Differential effect of NMDA and AMPA receptor blockade on protein synthesis in the rat infarct borderzone

Neuroprotective effects of the AMPA antagonist PNQX in oxygen-glucose deprivation in mouse hippocampal slice cultures and global cerebral ischemia in gerbils

PNQX (9-methyl-amino-6-nitro-hexahydro-benzo(F)quinoxalinedione) is a selective AMPA antagonist with demonstrated neuroprotective effects in focal ischemia in rats. Here we report corresponding effects in mouse hippocampal slice cultures subjected to oxygen and glucose deprivation (OGD) and in transient global cerebral ischemia in gerbils. For in vitro studies, hippocampal slice cultures derived from 7-day-old mice and grown for 14 days, were submersed in oxygen-glucose deprived medium for 30 min and exposed to PNQX for 24 h, starting together with OGD, immediately after OGD, or 2 h after OGD. For comparison, other cultures were exposed to the NMDA antagonist MK-801 using the same protocol. Both PNQX and MK-801 displayed significant neuroprotective effects in all hippocampal subfields when present during and after OGD. When added just after OGD, only PNQX retained some neuroprotective effect. When added 2 h after OGD neither PNQX nor MK-801 had an effect. Transient global cerebral ischemia was induced in Mongolian gerbils by occlusion of both common carotid arteries for 4.5 min, with PNQX (10 mg/kg) being injected i.p. 30, 60 and 90 min after the insult. Subsequent analysis of brain sections stained for the neurodegeneration marker Fluoro-Jade B and immunostained for the astroglial marker glial fibrillary acidic protein revealed a significant PNQX-induced decrease in neuronal cell death and astroglial activation. We conclude that, PNQX provided neuroprotection against both global cerebral ischemia in gerbils in vivo and oxygen-glucose deprivation in mouse hippocampal slice cultures.

Somatostatin receptor 2 is activated in cortical neurons and contributes to neurodegeneration after focal ischemia

Somatostatin receptor 2 (SSTR2) mediates neuromodulatory signals of somatostatin and cortistatin in the cerebral cortex. Recently, SSTR2 has been shown to enhance conserved death ligand- and mitochondria-mediated apoptotic pathways in non-neuronal cells. Whether somatostatin receptors are activated in cerebrocortical neurons and contribute to neurodegeneration after experimental focal ischemia was unknown until now. Here we examined internalization of SSTR2 in a rat model of middle cerebral artery occlusion (MCAO) by confocal microscopy. At 3 and 6 hr after MCAO, SSTR2 was internalized excessively in cerebrocortical neurons adjacent to the infarct, which was prevented by intracerebroventricular application of the SSTR2-selective antagonist BIM-23627. SSTR2 internalization was associated with a transient depletion of somatostatin from axonal terminals and increased expression of SSTR2 mRNA. The initial loss of somatostatin was followed by an increase in somatostatin mRNA levels, whereas cortistatin mRNA expression was decreased. In SSTR2-deficient mice with lacZ under the control of the SSTR2 promoter, MCAO-induced upregulation of SSTR2 gene expression was less pronounced than in wild types. SSTR2-deficient mice exhibited a 40% reduction of infarct size after permanent distal MCAO and a 63% reduction after transient proximal MCAO. In summary, we provide direct evidence for activation of SSTR2 by an endogenous ligand after focal ischemia. Activation of functional SSTR2 receptors contributes to increased SSTR2 gene expression and postischemic neurodegeneration.

Widespread and long-lasting alterations in GABA(A)-receptor subtypes after focal cortical infarcts in rats: mediation by NMDA-dependent processes

Impairment of inhibitory neurotransmission has been reported to occur in widespread, structurally intact brain regions after focal ischemic stroke. These long-lasting alterations contribute to the functional deficit and influence long-term recovery. Inhibitory neurotransmission is primarily mediated by gamma-aminobutyric acid (GABA)A receptors assembled of five subunits that allow a variety of adaptive changes. In this study, the regional distribution of five major GABA(A)-receptor subunits (alpha1, alpha2, alpha3, alpha5, and gamma2) was analyzed immunohistochemically 1, 7, and 30 days after photochemically induced cortical infarcts. When compared with sham-operated controls, a general and regionally differential reduction in immunostaining was found within the cortex, hippocampus, and thalamus of both hemispheres for almost all subunits. Within ipsilateral and contralateral neocortical areas, a specific pattern of changes with a differential decrease of subunits alpha1, alpha2, alpha5, and gamma2 and a significant upregulation of subunit alpha3 was observed in the contralateral cortex homotopic to the infarct. This dysregulation was most prominent at day 7 and still present at day 30. Interestingly, a single application of the noncompetitive N-methyl-D-aspartate-receptor antagonist MK-801 during lesion induction completely blocked these bihemispheric alterations. Cortical spreading depressions induced by topical application of KCl do not change GABA(A)-receptor subunit expression. As alterations in subtype distribution crucially influence inhibitory function, ischemia-induced modifications in GABA(A)-receptor subtype expression may be of relevance for functional recovery after stroke.

Post-ischemic administration of DY-9760e, a novel calmodulin antagonist, reduced infarct volume in the permanent focal ischemia model of spontaneously hypertensive rat

We assessed the effect of a novel calmodulin antagonist, DY-9760e (3-[2-[4-(3-chloro-2-methylphenyl)-1-piperazinyl]ethyl]-5,6-dimethoxy-1-(4-imidazolylmethyl)-1H-indazole dihydrochloride 3.5 hydrate) in a spontaneously hypertensive rat (SHR) permanent focal cerebral ischemia. In experiment I, the left middle cerebral artery was permanently occluded in 62 SHRs. DY-9760e (0.5 mg kg(-1) h(-1)) or vehicle alone were administered continuously i.v. for 6 h, beginning 0, 30, or 60 min after the arterial occlusion. The infarct volume was measured 24 h of ischemia. In experiment II, the effect of DY-9760e on CBF was assessed in 10 SHRs. Administration without a delay resulted in a mean infarct volume of 166.7 +/- 21.0 mm3 (vehicle; n = 10) and 125.1 +/- 31.8 mm3 (DY-9760e; n = 9). Administration with a 30 min delay resulted in a mean infarct volume of 173.2 +/- 32.4 mm3 (vehicle; n = 12) and 143.3 +/- 35.3 mm3 (DY-9760e; n = 11). Dy-9760e significantly reduced the infarct under these conditions (p < 0.05). The administration with a 60 min delay failed to reduce the infarct. DY-9760e had no effect on the CBF. Continuous i.v. administration of DY-9760e reduced infarct volume in a SHR permanent focal ischemia without affecting ischemic CBF.

Differential effects of NMDA-receptor antagonists on long-term potentiation and hypoxic/hypoglycaemic excitotoxicity in hippocampal slices

Whole-cell patch clamp recording from cultured hippocampal neurones was used to investigate the NMDA antagonistic effects of the glycineB antagonist 5,7-DCKA and the competitive antagonist CGP 37849. Extracellular field potential recording from area CA1 of hippocampal slices was used to investigate their effects on the induction of LTP and hypoxia/hypoglycaemia-induced suppression of fEPSPs. Additionally, memantine and (+)MK-801 were tested in the later model. 5,7-DCKA inhibited NMDA-induced plateau currents (IC50=0.24+/-0.02 microM) with around nine times higher potency than against peak (IC50=2.14+/-0.17 microM). In contrast, CGP 37849 slowed the onset of NMDA-induced currents considerably and antagonized currents at the time point when the peak component occurred in control responses (IC50=0.18+/-0.01 microM) with around seven times higher potency than against plateau (IC50=1.26+/-0.19 microM). Both 5,7-DCKA and CGP 37849 inhibited the induction of LTP (IC50s=2.53+/-0.13 and 0.37+/-0.04 microM respectively) with potencies close to those inhibiting peak currents in patch clamp studies. 5,7-DCKA and CGP 37849 also blocked the hypoxia/hypoglycaemia-induced suppression of fEPSPs but CGP 37849 (EC50=4.3+/-0.33 microM) was far less potent than against the induction of LTP whilst 5,7-DCKA (EC50=1.47+/-0.04 microM) had similar potency in these two models. Memantine and (+)MK-801 also blocked hypoxia/hypoglycaemia-induced suppression of fEPSPs with EC50s of 14.1+/-0.52 and 0.53+/-0.02 microM respectively. Whereas memantine blocked this effect with similar potency as we previously reported for LTP, (+)MK-801 was four time less potent in this model. The calculated relative therapeutic indices (IC50 LTP over EC50 hypoxia/hypoglycaemia) for 5,7-DCKA, CGP 37849, memantine and (+)MK-801 were 1.72, 0.09, 0.82 and 0.24 respectively. These results show that even in a severe model of hypoxia/hypoglycaemia, glycineB site antagonists and moderate affinity channel blockers exhibit a better therapeutic index than competitive antagonists and high affinity channel blockers. It is likely that in milder forms of pathology the observed differences in therapeutic indices remain the same but the absolute values are expected to be higher.

Neuroprotective electrical stimulation of cerebellar fastigial nucleus attenuates expression of periinfarction depolarizing waves (PIDs) and inhibits cortical spreading depression

In rat, electrical stimulation of the cerebellar fastigial nucleus (FN) for 1 h reduces the volume of focal ischemic infarctions produced by occluding the middle cerebral artery (MCAO), even 10 days later. The mechanism by which this 'central neurogenic neuroprotection' salvages ischemic brain is not known but does not result from changes in cerebral perfusion. MCAO also triggers periodic periinfarction depolarizing waves (PIDs) in the ischemic penumbra, the territory of salvage. These may contribute to neuronal death and promote infarct expansion. Conceivably, FN stimulation, which can otherwise modify cortical excitability, may alter the development of PIDs. We investigated in anesthetized rats whether FN stimulation modifies PIDs expression and, if so, the threshold for evoking cortical spreading depression (CSD), a process sharing characteristics with PIDs and an index of cortical excitability. Stimulation of FN immediately or 72 h before MCAO decreased infarction volumes by approximately 45% (p<0.01), increased PID latency >10-fold, and decreased the number of PIDs by >50% (p<0.001). In normal rats, stimulation of FN increased the threshold current for eliciting CSD by 175% and slowed its propagation velocity by 35% (p<0.01 for each) immediately, but not 72 h, after FN stimulation. We conclude: FN stimulation elicits long-lasting suppression of PIDs in parallel with neuroprotection. However, PIDs suppression over time is unlikely to result from a major increase in cortical tolerance to depolarization and probably is not the principal mechanism of salvage.

Focal cerebral ischemia in rats induces expression of P75 neurotrophin receptor in resistant striatal cholinergic neurons

Expression of p75 neurotrophin receptor and survival of medium-sized spiny projection neurons and cholinergic interneurons in the rat striatum were studied using immunocytochemistry at different times after transient, unilateral middle cerebral artery occlusion. Thirty minutes of middle cerebral artery occlusion caused a major loss of projection neurons, identified by their immunoreactivity to dopamine- and adenosine 3':5'-monophosphate-regulated phosphoprotein with a molecular weight of 32,000, in the lateral part of the striatum, as observed at 48 h following the insult with no further change at one week. In contrast, no reduction of the number of choline acetyltransferase-positive, cholinergic interneurons, which also expressed TrkA, was detected at either time-point. At 48 h following middle cerebral artery occlusion, expression of p75 neurotrophin receptor was observed in striatal cells which, by the use of double-label immunostaining, were identified as the cholinergic interneurons. No p75 neurotrophin receptor immunoreactivity remained in cholinergic cells after one week of reperfusion. Based on current hypotheses regarding the function of the p75 neurotrophin receptor, the transient expression of this receptor in striatal cholinergic interneurons might contribute to their high resistance to ischemic neuronal death. However, the expression of p75 neurotrophin receptor could also be a first step in a pathway leading to apoptosis, which is inhibited after the present insult due to concomitant activation of TrkA.

Structural alterations and changes in cytoskeletal proteins and proteoglycans after focal cortical ischemia

In order to study structural alterations which occur after a defined unilateral cortical infarct, the hindlimb region of the rat cortex was photochemically lesioned. The infarcts caused edema restricted to the perilesional cortex which affected allocortical and isocortical areas differently. Postlesional changes in cytoskeletal marker proteins such as microtubule-associated protein 2, non-phosphorylated (SMI32) and phosphorylated (SMI35, SMI31 and 200,000 mol. wt) neurofilaments and 146,000 mol. wt glycoprotein Py as well as changes in proteoglycans visualized with Wisteria floribunda lectin binding (WFA) were studied at various time points and related to glial scar formation. The results obtained by the combination of these markers revealed six distinct regions in which transient, epitope-specific changes occurred: the core, demarcation zone, rim, perilesional cortex, ipsilateral thalamus and contralateral homotopic cortical area. Within the core immunoreactivity for microtubule-associated protein 2 and SMI32 decreased and the cellular components showed structural disintegration 4 h post lesion, but partial recovery of somatodendritic staining was seen after 24 h. Microtubule-associated protein 2 and SMI32 persisted up to days 7 and 5 respectively in the core, whereas the number of glial fibrillary acidic protein- and WFA-positive cells decreased between days 7 and 14. The demarcation zone showed a dramatic loss of immunoreactivity for all epitopes 4 h post lesion which was not followed by a phase of recovery. In the inner region of the demarcation zone there was an invasion and accumulation of non-neuronal WFA-positive cells which formed a tight capsule around the core. Neuronal immunoreactivities for microtubule-associated protein 2, SMI31 and Py as well as astrocytic glial fibrillary acidic protein increased strongly within an approximately 0.4-1.0 mm-wide rim region directly bordering the demarcation zone. Py immunoreactivity increased significantly in the perilesional cortex, whereas glial fibrillary acidic protein-positive astrocytes became transiently more numerous in the entire lesioned hemisphere including strongly enhanced immunoreactivity in the thalamus by days 5-7 post lesion. Glial fibrillary acidic protein immunoreactivity increased in the corpus callosum and the homotopic cortical area of the unlesioned hemisphere by days 5-7. In this homotopic area additional changes in SMI31 immunoreactivity occurred. Our results showed that a cortical infarct is not only a locally restricted lesion, but leads to a variety of cytoskeletal and other structural changes in widely-distributed functionally-related areas of the brain.