Preservation of redox, polyamine, and glycine modulatory domains of the N-methyl-D-aspartate receptor in Alzheimer's disease

The glutamatergic system in Alzheimer's disease: a systematic review with meta-analysis

Glutamatergic neurotransmission system dysregulation may play an important role in the pathophysiology of Alzheimer's disease (AD). However, reported results on glutamatergic components across brain regions are contradictory. Here, we conducted a systematic review with meta-analysis to examine whether there are consistent glutamatergic abnormalities in the human AD brain. We searched PubMed and Web of Science (database origin-October 2023) reports evaluating glutamate, glutamine, glutaminase, glutamine synthetase, glutamate reuptake, aspartate, excitatory amino acid transporters, vesicular glutamate transporters, glycine, D-serine, metabotropic and ionotropic glutamate receptors in the AD human brain (PROSPERO #CDRD42022299518). The studies were synthesized by outcome and brain region. We included cortical regions, the whole brain (cortical and subcortical regions combined), the entorhinal cortex and the hippocampus. Pooled effect sizes were determined with standardized mean differences (SMD), random effects adjusted by false discovery rate, and heterogeneity was examined by I2 statistics. The search retrieved 6 936 articles, 63 meeting the inclusion criteria (N = 709CN/786AD; mean age 75/79). We showed that the brain of AD individuals presents decreased glutamate (SMD = -0.82; I2 = 74.54%; P < 0.001) and aspartate levels (SMD = -0.64; I2 = 89.71%; P = 0.006), and reuptake (SMD = -0.75; I2 = 83.04%; P < 0.001. We also found reduced α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR)-GluA2/3 levels (SMD = -0.63; I2 = 95.55%; P = 0.046), hypofunctional N-methyl-D-aspartate receptor (NMDAR) (SMD = -0.60; I2 = 91.47%; P < 0.001) and selective reduction of NMDAR-GluN2B subunit levels (SMD = -1.07; I2 = 41.81%; P < 0.001). Regional differences include lower glutamate levels in cortical areas and aspartate levels in cortical areas and in the hippocampus, reduced glutamate reuptake, reduced AMPAR-GluA2/3 in the entorhinal cortex, hypofunction of NMDAR in cortical areas, and a decrease in NMDAR-GluN2B subunit levels in the entorhinal cortex and hippocampus. Other parameters studied were not altered. Our findings show depletion of the glutamatergic system and emphasize the importance of understanding glutamate-mediated neurotoxicity in AD. This study has implications for the development of therapies and biomarkers in AD.

An integrated strategy using LC-MS/MS combined with in vivo microdialysis for the simultaneous determination of lignans of Schisandra chinensis (Turcz.) Baill. Fructus and endogenous neurotransmitters: application in pharmacokinetic and pharmacodynamic studies

Schisandra chinensis (Turcz.) Baill Fructus (SCF) is the ripe fruit of Schisandra chinensis (Turcz.) Baill, and is often used as a neuroprotective drink. Modern pharmacological studies have shown that lignans are the main bioactive components responsible for neuroprotection and have potential in the treatment of Alzheimer's disease (AD). However, the mechanism of action of SCF in the treatment of AD from the pharmacokinetics-pharmacodynamics (PK-PD) perspective remains not well established. The purpose of this study is to investigate and compare the pharmacokinetic differences of lignans in normal and AD rats, as well as to investigate their effects on neurotransmitters and their role in the treatment of AD. To achieve this goal, an integrated strategy using LC-MS/MS combined with in vivo microdialysis for the simultaneous determination of lignans of SCF and endogenous neurotransmitters has been developed and validated. The results show that the pharmacokinetic behaviors of ten lignans in the AD group were significantly different from those in the normal group. The AD group had better absorption and slower elimination than the normal group. In addition, the pharmacodynamic results of the Morris water maze (MWM) test, biochemical tests, histopathological examination, as well as immunohistochemistry analysis showed that lignans could improve the learning and memory of AD rats. The oral administration of SCF could restore the levels of the neurotransmitter parameters; seven neurotransmitters showed clockwise or counterclockwise changes with the four lignans in the hippocampal region. Taken together, the PK and PD studies based on in vivo microdialysis sampling might offer novel insights into the mechanisms of action of SCF against AD.

Synaptic plasticity is impaired in rats with a low glutathione content

Long-term potentiation (LTP) is a sustained increase in the efficacy of synaptic transmission, based on functional changes involving pre- and postsynaptic mechanisms, and has been considered a cellular model for learning and memory. The sulphurated tripeptide glutathione acts as a powerful antioxidant agent within the nervous system. Recent in vitro studies suggest that the cellular redox status might influence the mechanisms involved in synaptic plasticity. It is not known, however, how glutathione depletion might affect LTP. In the present study, we evaluated the input-output relationships, LTP, and paired-pulse interactions in rats with low glutathione levels induced by systemic injection of diethylmaleate. Our results in anesthetized rats show that the basic synaptic transmission between the perforant pathway and the dentate gyrus granule cells was not affected by glutathione depletion. However, in the same synapses it was not possible to induce prolonged changes in synaptic efficacy (LTP). Paired-pulse facilitation was also absent in the treated animals, suggesting an impairment of short-term synaptic interactions. These findings indicate that low content of glutathione can impair short-term and long-term mechanisms of synaptic plasticity and stress the importance of the redox balance in the normal function of brain circuitry.

Preservation of N-methyl-D-aspartate receptor binding sites with age in rat neocortex

This study used [3H]dizocilpine ([3H]MK-801) binding to examine glycine, polyamine, and zinc subsites of the N-methyl-D-aspartate (NMDA) receptor in well-washed membranes derived from the neocortex of Fischer 344/Norwegian brown rats aged 3, 12, 24 and 37 months. [3H]dizocilpine binding in the presence of 100 microM glutamate was enhanced by the addition of 30 microM glycine. Binding in the presence of both glutamate and glutamate plus glycine were unaffected by age. The competitive polyamine site antagonist arcaine inhibited [3H]dizocilpine binding in a dose-dependent fashion and 50 microM spermidine caused a rightward shift in this dose response curve. IC50 values derived from these plots were not significantly affected by age. Similarly, zinc inhibited binding in a dose-dependent fashion and was also unaffected by age. These data indicate that the NMDA receptor is spared in aging.

Age-associated memory impairment. Assessing the role of nitric oxide

Several neurotransmitter systems have been investigated to assess hypothesized mechanisms underlying the decline in recent memory abilities in normal aging and in Alzheimer's disease. Examining the performance of F344 rats in a 14-unit T-maze (Stone maze), we have focused on the muscarinic cholinergic (mACh) and the N-methyl-D-aspartate (NMDA) glutamate (Glu) systems and their interactions. Maze learning is impaired by antagonists to mACh or NMDA receptors. We have also shown that stimulation of mACh receptors can overcome a maze learning deficit induced by NMDA blockade, and stimulation of the NMDA receptor can overcome a similar blockade of mACh receptors. No consistent evidence in rats has been produced from our laboratory to reveal significant age-related declines in mACh or NMDA receptor binding in the hippocampus (HC), a brain region that is greatly involved in processing of recent memory. Thus, we have directed attention to the possibility of a common signal transduction pathway, the nitric oxide (NO) system. Activated by calcium influx through the NMDA receptor, NO is hypothesized to be a retrograde messenger that enhances presynaptic Glu release. Maze learning can be impaired by inhibiting the synthetic enzyme for NO, nitric oxide synthase (NOS), or enhanced by stimulating NO release. However, we have found no age-related loss of NOS-containing HC neurons or fibers in rats. Additionally, other laboratories have reported no evidence of an age-related loss of HC NOS activity. In a microdialysis study we have found preliminary evidence of reduced NO production following NMDA stimulation. We are currently working to identify the parameters of this phenomenon as well as testing various strategies for safely stimulating the NO system to improve memory function in aged rats.

Analysis of NMDA receptors in the human spinal cord

NMDA receptors in postmortem human spinal cord were analyzed using [3H]MK-801 ligand binding and immunoblotting with NMDA receptor subunit-specific antibodies. The average KD for [3H]MK-801 binding was 1.77 nM with a Bmax of 0.103 pmol/mg. The EC50 for stimulation of -3H-MK-801 binding with L-glutamate was 0.34 microM. None of these parameters were affected by postmortem intervals up to 72 h. Immunoblotting of native NMDA receptors showed that NR1, NR2A, NR2C, and NR2D subunits could all be found in the human spinal cord of which NR1 was preferentially located to the dorsal half. Immunoprecipitation of solubilized receptors revealed that NR1, NR2C, and NR2D subunits coprecipitated with the NR2A subunit, indicating that native human spinal cord NMDA receptors are heteroligimeric receptors assembled by at least three different receptor subunits. These results provide a basis for the development of drugs selectively aimed at spinal cord NMDA receptors for the future treatment of spinal cord disorders.

Washable endogenous substances and regional heterogeneity in agonist enhanced [3H]MK-801 binding in rat brain

NMDA receptor/ion channel function is modulated through a number of distinct sites that regulate channel opening. Published studies report widely varying results in modulatory site agonist effects due to assay conditions and technique. Also, NMDA receptor regulation at these sites by endogenous substances remains poorly characterized. The objectives of the present study in Sprague-Dawley rat forebrain sections were: (i) determine the contribution of various prewash variables on agonist stimulation of the NMDA receptor, (ii) compare regional differences in functional glycine, spermidine and NMDA binding sites under optimized prewash conditions, and (iii) define the influence of endogenous substances at each modulatory site by analyzing changes in binding at different prewash durations. We demonstrate that prewash conditions have a critical influence on [3H]MK-801 binding in rat tissue sections and that this effect was differentially expressed across brain regions. An extended prewash duration caused a regionally specific decrease in unenhanced [3H]MK-801 binding, while a short prewash caused a regionally specific biphasic effect on enhanced [3H]MK-801 binding. After prolonged prewash, binding was restored to previous (unwashed) binding levels with exogenously added glycine, NMDA, or spermidine alone or combinations of agonists. These data suggest that washable endogenous substances contribute to the full functionality of the NMDA receptor and the regional heterogeneity in [3H]MK-801 binding is dependent on the interaction of receptor protein subtypes and the presence of one or more endogenous substances.

Age-related changes in bindings of second messengers in the rat brain

Age-related alterations in bindings of major second messengers in the brain were studied in 3-week- and 6-, 12-, 18- and 24-month-old Fisher 344 rats using receptor autoradiography. [3H]Phorbol 12,13-dibutyrate (PDBu) and [3H]forskolin were used to label protein kinase C (PKC) and adenylate cyclase, respectively. In immature rats (3-week-old), [3H]PDBu binding showed a significant decrease only in the cerebellum as compared to adult rats (6-month-old), whereas [3H]forskolin binding exhibited a significant reduction in the neocortex, nucleus accumbens, thalamus and substantia nigra. In aged rats, [3H]PDBu binding showed no significant change in all brain areas. In contrast, [3H]forskolin binding showed a conspicuous reduction in various brain areas in 18-month-old rats as compared to adult animals. The age-related reduction was especially observed in the cerebral cortex, hippocampal CA3 pyramidal cell layer, dentate gyrus, thalamus and molecular layer of cerebellum of 24-month-old rats. The results indicate that adenylate cyclase system in the rat brain is more susceptible to aging processes than phosphoinositide cycle system. Furthermore, our data demonstrate that the change in the adenylate cyclase system is more pronounced than that in the phosphoinositide cycle system in immature rat brain. These findings suggest that the adenylate cyclase system is primarily affected in aging processes and this may lead to age-related neurological deficits.

Excitatory amino acids in health and disease

PURPOSE

To review the role of excitatory neurotransmitters in normal mammalian brain function, the concept of excitotoxic neuronal death as an important final common path in a variety of diseases, and modification of excitatory synaptic transmission as an important new pharmacological principle. These principles are discussed, with special emphasis on diseases of importance to older adults.

DATA SOURCES

A MEDLINE search from 1966 to May 1995 was undertaken, as well as a manual search of current issues of clinical and basic neuroscience journals, for articles that addressed glutamate N-methyl-D-aspartate and/or excitotoxicity.

STUDY SELECTION

A total of 5398 original and 68 review articles were identified that addressed animal and human experimentation relevant to excitotoxic neuronal death. There were 364 articles with potential significance for clinical application identified; 132 of the most recent references are provided.

DATA EXTRACTION

All articles were classified into three categories: general receptor, biology pathogenesis of disease, and pharmacotherapy.

RESULTS

Glutamic and aspartic acids are the physiological mediators of most excitatory synaptic transmission. This is critical to several normal nervous system functions, including memory and long-term modification of synaptic transmission and nociception. Activation of the inotropic NMDA and non-NMDA receptors increases transmembrane calcium and sodium fluxes, and the metabotropic glutamate receptor activation results in generation of inositol triphosphate and inhibition of adenylate cyclase. Numerous modulatory sites exist, especially on the NMDA receptor. Nitric oxide, arachidonic acid, superoxide, and intracellular calcium overload are the ultimate mediators of neuronal death. Glutamate re-uptake transporters belong to a unique family of amino acid transport systems, the malfunction of which is intricately involved in disease pathogenesis. Ischemic stroke, hypoglycemia, Parkinson's disease, alcohol intoxication and withdrawal, Alzheimer's disease, epilepsy, and chronic pain syndromes are only some of the important clinical neurological disorders with a major pathogenic role for the excitatory amino acids.

CONCLUSIONS

Pharmacological manipulation of the excitatory amino acid receptors is likely to be of benefit in important and common diseases of the nervous system. Only a few of the currently available drugs that modify excitatory neurotransmission, such as remacemide, lamotrigine, and tizanidine, have an acceptable therapeutic index. The identification of numerous receptor subtypes, topographic variabilities of distribution, and multiple modulatory sites will provide a true challenge to the neuropharmacologist.

Glucose deprivation increases aspartic acid release from synaptosomes of aged mice

To investigate the possible existence of age-related changes in excitatory amino acid release in brain, and the influence of glucose deprivation on this process, we have determined the release of endogenous aspartate and glutamate from synaptosomes freshly isolated from the cerebrum of young (12 months old) and aged (24 months old) mice. We found that there are no age-related changes in the synaptosomal release of aspartic and glutamic acids. However, in the absence of glucose in the medium of incubation aspartate and glutamate release was higher in old than in young animals (P < 0.05, and P = 0.09 respectively). Our results suggest that the ability of cerebral synaptosomes to release glutamate and aspartate remains functionally intact in old cerebrum, but there is an age-dependent dysfunction in this process linked to energy metabolism disturbances.

Similar effects of glycine, zinc and an oxidizing agent on [3H]dizocilpine binding to the N-methyl-D-aspartate receptor in neocortical tissue from suicide victims and controls

This study used [3H]dizocilpine (MK-801) binding to the N-methyl-D-aspartate (NMDA) receptor to examine glycine, redox and zinc modulatory sites in membranes derived from the frontal and parietal cortex of control subjects (n = 8) and suicide victims (n = 6). [3H]dizocilpine binding in the presence of glutamate and glutamate plus glycine was similar in control and suicide subjects. The sulphydryl redox site was assessed using the oxidizing agent 5,5'-dithio-bis (2-nitrobenzoic acid), which inhibited binding in a dose-dependent fashion. Both redox and zinc sites were unaffected in the frontal and parietal cortex of suicide victims. These data indicate that the NMDA receptor and its glycine, redox and zinc subsites are preserved in the neocortex of suicide victims.