects of reversible anticholinesterase physostigmine. We found that physostigmine but not PBN significantly reversed the cognitive impairment in scopolamine-challenged rats, prevented the induction of c-fos and c-jun mRNAs by SKF-82958 and attenuated the augmentation of the SKF-82958-induced expression of these genes by scopolamine. The present experiments did not reveal a significant in vivo anticholinesterase activity of PBN.

Marked elevation in cortical urate and xanthine oxidoreductase activity in experimental bacterial meningitis

Experimental bacterial meningitis due to Streptococcus pneumoniae in infant rats was associated with a time-dependent increase in CSF and cortical urate that was approximately 30-fold elevated at 22 h after infection compared to baseline. This increase was mirrored by a 20-fold rise in cortical xanthine oxidoreductase activity. The relative proportion of the oxidant-producing xanthine oxidase to total activity did not increase, however. Blood plasma levels of urate also increased during infection, but part of this was as a consequence of dehydration, as reflected by elevated ascorbate concentrations in the plasma. Administration of the radical scavenger alpha-phenyl-tert-butyl nitrone, previously shown to be neuroprotective in the present model, did not significantly affect either xanthine dehydrogenase or xanthine oxidase activity, and increased even further cortical accumulation of urate. Treatment with the xanthine oxidoreductase inhibitor allopurinol inhibited CSF urate levels earlier than those in blood plasma, supporting the notion that urate was produced within the brain. However, this treatment did not prevent the loss of ascorbate and reduced glutathione in the cortex and CSF. Together with data from the literature, the results strongly suggest that xanthine oxidase is not a major cause of oxidative stress in bacterial meningitis and that urate formation due to induction of xanthine oxidoreductase in the brain may in fact represent a protective response.

Synthesis, structure, and neuroprotective properties of novel imidazolyl nitrones

A new series of imidazolyl nitrones spin traps has been synthesized and evaluated pharmacologically. The salient structural feature of these molecules is the presence of an imidazole moiety substituted by aromatic or heteroaromatic cycles. This connectivity imparts to the nitrone superior neuroprotective properties in vivo and in parallel reduced side effects and toxicity. Thus compound 6a (a 2-phenylimidazolyl nitrone) administered intraperitoneally protects (80%) mice from lethality induced by an intracerebroventricular administration of tert-butyl hydroperoxide (t-BHP) an oxidant capable of inducing neurodegenerative processes. Administration of the archetypal nitrone phenyl-tert-butyl nitrone (PBN) at an equimolar dose also affords some protection (60%) in this test. However, this activity is accompanied by hypothermia, whereas no such effect is apparent for 6a. Moreover, previously prepared nonsubstituted or alkyl-substituted imidazolyl nitrones were shown to be extremely toxic to rats in contrast to the compounds prepared in this study. The observed activities in vivo correlate well with the calculated partition coefficients (ClogP) and HOMO energy level.

Neuroprotection by 2-h postischemia administration of two free radical scavengers, alpha-phenyl-n-tert-butyl-nitrone (PBN) and N-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN), in rats subjected to focal embolic cerebral ischemia

Oxygen free radical generation may have important secondary damaging effects after the onset of cerebral ischemia. Free radical scavengers have been used successfully in attenuating neuronal damage in the reperfusion period in transient forebrain ischemia. There are limited data on effectiveness in models of focal ischemia. Two free radical scavengers, alpha-phenyl-n-tert-butyl-nitrone (PBN) and N-tert-butyl-(2-sulfophenyl)-nitrone (S-PBN), have been shown to reduce oxidative-stress-induced neuronal injury. Whereas PBN has been demonstrated to reduce infarct volume in focal ischemia, neuroprotection has not been evaluated with S-PBN. The present study was designed to evaluate the neuroprotective effect of PBN and S-PBN compared to vehicle in a focal embolic middle cerebral artery (MCA) cerebral ischemia model in rats. Wistar rats were randomly divided into three groups (n = 10 each group). Animals in the control group received vehicle and those in the treatment groups were treated with PBN or S-PBN (both 100 mg/kg/day x 3 days, intraperitoneally) starting 2 h after the introduction of an autologous thrombus into the right-side MCA. The neurological outcome was observed and compared before and after treatment and between groups. The percentage of cerebral infarct volume was estimated from 2,3, 5-triphenyltetrazolium chloride stained coronal slices 72 h after the ischemic insult. Two-hour postischemia administration of PBN or S-PBN significantly improved neurobehavioral scores at 24 h following MCA embolization (both P < 0.01). The percentage of infarct volume for animals receiving vehicle was 32.8 +/- 9.4%. Two-hour delayed administration of PBN and S-PBN achieved a 35.4% reduction in infarct volume in treatment groups when compared with animals receiving vehicle (PBN vs control, 21.2 +/- 10.9% vs 32.8 +/- 9.4%; P < 0.05; S-PBN vs control, 21.2 +/- 13.1%, (P < 0.05). These data indicate that free radical generation may be involved in brain damage in this model and 2-h delayed postischemia treatment with PBN and S-PBN may have neuroprotective effects in focal cerebral ischemia. As S-PBN does not normally cross the blood-brain barrier, the neuroprotection evident in this study may be explained by entry into the brain via damaged vessels.

MPTP-induced deficits in motor activity: neuroprotective effects of the spintrapping agent, alpha-phenyl-tert-butyl-nitrone (PBN)

In Experiment 1, groups of mice were administered either saline or MPTP (2 x 30 mg/kg, s.c., separated by a 24-hr interval) 30 min after being injected either PBN (15, 50 or 150 mg/kg, s.c., low, medium and high doses, respectively) or L-Deprenyl (0.25 or 10.0 mg/kg, s.c., low and high doses, respectively), the reference compound used, or saline. Tests of spontaneous motor activity 14 days later indicated that the MPTP-induced hypokinesia for locomotion and rearing was alleviated by prior administration with PBN (50 or 150 mg/kg) or L-Deprenyl (10.0 mg/kg); lower doses of PBN (15 mg/kg) and L-Deprenyl (0.25 mg/kg) did not affect the MPTP-induced deficits. Dopamine (DA) concentrations in the striatum confirmed a more severe loss of DA in the MPTP, PBN (15) + MPTP and Deprenyl(0.25) + MPTP groups than in the control group. Significant protection of DA was observed in the PBN(50) + MPTP, PBN(150) + MPTP and Deprenyl(10) + MPTP groups that did not exhibit an hypokinetic behaviour. In Experiment 2, the effects of repeated treatment with PBN (50 mg/kg, s.c. over 12 days), post-MPTP, were studied in aged (15-month-old) and young (3-month-old) mice. Subchronic administration of PBN increased substantially the motor activity of old and young mice that had received MPTP. Aged control (saline) mice showed an activity deficit compared to young control mice; this deficit was abolished by repeated PBN treatment. The results suggest that moderate-to-high doses of PBN whether injected in a single dose prior to MPTP or subchronically following MPTP injections may afford protective effects against both the functional changes and DA-loss caused by MPTP treatment, possibly through an antioxidant mechanism.

Atypical effect of some spin trapping agents: reversible inhibition of acetylcholinesterase

N-tert-butyl-alpha-phenylnitrone (PBN), a widely used nitrone-based free radical trap was recently shown to prevent acetylcholinesterase (AChE) inhibitors induced muscle fasciculations and brain seizures while being ineffective against glutamergic or cholinergic receptor agonist induced seizures. In the present study we compared the effects on AChE activity of four free radical spin traps PBN, alpha-(4-pyridil-1)-N-tert-butyl nitrone (POBN), N-tert-butyl-alpha-(2-sulfophenyl)-nitrone (S-PBN) and 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). The kinetics of AChE inhibition were studied in vitro using a spectrophotometric kinetic assay with AChE from rat brain, diaphragm, electric eel and mouse brain. Spin trapping compounds S-PBN and DEPMPO, in concentrations up to 3 mM did not inhibit hydrolysis of ACh, while PBN and POBN inhibited hydrolysis of ACh in a reversible and concentration-dependent manner. Double reciprocal plots of the reaction velocity against varying ACh concentrations at each inhibitor concentration were linear and generally indicated mixed type inhibition. PBN was the most potent inhibitor of mouse AChE with Ki and Ki' of 0.58 and 2.99 mM, respectively, and the weakest inhibitor of electric eel AChE. In contrast, POBN showed the highest affinity for electric eel enzyme, with Ki and Ki' values of 1.065 and 3.15 mM, respectively. These findings suggest that the effect of PBN and POBN on AChE activity does not depend on trapping of damaging reactive oxygen and that in addition to their antioxidant action other pharmacological effects of these compounds should be considered when neuroprotective actions of PBN or POBN are investigated.

The effect of alpha-phenyl-tert-butyl nitrone (PBN) on free radical formation in transient focal ischaemia measured by microdialysis and 3,4-dihydroxybenzoate formation

alpha-phenyl-tert-butyl nitrone (PBN) reduces infarct size, improves recovery of brain energy metabolism and delays the secondary increase in extracellular potassium after focal ischaemia, presumably by trapping OH radicals. We investigated the effect of PBN on the formation of 3,4-dihydroxybenzoic acid (3,4-DHBA) as a measure of OH radical formation, during and following middle cerebral artery occlusion (MCAO). Rats, subjected to 2 h of ischaemia followed by 3 h of recirculation, were injected with either vehicle or PBN (100 mg kg-1 i.p.) prior to MCAO or immediately after recirculation, respectively. The in vivo microdialysis technique was used to collect samples for analysis of 3,4-DHBA by HPLC. The basal levels of 3,4-DHBA were 56-77 nmol L-1 in the four groups. During ischaemia, the formation of 3,4-DHBA decreased by about 50% in all groups. Upon recirculation, a 3-fold rise in 3,4-DHBA formation was seen. At 2 h of recirculation the mean value of 3,4-DHBA in the pretreated, vehicle-injected animals was 125 +/- 18 nmol L-1 and in the PBN-injected 145 +/- 48 nmol L-1, respectively. When the animals were treated after MCAO either with vehicle or PBN the values at 2 h recirculation were 155 +/- 148 and 189 +/- 145 nmol L-1, respectively. No statistically significant difference between vehicle- and PBN-injected groups was seen. We conclude that during reperfusion following MCAO, hydroxyl radical formation increases. The increase is not ameliorated by PBN which suggests that PBN does not protect the brain by a general scavenging of OH radicals, although tissue specific actions cannot be excluded.

Effect of alpha-phenyl-N-tert-butylnitrone on brain cell membrane function and energy metabolism in experimental Escherichia coli meningitis in the newborn piglet

We evaluated the efficacy of alpha-phenyl-N-tertbutylnitrone as an adjunctive therapy in experimental bacterial meningitis in the newborn piglet. Meningitis was induced by intracisternal injection of 10(8) colony-forming units of Escherichia coli in 100 microl of saline. Alpha-Phenyl-N-tert-butylnitrone 100 mg/kg was given as a bolus intravenous injection 30 min before induction of meningitis. Although it completely abolished the elevated CSF tumor necrosis factor-a level observed in the meningitis group, alpha-phenyl-N-tert-butylnitrone did not down-modulate parameters of inflammatory responses such as increased intracranial pressure, hypoglycorrhachia, elevated CSF lactate level, and CSF leukocytosis observed in this group. However, alpha-phenyl-N-tert-butylnitrone treatment mitigated alterations in brain cell membrane structure and function during meningitis, evidenced by amelioration of increased brain cell membrane lipid peroxidation products (conjugated dienes) and decreased Na+, K+-ATPase activity. Reduced mean arterial blood pressure, cerebral perfusion pressure, brain glucose concentration, and cerebral energy stores and marginally increased brain lactate level observed in the meningitis group were also ameliorated. These results suggest that although it failed to attenuate the inflammatory responses, alpha-phenyl-N-tert-butylnitrone was effective in ameliorating brain injury in neonatal bacterial meningitis.

Restoration and putative protection in Parkinsonism

Synergistic antiparkinsonian actions of different classes of putative therapeutic agents co-administered with a subthreshold dose of L-3,4-dihydroxyphenylalanine (L-Dopa) (5 mg/kg) in drug-naive 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mice as well as the restorative actions of those compounds in suprathreshold L-Dopa-tolerant MPTP-treated mice subjected to "wearing-off" of L-Dopa efficacy were assessed in a series of experiments. The classes of compounds studied included the noncompetitive NMDA antagonists, memantine, amantadine and MK-801, the anticonvulsive and putative anticonvulsive agents, lamotrigine, FCE 26743, phenytoin, the monoamine oxidase inhibitors, L-Deprenyl, amiflamine, alpha-ethyltryptamine, clorgyline and guanfacine. In this final case, the restorative effects of clonidine and guanfacine were antagonized by the alpha(2)-adrenoceptor antagonist, yohimbine, but not the alpha(1)-adrenoceptor antagonist, prazosin. Within each class of potentially therapeutic agents a differential restorative efficacy was obtained, but the combination of different doses of apomorphine with clonidine failed to restore motor activity. Finally, the neuroprotective actions of acute and subchronic administration of the nitrone spin-trapping compound, alpha-phenyl-tert-butyl nitrone upon the spontaneous motor behaviour and striatal dopamine concentrations of MPTP-treated mice was examined.

Evidence for enhanced neuro-inflammatory processes in neurodegenerative diseases and the action of nitrones as potential therapeutics

A brief review is presented on observations leading to the current notions regarding neuro-inflammatory processes. The greatest focus is on Alzheimer's disease (AD) since this is where the most convincing data has been obtained. A brief summary of observations on the neuroprotective action of alpha-phenyl-tert-butyl-nitrone (PBN) as well as results of research designed to understand its mechanism of action is presented. We hypothesize that the mechanism of action of PBN involves inhibition of signal transduction processes, which are involved in the upregulation of genes mediated by pro-inflammatory cytokines and H2O2 that cause formation of toxic gene products. Results from recent experiments on Kainic acid (KA) mediated brain damage are provided to suggest the validity of the in vivo action of PBN to inhibit neuro-inflammatory processes. The accumulating scientific facts are helping to provide concepts that may become the basis for novel therapeutic approaches to the treatment of several neurodegenerative diseases.

Nitrone spin trapping compound N-tert-butyl-alpha-phenylnitrone prevents seizures induced by anticholinesterases

The neuroprotection afforded by spin trapping agents such as N-tert-butyl-alpha-phenylnitrone (PBN) has lent support to the hypothesis that increased production of reactive oxygen species (ROS) is a major contributing factor to excitotoxicity, aging and cognitive decline. Little is known, however, about the pharmacological properties of PBN. We have compared the acute effects of PBN on the development of seizures induced by the irreversible acetylcholinesterase (AChE) inhibitor diisopropylphosphorofluoridate (DFP), the reversible AChE inhibitor physostigmine (PHY), the muscarinic cholinergic receptor agonist pilocarpine (PIL) and the glutamatergic receptor agonist kainic acid (KA). Rats were sacrificed 90 min after the injection of seizure-inducing agents. In situ hybridization was used to detect the induction of immediate early gene (IEG) c-fos and c-jun mRNA's and the levels of AChE mRNA. The activity of AChE was visualized by AChE staining and quantified using an in vitro AChE assay. The seizures correlated with the induction of IEG mRNA's with all agents used. The pre-treatment with 150 mg/kg of PBN prevented DFP- and PHY-induced seizures and the related expression of IEG mRNA's, but had no effect on PIL- or KA-induced seizures and associated IEG mRNA's changes. PBN prevented seizures and significantly protected AChE activity against DFP inhibition when given before, but not when given after DFP. This study shows that PBN specifically protects against anticholinesterase-induced seizures by reversible protection of AChE activity and not by the blockade of muscarinic or glutamate receptors, reactivation of AChE or scavenging of ROS. The anticholinesterase properties should be considered when using PBN in studies of cholinergic dysfunction.

Production of hydroxyl free radical by brain tissues in hyperglycemic rats subjected to transient forebrain ischemia

Preischemic hyperglycemia is known to aggravate brain damage resulting from transient ischemia. In the present study, we explored whether this aggravation is preceded by an enhanced formation of reactive oxygen species (ROS) during the early reperfusion period. To that end, normo- and hyperglycemic rats were subjected to 15 min of forebrain ischemia and allowed recovery periods of 5, 15, and 60 min. Sodium salicylate was injected intraperitoneally in a dose of 100 mg/kg, and tissues were sampled during recirculation to allow analyses of salicylic acid (SA) and its hydroxylation products, 2,3- and 2,5-dihydroxybenzoate (DHBA). Tissue sampled from thalamus and caudoputamen in normoglycemic animals failed to show an increase in 2,3- or 2,5-DHBA after 5 and 15 min of recirculation. However, such an increase was observed in the neocortex after 60 min of recirculation, with a suggested increase in the hippocampus as well. Hyperglycemia had three effects. First, it increased 2,5-DHBA in the thalamus and caudoputamen to values exceeding normoglycemic ones after 15 min of recirculation. Second, it increased basal values of 2,5- and total DHBA in the neocortex. Third, it increased the 60-min values for 2,5- and total DHBA in the hippocampus. These results hint that, at least in part, hyperglycemia may aggravate damage by enhancing basal- and ischemia-triggered production of ROS.

An optimal redox status for the survival of axotomized ganglion cells in the developing retina

The neuronal redox status influences the expression of genes involved in neuronal survival. We previously showed that antioxidants may reduce the number of dying ganglion cells following axotomy in chick embryos. In the present study, we show that various antioxidants, including the new spin trap azulenyl nitrone and 1,3-dimethyl-2-thiourea, protect axotomized ganglion cells, confirming that neuronal death involves an imbalance of the cellular redox status towards oxidation. However, high concentrations of antioxidants did not protect ganglion cells, suggesting that excessive reduction is detrimental for neurons. Simultaneous injections of two different antioxidants gave results only partly supporting this view. Combinations of azulenyl nitrone and N-acetyl cysteine in fact gave greater protection than either antioxidant alone, whereas N-acetyl cysteine lost its neuroprotective effects and diminished those of alpha-phenyl-N-tert-butyl nitrone when the two compounds were injected simultaneously. The results of the combined treatments suggest that azulenyl nitrone and alpha-phenyl-N-tert-butyl nitrone do not have the same chemical effects within the ganglion cells. Moreover, N-acetyl cysteine's own antioxidant properties enhance the spin trapping effects of azulenyl nitrone but potentiate the toxicity of alpha-phenyl-N-tert-butyl nitrone. Our main conclusion is that neuronal survival requires the maintenance of the redox status near an optimal set-point. "Reductive stress" may be as dangerous as oxidative stress.

High-performance liquid chromatography study of the pharmacokinetics of various spin traps for application to in vivo spin trapping

In vivo spin trapping is potentially a very useful tool to investigate the role of free radicals in physiologic processes and disease development. Unfortunately, knowledge on the stability and distribution of spin traps in living systems is limited. Therefore, in our study, we selected 11 acyclic and cyclic nitrone spin traps with diverse properties to determine their pharmacokinetics in mice. At varying times after intraperitoneal administration, we measured the concentration of the spin traps in the liver, heart, and blood. Our results showed that most spin traps were rapidly absorbed and were approximately evenly distributed throughout the mouse body. It was also found that most of the traps were relatively stable in vivo with more than half of the injected amount still available for spin trapping free radicals after an hour. Two of the 11 tested spin traps, however, decomposed after injection. These results indicate that for a successful in vivo spin trapping experiment, the stability of the spin trap is not of major concern, but the time course of distribution may be important.

Neuroprotective effects of a novel nitrone, NXY-059, after transient focal cerebral ischemia in the rat

Recent results have demonstrated that the spin trapping agent alpha-phenyl-N-tert-butyl nitrone (PBN) reduces infarct volume in rats subjected to 2 hours of middle cerebral artery occlusion, even when given 1 to 3 hours after the start of recirculation. In the current study, the authors assessed the effect of NXY-059, a novel nitrone that is more soluble than PBN. Loading doses were given of 0.30, 3.0, or 30 mg x kg(-1) followed by 0.30, 3.0, or 30 mg x kg(-1) x h(-1) for 24 or 48 hours. Dose-response studies showed that when treatment was begun 1 hour after recirculation, 0.30 mg x kg(-1) had a small and 30 mg x kg(-1) a marked effect on infarct volume. At equimolar doses (3.0 mg x kg(-1) for NXY-059 and 1.4 mg x kg(-1) for PBN), NXY-059 was more efficacious than PBN. Similar results were obtained when a recovery period of 7 days was allowed. The window of therapeutic opportunity for NXY-059 was 3 to 6 hours after the start of recirculation. Studies of the transfer constant of [14C]NXY-059 showed that, in contrast to PBN, this more soluble nitrone penetrates the blood-brain barrier less extensively. This fact, and the pronounced antiischemic effect of NXY-059, suggest that the delayed events leading to infarction may be influenced by reactions occurring at the blood-endothelial interface.

Free radical scavengers: chemical concepts and clinical relevance

Free radicals are involved in the pathology of many CNS disorders, like Parkinson's disease, Alzheimer's disease, or stroke. This discovery lead to the development of many radical scavengers for the clinical treatment of neurodegenerative diseases. In this review, the different chemical concepts for free radical scavenging will be discussed: nitrons, thiols, iron chelators, phenols, and catechols. Especially catechols, like the naturally occurring flavonols, the synthetic drug nitecapone, or the endogenous catacholamines and their metabolites, are of great interest, as they combine iron chelating with radical scavenging activity. We present data on the radical scvenging activity of dopamine and apomorphine, which prevent lipid peroxidation in rat brain mitochondria and protect PC12 cells against H2O2-toxicity.

Novel free radical spin traps protect against malonate and MPTP neurotoxicity

Both malonate and 1-methyl-4-phenyl-1,2,5,6 tetrahydropyridine (MPTP) are neurotoxins which cause energy depletion, secondary excitotoxicity, and free radical generation. Malonate is a reversible inhibitor of succinate dehydrogenase, while MPTP is metabolized to 1-methyl-4-phenylpyridinium, an inhibitor of mitochondrial complex I. We examined the effects of pretreatment with the cyclic nitrone free radical spin trap MDL 101,002 on malonate and MPTP neurotoxicity. MDL 101,002 produced dose-dependent neuroprotection against malonate-induced striatal lesions. MDL 101, 002 produced significant protection against MPTP induced depletions of dopamine and its metabolites. MDL 101,002 also significantly attenuated MPTP-induced increases in striatal 3-nitrotyrosine concentrations. The free radical spin trap tempol also produced significant protection against MPTP neurotoxicity. These findings provide further evidence that free radical spin traps produce neuroprotective effects in vivo and suggest that they may be useful in the treatment of neurodegenerative diseases.

N-tert-butyl-alpha-phenylnitrone protects against 3,4-methylenedioxymethamphetamine-induced depletion of serotonin in rats

The present study examined the effect of N-tert-butyl-alpha-phenylnitrone (PBN) on 3,4-methylenedioxmathamphetamine (MDMA)-induced depletion of serotonin in the CNS. Rats were treated with two concurrent injections of MDMA (20 mg/kg, s.c.), PBN (50-400 mg/kg dissolved in ethanol, 50 mg/ml of 25% ethanol, i.p.), saline or 25% ethanol, alone or in combination, 6 h apart, and sacrificed 5 days later. Rectal temperature was measured prior to and hourly following the drug injection for 5 h. Monoamine levels in the tissue were measured by HPLC. Density of the 5-HT transporters was assayed by [3H]paroxetine binding. Rectal temperature of rats increased after MDMA, decreased after PBN, ethanol, PBN plus ethanol, and MDMA plus ethanol, and was not significantly altered after MDMA plus PBN. Levels of 5-HT and 5-HIAA in the frontal cortex, hippocampus, striatum, and brain stem of rats decreased significantly after MDMA or MDMA plus ethanol, but not after MDMA plus PBN, PBN plus ethanol (PBN dissolved in ethanol), or ethanol as compared to the saline controls. Levels of 5-HT and 5-HIAA in the brain tissues of rats treated with MDMA plus PBN were elevated as compared to those treated with MDMA plus saline. Similar results were observed in the density of 5-HT transporters in the frontal cortex and hippocampus. These results indicate that scavenging of free radicals of MDMA metabolites or reactive oxygen species by PBN and with lowering of body temperature protected against MDMA-induced depletion of serotonin transmitter.

Pharmacologic properties of phenyl N-tert-butylnitrone

Phenyl N-tert-butylnitrone (PBN) is the parent of a family of nitrones used as spin-trapping agents to trap free radicals. PBN's pharmacological effects in animal models are extensive, ranging from protection against death after endotoxin shock, protection from ischemia-reperfusion injury, to increasing the life span of mice. Recent additions to the list include protection from bacterial meningitis, thalidomide-induced teratogenicity, drug-induced diabetogenesis, and choline-deficient hepatocarcinogenesis. Because PBN reacts with oxygen radicals to produce less reactive species, it has been suggested that this is the basis of its pharmacological effects. However, there has been no hard evidence for this notation. Nevertheless, many investigators have used the presence of PBN's pharmacologic effect as evidence for free radical involvement in their models. Mechanistic studies on the PBN's antisepsis action revealed that PBN inhibits expression of various pro-inflammatory genes, suggesting that the protective action involves more than a straightforward free radical-scavenging mechanism. Previous and recent developments in the investigations on the pharmacologic properties of PBN are described in this review.

Treatment with the spin-trap agent alpha-phenyl-N-tert-butyl nitrone does not enhance the survival of embryonic or adult dopamine neurons

Reactive oxygen species are thought to be involved in the death of dopaminergic neurons in Parkinson's disease as well as in transplanted embryonic dopaminergic neurons. The spin-trap agent alpha-phenyl-N-tert-butyl nitrone (PBN) reacts directly with radical species and may thereby prevent them from damaging important cellular molecules such as membrane lipids. We found that PBN does not increase the survival of cultured embryonic dopaminergic neurons subjected to serum deprivation, whereas the antioxidant and lipid peroxidation inhibitor lazaroid U-83836E does. Moreover, PBN does not increase the survival of grafted embryonic dopaminergic neurons or graft efficacy (monitored as changes in drug-induced motor asymmetry in hemiparkinsonian rats) when the spin-trap agent is given intraperitoneally to the graft recipient or is added to the solutions used when preparing tissue for transplantation. Another spin-trap agent, alpha-(4-pyridyl-1-oxide)-N-tert-butyl nitrone (POBN) also failed to protect neurons when given to graft recipients in the same experimental paradigm. Finally, we found that adult nigral neurons subjected to a progressive retrograde 6-OHDA lesion are not protected by systemic treatment with PBN. Even though reduction of oxidative stress by overexpression of superoxide dismutase or addition of lazaroids have previously been shown to enhance the survival of cultured and grafted dopaminergic neurons, spin-trap agents PBN and POBN do not provide protection in these experimental paradigms. This may be due to antioxidants and spin-trap agents interfering in different steps of free radical-induced cell damage.

Free radicals in rabbit spinal cord ischemia: electron spin resonance spectroscopy and correlation with SOD activity

1. In nonanesthetized rabbits temporal occlusion of the abdominal aorta was used to induce oxidative stress in the lower part of the body including distal segments of the spinal cord. 2. Spinal cord samples were taken from the animals exposed to 25-min aortic occlusion (AO) or to occlusion followed by 1- or 2-hr reperfusion (AO/R1 or AO/R2, respectively) or from sham-operated animals (C). The presence of free radicals (FR) in the spinal cord samples frozen in liquid N2 was assessed by ESR spectroscopy without spin trapping. Moreover, superoxide dismutase (SOD) activity and conjugated diene (CD) levels were measured in the samples. 3. In the AO group FR were detected in the spinal cord regions close to the occlusion (lower thoracic and distal segments) along with a decrease in SOD activity. The calculated g value (g = 2.0291) indicated that the paramagnetic signal recorded might be attributed to superoxide radicals. FR were absent in the AO/R1 group. Concurrently, the SOD activity revealed a significant tendency to return to the control level. FR appeared again in the AO/R2 group, mostly in the upper and middle lumbar regions, along with a decrease in SOD activity. No sample from the C group revealed FR. A significant increase in CD levels was observed in the thoracolumbar region only in the AO/R2 group. The temporary absence of FR in the AO/R1 group suggests activation of defense antioxidant mechanisms (e.g., specific enzymatic systems such as SOD), which might have been exhausted later. 4. Changes in SOD activity similar to those observed in the thoracolumbar region, though less noticeable, occurred in the obviously noncompromised tissue (upper cervical region). This points to a kind of generalized response of the animal to aortic occlusion. 5. Direct ESR spectroscopy revealed the presence of FR as well as their time course in the spinal cord during the early phase of ischemia/reperfusion injury and the inverse relationship between FR and SOD activity.

Superior neuroprotective efficacy of a novel antioxidant (U-101033E) with improved blood-brain barrier permeability in focal cerebral ischemia

BACKGROUND AND PURPOSE

The vascular endothelium and parenchyma of the brain have the potential to generate free radicals under pathological conditions, but it is unclear which of these two sites prevails in the production of free radicals and should be the primary target of therapeutic intervention. To clarify this issue, we compared the neuroprotective properties of a 21-aminosteroid (U-74389G) that acts on the microvasculature and a pyrrolopyrimidine (U-101033E), a novel antioxidant compound that has significantly improved potential to enter the brain parenchyma.

METHODS

In Sprague-Dawley rats the middle cerebral artery was occluded for 90 minutes by an intraluminal filament. Local cortical blood flow was recorded by bilateral laser Doppler flowmetry throughout ischemia and 1 hour of reperfusion. Three groups of rats were studied: controls that received vehicle only and animals that received either U-74389G or U-101033E. Neurological examinations were performed daily, and infarct size was assessed histologically 7 days after ischemia.

RESULTS

U-101033E reduced infarct volume significantly by 51%, whereas U-74389G led to a nonsignificant decrease in infarct volume. U-101033E improved neurological function immediately after ischemia, whereas U-74389G led to improvement only at the end of the observation period. Laser Doppler measurements showed no significant difference in local cortical blood flow among the treatment groups.

CONCLUSIONS

We conclude that for treatment of transient focal ischemia, an antioxidant that crosses the blood-brain barrier might be superior to agents that predominantly act on the endothelium of the cerebral microvasculature.

Methodological considerations of intracerebral microdialysis in pharmacokinetic studies on drug transport across the blood-brain barrier

For the study of the pharmacokinetics of drugs in the brain a number of in vivo techniques is available, including autoradiography, imaging techniques, cerebrospinal fluid sampling and in vivo voltammetry, which all have their specific advantages and limitations. Intracerebral microdialysis is a relatively new in vivo technique. It permits monitoring of local concentrations of drugs and metabolites at specific sites in the brain which makes it an attractive tool for pharmacokinetic research. In the use of this technique a number of factors should be considered. These include: type of probe, surgical trauma, post-surgery interval, perfusion flow rate, as well as composition and temperature of the perfusion medium. In particular in studies on drug transport across the blood-brain barrier (BBB), effects of insertion of the probe on BBB functionality is important. It appears that BBB functionality is not significantly affected if surgical and experimental conditions are well-controlled. The relationship between dialysate concentrations and those in the extracellular fluid of the periprobe tissue, the recovery of the drug, depends on periprobe processes governing the actual concentration of the drug at that site. These include extracellular-microvascular exchange, metabolism, and diffusion of the drug. Several methods have been proposed to determine recovery values. In particular the no net flux method and the extended no net flux method are useful in practice. Several microdialysis studies on BBB transport of drugs are presented showing that intracerebral microdialysis is capable to assess local BBB transport profiles. Compared with other in vivo techniques, intracerebral microdialysis is the only (affordable) technique that offers the possibility to monitor local BBB transport of drugs in unanaesthetized animals, under physiological and pathological conditions.

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.

Reactive oxygen intermediates contribute to necrotic and apoptotic neuronal injury in an infant rat model of bacterial meningitis due to group B streptococci

Reactive oxygen intermediates (ROI) contribute to neuronal injury in cerebral ischemia and trauma. In this study we explored the role of ROI in bacterial meningitis. Meningitis caused by group B streptococci in infant rats led to two distinct forms of neuronal injury, areas of necrosis in the cortex and neuronal loss in the dentate gyrus of the hippocampus, the latter showing evidence for apoptosis. Staining of brain sections with diaminobenzidine after perfusion with manganese buffer and measurement of lipid peroxidation products in brain homogenates both provided evidence that meningitis led to the generation of ROI. Treatment with the radical scavenger alpha-phenyl-tert-butyl nitrone (PBN) (100 mg/kg q8h i.p.) beginning at the time of infection completely abolished ROI detection and the increase in lipidperoxidation. Cerebral cortical perfusion was reduced in animals with meningitis to 37.5+/-21.0% of uninfected controls (P < 0.05), and PBN restored cortical perfusion to 72.0+/-8.1% of controls (P < 0.05 vs meningitis). PBN also completely prevented neuronal injury in the cortex and hippocampus, when started at the time of infection (P < 0.02), and significantly reduced both forms of injury, when started 18 h after infection together with antibiotics (P < 0.004 for cortex and P < 0.001 for hippocampus). These data indicate that the generation of ROI is a major contributor to cerebral ischemia and necrotic and apoptotic neuronal injury in this model of neonatal meningitis.

alpha-Phenyl-N-tert-butyl nitrone attenuates methamphetamine-induced depletion of striatal dopamine without altering hyperthermia

Methamphetamine (MA) administration to adult rats (4 x 10 mg/kg s.c.) induces neurotoxicity predominately characterized by a persistent reduction of neostriatal dopamine (DA) content. Hyperthermia following MA administration potentiates the resulting DA depletion. DA-derived free radicals are postulated to be a mechanism through which MA-induced neurotoxicity is produced. The spin trapping agent PBN reacts with free radicals to form nitroxyl adducts, thereby preventing damaging free radical reactions with cellular substrates. MA with saline pretreatment (Sal-MA) reduced neostriatal DA by 55% (P < 0.01 vs. Sal-Sal). MA with PBN pretreatment (PBN-MA) at 36 or 60 mg/kg reduced neostriatal DA by 36 and 22%, respectively (P < 0.05 and P < 0.01 vs Sal-MA) indicating partial protection. PBN pretreatment did not alter MA-induced hyperthermia. Thus, PBN does not attenuate MA-induced neurotoxicity by reducing MA-induced hyperthermia. These results support a role for free radicals in the generation of MA-induced dopaminergic neurotoxicity.