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  • Selected Abstracts

    Activation of receptors negatively coupled to adenylate cyclase is required for induction of long-term synaptic depression at Schaffer collateral-CA1 synapses

    Linda A. Santschi
    Abstract Chemical LTD (CLTD) of synaptic transmission is triggered by simultaneously increasing presynaptic [cGMP] while inhibiting PKA. Here, we supply evidence that class II, but not III, metabotropic glutamate receptors (mGluRs), and A1 adenosine receptors, both negatively coupled to adenylate cyclase, play physiologic roles in providing PKA inhibition necessary to promote the induction of LTD at Schaffer collateral-CA1 synapses in hippocampal slices. Simultaneous activation of group II mGluRs with the selective agonist (2S,2,R,3,R)-2-(2,,3,-dicarboxy-cyclopropyl) glycine (DCGIV; 5 ,M), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 ,M), resulted in a long-lasting depression of synaptic strength. When zaprinast (20 ,M) was combined with a cell-permeant PKA inhibitor H-89 (10 ,M), the need for mGluR IIs was bypassed. DCGIV, when combined with a "submaximal" low frequency stimulation (1 Hz/400 s), produced a saturating LTD. The mGluR II selective antagonist, (2S)-alpha-ethylglutamic acid (EGLU; 5 ,M), blocked induction of LTD by prolonged low frequency stimulation (1 Hz/900 s). In contrast, the mGluR III selective receptor blocker, (RS)-a-Cyclopropyl-[3- 3H]-4-phosphonophenylglycine (CPPG; 10 ,M), did not impair LTD. The selective adenosine A1 receptor antagonist, 1,3-dipropyl-8-cyclopentylxanthine (DPCPX; 100 nM), also blocked induction of LTD, while the adenosine A1 receptor agonist N6 -cyclohexyl adenosine (CHA; 50 nM) significantly enhanced the magnitude of LTD induced by submaximal LFS and, when paired with zaprinast (20 ,M), was sufficient to elicit CLTD. Inhibition of PKA with H-89 rescued the expression of LTD in the presence of either EGLU or DPCPX, confirming the hypothesis that both group II mGluRs and A1 adenosine receptors enhance the induction of LTD by inhibiting adenylate cyclase and reducing PKA activity. 2005 Wiley Periodicals, Inc. J Neurobiol, 2006 [source]

    Are CB1 receptor antagonists nootropic or cognitive impairing agents?

    Stephen A. Varvel
    Abstract For more than a decade, a considerable amount of research has examined the effects of rimonabant (SR 141716) and other CB1 receptor antagonists in both in vivo and in vitro models of learning and memory. In addition to its utility in determining whether the effects of drugs are mediated though a CB1 receptor mechanism of action, these antagonists are useful in providing insight into the physiological function of the endogenous cannabinoid system. Several groups have reported that CB1 receptor antagonists enhance memory duration in a variety of spatial and operant paradigms, but not in all paradigms. Conversely, disruption of CB1 receptor signaling also impairs extinction learning in which the animal actively suppresses a learned response when reinforcement has been withheld. These extinction deficits occur in aversively motivated tasks, such as in fear conditioning or escape behavior in the Morris water maze task, but not in appetitively motivated tasks. Similarly, in electrophysiological models, CB1 receptor antagonists elicit a variety of effects, including enhancement of long-term potentiation (LTP), while disrupting long-term depression (LTD) and interfering with transient forms of plasticity, including depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE). The collective results of the in vivo and in vitro studies employing CB1 receptor antagonists, demonstrate that these receptors play integral roles in different components of cognitive processing. Functionally, pharmacological blockade of CB1 receptors may strengthen memory duration, but interferes with extinction of learned behaviors that are associated with traumatic or aversive memories. Drug Dev Res 70:555,565, 2009. 2009 Wiley-Liss, Inc. [source]

    Electrical and Chemical Long-term Depression Do Not Attenuate Low-Mg2+,induced Epileptiform Activity in the Entorhinal Cortex

    EPILEPSIA, Issue 4 2005
    Jrg Solger
    Summary:,Purpose: Low-frequency electrical and magnetic stimulation of cortical brain regions has been shown to reduce cortical excitability and to decrease the susceptibility to seizures in humans and in vivo models of epilepsy. The induction of long-term depression (LTD) or depotentiation of a seizure-related long-term potentiation has been proposed to be part of the underlying mechanism. With the low-Mg2+ -model of epilepsy, this study investigated the effect of electrical LTD, chemical LTD, and depotentiation on the susceptibility of the entorhinal cortex to epileptiform activity. Methods: The experiments were performed on isolated entorhinal cortex slices obtained from adult Wistar rats and mice. With extracellular recording techniques, we studied whether LTD induced by (a) three episodes of low-frequency paired-pulse stimulation (3 900 paired pulses at 1 Hz), and by (b) bath-applied N -methyl- d -aspartate (NMDA, 20 ,M) changes time-to-onset, duration, and frequency of seizure-like events (SLEs) induced by omitting MgSO4 from the artificial cerebrospinal fluid. Next we investigated the consequences of depotentiation on SLEs themselves by applying low-frequency stimulation after onset of low-Mg2+,induced epileptiform activity. Results: LTD, induced either by low-frequency stimulation or by bath-applied NMDA, had no effect on time-to-onset, duration, and frequency of SLEs compared with unconditioned slices. Low-frequency stimulation after onset of SLEs did not suppress but induced SLEs that lasted for the time of stimulation and were associated with a simultaneous increase of the extracellular K+ concentration. Conclusions: Our study demonstrates that neither conditioning LTD nor brief low-frequency stimulation decreases the susceptibility of the entorhinal cortex to low-Mg2+,induced epileptiform activity. The present study does not support the hypothesis that low-frequency brain stimulation exerts its anticonvulsant effect via the induction of LTD or depotentiation. [source]

    N -methyl- d -aspartate receptor- and metabotropic glutamate receptor-dependent long-term depression are differentially regulated by the ubiquitin-proteasome system

    Ami Citri
    Abstract Long-term depression (LTD) in CA1 pyramidal neurons can be induced by activation of either N -methyl- d -aspartate receptors (NMDARs) or metabotropic glutamate receptors (mGluRs), both of which elicit changes in synaptic efficacy through ,-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate receptor (AMPAR) endocytosis. To address the role of the ubiquitin-proteasome system in regulating AMPAR endocytosis during these forms of LTD, we examined the effects of pharmacological inhibitors of proteasomal degradation and protein ubiquitination on endocytosis of glutamate receptor 1 (GluR1) -containing AMPARs in dissociated rat hippocampal cultures as well as LTD of excitatory synaptic responses in acute rat hippocampal slices. Our findings suggest that the contribution of the ubiquitin-proteasome system to NMDAR-induced vs. mGluR-induced AMPAR endocytosis and the consequent LTD differs significantly. NMDAR-induced AMPAR endocytosis and LTD occur independently of proteasome function but appear to depend, at least in part, on ubiquitination. In contrast, mGluR-induced AMPAR endocytosis and LTD are enhanced by inhibition of proteasomal degradation, as well as by the inhibitor of protein ubiquitination. Furthermore, the decay of mGluR-induced membrane depolarization and Erk activation is delayed following inhibition of either ubiquitination or proteasomal degradation. These results suggest that, although NMDAR-dependent LTD may utilize ubiquitin as a signal for AMPAR endocytosis, mGluR-induced signaling and LTD are limited by a feedback mechanism that involves the ubiquitin-proteasome system. [source]

    Stress reverses plasticity in the pathway projecting from the ventromedial prefrontal cortex to the basolateral amygdala

    Mouna Maroun
    Abstract We have previously shown that high-frequency stimulation to the basolateral amygdala (BLA) induces long-term potentiation (LTP) in the ventromedial prefrontal cortex (vmPFC) and that prior exposure to inescapable stress inhibits the induction of LTP in this pathway [Maroun & Richter-Levin (2003)J. Neurosci., 23, 4406,4409]. Here, we show that the reciprocal pathway projecting from the vmPFC to the BLA is resistant to the induction of LTP. Conversely, long-term depression (LTD) is robustly induced in the BLA in response to low-frequency stimulation to the vmPFC. Furthermore, prior exposure to inescapable stress reverses plasticity in this pathway, resulting in the promotion of LTP and the inhibition of LTD. Our findings suggest that, under normal and safe conditions, the vmPFC is unable to exert excitatory synaptic plasticity over the BLA; rather, LTD, which encodes memory of safety in the BLA, is favoured. Following stressful experiences, LTP in the BLA is promoted to encode memory of fear. [source]

    Long-term depression activates transcription of immediate early transcription factor genes: involvement of serum response factor/Elk-1

    Antje Lindecke
    Abstract Long-term depression (LTD) is one of the paradigms used in vivo or ex vivo for studying memory formation. In order to identify genes with potential relevance for memory formation we used mouse organotypic hippocampal slice cultures in which chemical LTD was induced by applications of 3,5-dihydroxyphenylglycine (DHPG). The induction of chemical LTD was robust, as monitored electrophysiologically. Gene expression analysis after chemical LTD induction was performed using cDNA microarrays containing >7000 probes. The DHPG-induced expression of immediate early genes (c-fos, junB, egr1 and nr4a1) was subsequently verified by TaqMan polymerase chain reaction. Bioinformatic analysis suggested a common regulator element [serum response factor (SRF)/Elk-1 binding sites] within the promoter region of these genes. Indeed, here we could show a DHPG-dependent binding of SRF at the SRF response element (SRE) site within the promoter region of c-fos and junB. However, SRF binding to egr1 promoter sites was constitutive. The phosphorylation of the ternary complex factor Elk-1 and its localization in the nucleus of hippocampal neurones after DHPG treatment was shown by immunofluorescence using a phosphospecific antibody. We suggest that LTD leads to SRF/Elk-1-regulated gene expression of immediate early transcription factors, which could in turn promote a second broader wave of gene expression. [source]

    Group I metabotropic glutamate receptors regulate the frequency,response function of hippocampal CA1 synapses for the induction of LTP and LTD

    Els J. M. Van Dam
    Abstract Synaptically released glutamate binds to ionotropic or metabotropic glutamate receptors. Metabotropic glutamate receptors (mGluRs) are G-protein-coupled receptors and can be divided into three subclasses (Group I,III) depending on their pharmacology and coupling to signal transduction cascades. Group I mGluRs are coupled to phospholipase C and are implicated in several important physiological processes, including activity-dependent synaptic plasticity, but their exact role in synaptic plasticity remains unclear. Synaptic plasticity can manifest itself as an increase or decrease of synaptic efficacy, referred to as long-term potentiation (LTP) and long-term depression (LTD). The likelihood, degree and direction of the change in synaptic efficacy depends on the history of the synapse and is referred to as ,metaplasticity'. We provide direct experimental evidence for an involvement of group I mGluRs in metaplasticity in CA1 hippocampal synapses. Bath application of a low concentration of the specific group I agonist 3,5-dihydroxyphenylglycine (DHPG), which does not affect basal synaptic transmission, resulted in a leftward shift of the frequency,response function for the induction of LTD and LTP in nave synapses. DHPG resulted in the induction of LTP at frequencies which induced LTD in control slices. These alterations in the induction of LTD and LTP resemble the metaplastic changes observed in previously depressed synapses. In addition, in the presence of DHPG additional potentiation could be induced after LTP had apparently been saturated. These findings provide strong evidence for an involvement of group I mGluRs in the regulation of metaplasticity in the CA1 field of the hippocampus. [source]

    Bidirectional synaptic plasticity as a consequence of interdependent Ca2+ -controlled phosphorylation and dephosphorylation pathways

    Pablo D'Alcantara
    Abstract Postsynaptic Ca2+ signals of different amplitudes and durations are able to induce either long-lasting potentiation (LPT) or depression (LTD). The bidirectional character of synaptic plasticity may result at least in part from an increased or decreased responsiveness of the glutamatergic ,-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor (AMPA-R) due to the modification of conductance and/or channel number, and controlled by the balance between the activities of phosphorylation and dephosphorylation pathways. AMPA-R depression can be induced by a long-lived Ca2+ signal of moderate amplitude favouring the activation of the dephosphorylation pathway, whereas a shorter but higher Ca2+ signal would induce AMPA-R potentiation resulting from the preferential activation of the phosphorylation pathway. Within the framework of a model involving calcium/calmodulin-dependent protein kinase II (CaMKII), calcineurin (PP2B) and type 1 protein phosphatase (PP1), we aimed at delineating the conditions allowing a biphasic U-shaped relationship between AMPA-R and Ca2+ signal amplitude, and thus bidirectional plasticity. Our theoretical analysis shows that such a property may be observed if the phosphorylation pathway: (i) displays higher cooperativity in its Ca2+ -dependence than the dephosphorylation pathway; (ii) displays a basal Ca2+ -independent activity; or (iii) is directly inhibited by the dephosphorylation pathway. Because the experimentally observed inactivation of CaMKII by PP1 accounts for this latter characteristic, we aimed at verifying whether a realistic model using reported parameters values can simulate the induction of either LTP or LTD, depending on the time and amplitude characteristics of the Ca2+ signal. Our simulations demonstrate that the experimentally observed bidirectional nature of Ca2+ -dependent synaptic plasticity could be the consequence of the PP1-mediated inactivation of CaMKII. [source]

    Hippocampal long-term depression as an index of spatial working memory

    Kazuhito Nakao
    Abstract Long-term potentiation (LTP), a form of synaptic plasticity in the hippocampus, is a cellular model for the neural basis of learning and memory, but few studies have investigated the contribution of long-term depression (LTD), a counterpart of LTP. To address the possible relationship between hippocampal LTD and spatial performance, the spatial cognitive ability of a rat was assessed in a spontaneous alternation test and, thereafter, LTD in response to low-frequency burst stimulation (LFBS) was monitored in the dentate gyrus of the same rat under anaesthesia. To enhance a divergence in the ability for spatial performance, some of the animals received fimbria,fornix (FF) transection 14 days before the experiments. LTD was reliably induced by application of LFBS to the medial perforant path of intact rats, while no apparent LTD was elicited in rats with FF lesions. The behavioural parameters of spatial memory showed a significant correlation with the magnitude of LTD. We found no evidence that the cognitive ability correlated with other electrophysiological parameters, e.g. basal synaptic responses, stimulus intensity to produce half-maximal responses, paired-pulse facilitation or paired-pulse depression. These results suggest that the magnitude of LTD in the dentate gyrus serves as a reliable index of spatial cognitive ability, providing insights into the functional significance of hippocampal LTD. [source]

    NMDA receptor-mediated metaplasticity during the induction of long-term depression by low-frequency stimulation

    Bruce Mockett
    Abstract Metaplasticity refers to the activity-dependent modification of the ability of synapses to undergo subsequent synaptic plasticity. Here, we have addressed the question of whether metaplasticity contributes to the induction of long-term depression (LTD) by low-frequency stimulation (LFS). The experiments were conducted using standard extracellular recording techniques in stratum radiatum of area CA1 in hippocampal slices made from adult Sprague,Dawley rats. The degree of LTD induction was found to be a nonlinear function of the number of pulses during a 1-Hz LFS. Little LTD was observed following 600 or 900 pulses, but a significant LTD occurred following 1200 pulses of LFS, whether delivered in one episode, or in two bouts of 600 pulses given 10 min apart. A similar pattern was observed for 3 Hz LFS. The data support the suggestion that pulses occurring early in the LFS train prime synapses for LTD induction, as triggered by later occurring stimuli. The priming effect lasted at least 120 min, when tested by giving two bouts of 1 Hz LFS (600 pulses each) at different intervals. Neither heterosynaptic nor homosynaptic stimulation by itself was sufficient to prime LTD. However, a combination of the stimuli, induced by increased stimulus strength during the LFS, appeared necessary for inducing the effect. An N -methyl- d -aspartate (NMDA) receptor antagonist markedly reduced total LTD induction, regardless of whether it was administered during the first or second LFS in a protocol employing two bouts of 600 pulse LFS, 30 min apart. These findings strongly support the hypothesis that NMDA receptor-dependent metaplasticity processes contribute to the induction of LTD during standard LFS protocols. [source]

    Properties of LTD and LTP of retinocollicular synaptic transmission in the developing rat superior colliculus

    Fu-Sun Lo
    Abstract The developing retinocollicular pathway undergoes synaptic refinement in order to form the precise retinotopic pattern seen in adults. To study the mechanisms which underlie refinement, we investigated long-term changes in retinocollicular transmission in rats aged P0,P25. Field potentials (FPs) in the superior colliculus (SC) were evoked by stimulation of optic tract fibers in an in vitro isolated brainstem preparation. High intensity stimulation induced long-term depression (LTD) in the SC after both low (1000 stimuli at 1 Hz) and higher (1000 stimuli at 50 Hz) frequency stimulation. The induction of LTD was independent of activation of NMDA and GABAA receptors, because d -APV (100 M) and bicuculline (10 M) did not block LTD. Induction of LTD was dependent upon activation of l -type Ca2+ channels as 10 M nitrendipine, an l -type Ca2+ channel blocker, significantly decreased the magnitude of LTD. LTD was down-regulated during development. LTD magnitude was greatest in rats aged P0,P9 and significantly less in rats aged P10,P25. Long-term potentiation (LTP) was induced by low intensity stimulation and only after high frequency tetanus (1000 stimuli at 50 Hz). LTP was NMDA receptor dependent because d -APV (100 ,M) completely abolished it. LTP induction was also blocked by the l -type Ca2+ channel blocker nitrendipine. The magnitude of LTP first increased with age, being significantly greater at P7,P13 than at P0,3 and then decreased at P23,25. In summary, both LTD and LTP are present during retinocollicular pathway refinement, but have different transmitter and ionic mechanisms and time courses of expression. [source]

    Impairment of eyeblink conditioning in GluR,2-mutant mice depends on the temporal overlap between conditioned and unconditioned stimuli

    Yasushi Kishimoto
    Abstract Mice lacking the glutamate receptor subunit ,2 (GluR,2) are deficient in cerebellar long-term depression (LTD) at the parallel fibre,Purkinje cell synapses. We conducted delay and trace eyeblink conditioning with these mice, using various temporal intervals between the conditioned stimulus (CS) and unconditioned stimulus (US). During trace conditioning in which a stimulus-free trace interval (TI) of 250, 100 or 50 ms intervened between the 352-ms tone CS and 100-ms US, GluR,2-mutant mice learned as successfully as wild-type mice. Even in the paradigm with TI = 0 ms, in which the end of CS and onset of US are simultaneous, there was no difference between the GluR,2-mutant and wild-type mice in their acquisition of a conditioned response. However, in the delay paradigm in which the 452-ms CS overlapped temporally with the coterminating 100-ms US, GluR,2-mutant mice exhibited severe learning impairment. The present study together with our previous work [Kishimoto, Y., Kawahara, S., Suzuki, M., Mori, H., Mishina, M. & Kirino, Y. (2001) Eur. J. Neurosci.,13, 1249,1254], indicates that cerebellar LTD-independent learning is possible in paradigms without temporal overlap between the CS and US. On the other hand, GluR,2 and cerebellar LTD are essential for learning when there is CS,US temporal overlap, suggesting that the cerebellar neural substrates underlying eyeblink conditioning may change, depending on the temporal overlap of the CS and US. [source]

    Differential induction of LTP and LTD is not determined solely by instantaneous calcium concentration: an essential involvement of a temporal factor

    Tomoyuki Mizuno
    Abstract Two opposite types of synaptic plasticity in the CA1 hippocampus, long-term potentiation (LTP) and long-term depression (LTD), require postsynaptic Ca2+ elevation. To explain these apparently contradictory phenomena, the current view assumes that a moderate postsynaptic increase in Ca2+ leads to LTD, whereas a large increase leads to LTP. No detailed study has so far been attempted to investigate whether the instantaneous Ca2+ elevation level differentially induces LTP or LTD. We therefore used low-frequency (1 Hz) stimulation of Schaffer collateral/commissural fibers in rat hippocampal slices, during a Mg2+ -free period, as the conditioning stimulus to investigate this. This allowed low-frequency afferent stimulation to cause a postsynaptic Ca2+ influx because the voltage-dependent block of N -methyl- d -aspartate (NMDA) receptor-channels by Mg2+ was removed. When delivered during the Mg2+ -free period, a single pulse, as well as 2,600 pulses, induced LTP that was occluded with tetanus-induced LTP. To decrease the Ca2+ influx, ,-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors were completely blocked by the addition of 10 m 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to the conditioning medium, in which 1 Hz afferent stimuli (1,600 pulses) induced less LTP and never induced LTD. To further reduce the Ca2+ influx, NMDA receptors were partially blocked with d -(,)-2-amino-5-phosphonopentanoic acid (d -AP5). A small number of 1 Hz stimuli, however, never induced LTD. Only when the conditioning stimuli exceeded 200 pulses was LTD induced. The present findings provide definitive evidence that protracted conditioning is a prerequisite for the induction of LTD. Thus, not only the amplitude but also the duration of postsynaptic Ca2+ elevation could be essential factors for differentially inducing LTP or LTD. [source]

    Deficient long-term synaptic depression in the rostral cerebellum correlated with impaired motor learning in phospholipase C ,4 mutant mice

    Mariko Miyata
    Abstract Long-term depression (LTD) at parallel fibre,Purkinje cell synapse of the cerebellum is thought to be a cellular substrate for motor learning. LTD requires activation of metabotropic glutamate receptor subtype 1 (mGluR1) and its downstream signalling pathways, which invariably involves phospholipase C,s (PLC,s). PLC,s consist of four isoforms (PLC,1,4) among which PLC,4 is the major isoform in most Purkinje cells in the rostral cerebellum (lobule 1 to the rostral half of lobule 6). We studied mutant mice deficient in PLC,4, and found that LTD was deficient in the rostral but not in the caudal cerebellum of the mutant. Basic properties of parallel fibre,Purkinje cell synapses and voltage-gated Ca2+ channel currents appeared normal. The mGluR1-mediated Ca2+ release induced by repetitive parallel fibre stimulation was absent in the rostral cerebellum of the mutant, suggesting that their LTD lesion was due to the defect in the mGluR1-mediated signalling in Purkinje cells. Importantly, the eyeblink conditioning, a simple form of discrete motor learning, was severely impaired in PLC,4 mutant mice. Wild-type mice developed the conditioned eyeblink response, when pairs of the conditioned stimulus (tone) and the unconditioned stimulus (periorbital shock) were repeatedly applied. In contrast, PLC,4 mutant mice could not learn the association between the conditioned and unconditioned stimuli, although their behavioural responses to the tone or to the periorbital shock appeared normal. These results strongly suggest that PLC,4 is essential for LTD in the rostral cerebellum, which may be required for the acuisition of the conditioned eyeblink response. [source]

    Synaptic plasticity in the basolateral amygdala in transgenic mice expressing dominant-negative cAMP response element-binding protein (CREB) in forebrain

    G. Rammes
    Abstract Electrophysiological and behavioural experiments were performed in transgenic mice expressing a dominant-negative form of cAMP response element-binding protein (CREBA133) in the limbic system. In control littermate in vitro slice preparation, tetanizing the lateral amygdala,basolateral amygdala (BLA) pathway with a single train (100 Hz for 1 s) produced short-term potentiation (STP) in the BLA. Five trains (10-s interstimulus interval) induced long-term potentiation (LTP), which was completely blocked by the N-methyl- d -aspartate (NMDA) receptor antagonist d(,)-2-amino-5-phosphonopentanoic acid (AP5; 50 ,m). When GABAergic (,-aminobutyric acid) inhibition was blocked by picrotoxin (10 ,m), LTP became more pronounced. Low-frequency stimulation (1 Hz for 15 min) induced either long-term depression (LTD) or depotentiation. LTD remained unaffected by AP5 (50 ,m) or by the L- and T-type Ca2+ -channel blockers nifedipine (20 ,m) and Ni2+ (50 ,m), but was prevented by picrotoxin (10 ,m), indicating a GABAergic link in the expression of LTD in the BLA. When conditioned fear was tested, a mild impairment was seen in one of three transgenic lines only. Although high levels of mRNA encoding CREBA133 lead to downregulation of endogenous CREB, expression of LTP and depotentiation were unaltered in BLA of these transgenic animals. These results could suggest that residual CREB activity was still present or that CREB per se is dispensable. Alternatively, other CREB-like proteins were able to compensate for impaired CREB function. [source]

    Tyrosine phosphorylation of the GluR2 subunit is required for long-term depression of synaptic efficacy in young animals in vivo

    HIPPOCAMPUS, Issue 8 2007
    Christopher J. Fox
    Abstract The study of the intracellular mechanics that underlay changes in synaptic efficacy is a rapidly evolving field of research. It is currently believed that NMDA receptors play a significant role in the induction of synaptic plasticity, whereas AMPA receptors play a significant role in its expression. For AMPA receptors, it has been shown that tyrosine phosphorylation of the GluR2 carboxyl termini is required for the expression of long-term depression of synaptic efficacy (LTD) in vitro (Ahmadian et al. (2004) EMBO J 23:1040,1050). In the present study, we sought to determine whether similar mechanisms are involved in vivo, where different stimulation parameters are required for the induction of LTD. We initially used a paired-burst (PB) paradigm that reliably induces LTD in vivo. In these animals we were able to prevent the induction and expression of PB-LTD by administering a peptide (GluR-3Y) that acted as a competitive inhibitor of tyrosine phosphorylation. In a separate set of animals, we exposed animals to brief periods of stress (S) before using low-frequency stimuli to induce LTD (S-LTD). Again, GluR2,3Y blocked both the induction and expression of S-LTD. In contrast, an inert version of the peptide, with alanine replacing the three tyrosine residues, did not inhibit LTD induction. In addition, we demonstrated that GluR2,3Y did not affect the induction of long-term potentiation in vivo. These findings support the hypothesis that tyrosine phosphorylation and AMPA receptor endocytosis are necessary steps for the induction and maintenance of two forms of LTD in the CA1 region. 2007 Wiley-Liss, Inc. [source]

    Contribution of NR2A and NR2B NMDA subunits to bidirectional synaptic plasticity in the hippocampus in vivo

    HIPPOCAMPUS, Issue 11 2006
    Christopher J. Fox
    Abstract It has recently been proposed that activation of the NR2A subunit results in Long-term potentiation (LTP) induction, whereas activation of the NR2B subunit results in long-term depression (LTD) induction. The present study undertakes to replicate these findings in vivo to determine if a role for specific subunits in synaptic plasticity can be shown in the intact brain. Field recordings were made from the CA1 subfield of the hippocampus using Schaffer collateral stimulation in anesthetized male Sprague-Dawley rats. Antagonists of the N -methyl- D -aspartate receptors NR2A and NR2B subunits were administered by either intraperitoneal (i.p.) or intrahippocampal (i.h.) injections to assess their involvement in LTP (100 Hz stimuli) and LTD (200 Paired-burst stimuli). i.h. injection of Ro25,6981 (100 ,M) significantly attenuated hippocampal LTP expression and completely blocked LTD expression. When administered i.p., Ro25,6981 (6 mg/kg) again blocked LTD, but did not significantly diminish the expression of LTP. When NVP-AAM077 was administered i.h. (80 ,M) both LTP and LTD were completely abolished. The administration of this compound i.p. (1.2 mg/kg) also significantly attenuated LTP, but did not affect LTD. These data suggest that both NR2A and NR2B subunits can play roles in LTP and LTD in the hippocampus in vivo. 2006 Wiley-Liss, Inc. [source]

    Alterations of postsynaptic density proteins in the hippocampus of rat offspring from the morphine-addicted mother: Beneficial effect of dextromethorphan

    HIPPOCAMPUS, Issue 6 2006
    San Nan Yang
    Abstract Infants passively exposed to morphine or heroin through their addicted mothers usually develop characteristic withdrawal syndrome of morphine after birth. In such early life, the central nervous system exhibits significant plasticity and can be altered by various prenatal influences, including prenatal morphine exposure. Here we studied the effects of prenatal morphine exposure on postsynaptic density protein 95 (PSD-95), an important cytoskeletal specialization involved in the anchoring of the NMDAR and neuronal nitric oxide synthase (nNOS), of the hippocampal CA1 subregion from young offspring at postnatal day 14 (P14). We also evaluated the therapeutic efficacy of dextromethorphan, a widely used antitussive drug with noncompetitive antagonistic effects on NMDARs, for such offspring. The results revealed that prenatal morphine exposure caused a maximal decrease in PSD-95 expression at P14 followed by an age-dependent improvement. In addition, prenatal morphine exposure reduced not only the expression of nNOS and the phosphorylation of cAMP responsive element-binding protein at serine 133 (CREBSerine-133), but also the magnitude of long-term depression (LTD) at P14. Subsequently, the morphine-treated offspring exhibited impaired performance in long-term learning and memory at later ages (P28,29). Prenatal coadministration of dextromethorphan with morphine during pregnancy and throughout lactation could significantly attenuate the adverse effects as described above. Collectively, the study demonstrates that maternal exposure to morphine decreases the magnitude of PSD-95, nNOS, the phosphorylation of CREBSerine-133, and LTD expression in hippocampal CA1 subregion of young offspring (e.g., P14). Such alterations within the developing brain may play a role for subsequent neurological impairments (e.g., impaired performance of long-term learning and memory). The results raise a possibility that postsynaptic density proteins could serve an important role, at least in part, for the neurobiological pathogenesis in offspring from the morphine-addicted mother and provide tentative therapeutic strategy. 2006 Wiley-Liss, Inc. [source]

    Prenatal stress modifies hippocampal synaptic plasticity and spatial learning in young rat offspring

    HIPPOCAMPUS, Issue 5 2006
    Jianli Yang
    Abstract Clinical studies demonstrate that prenatal stress causes cognitive deficits and increases vulnerability to affective disorders in children and adolescents. The underlying mechanisms are not yet fully understood. Here, we reported that prenatal stress (10 unpredictable, 1 s, 0.8 mA foot shocks per day during gestational days 13,19) impaired long-term potentiation (LTP) but facilitated long-term depression (LTD) in hippocampal CA1 region in slices of the prenatal stressed offspring (5 weeks old). Cross-fostering neonate offspring by the prenatal stressed or control mothers did not change the effects of prenatal stress on the hippocampal LTP and LTD. Furthermore, prenatal stress enhanced the effects of acute stress on the hippocampal LTP and LTD and impaired spatial learning and memory in the Morris water maze in the young rat offspring. Therefore, prenatal stress alters synaptic plasticity and enhances the effects of acute stress on synaptic plasticity in the hippocampus, which may be the mechanism for the impaired spatial learning and memory in young rat offspring. 2006 Wiley-Liss, Inc. [source]

    Competitive interactions between endogenous LTD and LTP in the hippocampus underlie the storage of emotional memories and stress-induced amnesia

    HIPPOCAMPUS, Issue 8 2005
    David M. Diamond
    Abstract This speculative review serves two purposes. First, it as an extension of the ideas we developed in a previous review (Diamond et al., Hippocampus, 2004;14:281,291), and second, it is a rebuttal to Abraham's (Hippocampus, 2004;14:675,676) critique of that review. We had speculated on the functional significance of the finding that post-training LTP induction produces retrograde amnesia. We noted the similarities between the findings that strong tetanizing stimulation can produce LTP and retrograde amnesia, and that a strong emotional experience can produce a long-lasting memory and retrograde amnesia, as well. The commonalities between LTP induction and emotional learning provided the basis of our hypothesis that an emotional experience generates endogenous LTD/depotentiation, which reverses synaptic plasticity formed during previous learning experiences, and endogenous LTP, which underlies the storage of new information. Abraham raised several concerns with our review, including the criticism that our speculation "falters because there is no evidence that stress causes LTD or depotentiation," and that research on stress and hippocampus has "failed to report any LTP-like changes." Abraham's points are well-taken because stress, in isolation, does not appear to generate long-lasting changes in baseline measures of hippocampal excitability. Here, within the context of a reply to Abraham's critique, we have provided a review of the literature on the influence of stress, novelty, fear conditioning, and the retrieval of emotional memories on cognitive and physiological measures of hippocampal functioning. An emphasis of this review is our hypothesis that endogenous forms of depotentiation, LTD and LTP are generated only when arousing experiences occur in conjunction with memory-related activation of the hippocampus and amygdala. We conclude with speculation that interactions among the different forms of endogenous plasticity underlie a form of competition by synapses and memories for access to retrieval resources. 2005 Wiley-Liss, Inc. [source]

    Long-term synaptic depression in the adult entorhinal cortex in vivo

    HIPPOCAMPUS, Issue 7 2003
    Raby Bouras
    Abstract The piriform cortex provides a major input to the entorhinal cortex. Mechanisms of long-term depression (LTD) of synaptic transmission in this pathway may affect olfactory and mnemonic processing. We have investigated stimulation parameters for the induction of homosynaptic LTD and depotentiation in this pathway using evoked synaptic field potential recordings in the awake rat. In this study, 15 min of 1-Hz stimulation induced a transient (<5 min) depression of evoked responses but did not induce LTD or depotentiation. To determine whether inhibitory and/or facilitatory mechanisms contribute to LTD induction, repetitive delivery of pairs of stimulation pulses was also assessed. Repetitive paired-pulse stimulation with a 10-ms interval between pulses, which activates inhibitory mechanisms during the second response, did not reliably induce LTD. However, repetitive paired-pulse stimulation using a 30-ms interval, which evokes marked paired-pulse facilitation, resulted in synaptic depression that lasted ,1 day, and which was reversible by tetanization. The selective induction of LTD by stimulation that evokes paired-pulse facilitation suggests that strong synaptic activation is required for LTD induction. The N -methyl- D -aspartate (NMDA) receptor antagonist MK-801 (0.1 mg/kg) blocked the induction of LTD, indicating that NMDA receptor activation is required for LTD induction in this pathway. These results indicate that LTD in piriform cortex inputs to the entorhinal cortex in the awake rat is effectively induced by strong repetitive synaptic stimulation, and that this form of LTD is dependent on activation of NMDA receptors. 2003 Wiley-Liss, Inc. [source]

    Theory of chemical bonds in metalloenzymes.


    Abstract Reaction mechanisms of oxygen evolution in native and artificial photosynthesis II (PSII) systems have been investigated on the theoretical grounds, together with experimental results. First of all, our previous broken-symmetry (BS) molecular orbitals (MO) calculations are reviewed to elucidate the instability of the d,-p, bond in high-valent (HV) Mn(X)O systems and the d,-p,-d, bond in HV MnOMn systems. The triplet instability of these bonds entails strong or intermediate diradical characters: ,Mn(IV)O, and ,MnOMn,; the BS MO resulted from strong electron correlation, leading to the concept of electron localizations and local spins. The BS computations have furthermore revealed guiding principles for derivation of selection rules for radical reactions of local spins. As a continuation of these theoretical results, the BS MO interaction diagrams for oxygen-radical coupling reactions in the oxygen evolution complex (OEC) in the PSII have been depicted to reveal scope and applicability of local singlet diradical (LSD) and local triplet diradical (LTD) mechanisms that have been successfully utilized for theoretical understanding of oxygenation reactions mechanisms by p450 and methane monooxygenase (MMO). The manganese-oxide cluster models examined are London, Berlin, and Berkeley models of CaMn4O4 and related clusters Mn4O4 and Mn3Ca. The BS MO interaction diagrams have revealed the LSD and/or LTD mechanisms for generation of molecular oxygen in the total low-, intermediate and high-spin states of these clusters. The spin alignments are found directly corresponding to the spin-coupling mechanisms of oxygen-radical sites in these clusters. The BS UB3LYP calculations of the clusters have been performed to confirm the comprehensive guiding principles for oxygen evolution; charge and spin densities by BS UB3LYP are utilized for elucidation and confirmation of the LSD and LTD mechanisms. Applicability of the proposed selection rules are examined in comparison with a lot of accumulated experimental and theoretical results for oxygen evolution reactions in native and artificial PSII systems. 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2010 [source]

    Commonalities in the neurobiology between autism and fragile X

    R. Hagerman
    There is a close association between autism and fragile X syndrome (FXS) with 30% of males with FXS having autism and 2 to 7% of children with autism having the fragile X mutation. The protein that is missing or deficient in FXS, FMRP, is an RNA binding and transport protein which regulates the translation of many messages important for synaptic plasticity. Typically FMRP inhibits the translation of these messages, such that protein production increases when FMRP is absent. Some of these proteins are known to also cause autism when they are mutated including neuroligin 3 and 4 and the SHANK protein. Therefore, when FMRP is missing there is dysregulation of other proteins that are known to cause autism. FMRP is an important inhibitor of protein production in the metabotropic glutamate receptor 5 pathway (mGluR5) which leads to long term depression (LTD) or the weakening of synaptic connections. Therefore, when FMRP is missing there is enhanced mGluR5 activity leading to enhanced LTD and weak or immature synaptic connections. The use of mGluR5 antagonists to reverse the LTD in the animal models of FXS has led to reversal of the learning, behaviour and dendritic spine abnormalities in these animals. There are now initial studies taking place in humans regarding the use of mGluR5 antagonists to improve behaviour and cognition in FXS. It is likely that these mGluR5 antagonists will also be helpful in a subgroup of patients with non fragile X autism who have similar problems with hyperactivity, hyperarousal and anxiety to those seen in FXS. A second cause of autism is the fragile X premutation but this mechanism of involvement is related to RNA toxicity which perhaps stimulates neuroimmune problems and may mimic other causes of autism. Neurons with the premutation are more vulnerable to environmental toxicity and oxidative stress leading to early cell death. [source]

    Ethanol Acutely Modulates mGluR1-Dependent Long-Term Depression in Cerebellum

    ALCOHOLISM, Issue 7 2010
    Li-Da Su
    Background:, Acute and chronic ethanol exposure produces profound impairments in motor functioning. Individuals with lower sensitivity to the acute motor impairing effects of ethanol have an increased risk of developing alcohol dependence and abuse, and infants with subtle delays in motor coordination development may have an increased risk for subsequently developing alcoholism. Thus, understanding the mechanism by which ethanol disrupts motor functioning is very important. Methods:, Parasagittal slices of the cerebellar vermis (250 ,M thick) were prepared from P17 to 20 Sprague,Dawley rats. Whole-cell recordings of Purkinje cells were obtained with an Axopatch 200B amplifier. Parallel fiber-Purkinje cell synaptic currents were sampled at 1 kHz and digitized at 10 kHz, and synaptic long-term depression (LTD) was observed in either external or internal application of ethanol for comparison. Results:, We determined whether ethanol acutely affects parallel fiber LTD using whole-cell patch-clamp recordings from Purkinje cells. Application of ethanol both externally (50 mM) and internally (17 and 10 mM) significantly suppressed mGluR-mediate slow currents. Short-term external ethanol exposure (50 but not 17 mM) during tetanus blocked mGluR-dependent parallel fiber LTD. Furthermore, internal 17 and 10 mM ethanol completely inhibited this LTD. Conclusions:, The results of the current study demonstrate that ethanol acutely suppresses parallel fiber LTD and may influence the mGluR-mediated slow current intracellularly. This study, plus previous evidence by Carta and colleagues (2006) and Belmeguenai and colleagues (2008), suggests significant actions of ethanol on mGluR-mediated currents and its dependent plasticity in brain. [source]

    Alterations of Rat Corticostriatal Synaptic Plasticity After Chronic Ethanol Exposure and Withdrawal

    ALCOHOLISM, Issue 5 2006
    Jian Xun Xia
    Background: The purpose of this study was to investigate the effects of chronic ethanol exposure (CEE) and withdrawal on corticostriatal plasticity in rats. Methods: We established an animal model of alcoholism using the method of Turchan et al. (1999). A synaptic model of long-term memory (long-term depression, LTD) was used as an index and the striatum, which is related to habit learning, was selected as a target region in the present study. The effects of CEE and withdrawal on the LTD were studied in striatal slices of ethanol-dependent rats using the extracellular recording method. Results: A stable LTD can be induced after high-frequency stimulation (HFS) in the slices of control rats. Chronic ethanol exposure and withdrawal suppressed the induction of corticostriatal LTD to different extents, with the strongest suppressive effects on LTD occurring in the slices of rats exposed to ethanol for 10 days and in those withdrawn from ethanol for 1 day. Notably, 3 days of withdrawal resulted in the shift of corticostriatal synaptic plasticity from LTD to long-term potentiation, and the peak latencies of the population spikes were obviously shortened compared with those of control rats. After 7 days of withdrawal, ethanol's effects tended to disappear. Conclusions: These results suggest that the alterations of corticostriatal synaptic plasticity produced by CEE and withdrawal may play a prominent role in alcohol abuse and alcoholism. [source]

    Nonrobustness of the normal approximation of lead-time demand in a (Q, R) system

    Hon-Shiang Lau
    For computing an optimal (Q, R) or kindred inventory policy, the current literature provides mixed signals on whether or when it is safe to approximate a nonnormal lead-time-demand ("LTD") distribution by a normal distribution. The first part of this paper examines this literature critically to justify why the issue warrants further investigations, while the second part presents reliable evidence showing that the system-cost penalty for using the normal approximation can be quite serious even when the LTD-distribution's coefficient of variation is quite low,contrary to the prevalent view of the literature. We also identify situations that will most likely lead to large system-cost penalty. Our results indicate that, given today's technology, it is worthwhile to estimate an LTD-distribution's shape more accurately and to compute optimal inventory policies using statistical distributions that more accurately reflect the LTD-distributions' actual shapes. 2003 Wiley Periodicals, Inc. Naval Research Logistics, 2003 [source]

    Functional contributions of synaptically localized NR2B subunits of the NMDA receptor to synaptic transmission and long-term potentiation in the adult mouse CNS

    Hideki Miwa
    The NMDA-type glutamate receptor is a heteromeric complex composed of the NR1 and at least one of the NR2 subunits. Switching from the NR2B to the NR2A subunit is thought to underlie functional alteration of the NMDA receptor during synaptic maturation, and it is generally believed that it results in preferential localization of NR2A subunits on the synaptic site and that of NR2B subunits on the extracellular site in the mature brain. It has also been proposed that activation of the NR2A and NR2B subunits results in long-term potentiation (LTP) and long-term depression (LTD), respectively. Furthermore, recent reports suggest that synaptic and extrasynaptic receptors may have distinct roles in synaptic plasticity as well as in gene expression associated with neuronal death. Here, we have investigated whether NR2B subunit-containing receptors are present and functional at mature synapses in the lateral nucleus of the amygdala (LA) and the CA1 region of the hippocampus, comparing their properties between the two brain regions. We have found, in contrast to the above hypotheses, that the NR2B subunit significantly contributes to synaptic transmission as well as LTP induction. Furthermore, its contribution is greater in the LA than in the CA1 region, and biophysical properties of NMDA receptors and the NR2B/NR2A ratio are different between the two brain regions. These results indicate that NR2B subunit-containing NMDA receptors accumulate on the synaptic site and are responsible for the unique properties of synaptic function and plasticity in the amygdala. [source]

    LTP of GABAergic synapses in the ventral tegmental area and beyond

    Fereshteh S. Nugent
    One of the mechanisms by which the experience-dependent reorganization of neural circuitry can occur is through changes in synaptic strength. Almost every excitatory synapse in the mammalian brain exhibits LTP (long-term potentiation) or LTD (long-term depression), two cellular mechanisms of synaptic plasticity. However, LTP and LTD have been reported much more rarely at fast inhibitory GABAA receptor synapses. Our recent study suggests that in vivo morphine initiates a long-lasting alteration of GABAergic synapses in the ventral tegmental area (VTA) by blocking the mechanisms required for LTP of GABAergic synapses. Here we put this work into the context of other examples of synaptic plasticity at GABAergic synapses. [source]

    Developmental shift from long-term depression to long-term potentiation in the rat medial vestibular nuclei: role of group I metabotropic glutamate receptors

    Julien Puyal
    The effects of high frequency stimulation (HFS) of the primary vestibular afferents on synaptic transmission in the ventral part of the medial vestibular nuclei (vMVN) were studied during postnatal development and compared with the changes in the expression of the group I metabotropic glutamate receptor (mGluR) subtypes, mGluR1 and mGluR5. During the first stages of development, HFS always induced a mGluR5- and GABAA -dependent long-term depression (LTD) which did not require NMDA receptor and mGluR1 activation. The probability of inducing LTD decreased progressively throughout the development and it was zero at about the end of the second postnatal week. Conversely, long-term potentiation (LTP) appeared at the beginning of the second week and its occurrence increased to reach the adult value at the end of the third week. Of interest, the sudden change in the LTP frequency occurred at the time of eye opening, about the end of the second postnatal week. LTP depended on NMDA receptor and mGluR1 activation. In parallel with the modifications in synaptic plasticity, we observed that the expression patterns and localizations of mGluR5 and mGluR1 in the medial vestibular nuclei (MVN) changed during postnatal development. At the earlier stages the mGluR1 expression was minimal, then increased progressively. In contrast, mGluR5 expression was initially high, then decreased. While mGluR1 was exclusively localized in neuronal compartments and concentrated at the postsynaptic sites at all stages observed, mGluR5 was found mainly in neuronal compartments at immature stages, then preferentially in glial compartments at mature stages. These results provide the first evidence for a progressive change from LTD to LTP accompanied by a distinct maturation expression of mGluR1 and mGluR5 during the development of the MVN. [source]

    The Impact of Obesity on Long-term Outcomes in Liver Transplant Recipients,Results of the NIDDK Liver Transplant Database

    J. Leonard
    The impact of obesity on outcomes following liver transplantation has been difficult to determine, in part due to the confounding effects of ascites on BMI. We evaluated the impact of pretransplant recipient obesity on outcomes following liver transplantation using the NIDDK Liver Transplantation Database. Pretransplant BMI, corrected for ascites, was categorized as underweight (BMI <18 kg/m2), normal weight (BMI 18,25 kg/m2), overweight (BMI 25.1,30 kg/m2), Class I obese (BMI 30.1,35 kg/m2), Class II obese (BMI 35.1,40 kg/m2) and Class III obese (BMI >40 kg/m2). Primary outcomes were patient and graft survival. Secondary outcomes included days in hospital and days in ICU. Data from 704 adult liver transplant recipients from the NIDDK LTD and a further 609 patients from the Mayo Clinic were analyzed. Early and late patient and graft survival was similar across all BMI categories. Correcting for ascites volume resulted in 11,20% of patients moving into a lower BMI classification. The relative risk for mortality increased by 7% for each liter of ascites removed. We conclude that corrected BMI is not independently predictive of patient or graft survival. Obesity, within the ranges observed in this study, should not be considered to be a contraindication to liver transplantation in the absence of other relative contraindications. [source]