Factors governing the potentiation of NMDA receptor-mediated responses in hippocampus.

A modified medium containing an AMPA receptor antagonist and low concentrations of magnesium was used to investigate the factors governing the potentiation of synaptic responses mediated by NMDA receptors. When long-term potentiation (LTP) was induced in standard medium and NMDA responses were analyzed by changing to the modified medium, no statistically significant differences were observed between potentiated and control pathways. Returning the slices to the standard medium showed that LTP was still present, indicating that the potentiation effect was not reversed by the modified medium. High-frequency stimulation applied in the modified medium produced an enhancement of synaptic responses, but this was not occluded by prior potentiation in standard medium. The degree of potentiation induced in the modified medium and expressed by NMDA responses was larger in the presence than in the absence of inhibition and, unlike LTP, was proportionately larger when recorded in the stratum pyramidale than in the stratum radiatum. These results indicate that the potentiation of NMDA receptor-mediated responses triggered by high-frequency stimulation applied in modified medium differs in several respects from the LTP induced in standard conditions. They confirm that LTP is expressed to a markedly different degree by NMDA and non-NMDA receptors and suggest that events that do not necessarily accompany LTP affect the potentiation of NMDA receptor-dependent synaptic responses.     

Simultaneous Expression of Long-term Depression of NMDA and Long-term Potentiation of AMPA Receptor-mediated Synaptic Responses in the CA1 Area of the Kainic Acid-lesioned Hippocampus

This study investigates the plasticity of the excitatory synapses in an experimental model of epilepsy, the kainic acid-lesioned rat hippocampus. Stimulation of afferents in the CA1 area of lesioned hippocampi produced an epileptiform burst of action potentials, with an underlying synaptic potential composed of mixed α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA; 80%) and N -methyl-D-aspartate (NMDA; 20%) receptor-mediated components. Tetanic stimulation yielded a long-term potentiation (LTP) of the mixed AMPA/NMDA receptor-mediated population excitatory postsynaptic potentials. However, the same type of tetanus resulted in a long-term depression (LTD) of pharmacologically isolated NMDA receptor-mediated responses. This LTD occurred independently of the antagonism of AMPA receptors. This suggests that tetanic stimulation produced LTP of AMPA and LTD of NMDA receptor-mediated responses simultaneously. Finally, both LTP and LTD were shown to be NMDA dependent. This property has profound functional implications for the control of excitatory networks in temporal lobe epilepsy. This work was supported by the Wellcome Trust and the Fondation Simone et Cino Del Duca.     

Glutamate receptor changes associated with transient anoxia/hypoglycaemia in hippocampal slice cultures

Transient anoxia/hypoglycaemia in organotypic hippocampal slice cultures, a model of transient brain ischaemia, ultimately results in delayed cell death. Although the mechanisms underlying this delayed death remain unknown, an increase in excitatory drive has been postulated. We report here that transient anoxia/hypoglycaemia in rat hippocampal slice cultures resulted in a 70-80% enhancement of evoked, alpha-amino-3-hydroxy-5-methyl-4-isoxazolpropionic acid (AMPA) receptor-mediated, excitatory responses lasting over 60 min. This effect was prevented by blockade of N-methyl-d-aspartate (NMDA) receptors, did not involve changes of paired-pulse facilitation ratio, but was associated with a 50% increase in amplitude, but not frequency, of spontaneous miniature excitatory postsynaptic currents (mEPSCs). Consistent with this, paired recordings revealed the appearance of AMPA receptor-mediated EPSCs at previously silent synapses and occlusion by prior induction of long-term potentiation (LTP). Transient anoxia/hypoglycaemia further resulted in a 63% potentiation of evoked NMDA receptor-dependent synaptic responses, accounting for the 20% increase in ratio of AMPA to NMDA responses. No change in rectification properties of AMPA receptor-mediated currents could be detected within the first hour following anoxia/hypoglycaemia-induced potentiation. Western blot analyses of slice cultures exposed to either control conditions or a short anoxia/hypoglycaemia revealed a marked, 50-70% increase of GluR1, GluR2/3 and NR1 subunits 1 h, but not 15 min, after the anoxic/hypoglycaemic episode. This increase was blocked by an inhibitor of protein synthesis. Together these results indicate that a transient anoxia/hypoglycaemia is associated with a marked enhancement of excitatory transmission sharing similarities with the mechanisms underlying LTP, and is correlated with an increased synthesis of excitatory receptor subunits.     

Acute alcohol blocks neurosteroid modulation of synaptic transmission and long-term potentiation in the rat hippocampal slice

Effects of ethanol (22 mM) on the modulation of synaptic transmission and long-term potentiation (LTP) by the neurosteroid dehydroepiandrosterome sulfate (DHEAS; 10 μM) was examined in the in vitro rat hippocampal slice preparation. The synaptic responses were elicited by Schaffer collateral stimulation and recorded extracellularly in the somatic and dendritic regions of CA1 pyramidal neurons. LTP induction produced an increase ( 55% to 75%) in the amplitude of synaptic responses in ethanol and ethanol plus DHEAS (ethanol/DHEAS) treated slices. These increases were significantly smaller than the 130% increase observed previously in slices treated with DHEAS, but were not significantly different from the 82% increase observed in control slices. These results indicate that an ethanol/DHEAS interaction prevents the enhancement of LTP normally observed with DHEAS treatment of hippocampal slices. An ethanol/DHEAS interaction also altered DHEAS's effects on individual synaptic components of the synaptic response to Schaffer collateral stimulation. Ethanol applied before but not after DHEAS prevented DHEAS's enhancement of the NMDA receptor-mediated synaptic component. DHEAS's depression of the GABA_A receptor-mediated synaptic component was also blocked by ethanol. Ethanol or DHEAS individually had no effect on the AMPA receptor-mediated synaptic component, but application of ethanol after DHEAS resulted in a small enhancement of this synaptic component, an effect that was not observed if ethanol was applied before DHEAS. These results show that ethanol and DHEAS interact, altering DHEAS's effects on synaptic transmission and LTP in the hippocampas. Such an interaction may be involved in ethanol's actions on the CNS and raises the possibility that ethanol and DHEAS may act via a common site or pathway.     

Chronic liver failure in rats impairs glutamatergic synaptic transmission and long-term potentiation in hippocampus and learning ability

Cognitive function is impaired in patients with liver disease by unknown mechanisms. Long-term potentiation (LTP) in the hippocampus is considered the basis of some forms of learning and memory. The aims of this work were to assess (i) whether chronic liver failure impairs hippocampal LTP; (ii) if this impairment may be due to alterations in glutamatergic neurotransmission, and (iii) if impairment of LTP is associated with reduced learning ability. It is shown that liver failure in Wistar rats induces the following alterations in the hippocampus; (i) alters the phosphorylation of NMDA and AMPA receptors; (ii) reduces the expression of NMDA and AMPA receptors in membranes, (iii) reduces the magnitude of excitatory postsynaptic potentials (EPSPs) induced by activation of NMDA or AMPA receptors, and (iv) impairs NMDA receptor-dependent LTP. Liver failure also impairs learning of the Morris water maze task. Impairment of glutamatergic synaptic transmission and NMDA receptor-mediated responses may be involved in the alterations of cognitive function in patients with liver disease.     

Genetic enhancement of trace fear memory and cingulate potentiation in mice overexpressing Ca2+/calmodulin-dependent protein kinase IV

Long-term potentiation (LTP) is a key cellular model for studying mechanisms for learning and memory. Previous studies reported that the Ca(2+)/calmodulin-dependent protein kinase IV (CaMKIV) is critical for gene regulation, and behavioral learning and memory. Less is known about the roles of CaMKIV in cortical plasticity and trace fear memory. Here we have found that LTP was significantly enhanced in the anterior cingulate cortex (ACC) of the mice overexpressing CaMKIV. By contrast, neither alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated basal excitatory synaptic transmission nor N-methyl-d-aspartate (NMDA) receptor-mediated excitatory postsynaptic currents were affected. Furthermore, paired-pulse ratio in the transgenic mice is normal. In behavioral tests, we found that the CaMKIV transgenic mice exhibited significant enhancement in trace fear memory, while the acute sensory thresholds were not affected. Our results provide strong evidence that forebrain CaMKIV contributes to trace fear memory by enhancing synaptic potentiation in the ACC.     

Tissue plasminogen activator is required for corticostriatal long-term potentiation

Several experimental data indicate that tissue plasminogen activator (tPA) is involved in memory formation and synaptic plasticity in different brain areas. In the attempt to highlight the role of this serine protease in striatal neuron activity, mice lacking tPA have been used for electrophysiological, immunohistochemical and Western blot experiments. Disruption of tPA gene prevented corticostriatal long-term potentiation, an NMDA-dependent form of synaptic plasticity requiring the stimulation of both dopamine and acetylcholine receptors. Spontaneous and evoked glutamatergic transmission was intact in the striatum of tPA-deficient mice, as was the nigrostriatal dopamine innervation and the expression of dopamine D1 receptors. Conversely, the sensitivity of striatal cholinergic interneurons to dopamine D1 receptor stimulation was lost in these mutants, suggesting that tPA facilitates long-term potentiation (LTP) induction in the striatum by favouring the D1 receptor-mediated excitation of acetylcholine-producing interneurons. The demonstration that tPA ablation interferes with the induction of corticostriatal LTP and with the dopamine receptor-mediated control of cholinergic interneurons might help to explain the altered striatum-dependent learning deficits observed in tPA-deficient mice and provides new insights into the molecular mechanisms underlying synaptic plasticity in the striatum.     

Interleukin-18 stimulates synaptically released glutamate and enhances postsynaptic AMPA receptor responses in the CA1 region of mouse hippocampal slices

The present study examined the effects of the proinflammatory cytokine interleukin-18 (IL-18) on mouse hippocampal synaptic transmission. IL-18 (100 ng/ml) significantly increased amplitude and frequency of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor-mediated miniature excitatory postsynaptic currents (AMPA-mEPSCs), that are monitored from CA1 pyramidal neurons of mouse hippocampal slices. IL-18 (100 ng/ml) enhanced slope of basal field excitatory postsynaptic potentials (fEPSPs) that are recorded from the CA1 region of mouse hippocampal slices. There was no significant difference in the expression of Schaffer collateral/CA1 long-term potentiation (LTP) between in the presence and absence of IL-18, although IL-18 tended to inhibit saturation levels of the potentiation induced by tetanic stimulation in a dose-dependent manner at concentrations ranged from 10 ng/ml to 1 microg/ml. Paired-pulse facilitation in the presence of IL-18 (100 ng/ml) was not influenced after tetanic stimulation, while that in the absence of IL-18 was depressed. The results of the present study, thus, suggest that IL-18 stimulates synaptically released glutamate and enhances postsynaptic AMPA receptor responses in CA1 pyramidal neurons of mouse hippocampal slices, thereby facilitating basal hippocampal synaptic transmission without affecting the LTP.     

Non-fibrillar beta-amyloid abates spike-timing-dependent synaptic potentiation at excitatory synapses in layer 2/3 of the neocortex by targeting postsynaptic AMPA receptors

Cognitive decline in Alzheimer's disease (AD) stems from the progressive dysfunction of synaptic connections within cortical neuronal microcircuits. Recently, soluble amyloid beta protein oligomers (Abeta(ol)s) have been identified as critical triggers for early synaptic disorganization. However, it remains unknown whether a deficit of Hebbian-related synaptic plasticity occurs during the early phase of AD. Therefore, we studied whether age-dependent Abeta accumulation affects the induction of spike-timing-dependent synaptic potentiation at excitatory synapses on neocortical layer 2/3 (L2/3) pyramidal cells in the APPswe/PS1dE9 transgenic mouse model of AD. Synaptic potentiation at excitatory synapses onto L2/3 pyramidal cells was significantly reduced at the onset of Abeta pathology and was virtually absent in mice with advanced Abeta burden. A decreased alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/N-methyl-D-aspartate (NMDA) receptor-mediated current ratio implicated postsynaptic mechanisms underlying Abeta synaptotoxicity. The integral role of Abeta(ol)s in these processes was verified by showing that pretreatment of cortical slices with Abeta((25-35)ol)s disrupted spike-timing-dependent synaptic potentiation at unitary connections between L2/3 pyramidal cells, and reduced the amplitude of miniature excitatory postsynaptic currents therein. A robust decrement of AMPA, but not NMDA, receptor-mediated currents in nucleated patches from L2/3 pyramidal cells confirmed that Abeta(ol)s perturb basal glutamatergic synaptic transmission by affecting postsynaptic AMPA receptors. Inhibition of AMPA receptor desensitization by cyclothiazide significantly increased the amplitude of excitatory postsynaptic potentials evoked by afferent stimulation, and rescued synaptic plasticity even in mice with pronounced Abeta pathology. We propose that soluble Abeta(ol)s trigger the diminution of synaptic plasticity in neocortical pyramidal cell networks during early stages of AD pathogenesis by preferentially targeting postsynaptic AMPA receptors.     

Extracellular Alpha-Synuclein Oligomers Modulate Synaptic Transmission and Impair LTP Via NMDA-Receptor Activation

Parkinson's disease (PD) is the most common representative of a group of disorders known as synucleinopathies, in which misfolding and aggregation of α-synuclein (a-syn) in various brain regions is the major pathological hallmark. Indeed, the motor symptoms in PD are caused by a heterogeneous degeneration of brain neurons not only in substantia nigra pars compacta but also in other extrastriatal areas of the brain. In addition to the well known motor dysfunction in PD patients, cognitive deficits and memory impairment are also an important part of the disorder, probably due to disruption of synaptic transmission and plasticity in extrastriatal areas, including the hippocampus. Here, we investigated the impact of a-syn aggregation on AMPA and NMDA receptor-mediated rat hippocampal (CA3-CA1) synaptic transmission and long-term potentiation (LTP), the neurophysiological basis for learning and memory. Our data show that prolonged exposure to a-syn oligomers, but not monomers or fibrils, increases basal synaptic transmission through NMDA receptor activation, triggering enhanced contribution of calcium-permeable AMPA receptors. Slices treated with a-syn oligomers were unable to respond with further potentiation to theta-burst stimulation, leading to impaired LTP. Prior delivery of a low-frequency train reinstated the ability to express LTP, implying that exposure to a-syn oligomers drives the increase of glutamatergic synaptic transmission, preventing further potentiation by physiological stimuli. Our novel findings provide mechanistic insight on how a-syn oligomers may trigger neuronal dysfunction and toxicity in PD and other synucleinopathies.     

Receptor changes and LTP: an analysis using aniracetam, a drug that reversibly modifies glutamate (AMPA) receptors.

The hypothesis that long-term potentiation (LTP) involves receptor modifications was tested with aniracetam, a nootropic drug that selectively increases currents mediated by the AMPA subclass of glutamate receptors. Aniracetam had different effects on the waveform of synaptic potentials in hippocampus before and after induction of LTP: (1) the drug caused a slight reduction (or delay) of the initial segment of the response after LTP; and (2) the facilitatory effects of aniracetam occurred at a later time point in the response after LTP than before. The interactions between LTP and aniracetam were still present when synaptic responses were greatly reduced by partial blockade of postsynaptic receptors and were not reproduced by increasing release or the number of stimulated synapses. A mathematical treatment of synaptic currents produced the following results: (1) if aniracetam facilitates AMPA receptor currents simply by reducing desensitization, then its complex interaction with LTP emerges when potentiation changes the kinetic and conductance properties of receptor channels; (2) if aniracetam also significantly increases conductance, then the experimental data can be reproduced by modeling LTP as an increase in channel conductance alone.     

Involvement of ryanodine receptors in tetanic sciatic stimulation-induced long-term potentiation of spinal dorsal horn and persistent pain in rats

Tetanic stimulation of the sciatic nerve induces long‐term potentiation (LTP) of C‐fiber‐evoked field potentials in the spinal dorsal horn and persistent pain, suggesting that spinal LTP may be a substrate for central sensitization of the pain pathway. However, its cellular mechanism remains unclear. The present study provides electrophysiological and behavioral evidence for the involvement of ryanodine receptor (RyR) in the induction of spinal LTP and persistent pain in rats. The specific inhibitor of ryanodine receptor, ryanodine and dantrolene, dose dependently blocked the induction, but not maintenance, of spinal LTP and reduced persistent pain behaviors induced by tetanic sciatic stimulation. Both cyclic ADP ribose (cADPR), an endogenous agonist of RyR, and (±)‐1,4‐dihydro‐2,6‐dimethyl‐5‐nitro‐4‐[2‐(trifluromethyl)‐phenyl]‐3‐pyridine carboxylic acid methyl ester (Bay K 8644), an agonist of L‐type calcium channel, attenuated ryanodine‐induced inhibition. Immunohistochemistry and electron microscopic observation showed that RyR subtypes RyR1 and RyR3 were located in the spinal dorsal horn. The results suggest that RyRs are involved in synaptic plasticity of the spinal pain pathway and may be a novel target for treating pain. © 2012 Wiley Periodicals, Inc.     

Myristoylated alanine rich C kinase substrate (MARCKS) heterozygous mutant mice exhibit deficits in hippocampal mossy fiber-CA3 long-term potentiation

The myristoylated alanine-rich C kinase substrate (MARCKS) is a primary protein kinase C (PKC) substrate in brain thought to transduce PKC signaling into alterations in the filamentous (F) actin cytoskeleton. Within the adult hippocampus, MARCKS is highly expressed in the dentate gyrus (DG)-CA3 mossy fiber pathway, but is expressed at low levels in the CA3-CA1 Schaffer collateral-CA1 pathway. We have previously demonstrated that 50% reductions in MARCKS expression in heterozygous Marcks mutant mice produce robust deficits in spatial reversal learning, but not contextual fear conditioning, suggesting that only specific aspects of hippocampal function are impaired by reduction in MARCKS expression. To further elucidate the role of MARCKS in hippocampal synaptic plasticity, in the present study we examined basal synaptic transmission, paired-pulse facilitation, post-tetanic potentiation, and long-term potentiation (LTP) in the hippocampal mossy fiber-CA3 and Schaffer collateral-CA1 pathways of heterozygous Marcks mutant and wild-type mice. We found that LTP is significantly impaired in the mossy fiber-CA3 pathway, but not in the Schaffer collateral-CA1 pathway, in heterozygous Marcks mutant mice, whereas basal synaptic transmission, paired-pulse facilitation, and post-tetanic potentiation are unaffected in both pathways. These findings indicate that a 50% reduction in MARCKS expression impairs processes required for long-term, but not short-term, synaptic plasticity in the mossy fiber-CA3 pathway. The implications of these findings for the role of the mossy fiber-CA3 pathway in hippocampus-dependent learning processes are discussed.     

Glycine-induced changes in synaptic efficacy in hippocampal slices involve changes in AMPA receptors

Brief applications of high glycine concentrations to hippocampal slices have been shown to produce long-lasting changes in synaptic efficacy. In the present study, we show that glycine application transiently and reversibly increases the amplitude and prolongs the duration of synaptic potentials mediated by (NMDA) receptors. The long-lasting changes in synaptic potentials mediated by AMPA receptors are correlated with changes in the binding of [³H]α-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid ([³H]AMPA) to membranes prepared from glycine-treated slices. The changes in binding properties of AMPA receptors in adult slices are due to an increase in affinity of the agonist for the receptor. Furthermore, glycine-induced increases in [³H]AMPA binding and in synaptic potentials in adult hippocampal slices are markedly reduced in the presence of low extracellular calcium or of the phospholipase inhibitor bromophenacylbromide. Finally, glycine-induced potentiation of synaptic potentials is associated with an increased potency of the glutamate receptor antagonist, 6,7-dinitroquinoxaline (DNQX), to inhibit synaptic potentials. The results indicate that glycine-induced changes in synaptic efficacy are likely triggered by the activation of NMDA receptors and expressed by changes in the properties of AMPA receptors. As similar events underly long-term potentiation (LTP), this phenomenon might provide important clues to elucidate the molecular mechanisms involved in LTP maintenance.     

Activation of NMDA receptors in hippocampal area CA1 by low and high frequency orthodromic stimulation and their contribution to induction of long-term potentiation

N-methyl-D-aspartate (NMDA) receptors are important in many instances of synaptic plasticity. In hippocampal area CA1, long-term potentiation (LTP) can be induced by both NMDA receptor-dependent and -independent mechanisms. Using intracellular recordings and single-electrode voltage clamp, we isolated and characterized NMDA receptor-mediated synaptic responses. NMDA receptor-mediated responses evoked by low frequency orthodromic stimulation were inhibited in a dose-dependent manner by the competitive antagonist D,L-2-amino-5-phosphonovaleric acid (APV). High frequency (tetanic) stimulation, which facilitates synaptic release of glutamate, failed to overcome the blockade of NMDA receptors by APV. Using extracellular recordings of field potentials, we studied the contribution of NMDA receptors to LTP induced by different patterns of tetanic stimulation. LTP was inhibited in a dose-dependent manner by APV, but was more sensitive to APV than were NMDA receptor-mediated synaptic responses. This most likely reflects a threshold for NMDA receptor activation in LTP induction. A component of LTP that resisted blockade by APV was induced by high (200 Hz), but not low (25 Hz), frequency tetanization. This NMDA receptor-independent component of LTP persisted for > 4 hours and accounted for approximately half the potentiation induced by 200 Hz tetanization. Procedures necessary to induce LTP at the Schaffer collateral/ commissural synapses in area CA1 by both NMDA receptor-dependent and -independent mechanisms are now well characterized. Using the same neuronal population, it will be possible to ask if processes involved in the maintenance of LTP are shared even when LTP is induced through two different mechanisms. © 1994 Wiley-Liss, Inc.     

Enhancement of AMPA-mediated Synaptic Transmission by the Protein Phosphatase Inhibitor Calyculin A in Rat Hippocampal Slices

Using the phosphatase inhibitor calyculin A, we have examined the influence of phosphorylation on synaptic transmission and plasticity in rat CA1 hippocampal slices. Bath application of 0.5 – 1 μM of calyculin A resulted in an increase of 42.6 ±2.9% in synaptic responses. The effect produced by calyculin A was not accompanied by changes in fibre volley, was not associated with changes in paired-pulse facilitation, and could be reproduced by intracellular injection of the compound, thereby indicating a postsynaptic action. Also, the synaptic enhancement produced by calyculin A was expressed only by potentials mediated by amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, but not by the NMDA responses recorded in the presence of the AMPA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and low magnesium. The effect of calyculin A could be prevented by KN-62, an inhibitor of calcium/calmodulin-dependent protein kinase II. Long-term potentiation could still be induced in the presence of calyculin A, but the effect of the compound was slightly reduced on potentiated compared with control pathways. These results indicate that calyculin A can selectively increase the efficacy of AMPA receptor-mediated synaptic transmission at excitatory synapses.     

The NMDA receptor-mediated components of responses evoked by patterned stimulation are not increased by long-term potentiation

The participation of N-methyl-D-aspartate (NMDA) receptors in synaptic transmission before and after induction of long-term potentiation (LTP) was studied in field CA1 of hippocampal slices. NMDA receptor-mediated postsynaptic responses were determined by comparing responses recorded in the presence and absence of the selective antagonist, D-2-amino-5-phosphonopentanoate (D-AP5, 50 microM). In the presence of physiological magnesium concentrations (1 mM), robust D-AP5-sensitive responses could be evoked by high frequency bursts (4 pulses, 100 Hz) when burst stimulation was preceded 200 ms earlier by 'priming' stimulation (2 pulses, 15 ms apart) of a separate input. Induction of LTP resulted in a substantial potentiation (35%) of non-NMDA-mediated responses to primed bursts but not of NMDA-mediated responses. These results suggest that long-term postsynaptic modifications are at least partly responsible for the expression of LTP.     

Induction and expression of GluA1 (GluR-A)-independent LTP in the hippocampus

Long-term potentiation (LTP) at hippocampal CA3–CA1 synapses is thought to be mediated, at least in part, by an increase in the postsynaptic surface expression of α-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) receptors induced by N -methyl- d -aspartate (NMDA) receptor activation. While this process was originally attributed to the regulated synaptic insertion of GluA1 (GluR-A) subunit-containing AMPA receptors, recent evidence suggests that regulated synaptic trafficking of GluA2 subunits might also contribute to one or several phases of potentiation. However, it has so far been difficult to separate these two mechanisms experimentally. Here we used genetically modified mice lacking the GluA1 subunit ( Gria1 ^−/− mice) to investigate GluA1-independent mechanisms of LTP at CA3–CA1 synapses in transverse hippocampal slices. An extracellular, paired theta-burst stimulation paradigm induced a robust GluA1-independent form of LTP lacking the early, rapidly decaying component characteristic of LTP in wild-type mice. This GluA1-independent form of LTP was attenuated by inhibitors of neuronal nitric oxide synthase and protein kinase C (PKC), two enzymes known to regulate GluA2 surface expression. Furthermore, the induction of GluA1-independent potentiation required the activation of GluN2B (NR2B) subunit-containing NMDA receptors. Our findings support and extend the evidence that LTP at hippocampal CA3–CA1 synapses comprises a rapidly decaying, GluA1-dependent component and a more sustained, GluA1-independent component, induced and expressed via a separate mechanism involving GluN2B-containing NMDA receptors, neuronal nitric oxide synthase and PKC.     

A novel role for calcium-independent phospholipase A in alpha-amino-3-hydroxy-5-methylisoxazole-propionate receptor regulation during long-term potentiation

A considerable body of evidence indicates that phospholipase A(2) (PLA(2)) enzymes participate in long-term potentiation (LTP) of excitatory synaptic transmission. In the present study, we have undertaken experiments to identify which calcium-independent isoform of PLA(2) is involved in synaptic plasticity and to determine whether calcium-independent PLA(2) (iPLA(2)) contributes to post-synaptic processes of LTP. Using field recordings from rat CA1 hippocampal slices, we found that theta-burst stimulation (TBS)-induced LTP of field excitatory post-synaptic potentials (fEPSPs) was abolished by the iPLA(2) inhibitor bromoenol lactone (BEL) but not by the Ca(2+)-dependent PLA(2) inhibitor arachidonyl trifluoromethyl ketone (AACOCF(3)). The ionic currents generated during TBS were not affected during iPLA(2) inhibition as BEL by itself had no effect on the magnitude of facilitation during burst responses. In addition, (R)-BEL, an enantioselective inhibitor of iPLA(2)gamma, precluded TBS-induced LTP, an action that was not replicated by the iPLA(2)beta inhibitors (S)-BEL and methyl arachidonyl fluorophosphonate. (R)-BEL was, however, ineffective on pre-established LTP. Finally, BEL also prevented the potentiation of fEPSPs elicited by brief exposure to 50 microM N-methyl-d-aspartate, as well as the associated up-regulation of alpha-amino-3-hydroxy-5-methylisoxazole-propionate (AMPA) receptor GluR1 subunit levels and the increase of (3)H-AMPA binding in crude synaptic fractions. Collectively, these results unravel a new role for iPLA(2)gamma in LTP, which appears to favor the insertion of AMPA receptors at post-synaptic membranes.     

Modeling glutamatergic synapses: insights into mechanisms regulating synaptic efficacy

The hippocampal formation is critically involved for the long-term storage of various forms of information, and it is widely believed that the phenomenon of long-term potentiation (LTP) of synaptic transmission is a molecular/cellular mechanism participating in memory formation. Although several high level models of hippocampal function have been developed, they do not incorporate detailed molecular information of the type necessary to understand the contribution of individual molecular events to the mechanisms underlying LTP and learning and memory. We are therefore developing new technological tools based on mathematical modeling and computer simulation of the molecular processes taking place in realistic biological networks to reach such an understanding. This article briefly summarizes the approach we are using and illustrates it by presenting data regarding the effects of changing the number of AMPA receptors on various features of glutamatergic transmission, including NMDA receptor-mediated responses and paired-pulse facilitation. We conclude by discussing the significance of these results and providing some ideas for future directions with this approach.