Ammonia-mediated LTP inhibition: effects of NMDA receptor antagonists and L-carnitine

Because hyperammonemia is thought to contribute to the pathogenesis of hepatic encephalopathy, we examined the effects of ammonia on ATP levels, neuronal morphology, and synaptic function in rat hippocampal slices. Although ammonia did not alter ATP levels supported by 10 mM glucose, ammonia significantly depressed ATP levels in the presence of 3.3 mM glucose or 10 mM pyruvate, suggesting effects on respiratory energy metabolism. Ammonia also impaired synaptic function and neuronal integrity sustained by pyruvate. In 10 mM glucose, ammonia inhibited the induction and maintenance of long-term potentiation (LTP) in a concentration-dependent fashion. These inhibitory effects of ammonia were overcome by L-carnitine. DL-APV, an antagonist of NMDA receptors, also diminished the effects of ammonia on ATP levels and LTP induction, indicating that ammonia impairs neuronal function via altered metabolism and untimely NMDA receptor activation. These results suggest that L-carnitine and NMDA receptor antagonists have the potential to preserve neuronal function during hyperammonemia.     

Pharmacological manipulation of cyclic GMP levels in brain restores learning ability in animal models of hepatic encephalopathy: therapeutic implications

Hepatic encephalopathy (HE) is a complex neuropsychiatric syndrome present in patients with liver disease that includes impaired intellectual function. To develop therapeutic treatments to restore cognitive function, it is important to understand the molecular mechanisms that impair cognitive function in HE. This review summarizes data showing that: (a) cognitive function and learning are impaired in patients with liver disease and in animal models of chronic liver failure or hyperammonemia; (b) the glutamate–NO–cGMP pathway modulates some forms of learning; and (c) the function of this pathway is impaired in brain in vivo in rats with chronic hyperammonemia or liver failure and from patients who died from HE. Learning ability of hyperammonemic rats was restored by increasing cGMP by: (1) continuous intracerebral administration of zaprinast, an inhibitor of the cGMP-degrading phosphodiesterase; (2) chronic oral administration of sildenafil, an inhibitor of the phosphodiesterase that crosses the blood–brain barrier; and (3) continuous intracerebral administration of cGMP. The data summarized indicate that impairment of learning ability in rats with chronic liver failure or hyperammonemia is due to impairment of the glutamate–NO–cGMP pathway. Moreover, increasing extracellular cGMP by pharmacological means may be a new therapeutic approach to improve cognitive function in patients with HE.     

Chronic moderate hyperammonemia impairs active and passive avoidance behavior and conditional discrimination learning in rats

The cerebral dysfunction associated with hepatic encephalopathy is generally considered to have hyperammonemia as one of its main causes. Hyperammonemia impairs the neuronal glutamate-nitric oxide-cyclic GMP pathway and the induction of NMDA receptor-dependent long-term potentiation in the hippocampus. We studied the performance of pre/neonatally and postnatally exposed rats to hyperammonemia on active avoidance, passive avoidance, and conditional discrimination tasks. Pre/neonatal hyperammonemia slowed learning of active avoidance behaviors and impaired memory for the passive avoidance task while postnatal hyperammonemia impaired learning on the conditional discrimination task. Hyperammonemia thus may produce cognitive disturbances that relate to the effects of ammonia on the neuronal glutamate-nitric oxide-cyclic GMP pathway.     

Chronic hyperammonemia impairs the glutamate–nitric oxide–cyclic GMP pathway in cerebellar neurons in culture and in the rat in vivo

The aim of this work was to assess whether ammonia concentrations similar to the increase found in the brain of hyperammonemic rats (100 μm ), impair N-methyl-d -aspartate (NMDA) receptor-mediated signal transduction. We first measured glutamate neurotoxicity, which in these neurons is mediated by activation of NMDA receptors, as an initial parameter reflecting activation of NMDA receptor-mediated pathways. Long-term treatment of cultured neurons with ammonia prevents glutamate-induced neuronal death. The EC₅₀ was 20 μm , and at 100 μm the protection was complete. The induction of the protective effect was not immediate, but took several hours. Treatment with 100 μm ammonia did not prevent a glutamate- or NMDA-induced rise of intracellular calcium. Ammonia impaired the glutamate–nitric oxide–cGMP (3′,5′-cyclic guanosine monophosphate) pathway in a dose- and time-dependent manner. Glutamate-induced formation of cGMP was reduced by 42%, while activation of nitric oxide synthase was not affected. Ammonia reduced by 31% cGMP formation induced by S-nitroso-N-acetyl-penicillamine (SNAP), a NO-generating agent, confirming that the interference occurs at the level of guanylate cyclase activation by nitric oxide. To assess whether chronic moderate hyperammonemia in vivo also impairs the glutamate–nitric oxide–cGMP pathway, we determined by in vivo brain microdialysis in freely moving rats the formation of cGMP induced by NMDA. In hyperammonemic rats, the formation of cGMP induced by NMDA and SNAP was reduced by ca. 60 and 41%, respectively, indicating that chronic hyperammonemia in the animal in vivo also impairs the glutamate–nitric oxide–cGMP pathway. Impairment of this pathway can contribute to the neurological alterations found in hyperammonemia and hepatic encephalopathy.     

Restoration of learning ability in hyperammonemic rats by increasing extracellular cGMP in brain

Intellectual function is impaired in patients with hyperammonemia and hepatic encephalopathy. Chronic hyperammonemia with or without liver failure impairs the glutamate-nitric oxide-cGMP pathway function in brain in vivo and reduces extracellular cGMP in brain as well as the ability of rats to learn a Y maze conditional discrimination task. We hypothesized that the decrease in extracellular cGMP may be responsible for the impairment in learning ability and intellectual function and that pharmacological modulation of the levels of cGMP may restore learning ability. The aim of this work was to try to reverse the impairment in learning ability of hyperammonemic rats by pharmacologically increasing extracellular cGMP in brain. We assessed whether learning ability may be restored by increasing extracellular cGMP in brain by continuous intracerebral administration of: (1) zaprinast, an inhibitor of the phosphodiesterase that degrades cGMP or (2) cGMP. We carried out tests of conditional discrimination learning in a Y maze with control and hyperammonemic rats treated or not with zaprinast or cGMP. Learning ability was reduced in hyperammonemic rats, which needed more trials than control rats to learn the task. Continuous intracerebral administration of zaprinast or cGMP restored the ability of hyperammonemic rats to learn this task. Pharmacological modulation of extracellular cGMP levels in brain may be a useful therapeutic approach to improve learning and memory performance in individuals in whom cognitive abilities are impaired by different reasons, for example in patients with liver disease who present hyperammonemia and decreased intellectual function.     

Pregnenolone Sulfate Restores the Glutamate-Nitric-Oxide-cGMP Pathway and Extracellular GABA in Cerebellum and Learning and Motor Coordination in Hyperammonemic Rats

相似文献   

Taurine rescues hippocampal long-term potentiation from ammonia-induced impairment

Hyperammonemia, a major pathophysiological factor in hepatic encephalopathy, impairs long-term potentiation (LTP) of synaptic transmission, a cellular model of learning and memory, in the hippocampus. We have now studied the protective action of taurine on this paradigm by analyzing LTP characteristics in mouse hippocampal slices treated with ammonium chloride (1 mM) in the presence of taurine (1 mM), an ubiquitous osmolyte, antioxidant, and neuromodulator, as well as other substances with such properties. Ammonia-treated slices displayed a significant impairment of LTP maintenance. Taurine and the mitochondrial enhancer l-carnitine, but not the antioxidants (ascorbate, carnosine, and the novel compound GVS-111) or the osmolyte betaine prevented this impairment. The protective effect of taurine was preserved under the blockade of inhibitory GABA_A and glycine receptors. It is suggested that taurine may rescue the mechanisms of hippocampal synaptic plasticity by improving mitochondrial function under hyperammonemic conditions.     

Genetic and pharmacological demonstration of a role for cyclic AMP-dependent protein kinase-mediated suppression of protein phosphatases in gating the expression of late LTP

Protein kinases and phosphatases play antagonistic roles in regulating hippocampal long-term potentiation (LTP), with kinase inhibition and phosphatase activation both impairing LTP. The late phase of LTP (L-LTP) requires activation of cAMP-dependent protein kinase (PKA) for its full expression. One way in which PKA may critically modulate L-LTP is by relieving an inhibitory constraint imposed by protein phosphatases. Using mutant PKA mice [R(AB) transgenic mice] that have genetically reduced hippocampal PKA activity, we show that deficient L-LTP in area CA1 of mutant hippocampal slices is rescued by acute application of two inhibitors of protein phosphatase-1 and protein phosphatase-2A (PP1/2A) (okadaic acid and calyculin A). Furthermore, synaptic facilitation induced by forskolin, an adenylyl cyclase activator, was impaired in R(AB) transgenics and was also rescued by a PP1/2A inhibitor in mutant slices. Inhibition of PP1/2A did not affect early LTP (E-LTP) or basal synaptic transmission in mutant and wildtype slices. Our data show that genetic inhibition of PKA impairs L-LTP by reducing PKA-mediated suppression of PP1/2A.     

A critical role of NO/cGMP/PKG dependent pathway in hippocampal post-ischemic LTP: modulation by zonisamide

Nitric oxide (NO) is an intercellular retrograde messenger involved in several physiological processes such as synaptic plasticity, hippocampal long-term potentiation (LTP), and learning and memory. Moreover NO signaling is implicated in the pathophysiology of brain ischemia. In this study, we have characterized the role of NO/cGMP signaling cascade in the induction and maintenance of post-ischemic LTP (iLTP) in rat brain slices. Moreover, we have investigated the possible inhibitory action of zonisamide (ZNS) on this pathological form of synaptic plasticity as well as the effects of this antiepileptic drug (AED) on physiological activity-dependent LTP. Finally, we have characterized the possible interaction between ZNS and the NO/cGMP/PKG-dependent pathway involved in iLTP. Here, we provided the first evidence that an oxygen and glucose deprivation episode can induce, in CA1 hippocampal slices, iLTP by modulation of the NO/cGMP/PKG pathway. Additionally, we found that while ZNS application did not affect short-term synaptic plasticity and LTP induced by high-frequency stimulation, it significantly reduced iLTP. This reduction was mimicked by bath application of NO synthase inhibitors and a soluble guanyl cyclase inhibitor. The effect of ZNS was prevented by either the application of a NO donor or drugs increasing intracellular levels of cGMP and activating PKG. These findings are in line with the possible use of AEDs, such as ZNS, as a possible neuroprotective strategy in brain ischemia. Moreover, these findings strongly suggest that NO/cGMP/PKG intracellular cascade might represent a physiological target for neuroprotection in pathological forms of synaptic plasticity such as hippocampal iLTP.     

Both arachidonic acid and 1-oleoyl-2-acetyl glycerol in low magnesium solution induce long-term potentiation in hippocampal CA1 neurons in vitro.

The effects of phospholipase blockers on tetanus-induced long-term potentiation (LTP) and of diacylglycerol (DG) and arachidonic acid (AA) on synaptic transmission were studied in CA1 neurons of guinea pig hippocampal slices to evaluate the role of protein kinase C (PKC) and AA on the maintenance of LTP. Tetanus-induced LTP was suppressed by perfusion with neomycin (1 mM) or 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC, 0.1 mM), blockers of phospholipase. 1-Oleoyl-2-acetyl-glycerol (OAG, 100 micrograms/ml) and AA (100 microM) produced a temporal increase in both the amplitude of the population spike (PS) and the slope of the field excitatory postsynaptic potentials (EPSPs) but failed to produce LTP. Application of OAG or AA in low-Mg2+ (0.1 mM) solution induced LTP. OAG- and AA-induced LTP was blocked by DL-2-amino-phosphopentanoic acid (AP5; 50 microM). The administration of a potent activator of PKC, phorbol-12,13-dibutyrate (PDBu), in low-Mg2+ (0.1 mM) solution enhanced the PS and EPSPs for 2 or 3 h but this enhancement did not persist. These results suggest that PKC activation is not as important as AA for the maintenance of LTP and that OAG and AA play important roles in the maintenance of LTP in synergy with the influx of Ca2+ through NMDA receptor-coupled channels.     

Both arachidonic acid and 1-oleoyl-2-acetyl glycerol in low magensium solution induce long-term potentiation in hippocampal CA1 neurons in vitro

The effects of phospholipase blockers on tetanus-induced long-term potentiation (LTP) and of diacylglycerol (DG) and arachidonic acid (AA) on synaptic transmission were studied in CA1 neurons of guinea pig hippocampal slices to evaluate the role of protein kinase C (PKC) and AA on the maintenance of LTP. Tetanus-induced LTP was suppressed by perfusion with neomycin (1 mM) or 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (NCDC, 0.1 mM), blockers of phospholipase. 1-Oleoyl-2-acetyl-glycerol (OAG, 100 μg/ml) and AA (100 μM) produced a temporal increase in both the amplitude of the population spike (PS) and the slope of the field excitatory postsynaptic potentials (EPSPs) but failed to produce LTP. Application of OAG or AA in low-Mg²⁺ (0.1 mM) solution induced LTP. OAG- and AA-induced LTP was blocked by -2-amino-phosphopentanoic acid (AP5; 50 μM). The administration of a potent activator of PKC, phorbol-12,13-dibutyrate (PDBu), in low-Mg²⁺ (0.1 mM) solution enhanced the PS and EPSPs for 2 or 3 h but this enhancement did not persist. These results suggest that PKC activation is not as important as AA for the maintenance of LTP and that OAG and AA play important roles in the maintenance of LTP in synergy with the influx of Ca²⁺ through NMDA receptor-coupled channels.     

Quantitative correlation between tetanus-induced decreases in extracellular calcium and LTP

Decreases in the extracellular calcium concentration ([Ca2+]o), induced by tetanization of the Schaffer collaterals in rat hippocampal slices, were measured by means of Ca2+-sensitive microelectrodes. The amount of long term potentiation (LTP) of the evoked field potentials, induced by this tetanus, was determined. A positive correlation was found between the amplitude of the tetanus induced decrease in [Ca2+]o and the amount of LTP that was elicited. The N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-phosphonovalerate decreased both the tetanus-induced decreases in [Ca2+]o and the amount of LTP that was induced. We conclude that the amount of Ca2+ that enters the cell during a tetanus is of major importance in the induction process of LTP.     

Maintenance of long‐term potentiation in hippocampal mossy fiber—CA3 pathway requires fine‐tuned MMP‐9 proteolytic activity

Mechanisms of synaptic plasticity involve proteolytic activity mediated by a complex system of proteases, including members of metalloproteinase (MMP) family. In particular, MMP‐9 is critical in LTP maintenance in the Schaffer collateral‐CA1 pathway and in the acquisition of hippocampus‐dependent memory. Recent studies from this laboratory revealed that in the mossy fiber‐CA3 (MF‐CA3) projection, where LTP induction and expression are largely presynaptic, MMPs blockade disrupts LTP maintenance and that LTP induction is associated with increased MMP‐9 expression. Here we used acute brain slices from MMP‐9 knock‐out mice and transgenic rats overexpressing MMP‐9 to determine how manipulations in endogenous MMP‐9 affect LTP in the MF‐CA3 projection. Both types of transgenic models showed a normal basal synaptic transmission and short‐term plasticity. Interestingly, the maintenance of LTP induced in slices from knock‐out mice and overexpressing rats was nearly abolished. However, in the presence of active MMP‐9, a gradual fEPSP autopotentiation was observed and tetanization evoked a marked LTP in knock‐out mice. Additionally, in MMP‐9‐treated slices from wild‐type mice, fEPSP autopotentiation also occurred and partially occluded LTP. This indicates that exogenous protease can restore LTP in null mice whereas in the wild‐type, MMP‐9 excess impairs LTP. We expected that LTP maintenance in transgenic rats could be re‐established by a partial MMP blockade but non‐saturating concentrations of MMP inhibitor were ineffective. In conclusion, we demonstrate that LTP maintenance in MF‐CA3 pathway requires fine‐tuned MMP‐9 activity and raises the possibility that altered MMP‐9 level might be detrimental for cognitive processes as observed in some neuropathologies. © 2013 Wiley Periodicals, Inc.     

The amnesic substance 2-deoxy-d-galactose suppresses the maintenance of hippocampal LTP

Male Wistar rats were intraventricularly injected with 2-doexy-d-galactose (do-gal), a substance interfering with the fucosylation of glycomacromolecules and impairing memory consolidation in various learning tasks. Do-gal was found to have no influence on the monosynaptically evoked field potential (MEFP) recorded in the dentate gyrus upon stimulation of the perforant pathway. However, hippocampal long-term potentiation (LTP) induced in do-gal-pretreated animals by fractionated tetanization of the perforant pathway declined to control levels 2 h after tetanization, whereas it remained constant for 24 h in saline-treated rats. Similar effects were observed in the CA1 region of hippocampal slices. The results indicate a participation of fucosylated macromolecules in the maintenance of LTP. The possible significance of processes involved in LTP memory formation is discussed.     

The amnesic substance 2-deoxy-D-galactose suppresses the maintenance of hippocampal LTP

Male Wistar rats were intraventricularly injected with 2-deoxy-D-galactose (do-gal), a substance interfering with the fucosylation of glycomacromolecules and impairing memory consolidation in various learning tasks. Do-gal was found to have no influence on the monosynaptically evoked field potential (MEFP) recorded in the dentate gyrus upon stimulation of the perforant pathway. However, hippocampal long-term potentiation (LTP) induced in do-gal-pretreated animals by fractionated tetanization of the perforant pathway declined to control levels 2 h after tetanization, whereas it remained constant for 24 h in saline-treated rats. Similar effects were observed in the CA1 region of hippocampal slices. The results indicate a participation of fucosylated macromolecules in the maintenance of LTP. The possible significance of processes involved in LTP for memory formation is discussed.     

Nitric oxide/cyclic guanosine monophosphate signaling in the central complex of the grasshopper brain inhibits singing behavior

Grasshopper sound production, in the context of mate finding, courtship, and rivalry, is controlled by the central body complex in the protocerebrum. Stimulation of muscarinic acetylcholine receptors in the central complex has been demonstrated to stimulate specific singing in various grasshoppers including the species Chorthippus biguttulus. Sound production elicited by stimulation of muscarinic acetylcholine receptors in the central complex is inhibited by co-applications of various drugs activating the nitric oxide/cyclic guanosine monophosphate (cGMP) signaling pathway. The nitric oxide-donor sodium nitroprusside caused a reversible suppression of muscarine-stimulated sound production that could be blocked by 1H-[1,2,4]oxadiazolo-[4,3-a]quinoxaline-1-one (ODQ), which prevents the formation of cGMP by specifically inhibiting soluble guanylyl cyclase. Furthermore, injections of both the membrane-permeable cGMP analog 8-Br-cGMP and the specific inhibitor of the cGMP-degrading phosphodiesterase Zaprinast reversibly inhibited singing. To identify putative sources of nitric oxide, brains of Ch. biguttulus were subjected to both nitric oxide synthase immunocytochemistry and NADPH-diaphorase staining. Among other areas known to express nitric oxide synthase, both procedures consistently labeled peripheral layers in the upper division of the central body complex, suggesting that neurons supplying this neuropil contain nitric oxide synthase and may generate nitric oxide upon activation. Exposure of dissected brains to nitric oxide and 3-(5'hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1) induced cGMP-associated immunoreactivity in both the upper and lower division. Therefore, both the morphological and pharmacological data presented in this study strongly suggest a contribution of the nitric oxide/cGMP signaling pathway to the central control of grasshopper sound production.     

Caspase-3 and calpain: Differently directed involvement in presynaptic long-term plasticity

We studied the effects of the proteolytic enzymes caspase-3 and calpain on the changes in LTP-dependent modifications of paired plasticity in the CA1 field of hippocampal slices. Paired pulse facilitation (PPF) was measured for 1 h after high-frequency stimulation of Schaffer collaterals (100 Hz, 1 s). After LTP induction, we observed a significant decrease in PPF, whereas when LTP was not induced after tetanization a decrease in PPF was also absent. At 1 hour after tetanization, the residual decline in PPF significantly correlated with the efficacy of LTP maintenance. After the end of the experiments we measured the activity of caspase-3 and calpain in the same slices. Potentiation rarely occurred with the low activity of both enzymes and most often came to the end with depression. The probability of LTP induction did not depend on specific activation of any proteases; however, the probability of long-term maintenance of the modifications was significantly higher when the activity of caspase-3, but not of calpain, was elevated. High activity of both proteases simultaneously did not influence the probability of LTP induction; however, under these conditions, LTP maintenance was worse compared to the slices with higher activity of caspase-3. The effect of an LTP-dependent long-term decrease in PPF was predominant in slices with high activity of caspase-3. In contrast, a high activity level of calpain was associated with a less expressed and short-term decrease in PPF. A low activity level of calpain was related to a stable decrease in PPF after LTP induction. Both these effects disappeared under the conditions of high activity of both proteases. In these slices, potentiation was followed by a gradual PPF increase without its initial decrease. Analysis using “caspase-3,” “calpain,” and “LTP maintenance” as factors revealed a significant interaction between the factors. Our data suggest that caspase-3 promotes whereas calpain prevents the maintenance of presynaptic mechanisms of plasticity. The specific effects of proteases may be related to different loci of plasticity or concurrent interaction in the presynaptic zone.     

Inhibition of TEA-Induced LTP by Aluminum

Brief application of tetraethylammonium (TEA) to hippocampal slices causes long-term potentiation (TEA LTP) at synapses of CA1 pyramidal neurons characterized by a long-lasting increase of field excitatory postsynaptic potential (fEPSP) slope and population spike (PS) amplitude. Since this kind of potentiation requires the activation of voltage-dependent calcium channels, we examined the effect of the inorganic calcium channel blocker aluminum, which has been shown to impair tetanus-induced LTP (eLTP). We found that Al inhibited in a concentration-dependent manner both fEPSP slope and PS amplitude potentiation by TEA; 0.68 μg/ml Al attenuated TEA LTP, while a complete block of long-lasting potentiation was obtained for 2.7 μg/ml Al. Occlusion experiments revealed that both concentrations of Al allowed the induction of eLTP 60 min after TEA/Al exposure. However, longer application (15 min) of 2.7 μg/ml Al before the induction of TEA LTP prevented the subsequent induction of eLTP although no significant differences concerning the action on TEA LTP were observed. This indicates a general loss of neuronal plasticity which might be due to progressive neuronal cell damage. Since the effective concentration range of Al is directly comparable to the action of Al on eLTP, our data provide evidence for shared mechanisms of both potentiations. Although based on different induction mechanisms, Ca²⁺is assumed to be a general intracellular trigger for both forms of LTP and thus it can be hypothesized that the neurotoxic action of Al is due to interference with Ca²⁺-dependent processes by inhibition of calcium conductances.     

Long-term potentiation inhibition by low-level N-methyl-D-aspartate receptor activation involves calcineurin, nitric oxide, and p38 mitogen-activated protein kinase

Low-level activation of N-methyl-d-aspartate receptors (NMDARs) results in a decrease in the ability of tetanic stimulation to induce long-term potentiation (LTP). This NMDAR-mediated LTP inhibition is observed with low micromolar concentrations of NMDA or chelation of ambient extracellular zinc. In rat hippocampal slices, we examined whether LTP inhibition by 1 muM NMDA and zinc chelation share common mechanisms. We found that both forms of LTP inhibition involve nitric oxide (NO) synthase (NOS) and calcineurin. Furthermore, both forms of LTP inhibition are overcome by block of p38 mitogen-activated protein kinase (MAPK), but not by inhibition of extracellular signal-regulated kinase 1/2 or c-Jun-N-terminal kinase. A p38 antagonist also overcame the block of LTP by sodium nitroprusside, an agent that releases NO, suggesting that NO release occurs upstream of MAPK activation. Despite the involvement of p38 MAPK in NMDAR-mediated LTP inhibition, p38 antagonism did not enhance LTP induction in response to weak tetanic stimulation under baseline conditions. These results indicate that p38 MAPK is part of a complex NMDAR-driven signaling pathway involving calcineurin and NO that helps to regulate synaptic plasticity in the CA1 region.