Disruption of MET Receptor Tyrosine Kinase,an Autism Risk Factor,Impairs Developmental Synaptic Plasticity in the Hippocampus

As more genes conferring risks to neurodevelopmental disorders are identified, translating these genetic risk factors into biological mechanisms that impact the trajectory of the developing brain is a critical next step. Here, we report that disrupted signaling mediated MET receptor tyrosine kinase (RTK), an established risk factor for autism spectrum disorders, in the developing hippocampus glutamatergic circuit leads to profound deficits in neural development, synaptic transmission, and plasticity. In cultured hippocampus slices prepared from neonatal mice, pharmacological inhibition of MET kinase activity suppresses dendritic arborization and disrupts normal dendritic spine development. In addition, single‐neuron knockdown (RNAi) or overexpression of Met in the developing hippocampal CA1 neurons leads to alterations, opposite in nature, in basal synaptic transmission and long‐term plasticity. In forebrain‐specific Met conditional knockout mice (Met^fx/fx;emx1^cre), an enhanced long‐term potentiation (LTP) and long‐term depression (LTD) were observed at early developmental stages (P12–14) at the Schaffer collateral to CA1 synapses compared with wild‐type littermates. In contrast, LTP and LTD were markedly reduced at young adult stage (P56–70) during which wild‐type mice show robust LTP and LTD. The altered trajectory of synaptic plasticity revealed by this study indicate that temporally regulated MET signaling as an intrinsic, cell autonomous, and pleiotropic mechanism not only critical for neuronal growth and functional maturation, but also for the timing of synaptic plasticity during forebrain glutamatergic circuits development.     

Effects of Behavioral Depression and Chronic Influence of Antidepressants on NMDA/Glutamate Receptor-Mediated Responses of Neurons of the Rat <Emphasis Type="Italic">Gyrus Dentatus</Emphasis>

In studies on superfused slices obtained from the rat hippocampus, we extracellularly recorded field EPSP (fEPSPs), pharmacologically isolated components of these fEPSP, which were related to activation of synaptic NMDA-glutamate receptors, and neuronal responses of the dentate gyrus, which were mediated by activation of extrasynaptic NMDA-glutamate receptors. Recordings were performed in junctions formed by fibers of the medial perforant pathway and dendrites of granular cells of the gyrus dentatus. In the course of the development of behavioral depression in rats, which was caused by zoosocial isolation or chronic injection of dexamethasone, the amplitude of fEPSPs decreased, on average, by 19.8 and 26.0%, respectively, while the NMDA components of fEPSP increased by 28.6 and 33.4%; the rise in the amplitude of neuronal responses recorded in the gyrus dentatus, which were mediated by the activation of extrasynaptic NMDA receptors (by 75.4 and 92.3%), was clearly expressed. In intact rats, chronic (over 14 days) injections of tricyclic antidepressants, imipramine and amitriptyline, as well as of an atypical antidepressant, maprotiline, resulted in an increase in the fEPSP amplitude by 22.1 to 25.9%. At the same time, the amplitude of NMDA components of fEPSP dropped by 27.0 to 29.1%, while the amplitude of responses mediated by the activation of extrasynaptic NMDA-glutamate receptors decreased by 46.1 to 49.8%. Changes in the neuronal responses in the gyrus dentatus, which were mediated by the activation of NMDA-glutamate receptors, are related to an increase or decrease in the number of such receptors. It is supposed that an increase in the total number of NMDA-glutamate receptors under conditions of behavioral depression results in a deterioration of the energy supply of cerebral neurons, while an increase in the extrasynaptic pool of NMDA-glutamate receptors leads to suppression of biosynthesis of neurotrophins and to injury of neurons. As a general result, informational processes in the brain are subjected to significant negative influences. Chronic injections of antidepressants promote a permanent rise in the concentration of noradrenaline and serotonin in extracellular environments and the activation of monoamine receptors positively conjugated with adenylate cyclase; suppression of expression of informational RNAs, which encode subunits of NMDA-glutamate receptors, and a decrease in the number of these receptors. These events improve the functional state of neurons. Neirofiziologiya/Neurophysiology, Vol. 37, No. 2, pp. 124–133, March–April, 2005.     

Interaction between noradrenergic and glucocorticoid brain systems: Probable involvement in the development of depression

Electrophysiological and biochemical experiments on slices of the rat dorsal hippocampus demonstrated that dexamethasone (100 nM) augmented and prolonged the depressive effect of noradrenaline on synaptic transmission in the CA1 zone; this effect is related to weakening of the uptake of noradrenaline by neurons. The effect of dexamethasone is mediated by glucocorticoid receptors. Inhibitors of presynaptic translocase of noradrenaline, cocaine and imipramine, increased, similarly to dexamethasone, the effects of noradrenaline; an additive synergism was observed upon combined applications of dexamethasone and cocaine. The effect of dexamethasone decreased with an increase in the extracellular concentration of glucose, but increased upon application of the Na/K-ATPase inhibitor strophantin. The potentiating influence of dexamethasone on the effects of noradrenaline was weaker in slices obtained from rats with behavioral depression induced by social isolation or chronic introduction of dexamethasone. We hypothesize that glucocorticoids stabilize noradrenergic neurotransmission in the brain under the action of stressogenic influences. The role of glucocorticoid mechanisms in the development of depression is discussed.Neirofiziologiya/Neurophysiology, Vol. 36, Nos. 5/6, pp. 377–385, September–December, 2004.This revised version was published online in April 2005 with a corrected cover date and copyright year.     

Modifications of plastic properties and metaplasticity of glutamatergic synapses in the rat cortex and hippocampus under conditions of reserpine-induced behavioral depression

Intraperitoneal injection of 1 mg/kg reserpine into rats caused the development of behavioral depression that was especially clearly pronounced 24 h after injection. Under such conditions, induction of long-term potentiation of synaptic transmission was suppressed, the development of long-term depression in glutamatergic synapses of pyramidal neurons of the hippocampal CA1 area and layers II/III of the parietal cortex was facilitated, and metaplasticity threshold (θ_M) was shifted to the right. Such modifications of plasticity and metaplasticity of glutamatergic synapses were determined by changes in the functional state of postsynaptic NMDA receptors, which was confirmed by a decrease in the duration of NMDA component of field EPSPs generated in the studied neurons and by an increase in the sensitivity of this component to the action of a nonselective blocker of NMDA receptors, ketamine. Simultaneously, the sensitivity to zinc and haloperidol, which are selective with respect to NMDA receptors with the subunit composition NR1/NR2B, decreased. It is hypothesized that, under conditions of depression, either replacement of a part of NR2B subunits in the structure of NMDA receptors by NR2A subunits or biochemical inactivation of NMDA receptors containing NR2B subunit, as well as a decrease in the clearance of transmitter in glutamatergic synapses, occur; these events determine the impairment of plastic properties of the latter contacts. Neirofiziologiya/Neurophysiology, Vol. 39, No. 3, pp. 214–221, May–June, 2007.     

GluA1 trafficking and metabotropic NMDA: addressing results from other laboratories inconsistent with ours

We have previously shown that when over-expressed in neurons, green fluorescent protein (GFP) tagged GluA1 (GluA1-GFP) delivery into synapses is dependent on plasticity. A recent study suggests that GluA1 over-expression leads to its incorporation into the synapse, in the absence of additional long-term potentiation-like manipulations. It is possible that a GFP tag was responsible for the difference. Using rectification index as a measure of synaptic delivery of GluA1, we found no difference in the synaptic delivery of GluA1-GFP versus untagged GluA1. We recently published a study showing that while D-APV blocks NMDAr-dependent long-term depression (LTD), MK-801 and 7-chloro kynurenate (7CK) fail to block LTD. We propose a metabotropic function for the NMDA receptor in LTD induction. In contrast to our observations, recent unpublished data suggest that the above antagonists are equally effective in blocking LTD. We noticed different methodology in their study. Here, we show that their methodology has complex effects on synaptic transmission. Therefore, it is not possible to conclude that 7CK is effective in blocking LTD from their type of experiment.     

诱导成年大鼠海马CA1区长时程压抑的强直刺激型式

标准低频率连续刺激(1～2 Hz,15 min)能够诱导幼年大鼠(＜4周)海马CA1区同突触长时程压抑(long-term depression,LTD),而只有较高频率且持续时间较长的连续刺激才能诱导出成年动物该部位稳定的LTD.本研究采用成年大鼠海马脑片标本,电刺激Schaffer侧枝传入纤维,在CA1区锥体细胞层记录群体锋电位,选用两种新的刺激参数以观测不同刺激型式在诱导成年大鼠LTD中的作用.诱导LTD的刺激参数为(1)2 Hz,5串,串长60 s,串间隔60 s;(2)5 Hz,5串,串长24 s,串间隔96 s;(3)对照组参数2 Hz,300 s.结果显示,对照参数未能诱导出LTD;而两种频率不同但脉冲总数与刺激总时程相同的多串刺激,即参数(1)与参数(2),均在成年大鼠海马CA1区诱导产生了LTD.两种参数所诱导的LTD特征具有参数特异性,该特征主要表现为LTD诱导潜伏期和LTD的幅度参数(1)、(2)诱导的LTD的潜伏期分别为15～25 min和30～40 min;强直刺激后80 min时LTD的幅度分别为(57.5±2.8)%和(67.7±3.4)%.以上结果表明特定型式的低频率刺激能够诱导成年大鼠海马CA1区的LTD,提示LTD的诱导与刺激的组合型式相关,并且2 Hz较5 Hz的多串刺激在诱导LTD中更为有效.     

Actin-associated neurabin-protein phosphatase-1 complex regulates hippocampal plasticity

Protein phosphatase-1 (PP1) has been implicated in the control of long-term potentiation (LTP) and depression (LTD) in rat hippocampal CA1 neurons. PP1 catalytic subunits associate with multiple postsynaptic regulatory subunits, but the PP1 complexes that control hippocampal LTP and LTD in the rat hippocampus remain unidentified. The neuron-specific actin-binding protein, neurabin-I, is enriched in dendritic spines, and tethers PP1 to actin-rich postsynaptic density to regulate morphology and maturation of spines. The present studies utilized Sindbis virus-mediated expression of wild-type and mutant neurabin-I polypeptides in organotypic cultures of rat hippocampal slices to investigate their role in synaptic plasticity. While wild-type neurabin-I elicited no change in basal synaptic transmission, it enhanced LTD and inhibited LTP in CA1 pyramidal neurons. By comparison, mutant neurabins, specifically those unable to bind PP1 or F-actin, decreased basal synaptic transmission, attenuated LTD and increased LTP in slice cultures. Biochemical and cell biological analyses suggested that, by mislocalizing synaptic PP1, the mutant neurabins impaired the functions of endogenous neurabin-PP1 complexes and modulated LTP and LTD. Together, these studies provided the first biochemical and physiological evidence that a postsynaptic actin-bound neurabin-I-PP1 complex regulates synaptic transmission and bidirectional changes in hippocampal plasticity.     

NR2A-containing NMDA receptors depress glutamatergic synaptic transmission and evoked-dopamine release in the mouse striatum

NMDA receptors play essential roles in the physiology and pathophysiology of the striatum, a brain nucleus involved in motor control and reward-motivated behaviors. NMDA receptors are composed of NR1 and NR2A–D subunits. Functional properties of NMDA receptors are determined by the type of NR2 subunit they contain. In this study, we have examined the involvement of NR2B and NR2A in the modulatory effect of NMDA on glutamatergic and dopaminergic synaptic transmission in the striatum. We found that bath application of NMDA decreased the amplitude of the field excitatory post-synaptic potential/population spike (fEPSP/PS) measured in corticostriatal mouse brain slices. This depression was not affected by the NR2B-selective antagonists Ifenprodil and Ro 25-6981, but was abolished by the NR2A antagonist NVP-AAM077. Activation of corticostriatal neurons by NMDA did not contribute to synaptic depression because similar results were obtained in decorticated striatal slices. Synaptic depression was not dependent on GABA release because the GABA_A receptor antagonist bicuculline did not affect NMDA-induced decrease of the fEPSP/PS. NMDA also depressed evoked-dopamine release through NR2A- but not NR2B-containing NMDA receptors. Our results identify an important role for NR2A-containing NMDA receptors intrinsic to the striatum in regulating glutamatergic synaptic transmission and evoked-dopamine release.     

Activation of metabotropic glutamate receptors in conjunction with postsynaptic depolarization triggers a long-term depression of the N-methyl-D-aspartate receptor-mediated synaptic potential in the rat hippocampus

The mechanism responsible for long-term depression (LTD) of pharmacologically isolated N-methyl-D-aspartate (NMDA) receptor-mediated excitatory postsynaptic potential (EPSP_NMDA) was studied. Intracellular recordings were made from CA1 cells of rat hippocampal slices in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (10 µM) and picrotoxin (50 µM), which block non-NMDA and GABA_A receptors, respectively. Intracellular injections of depolarizing pulses (500 ms, 0.3–0.7 nA) at 1 Hz for 5 min in the absence of synaptic stimulation caused a persistent increase in the amplitude of EPSP_NMDA. However, coupling postsynaptic depolarization with synaptic activity induced LTD. The EPSP_NMDA LTD could be blocked byL-2-amino-3-phosphonopropionic acid (50 µM) or (RS)--methyl-4-carboxyphenylglycine (200 µM), specific antagonists for metabotropic glutamate receptors (mGluR). Furthermore, application oftrans-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD, 50 µM), a specific mGluR agonist, in conjunction with postsynaptic depolarizing elicited LTD. In contrast, the mGluR agonists quisqualate or t-ACPD when given alone produced a sustained enhancement of EPSP_NMDA. Finally, coupled depolarization did not evoke LTD in slices pretreated with the protein kinase C (PKC) inhibitor calphostin c (60 nM). The present results demonstrate that activation of mGluR is necessary for the induction of LTD of EPSP_NMDA and suggest that NMDA receptors are subject to bidirectional regulation by mGluR. Furthermore, the induction of LTD is likely to involve the stimulation of PKC.     

Phosphorylation of proteins involved in activity-dependent forms of synaptic plasticity is altered in hippocampal slices maintained in vitro

The acute hippocampal slice preparation has been widely used to study the cellular mechanisms underlying activity-dependent forms of synaptic plasticity such as long-term potentiation (LTP) and long-term depression (LTD). Although protein phosphorylation has a key role in LTP and LTD, little is known about how protein phosphorylation might be altered in hippocampal slices maintained in vitro. To begin to address this issue, we examined the effects of slicing and in vitro maintenance on phosphorylation of six proteins involved in LTP and/or LTD. We found that AMPA receptor (AMPAR) glutamate receptor 1 (GluR1) subunits are persistently dephosphorylated in slices maintained in vitro for up to 8 h. alpha calcium/calmodulin-dependent kinase II (alphaCamKII) was also strongly dephosphorylated during the first 3 h in vitro but thereafter recovered to near control levels. In contrast, phosphorylation of the extracellular signal-regulated kinase ERK2, the ERK kinase MEK, proline-rich tyrosine kinase 2 (Pyk2), and Src family kinases was significantly, but transiently, increased. Electrophysiological experiments revealed that the induction of LTD by low-frequency synaptic stimulation was sensitive to time in vitro. These findings indicate that phosphorylation of proteins involved in N-methyl-D-aspartate (NMDA) receptor-dependent forms of synaptic plasticity is altered in hippocampal slices and suggest that some of these changes can significantly influence the induction of LTD.     

Ionotropic glutamate receptors

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds—the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and theN-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.     

A single in vivo cocaine administration impairs 5‐HT1B receptor‐induced long‐term depression in the nucleus accumbens

The nucleus accumbens (NAc) is a crucial forebrain nucleus implicated in reward‐based decision‐making. While NAc neurons are richly innervated by serotonergic fibers, information on the functional role of serotonin 5‐hydroxytryptamine (5‐HT) in the NAc is still sparse. Here, we demonstrate that brief application of 5‐HT or 5‐HT_1B receptor agonist CP 93129 induced a long‐term depression (LTD) of glutamatergic transmission in NAc neurons. This LTD was presynaptically mediated and inducible by endogenous 5‐HT. Remarkably, a single cocaine exposure impaired the induction of LTD by 5‐HT or CP 93129. The inhibition was blocked when a selective dopamine D₁ receptor antagonist SCH23390 was coadministered with cocaine. Cocaine treatment resulted in increased phosphorylation of presynaptic proteins, rabphilin 3A and synapsin 1, and significantly attenuated CP 93129‐induced decrease in rabphilin 3A and synapsin 1 phosphorylation. Application of cAMP‐dependent protein kinase inhibitor KT5720 caused a prominent synaptic depression in NAc neurons of mice with a history of cocaine exposure. Our results reveal a novel 5‐HT_1B receptor‐mediated LTD in the NAc and suggest that cocaine exposure may result in elevated phosphorylation of presynaptic proteins involved in regulating glutamate release, which counteracts the presynaptic depressant effects of 5‐HT_1B receptors and thereby impairs the induction of LTD by 5‐HT.     

Mechanisms of Long-Lasting Depression of Synaptic Transmission in the CA1 Region of the Rat Hippocampus

Low-frequency tetanic stimulation (2 sec^-1, 5 min) of Schaffer collaterals (SchC) in superfused slices of the dorsal hippocampus of 12- to 15-day-old rats was demonstrated to evoke homosynaptic long-lasting depression (LLD) of synaptic transmission. The same procedure applied to hippocampal slices of mature (8-week-old or older) rats failed to elicit LLD. Low-frequency tetanic stimulation of the alveus in hippocampal slices, applied under conditions of intensified NMDA glutamate receptor functioning, led to the development of heterosynaptic LLD of synaptic transmission in the SchC–dendrites of the CA1 pyramidal neurons system. Both LLD cases were either absent or weakened when hippocampal slices were treated with a competitive blocker of the NMDA glutamate receptors, D-2-amino-5-phosphonovalerate (50 M). Morphine hydrochloride (10 M), as well as inhibitors of calmodulin and calcineurin (trifluoroperasine and cyclosporin A in concentrations of 1 and 50 M, respectively), interfered with induction of LLD or decreased its intensity. A blocker of the L-type voltage-dependent Ca²⁺ channels, nifedipine (10 M), did not influence homosynaptic LLD, but decreased heterosynaptic depression. Both types of depression of synaptic transmission were facilitated upon application of substances possessing a nootropic activity, 1 mM pyracetam or 5 M carbacetam. A blocker of NO synthase, N-nitro-L-arginine (10 M) did not alter either type of LLD. When hippocampal slices were influenced with a blocker of the A1 adenosine receptors, 1,3-dipropyl-8-phenylxanthine (1 M, 15 min), both LLD forms were intensified, and the development of homosynaptic LLD of synaptic transmission became possible in hippocampal slices of mature rats. When hippocampal slices were treated with an inhibitor of protein kinase C, polymyxin B (50 M, 15 min), intensification of LLD and, in particular, the development of homosynaptic LLD of synaptic transmission were observed. When an inhibitor of phospholipase A2, mepacrine (25 M, 15 min), was applied to hippocampal slices, both forms of LLD of synaptic transmission were significantly suppressed.     

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

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 μM), while raising [cGMP] with the type V phosphodiesterase inhibitor, zaprinast (20 μM), resulted in a long‐lasting depression of synaptic strength. When zaprinast (20 μM) was combined with a cell‐permeant PKA inhibitor H‐89 (10 μM), 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 μM), 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 μM), 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 μM), 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     

Group I mGluRs and Long-Term Depression: Potential Roles in Addiction?

Addiction is an enormous societal problem. A number of recent studies have focused on adaptations at glutamatergic synapses that may play a role in the behavioral responses to drugs of abuse. These studies have largely focused on NMDA receptor-dependent forms of synaptic plasticity such as NMDA receptor-dependent long-term potentiation (LTP) and long-term depression (LTD). A growing body of evidence, however, suggests that metabotropic glutamate receptors (mGluRs) also play important roles in the behavioral responses to drugs of abuse and participate in producing synaptic plasticity at glutamate synapses. In this review, we focus first on the evidence supporting a role for mGluRs in addiction and then on the properties of mGluR-dependent forms of synaptic plasticity, focusing in particular on Gq-linked receptor-induced LTD.     

Role of AMPA receptor trafficking in NMDA receptor-dependent synaptic plasticity in the rat lateral amygdala

Stimulated exocytosis and endocytosis of post-synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype of glutamate receptors (AMPARs) have been proposed as primary mechanisms for the expression of hippocampal CA1 long-term potentiation (LTP) and long-term depression (LTD), respectively. LTP and LTD, the two most well characterized forms of synaptic plasticity, are thought to be important for learning and memory in behaving animals. Both LTP and LTD can also be induced in the lateral amygdala (LA), a critical structure involved in fear conditioning. However, the role of AMPAR trafficking in the expression of either LTP or LTD in this structure remains unclear. In this study, we show that NMDA receptor-dependent LTP and LTD can be reliably induced at the synapses of the auditory thalamic inputs to the LA in brain slices. The expression of LTP was prevented by post-synaptic blockade of vesicle-mediated exocytosis with application of a light chain of Clostridium tetanus neurotoxin and was associated with increased cell-surface AMPAR expression. In contrast, the expression of LTD was prevented by post-synaptic application of a glutamate receptor 2-derived interference peptide, which specifically blocks the stimulated clathrin-dependent endocytosis of AMPARs, and was correlated with a reduction in plasma membrane-surface expression of AMPARs. These results strongly suggest that regulated trafficking of post-synaptic AMPARs is also involved in the expression of LTP and LTD in the LA.     

Insulin modulates hippocampal activity-dependent synaptic plasticity in a N-methyl-d-aspartate receptor and phosphatidyl-inositol-3-kinase-dependent manner

Insulin and its receptor are both present in the central nervous system and are implicated in neuronal survival and hippocampal synaptic plasticity. Here we show that insulin activates phosphatidylinositol 3-kinase (PI3K) and protein kinase B (PKB), and results in an induction of long-term depression (LTD) in hippocampal CA1 neurones. Evaluation of the frequency-response curve of synaptic plasticity revealed that insulin induced LTD at 0.033 Hz and LTP at 10 Hz, whereas in the absence of insulin, 1 Hz induced LTD and 100 Hz induced LTP. LTD induction in the presence of insulin required low frequency synaptic stimulation (0.033 Hz) and blockade of GABAergic transmission. The LTD or LTP induced in the presence of insulin was N-methyl-d-aspartate (NMDA) receptor specific as it could be inhibited by alpha-amino-5-phosphonopentanoic acid (APV), a specific NMDA receptor antagonist. LTD induction was also facilitated by lowering the extracellular Mg(2+) concentration, indicating an involvement of NMDA receptors. Inhibition of PI3K signalling or discontinuing synaptic stimulation also prevented this LTD. These results show that insulin modulates activity-dependent synaptic plasticity, which requires activation of NMDA receptors and the PI3K pathway. The results obtained provide a mechanistic link between insulin and synaptic plasticity, and explain how insulin functions as a neuromodulator.     

Long-term depression-inducing stimuli promote cleavage of the synaptic adhesion molecule NGL-3 through NMDA receptors,matrix metalloproteinases and presenilin/γ-secretase

Long-term depression (LTD) reduces the functional strength of excitatory synapses through mechanisms that include the removal of AMPA glutamate receptors from the postsynaptic membrane. LTD induction is also known to result in structural changes at excitatory synapses, including the shrinkage of dendritic spines. Synaptic adhesion molecules are thought to contribute to the development, function and plasticity of neuronal synapses largely through their trans-synaptic adhesions. However, little is known about how synaptic adhesion molecules are altered during LTD. We report here that NGL-3 (netrin-G ligand-3), a postsynaptic adhesion molecule that trans-synaptically interacts with the LAR family of receptor tyrosine phosphatases and intracellularly with the postsynaptic scaffolding protein PSD-95, undergoes a proteolytic cleavage process. NGL-3 cleavage is induced by NMDA treatment in cultured neurons and low-frequency stimulation in brain slices and requires the activities of NMDA glutamate receptors, matrix metalloproteinases (MMPs) and presenilin/γ-secretase. These results suggest that NGL-3 is a novel substrate of MMPs and γ-secretase and that NGL-3 cleavage may regulate synaptic adhesion during LTD.