Aging reduces neostriatal responsiveness to N-methyl-D-aspartate and dopamine: an in vitro electrophysiological study

Major Contribution of Somatostatin-Expressing Interneurons and Cannabinoid Receptors to Increased GABA Synaptic Activity in the Striatum of Huntington's Disease Mice

Huntington’s disease (HD) is a heritable neurological disorder that affects cognitive and motor performance in patients carrying the mutated huntingtin (HTT) gene. In mouse models of HD, previous reports showed a significant increase in spontaneous GABA_A receptor-mediated synaptic activity in striatal spiny projection neurons (SPNs). In this study, using optogenetics and slice electrophysiology, we examined the contribution of γ-aminobutyric acid (GABA)-ergic parvalbumin (PV)- and somatostatin (SOM)-expressing interneurons to the increase in GABA neurotransmission using the Q175 (heterozygote) mouse model of HD. Patch clamp recordings in voltage-clamp mode were performed on SPNs from brain slices of presymptomatic (2 months) and symptomatic (8 and 12 months) Q175 mice and wildtype (WT) littermates. While inhibitory postsynaptic currents (IPSCs) evoked in SPNs following optical activation of PV- and SOM-expressing interneurons differed in amplitude, no genotype-dependent differences were observed at all ages from both interneuron types; however, responses evoked by either type were found to have faster kinetics in symptomatic mice. Since SOM-expressing interneurons are constitutively active in striatal brain slices, we then examined the effects of acutely silencing these neurons in symptomatic mice with enhanced Natronomonas pharaonis halorhodopsin (eNpHR). Optically silencing SOM-expressing interneurons resulted in a greater decrease in the frequency of spontaneous IPSCs (sIPSCs) in a subset of SPNs from Q175 mice compared to WTs, suggesting that SOM-expressing interneurons are the main contributors to the overall increased GABA synaptic activity in HD SPNs. Additionally, the effects of activating GABA_B and cannabinoid (CB1) receptors were investigated to determine whether these receptors were involved in modulating interneuron-specific GABA synaptic transmission and if this modulation differed in HD mice. When selectively activating PV- and SOM-expressing interneurons in the presence of the CB1 receptor agonist WIN-55,212, the magnitudes of the evoked IPSCs in SPNs decreased for both interneuron types although this change was less prominent in symptomatic Q175 SPNs during SOM-expressing interneuron activation. Overall, these findings show that dysfunction of SOM-expressing interneurons contributes to the increased GABA synaptic activity found in HD mouse models and that dysregulation of the endocannabinoid system may contribute to this effect.

Effects of Aging in the Striatum and Substantia Nigra of a Parkinson's Disease Animal Model

Aging is a multifactorial process associated with functional deficits, and the brain is more prone to developing chronic degenerative diseases such as Parkinson's disease. Several groups have tried to correlate the age-related ultrastructural alterations to the neurodegeneration process using in vivo pharmacological models, but due to the limitations of the animal models, particularly in aged animals, the results are difficult to interpret. In this work, we investigated neurodegeneration induced by rotenone, as a pharmacological model of Parkinson's disease, in both young and aged Wistar rats. We assessed animal mobility, tyrosine hydroxylase staining in the substantia nigra pars compacta (SNpc), and TdT-mediated dUTP-biotin nick end labeling-positive nuclei and reactive oxygen species production in the striatum. Interestingly, the mobility impairment, dopaminergic neuron loss, and elevated number of apoptotic nuclei in the striatum of aged control rats were similar to young rotenone-treated animals. Moreover, we observed many ultrastructural alterations, such as swollen mitochondria in the striatum, and massive lipofuscin deposits in the SNpc of the aged rotenone-treated animals. We conclude that the rotenone model can be employed to explore age-related alterations in the ontogeny that can increase vulnerability in the striatum and SNpc, which may contribute to Parkinson's disease pathogenesis.

Age-related alterations in the expression of genes and synaptic plasticity associated with nitric oxide signaling in the mouse dorsal striatum

Age-related alterations in the expression of genes and corticostriatal synaptic plasticity were studied in the dorsal striatum of mice of four age groups from young (2-3 months old) to old (18-24 months of age) animals. A significant decrease in transcripts encoding neuronal nitric oxide (NO) synthase and receptors involved in its activation (NR1 subunit of the glutamate NMDA receptor and D1 dopamine receptor) was found in the striatum of old mice using gene array and real-time RT-PCR analysis. The old striatum showed also a significantly higher number of GFAP-expressing astrocytes and an increased expression of astroglial, inflammatory, and oxidative stress markers. Field potential recordings from striatal slices revealed age-related alterations in the magnitude and dynamics of electrically induced long-term depression (LTD) and significant enhancement of electrically induced long-term potentiation in the middle-aged striatum (6-7 and 12-13 months of age). Corticostriatal NO-dependent LTD induced by pharmacological activation of group I metabotropic glutamate receptors underwent significant reduction with aging and could be restored by inhibition of cGMP hydrolysis indicating that its age-related deficit is caused by an altered NO-cGMP signaling cascade. It is suggested that age-related alterations in corticostriatal synaptic plasticity may result from functional alterations in receptor-activated signaling cascades associated with increasing neuroinflammation and a prooxidant state.

Inhibition of cytoplasmic p53 differentially modulates Ca(2+) signaling and cellular viability in young and aged striata

The p53 protein, a transcription factor with many gene targets, can also trigger apoptosis in the cytoplasm. The disruption of cell homeostasis, such as Ca(2+) signaling and mitochondrial respiration, contributes to the loss of viability and ultimately leads to cell death. However, the link between Ca(2+) signaling and p53 signaling remains unclear. During aging, there are alterations in cell physiology that are commonly associated with a reduced adaptive stress response, thus increasing cell vulnerability. In this work, we examined the effects of a cytoplasmic p53 inhibitor (pifithrin μ) in the striatum of young and aged rats by evaluating Ca(2+) signaling, mitochondrial respiration, apoptotic protein expression, and tissue viability. Our results showed that pifithrin μ differentially modulated cytoplasmic and mitochondrial Ca(2+) in young and aged rats. Cytoplasmic p53 inhibition appeared to reduce the mitochondrial respiration rate in both groups. In addition, p53 phosphorylation and Bax protein levels were elevated upon cytoplasmic p53 inhibition and could contribute to the reduction of tissue viability. Following glutamate challenge, pifithrin μ improved cell viability in aged tissue, reduced reactive oxygen species (ROS) generation, and reduced mitochondrial membrane potential (ΔΨm). Taken together, these results indicate that cytoplasmic p53 may have a special role in cell viability by influencing cellular Ca(2+) homeostasis and respiration and may produce differential effects in the striatum of young and aged rats.

Biological sources of inflexibility in brain and behavior with aging and neurodegenerative diseases

Almost unequivocally, aging and neurodegeneration lead to deficits in neural information processing. These declines are marked by increased neural noise that is associated with increased variability or inconsistency in behavioral patterns. While it is often viewed that these problems arise from dysregulation of dopamine (DA), a monoamine modulator, glutamate (GLU), an excitatory amino acid that interacts with DA, also plays a role in determining the level of neural noise. We review literature demonstrating that neural noise is highest at both high and low levels of DA and GLU, allowing their interaction to form a many-to-one solution map for neural noise modulation. With aging and neurodegeneration, the range over which DA and GLU can be modulated is decreased leading to inflexibility in brain activity and behavior. As the capacity to modulate neural noise is restricted, the ability to shift noise from one brain region to another is reduced, leading to greater uniformity in signal-to-noise ratios across the entire brain. A negative consequence at the level of behavior is inflexibility that reduces the ability to: (1) switch from one behavior to another; and (2) stabilize a behavioral pattern against external perturbations. In this paper, we develop a theoretical framework where inflexibility across brain and behavior, rather than inconsistency and variability is the more important problem in aging and neurodegeneration. This theoretical framework of inflexibility in aging and neurodegeneration leads to the hypotheses that: (1) dysfunction in either or both of the DA and GLU systems restricts the ability to modulate neural noise; and (2) levels of neural noise and variability in brain activation will be dedifferentiated and more evenly distributed across the brain; and (3) changes in neural noise and behavioral variability in response to different task demands and changes in the environment will be reduced.

Huntington's disease and the striatal medium spiny neuron: cell-autonomous and non-cell-autonomous mechanisms of disease

Huntington's disease is an autosomal dominant disorder caused by a mutation in the gene encoding the protein huntingtin on chromosome 4. The mutation is an expanded CAG repeat in the first exon, encoding a polyglutamine tract. If the polyglutamine tract is > 40, penetrance is 100% and death is inevitable. Despite the widespread expression of huntingtin, HD has long been considered primarily as a disease of the striatum. It is characterized by selective vulnerability with dysfunction followed by death of the medium size spiny neuron. Considerable effort is being expended to determine whether striatal damage is cell-autonomous, non-cell-autonomous, requiring cell-cell and region to region communication, or both. We review data supporting both mechanisms. We also attempt to organize the data into common mechanisms that may arise outside the medium, spiny neuron, but ultimately have their greatest impact in the striatum.

Forebrain striatal-specific expression of mutant huntingtin protein in vivo induces cell-autonomous age-dependent alterations in sensitivity to excitotoxicity and mitochondrial function

HD (Huntington's disease) is characterized by dysfunction and death of striatal MSNs (medium-sized spiny neurons). Excitotoxicity, transcriptional dysregulation and mitochondrial abnormalities are among the mechanisms that are proposed to play roles in HD pathogenesis. To determine the extent of cell-autonomous effects of mhtt (mutant huntingtin) protein on vulnerability to excitotoxic insult in MSNs in vivo, we measured the number of degenerating neurons in response to intrastriatal injection of QA (quinolinic acid) in presymptomatic and symptomatic transgenic (D9-N171-98Q, also known as DE5) mice that express mhtt in MSNs but not in cortex. After QA, the number of degenerating neurons in presymptomatic DE5 mice was not significantly different from the number in WT (wild-type) controls, suggesting the early, increased vulnerability to excitotoxicity demonstrated in other HD mouse models has a largely non-cell-autonomous component. Conversely, symptomatic DE5 mice showed significantly fewer degenerating neurons relative to WT, implying the resistance to excitotoxicity observed at later ages has a primarily cell-autonomous origin. Interestingly, mitochondrial complex II respiration was enhanced in striatum of symptomatic mice, whereas it was reduced in presymptomatic mice, both relative to their age-matched controls. Consistent with the QA data, MSNs from symptomatic mice showed decreased NMDA (N-methyl-d-aspartate) currents compared with age-matched controls, suggesting that in addition to aging, cell-autonomous mechanisms mitigate susceptibility to excitotoxicity in the symptomatic stage. Also, symptomatic DE5 mice did not display some of the electrophysiological alterations present in other HD models, suggesting that blocking the expression of mhtt in cortical neurons may restore corticostriatal function in HD.

Differential susceptibility to excitotoxic stress in YAC128 mouse models of Huntington disease between initiation and progression of disease

Huntington disease (HD) is a neurodegenerative disorder caused by an expanded CAG tract in the HD gene. Polyglutamine expansion of huntingtin (htt) results in early, progressive loss of medium spiny striatal neurons, as well as cortical neurons that project to the striatum. Excitotoxicity has been postulated to play a key role in the selective vulnerability of striatal neurons in HD. Early excitotoxic neuropathological changes observed in human HD brain include increased quinolinate (QUIN) concurrent with proliferative changes such as increased spine density and dendritic length. In later stages of the disease, degenerative-type changes are apparent, such as loss of dendritic arborization, a reduction in spine density and reduced levels of 3-hydroxykynurenine and QUIN. It is currently unknown whether sensitivity to excitotoxic stress varies between initiation and progression of disease. Here, we have assessed the excitotoxic phenotype in the YAC128 mouse model of HD by examining the response to excitotoxic stress at different stages of disease. Our results demonstrate that YAC128 mice display enhanced sensitivity to NMDA ex vivo and QUIN in vivo before obvious phenotypic changes. In contrast, 10-month-old symptomatic YAC128 mice are resistant to QUIN-induced neurotoxicity. These findings are paralleled by a significant increase in NMDAR-mediated membrane currents in presymptomatic YAC128 dissociated medium spiny neurons progressing to reduced NMDAR-mediated membrane currents with disease progression. These data highlight the dynamic nature of the mutant htt-mediated excitotoxic phenotype and suggests that therapeutic approaches to HD may need to be altered, depending on the stage and development of the disease.

Effects of development and dopamine depletion on striatal NMDA receptor-mediated calcium uptake

Calcium (Ca(2+)) is the currency of N-methyl-D-aspartate (NMDA) receptor mediated signal transduction pathways involved in the modification of synaptic efficacy during regulation of excitatory inputs into the striatum. The aim of the present study was to investigate the effects of development and dopamine depletion on NMDA receptor function. NMDA receptors were stimulated by incubation of striatal sections (350 microm) in buffer containing NMDA (100 microm) for 2 min, the slices were washed and uptake of radioactively labelled calcium ((45)Ca(2+)) was measured. Dopamine depletion has been reported to result in alterations of glutamate receptor expression and upregulation of NMDA receptor activity. However, the results of the present study show that dopamine depletion does not alter NMDA-stimulated Ca(2+) uptake into rat striatal slices in vitro. Unilateral striatal dopamine depletion was achieved by infusion of 6-hydroxydopamine (6-OHDA, 13.5 microg/4.5 microl) into the medial forebrain bundle (MFB) of the left hemisphere of ten rats. NMDA-stimulated (45)Ca(2+) uptake into striata following dopamine depletion was not significantly different from NMDA-stimulated (45)Ca(2+) uptake into striata obtained from sham-operated rats. Other factors that induce changes in NMDA receptor function include development and aging. In young rats aged 7 weeks old (n = 7) and 16 weeks old (n = 6) a significant 2-3 fold decrease in striatal NMDA receptor function was observed with increasing age over the 9 week period of development. To our knowledge these are the first results to show developmental decreases of NMDA receptor function in the striatum of juvenile rats.

Glutamate-dopamine-GABA interactions in the aging basal ganglia

The study of neurotransmitter interactions gives a better understanding of the physiology of specific circuits in the brain. In this review we focus mostly on our own results on the interaction of the neurotransmitters glutamate, dopamine and GABA in the basal ganglia during the normal process of aging. We review first the studies on the action of endogenous glutamate on the extracellular concentrations of dopamine and GABA in the neostriatum and nucleus accumbens during aging. It was found that there exists an age-related change in the interaction of glutamate, dopamine and GABA and that these effects of aging exhibit a dorsal-to-ventral pattern of effects with no changes in the dorsal parts (dorsal striatum) and changes in the most ventral parts (nucleus accumbens). Second we reviewed the data on the effects of different ionotropic and metabotropic glutamate receptor agonists on the extracellular concentrations of dopamine and GABA in the nucleus accumbens. The results obtained clearly show the different contribution of each glutamate receptor subtype in the age-related changes produced on the interaction of glutamate, dopamine and GABA in this area of the brain. Third the effects of an enriched environment on the action of AMPA and NMDA-receptor agonists in the nucleus accumbens of rats during aging are also evaluated. Finally, and since the nucleus accumbens has been suggested to play a role in emotion and motivation and also motor behaviour, we speculated on the possibility of a specific contribution for the different glutamatergic pathways terminating in the nucleus accumbens and their interaction with a decreased dopamine playing a relevant role in motor behaviour during aging.

Mice heterozygous for neurotrophin-3 display enhanced vulnerability to excitotoxicity in the striatum through increased expression of N-methyl-D-aspartate receptors

The striatum is one of the brain areas most vulnerable to excitotoxicity, a lesion that can be prevented by neurotrophins. In the present study, intrastriatal injection of the N-methyl-d-aspartate receptor (NMDAR) agonist quinolinate (QUIN) was performed in mice heterozygous for neurotrophin-3 (NT3 +/-) or brain-derived neurotrophic factor (BDNF +/-) to analyze the role of endogenous neurotrophins on the regulation of striatal neurons susceptibility to excitotoxic injury. QUIN injection induced a decrease in dopamine- and cyclic AMP-regulated phosphoprotein of 32 kDa (DARPP-32) protein levels that was higher in NT-3 +/- than in BDNF+/- or wild type animals. This enhanced susceptibility was specific for enkephalin- and tachykinin-positive projection neurons, and also for parvalbumin-positive interneurons. However the excitotoxic damage in large interneurons was not modified in NT-3 +/- mice compared with wild type animals. This effect can be related to the regulation of NMDARs by endogenous NT-3. Thus, our results show that there is an age-dependent regulation of NMDAR subunits NR1 and NR2A, but not NR2B, in NT-3 +/- mice. The deficit of endogenous NT-3 induced a decrease in NR1 and NR2A subunits at postnatal day (P) 0 and P3 mice respectively, whereas an upregulation was observed in 12 week old NT-3 +/- mice. This differential effect was also observed after administration of exogenous NT-3. In primary striatal cultures, NT-3 treatment induced an enhancement in NR2A, but not NR2B, protein levels. However, intrastriatal grafting of NT-3 secreting-cells in adult wild type mice produced a down-regulation of NR2A subunit. In conclusion, NT-3 regulates the expression of NMDAR subunits modifying striatal neuronal properties that confers the differential vulnerability to excitotoxicity in projection neurons and interneurons in the striatum.

Functional degradation of visual cortical cells in aged rats

Functional degradation of mammalian visual cortex is associated with aging. It has been hypothesized that much of the decline might be mediated by a degradation of cortical inhibitory system during senescence. In the present work, we compared the properties of adaptation, onset latency and signal-to-noise ratio in primary visual cortex of young and old rats using extracellular single-unit techniques. The short-term synaptic plasticity of young and old rats was also studied using field potential recording techniques. We found significant increased adaptation, prolonged onset latency, lower signal-to-noise ratio and decreased short-term synaptic plasticity in aged rats. The results are in accordance with previously reported functional declines in old monkeys and old cats, indicating a universal mechanism of degradation in cortical function that accompanies old age in different mammalian species.

Functional degradation of visual cortical cells in old cats

Visual function declines with age. Using extracellular single-unit in vivo recordings, we compared the function of primary visual cortical (area 17) cells in young and old paralyzed, anesthetized cats. The results reveal that cortical neurons in old cats exhibit higher visually evoked responses, higher spontaneous activities, lower signal-to-noise ratios, and weaker orientation and direction selectivity than do cells in young adult cats. These findings are consistent with previously reported age related declines in cortical function in senescent macaque monkeys. Thus, similar declines in cortical function accompany old age in different mammalian species with well developed cortices.

Dopamine and GABA increases produced by activation of glutamate receptors in the nucleus accumbens are decreased during aging

The aim of the present study was to investigate the effects of aging on the increases of dopamine and GABA induced by activation of ionotropic and metabotropic glutamate receptors in the nucleus accumbens of the freely moving rat. The effects of local perfusion of the agonists NMDA (10, 100 and 500 microM), AMPA (1, 20 and 100 microM) and ACPD (100, 500 and 1000 microM) on extracellular concentration of dopamine and GABA in the nucleus accumbens of young (2-4 months), middle-aged (10-14 months) and aged (24-32 months) male Wistar rats were studied using microdialysis. In young rats, perfusion of the agonists NMDA and AMPA, but not ACPD, produced an increase of dialysate concentrations of dopamine. Perfusion of the three glutamate agonists (NMDA, AMPA and ACPD) produced an increase of dialysate GABA. This increase was delayed in time compared with the increase of dopamine. In the nucleus accumbens of middle-aged and aged rats, the increases of dopamine induced by NMDA were significantly lower than those in young rats. Also the increases of dopamine induced by AMPA were lower in aged rats than those in young rats. The effects of AMPA, NMDA and ACPD on dialysate GABA were significantly lower in aged rats than in young rats. These findings suggest that aging changes the interaction between the neurotransmitters glutamate and dopamine and glutamate and GABA in the nucleus accumbens of the freely moving rat.

Increased sensitization to morphine-induced oral stereotypy in aged rats

Sensitization develops to the stereotypic biting behavior that appears with the repeated administration of high dose morphine to rats. Because there is evidence that this behavior is dopamine-mediated and that there are age-related changes in dopamine systems, we compared the development and expression of morphine-induced biting behavior in aged (24 months) and young rats (5 months). Animals were treated with four sensitizing 10 mg/kg doses of morphine or saline, followed by three weekly challenges with 4 mg/kg doses of morphine or saline. By the fourth sensitizing morphine dose and after the administration of each low dose challenge, the biting time was significantly greater for aged than for young morphine pre-treated rats. After the first weekly low dose challenge, the aged but not young animals expressed more biting than when they did after the last 10 mg/kg dose. These results indicate that sensitization to morphine-induced oral stereotypy is significantly greater in aged as compared to young rats. Age-related enhanced sensitivity to morphine-induced oral stereotypy might be related to age-induced increases in vulnerability to opioid-induced insults to the basal ganglia, and may be a model for certain diseases of this pathway.

Aged F344 rats exhibit altered electrophysiological activity in locomotor-unrelated but not locomotor-related striatal neurons

Multi-wire electrode arrays were chronically implanted and striatal electrophysiological activity was recorded in young (4-9 months) versus aged (24-29 months) Fischer 344 (F344) rats in order to determine whether locomotor-related striatal neurons exhibit age-related changes in electrophysiological activity during freely-moving conditions. Individual neurons were classified as locomotor-related if they exhibited significant differences in their firing rates between periods of locomotion versus periods of non-movement. While the activity of locomotor-related striatal neurons did not differ between young and aged rats, neurons that were not related to locomotion exhibited significantly greater activity in the aged rats during both periods of non-movement and bouts of locomotion. These results suggest that in the aged striatum, increased activity of nonlocomotor-related neurons may contribute to hypokinesia through their influence on basal ganglia output nuclei. Such studies may aid in the understanding of movement disorders seen in aging and Parkinson's disease.

Enkephalinergic striatal projection neurons become less affected by quinolinic acid than substance P-containing striatal projection neurons as rats age

While the excitotoxic vulnerability of striatal neurons is known to be greater in juvenile than adult animals, it is uncertain if striatal neuron types decline differentially in their vulnerability with age. To examine this issue, we unilaterally injected quinolinic acid (QA), an N-methyl-d-aspartate (NMDA) receptor agonist, into the striatum of juvenile and adult rats, and used in situ hybridization histochemistry with oligonucleotide probes for preproenkephalin and preprotachykinin mRNA to label surviving enkephalinergic (ENK) and substance P-containing (SP) neurons in adjacent sections through the injection center. The results confirmed that the region of severe damage is greater in young than adult animals, but revealed that at the very center of the QA injection, labeled neuron abundance was lower in adult than juvenile striatum. In juvenile rats, the vulnerability of the ENK neurons at all distances from the injection center was the same as that of the SP neurons. By contrast, in adult rats, the ENK neuron survival was greater than the SP neuron survival at all distances beyond the lesion center. The SP neuron survival outside the injection center in the adult rats was similar to that in juvenile rats, while the ENK neuron survival beyond the injection center was better in adult than juvenile rats. These data indicate that there is an age-dependent decrease in the vulnerability of ENK but not SP striatal projection neurons to QA-mediated injury in rats. The results also raise the possibility that, if an excitotoxic process is involved in HD pathogenesis, a differential age-related decline in the sensitivity of striatal projection neuron types to this process may contribute to the more uniform striatal neuron loss in juvenile-onset Huntington's disease (HD) and the more differential loss in adult-onset HD.

Neural cells without functional N-Methyl-D-Aspartate (NMDA) receptors contribute extensively to normal postnatal brain development in efficiently generated chimaeric NMDA R1 -/- <--> +/+ mice

Embryonic stem (ES) cells have revolutionised our understanding of animal physiology. Analysis of chimaeric mice generated from these cells allows us to study the role of genes in development and function of the nervous system. The NMDA receptor, one of the two major ionotropic glutamate receptors, has been proposed to play fundamental roles in the survival, migration, differentiation, and activity-dependent maturation of neural cells. The NMDA receptor subunit 1 (NR1) gene is indispensable for receptor function, and knock-out mice die at birth, inhibiting the study of glutamate signalling in postnatal neurons. Homozygous NR1-/- ES cells were derived from matings of heterozygous mice under feeder-free conditions. Chimaeras were made by incorporating these ES cells into wild-type blastocysts and by the classical aggregation of morulae between wild-type and NR1-/- embryos. The resulting chimaeras survive and develop normally. NR1-/- neurons, identified by their lacZ label, were analysed and quantified in developing and adult brains with varying knock-out contributions in every single brain region. Specifically, postnatal ontogenesis of cerebellum and hippocampus was normal. Accordingly, in chimaeric mice, NMDA receptor-initiated signals are not required for the migration, differentiation, and survival of most types of neurons in the central nervous system, in a cell-autonomous way.

Metabotropic glutamate receptor 5 (mGluR5)-mediated phosphoinositide hydrolysis and NMDA-potentiating effects are blunted in the striatum of aged rats: a possible additional mechanism in striatal senescence

The aim of the present work was to verify whether an impairment of subtype 5 metabotropic glutamate receptor-mediated neurotransmission did occur in the aged striatum. To this end, the ability of the subtype 5 metabotropic glutamate receptor agonist, RS-2-chloro-5-hydroxyphenylglycine, to stimulate phosphoinositide hydrolysis and to potentiate N-methyl-d-aspartate-induced effects in striatal slices from young (3 months) and aged (24 months) rats was compared. The ability of RS-2-chloro-5-hydroxyphenylglycine to induce maximal phosphoinositide turnover and to potentiate N-methyl-d-aspartate effects was significantly reduced in slices from old vs. young rats. These changes were associated with a significant reduction in the expression of subtype 5 metabotropic glutamate receptor protein (-28.8%) and phospholipase C-beta1 (-55.8%) in old striata, while receptor messenger ribonucleic acid expression was unchanged. These results show that the signalling associated with subtype 5 metabotropic glutamate receptors undergoes significant age-related changes and that a reduced expression of subtype 5 metabotropic glutamate receptors and, more importantly, phospholipase C-beta1 may account for the functional decline of subtype 5 metabotropic glutamate receptors.

Aged F344 rats exhibit an increased proportion of dopamine agonist-excited striatal neurons

In order to study age-related differences in striatal electrophysiological activity in freely-moving animals, multi-wire electrode arrays were chronically implanted in the striatum of young (6-8 months) and aged (24-26 months) Fischer 344 rats. After recording baseline activity, d-amphetamine (D-AMPH; 1.0 mg/kg) and apomorphine (APO; 0.5 mg/kg) were administered to the two age groups. For both the D-AMPH and APO series, the percentage of striatal neurons that increased firing rates as a result of the DA agonists was 19% higher in the old animals than in the young animals. In addition, D-AMPH increased the firing rates of D-AMPH-excited neurons to a greater extent in the old animals than in the young animals. While the rate-increasing effects of APO did not differ significantly as a function of age, its effects were slightly greater in the old animals as well. These results suggest that age-related decreases in nigrostriatal DA function may result in alterations in the way in which the striatum integrates corticostriatal and nigrostriatal inputs to influence motor function.

Age-related changes in the N-methyl-D-aspartate receptor binding sites within the human basal ganglia

The present study examined the regional differences in dopamine transporter binding sites and NMDA receptor complex binding based on autoradiographic images obtained in postmortem sections of human normal brain tissues. In middle-aged control tissues, high and comparable levels of [(3)H]CFT binding were observed in the caudate nucleus, putamen, and accumbens nucleus without significant alteration along the rostrocaudal axis and ventral and dorsal parts of these nuclei. In aging normal brain tissues, dopamine binding sites for [(3)H]CFT were significantly reduced in the caudate nucleus, putamen, and accumbens nucleus. l-[(3)H]Glutamate, [(3)H]MK-801, and [(3)H]glycine binding to the NMDA receptor complex was lower in aging brain tissues than in middle-aged controls. Significant correlation did occur between age and [(3)H]CFT binding and between age and l-[(3)H]glutamate, [(3)H]MK-801, and [(3)H]glycine binding sites. These results demonstrate that the basal ganglia have age-associated reductions in dopamine transporter uptake and NMDA receptors. These data support hypoactive activity of the NMDA receptor complex system with advancing age. The dopamine transporter uptake and NMDA receptors appear to be vulnerable to the aging process in the basal ganglia.

Age-related changes in striatal function of freely-moving F344 rats

Multi-wire electrode arrays were used to record extracellular electrophysiological activity in striatal medium spiny-like neurons of freely-moving young (6-8 months) and aged (24-26 months) Fischer 344 rats. While overall basal firing rates did not differ between the two groups, d-amphetamine (5.0 mg/kg) increased firing rates more in the young rats. D-Amphetamine had heterogeneous effects on firing rates, however, exciting 63% of the neurons while inhibiting 37%. Neurons were classified according to their response to d-amphetamine (excited vs. inhibited) to examine age-related differences in firing rates and bursting activity. In the d-amphetamine-excited neurons, pre-drug intraburst firing rates were higher in the old rats. This effect was reversed by d-amphetamine. D-Amphetamine increased the percentage of spikes within bursts to a greater extent in the aged animals and decreased burst durations greater in the young group. In d-amphetamine-inhibited neurons, firing rates were diminished in the old rats more than they were in the young rats. These results demonstrate age-related alterations in striatal electrophysiological activity that may help explain motor deficits seen in senescence.

Long-term survival, migration, and differentiation of neural cells without functional NMDA receptors in vivo

The NMDA receptor, one of the two major ionotropic glutamate receptors, has been proposed to play fundamental roles in the survival, migration, differentiation, and activity-dependent maturation of neural cells. The NR1 gene encodes the major subunit that is responsible for channel function, and NR1 -/- mice die at birth, inhibiting the study of glutamate signaling in postnatal neurons. The properties of cells lacking the NR1 subunit of NMDA receptors were studied by transplanting dissociated telencephalic, diencephalic, and mesencephalic cells of E14 mouse embryos with a targeted deletion of the NR1 gene into the ventricles of embryonic rats using intrauterine transplantation (Brüstle et al., 1995, Neuron 15, 1275-1285). The transplanted cells took part in the normal development of the host brain where they survived after migration into a large number of brain structures. Morphological and immunohistochemical analysis suggests that NR1 -/- cells can differentiate normally in these sites. The results provide evidence that NMDA-receptor-initiated signals are not required for the postnatal differentiation and survival of many types of neurons in the central nervous system, in a noncell autonomous fashion after transplantation into a wild-type environment.

Glutamatergic neurotransmission in aging: a critical perspective

The effects of aging on glutamate neurotransmission in the brain is reviewed and evaluated. Glutamate is the neurotransmitter in most of the excitatory synapses and appears to be involved in functions such as motor behaviour, cognition and emotion, which alter with age. However, relatively few studies have been conducted to study the relationship between glutamate and aging of the brain. The studies presented here indicate the existence of a number of changes in the glutamatergic system during the normal process of aging. First, an age-related decrease of glutamate content in tissue from cerebral cortex and hippocampus has been reported, although it may be mainly a consequence of changes in metabolic activity rather than glutamatergic neurotransmission. On the other hand, studies in vitro and in vivo have shown no changes in glutamate release during aging. Since glutamate sampled in most of these studies is the result of a balance between release and uptake processes, the lack of changes in glutamate release may be due to compensatory changes in glutamate uptake. In fact, a reduced glutamate uptake capacity, as well as a loss in the number of high affinity glutamate transporters in glutamatergic terminals of aged rats, have been described. However, the most significant and consistent finding is the decrease in the density of glutamatergic NMDA receptors with age. A new perspective, in which glutamate interacts with other neurotransmitters to conform the substrates of specific circuits of the brain and its relevance to aging, is included in this review. In particular, studies from our laboratory suggest the existence of age-related changes in the interaction between glutamate and other neurotransmitters, e.g. dopamine and GABA, which are regionally specific.

Modification of ion channels and calcium homeostasis of basal forebrain neurons during aging

In this paper we review the last several years of work from our lab with attention to changes in the properties of basal forebrain neurons during aging. These neurons play a central role in behavioral functions, such as: attention, arousal, cognition and autonomic activity, and these functions can be adversely affected during aging. Therefore, it is fundamental to define the cellular mechanisms of aging in order to understand the basal forebrain and to correct deficits associated with aging. We have examined changes in the physiological properties of basal forebrain neurons during aging with whole-cell and single-channel patch-clamp, as well as, microfluorimetric measurements of intracellular calcium concentrations. These studies contribute to the understanding of integration within the basal forebrain and to the identification of age-related changes within central mammalian neurons. Although extensive functional/behavioral decline is often assumed to occur during aging, our data support an interpretation of compensatory increases in function for excitatory amino acid receptors, GABA(A) receptors, voltage-gated calcium currents and calcium homeostatic mechanisms. We believe that these changes occur to compensate for decrements accruing with age, such as decreased synaptic contacts, ion imbalances or neuronal loss. The basal forebrain must retain functionality into late aging if senescence is to be productive. Thus, it is critical to recognize the potential cellular and subcellular targets for therapeutic interventions intended to correct age-related behavioral deficits.

Age-related decline in the functional response of striatal group I mGlu receptors

In order to verify whether striatal group I metabotropic glutamate (mGlu) receptors undergo functional alteration in ageing, the effects induced by the selective agonist 3,5-dihydroxyphenylglycine (DHPG) in the striatum of young (3 months) and aged (24-25 months old) rats were compared. The ability of DHPG to stimulate phosphoinositide (PI) hydrolysis (striatal slices), to influence striatal dopamine release (in vivo microdialysis) and to potentiate the effects of NMDA on extracellular field potential amplitude (extracellular recordings on striatal slices) was reduced in the striatum of old vs young rats. These results show an age-dependent reduction in the functional response of striatal group I mGlu receptors, which may be one of the factors underlying the reduced ability aged striatum to integrate information.

Neurochemical parameters of the main neurotransmission systems in aging mice

The present work was designed to study the effect of aging on some parameters of the glutamatergic, aminergic and cholinergic neurotransmission, in the main brain areas of mice of the long-surviving BALB/c-nu strain. We have assayed: (1) the density of three ionotropic receptors for excitatory aminoacids (EAA) which selectively bind kainic acid (KA), N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA); (2) the content of dopamine (DA), norepinephrine (NE) and serotonin (5-HT) and the levels of the DA metabolite dihydrophenylacetic acid (DOPAC) and the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA); (3) the level of the choline acetyltransferase (ChAT), the enzyme catalyzing the synthesis of acetylcholine. The parameters were measured in animals at the age of 6, 12, 18 and 24 months; the brain zones under test were the frontal cortex (FC), the corpus striatum (STR), the hippocampus (HIP), the medio-dorsal cortex (DC) and the cerebellum (CER). Significant age-related variations for the density of KA-type and NMDA-type receptors were found in STR and a decrease of the NMDA parameter was found in DC. Neither the monoamine and metabolite contents nor the ChAT levels showed any significant variation in all the tested areas. These findings suggest that an unbalance among different neurotransmission activities could take place with normal aging in rodents: it could be involved in the onset of the motor deficit which occurs in the elderly of these and other mammals.

Indirect modulation of dopamine D2 receptors as potential pharmacotherapy for schizophrenia: II. Glutamate (Ant)agonists

OBJECTIVE

To summarize the published preclinical and clinical data that suggest the possible use of glutamate receptor agonists or antagonists as novel antipsychotic agents.

DATA SOURCES

Primary and review articles were identified by MEDLINE search (from 1966 to December 1999) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All of the articles identified from the data sources were evaluated and all information deemed relevant was included.

DATA SYNTHESIS

The standard antipsychotic drugs, whose clinical activity correlates with affinity for dopamine D2 receptors, alleviate some of the positive symptoms of schizophrenia, but have limited impact on negative symptoms. Several lines of evidence implicate glutamate-receptor system dysfunction(s) in schizophrenia, either as causative or contributory factors. In addition, several standard antipsychotic drugs modulate glutamate or glutamate receptor activity, suggesting an alternative view of their mechanism of antipsychotic action. Preliminary studies have shown that drugs which modulate glutamate brain concentrations have positive effects in animal models of schizophrenia.

CONCLUSIONS

A role for glutamate in the pathogenesis or pharmacotherapy of schizophrenia is suggested from anatomic (interactions between glutamatergic and dopaminergic systems in relevant brain regions), physiologic (implication of glutamate-receptor dysfunction), and pharmacologic (modulation of glutamate or glutamate receptors) evidence. Therefore, compounds that function at glutamate receptors might represent a novel approach to the treatment of the disease or to the amelioration of symptoms, either as monotherapy or as an adjunct to dopamine D2 receptor antagonists.

Caloric restriction prevents age-related deficits in LTP and in NMDA receptor expression

A major focus of aging research has been the search for treatments that will prevent or ameliorate the memory deficits associated with aging. One paradigm, lifelong caloric restriction, has been reported to reduce some of the effects of aging. In the current report, we examined the effects of this treatment on age-related deficits in LTP, a putative cellular building block for memory formation. We report here that lifelong caloric restriction completely prevents the age-related deficit in LTP. In addition, we report that there is a dramatic decrease in the expression of the NMDA receptor subunit NR1 in aged rats and this age-related defect is also prevented by caloric restriction. These data provide a molecular and cellular mechanism by which life long caloric restriction may ameliorate some of the cognitive deficits associated with the aging process.

Alteration of NMDA receptor-mediated synaptic responses in CA1 area of the aged rat hippocampus: contribution of GABAergic and cholinergic deficits

Synaptic responses mediated by the N-methyl-D-aspartate receptor (NMDAr) and non-NMDAr activation were compared in CA1 hippocampal region of young (3-4 months old) and aged (25-33 months old) Sprague-Dawley rats with the use of ex vivo extracellular recordings techniques. In aged rats, the amplitude of the NMDAr-mediated field excitatory postsynaptic potentials (fEPSPs) was not altered, whereas their duration was significantly increased. In contrast, the magnitude of non-NMDAr-mediated fEPSPs was significantly smaller. The presynaptic fiber volley was not affected by age. Considering that the depression of non-NMDAr-mediated responses was previously attributed to fewer synaptic contacts between glutamatergic afferent fibers and pyramidal cells in aged animals (see Barnes et al., Hippocampus 1992;2:457-468), the absence of age-related changes in the amplitude of NMDAr-mediated fEPSPs suggests that compensatory mechanisms may occur. The contribution of gamma-aminobutyric acid (GABA) and acetylcholine to these mechanisms was addressed. The NMDAr-mediated fEPSPs were then recorded (1) in young and aged rats before and after blockade of the GABA(B) receptor-mediated inhibition by the specific antagonist CGP 55845 and (2) in young rats after a selective cholinergic denervation of the hippocampus by the immunotoxin 192 IgG-saporin. The results did not indicate statistically relevant age-related effects of CGP 55845. In contrast, the loss of the cholinergic innervation by the immunotoxin induced a significant increase in both the amplitude and duration of the NMDAr-mediated fEPSPs. Our results indicate that the functional properties of the ionotropic glutamate receptor subtypes located on CA1 pyramidal cells are differentially affected by aging and suggest that the cholinergic deficit that occurs during aging may be involved in the maintenance of robust NMDAr-mediated synaptic responses.

Role of endogenous nitric oxide in the brain stem on the rapid adaptation of baroreflex

It has been shown that nitric oxide in the brain stem plays an important role in the control of sympathetic nerve activity. We examined the role of endogenous nitric oxide in the brain stem in the rapid central adaptation of baroreflex control of sympathetic nerve activity in anesthetized rabbits. Bilateral carotid sinuses were isolated, and a stepwise increase in pressure of 25 or 50 mm Hg for 50 to 60 seconds was applied to the carotid sinuses while the arterial pressure and renal sympathetic nerve activity were recorded. The renal sympathetic nerve activity was inhibited by the stepwise increase in carotid sinus pressure, but thereafter it gradually returned toward the baseline level despite the fact that carotid sinus pressure was kept constant. This procedure was performed after intracisternal injection of N(omega)-nitro-L-arginine methyl ester (L-NAME, 8 micromol), N(omega)-nitro-D-arginine methyl ester (D-NAME, 8 micromol), L-arginine (40 micromol), or the vehicle solution. The magnitude of the immediate and maximal inhibition of renal sympathetic nerve activity caused by a stepwise increase in carotid sinus pressure was similar between the vehicle and L-NAME treatment, but the rate of recovery of the renal sympathetic nerve activity after immediate inhibition was faster after L-NAME than after vehicle. L-Arginine reversed the effects of L-NAME. However, D-NAME or L-arginine alone had no such effects on the rate of recovery of the nerve activity. These results thus suggest that endogenous nitric oxide in the brain stem attenuates rapid adaptation of the arterial baroreflex control of the sympathetic nerve activity in rabbits.

Pharmacological characterization of ionotropic excitatory amino acid receptors in young and aged rat basal forebrain

Ionotropic glutamate receptors were characterized in acutely dissociated medial septum/nucleus of diagonal band neurons from one- to four-month- and 24-26-month-old male Fischer 344 rats. Whole-cell patch-clamp recordings were used to study glutamate, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate, kainate and N-methyl-D-aspartate-induced currents. Pharmacological properties of these ionotropic receptors were studied across different age groups by comparing concentration response curves and EC50 for agonist-induced currents, as well as dissociation constants (Kb) for competitive receptor antagonists. Our results suggest that non-N-methyl-D-aspartate receptors on medial septum/nucleus of diagonal band neurons were predominantly of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate type and display biophysical and pharmacological properties similar to other central neurons. However, peak alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents were enhanced in aged (35.0+/-4.4 pA/pF) compared to young cells (16.2+/-1.7 pA/pF, P<0.005), and the EC50 shifted to the right (4.4+/-0.6 in young compared to 8.8+/-1.3 microM in aged, P<0.05). The Kb for 6,7-dinitroquinoxaline-2,3-dione inhibition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-induced currents likewise shifted to the right (0.16+/-0.02 in young and 0.29+/-0.04 microM in aged, P<0.05) suggesting an age-related decrease in affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors. N-Methyl-D-aspartate-induced currents were generated in standard physiological solutions with the addition of 1 microM glycine and the removal of Mg2+. The N-methyl-D-aspartate responses were predictably modulated by magnesium and glycine, and were antagonized by the competitive antagonist 2-amino-5-phosphonovaleric acid. No age-related change in N-methyl-D-aspartate maximum, EC50, magnesium sensitivity, glycine sensitivity or Kb for 2-amino-5-phosphonovaleric acid was observed. Overall, our results suggest that ionotropic glutamate receptors in the medial septum/nucleus of diagonal band have a similar pharmacological profile compared to glutamate receptors in other brain regions. More importantly, these data suggest that while medial septum/nucleus of diagonal band cells maintain N-methyl-D-aspartate receptors during ageing, a significant increase in current density and decrease in receptor affinity for alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors, during this same time period, may provide a mechanism for age-related changes in neuronal plasticity and excitotoxicity in the basal forebrain.

Age-related decrease in the N-methyl-D-aspartateR-mediated excitatory postsynaptic potential in hippocampal region CA1

Glutamatergic fast synaptic transmission is known to be altered with age in a region-specific manner in hippocampus of memory-impaired old rats. In the present experiment, presynaptic fiber potentials and non-N-methyl-D-aspartate (NMDAR) and NMDAR-mediated synaptic responses in CA1 were compared in three ages of behaviorally characterized male F-344 rats. In the CA1 region, old rats showed approximately equivalent reductions in non-NMDAR- and NMDAR-excitatory postsynaptic potential amplitudes for a given size of presynaptic fiber potential. There was no change in magnitude of the presynaptic response itself at any stimulus level. These results are consistent with the hypothesis that there is a reduction in the number of Schaffer collateral synapses per presynaptic axon. This pattern of results in CA1 is very different from what is known to occur at the perforant path-granule cell synapse. In fascia dentata the non-NMDAR-mediated excitatory postsynaptic potential is increased in amplitude, although the NMDAR-mediated excitatory postsynaptic potential is reduced for a given presynaptic input. These data suggest that age-related functional alterations in neurotransmitter receptor subtypes occur differentially between closely-related anatomical subregions.

Modulatory actions of dopamine on NMDA receptor-mediated responses are reduced in D1A-deficient mutant mice

The role of D1 dopamine (DA) receptors in mediating the ability of DA to modulate responses attributable to activation of NMDA receptors was examined in mice lacking D1A dopamine receptors. Specifically, experiments were designed to test the hypothesis that the ability of DA to potentiate responses mediated by activation of NMDA receptors was attributable to activation of D1 receptors. Based on this hypothesis, we would predict that in the D1A mutant mouse, either DA would not induce enhancement of NMDA-mediated responses, or the enhancement would be severely attenuated. The results provided evidence to support the hypothesis. In mutant mice, DA and D1 receptor agonists did not potentiate responses mediated by activation of NMDA receptors. In contrast, in control mice, both DA and D1 receptor agonists markedly potentiated responses mediated by activation of NMDA receptors. The effects of DA in attenuating responses mediated by activation of non-NMDA receptors also were altered in the mutant, suggesting that this action of DA may require coupling or interactions between D1 and D2 receptors. The present studies also provided an opportunity to assess some of the basic electrophysiological and morphological properties of neostriatal neurons in mice lacking D1A DA receptors. Resting membrane potential, action potential parameters, input resistance, excitability, somatic size, dendritic extent, and estimates of spine density in mutants and controls were similar, suggesting that these basic neurophysiological and structural properties have not been changed by the loss of the D1A DA receptor.