Alternative excitotoxic hypotheses

New insights into progressive supranuclear palsy

Increased oxidative damage and mitochondrial dysfunction have been suggested to play crucial roles in the pathogenesis of several neurodegenerative diseases, including Parkinson's disease and Alzheimer's disease. In this review, we will focus on progressive supranuclear palsy (PSP), a rare parkinsonian disorder with tau pathology. Particular emphasis is placed on the genetic and biochemical data that has emerged, offering new perspectives into the pathogenesis of this devastating disease, especially the contributory roles of oxidative damage and mitochondrial dysfunction.

Neuroprotective possibilities for Huntington's disease

Huntington's disease (HD) is a devastating genetic disorder. Despite the absence of effective therapy, there has been an explosion in interest for developing treatment strategies aimed at lessening or preventing the neuronal death that occurs in this disease. In large part, the renewed interest in neuroprotective strategies has been spurred by our increasing understanding of the genetic and molecular events that drive the underlying neuropathology of HD. This escalating understanding of the biological underpinnings of HD is derived from several convergent sources represented by investigators with clinical, genetic, molecular, physiological and neurobehavioural backgrounds. The diversity of data being generated has, in turn, produced a unique time in HD research where an impressive number of potential therapeutics are coming to the forefront. This review outlines several of these possibilities including the use of intracerebrally delivered neurotrophic factors, pharmacologically altering cellular energy production, the use of antiglutamatergic drugs, the use of caspase inhibitors and inhibitors of protein aggregation. This review also touches on the interesting possibility of whether or not the neurodegeneration in HD is at least partially reversible in nature. All of these possibilities are highlighted in the context that HD is a neurodegenerative disorder in which genetic detection provides a clear and unequivocal opportunity for neuroprotection.

Invited review: Physiological and pathophysiological responses to intermittent hypoxia

This mini-review summarizes the physiological adaptations to and pathophysiological consequences of intermittent hypoxia with special emphasis given to the pathophysiology associated with obstructive sleep apnea. Intermittent hypoxia is an effective stimulus for evoking the respiratory, cardiovascular, and metabolic adaptations normally associated with continuous chronic hypoxia. These adaptations are thought by some to be beneficial in that they may provide protection against disease as well as improve exercise performance in athletes. The long-term consequences of chronic intermittent hypoxia may have detrimental effects, including hypertension, cerebral and coronary vascular problems, developmental and neurocognitive deficits, and neurodegeneration due to the cumulative effects of persistent bouts of hypoxia. Emphasis is placed on reviewing the available data on intermittent hypoxia, making extensions from applicable information from acute and chronic hypoxia studies, and pointing out major gaps in information linking the genomic and cellular responses to intermittent hypoxia with physiological or pathophysiological responses.

Striatal tissue transplantation in non-human primates

The caudate nucleus and putamen form part of a complex but topographically connected circuitry that links the cortex, the basal ganglia and the thalamus. Within this complex system lie a series of functionally and anatomically segregated loops that allow the concurrent processing of a wide range of cognitive and motor information (Alexander et al., 1986; Alexander and Crutcher, 1990). As a constituent of these loops it has been shown that the striatum is involved in movement initiation, response selection and attentional processes (Robbins and Brown, 1990; Alexander, 1994; Lawrence et al., 1998). Although it is the medium spiny GABAergic projection neurones that are primarily lost in HD, it is not sufficient merely to replace the GABA. Instead it is crucial for striatal tissue transplants to integrate with the host tissue in such a way that the cortico-striatal-thalamic circuitry is restored and is functional. Rodent studies have progressed a long way in establishing the principle that striatal grafts can, at least partially, restore function and integrate appropriately with the host (Dunnett and Svendsen, 1993; Björklund et al., 1994; Sanberg et al., 1998) but the limited behavioural repertoire and the undifferentiated striatum meant that it was inevitable that studies should progress into primate models. Anatomical tracing studies have demonstrated that motor, premotor and somatosensory cortical areas send corticostriatal projections primarily to the putamen region in primates, whereas the head and body of the caudate nucleus mostly receive efferent input from associative cortical areas (Kemp and Powell, 1970; Kunzle, 1975, 1977, 1978; Selemon and Goldman-Rakic, 1985). Based on such anatomical, and functional, studies Alexander and colleagues have proposed the existence of at least five cortico-striatal-thalamic loops including a motor, a dorsolateral-prefrontal and an orbito-frontal loop (Alexander et al., 1986). The concentration of motor inputs to the putamen region suggests a particular involvement of this structure in the motor loop. Indeed, unilateral lesions of the putamen disrupt motor performance in the marmoset and generate apomorphine-induced dyskinesias in larger primates (Burns et al., 1995; Kendall et al., 2000). The implantation of striatal grafts into marmosets that had previously received unilateral putamen lesions ameliorated some of the motor impairments, which suggested at least partial restoration of the motor loop. In support of this we found direct evidence of host-graft cortico-striatal connectivity using an anterograde tracer injected in the primary motor cortical region (Kendall et al., 1998a). In larger primates, with lesions of the caudate and putamen, striatal [figure: see text] allografts and xenografts have been shown to reduce apomorphine-induced dyskinesias (Isacson et al., 1989; Hantraye et al., 1992; Palfi et al., 1998). The mechanism by which dyskinesias are elicited is not fully understood but alterations in firing patterns within both segments of the globus pallidus have been identified during dyskinetic movements (Matsumura et al., 1995). It seems likely that it would actually require re-establishment of afferent connections between the implanted putamen and the globus pallidus as well as of functioning dopamine receptors within the graft for the reduction in the dyskinetic profile to be observed. Certainly there is evidence, from rodent studies and the marmoset study described here, that close proximity of the graft to the globus pallidus yields better functional recovery (Isacson et al., 1986). In addition, anatomical tracing studies in rats have demonstrated connections between the implanted tissue and the host globus pallidus (Wictorin et al., 1989b, 1990) However, the relationship between graft placement and functional recovery remains to be fully substantiated.

Glutamatergic influences on the basal ganglia

Glutamate is the predominant excitatory neurotransmitter of the basal ganglia, where it acts on ionotropic and metabotropic receptors. In the best studied of the basal ganglia disorders, Parkinson's disease, there is compelling evidence that the activities of glutamatergic pathways are altered. Of particular importance, the glutamatergic subthalamic nucleus becomes overactive. Pharmacologic blockade of subthalamic neurotransmission has antiparkinsonian symptomatic effects and may also help to protect the remaining dopamine neurons of the substantia nigra from excitotoxic neurodegeneration. Development of drugs to manipulate the glutamatergic system with appropriate pharmacologic and anatomic selectivity is likely to dramatically improve our ability to treat disorders of the basal ganglia.

Mitochondrial dysfunction and oxidative stress in aging and neurodegenerative disease

A major risk factor for neurodegenerative diseases such as Parkinson's disease (PD), Huntington's disease (HD), amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and progressive supranuclear palsy (PSP) is aging. Two processes that have been implicated in aging are free radical-induced oxidative damage and mitochondrial dysfunction. A progressive impairment of mitochondrial function and/or increased oxidative damage has been suggested to play critical roles in the pathogenesis of these neurodegenerative diseases. For example, decreased complex I activity, increased oxidative damage and altered activities of antioxidant defense enzymes have been demonstrated in PD. In AD, decrements in complex IV activity and increased oxidative damage have been reported. Reductions in complex II activity, increased cortical lactate levels and oxidative damage have been described in HD. Some familial ALS cases are associated with mutations in the gene for Cu,Zn superoxide dismutase (SOD1) while increased oxidative damage is observed in sporadic ALS. Studies in PSP have demonstrated regionally specific reductions in brain and muscle mitochondrial function, hypofrontality and increased oxidative damage. Altogether, the age-dependent onset and progressive course of these neurodegenerative diseases may ultimately highlight an association between aging, mitochondrial impairment and oxidative stress.

Role of corticostriatal and nigrostriatal inputs in malonate-induced striatal toxicity

The striatal neuronal loss evident following cellular metabolic compromise may be dependent upon the presence of glutamate and dopamine within the striatum. In order to investigate the relative roles of corticostriatal and nigrostriatal projections in malonate-induced neuronal loss, the extent of toxicity was quantified in animals with cortical lesions to deplete the striatum of glutamate, nigrostriatal lesions to deplete the striatum of dopamine, or both. We found that malonate-induced striatal toxicity was significantly reduced following lesions of either the glutamatergic or dopaminergic afferents to the striatum. The extent of attenuation following the loss of both inputs within the same animal was similar to that seen following lesions of either alone. These data suggest that malonate-induced toxicity in the striatum depends upon the integrity of interactive influences from both glutamatergic and dopaminergic afferents.

Mutant huntingtin enhances excitotoxic cell death

Evidence suggests overactivation of NMDA-type glutamate receptors (NMDARs) contributes to selective degeneration of medium-sized spiny striatal neurons in Huntington's disease (HD). Here we determined whether expression of huntingtin containing the polyglutamine expansion augments NMDAR-mediated excitotoxicity. HEK293 cells coexpressing mutant huntingtin (htt-138Q) and either NR1A/NR2A- or NR1A/NR2B-type NMDARs exposed to 1 mM NMDA showed a significant increase in excitotoxic cell death compared to controls (cells coexpressing htt-15Q or GFP), but the difference was larger for NR1A/NR2B. Moreover, agonist-dependent cell death showed apoptotic features for cells coexpressing htt-138Q and NR1A/NR2B, but not for cells expressing htt-138Q and NR1A/NR2A. Further, NR1A/NR2B-mediated apoptosis was not seen with coexpression of an N-terminal fragment of mutant htt. Since NR1A/NR2B is the predominant NMDAR subtype in neostriatal medium-sized spiny neurons, enhancement of NMDA-induced apoptotic death in NR1A/NR2B-expressing cells by full-length mutant htt may contribute to selective neurodegeneration in HD.

Transient global ischemia in rats yields striatal projection neuron and interneuron loss resembling that in Huntington's disease

The various types of striatal projection neurons and interneurons show a differential pattern of loss in Huntington's disease (HD). Since striatal injury has been suggested to involve similar mechanisms in transient global brain ischemia and HD, we examined the possibility that the patterns of survival for striatal neurons after transient global ischemic damage to the striatum in rats resemble that in HD. The perikarya of specific types of striatal interneurons were identified by histochemical or immunohistochemical labeling while projection neuron abundance was assessed by cresyl violet staining. Projectionneuron survival was assessed by neurotransmitter immunolabeling of their efferent fibers in striatal target areas. The relative survival of neuron types was determined quantitatively within the region of ischemic damage, and the degree of fiber loss in striatal target areas was quantified by computer-assisted image analysis. We found that NADPHd(+) and cholinergic interneurons were largely unaffected, even in the striatal area of maximal damage. Parvalbumin interneurons, however, were as vulnerable as projection neurons. Among immunolabeled striatal projection systems, striatoentopeduncular fibers survived global ischemia better than did striatopallidal or striatonigral fibers. The order of vulnerability observed in this study among the striatal projection systems, and the resistance to damage shown by NADPHd(+) and cholinergic interneurons, is similar to that reported in HD. The high vulnerability of projection neurons and parvalbumin interneurons to global ischemia also resembles that seen in HD. Our results thus indicate that global ischemic damage to striatum in rat closely mimics HD in its neuronal selectivity, which supports the notion that the mechanisms of injury may be similar in both.

Cyclosporin A protects striatal neurons in vitro and in vivo from 3-nitropropionic acid toxicity

The neuroprotective properties of cyclosporin A (CsA) are mediated by its ability to prevent mitochondrial permeability transition during exposure to high levels of calcium or oxidative stress. By using the mitochondrial toxin 3-nitropropionic acid (3NP), the present study assessed whether CsA could protect striatal neurons in vitro and in vivo. In vitro, 3NP produced a 20-30% reduction of striatal glutamic acid decarboxylase-immunoreactive (GAD-ir) neurons. A single treatment with CsA protected GAD-ir neurons from 3NP toxicity at lower (0.2 or 1.0 microM), but not at higher (5.0 microM) doses. Similar findings were seen when the cultures were treated twice with cyclosporin. In vivo experiments used the Lewis rat model of Huntington's disease (HD) in which a low 3NP dose was delivered subcutaneously through an osmotic minipump. Rats received unilateral or bilateral intrastriatal saline injections to disrupt the blood-brain barrier (BBB) and facilitate CsA reaching vulnerable neurons. In the first experiment, CsA treated 3NP-lesioned rats displayed significantly more dopamine-and adenosine-3;, 5;-monophosphate-regulated phosphoprotein (DARPP32-ir) neurons ipsilateral to BBB disruption compared to the contralateral intact striatum, indicating that disruption of the BBB maybe necessary for CsA's neuroprotective effects. In the second experiment, stereological counts of DARPP32-ir neurons revealed that CsA protected striatal neurons in a dose-dependent manner following bilateral disruption of the striatal BBB. Rats treated with the higher (15 or 20 mg/kg) but not lower (5 mg/kg) doses of CsA displayed greater numbers of DARRP32-ir striatal neurons relative to vehicle-treated 3NP-lesioned rats. Thus, under conditions in which CsA can gain access to striatal neurons, significant protection from 3NP toxicity is observed. Therefore, CsA or more lipophilic analogues of this compound, may be of potential therapeutic benefit by protecting vulnerable neurons from the primary pathological event observed in HD.

Effect of exogenous and endogenous antioxidants on 3-nitropionic acid-induced in vivo oxidative stress and striatal lesions: insights into Huntington's disease

3-Nitropropionic acid (3-NP) is an irreversible inhibitor of complex II in the mitochondria. 3-NP toxicity has gained acceptance as an animal model of Huntington's disease (HD). In the present study, we confirmed that rats injected with 3-NP (20 mg/kg, i.p., daily for 4 days) exhibit increased oxidative stress in both striatum and cortical synaptosomes as well as lesions in the striatum. Synaptosomal membrane proteins from rats injected with 3-NP exhibited a decrease in W/S ratio, the relevant electron paramagnetic resonance (EPR) parameter used to determine levels of protein oxidation, and western blot analysis for protein carbonyls revealed direct evidence of increased synaptosomal protein oxidation. Treatment of rats with the brain-accessible free radical spin trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO; 30 mg/kg, i.p., daily 2 h before 3-NP injection) or with N-acetylcysteine (NAC; 100 mg/kg, i.p., daily 2 h before 3-NP injection), a known glutathione precursor, before 3-NP treatments protects against oxidative damage induced by 3-NP as measured by EPR and western blot analysis for protein carbonyls. Furthermore, both DEMPMPO and NAC treatments before 3-NP administration significantly reduce striatal lesion volumes. These data suggest oxidative damage is a prerequisite for striatal lesion formation and that antioxidant treatment may be a useful therapeutic strategy against 3-NP neurotoxicity and perhaps against HD as well.

Motor neuron disease in vitro: the use of cultured motor neurons to study amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a lethal neurodegenerative disease in which motor neurons in the nervous system die. The cause is unknown, and no effective treatment exists. Mutations in the gene for superoxide dismutase found in a subpopulation have led to an animal model, but research with these mice has not produced complete insight into the disease mechanism. Studies with isolated motor neurons may produce important information. This review discusses approaches to culture motor neurons - single cells, cocultured with other cells, and in intact preparations, such as the spinal or cortical slice. Motor neurons in monoculture are suitable for acute but not for chronic studies, whereas cocultures and slices survive up to months and are used for chronic studies. Results with toxic substances believed to play a role in the disease, such as oxidants and glutamate, and of studies where the energy status of the cells is manipulated, are presented.

Major strain differences in response to chronic systemic administration of the mitochondrial toxin 3-nitropropionic acid in rats: implications for neuroprotection studies

Chronic systemic treatment with 3-nitropropionic acid in rats produces persistent dystonia and bradykinesia, and striatal lesions reminiscent of Huntington's disease. However, the interpretation of results obtained with this model are complicated by a heterogeneous distribution of the response to a given toxic dose of 3-nitropropionic acid: approximately half of the animals develop selective striatal lesions, which in certain cases are associated with extrastriatal lesions, and the other half are apparently spared. Thus, the chronic 3-nitropropionic acid lesion model can be difficult for neuroprotection studies in which a consistent response to neurotoxic treatment is prerequisite. We hypothesized that some of the variability in the model was related to the use of Sprague-Dawley rats, since inter-individual variability in response to various stressful conditions has been described previously in this rat strain. We therefore compared 3-nitropropionic acid toxicity in rat strains known to be highly (Fisher 344) or poorly (Lewis) responsive to stress and compared the distribution of responses to that of Sprague-Dawley rats. In a protocol of intraperitoneal injection, toxicity of 3-nitropropionic acid was highest in Fisher rats, intermediate in Sprague-Dawley rats and lowest in Lewis rats. In addition, survival curves showed a more heterogeneous response to 3-nitropropionic acid toxicity in Sprague-Dawley rats than that observed in Lewis and Fisher rats. These differences between Sprague-Dawley and Lewis rats were confirmed in a protocol of subcutaneous 3-nitropropionic acid intoxication using osmotic minipumps, where doses up to 36-45mg/kg per day for five days were necessary to induce striatal lesions in Lewis rats as compared to 12-14mg/kg per day for five days in Sprague-Dawley rats. The selectivity of the striatum to lesions, and homogeneous progression of symptoms and neurodegeneration, were more consistently observed in Lewis as compared to Sprague-Dawley rats. These results suggest that vulnerability to 3-nitropropionic acid may depend on genetic factors, which could also influence the physiological response to stress. The present findings also establish an improved model of progressive striatal degeneration in the rat adapted for the testing of new neuroprotective strategies.

Functional changes of the basal ganglia circuitry in Parkinson's disease

The basal ganglia circuitry processes the signals that flow from the cortex, allowing the correct execution of voluntary movements. In Parkinson's disease, the degeneration of dopaminergic neurons of the substantia nigra pars compacta triggers a cascade of functional changes affecting the whole basal ganglia network. The most relevant alterations affect the output nuclei of the circuit, the medial globus pallidus and substantia nigra pars reticulata, which become hyperactive. Such hyperactivity is sustained by the enhanced glutamatergic inputs that the output nuclei receive from the subthalamic nucleus. The mechanisms leading to the subthalamic disinhibition are still poorly understood. According to the current model of basal ganglia organization, the phenomenon is due to a decrease in the inhibitory control exerted over the subthalamic nucleus by the lateral globus pallidus. Recent data, however, suggest that additional if not alternative mechanisms may underlie subthalamic hyperactivity. In particular, given the reciprocal innervation of the substantia nigra pars compacta and the subthalamic nucleus, the dopaminergic deficit might influence the subthalamic activity, directly. In addition, the increased excitatory drive to the dopaminergic nigral neurons originating from the hyperactive subthalamic nucleus might sustain the progression of the degenerative process. The identification of the role of the subthalamic nucleus and, more in general, of the glutamatergic mechanisms in the pathophysiology of Parkinson's disease might lead to a new approach in the pharmacological treatment of the disease. Current therapeutic strategies rely on the use of L-DOPA and/or dopamine agonists to correct the dopaminergic deficit. Drugs capable of antagonizing the effects of glutamate might represent, in the next future, a valuable tool for the development of new symptomatic and neuroprotective strategies for therapy of Parkinson's disease.

Amino-terminal fragments of mutant huntingtin show selective accumulation in striatal neurons and synaptic toxicity

Huntington disease (HD) is caused by expansion of a glutamine repeat in the amino-terminal region of huntingtin. Despite its widespread expression, mutant huntingtin induces selective neuronal loss in striatal neurons. Here we report that, in mutant mice expressing HD repeats, the production and aggregation of N-terminal huntingtin fragments preferentially occur in HD-affected neurons and their processes and axonal terminals. N-terminal fragments of mutant huntingtin form aggregates and induce neuritic degeneration in cultured striatal neurons. N-terminal mutant huntingtin also binds to synaptic vesicles and inhibits their glutamate uptake in vitro. The specific processing and accumulation of toxic fragments of N-terminal huntingtin in HD-affected striatal neurons, especially in their neuronal processes and axonal terminals, may contribute to the selective neuropathology of HD.

Tissue transglutaminase: a possible role in neurodegenerative diseases

Tissue transglutaminase is a multifunctional protein that is likely to play a role in numerous processes in the nervous system. Tissue transglutaminase posttranslationally modifies proteins by transamidation of specific polypeptide bound glutamines. This action results in the formation of protein crosslinks or the incorporation of polyamines into substrate proteins, modifications that likely have significant effects on neural function. Tissue transglutaminase is a unique member of the transglutaminase family as in addition to catalyzing the calcium-dependent transamidation reaction, it also binds and hydrolyzes ATP and Guanosine 5'-triphosphate and may play a role in signal transduction. Tissue transglutaminase is a highly regulated and inducible enzyme that is developmentally regulated in the nervous system. In vitro, numerous substrates of tissue transglutaminase have been identified, and several of these proteins have been shown to be in situ substrates as well. Several specific roles for tissue transglutaminase have been described and there is evidence that tissue transglutaminase may also play a role in apoptosis. Recent findings have provided evidence that dysregulation of tissue transglutaminase may contribute to the pathology of several neurodegenerative conditions including Alzheimer's disease and Huntington's disease. In both of these diseases tissue transglutaminase and transglutaminase activity are elevated compared to age-matched controls. Further, immunohistochemical studies have demonstrated that there is an increase in tissue transglutaminase reactivity in affected neurons in both Alzheimer's and Huntington's disease. Although intriguing, many issues remain to be addressed to definitively establish a role for tissue transglutaminase in these neurodegenerative diseases.

Creatine reduces 3-nitropropionic-acid-induced cognitive and motor abnormalities in rats

This study assessed whether creatine could attenuate 3-nitropropionic acid (3NP)-induced neuropathological and behavioral abnormalities that are analogous to those observed in Huntington's disease (HD). Rats were fed diets containing either 1% creatine or normal rat chow for 2 weeks prior to the onset of 3NP administration, and for the duration of the study. 3NP was administered systemically in gradually increasing concentrations over an 8-week testing period. Results show that creatine can attenuate 3NP-induced striatal lesions, striatal atrophy, ventricular enlargement, cognitive deficits, and motor abnormalities on a balance beam task. Collectively, these findings indicate that creatine provides significant protection against 3NP-induced behavioral and neuropathological abnormalities and may have therapeutic potential for HD.

Activation of group II metabotropic glutamate receptors inhibits synaptic excitation of the substantia Nigra pars reticulata

Loss of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) leads to increased activity of glutamatergic neurons in the subthalamic nucleus (STN). Recent studies reveal that the resultant increase in STN-induced excitation of basal ganglia output nuclei is responsible for the disabling motor impairment characteristic of PD. On the basis of this, it is possible that any manipulation that reduces activity at excitatory STN synapses onto basal ganglia output nuclei could be useful in the treatment of PD. We now report that group II metabotropic glutamate receptors (mGluRs) are presynaptically localized on STN terminals and that activation of these receptors inhibits excitatory transmission at STN synapses. In agreement with the hypothesis that this could provide a therapeutic benefit in PD, a selective agonist of group II mGluRs induces a dramatic reversal of catalepsy in a rat model of PD. These results raise the exciting possibility that selective agonists of group II mGluRs could provide an entirely new approach to the treatment of PD. These novel therapeutic agents would provide a noninvasive pharmacological treatment that does not involve the manipulation of dopaminergic systems, thus avoiding the problems associated with current therapies.

Neuronal cell death in Huntington's disease: a potential role for dopamine

Huntington's disease is an inherited neurodegenerative disorder, the cause of which is unknown. Excitotoxicity, mitochondrial dysfunction and oxidative stress are all likely to contribute to the striatal cell death that occurs in this disorder. There are accumulating data indicating that under specific circumstances, dopamine, which occurs in high concentrations in the basal ganglia, might be neurotoxic. In this article, the current models used to study Huntington's disease are reviewed and the recent findings that implicate dopamine in the pathophysiology of this progressive disorder are discussed. Although many questions remain unanswered, the dopaminergic system could contribute to striatal vulnerability in Huntington's disease and provide a novel avenue for the development of new therapies.

Expression of NMDA receptor subunit mRNAs in neurochemically identified projection and interneurons in the human striatum

N-methyl-D-aspartate (NMDA) receptors are composed of subunits from two families: NR1 and NR2. We used a dual-label in situ hybridization technique to assess the levels of NR1 and NR2A-D messenger ribonucleic acid (mRNA) expressed in projection neurons and interneurons of the human striatum. The neuronal populations were identified with digoxigenin-tagged complementary RNA probes for preproenkephalin (ENK) and substance P (SP) targeted to striatal projection neurons, and somatostatin (SOM), glutamic acid decarboxylase 67 kD (GAD(67)), and choline acetyltransferase (ChAT) targeted to striatal interneurons. Intense NR1 signals were found over all striatal neurons. NR2A signals were high over GAD(67)-positive neurons and intermediate over SP-positive neurons. ENK-positive neurons displayed low NR2A signals, whereas ChAT- and SOM-positive neurons were unlabeled. NR2B signals were intense over all neuronal populations in striatum. Signals for NR2C and NR2D were weak. Only ChAT-positive neurons displayed moderate signals, whereas all other interneurons and projection neurons were unlabeled. Moderate amounts of NR2D signal were detected over SOM- and ChAT-positive neurons; GAD(67)- and SP-positive striatal neurons displayed low and ENK-positive neurons displayed no NR2D hybridization signal. These data suggest that all human striatal neurons have NMDA receptors, but different populations have different subunit compositions that may affect function as well as selective vulnerability.

Remacemide: current status and clinical applications

Remacemide (RMC) is a non-competitive, low-affinity N-methyl-D-aspartate (NMDA) receptor antagonist that does not cause the behavioural and neuropathological side effects seen with other NMDA receptor antagonists. RMC and its active metabolite, AR-R 12495 AR, which has moderate affinity for the NMDA receptor, also interact with voltage-dependent neuronal sodium channels. Both agents show efficacy in a variety of animal models of epilepsy, parkinsonism and cerebral ischaemia. There is no evidence for teratogenicity or genotoxicity. RMC delays the absorption of L-dopa and elevates the concentrations of drugs metabolised by the hepatic cytochrome P450 3A4 isoform. RMC and AR-R 12495 AR have moderate protein binding and linear pharmacokinetics. Controlled studies show evidence of efficacy in treating epilepsy and Parkinson's disease. Post-surgical outcomes in RMC-treated patients at risk for intra-operative cerebral ischaemia are also encouraging. Adverse effects are related to the gastrointestinal and central nervous systems. RMC is a promising drug with numerous potential applications for both acute or chronic conditions associated with glutamate-mediated neurotoxicity.

Neuroprotective effects of creatine administration against NMDA and malonate toxicity

We examined whether creatine administration could exert neuroprotective effects against excitotoxicity mediated by N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid. Oral administration of 1% creatine significantly attenuated striatal excitotoxic lesions produced by NMDA, but had no effect on lesions produced by AMPA or kainic acid. Both creatine and nicotinamide can exert significant protective effects against malonate-induced striatal lesions. We, therefore, examined whether nicotinamide could exert additive neuroprotective effects with creatine against malonate-induced lesions. Nicotinamide with creatine produced significantly better neuroprotection than creatine alone against malonate-induced lesions. Creatine can, therefore, produce significant neuroprotective effects against NMDA mediated excitotoxic lesions in vivo and the combination of nicotinamide with creatine exerts additive neuroprotective effects.

Oxidative stress, mitochondrial permeability transition and activation of caspases in calcium ionophore A23187-induced death of cultured striatal neurons

Disruption of intracellular calcium homeostasis is thought to play a role in neurodegenerative disorders such as Huntington's disease (HD). To study different aspects of putative pathogenic mechanisms in HD, we aimed to establish an in vitro model of calcium-induced toxicity in striatal neurons. The calcium ionophore A23187 induced a concentration- and time-dependent cell death in cultures of embryonic striatal neurons, causing both apoptosis and necrosis. Cell death was significantly reduced by the cell-permeant antioxidant manganese(III)tetrakis(4-benzoic acid) porphyrin (MnTBAP). Cyclosporin A and its analogue N-MeVal-4-cyclosporin also reduced the incidence of cell death, suggesting the participation of mitochondrial permeability transition in this process. Furthermore, addition of either of two types of caspase inhibitors, Ac-YVAD-CHO (acetyl-Tyr-Val-Ala-Asp-aldehyde) and Ac-DEVD-CHO (acetyl-Asp-Glu-Val-Asp-aldehyde), to the striatal cells blocked A23187-induced striatal cell death in a concentration-dependent manner. These results suggest that oxidative stress, opening of the mitochondrial permeability transition pore and activation of caspases are important steps in A23187-induced cell death.

Behavioral disturbances without amyloid deposits in mice overexpressing human amyloid precursor protein with Flemish (A692G) or Dutch (E693Q) mutation

The contribution of mutations in the amyloid precursor protein (APP) gene known as Flemish (APP/A692G) and Dutch (APP/E693Q) to the pathogenesis of Alzheimer's disease and hereditary cerebral hemorrhage with amyloidosis of the Dutch type, respectively, was studied in transgenic mice that overexpress the mutant APP in brain. These transgenic mice showed the same early behavioral disturbances and defects and increased premature death as the APP/London (APP V717I), APP/Swedish (K670N, M671L), and other APP transgenic mice described previously. Pathological changes included intense glial reaction, extensive microspongiosis in the white matter, and apoptotic neurons in select areas of the brain, while amyloid deposits were absent, even in mice over 18 months of age. This contrasts with extensive amyloid deposition in APP/London transgenic mice and less pronounced amyloid deposition in APP/Swedish transgenic mice generated identically. It demonstrated, however, that the behavioral deficiencies and the pathological changes in brain resulting from an impaired neuronal function are caused directly by APP or its proteolytic derivative(s). These accelerate or impinge on the normal process of aging and amyloid deposits per se are not essential for this phenotype.

Brainstem activation of platelet-derived growth factor-beta receptor modulates the late phase of the hypoxic ventilatory response

The early phase of the biphasic ventilatory response to hypoxia in mammals is critically dependent on NMDA glutamate receptor activation within the nucleus of the solitary tract. However, the mechanisms underlying the subsequent development of the typical ventilatory roll-off are unclear and could underlie important roles in the functional and molecular adaptation to oxygen deprivation. Because the growth factor platelet-derived growth factor (PDGF)-BB can modulate the open channel probability of NMDA receptors by activating PDGF-beta receptors, its contribution to hypoxic ventilatory roll-off was examined. Administration of PDGF-BB, but not PDGF-AA, in the nucleus of the solitary tract was associated with significant attenuations of the early hypoxic ventilatory response in conscious rats. Furthermore, marked reductions in the magnitude of hypoxic ventilatory roll-off occurred in mice heterozygous for a mutation in the PDGF-beta receptor. Administration of a PDGF-beta receptor antagonist to wild-type littermates elicited similar declines in hypoxic ventilatory roll-off. The relative abundance of PDGF-beta receptors was confirmed in the nucleus of the solitary tract and other nuclei implicated in the hypoxic ventilatory response. In nucleus of the solitary tract lysates, PDGF-beta receptor tyrosine phosphorylation was temporally correlated with hypoxic ventilatory roll-off formation. Increased PDGF-B chain mRNA expression was induced by hypoxia in the nucleus of the solitary tract, and PDGF-B chain immunoreactivity colocalized with approximately 40% of nucleus of the solitary tract neurons, demonstrating hypoxia-induced c-Fos enhancements. Thus, PDGF-BB release and PDGF-beta receptor activation in the nucleus of the solitary tract are critical components of hypoxic ventilatory roll-off and may have important functional implications in processes underlying survival and acclimatization to hypoxic environments.

Characterization of iodoacetate-mediated neurotoxicity in vitro using primary cultures of rat cerebellar granule cells

The neuroprotective efficacy of antioxidant molecules against iodoacetate (IAA) neurotoxicity in rat cerebellar granule cell (CGC) cultures was investigated. Transient exposure to IAA caused a concentration-dependent decrease in cell viability (ED50 = 9.8 microM). Dizocilpine maleate (MK-801), and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxobenzo[f]quinoxaline-7-sulfonamide (NBQX), failed to prevent IAA toxicity. Certain antioxidant molecules were shown to be neuroprotective against IAA when combined with MK-801 but were ineffective when administered alone. (S)-(-)-Trolox, butylated hydroxytoluene (BHT), and U-83836E exhibited EC50 values of 78, 5.9, and 0.25 microM, respectively, in the presence of 10 microM MK-801. IAA also induced an increase in intracellular oxidative stress, which was quenched by the antioxidants (in the presence of MK-801) in cultures loaded with the oxidant sensitive dye 2'7'-dichlorodihydrofluorescein diacetate (DCFH-DA).

Neuroprotective and neurorestorative strategies for neuronal injury

Mechanisms of neuronal cell death in apoptosis and necrosis are examined. Neurotoxic processes underlying cellular destruction may involve N-methyl-D-aspartate (NMDA) receptor activation and/or activation of neuronal nitric oxide synthase but the depletion of energy and generation of free radicals appears to be critical. In Alzheimer's disease the damaging effects of peroxynitrite and exposure to beta-amyloid peptide is evident. Mitochondrial dysfunction is involved in several neurodegenerative diseases including Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease as well as Alzheimer's disease and in these disorders the innovations offered by techniques ranging from transgenic mouse models of the disorder to cell culture preparations are remarkable. Agents of neuroprotection and neurorestoration possess either characteristics specific to particular disorders or have a general applicability or both. The vast array of agents available are for the most part the objectives of laboratory examinations but an increasing selection of compounds are reaching the clinical necessities thereby influencing current strategic notions to modify tactical contingencies. Among the agents listed are included: inhibitors of the enzyme poly-ADP-ribose polymerase, inhibition of apoptotic cell death, agents acting on mitochondrial permeability transition, excitatory amino acid antagonists, applications of neurotrophins, immunophilins, agents influencing heme oxygenase-1 expression and iron sequestration in aging astroglia, improvements in mitochondrial energy production or buffering, and finally dopaminemimetics with differential affinities for dopamine receptors.

Mitochondria and neuronal survival

Mitochondria play a central role in the survival and death of neurons. The detailed bioenergetic mechanisms by which isolated mitochondria generate ATP, sequester Ca(2+), generate reactive oxygen species, and undergo Ca(2+)-dependent permeabilization of their inner membrane are currently being applied to the function of mitochondria in situ within neurons under physiological and pathophysiological conditions. Here we review the functional bioenergetics of isolated mitochondria, with emphasis on the chemiosmotic proton circuit and the application (and occasional misapplication) of these principles to intact neurons. Mitochondria play an integral role in both necrotic and apoptotic neuronal cell death, and the bioenergetic principles underlying current studies are reviewed.

Effects of in vivo sodium azide administration on the immunohistochemical localization of kynurenine aminotransferase in the rat brain

Endogenous excitotoxins that act on receptors of cerebral excitatory amino acids play important roles in the pathogenesis of excitotoxic brain diseases. Activation of excitatory amino acid receptors results in neuronal death characteristic of these disorders. Kynurenic acid, a powerful endogenous excitatory amino acid receptor antagonist, which is therefore widely regarded as a potent neuroprotective agent, is produced from its biological precursor, L-kynurenine, by the action of the enzyme kynurenine aminotransferase-I. The chemical hypoxia induced by mitochondrial toxins produces a secondary excitotoxicity, leading to the activation of N-methyl-D-aspartate receptors. Accordingly, sodium azide, an inhibitor of cytochrome oxidase, induces the release of excitotoxins via an energy impairment and this, in turn, results in neurodegeneration. Since energy-dependent secondary excitotoxic mechanisms also account for the pathogenesis of neurodegenerative diseases, a study was made of the effects of sodium azide on the immunohistochemical localization of kynurenine aminotransferase-I. After in vivo administration of sodium azide for five days, a markedly decreased glial kynurenine aminotransferase-I immunoreactivity was found by immunohistochemical techniques in the glial cells of the striatum, hippocampus, dentate gyrus and temporal cortex; at the same time, kynurenine aminotransferase-I started to be expressed by nerve cells which had not been immunoreactive previously. The accumulation of kynurenine aminotransferase-I reaction product around the ribosomes of neuronal endoplasmic reticulum suggests de novo synthesis of kynurenine aminotransferase-I in the reactive nerve cells.

The early cellular pathology of Huntington's disease

Huntington's disease (HD) is an inherited neurodegenerative disorder that affects about one in 10,000 individuals in North America. The genetic defect responsible for the disease is an expansion of a CAG repeat that encodes a polyglutamine tract in the expressed protein, huntingtin. The disease is characterized by involuntary movements, cognitive impairment, and emotional disturbance. Despite the widespread expression of huntingtin, the brains of HD patients show selective neuronal loss in the striatum and the deep layers of the cerebral cortex. Recent studies have shown that polyglutamine expansion causes huntingtin to aggregate, to accumulate in the nucleus, and to interact abnormally with other proteins. Several cellular and animal models for HD have revealed that intranuclear accumulation of mutant huntingtin and the formation of neuropil aggregates precede neurological symptoms and neurodegeneration. Intranuclear huntingtin may affect nuclear function and the expression of genes important for neuronal function, whereas neuropil aggregates may interfere with neuritic transport and function. These early pathological events, which occur in the absence of neurodegeneration, may contribute to the neurological symptoms of HD and ultimately lead to neuronal cell death.

Group I metabotropic glutamate receptors mediate an inward current in rat substantia nigra dopamine neurons that is independent from calcium mobilization

Metabotropic glutamate receptors modulate neuronal excitability via a multitude of mechanisms, and they have been implicated in the pathogenesis of neurodegenerative processes. Here we investigated the responses mediated by group I metabotropic glutamate receptors (mGluRs) in dopamine neurons of the rat substantia nigra pars compacta, using whole cell patch-clamp recordings in combination with microfluorometric measurements of [Ca(2+)](i) and [Na(+)](i). The selective group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (3,5-DHPG) was bath-applied (20 microM, 30 s to 2 min) or applied locally by means of short-lasting (2-4 s) pressure pulses, delivered through an agonist-containing pipette positioned close to the cell body of the neuron. 3,5-DHPG evoked an inward current characterized by a transient and a sustained component, the latter of which was uncovered only with long-lasting agonist applications. The fast component coincided with a transient elevation of [Ca(2+)](i), whereas the total current was associated with a rise in [Na(+)](i). These responses were not affected either by the superfusion of ionotropic excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and D-2-amino-5-phosphono-pentanoic acid (D-APV), nor by the sodium channel blocker tetrodotoxin (TTX). (S)-alpha-methyl-4-carboxyphenylglycine (S-MCPG) and the more selective mGluR1 antagonist 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate (CPCCOEt) depressed both 3,5-DHPG-induced inward current components and, although less effectively, the associated [Ca(2+)](i) elevations. On repeated agonist applications the inward current and the calcium transients both desensitized. The time constant of recovery from desensitization differed significantly between these two responses, being 67.4+/-4.4 s for the inward current and 28.6+/-2.7 s for the calcium response. Bathing the tissue in a calcium-free/EGTA medium or adding thapsigargin (1 microM) to the extracellular medium prevented the generation of the [Ca(2+)](i) transient, but did not prevent the activation of the inward current. These electrophysiological and fluorometric results show that the 3, 5-DHPG-induced inward current and the [Ca(2+)](i) elevations are mediated by independent pathways downstream the activation of mGluR1.

Changes in amino acid concentrations over time and space around an impact injury and their diffusion through the rat spinal cord

Release of amino acids, particularly the neurotoxin glutamate, in and around the site of an experimental spinal cord injury was characterized over time by microdialysis. Increases in amino acid concentrations caused by injury decline steeply and then slowly over distance from the impact area, becoming undetectable beyond about 5 mm from the injury epicenter. Diffusion profiles determined in the cord by administering amino acids through one microdialysis fiber and sampling them in a parallel fiber declined steeply with distance. Distant increases coincided temporally with those in the injury epicenter. We conclude that elevated amino acids more than about 1 mm into the periimpact zone are predominantly released in that region rather than diffusing into it from the trauma epicenter. In the outer areas of lesion development, glutamate does not appear to reach concentrations ordinarily toxic, and elevated concentrations do not persist nearly as long as the therapeutic window of NBQX in any part of the lesion. Therefore, the mechanisms whereby excitatory amino acid antagonists reduce the dimensions of injury lesions are unclear. However, sensitization of neurons following impact injury could be important in amino acid neurotoxicity.

Cellular delivery of trophic factors for the treatment of Huntington's disease: is neuroprotection possible?

The elucidation of the genetic defect in patients with Huntington's disease (HD) has allowed for the detection of individuals at risk for HD prior to the onset of symptoms. Thus "neuroprotection strategies" aimed at preventing the neuropathological and behavioral sequelae of this disease might be powerful therapeutically since they could be introduced to healthy patients before the initiation of a massive degenerative cascade principally localized to the striatum. A variety of trophic factors potently protect vulnerable striatal neurons in animal models of HD. A number of experimental variables are critical in determining the success of trophic factors in animal models. In this regard, the method of trophic factor delivery may be crucial, as delivery via genetically modified cells often produces greater and more widespread effects on striatal neurons than infusions of that same factor. The mechanisms by which cellularly delivered trophic factors forestall degeneration and prevent behavioral deficits are complex and often appear to be unrelated to the trophic factor binding to its cognate receptor. In this regard, cells genetically modified to secrete nerve growth factor (NGF) or ciliary neurotrophic factor (CNTF) protect degenerating striatal neurons which do not express either NGF or CNTF receptors. This review will discuss some of the non-receptor-based events that might underlie these effects and present the hypothesis that cellular delivery of certain trophic factors using genetically modified cells may be ready for clinical testing in HD patients.

Group I metabotropic glutamate receptors: implications for brain diseases

Glutamate is the major excitatory neurotransmitter in the brain and plays a unique role in a variety of central nervous system (CNS) functions. The discovery of the metabotropic receptors (mGluRs), a family of G-protein coupled receptors than can be activated by glutamate, has led to an impressive number of studies in recent years aimed at understanding their biochemical, physiological and pharmacological characteristics. The eight mGluRs now known are divided into three groups according to their sequence homology, signal transduction mechanisms, and agonist selectivity. Group I mGluRs include mGluR1 and mGluR5, which are linked to the activation of phospholipase C; Groups II and III include all others and are negatively coupled to adenylyl cyclases. The availability in recent years of agents selective for Group I mGluRs has made possible the study of the physiological roles of these receptors in the CNS. In addition to mediating glutamatergic neurotransmission, Group I mGluRs can modulate other neurotransmitter receptors, including GABA and the ionotropic glutamate receptors. Group I mGluRs are involved in many CNS functions and may participate in a variety of disorders such as pain, epilepsy, ischemia, and chronic neurodegenerative diseases. This class of receptor may provide important pharmacological therapeutic targets and elucidating its functions will be relevant to develop new treatments for neurological and psychiatric disorders in which glutamatergic neurotransmission is abnormally regulated. In this review anatomical, physiological and pharmacological results are presented with a special emphasis on the role of Group I mGluRs in functional and pathological processes.

Differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptor subunits by calretinin-immunoreactive neurons in the human striatum

We recently reported the existence of medium and large intemeurons immunoreactive for the calcium-binding protein calretinin in the human striatum. We also showed a selective sparing of all medium, but not all large, calretinin-immunoreactive striatal neurons in Huntington's disease striatum. Because glutamate receptor-mediated excitotoxicity has been implicated in the massive loss of striatal projection neurons that characterizes Huntington's disease, we have applied a double-antigen localization procedure to post mortem tissue from eight normal human subjects to determine the expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate glutamate receptor subunits 1/2/4 by the calretinin-immunoreactive interneurons. The two types of calretinin-immunoreactive neurons were found to display various patterns of glutamate receptor subunit expression and a specific regionalization was also noted in the expression of these glutamate receptor subunits. Approximately half of the large calretinin-immunoreactive neurons displayed immunoreactivity for glutamate receptor subunits 1 and 2, and about the same proportion of medium calretinin-immunoreactive neurons expressed glutamate receptor subunits 1 and 4. These double-labeled neurons were rather uniformly distributed in the caudate nucleus and putamen. In contrast, as much as 70.1% of the large calretinin-immunoreactive neurons displayed glutamate receptor subunit 4 immunoreactivity in the postcommissural portion of the putamen, an area that corresponds to the sensorimotor striatal territory. For their part, the medium calretinin-immunoreactive neurons were markedly enriched with glutamate receptor subunit 2, 76% of them being double labeled in the caudate nucleus, which corresponds to the striatal associative territory, compared with 85.5% in the postcommissural putamen. Receptor subunit composition plays a key role in determining the functional properties of glutamate receptors, including their permeability to calcium and susceptibility to excitotoxic insults. Thus, the differential expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate glutamate receptor subunits reported here may help to explain the selective sparing of certain types of calretinin-immunoreactive striatal interneurons in Huntington's disease, although other factors, such as post-transcriptional editing, are also likely to be involved.

Serial 1H-NMR spectroscopy study of metabolic impairment in primates chronically treated with the succinate dehydrogenase inhibitor 3-nitropropionic acid

Previous studies in primates have shown that chronic systemic administration of the succinate dehydrogenase (SDH) inhibitor, 3-nitropropionic acid (3NP), replicates most of the motor, cognitive, and histopathological features of Huntington's disease. In the present study, serial 1H-NMR spectroscopy (1H-MRS) assessment of striatal and occipital cortex concentrations of N-acetylaspartate, phosphocreatine/creatine, choline, and lactate, were obtained every 2-weeks during the entire course of a chronic 3NP treatment in baboons. A region-selective increase in lactate was detected in the striatum of the 3NP-treated animals, either immediately before or in conjunction with a lesion in the dorsolateral putamen detected by T2-MR imaging. Absolute 1H-MRS quantitation demonstrated a progressive and region-specific decrease in striatal N-acetylaspartate, creatine, and choline, occuring as early as 3 weeks before the first detection of lactate. These results demonstrate that 1H-MRS can be used to monitor early stages of brain metabolic impairment. In addition, given that 3NP-induced SDH inhibition following systemic injection similarly affects all brain regions, the striatal selective decreases in N-acetylaspartate or creatine concentrations are not simply related to the level of mitochondrial impairment but to a preferential vulnerability of the striatum to 3NP-induced toxicity.

Temporal evolution of 3-nitropropionic acid-induced neurodegeneration in the rat brain by T2-weighted, diffusion-weighted, and perfusion magnetic resonance imaging

An appropriate detecting technique is necessary for the early detection of neurodegenerative diseases. 3-Nitropropionic acid-intoxicated rats serve as the animal model for one neurodegenerative disease, Huntington's disease. Non-invasive diffusion- and T2-weighted magnetic resonance imaging were applied to study temporal evolution and spatial distribution of brain lesions which were produced by intravenous injection of 3-nitropropionic acid in rats. Lesions in the striatum, hippocampus, and corpus callosum but not in the cortex were observed 3 and 4.5 h after 3-nitropropionic acid injection (30 mg/kg) on the diffusion- and T2-weighted images, respectively (n = 6). The results demonstrated that the diffusion-weighted imaging is not only superior to T2-weighted imaging in detecting onset of 3-nitropropionic acid-induced excitotoxic brain damage but also differentiates lesion and non-lesion areas with better spatial resolution than T2-weighted imaging. Additionally, to correlate structural alterations with pathophysiological conditions, dynamic susceptibility contrast magnetic resonance imaging was performed before and 4 h after 3-nitropropionic acid administration (n = 8). The relative cerebral blood volume was significantly elevated in the striatum (P < 0.001) but not in the cortex after 3-nitropropionic acid administration. The changes in regional relative cerebral blood volume were well correlated to the changes in signal intensities in the corresponding areas on the diffusion- and T2-weighted images. The combined structural and functional information in this study may provide new insights and therapeutic strategies in treating neurodegenerative diseases.

Transgenic mice expressing a Huntington's disease mutation are resistant to quinolinic acid-induced striatal excitotoxicity

Huntington's disease (HD) is a hereditary neurodegenerative disorder presenting with chorea, dementia, and extensive striatal neuronal death. The mechanism through which the widely expressed mutant HD gene mediates a slowly progressing striatal neurotoxicity is unknown. Glutamate receptor-mediated excitotoxicity has been hypothesized to contribute to the pathogenesis of HD. Here we show that transgenic HD mice expressing exon 1 of a human HD gene with an expanded number of CAG repeats (line R6/1) are strongly protected from acute striatal excitotoxic lesions. Intrastriatal infusions of the N-methyl-D-aspartate (NMDA) receptor agonist quinolinic acid caused massive striatal neuronal death in wild-type mice, but no damage in transgenic HD littermates. The remarkable neuroprotection in transgenic HD mice occurred at a stage when they had not developed any neurological symptoms caused by the mutant HD gene. At this stage there was no change in the number of striatal neurons and astrocytes in untreated R6/1 mice, although the striatal volume was decreased by 17%. Moreover, transgenic HD mice had normal striatal levels of NMDA receptors, calbindin D28k (calcium buffer), superoxide dismutase activity (antioxidant enzyme), Bcl-2 (anti-apoptotic protein), heat shock protein 70 (stress-induced anti-apoptotic protein), and citrate synthase activity (mitochondrial enzyme). We propose that the presence of exon 1 of the mutant HD gene induces profound changes in striatal neurons that render these cells resistant to excessive NMDA receptor activation.

Time-course analysis and comparison of acute and chronic intrastriatal quinolinic acid administration on forelimb reaching deficits in the rat

Rats were trained to use a single forelimb for a food pellet retrieval task. During baseline testing all rats exhibited > 90% use of a preferred limb for the task. Following baseline, rats were subjected to chronic administration (18 day) or acute injection of quinolinic acid (QUIN) or vehicle to the striatum contralateral to the preferred limb. Rats were tested 48 h after insertion of chronic delivery probes or after acute injection and retested every 48 h over an 18-day period. Compared to vehicle, rats receiving chronic QUIN (7.6 nmol/h) exhibited an increase in the number of reach attempts required to meet task criteria. Chronic QUIN did not produce a significant change in latency to initiate the task or an increase in latency to complete the task. No rats exposed to chronic QUIN exhibited a switch in limb preference for the task. Unlike animals exposed to chronic QUIN, a significant number of animals receiving acute QUIN injections switched to exclusive use of the ipsilateral (nonpreferred) limb for the task. Quantitative histological analysis revealed no significant difference in lesion volume between acute and chronic lesion animals. These findings suggest that behavioral manifestations of histopathologically similar lesions may be vastly different depending on the methods used to produce these lesions. More specifically, the acute injection model resulted primarily in forelimb disuse, whereas the chronic model resulted in continued abnormal use of the affected limb. Understanding adaptive strategies used in these models may be particularly important when testing newly developed transgenic models of neurodegenerative diseases and the therapeutic potential of newly developed neuroprotectants.

Altered neurotransmitter receptor expression in transgenic mouse models of Huntington's disease

Alterations in neurotransmitter receptors are a pathological hallmark of the neurodegeneration seen in Huntington's disease (HD). However, the significance of these alterations has been uncertain, possibly reflecting simply the loss of brain cells. It is not known for certain whether the alteration of neurotransmitter receptors occurs before the onset of symptoms in human HD. Recently we developed transgenic mice that contain a portion of a human HD gene and develop a progressive abnormal neurological phenotype. Neurotransmitter receptors that are altered in HD (receptors for glutamate, dopamine, acetylcholine and adenosine) are decreased in the brain transgenic mice, in some cases before the onset of behavioural or motor symptoms. In transgenic mice, neurotransmitter receptor alterations occur before neuronal death. Further, receptor alterations are selective in that certain receptors, namely N-methyl-D-aspartate and gamma-aminobutyric acid receptors, are unaltered. Finally, receptor decreases are preceded by selective decreases in the corresponding mRNA species, suggesting the altered transcription of specific genes. These results suggest that (i) receptor decreases precede, and therefore might contribute to, the development of clinical symptoms, and (ii) altered transcription of specific genes might be a key pathological mechanism in HD.

Subtype-specific enhancement of NMDA receptor currents by mutant huntingtin

Evidence suggests that NMDA receptor-mediated neurotoxicity plays a role in the selective neurodegeneration underlying Huntington's disease (HD). The gene mutation that causes HD encodes an expanded polyglutamine tract of >35 in huntingtin, a protein of unknown function. Both huntingtin and NMDA receptors interact with cytoskeletal proteins, and, for NMDA receptors, such interactions regulate surface expression and channel activity. To determine whether mutant huntingtin alters NMDA receptor expression or function, we coexpressed mutant or normal huntingtin, containing 138 or 15 glutamine repeats, respectively, with NMDA receptors in a cell line and then assessed receptor channel function by patch-clamp recording and surface expression by western blot analysis. It is interesting that receptors composed of NR1 and NR2B subunits exhibited significantly larger currents when coexpressed with mutant compared with normal huntingtin. Moreover, this effect was selective for NR1/NR2B, as NR1/NR2A showed similar currents when coexpressed with mutant versus normal huntingtin. However, ion channel properties and total surface expression of the NR1 subunit were unchanged in cells cotransfected with NR1/NR2B and mutant huntingtin. Our results suggest that mutant huntingtin may increase numbers of functional NR1/NR2B-type receptors at the cell surface. Because NR1/NR2B is the predominant NMDA receptor subtype expressed in medium spiny neostriatal neurons, our findings may help explain the selective vulnerability of these neurons in HD.

A YAC mouse model for Huntington's disease with full-length mutant huntingtin, cytoplasmic toxicity, and selective striatal neurodegeneration

We have produced yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 and YAC72) huntingtin (htt) in a developmental and tissue-specific manner identical to that observed in Huntington's disease (HD). YAC46 and YAC72 mice show early electrophysiological abnormalities, indicating cytoplasmic dysfunction prior to observed nuclear inclusions or neurodegeneration. By 12 months of age, YAC72 mice have a selective degeneration of medium spiny neurons in the lateral striatum associated with the translocation of N-terminal htt fragments to the nucleus. Neurodegeneration can be present in the absence of macro- or microaggregates, clearly showing that aggregates are not essential to initiation of neuronal death. These mice demonstrate that initial neuronal cytoplasmic toxicity is followed by cleavage of htt, nuclear translocation of htt N-terminal fragments, and selective neurodegeneration.

Bis(7)-tacrine, a novel dimeric AChE inhibitor, is a potent GABA(A) receptor antagonist

Heptylene-linked bis-(9-amino-1,2,3,4-tetrahydroacridine) (bis(7)-tacrine) is a potential palliative therapeutic agent for Alzheimer's disease (AD), on the basis of its superior acetylcholinesterase (AChE) inhibition and memory-enhancing potency relative to tacrine. In this study we report that bis(7)-tacrine exhibits a potentially complementary central nervous system action, antagonism of GABA(A) receptor function. Bis(7)-tacrine displaced [3H]muscimol from rat brain membranes with an apparent Ki of 6.0 microM; tacrine and physostigmine were shown to be 18 and 170 times less potent, respectively. In whole-cell patch-clamp recordings, bis(7)-tacrine inhibited GABA-induced inward current with an IC50 of 5.6 microM, and shifted the GABA concentration-response curve to the right in a parallel manner. These results suggest that bis(7)-tacrine is a competitive antagonist of the GABA(A) receptor.

Early phenotypic changes in transgenic mice that overexpress different mutants of amyloid precursor protein in brain

Transgenic mice overexpressing different forms of amyloid precursor protein (APP), i.e. wild type or clinical mutants, displayed an essentially comparable early phenotype in terms of behavior, differential glutamatergic responses, deficits in maintenance of long term potentiation, and premature death. The cognitive impairment, demonstrated in F1 hybrids of the different APP transgenic lines, was significantly different from nontransgenic littermates as early as 3 months of age. Biochemical analysis of secreted and membrane-bound APP, C-terminal "stubs," and Abeta(40) and Abeta(42) peptides in brain indicated that no single intermediate can be responsible for the complex of phenotypic dysfunctions. As expected, the Abeta(42) levels were most prominent in APP/London transgenic mice and correlated directly with the formation of amyloid plaques in older mice of this line. Plaques were associated with immunoreactivity for hyperphosphorylated tau, eventually signaling some form of tau pathology. In conclusion, the different APP transgenic mouse lines studied display cognitive deficits and phenotypic traits early in life that dissociated in time from the formation of amyloid plaques and will be good models for both early and late neuropathological and clinical aspects of Alzheimer's disease.

Neuroprotective therapies

Though effective symptomatic therapies for Parkinson's disease exist, currently no treatment is proven to slow the progression of the underlying disease. Our growing understanding of the mechanisms of neuronal models, however, offers hope that neuroprotective strategies will soon be a standard part of the treatment of PD. Current approaches to the development of neuroprotective strategies are based on the hypothesized roles of oxidative stress and excitotoxicity in the degenerative process. In this article, we review evidence in support of these hypotheses as well as attempts to achieve neuroprotection in PD based on these and other mechanisms.