Interaction of the NMDA receptor noncompetitive antagonist MK-801 with model and native membranes

Dietary Plant Polyphenols as the Potential Drugs in Neurodegenerative Diseases: Current Evidence, Advances, and Opportunities

Neurodegenerative diseases, including Alzheimer's disease (AD), Parkinson's disease (PD), and Huntington's disease (HD), are characterized by the progressive degeneration of neurons. Although the etiology and pathogenesis of neurodegenerative diseases have been studied intensively, the mechanism is still in its infancy. In general, most neurodegenerative diseases share common molecular mechanisms, and multiple risks interact and promote the pathologic process of neurogenerative diseases. At present, most of the approved drugs only alleviate the clinical symptoms but fail to cure neurodegenerative diseases. Numerous studies indicate that dietary plant polyphenols are safe and exhibit potent neuroprotective effects in various neurodegenerative diseases. However, low bioavailability is the biggest obstacle for polyphenol that largely limits its adoption from evidence into clinical practice. In this review, we summarized the widely recognized mechanisms associated with neurodegenerative diseases, such as misfolded proteins, mitochondrial dysfunction, oxidative damage, and neuroinflammatory responses. In addition, we summarized the research advances about the neuroprotective effect of the most widely reported dietary plant polyphenols. Moreover, we discussed the current clinical study and application of polyphenols and the factors that result in low bioavailability, such as poor stability and low permeability across the blood-brain barrier (BBB). In the future, the improvement of absorption and stability, modification of structure and formulation, and the combination therapy will provide more opportunities from the laboratory into the clinic for polyphenols. Lastly, we hope that the present review will encourage further researches on natural dietary polyphenols in the treatment of neurodegenerative diseases.

Prefrontal inhibition of neuronal Kv 7 channels enhances prepulse inhibition of acoustic startle reflex and resistance to hypofrontality

Background and Purpose

Dysfunction of the prefrontal cortex (PFC) is involved in the cognitive deficits in neuropsychiatric diseases, such as schizophrenia, characterized by deficient neurotransmission known as NMDA receptor hypofrontality. Thus, enhancing prefrontal activity may alleviate hypofrontality‐induced cognitive deficits. To test this hypothesis, we investigated the effect of forebrain‐specific suppression or pharmacological inhibition of native K_v7/KCNQ/M‐current on glutamatergic hypofrontality induced by the NMDA receptor antagonist MK‐801.

Experimental Approach

The forebrain‐specific inhibition of native M‐current was generated by transgenic expression, in mice, of a dominant‐negative pore mutant G279S of K_v7.2/KCNQ2 channels that suppresses channel function. A mouse model of cognitive impairment was established by single i.p. injection of 0.1 mg·kg⁻¹ MK‐801. Mouse models of prepulse inhibition (PPI) of acoustic startle reflex and Y‐maze spontaneous alternation test were used for evaluation of cognitive behaviour. Hippocampal brain slice recordings of LTP were used to assess synaptic plasticity. Hippocampus and cortex were dissected for detecting protein expression using western blot analysis.

Key Results

Genetic suppression of K_v7 channel function in the forebrain or pharmacological inhibition of K_v7 channels by the specific blocker XE991 enhanced PPI and also alleviated MK‐801 induced cognitive decline. XE991 also attenuated MK‐801‐induced LTP deficits and increased basal synaptic transmissions. Western blot analysis revealed that inhibiting K_v7 channels resulted in elevation of pAkt1 and pGSK‐3β expressions in both hippocampus and cortex.

Conclusions and Implications

Both genetic and pharmacological inhibition of K_v7 channels alleviated PPI and cognitive deficits. Mechanistically, inhibition of K_v7 channels promotes synaptic transmission and activates Akt1/GSK‐3β signalling.

Polarity of Hydrated Phosphatidylcholine Headgroups

The headgroup (H) stratum (sometimes called the polar region) of membrane bilayers is a relevant yet poorly understood solvation phase for small molecules and macromolecules interacting with the membranes. Solvation of compounds in bilayer strata is characterized experimentally by wide- and small-angle X-ray scattering, neutron diffraction, and various NMR techniques. The quantification is tedious and only available for a limited set of small molecules. Our recently published model of liposome partitioning of small molecules shows that solvation of compounds in the H-stratum of fluid phosphatidylcholine (PC) bilayers correlates well with their solvation in hydrated diacetyl phosphatidylcholine (DAcPC), and solvation in the core (C) depends in a similar way on that in n-hexadecane. These two correlations became a basis for a model describing the location of compounds in the H- and C-strata and at the connecting interface as a nonlinear function of the fragment solvation characteristics of the compounds. In this study, refractivity of hydrated DAcPC phases with varying water contents was measured and polarity was determined using the steady-state fluorescence of indole and Nile Red. The results were compared with the published data obtained by other techniques for PC bilayers in liposomes or on solid supports. The demonstrated qualitative agreement, as well as the polarity and refractivity dependencies on the DAcPC concentration, supports the suitability of hydrated DAcPC as the H-stratum surrogate. Interestingly, depending on hydrations typical for the H-strata of fluid PC bilayers, the dielectric constant could decrease significantly from 31.0 to 7.3 for 16 and 8 water molecules per headgroup, respectively. Although additional experiments are needed for confirmation, this observation could help set proper dielectric constant magnitudes in continuum-based computational models of accumulation and crossing of the PC bilayers with varying hydration levels thanks to the temperature or the structure of fatty acid chains.

Vesicular glutamate release from central axons contributes to myelin damage

The axon myelin sheath is prone to injury associated with N-methyl-d-aspartate (NMDA)-type glutamate receptor activation but the source of glutamate in this context is unknown. Myelin damage results in permanent action potential loss and severe functional deficit in the white matter of the CNS, for example in ischemic stroke. Here, we show that in rats and mice, ischemic conditions trigger activation of myelinic NMDA receptors incorporating GluN2C/D subunits following release of axonal vesicular glutamate into the peri-axonal space under the myelin sheath. Glial sources of glutamate such as reverse transport did not contribute significantly to this phenomenon. We demonstrate selective myelin uptake and retention of a GluN2C/D NMDA receptor negative allosteric modulator that shields myelin from ischemic injury. The findings potentially support a rational approach toward a low-impact prophylactic therapy to protect patients at risk of stroke and other forms of excitotoxic injury.

Neuronal activity can lead to vesicular release of glutamate. Here the authors demonstrate that vesicular release of glutamate occurs in axons during ischemic conditions, and that an allosteric modulator of GluN2C/D is protective in models of ischemic injury.

Structure-based prediction of drug distribution across the headgroup and core strata of a phospholipid bilayer using surrogate phases

Solvation of drugs in the core (C) and headgroup (H) strata of phospholipid bilayers affects their physiological transport rates and accumulation. These characteristics, especially a complete drug distribution profile across the bilayer strata, are tedious to obtain experimentally, to the point that even simplified preferred locations are only available for a few dozen compounds. Recently, we showed that the partition coefficient (P) values in the system of hydrated diacetyl phosphatidylcholine (DAcPC) and n-hexadecane (C16), as surrogates of the H- and C-strata of the bilayer composed of the most abundant mammalian phospholipid, PC, agree well with the preferred bilayer location of compounds. High P values are typical for lipophiles accumulating in the core, and low P values are characteristic of cephalophiles preferring the headgroups. This simple pattern does not hold for most compounds, which usually have more even distribution and may also accumulate at the H/C interface. To model complete distribution, the correlates of solvation energies are needed for each drug state in the bilayer: (1) for the H-stratum it is the DAcPC/W P value, calculated as the ratio of the C16/W and C16/DAcPC (W for water) P values; (2) for the C-stratum, the C16/W P value; (3) for the H/C interface, the P values for all plausible molecular poses are characterized using the fragment DAcPC/W and C16/W solvation parameters for the parts of the molecule embedded in the H- and C-strata, respectively. The correlates, each scaled by two Collander coefficients, were used in a nonlinear, mass-balance based model of intrabilayer distribution, which was applied to the easily measurable overall P values of compounds in the DMPC (M = myristoyl) bilayers and monolayers as the dependent variables. The calibrated model for 107 neutral compounds explains 94% of experimental variance, achieves similar cross-validation levels, and agrees well with the nontrivial, experimentally determined bilayer locations for 27 compounds. The resulting structure-based prediction system for intrabilayer distribution will facilitate more realistic modeling of passive transport and drug interactions with those integral membrane proteins, which have the binding sites located in the bilayer, such as some enzymes, influx and efflux transporters, and receptors. If only overall bilayer accumulation is of interest, the 1-octanol/W P values suffice to model the studied set.

Retrieval-induced NMDA receptor-dependent Arc expression in two models of cocaine-cue memory

The association of environmental cues with drugs of abuse results in persistent drug-cue memories. These memories contribute significantly to relapse among addicts. While conditioned place preference (CPP) is a well-established paradigm frequently used to examine the modulation of drug-cue memories, very few studies have used the non-preference-based model conditioned activity (CA) for this purpose. Here, we used both experimental approaches to investigate the neural substrates of cocaine-cue memories. First, we directly compared, in a consistent setting, the involvement of cortical and subcortical brain regions in cocaine-cue memory retrieval by quantifying activity-regulated cytoskeletal-associated (Arc) protein expression in both the CPP and CA models. Second, because NMDA receptor activation is required for Arc expression, we investigated the NMDA receptor dependency of memory persistence using the CA model. In both the CPP and CA models, drug-paired animals showed significant increases in Arc immunoreactivity in regions of the frontal cortex and amygdala compared to unpaired controls. Additionally, administration of a NMDA receptor antagonist (MK-801 or memantine) immediately after cocaine-CA memory reactivation impaired the subsequent conditioned locomotion associated with the cocaine-paired environment. The enhanced Arc expression evident in a subset of corticolimbic regions after retrieval of a cocaine-context memory, observed in both the CPP and CA paradigms, likely signifies that these regions: (i) are activated during retrieval of these memories irrespective of preference-based decisions, and (ii) undergo neuroplasticity in order to update information about cues previously associated with cocaine. This study also establishes the involvement of NMDA receptors in maintaining memories established using the CA model, a characteristic previously demonstrated using CPP. Overall, these results demonstrate the utility of the CA model for studies of cocaine-context memory and suggest the involvement of an NMDA receptor-dependent Arc induction pathway in drug-cue memory interference.

Common influences of non-competitive NMDA receptor antagonists on the consolidation and reconsolidation of cocaine-cue memory

RATIONALE

Environmental stimuli or contexts previously associated with rewarding drugs contribute importantly to relapse among addicts, and research has focused on neurobiological processes maintaining those memories. Much research shows contributions of cell surface receptors and intracellular signaling pathways in maintaining associations between rewarding drugs (e.g., cocaine) and concurrent cues/contexts; these memories can be degraded at the time of their retrieval through reconsolidation interference. Much less studied is the consolidation of drug-cue memories during their acquisition.

OBJECTIVE

The present experiments use the cocaine-conditioned place preference (CPP) paradigm in rats to directly compare, in a consistent setting, the effects of N-methyl-D-aspartate (NMDA) glutamate receptor antagonists MK-801 and memantine on the consolidation and reconsolidation of cocaine-cue memories.

METHODS

For the consolidation studies, animals were systemically administered MK-801 or memantine immediately following training sessions. To investigate the effects of these NMDA receptor antagonists on the retention of previously established cocaine-cue memories, animals were systemically administered MK-801 or memantine immediately after memory retrieval.

RESULTS

Animals given either NMDA receptor antagonist immediately following training sessions did not establish a preference for the cocaine-paired compartment. Post-retrieval administration of either NMDA receptor antagonist attenuated the animals' preference for the cocaine-paired compartment. Furthermore, animals given NMDA receptor antagonists post-retrieval showed a blunted response to cocaine-primed reinstatement.

CONCLUSIONS

Using two distinct NMDA receptor antagonists in a common setting, these findings demonstrate that NMDA receptor-dependent processes contribute both to the consolidation and reconsolidation of cocaine-cue memories, and they point to the potential utility of treatments that interfere with drug-cue memory reconsolidation.

Molecular mechanisms of excitotoxicity and their relevance to pathogenesis of neurodegenerative diseases

A pivotal role for excitotoxicity in neurodegenerative diseases is gaining increasingly more acceptance, but the underlying mechanisms through which it participates in neurodegeneration still need further investigation. Excessive activation of glutamate receptors by excitatory amino acids leads to a number of deleterious consequences, including impairment of calcium buffering, generation of free radicals, activation of the mitochondrial permeability transition and secondary excitotoxicity. Recent studies implicate excitotoxicity in a variety of neuropathological conditions, suggesting that neurodegenerative diseases with distinct genetic etiologies may share excitotoxicity as a common pathogenic pathway. Thus, understanding the pathways involved in excitotoxicity is of critical importance for the future clinical treatment of many neurodegenerative diseases. This review discusses the current understanding of excitotoxic mechanisms and how they are involved in the pathogenesis of neurodegenerative diseases.

Partitioning of organic compounds in phases imitating the headgroup and core regions of phospholipid bilayers

Solvation free energies of drugs, peptides, and other small molecules in the core and headgroup regions of phospholipid bilayers determine their conformations, accumulation, and transport properties. The transfer free energy includes the energy terms for the formation of a cavity for the solute, the interactions of the solute with phospholipids, electrostatic interactions of the solute with the membrane, and dipole potentials and entropy terms. The interaction energies with phospholipids can be estimated by correlating the partitioning in surrogate solvent systems and in the bilayer. As the headgroup surrogate, we use diacetylphosphatidylcholine (DAcPC), the acetylated headgroup of the most abundant mammalian phospholipid, phosphatidylcholine, which forms a homogeneous solution with acceptable viscosity when mixed with water in ratios similar to those in the fully hydrated bilayer. The two-phase system of n-hexadecane (C16) as the core surrogate and hydrated DAcPC was used to monitor partitioning of 16 nonionizable compounds. On the bilogarithmic scale, the C16/DAcPC partition coefficients correlate neither with those in the C16/water and 1-octanol/water systems nor with their difference, which is frequently used as a parameter of hydrogen bonding for prediction of the bilayer location of the solutes. The C16/DAcPC system provides a satisfactory emulation of the solvation properties of the bilayer regions, as reflected in correct predictions of the bilayer location for those of the studied chemicals, for which this information is available.

Competitive, reversible inhibition of cytosolic phospholipase A2 at the lipid-water interface by choline derivatives that partially partition into the phospholipid bilayer

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of phospholipids and is believed to play a key cellular role in the generation of arachidonic acid. When assaying the human recombinant cPLA2 using membranes isolated from [3H]arachidonate-labeled U937 cells as substrate, 2-(2'-benzyl-4-chlorophenoxy)ethyl-dimethyl-n-octadecyl-ammonium chloride (compound 1) was found to inhibit the enzyme in a dose-dependent manner (IC50 = 5 microM). It was over 70 times more selective for the cPLA2 as compared with the human nonpancreatic secreted phospholipase A2, and it did not inhibit other phospholipases. Additionally, it inhibited arachidonate production in N-formyl-methionyl-leucyl-phenylalanine-stimulated U937 cells. To further characterize the mechanism of inhibition, an assay in which the enzyme is bound to vesicles of 1,2-dimyristoyl-sn -glycero-3-phosphomethanol containing 6-10 mol % of 1-palmitoyl-2-[1-14C]arachidonoyl-sn-glycero-3-phosphocholine was employed. With this substrate system, the dose-dependent inhibition could be defined by kinetic equations describing competitive inhibition at the lipid-water interface. The apparent equilibrium dissociation constant for the inhibitor bound to the enzyme at the interface (KI*app) was determined to be 0.097 +/- 0.032 mol % versus an apparent dissociation constant for the arachidonate-containing phospholipid of 0.3 +/- 0.1 mol %. Thus, compound 1 represents a novel structural class of inhibitor of cPLA2 that partitions into the phospholipid bilayer and competes with the phospholipid substrate for the active site. Shorter n-alkyl-chained (C-4, C-6, C-8) derivatives of compound 1 were shown to have even smaller KI*app values. However, these short-chained analogs were less potent in terms of bulk inhibitor concentration needed for inhibition when using the [3H]arachidonate-labeled U937 membranes as substrate. This discrepancy was reconciled by showing that these shorter-chained analogs did not partition into the [3H]arachidonate-labeled U937 membranes as effectively as compound 1. The implications for in vivo efficacy that result from these findings are discussed.

N-methyl-D-aspartate receptor function observed by rate of ligand dialysis from proteoliposome solution

This study reports rate-of-dialysis of an iodinated N-methyl-D-aspartate antagonist drug, [125-I] MK-801. from solutions of lipid vesicles and from proteoliposomes containing purified membrane proteins. A 170 kd protein precipitated from proteoliposomes cross reacts with monoclonal antibodies against cloned NMDA-NR2(A) and NR2(B) subunits. Drug binding in proteoliposomes includes contributions from lipid and from protein, in addition to lipid. A significant change in drug binding was observed in proteoliposomes in response to 10 uM agonist, NMDA. Rate-of-dialysis from agonist-stimulated proteoliposomes was sensitive to perturbation by decreased aqueous ligand concentration in a manner consistent with a lipid-mediated receptor/antagonist equilibrium.