Isabelle Margaill

The proteolytic activity of tissue-plasminogen activator enhances NMDA receptor-mediated signaling.

Tissue-plasminogen activator (t-PA) is now available for the treatment of thrombo-embolic stroke but adverse effects have been reported in some patients, particularly hemorrhaging. In contrast, the results of animal studies have indicated that t-PA could increase neuronal damage after focal cerebral ischemia. Here we report for the first time that t-PA potentiates signaling mediated by glutamatergic receptors by modifying the properties of the N-methyl-D-aspartate (NMDA) receptor. When depolarized, cortical neurons release bio-active t-PA that interacts with and cleaves the NR1 subunit of the NMDA receptor. Moreover, the treatment with recombinant t-PA leads to a 37% increase in NMDA-stimulated fura-2 fluorescence, which may reflect an increased NMDA-receptor function. These results were confirmed in vivo by the intrastriatal injection of recombinant-PA, which potentiated the excitotoxic lesions induced by NMDA. These data provide insight into the regulation of NMDA-receptor–mediated signaling and could initiate therapeutic strategies to improve the efficacy of t-PA treatment in man.

Mechanisms Involved in the Neuroprotective Activity of a Nitric Oxide Synthase Inhibitor During Focal Cerebral Ischemia

Abstract: We have reported previously that posttreatment with NG‐nitro‐L‐arginine methyl ester (L‐NAME), an inhibitor of the nitric oxide synthase, reduced the volume of cortical and striatal infarct induced by middle cerebral artery occlusion in rats. In the present study, we investigated the mechanisms by which L‐NAME (3 mg/kg i.p.) is neuroprotective in this model of cerebral ischemia. First, we have shown the reversal of the neuroprotective effect of L‐NAME by a coinjection of L‐arginine. Second, in order to determine by which mechanism nitric oxide exacerbates neuronal damage produced by focal cerebral ischemia, we studied the effect of the inhibition of nitric oxide synthase by L‐NAME on the histological consequences of a focal injection of N‐methyl‐D‐aspartate (NMDA) in the striatum, and on the striatal overflow of glutamate and aspartate induced either by K+ depolarization or by focal cerebral ischemia. We have found that L‐NAME treatment reduced the excitotoxic damage produced by NMDA injection. By using microdialysis, we have shown that the K+‐ and the ischemia‐induced glutamate efflux was reduced by 52 and 30%, respectively, after the L‐NAME treatment. These results indicate that nitric oxide synthesis induced by the NMDA receptor overstimulation is one of the major events leading to neuronal damage. One possible mechanism by which nitric oxide may contribute to the excitotoxic process is by facilitating the ischemia‐induced glutamate overflow.

Selective inhibition of inducible nitric oxide synthase prevents ischaemic brain injury

The aim of this study was to investigate the effect of N‐(3‐(aminomethyl)benzyl)acetamidine (1400W), a selective inhibitor of inducible calcium‐independent nitric oxide synthase (iNOS), on the functional and histopathological outcomes of experimental transient focal cerebral ischaemia in rats. Transient ischaemia was produced by the occlusion for 2 h of both the left middle cerebral artery and common carotid artery. Treatments with 1400W (20 mg kg−1) or vehicle were started 18 h after occlusion of the arteries and consisted in seven subcutaneous injections at 8 h interval. Ischaemic outcomes and NOS activities (constitutive and calcium‐independent NOS) were evaluated 3 days after ischaemia. 1400W significantly reduced ischaemic lesion volume by 31%, and attenuated weight loss and neurological dysfunction. 1400W attenuated the calcium‐independent NOS activity in the infarct by 36% without affecting the constitutive NOS activity. These findings suggest that iNOS activation contributes to tissue damage and that selective inhibitors of this isoform may be of interest for the treatment of stroke.

Protective effect of melatonin in a model of traumatic brain injury in mice

ABSTRACT: The pineal hormone melatonin has recently been shown to exert neuroprotective activity in a variety of experimental neuropathologies in which free radicals are involved. This neuroprotective effect has been attributed to the antioxidant properties of melatonin. Considering that free radicals also play a deleterious role in traumatic brain injury (TBI), the purpose of the present study was to determine whether melatonin would have a beneficial effect in this pathology. Head injury was induced in mice and the neurological deficit was evaluated at 24 hr by a grip test. In this model, the free radical scavenger, α‐phenyl‐tert‐butyl‐nitrone (2 ± 100 mg/ kg, i.p.) given 5 min and repeated at 4 hr after TBI was neuroprotective. Melatonin (1.25 mg/kg, i.p.) given 5 min and repeated at 1,2, and 3 hr after head trauma also significantly reduced the neurological deficit. This beneficial effect was not due to melatonin‐induced hypothermia since repeated treatment with melatonin did not modify the colonic temperature of mice. This study shows that melatonin exerts a beneficial effect on the neurological deficit induced by traumatic brain injury in mice. The mechanisms of this neuroprotection remains to be established, and more particularly, the contribution of the antioxidant activity of melatonin.

Short Therapeutic Window for MK-801 in Transient Focal Cerebral Ischemia in Normotensive Rats

The present study investigates the role of N-methyl-D-aspartate (NMDA) receptors in a model of transient focal cerebral ischemia in normotensive rats. The left middle cerebral artery and both common carotid arteries were occluded for 60 min. Preliminary studies indicated that this gave reproducible infarctions of the cortex and striatum. These infarctions were the result of severe ischemia followed by complete reperfusion after clamp removal, as showed by striatal tissue Po2 monitoring. Microdialysis indicated that glutamate concentration increased immediately after occlusion and returned to the baseline value 40 min after clamp removal. MK-801 (1 mg kg−1 i.v.), an antagonist of the NMDA glutamatergic receptor, reduced the cortical infarct volume by 29% (p < 0.001) and the striatal infarct volume by 14% (p < 0.05) when given just prior to ischemia, but had no neuroprotective activity when given 30 min after the onset of ischemia. This short therapeutic window for MK-801 suggests that NMDA receptors play only a transient role in reversible focal ischemia in rats.

Progesterone receptors: a key for neuroprotection in experimental stroke.

Progesterone receptors (PR) are expressed throughout the brain. However, their functional significance remains understudied. Here we report a novel role of PR as crucial mediators of neuroprotection using a model of transient middle cerebral artery occlusion and PR knockout mice. Six hours after ischemia, we observed a rapid increase in progesterone and 5α-dihydroprogesterone, the endogenous PR ligands, a process that may be a part of the natural neuroprotective mechanisms. PR deficiency, and even haploinsufficiency, increases the susceptibility of the brain to stroke damage. Within a time window of 24 h, PR-dependent signaling of endogenous brain progesterone limits the extent of tissue damage and the impairment of motor functions. Longer-term improvement requires additional treatment with exogenous progesterone and is also PR dependent. The potent and selective PR agonist Nestorone is also effective. In contrast to progesterone, levels of the neurosteroid allopregnanolone, which modulates γ-aminobutyric acid type A receptors, did not increase after stroke, but its administration protected both wild-type and PR-deficient mice against ischemic damage. These results show that 1) PR are linked to signaling pathways that influence susceptibility to stroke, and 2) PR are direct key targets for both endogenous neuroprotection and for therapeutic strategies after stroke, and they suggest a novel indication for synthetic progestins already validated for contraception. Although allopregnanolone may not be an endogenous neuroprotective agent, its administration protects the brain against ischemic damage by signaling mechanisms not involving PR. Collectively, our data clarify the relative roles of PR and allopregnanolone in neuroprotection after stroke.

Changes in oxidative stress, iNOS activity and neutrophil infiltration in severe transient focal cerebral ischemia in rats

Oxidative stress, inducible nitric oxide synthase (iNOS) and neutrophils all contribute to post-ischemic brain damage. This study has determined the time courses of these three phenomena after ischemia in parallel with histological and functional outcomes. Ischemia was produced in rats by occluding the left middle cerebral artery and both common carotid arteries for 20 min. Regional cerebral blood flow (rCBF) rapidly decreased to 20% of its preischemic value during occlusion and stabilized at 60% following reperfusion. The striatal infarction was maximal 15 h after reperfusion (50+/-3 mm(3)), whereas the cortical infarction reached its maximum at 48 h (183+/-10 mm(3)). This drastic decrease in rCBF followed by incomplete reperfusion and massive infarction is, thus, extremely severe. The cortical infarction was strongly correlated with the neurologic deficit and loss of body weight. Oxidative stress, evaluated by the decrease in glutathione concentrations, appeared in the striatum at 6 h after reperfusion and in the cortex at 15 h. Calcium-independent NOS activity, considered as inducible NOS activity, was significantly enhanced at 24 h in the striatum and at 48 h in the cortex. Myeloperoxidase activity, a marker of neutrophil infiltration, was significantly increased at 48 h in both the striatum and cortex. These time courses show that the delayed iNOS activity and neutrophil infiltration that occur after the maturation of infarction in severe ischemia may not contribute to ischemic brain damage. By contrast, early oxidative stress may well be implicated in cerebral injury.

Dose- and time-dependence of L-NAME neuroprotection in transient focal cerebral ischaemia in rats.

In this study the effect of the dose and administration time of NG‐nitro‐l‐arginine methyl ester (l‐NAME), an NO‐synthase inhibitor, in a model of transient focal cerebral ischaemia in rats was investigated. Two injections of l‐NAME were given, of 1, 3 and 10 mg kg−1, 5 min and 3 h after the onset of ischaemia. None of the doses gave any striatal neuroprotection, but 1 and 3 mg kg−1l‐NAME reduced the infarcted volume in the cortex (by 26%, P<0.01 for 1 mg kg−1 and 21%, P<0.05 for 3 mg kg−1), whereas 10 mg kg−1 had no neuroprotective effect. Single injections of l‐NAME 1 mg kg−1, given 5 min or 3 h after ischaemia onset, had similar neuroprotective effects on the cortical infarction as did the repeated injections. l‐NAME 1 mg kg−1 given 3, 6 or 9 h after ischaemia induction reduced the cortical infarct volume by 19% (P<0.01) when given 3 h after ischaemia, by 21% (P<0.01) when given at 6 h, and by 16% (P<0.5) when given at 9 h, but had no neuroprotective activity when given 12 h after ischaemia. Thus a low dose of l‐NAME is neuroprotective in a model of transient focal ischaemia, with a wide therapeutic window, much larger than that found for MK‐801.

Anti-inflammatory effects of PJ34, a poly(ADP-ribose) polymerase inhibitor, in transient focal cerebral ischemia in mice

Activation of poly(ADP‐ribose) polymerase (PARP) is deleterious during cerebral ischemia. We assessed the influence of PARP activation induced by cerebral ischemia on the synthesis of proinflammatory mediators including the cytokines, tumour necrosis factor‐α (TNF‐α) and interleukin‐6 (IL‐6) and the adhesion molecules, E‐selectin and intercellular adhesion molecule‐1 (ICAM‐1).

Antisense Oligodeoxynucleotide to Inducible Nitric Oxide Synthase Protects against Transient Focal Cerebral Ischemia—Induced Brain Injury

Nitric oxide (NO) has been suspected to mediate brain damage during ischemia. Here the authors studied the effects of an antisense oligodeoxynucleotide (ODN) directed against the inducible isoform of NO synthase (iNOS) in a model of transient focal cerebral ischemia in rats. Treatment consisted of seven intracerebroventricular injections of a phosphodiester/phosphorothioate chimera ODN (3 nmol each) at 12-hour intervals, and was initiated 12 hours before a 2-hour occlusion of the left middle cerebral artery and common carotid artery. Outcomes were measured three days after ischemia. When compared with animals treated with vehicle or an appropriate random non-sense control ODN sequence, the antisense treatment reduced the lesion volume by 30% and significantly improved recovery of sensorimotor functions, as assessed on a neuroscore. This effect was associated with a decrease in iNOS expression, as assessed by Western blot, a 39% reduction in iNOS enzymatic activity evaluated as Ca2+-independent NOS activity, and a 37% reduction in nitrotyrosine formation, reflecting protein nitration by NO-derived peroxynitrite. These findings provide new evidence that inhibition of iNOS may be of interest for the treatment of stroke.