A benzodiazepine recognition site associated with the non-NMDA glutamate receptor.

GYKI 52466 is a benzodiazepine molecule that has muscle relaxant and anticonvulsant properties not attributable to a gamma-aminobutyric acid receptor-mediated mechanism. Here it is shown that GYKI 52466 exerts no blocking action at N-methyl-D-aspartate (NMDA) glutamate receptors, but acts noncompetitively to block ion currents and associated excitotoxicity, including ischemic neuronal degeneration, mediated through non-NMDA glutamate receptors. The inhibition of non-NMDA responses by GYKI 52466 is antagonized by cyclothiazide, hydrochlorothiazide, and diazoxide, benzothiadiazide drugs that inhibit non-NMDA receptor desensitization. These results suggest that non-NMDA receptor-ion channel complexes may contain a novel benzodiazepine recognition site where receptor desensitization is regulated; this postulated site represents a promising new target for rational development of drugs to treat neurological disorders.

Electrophysiologic effects of intracellular lysophosphoglycerides and their accumulation in cardiac lymph with myocardial ischemia in dogs.

Lysophosphatidylcholine (LPC) accumulates in ischemic tissue, and exogenous LPC (20-100 microM) induces electrophysiologic alterations in vitro. However, to determine whether compartmentalization is critical, intracellular pressure microinjection of LPC was performed with simultaneous recording of the transmembrane action potential. Intracellular LPC in concentrations as high as 500 microM (n = 18), calculated based on calibration of injectate volume and cellular volume, did not induce electrophysiologic alterations. The concentrations and efflux of phospholipids and lysophospholipids were assessed in lymph obtained from the supracardiac lymph vessel in anesthetized dogs to assess the extent of extracellular accumulation. Prior to ischemia, phosphatidylcholine (PC) was the major phospholipid in lymph (79 +/- 2%) with substantial quantities of sphingomyelin (11 +/- 2%) and LPC (6 +/- 1%). With ischemia, the concentration of LPC increased by 18%, and net efflux of LPC increased by 24% (P less than 0.01) with no net efflux of PC or other assayed phospholipids. The calculated concentration of LPC increased from 84 to 197 microM in lymph within the ischemic region, a concentration sufficient to induce electrophysiologic derangements.

Prophylaxis of early ventricular fibrillation by inhibition of acylcarnitine accumulation.

Hypoxia in isolated myocytes results in accumulation of long-chain acylcarnitines (LCA) in sarcolemma. Inhibition of carnitine acyltransferase I (CAT-I) with sodium 2-[5-(4-chlorophenyl)-pentyl]-oxirane-2-carboxylate (POCA) prevents both the accumulation of LCA in the sarcolemma and the initial electrophysiologic derangements associated with hypoxia. Another amphiphilic metabolite, lysophosphatidylcholine (LPC), accumulates in the ischemic heart in vivo, in part because of inhibition of its catabolism by accumulating LCA. It induces electrophysiologic alterations in vitro analogous to early changes induced by ischemia in vivo. The present study was performed to determine whether POCA could prevent accumulation of both LCA and LPC induced by ischemia in vivo and if so, whether attenuation of early arrhythmogenesis would result. LAD coronary artery occlusions were induced for 5 min in chloralose-anesthetized cats. Coronary occlusion in untreated control animals elicited prompt, threefold increases of LCA (73 +/- 8 to 286 +/- 60 pmol/mg protein) and twofold increase of LPC (3.3 +/- 0.4 to 7.5 +/- 0.9 nmol/mg protein) selectively in the ischemic zone, associated with ventricular tachycardia (VT) or ventricular fibrillation (VF) occurring within the 5-min interval before acquisition of myocardial samples in 64% of the animals. POCA prevented the increase of both LCA and LPC. It also prevented the early occurrence of VT or VF (within 5 min of occlusion) in all animals studied. The antiarrhythmic effect of POCA was not attributable to favorable hemodynamic changes or to changes in myocardial perfusion measured with radiolabeled microspheres. Thus, inhibition of CAT-I effectively reduced the incidence of lethal arrhythmias induced early after the onset of ischemia. Accordingly, pharmacologic inhibition of this enzyme provides a promising approach for prophylaxis of sudden cardiac death, that typically occurs very soon after the onset of acute ischemia, in man.

Slow ventricular conduction in mice heterozygous for a connexin43 null mutation.

To characterize the role of the gap junction protein connexin43 (Cx43) in ventricular conduction, we studied hearts of mice with targeted deletion of the Cx43 gene. Mice homozygous for the Cx43 null mutation (Cx43 -/-) die shortly after birth. Attempts to record electrical activity in neonatal Cx43 -/- hearts (n = 5) were unsuccessful. Ventricular epicardial conduction of paced beats, however, was 30% slower in heterozygous (Cx43 -/+) neonatal hearts (0.14+/-0.04 m/s, n = 27) than in wild-type (Cx43 +/+) hearts (0.20+/-0.07 m/s, n = 32; P < 0.001). This phenotype was even more severe in adult mice; ventricular epicardial conduction was 44% slower in 6-9 mo-old Cx43 -/+ hearts (0.18+/-0.03 m/s, n = 5) than in wild-type hearts (0.32+/-0.07 m/s, n = 7, P < 0.001). Electrocardiograms revealed significant prolongation of the QRS complex in adult Cx43 -/+ mice (13.4+/-1.8 ms, n = 13) compared with Cx43 +/+ mice (11.5+/-1.4 ms, n = 12, P < 0.01). Whole-cell recordings of action potential parameters in cultured disaggregated neonatal ventricular myocytes from Cx43 -/+ and +/+ hearts showed no differences. Thus, reduction in the abundance of a major cardiac gap junction protein through targeted deletion of a Cx43 allele directly leads to slowed ventricular conduction.

Repeated simulated ischemia and protection against gap junctional uncoupling.

Ischemic preconditioning increases the heart's tolerance to a subsequent longer ischemic period. The aim of this study was to investigate the effect of early and delayed preconditioning on gap junction communication, connexin abundance, and phosphorylation in cultured neonatal rat cardiac myocytes. Prolonged ischemia followed 5 minutes after preconditioning in the early protocol, whereas 20 hours separated preconditioning and prolonged ischemia in the delayed preconditioning protocol. Gap junctional intercellular communication (GJIC) was assessed by Lucifer yellow dye transfer. An initial reduction in communication in response to sublethal ischemia was observed. This may be one mechanism whereby neighboring cells are protected from damaging substances produced during the first phase of subsequent regional ischemia in early preconditioning protocols. With respect to delayed preconditioning, the transient decrease in GJIC disappeared prior to prolonged ischemia, indicating that other mechanisms are responsible for delayed protection. Both early and delayed preconditioning preserved intercellular coupling after prolonged ischemia and this correlated with presence of less connexin43 dephosphorylation assessed by immunoblot.

Connexin expression and turnover : implications for cardiac excitability.

Electrical activation of the heart requires current transfer from one cell to another via gap junctions, arrays of densely packed intercellular channels. The extent to which cardiac myocytes are coupled is determined by multiple mechanisms, including tissue-specific patterns of expression of diverse gap junction channel proteins (connexins), and regulatory pathways that control connexin synthesis, intracellular trafficking, assembly into channels, and degradation. Many connexins, including those expressed in the heart, have been found to turn over rapidly. Recent studies in the intact adult heart suggest that connexin43, the principal cardiac connexin, is surprisingly short-lived (half-life approximately 1.3 hours). Both the proteasome and the lysosome participate in connexin43 degradation. Other ion channel proteins, such as those forming selected voltage-gated K(+) channels, may also exhibit rapid turnover kinetics. Regulation of connexin degradation may be an important mechanism for adjusting intercellular coupling in the heart under normal and pathophysiological conditions.

Dephosphorylation and intracellular redistribution of ventricular connexin43 during electrical uncoupling induced by ischemia.

Electrical uncoupling at gap junctions during acute myocardial ischemia contributes to conduction abnormalities and reentrant arrhythmias. Increased levels of intracellular Ca(2+) and H(+) and accumulation of amphipathic lipid metabolites during ischemia promote uncoupling, but other mechanisms may play a role. We tested the hypothesis that uncoupling induced by acute ischemia is associated with changes in phosphorylation of the major cardiac gap junction protein, connexin43 (Cx43). Adult rat hearts perfused on a Langendorff apparatus were subjected to ischemia or ischemia/reperfusion. Changes in coupling were monitored by measuring whole-tissue resistance. Changes in the amount and distribution of phosphorylated and nonphosphorylated isoforms of Cx43 were measured by immunoblotting and confocal immunofluorescence microscopy using isoform-specific antibodies. In control hearts, virtually all Cx43 identified immunohistochemically at apparent intercellular junctions was phosphorylated. During ischemia, however, Cx43 underwent progressive dephosphorylation with a time course similar to that of electrical uncoupling. The total amount of Cx43 did not change, but progressive reduction in total Cx43 immunofluorescent signal and concomitant accumulation of nonphosphorylated Cx43 signal occurred at sites of intercellular junctions. Functional recovery during reperfusion was associated with increased levels of phosphorylated Cx43. These observations suggest that uncoupling induced by ischemia is associated with dephosphorylation of Cx43, accumulation of nonphosphorylated Cx43 within gap junctions, and translocation of Cx43 from gap junctions into intracellular pools.

Tolerance for ATP-insensitive K(ATP) channels in transgenic mice.

To examine the role of sarcolemmal K(ATP) channels in cardiac function, we generated transgenic mice expressing GFP-tagged Kir6.2 subunits with reduced ATP sensitivity under control of the cardiac alpha-myosin heavy chain promoter. Four founder mice were isolated, and both founders and progeny were all apparently normal and fertile. Electrocardiograms from conscious animals also appeared normal, although mean 24-hour heart rate was approximately 10% lower in transgenic animals compared with littermate controls. In excised membrane patches, K(ATP) channels were very insensitive to inhibitory ATP: mean K(1/2) ([ATP] causing half-maximal inhibition) was 2.7 mmol/L in high-expressing line 4 myocytes, compared with 51 micromol/L in littermate control myocytes. Counterintuitively, K(ATP) channel density was approximately 4-fold lower in transgenic membrane patches than in control. This reduction of total K(ATP) conductance was confirmed in whole-cell voltage-clamp conditions, in which K(ATP) was activated by metabolic inhibition. K(ATP) conductance was not obvious after break-in of either control or transgenic myocytes, and there was no action potential shortening in transgenic myocytes. In marked contrast to the effects of expression of similar transgenes in pancreatic beta-cells, these experiments demonstrate a profound tolerance for reduced ATP sensitivity of cardiac K(ATP) channels and highlight differential effects of channel activity in the electrical activity of the 2 tissues.

Accelerated onset and increased incidence of ventricular arrhythmias induced by ischemia in Cx43-deficient mice.

BACKGROUND: Myocardial ischemia causes profound changes in both active membrane currents and passive electrical properties. Because these complex changes develop and progress concomitantly, it has not been possible to elucidate the relative contributions of any one component to arrhythmogenesis induced by acute ischemia. Cx43+/- mice express 50% of the normal level of connexin43 (Cx43), the major ventricular electrical coupling protein, but are otherwise identical to wild-type (Cx43+/+) mice. Comparison of arrhythmogenesis in Cx43+/- and +/+ mice can provide insights into the role of changes in electrical coupling as an independent variable in the complex setting of acute ischemia. METHODS AND RESULTS: Acute ischemia was induced in isolated perfused mouse hearts by occlusion of the left anterior descending coronary artery. Spontaneous ventricular tachyarrhythmias (VT) occurred in more than twice as many Cx43+/- hearts than Cx43+/+ hearts. VT was induced in nearly 3 times as many Cx43+/- hearts. Multiple runs and prolonged runs of spontaneous VT were more frequent in Cx43+/- hearts. Onset of the first run of VT occurred significantly earlier in Cx43+/- hearts. Premature ventricular beats were also more frequent in Cx43+/- hearts. The size of the hypoperfused region was equivalent in both groups. CONCLUSIONS: Reduced expression of Cx43 accelerates the onset and increases the incidence, frequency, and duration of ventricular tachyarrhythmias after coronary artery occlusion. Thus diminished electrical coupling per se plays a critical role in arrhythmogenesis induced by acute ischemia.

Subcellular distribution of endogenous long chain acylcarnitines during hypoxia in adult canine myocytes.

OBJECTIVE: Previous studies from our laboratory showed a pronounced increase in the sarcolemmal accumulation of long chain acylcarnitines in isolated neonatal rat myocytes after a prolonged (60 minute) hypoxic interval. Because much shorter intervals of hypoxia are associated with electrophysiological alterations in adult cells, the present study was performed to assess the extent of sarcolemmal accumulation of long chain acylcarnitines during hypoxia in adult canine myocytes. METHODS: Cells were incubated (for 24 hours) with 3H-carnitine and the uniformity of incorporation into carnitine fractions was verified biochemically. Cells were exposed to hypoxia (PO2 < 15 mm Hg) for 10 or 20 minutes in the presence or absence of sodium 2-[5-(4-chlorphenyl)-pentyl]-oxirane-2-carboxylate (POCA; 10 microM), an inhibitor of carnitine acyltransferase I. Cells were processed for electron microscopical autoradiography with a technique to spatially fix endogenous long chain acylcarnitines with selective and complete removal of short chain and free carnitine. Grain distributions were analysed by the maximum likelihood method from digitised micrographs. RESULTS: Total mass of long chain acylcarnitines increased ninefold [42.3(3.3) to 374(42) pmol.mg-1 protein] by 10 minutes of hypoxia and 15-fold [to 632(36) nmol.mg-1 protein] by 20 minutes of hypoxia. Normoxic cells exhibited little long chain acylcarnitines in the sarcolemma, and modest amounts in mitochondria and cytoplasm. In hypoxic cells, content of long chain acylcarnitines in mitochondria and cytoplasm increased by a maximum of twofold. By contrast, long chain acylcarnitines increased 100-fold in the sarcolemma to 4.18 x 10(6) molecules.microns-3 after 10 minutes of hypoxia. The increase in long chain acylcarnitines with hypoxia was completely prevented by pretreatment with POCA. CONCLUSION: Hypoxia in adult ventricular myocytes induces a rapid and preferential increase in endogenous long chain acylcarnitines within the sarcolemma.

High resolution optical mapping reveals conduction slowing in connexin43 deficient mice.

UNLABELLED: Analysis of mice with genetically altered expression of cardiac connexins can provide insights into the role of individual gap junction channel proteins in cell-to-cell communication, impulse propagation, and arrhythmias. However, conflicting results have been reported regarding conduction velocity slowing in mice heterozygous for a null mutation in the gene encoding connexin43 (Cx43). METHODS: High-resolution optical mapping was used to record action potentials from 256 sites, simultaneously, on the ventricular surface of Langendorff perfused hearts from 15 heterozygous (Cx43+/-) and 8 wildtype (Cx43+/+) mice (controls). A sensitive method for measuring epicardial conduction velocity was developed to minimize confounding influences of subepicardial breakthrough and virtual electrode effects. RESULTS: Epicardial conduction velocity was significantly slower (23 to 35%, P<0.01) in Cx43+/- mice compared to wildtype. There was no change in conduction patterns or anisotropic ratio (Cx43+/- 1.54+/-0.33; Cx43+/+ 1.57+/-0.17) suggesting that Cx43 expression was reduced uniformly throughout myocardium. The magnitude of reductions in conduction velocity and Cx43 protein expression (45%) were similar in mice in which the null allele occurred in a pure C57BL/6J genetic background versus a mixed (C57BL/6J X 129) background. Action potential duration did not differ between mice of different genotypes. CONCLUSIONS: A approximately 50% reduction of Cx43 expression causes significant conduction velocity slowing in the Cx43+/- mouse heart. The apparent lack of conduction velocity changes reported in previous studies may be related to technical factors rather than variations in genetic background. High-resolution optical mapping is a powerful tool for investigating molecular determinants of propagation and arrhythmias in genetically engineered mice.

Ketone bodies do not directly alter excitatory or inhibitory hippocampal synaptic transmission.

OBJECTIVE: To determine the effect of the ketone bodies beta-hydroxybutyrate (betaHB) and acetoacetate (AA) on excitatory and inhibitory neurotransmission in the mammalian CNS. BACKGROUND: The ketogenic diet is presumed to be an effective anticonvulsant regimen for some children with medically intractable seizures. However, its mechanism of action remains a mystery. According to one hypothesis, ketone bodies have anticonvulsant properties. METHODS: The authors examined the effect of betaHB and AA on excitatory and inhibitory synaptic transmission in rat hippocampal-entorhinal cortex slices and cultured hippocampal neurons. In cultured neurons, their effect was also directly assayed on postsynaptic receptor properties. Finally, their ability to prevent spontaneous seizures was determined in a hippocampal-entorhinal cortex slice model. RESULTS: betaHB and AA did not alter synaptic transmission in these models. CONCLUSIONS: The anticonvulsant properties of the ketogenic diet do not result from a direct effect of ketone bodies on the primary voltage and ligand gated ion channels mediating excitatory or inhibitory neurotransmission in the hippocampus.

Neurologic complications of pediatric lung transplantation.

OBJECTIVE: To report neurologic complications in a large population of pediatric lung transplantation patients. METHODS: A retrospective review of the first 135 patients to undergo lung transplantation at St. Louis Children's Hospital from July 1990 to December 1997. RESULTS: Sixty-one (45%) patients had neurologic complications. The most common presenting symptoms were seizures (27%), followed by encephalopathy, headache, depression, and focal neurologic deficits. Cyclosporine toxicity (7%) and hypoxia-ischemia (7%) constituted the most commonly identified etiologies, followed by stroke, metabolic, and infectious causes. Risk factor analysis found that patients with interstitial lung disease had a higher frequency of hypoxic-ischemic events and patients with seizures had significantly elevated trough cyclosporine levels. Patients with stroke and hypoxia had a poor neurologic prognosis, whereas patients with cyclosporine toxicity uniformly had a good outcome. CONCLUSIONS: Neurologic complications occur frequently after lung transplantation in pediatric patients, with seizures being the most common presenting symptom. Except in patients with stroke and hypoxia, prognosis is generally favorable. Seizures not accompanied by an irreversible structural etiology are unlikely to require long-term treatment with antiepileptic medications. Cyclosporine neurotoxicity typically resolves without requiring discontinuation of immunosuppressive therapy.

Mechanisms contributing to the arrhythmogenic influences of alpha 1-adrenergic stimulation in the ischemic heart.

The majority of deaths associated with ischemic heart disease occur suddenly because of disturbances in cardiac rhythm culminating in ventricular fibrillation. Past research has focused on elucidating the biochemical membrane mechanisms responsible for the adverse electrophysiologic alterations in the ischemic heart, with major emphasis on the influence of adrenergic neural factors. It has been demonstrated that both alpha 1-and beta-adrenergic mechanisms contribute to arrhythmogenesis in the ischemic heart. In the normal heart, alpha 1-adrenergic input has very little effect on electrophysiologic indices. However, during early ischemia and reperfusion, enhanced alpha 1-adrenergic responsivity associated with a twofold reversible increase in alpha 1-adrenergic receptors in vivo has been demonstrated. Likewise, in a variety of species, alpha 1-adrenergic inhibition with prazosin markedly decreases the incidence of malignant ventricular arrhythmias associated with either myocardial ischemia or subsequent reperfusion. One major manifestation of alpha 1-adrenergic receptor activation during reperfusion of ischemic myocardium is an increase in intracellular calcium ion (Ca2+). It has been demonstrated that reperfusion of ischemic myocardium increases intracellular Ca2+ in reversibly injured tissue, and that the gain in intracellular Ca2+ is prevented by alpha 1-adrenergic inhibition with hydroxyphenylethyl aminomethyl tetralone, even when administered just prior to reperfusion. Subsequently, it was demonstrated that the alpha 1-adrenergic-induced increase in mitochondrial Ca2+ contributes to the decline in mitochondrial function. These findings suggest that even single-dose intervention with alpha 1-adrenergic inhibitors may improve markedly the functional recovery and extent of ultimate necrosis in humans after coronary thrombolysis. To investigate the mechanisms responsible for the increase in alpha 1-adrenergic receptors during ischemia, we used isolated adult canine ventricular myocytes exposed to hypoxia. Thirty minutes of hypoxia at 25 degrees C or 10 minutes of hypoxia at 37 degrees C resulted in a threefold reversible increase in the density of surface alpha 1-adrenergic receptors and a threefold increase in the cellular content of long-chain acylcarnitines. Inhibition of carnitine acyltransferase I abolished not only the accumulation of long-chain acylcarnitines during hypoxia but also the increase in alpha 1-adrenergic receptors. Exposure of normoxic myocytes to exogenous long-chain acylcarnitines (1 mumol/liter) for 10 minutes also increased alpha 1-adrenergic receptor number. These findings indicate that the sarcolemmal accumulation of long-cha

The adenylate nucleotide pool in the digestive gland-gonad complex of Biomphalaria glabrata infected by Schistosoma mansoni.

The levels of adenylate nucleotides were examined in the digestive gland and ovotestes of Biomphalaria glabrata during cercarial shedding of Schistosoma mansoni, 10 weeks post-infection. In general, parasitization resulted in decreases in the adenylate levels in both tissues, but the results were not statistically significant. Moreover, the energy charge ratio was not significantly altered. The mean energy charge in ovotestes from uninfected and infected individuals was 0.81 and 0.77, respectively, and that in the digestive gland, 0.70 and 0.66, respectively. Extensive variation was observed in energy charge of digestive gland from infected individuals and this was attributed to possible differences in the degree of infection.

Purine metabolism by the avian malarial parasite Plasmodium lophurae.

Extracts of normal duckling erythrocytes catabolized AMP to IMP, inosine and hypoxanthine; adenosine and adenine were not formed from AMP. When erythrocyte-free Plasmodium lophurae, prepared by antibody lysis, were incubated in the presence of [14C]hypoxanthine approximately 60% of the label was recovered as purine nucleotides and there was not evidence of extracellular alteration of added hypoxanthine. However, when adenosine was added to suspensions of antibody- or saponin-prepared parasites extensive conversion to inosine and hypoxanthine occurred. This conversion was found to be the result of parasite lysis with release of cytosolic purine salvage pathway enzymes; plasmodial surface membrane ecto-enzymes were not responsible for adenosine catabolism. It appears that in vivo the intracellular plasmodium utilizes the normal erythrocytic process of purine turnover to avail itself of hypoxanthine, the red cell's end product, and at the same time the parasite avoids direct competition for adenosine essential to erythrocyte survival. Since the blood plasma of infected ducklings contained increased amounts of hypoxanthine it is possible that P. lophurae also utilizes this as a purine source.

Purine metabolizing enzymes of Plasmodium lophurae and its host cell, the duckling (Anas domesticus) erythrocyte.

Adenosine kinase, adenosine deaminase, hypoxanthine phosphoribosyltransferase, inosine-nucleoside phosphorylase, 5'-AMP deaminase and 5'-IMP nucleotidase were identified in cell-free extracts of duckling erythrocytes; no evidence for 5'-AMP nucleotidase and xanthine oxidase activity was found. The Km values for the duckling red cell enzymes were similar to those reported for human erythrocytes. Plasmodium lophurae extracts demonstrated similar enzyme activities except for 5'-AMP deaminase and 5'-IMP nucleotidase which were absent. It is proposed that during infection erythrocytic AMP is catabolized to IMP, inosine and hypoxanthine; the hypoxanthine is taken up by the plasmodium, utilized to form IMP, and this in turn is converted into adenine and guanine nucleotides.

Nordihydroguaiaretic acid attenuates NMDA neurotoxicity--action beyond the receptor.

There is widespread interest in the neurotoxicity of the endogenous excitatory amino acid neurotransmitter glutamate. Excessive glutamate release or accumulation leads to neuronal injury or death in a variety of experimental models of ischemia, anoxia and hypoglycemia. This injury appears to be caused by overactivation of the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors since a variety of competitive and uncompetitive NMDA antagonists can attenuate this process, sometimes in a dramatic fashion. Given the clinical context in which this form of neuronal injury occurs, it would be desirable if we could identify agents that blocked NMDA toxicity, after initial receptor binding and ion channel fluxes had transpired. Because NMDA receptor activation initiates the arachidonic acid cascade, we have recently looked at whether the phospholipase A2 and lipoxygenase inhibitor nordihydroguaiaretic acid (NDGA) can reduce NMDA neurotoxicity in vitro. In the concentration range 1-30 microM, NDGA diminished the death of cultured rodent hippocampal neurons produced by 100 microM NMDA. When 30 microM NDGA was present both before and after NMDA exposure, death declined by over 50%. NDGA did not block NMDA-induced inward currents in voltage-clamped neurons, so the drug is not a direct NMDA receptor antagonist. It also had no effect on the elevation in intracellular calcium produced by NMDA exposure. It is likely that NDGA acts at a site(s) distal to the NMDA receptor and the neuronal membrane to limit NMDA toxicity. We are hopeful that strategies for limiting excitotoxicity, which halt destructive intracellular events, can be developed for use in human neurological diseases linked to excessive stimulation of glutamate receptors.

Rapid desensitization determines the pharmacology of glutamate neurotoxicity.

Glutamate (Glu), the major excitatory neurotransmitter in the nervous system, is toxic to neurons when it accumulates at high concentrations in the extracellular space. Even though Glu is a mixed agonist, capable of activating N-methyl-D-aspartate (NMDA) receptors and non-NMDA receptors, in many preparations Glu neurotoxicity is prevented by selective blockade of NMDA receptors. In cultures of hippocampal neurons, treatment with 500 microM Glu for 30 min killed more than 90% of the neurons. The simultaneous addition of the selective NMDA agonist methyl-10,11-dihydro-5-H-dibenzocyclo-hepten-5,10-imine (MK-801) reduced the cell loss to less than 30%. However, when Glu was combined with either diazoxide or cyclothiazide, two thiazides which dramatically diminish rapid Glu desensitization, MK-801 was no longer very protective and neuronal loss exceeded 80%. However, the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), in combination with MK-801, was able to prevent most Glu neurotoxicity in the presence of these thiazides. These experiments show that there are circumstances under which Glu neurotoxicity is produced by overactivation of non-NMDA receptors. Our observations offer a possible explanation for the recent finding that blockade of non-NMDA receptors is much more beneficial than NMDA receptor blockade in protecting the brain in some in vivo models of global ischemia.

Allosteric interactions between cyclothiazide and AMPA/kainate receptor antagonists.

1. Cyclothiazide blocks alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor desensitization and potentiates AMPA receptor gated currents. Interactions between cyclothiazide, and the non-competitive antagonist GYKI52466 (GYKI) and competitive antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo (F) quinoxaline (NBQX) were studied at native and recombinant AMPA/kainate receptors using whole-cell recording in order to characterize the modulation by cyclothiazide of these two antagonist sites. 2. GYKI 100 microM, which is sufficient to eliminate virtually hippocampal kainate (100 microM) currents, failed to prevent access of cyclothiazide to its site of potentiation, and was unable to enhance removal of cyclothiazide potentiation. However, cyclothiazide reduced GYKI (30 microM) block from 84 +/- 8.3% to 38 +/- 12%, and slowed the onset of the block with a time course much faster than the time course for onset and offset of potentiation induced by cyclothiazide. Cyclothiazide had qualitatively similar effects upon antagonism by NBQX 1 microM. 3. Kainate activated desensitizing currents in dorsal root ganglion (DRG) neurones, which were unaffected by cyclothiazide. GYKI blocked these kainate currents with lower affinity (IC50 > 120 microM) than for hippocampal neurones (IC50 < 30 microM), and cyclothiazide did not affect GYKI antagonism. 4. Steady-state AMPA currents from homomeric GluRA-Dflip receptors in HEK 293 cells were dramatically potentiated (up to 216 fold) by cyclothiazide via reduction of desensitization. In contrast, kainate-gated currents in HEK 293 cells expressing GluR6R receptors exhibited pronounced desensitization that was unaffected by cyclothiazide. GYKI retains its inhibition at both recombinant AMPA and kainate receptors. 5. These results indicate that cyclothiazide allosterically influences two important antagonist sites on AMPA receptors. In addition, AMPA/kainate receptor subunit composition influences the affinity of GYKI for the receptor.