Therapeutic potential of NMDA antagonists in neurodegenerative diseases

Novel dimeric bis(7)-tacrine proton-dependently inhibits NMDA-activated currents

Bis(7)-tacrine has been shown to prevent glutamate-induced neuronal apoptosis by blocking NMDA receptors. However, the characteristics of the inhibition have not been fully elucidated. In this study, we further characterize the features of bis(7)-tacrine inhibition of NMDA-activated current in cultured rat hippocampal neurons. The results show that with the increase of extracellular pH, the inhibitory effect decreases dramatically. At pH 8.0, the concentration-response curve of bis(7)-tacrine is shifted rightwards with the IC(50) value increased from 0.19+/-0.03 microM to 0.41+/-0.04 microM. In addition, bis(7)-tacrine shifts the proton inhibition curve rightwards. Furthermore, the inhibitory effect of bis(7)-tacrine is not altered by the presence of the NMDA receptor proton sensor shield spermidine. These results indicate that bis(7)-tacrine inhibits NMDA-activated current in a pH-dependent manner by sensitizing NMDA receptors to proton inhibition, rendering it potentially beneficial therapeutic effects under acidic conditions associated with stroke and ischemia.

Angiotensin II attenuates NMDA receptor-mediated neuronal cell death and prevents the associated reduction in Bcl-2 expression

While angiotensin II (Ang II) plays a major role in the regulation of blood pressure, fluid homeostasis and neuroendocrine function, recent studies have also implicated the peptide hormone in cell growth, differentiation and apoptosis. In support of this, we have previously demonstrated that Ang II attenuates N-methyl-D-aspartate (NMDA) receptor signaling [Molec. Brain Res. 48 (1997) 197]. To further examine the modulatory role of Ang II on NMDA receptor function, we investigated the effect of angiotensin receptor (AT) activation on NMDA-mediated cell death and the accompanying decrease in Bcl-2 expression. The viability of differentiated N1E-115 and NG108-15 neuronal cell lines was reduced following exposure to NMDA in a dose-dependent manner. MTT analysis (mitochondrial integrity) revealed a decrease in cell survival of 49.4+/-12.3% in NG108 cells and 79.9+/-6.8% in N1E cells following treatment with 10 mM NMDA for 20 h. Cytotoxicity in N1E cells was inhibited by the noncompetitive NMDA receptor antagonist, MK-801. Further, NMDA receptor-mediated cell death in NG108 cells was attenuated by treatment with Ang II. The Ang II effect was inhibited by both AT1 and AT2 receptor antagonists, losartan and PD123319, respectively, suggesting that both receptor subtypes may play a role in the survival effect of Ang II. Since it has been shown that activation of NMDA receptors alters the expression of Bcl-2 family proteins, Western blot analysis was performed in N1E cells to determine whether Ang II alters the NMDA-induced changes in Bcl-2 expression. A concentration-dependent decrease of intracellular Bcl-2 protein levels was observed following treatment with NMDA, and this reduction was inhibited by MK801. Addition of Ang II suppressed the NMDA receptor-mediated reduction in Bcl-2. The Ang II effect on NMDA-mediated changes in Bcl-2 levels was blocked by PD123319, but was not significantly changed by losartan, suggesting AT2 receptor specificity. Taken together, these results suggest that Ang II attenuates NMDA receptor-mediated neurotoxicity and that this effect may be due, in part, to an alteration in Bcl-2 expression.

Angiotensin II attenuates chemical hypoxia-induced caspase-3 activation in primary cortical neuronal cultures

In this study we determined whether caspase-3 is required in mouse cortical neurons for sodium azide-mediated apoptosis. Primary cortical neuronal cultures were treated with a cell permeable caspase-3 inhibitor, DEVD (1 nM-100 fM), prior to sodium azide-induced hypoxia. Treatment with the caspase-3 inhibitor resulted in a dose-dependent decrease in apoptosis, suggesting that sodium azide-induced apoptosis is mediated through a caspase-3 dependent pathway. Levels of cytochrome-c release and caspase-3 cleavage were assayed by Western analysis. Cytochrome-c release and caspase-3 cleavage were observed at 5 h (85.3+/-5.8%) and 8 h (53.4+/-14.9%), respectively. We have previously reported that angiotensin II, acting through the AT(2) receptor subtype, protects cultured mouse cortical neurons from sodium azide-induced apoptosis. We also examined whether the protective effect of angiotensin II is mediated through modulation of caspase-3. Pre-treatment of cells with angiotensin II and the AT(1) receptor antagonist, losartan, reduced levels of sodium azide-induced caspase-3 cleavage by 95.0+/-4.0%. Cells pre-treated with the AT(2) receptor antagonist, PD123319 showed a smaller reduction of caspase-3 cleavage (53.8+/-3.4%). Our findings indicate that sodium azide-induced apoptosis is caspase-3 dependent and that angiotensin II protects cortical neurons from chemical-induced apoptosis by reducing caspase-3 cleavage.

Angiotensin protects cortical neurons from hypoxic-induced apoptosis via the angiotensin type 2 receptor

The effects of angiotensin on mouse cortical neuronal cultures exposed to chemical-induced hypoxia was investigated. Cultures exposed to 10 mM sodium azide for 5 min showed a 17% increase in apoptosis when assayed 24 h postinsult. The N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 blocked sodium azide-induced cell death suggesting that the NMDA receptor contributes to the mediated cell death. Pretreatment of cultured neurons with angiotensin decreased sodium azide-induced apoptosis by 94%. When the AT(1) receptor was blocked by its receptor antagonist, losartan, angiotensin activation of the AT(2) receptor completely inhibited sodium azide-induced apoptosis. Pretreatment of neurons with the AT(2) receptor antagonist PD123319 resulted in angiotensin reducing sodium azide-induced apoptosis by 48%. These results demonstrate that angiotensin can significantly attenuate sodium azide-induced apoptosis primarily through activation of the AT(2) receptor and suggests that angiotensin may have a protective role in neurons undergoing ischemic injury.

Effects of cerestat and NBQX on functional and morphological outcomes in rat focal cerebral ischemia

This study investigated the ability of NBQX, an AMPA receptor antagonist, and cerestat, a NMDA receptor antagonist, to counteract neurological deficits and morphological damage induced by permanent occlusion of the left middle cerebral artery (MCAO model) in the rat. NBQX (3, 10, and 30 mg/kg, ip) injected at 10, 60, and 120 min postocclusion did not reduce the volume of infarct in the MCAO model of cerebral ischemia and had marginal effects on sensory dysfunctions (vibrissae stimulation and body proprioception) and no effects on motor dysfunctions (forelimb flexion and footfault test). Conversely, cerestat (0.3, 1, and 3 mg/kg, sc) injected at 10 and 120 min postocclusion significantly reduced the ischemic volume at the dose of 1 mg/kg, and, at the same dose, significantly attenuated behavioural deficits in the body proprioception and in the forelimb flexion tests.

Allosteric modulators of the AMPA receptor: novel 6-substituted dihydrophthalazines

Novel analogs of the allosteric AMPA receptor modulator SYM 2206 have been prepared. Structure/activity correlations of these novel analogs and other dihydrophthalazines (DHPs) reveal the important contribution of the heteroatom-based aryl substituents in this class of noncompetitive inhibitors. One of the analogs (6, SYM 2189) is equipotent with the early series, but with reduced sedation.

The N-methyl-D-aspartate (NMDA) receptor glycine site antagonist ACEA 1021 does not produce pathological changes in rat brain

ACEA 1021 is a potent, selective N-methyl-D-aspartate (NMDA) receptor glycine site antagonist under clinical evaluation as a neuroprotectant for stroke and head trauma. The potential of ACEA 1021 to produce morphologic changes in cerebrocortical neurons of the rat was assessed since it is known that noncompetitive (e.g., MK-801) and competitive (e.g., CGS 19755)NMDA receptor antagonists produce neuronal vacuolization and necrosis in the rat posterior cingulate/retrosplenial cortex. Male and female adult rats were treated intravenously with either vehicle (Tris) or 10 mg/kg or 50 mg/kg ACEA 1021. MK-801 (5 mg/kg, s.c.) served as positive control. Whereas MK-801 produced characteristic neuronal vacuolization and necrosis in the posterior cingulate/retrosplenial cortex, neither dose of ACEA 1021 had any effect on neuronal morphology. The absence of neuropathological changes in rats supports the further clinical evaluation of ACEA 1021 for stroke and head trauma, and suggests that glycine site antagonists may be devoid of neurotoxic potential.

Immunolesion by 192IgG-saporin of rat basal forebrain cholinergic system: a useful tool to produce cortical cholinergic dysfunction

Cholinergic lesion paradigms have been used to study the role of the cholinergic system in cortical arousal and cognitive function, and its implication in cognitive deficits that occur in Alzheimer's disease. In the last few years an increasing number of studies have applied neurotoxins including excitotoxins or cholinotoxins (e.g. AF64A) by stereotaxic injection into the Nbm to produce reductions in cortical cholinergic activity. One of the most serious limitations of these lesion paradigms is the fact that basal forebrain cholinergic neurons are always intermingled with populations of noncholinergic cells and that the cytotoxins used are far from being selective to cholinergic cells. Excitoxins when infused directly into the Nbm destroy non-specifically cell bodies but spare axons passing the injection site, whereas the specificity of AF64A to destroy cholinergic neurons depends on both the dosage applied and the site of injection. Recently, a monoclonal antibody to the low-affinity nerve growth factor (NGF) receptor, 192IgG, coupled to a cytotoxin, saporin, has been described as an efficient and selective immunotoxin for the NGF-receptor bearing cholinergic neurons in rat basal forebrain. Intraventricular administration of the 192IgG-saporin conjugate appears to induce a nearly complete and specific lesion of neocortical and hippocampal cholinergic afferents. Other neuronal systems in the basal forebrain are spared by the immunotoxin. Electrolytic, ibotenic acid, and cholinergic immunotoxic lesions of cholinergic basal forebrain nuclei resulted in slightly different effects on cortical cholinergic markers: Electrolytic lesion of the Nbm did not change M1-mAChR but resulted in reduced M2-mAChR in frontal and parietal cortices 1 week after lesion. Ibotenic acid lesion of the nucleus basalis did not alter M1-mAChR in any cortical region but led to enhanced M2-mAChR binding in the parietal cortex only. When applying the cholinergic immunotoxin 192IgG-saporin, both M1- and M2-mAChR binding sites were increased in a number of cortical areas 1 week after lesion. This comparison suggests that possibly the destruction of non-cholinergic basal forebrain cells by ibotenic acid and electrolytic lesion, might partly contribute to these different cortical effects. NMDA receptor binding was markedly reduced and AMPA, kainate, and GABAA receptor binding has been significantly increased in cortical regions displaying a reduced activity of AChE and decreased levels of high-affinity choline uptake sites due to immunolesion of the basal forebrain cholinergic system. Equivalent changes in cortical glutamate and GABA receptor subtype levels have been observed 7 days after electrolytic or ibotenic acid lesion of the Nbm. The data suggest that cholinergic immunolesion by 192IgG-saporin exhibits a valuable tool to produce specific cholinergic deficits in rats, which can be used as a model to study the effect of treatment with various drugs for compensating the impaired cortical cholinergic input.

192IgG-saporin-induced immunotoxic lesions of cholinergic basal forebrain system differentially affect glutamatergic and GABAergic markers in cortical rat brain regions

To study the effect of reduced cortical cholinergic activity on GABAergic and glutamatergic mechanisms in cholinoceptive cortical target regions a novel cholinergic immunotoxin (conjugate of the monoclonal antibody 192IgG against the low-affinity nerve growth factor receptor with the cytotoxic protein saporin) was applied, which specifically and selectively destroys cholinergic cells in rat basal forebrain nuclei. To correlate the responses to cholinergic immunolesion in cholinoceptive cortical target regions with cholinergic hypoactivity, quantitative receptor autoradiography to measure NMDA, AMPA and kainate glutamate receptor subtypes, GABAA and benzodiazepine receptors as well as choline uptake sites, and histochemistry to estimate acetylcholinesterase activity were performed in adjacent brain sections. One week after a single intraventricular injection of 4 micrograms of 192IgG-saporin, NMDA receptor binding was markedly reduced in cortical regions displaying a reduced activity of acetylcholinesterase and high-affinity choline uptake sites as a consequence of cholinergic lesion, whereas AMPA and kainate binding sites were significantly increased in these regions. Muscimol binding to GABAA receptors was increased in the caudal portions of frontal and parietal cortices as well as occipital and temporal cortex as compared to the corresponding brain regions from vehicle-injected control rats. Binding levels of benzodiazepine receptors were not affected by the lesion in any of the cortical regions studied. The differential changes in glutamate and GABA receptor subtypes following cholinergic immunolesion might be regarded as the consequence of a cortical reorganization compensating for the reduced cholinergic presynaptic input. The data further suggest that presynaptic cortical cholinergic deficits might affect both glutamatergic and GABAergic functions with different intensity and different directions.