Interactions of glutamate receptor agonists with long-term potentiation in the rat hippocampal slice

Ketone bodies attenuate excitotoxic cell injury in the rat hippocampal slice under conditions of reduced glucose availability

Calorie restriction and the ketogenic diet have been successfully used in models of neuroprotection. However, there are limitations in clinical application of these diets and attention has turned to understanding their mechanism of action. Ketone bodies are produced in both diets and maybe involved in their ability to attenuate neuronal injury. This study seeks to assess the effects of ketone bodies on neuronal transmission and their efficacy in reducing the impact of known excitotoxins. We made use of extracellular recordings from rat hippocampal slices and demonstrate that ketone bodies had no effect on neuronal transmission or induction of long-term potentiation (LTP). Perfusion of slices with N-methyl-d-aspartate (NMDA, 15 μM) produced neuronal depression suggestive of cell injury (antidromic recording also demonstrated similar depression), such that recovery of population spike potentials after 60 minutes was 27% in normal (10 mM) glucose but only 7% during reduced (2·5 mM) glucose availability. Experiments in ketone bodies demonstrated improved recovery (31%) but only under conditions of low glucose. Similarly, there was enhanced recovery of slices treated with kainic acid (KA, 30 μM) in reduced glucose media (13-27%), but no difference in normal glucose. These findings suggest that ketone bodies do not alter neuronal function but can alter the response to excitotoxins when energy supplies are impaired, probably by acting as an alternative energy substrate.

Sustained activation of renal N-methyl-D-aspartate receptors decreases vitamin D synthesis: a possible role for glutamate on the onset of secondary HPT

N-methyl-D-aspartate (NMDA) receptors (NMDAR) are tetrameric amino acid receptors that act as membrane calcium channels. The presence of the receptor has been detected in the principal organs responsible for calcium homeostasis (kidney, bone, and parathyroid gland), pointing to a possible role in mineral metabolism. The aim of this study was to test the effect of NMDAR activation in the kidney and on 1,25(OH)₂D₃ synthesis. We determined the presence of NMDAR subunits in HK-2 (human kidney cells) cells and proved its functionality. NMDA treatment for 4 days induced a decrease in 1α-hydroxylase levels and 1,25(OH)₂D₃ synthesis through the activation of the MAPK/ERK pathway in HK-2 cells. In vivo administration of NMDA for 4 days also caused a decrease in blood 1,25(OH)₂D₃ levels in healthy animals and an increase in blood PTH levels. This increase in PTH induced a decrease in the urinary excretion of calcium and an increase in urinary excretion of phosphorous and sodium as well as in diuresis. Bone turnover markers also increased. Animals with 5/6 nephrectomy showed low levels of renal 1α-hydroxylase as well as high levels of renal glutamate compared with healthy animals. In conclusion, NMDAR activation in the kidney causes a decrease in 1,25(OH)₂D₃ synthesis, which induces an increase on PTH synthesis and release. In animals with chronic kidney disease, high renal levels of glutamate could be involved in the downregulation of 1α-hydroxylase expression.

Adult-onset calorie restriction attenuates kainic acid excitotoxicity in the rat hippocampal slice

Lifelong calorie restriction is the only known intervention that has been shown to consistently increase life span and reduce the effects of aging on the brain. Given the difficulties of replicating lifelong calorie restriction within human populations, we have sought to assess the effects of short-term adult-onset calorie restriction upon acute excitotoxic insults in the rat hippocampus. Adult animals (approximately 6 months of age) underwent calorie restriction (alternate day feeding) for 7-10 weeks. Utilizing both electrophysiological and immunocytochemical techniques, we report that calorie restriction had no effect upon long-term potentiation (LTP), a measure of neuronal function. In control animals, application of kainic acid (20 microM) resulted in only 35% recovery of CA1 population spikes post-insult. However calorie-restricted animals showed significantly improved recovery after kainic acid treatment (64%). This data was supported by immunocytochemical studies which noted widespread loss of microtubule-associated protein (MAP 2) immunoreactivity in control slices following treatment with kainic acid; however MAP 2 staining was preserved in the CA1 and CA3 regions of calorie-restricted animals. Interestingly there was no significant difference in the recovery of population spikes between groups when slices were treated with N-methyl-d-aspartate (15 microM). We conclude that short-term adult-onset calorie restriction does not alter normal neuronal function and serves to protect the hippocampus from acute kainic acid excitotoxicity.

AMPA receptor activation reduces epileptiform activity in the rat neocortex

We have previously reported that topical application of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) to the rat neocortex prevents the effects of a subsequent application of N-methyl-d-aspartic acid (NMDA). Activation of NMDA receptors is involved in the pathogenesis of epileptic activity. Therefore, we examined if topically applied AMPA could affect changes in the somatosensory evoked potentials (SEPs) and electrocorticogram (ECoG) epileptic spikes caused by bicuculline. AMPA (50 microM) prevented the epileptiform activity to a level that was comparable to that caused by diazepam (3 mg/kg i.p.) or clomethiazole (100 mg/kg i.p.). Also, the epileptiform activity was suppressed by the AMPAR antagonist, CNQX, or the blocker of AMPAR desensitization, cyclothiazide. In the hippocampal slice, bicuculline-induced changes in the population spike potentials recorded from the CA1 cells were not affected by AMPA. We conclude that in the complex neuronal network of the rat neocortex, epileptiform activity can be suppressed in a variety of strategies that target the AMPA receptors: (1) blocking AMPA receptors, (2) promoting an apparent desensitization of AMPA receptors (possibly on the pyramidal neurons) or (3) reducing an apparent desensitization of AMPA receptors (possibly on the inhibitory GABA-ergic interneurons).

NMDA-induced preconditioning attenuates synaptic plasticity in the rat hippocampus

It was recently demonstrated that glutamate could precondition hippocampal slices against the damaging effects of hypoxia, and we have now extended this observation by investigating (i) the ability of glutamate receptor agonists to act as preconditioning agents and (ii) the effects of preconditioning on synaptic plasticity. Using rat hippocampal slices, 15 microM NMDA applied for 10 min (chemical insult) caused abolition of the population spike potentials (PS) followed by approximately 33% recovery at 60 min post-insult. In comparison, a 5 min preconditioning exposure of 10 microM NMDA given 30 min prior to the insult significantly improved the recovery to 69%. Preconditioning did not alter paired pulse facilitation; however, it significantly enhanced paired pulse depression and reduced population spike long-term potentiation (PS-LTP) and LTP in field recordings. This effect on PS-LTP appeared to be NMDA receptor dependent and was blocked by the nitric oxide synthase inhibitors nitro-L-arginine methyl ester (L-NAME) and 7-nitro indazole (7-NI) but not by the adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). We conclude that preconditioning by NMDA can improve recovery following acute insults but may have deleterious effects on neuronal plasticity.

Application of a systems biology approach to developmental neurotoxicology

Systems biology can be applied to enhance the understanding of complex biological processes such as apoptosis in the developing brain. Systems biology, as applied to toxicology, provides a structure to arrange information in the form of a biological model. The approach allows for the subsequent and iterative perturbation of the initial model with the use of toxicants, and the comparison of the resulting data against the proposed biological model. It is postulated that the exposure of the developing rat to NMDA antagonists, e.g., ketamine or phencyclidine (PCP), causes a compensatory up-regulation of NMDA receptors, thereby making cells bearing these receptors more vulnerable to excitotoxic effects of endogenous glutamate. Although comprehensive gene expression/proteomic studies and mathematical modeling remain to be accomplished, a biological model has been established and perturbed in an iterative manner to allow confirmation of the biological pathway for NMDA antagonist-induced brain cell death in the developing rat.

AMPA receptor regulation mechanisms: future target for safer neuroprotective drugs

The post-synaptic AMPA receptors play an important role in mediating fast excitatory transmission in the mammalian brain. Over-activated AMPA receptors induce excitotoxicity, implicated in a number of Chronic neurodegenerative disorders such as Parkinson's disease, Huntington's disease, and AIDS encephalitis. AMPA receptor antagonists offer protection against neurodegeneration in the experimental models even if they are given 24 h after the injury. Because AMPA receptors seem to be involved in the neurodegenerative diseases, modulating the activity of the AMPA receptors could be an attractive approach to reduce or prevent excitotoxicity. Studies conducted recently have exhibited a number of new mechanisms for AMPA receptor regulation. Modulations of these were found to have protective implications. AMPA receptor depolarization and desensitization are protective to the neurons. Receptor desensitization depends on the receptor subunit composition. The R/G editing site and the flip/flop cassettes in AMPA receptor subunits contribute to a great extent in receptor desensitization and recovery rates. Molecules that could quicken receptor desensitization or delay recovery could be of use. AMPA receptors limit neuronal entry of Ca2+ ions by regulating Ca2+-permeability. Ca2+-permeable receptor channels are made up of GluR1, GluR3, or GluR4 subunits, whereas presence of the GluR2 subunit restricts Ca2+ entry and renders the receptor Ca2+-impermeable. GluR2 levels, however, experience a fall after neuronal insult rendering the AMPA receptors Ca2+-permeable, thus factors that could interfere with this event might prove to be very beneficial against excitotoxicity. AMPA receptor clusters are stabilized by PSD-95, which requires palmitoylation at two sites. Targeting palmitoylation of the PSD-95 can also be a useful approach to disperse AMPA clusters at the synapse. In the perisynaptic region, mGluRs are present a little away from the synapse and are among the glutamate transporters, which require high-frequency firing for activation. On activation they might enhance the activity of NMDA receptors at the synapse to increase the levels of AMPA receptors. AMPA receptors surfaced at this juncture can contribute to heavy Ca2+ influx. Thus, blocking this pathway could be of considerable importance in preventing the excitotoxicity. A number of proteins such as the GRIP, PICK, and NSF also modulate the functions of AMPA receptors. Polyamines also block Ca2+ permeable AMPA receptors and thus are protective. NO and cGMP also play an important role in negatively regulating AMPA receptors and thus could offer protection. Modulation of AMPA receptor by different mechanisms has been discussed in the present review to implicate importance of these targets/pathways for safer and future neuroprotective drugs.

LTP-induced depression of response to hypoxia in hippocampus: effects of adenosine receptor activation

Previous work has shown that long-term potentiation (LTP) can reduce the effects of hypoxia in depressing population spikes in rat hippocampal slices. We have now investigated the role of adenosine in this phenomenon. There is no mutual inhibition between the depressant effects of hypoxia and adenosine, but LTP reduces responses to both hypoxia and adenosine, as does application of an A1 receptor antagonist. The effect of LTP is not due to a change in the balance of activation of A1 and A2A adenosine receptors since a selective A2A receptor antagonist did not prevent the interaction. We suggest that LTP may reduce the response to hypoxia by attenuating neuronal sensitivity to adenosine A1 receptors.

Age-related changes in tolerance to the marine algal excitotoxin domoic acid

During an incident of toxic mussel poisoning, the epileptogenic excitotoxin domoic acid (DOM) was associated with lasting neurological deficits mainly in older patients (), suggesting supersensitivity to excitotoxins is a feature of brain aging. Here, hippocampal slices from young (3 months) and aged (26-29 months) Sprague Dawley rats were assessed by CA1 field potential analysis before and after preconditioning with DOM. In naïve slices from young animals, DOM produced initial hyperexcitability followed by significant dose-dependent reductions in population spike amplitude during prolonged application. Following toxin washout, only small changes in neuronal activity were evident during a second application of DOM, suggesting that a resistance to the effects of DOM occurs in hippocampal slices which have undergone prior exposure to DOM. This inducible tolerance was not antagonized by the NMDA receptor blockers APV or MK-801, nor was it diminished by the group I, II or III mGluR blockers AIDA, CPPG and EGLU. Likewise, neither the AMPA/KA blocker CNQX nor the VSCC blocker nifedipine were effective in blocking tolerance induction in young slices. Field potential analysis revealed significant age-related reductions in CA1 EPSP strength, population spike amplitude and paired-pulse inhibition, but aged slices did not differ in sensitivity to DOM relative to young. However, aged CA1 failed to exhibit any tolerance to DOM following preconditioning, suggesting that a loss of inducible neuroprotective mechanisms may account for increased sensitivity to excitotoxins during aging.

The pharmacological manipulation of glutamate receptors and neuroprotection

The overactivation of glutamate receptors is a major cause of Ca(2+) overload in cells, potentially leading to cell damage and death. There is an abundance of agents and mechanisms by which glutamate receptor activation can be prevented or modulated in order to control these effects. They include the well-established, competitive and non-competitive antagonists at the N-methyl-D-aspartate (NMDA) receptors and modulators of desensitisation of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. More recently, it has emerged that some compounds can act selectively at different subunits of glutamate receptors, allowing a differential blockade of subtypes. It is also becoming clear that a number of endogenous compounds, including purines, can modify glutamate receptor sensitivity. The kynurenine pathway is an alternative but distinct pathway to the generation of glutamate receptor ligands. The products of tryptophan metabolism via the kynurenine pathway include both quinolinic acid, a selective agonist at NMDA receptors, and kynurenic acid, an antagonist at several glutamate receptor subtypes. The levels of these metabolites change as a result of the activation of inflammatory processes and immune-competent cells, and may have a significant impact on Ca(2+) fluxes and neuronal damage. Drugs which target some of these various sites and processes, or which change the balance between the excitotoxin quinolinic acid and the neuroprotective kynurenic acid, could also have potential as neuroprotective drugs.

Long-term potentiation protects rat hippocampal slices from the effects of acute hypoxia

We have previously shown that long-term potentiation (LTP) decreases the sensitivity of glutamate receptors in the rat hippocampal CA1 region to exogenously applied glutamate agonists. Since the pathophysiology of hypoxia/ischemia involves increased concentration of endogenous glutamate, we tested the hypothesis that LTP could reduce the effects of hypoxia in the hippocampal slice. The effects of LTP on hypoxia were measured by the changes in population spike potentials (PS) or field excitatory post-synaptic potentials (fepsps). Hypoxia was induced by perfusing the slice with (i) artificial CSF which had been pre-gassed with 95%N2/5% CO2; (ii) artificial CSF which had not been pre-gassed with 95% O2/5% CO2; or (iii) an oxygen-glucose deprived (OGD) medium which was similar to (ii) and in which the glucose had been replaced with sucrose. Exposure of a slice to a hypoxic medium for 1.5-3.0 min led to a decrease in the PS or fepsps; the potentials recovered to control levels within 3-5 min. Repeat exposure, 45 min later, of the same slice to the same hypoxic medium for the same duration as the first exposure caused a reduction in the potentials again; there were no significant differences between the degree of reduction caused by the first or second exposure for all three types of hypoxic media (P>0.05; paired t-test). In some of the slices, two episodes of LTP were induced 25 and 35 min after the first hypoxic exposure; this caused inhibition of reduction in potentials caused by the second hypoxic insult which was given at 45 min after the first; the differences in reduction in potentials were highly significant for all the hypoxic media used (P<0.01; paired t-test). The neuroprotective effects of LTP were not prevented by cyclothiazide or inhibitors of NO synthetase compounds that have been shown to be effective in blocking the effects of LTP on the actions of exogenously applied AMPA and NMDA, respectively. The neuroprotective effects of LTP were similar to those of propentofylline, a known neuroprotective compound. We conclude that LTP causes an appreciable protection of hippocampal slices to various models of acute hypoxia. This phenomenon does not appear to involve desensitisation of AMPA receptors or mediation by NO, but may account for the recognised inverse relationship between educational attainment and the development of dementia.