Enhanced release of synaptic glutamate underlies the potentiation of oxygen-glucose deprivation-induced neuronal injury after induction of NOS-2

Riluzole ameliorates learning and memory deficits in Aβ25-35-induced rat model of Alzheimer's disease and is independent of cholinoceptor activation

Alzheimer's disease (AD) is a major global public health concern and social care problem that is associated with learning, memory, and cognitive deficits. Riluzole is a glutamate modulator which has shown to improve memory performance in aged rats and may be of benefit in Alzheimer's disease. In the present study, its beneficial effect on attenuation of learning and memory deficits in Aβ25-35-induced rat model of AD was assessed. Riluzole administration at a dose of 10mg/kg/day p.o. improved spatial memory in Morris water maze and retention and recall in passive avoidance task and its protective effect was not neutralized following intracerebroventricular microinjection of muscarinic or nicotinic receptor antagonists. Further biochemical analysis showed that riluzole pretreatment of intrahippocampal Aβ-microinjected rats is able to attenuate hippocampal AChE activity and lower some oxidative stress markers, i.e. MDA and nitrite, with no significant change of the defensive enzyme catalase. Furthermore, riluzole prevented hippocampal CA1 neuronal loss and reduced 3-nitrotyrosine immunoreactivity. It is concluded that riluzole could exert a protective effect against memory decline induced by intrahippocampal Aβ25-35 through anti-oxidative, anti-cholinesterase, and neuroprotective potential and its beneficial effect is possibly independent of cholinoceptor activation.

Pre-conditioning induces the precocious differentiation of neonatal astrocytes to enhance their neuroprotective properties

Hypoxic preconditioning reprogrammes the brain's response to subsequent H/I (hypoxia–ischaemia) injury by enhancing neuroprotective mechanisms. Given that astrocytes normally support neuronal survival and function, the purpose of the present study was to test the hypothesis that a hypoxic preconditioning stimulus would activate an adaptive astrocytic response. We analysed several functional parameters 24 h after exposing rat pups to 3 h of systemic hypoxia (8% O₂). Hypoxia increased neocortical astrocyte maturation as evidenced by the loss of GFAP (glial fibrillary acidic protein)-positive cells with radial morphologies and the acquisition of multipolar GFAP-positive cells. Interestingly, many of these astrocytes had nuclear S100B. Accompanying their differentiation, there was increased expression of GFAP, GS (glutamine synthetase), EAAT-1 (excitatory amino acid transporter-1; also known as GLAST), MCT-1 (monocarboxylate transporter-1) and ceruloplasmin. A subsequent H/I insult did not result in any further astrocyte activation. Some responses were cell autonomous, as levels of GS and MCT-1 increased subsequent to hypoxia in cultured forebrain astrocytes. In contrast, the expression of GFAP, GLAST and ceruloplasmin remained unaltered. Additional experiments utilized astrocytes exposed to exogenous dbcAMP (dibutyryl-cAMP), which mimicked several aspects of the preconditioning response, to determine whether activated astrocytes could protect neurons from subsequent excitotoxic injury. dbcAMP treatment increased GS and glutamate transporter expression and function, and as hypothesized, protected neurons from glutamate excitotoxicity. Taken altogether, these results indicate that a preconditioning stimulus causes the precocious differentiation of astrocytes and increases the acquisition of multiple astrocytic functions that will contribute to the neuroprotection conferred by a sublethal preconditioning stress.

Reactive astrocytes as therapeutic targets for CNS disorders

Reactive astrogliosis has long been recognized as a ubiquitous feature of CNS pathologies. Although its roles in CNS pathology are only beginning to be defined, genetic tools are enabling molecular dissection of the functions and mechanisms of reactive astrogliosis in vivo. It is now clear that reactive astrogliosis is not simply an all-or-nothing phenomenon but, rather, is a finely gradated continuum of molecular, cellular, and functional changes that range from subtle alterations in gene expression to scar formation. These changes can exert both beneficial and detrimental effects in a context-dependent manner determined by specific molecular signaling cascades. Dysfunction of either astrocytes or the process of reactive astrogliosis is emerging as an important potential source of mechanisms that might contribute to, or play primary roles in, a host of CNS disorders via loss of normal or gain of abnormal astrocyte activities. A rapidly growing understanding of the mechanisms underlying astrocyte signaling and reactive astrogliosis has the potential to open doors to identifying many molecules that might serve as novel therapeutic targets for a wide range of neurological disorders. This review considers general principles and examines selected examples regarding the potential of targeting specific molecular aspects of reactive astrogliosis for therapeutic manipulations, including regulation of glutamate, reactive oxygen species, and cytokines.

Acute neuroprotection by the synaptic blocker botulinum neurotoxin E in a rat model of focal cerebral ischaemia

Evidence indicates that accumulation of excitotoxic mediators, such as glutamate, contributes to neuronal damage after an ischaemic insult. It is not clear, however, whether this accumulation is due to excess synaptic release or to impaired uptake. To test a role for synaptic release, here we investigated the neuroprotective potential of the synaptic blocker botulinum neurotoxin E (BoNT/E), that prevents vesicle fusion via the cleavage of the SNARE (soluble NSF-attachment receptor) protein SNAP-25 (synaptosomal-associated protein of 25 kDa). Focal ischaemia was induced in vivo by infusing the potent vasoconstricting peptide endothelin-1 (ET-1) into the CA1 area of the hippocampus in adult rats; BoNT/E or vehicle were administered into the same site 20 min later. Injection of ET-1 was found to produce a transient and massive increase in glutamate release that was potently antagonized by BoNT/E. To assess whether blocking transmitter release translates into neuroprotection, the extent of the ischaemic damage was determined 24 h and 6 weeks after the insult. We found that BoNT/E administration consistently reduced the loss of CA1 pyramidal neurons at 24 h. The neuroprotective effect of BoNT/E, however, was no longer significant at 6 weeks. These data provide evidence that blockade of synaptic transmitter release delays neuronal cell death following focal brain ischaemia, and underline the importance of assessing long-term neuroprotection in experimental stroke studies.

Can satellite glial cells be therapeutic targets for pain control?

Satellite glial cells (SGCs) undergo phenotypic changes and divide the following injury into a peripheral nerve. Nerve injury, also elicits an immune response and several antigen-presenting cells are found in close proximity to SGCs. Silencing SCG-specific molecules involved in intercellular transport (Connexin 43) or glutamate recycling (glutamine synthase) can dramatically alter nociceptive responses of normal and nerve-injured rats. Transducing SGCs with glutamic acid decarboxylase can produce analgesia in models of trigeminal pain. Taken together these data suggest that SGCs may play a role in the genesis or maintenance of pain and open a range of new possibilities for curing neuropathic pain.

Interleukin-1beta: a bridge between inflammation and excitotoxicity?

Interleukin-1 (IL-1) is a proinflammatory cytokine released by many cell types that acts in both an autocrine and/or paracrine fashion. While IL-1 is best described as an important mediator of the peripheral immune response during infection and inflammation, increasing evidence implicates IL-1 signaling in the pathogenesis of several neurological disorders. The biochemical pathway(s) by which this cytokine contributes to brain injury remain(s) largely unidentified. Herein, we review the evidence that demonstrates the contribution of IL-1beta to the pathogenesis of both acute and chronic neurological disorders. Further, we highlight data that leads us to propose IL-1beta as the missing mechanistic link between a potential beneficial inflammatory response and detrimental glutamate excitotoxicity.

TGF beta 1 and TNF alpha potentiate nitric oxide production in astrocyte cultures by recruiting distinct subpopulations of cells to express NOS-2

Nitric oxide (NO) synthase-2 (NOS-2), a key source of NO at sites of neuroinflammation, is induced in astrocyte cultures treated with lipopolysaccharide (LPS) plus interferon-gamma (IFN gamma). A recent study examining the regulation of astrocytic NOS-2 expression demonstrated that transforming growth factor-beta1 (TGF beta 1) potentiated LPS plus IFN gamma-induced NOS-2 expression via expansion of the pool of astrocytes that express NOS-2. Results in the current report indicate that this population-based mechanism of increasing NOS-2 expression is not restricted to TGF beta 1, since it also accounts for the potentiation of NO production in astrocyte cultures by tumor necrosis factor-alpha (TNFalpha). In contrast to TGF beta 1, which required 24h preincubation for optimal potentiation of NO production, TNF alpha was maximally effective when added concurrently with LPS plus IFN gamma. Nevertheless, under conditions that optimally potentiated NO production, both cytokines recruited similar numbers of astrocytes to express NOS-2 (% NOS-2-positive cells after LPS plus IFN gamma alone or with TNFalpha or TGF beta 1 was 9.5+/-1.2, 25.3+/-2.9, and 32.4+/-3.0, respectively). Interestingly, stimulation of astrocytes in the presence of both TGF beta 1 and TNFalpha additively increased the number of astrocytes that expressed NOS-2 protein (% NOS-2-positive cells was 61.0+/-4.2) relative to each cytokine alone. Potentiation of NO production by either TNF alpha or TGF beta 1 was not ablated by neutralizing antibodies to TGF beta 1 or TNFalpha, respectively. Thus, the two cytokines act independently to recruit separate pools of astrocytes to express NOS-2. These results are consistent with the notion that astrocytes possess an innate heterogeneity with respect to responsiveness to these cytokines.

TGF-beta1 reduces the heterogeneity of astrocytic cyclooxygenase-2 and nitric oxide synthase-2 gene expression in a stimulus-independent manner

Transforming growth factor-beta1 (TGF-beta1) is upregulated by inflammatory mediators in several neurological diseases/disorders where it either participates in the pathology or provides protection. Often, the biological outcome of TGF-beta1 is dependent upon changes in gene expression. Recently, we demonstrated that TGF-beta1 enhances astrocytic nitric oxide production induced by lipopolysaccharide (LPS) plus interferon-gamma (IFNgamma) by increasing the number of astrocytes in a population that express NOS-2. The purpose of this study was twofold: (1) to determine whether this effect occurs more generally by assessing the effect of TGF-beta1 on another pro-inflammatory gene, cyclooxygenase-2 (COX-2); and (2) to assess stimulus specificity. We found that TGF-beta1 augmented LPS plus IFNgamma-induced COX-2 mRNA and protein expression, by nearly tripling the number of astrocytes that express COX-2. The effect was not stimulus-specific as TGF-beta1 enhanced the number of astrocytes that expressed both COX-2 and NOS-2 protein when either IL-1beta or TNFalpha was used in lieu of LPS. Collectively, these results suggest that TGF-beta1 augments overall protein expression levels of select pro-inflammatory genes in astrocytes in a promiscuous manner by reducing the magnitude of noise in the cellular population.

System x(c)- activity and astrocytes are necessary for interleukin-1 beta-mediated hypoxic neuronal injury

The purpose of this study was to elucidate the cellular/biochemical pathway(s) by which interleukin-1beta (IL-1beta) contributes to the pathogenesis of hypoxic-ischemic brain damage. In vivo, IL-1 receptor type I (IL-1RI)-deficient mice showed smaller infarcts and less neurological deficits than wild-type animals after a 90 min reversible middle cerebral artery occlusion. In vitro, IL-1beta mediated an enhancement of hypoxic neuronal injury in murine cortical cultures that was lacking in cultures derived from IL-1RI null mutant animals and was blocked by the IL-1 receptor antagonist or an IL-1RI blocking antibody. This IL-1beta-mediated potentiation of hypoxic neuronal injury was associated with an increase in both cellular cystine uptake ([cystine]i) and extracellular glutamate levels ([glutamate]e) and was prevented by either ionotropic glutamate receptor antagonism or removal of L-cystine, suggesting a role for the cystine/glutamate antiporter (System x(c)-). Indeed, dual System x(c)-/metabotropic glutamate receptor subunit 1 (mGluR1) antagonism but not selective mGluR1 antagonism prevented neuronal injury. Additionally, cultures derived from mGluR1-deficient mice exhibited the same potentiation in injury after treatment with IL-1beta as wild-type cultures, an effect prevented by System x(c)-/mGluR1 antagonism. Finally, assessment of System x(c)- function and kinetics in IL-1beta-treated cultures revealed an increase in velocity of cystine transport (Vmax), in the absence of a change in affinity (Km). Neither the enhancement in [cystine]i, [glutamate]e, or neuronal injury were observed in chimeric cultures consisting of IL-1RI(+/+) neurons plated on top of IL-1RI(-/-) astrocytes, highlighting the importance of astrocyte-mediated alterations in System x(c)- as a novel contributor to the development and progression of hypoxic neuronal injury.

TGF-beta1 potentiates astrocytic nitric oxide production by expanding the population of astrocytes that express NOS-2

Both transforming growth factor-beta1 (TGF-beta1) and nitric oxide synthase-2 (NOS-2) are upregulated under various neuropathological states. Evidence suggests that TGF-beta1 can either attenuate or augment NOS-2 expression, with the prevailing effect dependent on the experimental paradigm employed and the cell-type under study. The purpose of the present study was to determine the effect of TGF-beta1 on astrocytic NOS-2 expression. In purified astrocyte cultures, TGF-beta1 alone did not induce NOS-2 or NO production. However, NO production induced by lipopolysaccharide (LPS) plus IFNgamma was enhanced by TGF-beta1 in a concentration-dependent manner between 10 and 1,000 pg/mL. The presence of IFNgamma was not necessary for this effect to occur, as TGF-beta1 enhanced NO production induced by LPS in a similar fashion. In cultures stimulated with LPS plus IFNgamma, the enhancement of NO production by TGF-beta1 was associated with a corresponding increase in NOS-2 mRNA and protein expression. Interestingly, immunocytochemical assessment of NOS-2 protein expression demonstrated that TGF-beta1 augmented astrocytic NO production, specifically by increasing the pool of astrocytes capable of expressing NOS-2 induced by either LPS (approximately threefold) or LPS plus IFNgamma (approximately sevenfold). In a broader sense, our results suggest that TGF-beta1 recruits a latent population of astrocytes to respond to stimulation by pro-inflammatory mediators.

Molecular changes in nNOS protein expression within the ventrolateral medulla following transient focal ischemia affect cardiovascular functions

The majority of human strokes involve an occlusion of the middle cerebral artery and subsequent damage to the brain tissues it perfuses. We have previously reported that reflex cardiovascular changes during a static muscle contraction are attenuated following transient middle cerebral artery occlusion (MCAO) and reperfusion [A. Ally, S.M. Nauli, T.J. Maher, Cardiovascular responses and neurotransmission in the ventrolateral medulla during skeletal muscle contraction following transient middle cerebral artery occlusion and reperfusion, Brain Res. 952 (2002) 176-187]. We hypothesized that the attenuation is a result of altered expression of neuronal nitric oxide synthase (nNOS) within the rostral (RVLM) and caudal ventrolateral medulla (CVLM). In this study, we have compared cardiovascular responses and nNOS protein expression within the four quadrants, i.e., left and right sides of both RVLM and CVLM in sham-operated rats (n = 10) and in rats with a temporary 90-min left-sided MCAO followed by 24 h reperfusion (n = 10). Increases in mean arterial pressure during a static muscle contraction were significantly attenuated in MCAO rats when compared to sham rats. The transient ischemia reduced nNOS expression within the ipsilateral RVLM quadrant compared to the contralateral RVLM or RVLM quadrants of control rats. In contrast, compared to sham rats and the right CVLM quadrant of MCAO rats, nNOS expression was significantly augmented in the ipsilateral CVLM in left-sided MCAO rats. These data suggest that the attenuation of cardiovascular responses during static muscle contraction in MCAO rats is partly due to a reduction in nNOS expression within the ipsilateral RVLM and an overexpression of nNOS abundance within the ipsilateral CVLM. Results demonstrate that nNOS expression within the medulla plays a significant role in mediating cardiovascular responses during static exercise in intact and pathophysiological conditions.