strychnine and Brain-Ischemia

During the past decade, converging lines of evidence have linked the abnormal release or leak of excitatory amino acids to the neurodegeneration associated with a wide range of pathologies including cerebral ischemias, Huntington's disease, and AIDS dementia (Coyle and Robinson, 1987; Lipton, 1994; Meldrum, 1994). Pharmacological studies indicate that activation of both ionotropic and metabotropic glutamate receptors can substantially contribute to excitotoxic cell damage (Choi, 1992; Pizzi et al., 1993; Sheardown et al., 1993; Xue et al., 1994). Based on these findings, therapeutic strategies based on blunting or blocking glutamatergic transmission may be useful in treating a variety of neurodegenerative disorders.

Topics: AIDS Dementia Complex; Amino Acids; Amino Acids, Cyclic; Animals; Brain Ischemia; Humans; Huntington Disease; Nerve Degeneration; Neurons; Neuroprotective Agents; Receptors, Glycine; Receptors, N-Methyl-D-Aspartate; Strychnine

To investigate the neuroprotective effect of L-serine and its underlying mechanisms, focal cerebral ischemia was induced in rats by occlusion of middle cerebral artery (MCAO) with a suture, and reperfusion was given by filament withdrawal 2 hr later. Meanwhile, rat hippocampal neurons were primarily cultured, and incubated in serum-free medium in an incubator containing 1% O(2) for hypoxic exposure of 5 hr, or incubated in serum-free medium containing 1 mM glutamate for glutamate exposure of 2 hr. Brain tissue injury and cell damage were then measured. L-serine dose-dependently decreased the neurology deficit score and infarct volume, elevated the cell viability and inhibited the leakage of lactate dehydrogenase. These effects were blocked by strychnine in both MCAO rats and cultured hippocampal neurons. Furthermore, L-serine (168 mg.kg(-1)) reduced the brain water content, permeability of blood-brain barrier, neuronal loss and the expression of activated caspase-3 in the cortex. In addition, L-serine effectively protected the brain from damage when it was administered within 6 hr after the end of MCAO. It is suggested that L-serine could exert a neuroprotective effect on the ischemic-reperfused brain and on the hypoxia- or glutamate-exposed hippocampal neurons, which may be mediated by activating glycine receptors.

Topics: Animals; Brain; Brain Ischemia; Cell Hypoxia; Cells, Cultured; Dose-Response Relationship, Drug; Glutamic Acid; Glycine Agents; Hippocampus; Infarction, Middle Cerebral Artery; Male; Neurons; Neuroprotective Agents; Rats; Rats, Sprague-Dawley; Serine; Strychnine; Time Factors

Recent evidence indicates that strychnine-sensitive glycine receptors are located in upper brain regions including the hippocampus. Because of excitatory effects of glycine via facilitation of NMDA-receptor function, however, the net effects of increased extracellular glycine on neuronal excitability in either physiological or pathophysiological conditions are mostly unclear. Here, we addressed the potential neuroprotective effect of either exogenous application of glycine and taurine, which are both strychnine-sensitive glycine-receptor agonists, or an endogenous increase of glycine via blockade of glycine transporter 1 (GlyT1) by assessing their ability to facilitate the functional recovery of field excitatory postsynaptic potentials (fEPSPs) after termination of brief oxygen/glucose deprivation (OGD) in the CA1 region in mouse hippocampal slices. Glycine and taurine promoted restoration of the fEPSPs after reperfusion, but this was never observed in the presence of strychnine. Interestingly, glycine and taurine appeared to generate neuroprotective effects only at their optimum concentration range. By contrast, blockade of GlyT1 by N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl]sarcosine or sarcosine did not elicit significant neuroprotection. These results suggest that activation of strychnine-sensitive glycine receptors potentially produces neuroprotection against metabolic stress such as OGD. However, GlyT1 inhibition is unlikely to elicit a sufficient increase in the extracellular level of glycine to generate neuroprotection.

Topics: Animals; Brain Ischemia; Excitatory Postsynaptic Potentials; Glycine; Hippocampus; In Vitro Techniques; Male; Mice; Neuroprotective Agents; Receptors, Glycine; Strychnine; Synaptic Transmission; Taurine

The role of the neutral amino acid glycine in excitotoxic neuronal injury is unclear. Glycine coactivates glutamate N-methyl-D-aspartate (NMDA) receptors by binding to a distinct recognition site on the NR1 subunit. Purely excitatory glycine receptors composed of NR1 and NR3/NR4 NMDA receptor subunits have recently been described, raising the possibility of excitotoxic effects mediated by glycine alone. We have previously shown that exposure to high concentrations of glycine causes extensive neurotoxicity in organotypic hippocampal slice cultures by activation of NMDA receptors. In the present study, we investigated further properties of in vitro glycine-mediated toxicity. Agonists of the glycine recognition site of NMDA receptors (D-serine and D-alanine) did not have any toxic effect in hippocampal cultures, whereas competitive blockade of the glycine site by 7-chlorokynurenic acid was neuroprotective. Stimulation (taurine, beta-alanine) or inhibition (strychnine) of the inhibitory strychnine-sensitive glycine receptors did not produce any neurotoxicity. The toxic effects of high-dose glycine were comparable in extent to those produced by the excitatory amino acid glutamate in our model. When combined with sublethal hypoxia/hypoglycemia, the threshold of glycine toxicity was decreased to less than 1 mM, which corresponds to the range of concentrations of excitatory amino acids measured during in vivo cerebral ischemia. Taken together, these results further support the assumption of an active role of glycine in excitotoxic neuronal injury.

Topics: Alanine; Animals; Animals, Newborn; Brain Ischemia; Dose-Response Relationship, Drug; Drug Resistance; Glutamic Acid; Glycine; Hippocampus; Kynurenic Acid; Nerve Degeneration; Neurons; Neuroprotective Agents; Neurotoxins; Organ Culture Techniques; Rats; Rats, Sprague-Dawley; Receptors, N-Methyl-D-Aspartate; Strychnine; Taurine

Although excitotoxic cell death is usually considered a Ca(2+)-dependent process, in certain neuronal systems there is strong evidence that excitotoxic cell death is independent of Ca2+ and is instead remarkably dependent on extracellular Cl-. We have shown (in isolated chick embryo retina) that at least some of the lethal Cl- entry is through GABA and glycine receptors. Here we show that when all the GABA and glycine receptors are blocked by using an appropriate cocktail of inhibitors, agonist-induced excitotoxic cell death can be completely prevented. To determine if ligand-gated Cl- channels contribute to excitotoxic cell death in other neurons, we examined KA-induced cell death in cultured rat cerebellar granule cells. GABA receptor blockade with either a competitive or noncompetitive antagonist provides complete neuroprotection. KA stimulates Cl- uptake by the granule cells, and this is blocked by the GABA antagonists. Granule cell cultures take up [3H]GABA and release it in response to KA treatment. A subpopulation of neurons in the cultures is shown to have GAD and high concentrations of GABA, and this presumably is the source of the GABA that leads to receptor activation and lethal Cl- entry. Finally, we show that retinal cell death due to 1 h of simulated ischemia (combined oxygen and glucose deprivation) is completely prevented by blocking the inhibitory receptors. These results indicate that, paradoxically, excitotoxic cell death is completely dependent on activation of inhibitory receptors, in at least some neuronal systems, and this pathological process may contribute to disease.

Topics: Action Potentials; Animals; Bicuculline; Brain Ischemia; Cell Hypoxia; Cells, Cultured; Cerebellar Cortex; Chick Embryo; Chloride Channels; Chlorides; Excitatory Amino Acid Agonists; GABA Antagonists; Glycine; Ion Channel Gating; Ion Transport; Kainic Acid; Neuroprotective Agents; Patch-Clamp Techniques; Picrotoxin; Rats; Rats, Sprague-Dawley; Receptors, GABA; Receptors, Glycine; Retina; Strychnine

1-Aminocyclopropanecarboxylic acid is a high affinity ligand with partial agonist properties at strychnine-insensitive glycine sites associated with the N-methyl-D-aspartate subtype of glutamate receptors. Since occupation of these sites appears required for operation of N-methyl-D-aspartate, receptor coupled cation channels, it was hypothesized that a glycine partial agonist could function as an N-methyl-D-aspartate antagonist. This hypothesis was examined by evaluating the in vivo and in vitro neuroprotective actions of 1-aminocyclopropanecarboxylic acid. 1-Aminocyclopropanecarboxlic acid (150-600 mg kg-1) administered to gerbils five minutes following twenty minutes of forebrain ischemia significantly improved seven day survival; the optimal dose (300 mg kg-1) increased 7 days survival > 4-fold, from 20% to 92%. Survival of hippocampal CA1 neurons (quantitated 7 days post-ischemia) was significantly (approximately 3-fold) increased by the 600 mg kg-1 dose. Seven day survival was not significantly increased when the interval between reperfusion and drug administration (300 mg kg-1) was increased from 5 to 30 min. In cerebellar granule cell cultures, NMDA combined with a saturating concentration of glycine (10 microM) resulted in a 500% increase in cGMP levels. cGMP levels were increased by 100% over basal when NMDA was combined with a saturating (10 microM) concentration of ACPC, indicating that in this measure, the efficacy of ACPC relative to glycine was approximately 0.2. Consistent with previous findings, 1-aminocyclopropanecarboxylic acid significantly reduced glutamate-induced neurotoxicity in cerebellar granule cell cultures. ACPC was most effective in blocking neurotoxicity at glutamate concentrations producing low to moderate levels of cell death.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Brain Ischemia; Cells, Cultured; Cycloleucine; Female; Gerbillinae; Neuroprotective Agents; Rats; Rats, Sprague-Dawley; Receptors, Glycine; Strychnine