Glycine-site antagonists and stroke

Measuring D-serine efflux from mouse cortical brain slices using online microdialysis-capillary electrophoresis

Efflux of a number of important neurochemicals, including D-serine, L-serine, taurine, glutamate, and gamma-aminobutyric acid (GABA), from mouse cortical brain slices housed in a 7 microL perfusion chamber was monitored using online microdialysis-CE (MD-CE). Analyte concentrations could be measured every 20-27 s using the MD-CE instrument. Stimulation with high potassium induced increased release of D-serine. Kainic acid (KA) induced D-serine release, but this release was not blocked by 6-cyano-7-nitroquinoxaline-2,3-dione, suggesting that alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid/KA receptors do not mediate D-serine release. Application of L-serine, the precursor of D-serine, resulted in increased extracellular D-serine concentrations. L-Cysteine also increased extracellular D-serine levels in a partially Na+-dependent manner. The observed effects upon application of L-serine and L-cysteine support the involvement of ASC neutral amino acid transporters in regulating the extracellular concentration of D-serine concentration through competitive inhibition of uptake or increased release through heteroexchange.

Characteristics and Regulation of Glycine Transport in Bergmann Glia

In the vertebrate CNS, glycine acts as an inhibitory neurotransmitter and as the obligatory coagonist of glutamate at N-methyl-D-aspartate receptors. These roles depend on extracellular glycine levels, regulated by Na+/Cl--dependent transporters GLYT1, present mainly in glial cells, and GLYT2, predominantly neuronal. In Bergmann glia, GLYT1 mediates both, glycine uptake and efflux, which, in turn, influences excitatory neurotransmission at Purkinje cell synapses. The biochemical properties of GLYTs and their regulation by signaling pathways in these cells are largely unknown. We characterized Gly uptake in confluent primary cultures of Bergmann glia from chick cerebellum. Transport was found to be energy- and Na+-dependent, and was resolved into a high (Km=25 microM) and a low affinity (Km=1.1 mM) components identified as GLYT1 and transport System A, respectively. Results show that high affinity transport by GLYT1 is regulated by calcium from intracellular stores, calmodulin, and myosin light chain kinase through an actin cytoskeleton-mediated action.

Monitoring d-Serine Dynamics in the Rat Brain Using Online Microdialysis-Capillary Electrophoresis

D-Serine was detected in dialysate collected from the rat striatum using an online microdialysis-CE-LIF instrument. Dialysate can be analyzed every 12.5 s using the online instrument, giving much better temporal resolution than previously possible for D-serine. Basal concentrations of D-serine (8 +/- 2 microM), glutamate (0.8 +/- 0.2 microM), GABA (0.11 +/- 0.04 microM), and L-serine (23 +/- 4 microM) were measured. Increases in the concentrations of these neurochemicals induced by the introduction of high-K+ aCSF were quantitated. Notably, an increase in D-serine concentration in response to high-K+ aCSF was observed for the first time. The identity of the D-serine peak was confirmed unambiguously using D-amino acid oxidase to selectively remove D-serine from a dialysate sample. The microdialysis-CE-LIF instrument was able to monitor this enzymatic reaction as it proceeded over a period of 60 min, demonstrating that online microdialysis-CE-LIF is not only useful in monitoring in vivo dynamics but can also be used to monitor other chemical systems.

Benzoazepine derivative as potent antagonists of the glycine binding site associated to the NMDA receptor

A series of benzoazepine derivatives, bearing suitable substituents at the C-3 position, was designed and evaluated by superimposition with the pharmacophore model of the glycine binding site. To fully explore the SAR of this class of compounds and to allow the preparation of new different compounds at the C-3 position, appropriate synthetic routes were set up. The benzoazepines were evaluated in terms of in vitro affinity using [3H]glycine binding assay and in vivo potency by inhibition of convulsions induced by N-methyl-D-aspartate (NMDA) in mice. This further analysis confirmed the preliminary results previously reported and that compound 27 is the most promising compound (Ki=32 nM, ED(50)=0.09 mg/kg, i.v.) in this series. Significant neuroprotective effect was observed after both pre- and post-ischaemia administration in the MCAo model. In particular, after post-ischaemia administration, it was found to be still effective when the administration was delayed up to 6 h after occlusion of the middle cerebral artery.

Possible mechanisms underlying the protective effects of SY-21, an extract of a traditional Chinese herb, on transient brain ischemia/reperfusion-induced neuronal death in rat hippocampus

We examined the neuroprotective actions of SY-21, a potent ingredient of a traditional Chinese herb, histologically. Transient brain ischemia (15 min) was induced by four-vessel occlusion in Sprague-Dawley rats. Administration of SY-21 20 min before or 6 h after brain ischemia significantly increased the number of surviving hippocampal CA1 pyramidal cells in rats subjected to transient brain ischemia followed by 5 days of reperfusion. Neuronal cell death resulting from ischemic events is associated with abnormal activation of the NMDA receptors. Tyrosine phosphorylation of the NMDA receptor subunit NR2A by Src or Fyn has been implicated in the up-regulation of NMDA receptor activity. In order to investigate the possible mechanism of the neuroprotective action of SY-21, we examined the effects of SY-21 on the ischemia/reperfusion-induced increases in tyrosine phosphorylation of the NMDA receptor subunit 2A (NR2A) and on the interactions involving NR2A, PSD-95 and Src/Fyn. We found that the increase in the tyrosine phosphorylation of NR2A induced by brain ischemia/reperfusion is suppressed by SY-21 administered 15 min before, or instantly after, brain ischemia. Also, SY-21 attenuated the increased interactions involving NR2A, PSD-95, Fyn and Src. These results demonstrate that SY-21 has a prominent neuroprotective action against brain ischemic insult, and the mechanism may involve the regulation of the tyrosine phosphorylation of NR2A by changing the above interactions.

Tricyclic indole-2-carboxylic acids: highly in vivo active and selective antagonists for the glycine binding site of the NMDA receptor

A series of tricyclic indole-2-carboxylic acid derivatives were synthesized and evaluated by the radioligand binding assay and the anticonvulsant effects in the mouse NMDA-induced seizure model. Among them, derivatives of 3S-(-)-4 such as 3a, 3f, and 3g which had certain zwitterionic anilides showed high affinity to the NMDA-glycine binding site. The absolute configuration of 3S-(-)-4 was confirmed by X-ray crystallographic analysis. In particular, 3g (SM-31900) was found to be a highly active glycine antagonist for both in vitro and in vivo assays (K(i) = 1.0 +/- 0.1 nM, ED(50) = 2.3 mg/kg, iv) and also showed high selectivity for the glycine site. In addition, 3g was soluble enough in aqueous media (>10 mg/mL at pH 7.4) to use for medications by intravenous injection.

NMDA receptor pathways as drug targets

Since the mid 1980s, there has been a great deal of enthusiasm within both academia and industry about the therapeutic potential of drugs targeting the NMDA subtype of glutamate receptors. That early promise is just beginning to translate into approvable drugs. Here we review the reasons for this slow progress and critically assess the future prospects for drugs that act on NMDA receptor pathways, including potential treatments for some major disorders such as stroke and Alzheimer's disease, for which effective therapies are still lacking.

Synthesis and pharmacological characterisation of a conformationally restrained series of indole-2-carboxylates as in vivo potent glycine antagonists

After the identification of GV150526, the indole-2-carboxylate template was further explored in order to identify novel potential anti-stroke agents. In particular, the SAR of the side chain present at the C-3 position of the indole nucleus was widely studied. In this paper, the synthesis and the pharmacological profile of a further class of conformationally restricted analogues of GV150526 as in vitro and in vivo potent glycine antagonists is reported. In particular, a pyrazolidinone derivative was identified as a potent neuroprotective agent in animal models of cerebral ischaemia.

Structure-activity relationships of 1,4-dihydro-(1H,4H)-quinoxaline-2,3-diones as N-methyl-D-aspartate (glycine site) receptor antagonists. 1. Heterocyclic substituted 5-alkyl derivatives

A series of 6,7-dichloro-1,4-dihydro-(1H, 4H)-quinoxaline-2,3-diones (1-17) were prepared in which the 5-position substituent was a heterocyclylmethyl or 1-(heterocyclyl)-1-propyl group. Structure-activity relationships were evaluated where binding affinity for the glycine site of the N-methyl-D-aspartate (NMDA) receptor was measured using the specific radioligand [3H]-L-689,560, and functional antagonism was demonstrated by inhibition of NMDA-induced depolarizations of rat cortical wedges. The ability to prevent NMDA-induced hyperlocomotion in mice in vivo was measured for selected compounds. Binding affinity increased significantly if the heterocyclic group, e.g. 1,2,3-triazol-1-yl could participate in accepting a hydrogen bond from the receptor. It was difficult to obtain compounds with adequate aqueous solubility and strategies to improve it were investigated. The most potent compound in this series, 6,7-dichloro-5-[1-(1,2,4-triazol-4-yl)propyl]-1,4-dihydro-(1H, 4H)-quinoxaline-2,3-dione (17) (binding IC50 = 2.6 nM; cortical wedge EC50 = 90 nM), inhibited NMDA-induced hyperlocomotion in mice (6/9 protected at 20 mg/kg iv). Pharmacokinetic parameters, including extent of brain penetration, for 11 and 17 are reported.

Therapeutic potential of anti-inflammatory drugs in focal stroke

The importance of cytokines, especially TNF-alpha and IL-1beta, are emphasised in the propagation and maintenance of the brain inflammatory response to injury. Much data supports the case that ischaemia and trauma elicit an inflammatory response in the injured brain. This inflammatory response consists of mediators (cytokines, chemokines and adhesion molecules) followed by cells (neutrophils early after the onset of brain injury and then a later monocyte infiltration). De novo upregulation of pro-inflammatory cytokines, chemokines and endothelial-leukocyte adhesion molecules occurs soon after focal ischaemia and trauma, as well as at the time when the tissue injury is evolving. The significance of this brain inflammatory response and its contribution to brain injury is now becoming more understood. In this review, we discuss the role of TNF-alpha and IL-1beta in traumatic and ischaemic brain injury and associated inflammation and the co-operative actions of chemokines and adhesion molecules in this process. We also address novel approaches to target cytokines and reduce the brain inflammatory response and thus brain injury, in stroke and neurotrauma. The mitogen-activated protein kinase (MAPK), p38, has been linked to inflammatory cytokine production and cell death following cellular stress. Stroke-induced p38 enzyme activation in the brain has been demonstrated and treatment with a second generation p38 MAPK inhibitor, SB-239063, provides a significant reduction in infarct size, neurological deficits and inflammatory cytokine expression produced by focal stroke. SB-239063 can also provide direct protection of cultured brain tissue to in vitro ischaemia. This robust SB-239063-induced neuroprotection emphasises a significant opportunity for targeting MAPK pathways in ischaemic stroke injury and also suggests that p38 inhibition should be evaluated for protective effects in other experimental models of nervous system injury and neurodegeneration.