On long-term depression induced by activation of G-protein coupled receptors

Metabotropic glutamate receptors and cognition: From underlying plasticity and neuroprotection to cognitive disorders and therapeutic targets

Metabotropic glutamate (mGlu) receptors are G protein-coupled receptors that play pivotal roles in mediating the activity of neurons and other cell types within the brain, communication between cell types, synaptic plasticity, and gene expression. As such, these receptors play an important role in a number of cognitive processes. In this chapter, we discuss the role of mGlu receptors in various forms of cognition and their underlying physiology, with an emphasis on cognitive dysfunction. Specifically, we highlight evidence that links mGlu physiology to cognitive dysfunction across brain disorders including Parkinson's disease, Alzheimer's disease, Fragile X syndrome, post-traumatic stress disorder, and schizophrenia. We also provide recent evidence demonstrating that mGlu receptors may elicit neuroprotective effects in particular disease states. Lastly, we discuss how mGlu receptors can be targeted utilizing positive and negative allosteric modulators as well as subtype specific agonists and antagonist to restore cognitive function across these disorders.

Neurobiological mechanisms associated with antipsychotic drug-induced dystonia

Dystonia is by far the most intrusive and invalidating extrapyramidal side effect of potent classical antipsychotic drugs. Antipsychotic drug-induced dystonia is classified in both acute and tardive forms. The incidence of drug-induced dystonia is associated with the affinity to inhibitory dopamine D2 receptors. Particularly acute dystonia can be treated with anticholinergic drugs, but the tardive form may also respond to such antimuscarinic treatment, which contrasts their effects in tardive dyskinesia. Combining knowledge of the pathophysiology of primary focal dystonia with the anatomical and pharmacological organization of the extrapyramidal system may shed some light on the mechanism of antipsychotic drug-induced dystonia. A suitable hypothesis is derived from the understanding that focal dystonia may be due to a faulty processing of somatosensory input, so leading to inappropriate execution of well-trained motor programmes. Neuroplastic alterations of the sensitivity of extrapyramidal medium-sized spiny projection neurons to stimulation, which are induced by the training of specific complex movements, lead to the sophisticated execution of these motor plans. The sudden and non-selective disinhibition of indirect pathway medium-sized spiny projection neurons by blocking dopamine D2 receptors may distort this process. Shutting down the widespread influence of tonically active giant cholinergic interneurons on all medium-sized spiny projection neurons by blocking muscarinic receptors may result in a reduction of the influence of extrapyramidal cortical-striatal-thalamic-cortical regulation. Furthermore, striatal cholinergic interneurons have an important role to play in integrating cerebellar input with the output of cerebral cortex, and are also targeted by dopaminergic nigrostriatal fibres affecting dopamine D2 receptors.

New insights into the mechanism of drug-induced dyskinesia

Dyskinesia is an extrapyramidal movement disorder characterized by involuntary, repetitive, irregular motions that affect the mouth and face and/ or the limbs and trunk. Tardive dyskinesia (TD) is a well-known complication of long term treatment with antipsychotic drugs. Dyskinesia is also induced with levodopa, a treatment for Parkinson's disease,and it occurs spontaneously as a symptom of Huntington's disease. Research on the pathogenesis of TD has focused on a dysfunction of either the dopaminergic or serotonergic system. However, recent evidence has suggested that we should focus on the possible damage of GABAergic medium spiny neurons (MSNs). MSNs are the first station in the corticostriato-thalamo-cortical circuit that regulates the amplitude and velocity of movements. Two pathways can be distinguished in this circuit: a direct pathway, which increases movements (hyperkinesia), and an indirect pathway,which decreases movements (hypokinesia). Both pathways are activated by glutamatergic corticostriatal neurons. Here,we discuss some evidence that supports the hypothesis that indirect pathway MSNs are damaged in dyskinesia.

Prenatal stress alters hippocampal synaptic plasticity in young rat offspring through preventing the proteolytic conversion of pro-brain-derived neurotrophic factor (BDNF) to mature BDNF

Prenatal stress (PS) has been associated with a higher risk of development of various neurological and psychiatric disorders later in life, but the underlying mechanisms are not yet fully understood. Here, using a chronic prenatal restraint stress model where the rat dams were immobilized for 45 min three times per day during the last week of pregnancy, we explored the long-lasting effects of PS on hippocampal synaptic plasticity in the offspring of both sexes. We found that PS switched the direction of synaptic plasticity in hippocampal CA1 region, favouring low-frequency stimulation-induced long-term depression (LTD) and opposing the induction of long-term potentiation (LTP) by high-frequency stimulation in young (5-week-old) rat offspring, but these changes disappeared at adult age (8 weeks old). Fostering of PS offspring to control dams did not alter the effects of PS on LTP and LTD. In addition, PS-induced changes in LTP and LTD induction were correlated with increasing endogenous pro-brain-derived neurotrophic factor (pro-BDNF) and decreasing of the mature form of BDNF (mBDNF) levels. Furthermore, PS resulted in a significant decrease in the activity and expression of tissue plasminogen activator (tPA), a key serine protease involved in the extracellular conversion of pro-BDNF to mBDNF. No significant differences were observed between the sexes for the effects of PS on hippocampal synaptic plasticity, the levels of pro-BDNF and mBDNF, and tPA expression. These results suggest that PS downregulates tPA levels within the hippocampus, inhibiting the proteolytic conversion of pro-BDNF to mBDNF, thereby leading to long-lasting alterations of the properties of synaptic plasticity.

Mushroom body efferent neurons responsible for aversive olfactory memory retrieval in Drosophila

Aversive olfactory memory is formed in the mushroom bodies in Drosophila melanogaster. Memory retrieval requires mushroom body output, but the manner in which a memory trace in the mushroom body drives conditioned avoidance of a learned odor remains unknown. To identify neurons that are involved in olfactory memory retrieval, we performed an anatomical and functional screen of defined sets of mushroom body output neurons. We found that MB-V2 neurons were essential for retrieval of both short- and long-lasting memory, but not for memory formation or memory consolidation. MB-V2 neurons are cholinergic efferent neurons that project from the mushroom body vertical lobes to the middle superiormedial protocerebrum and the lateral horn. Notably, the odor response of MB-V2 neurons was modified after conditioning. As the lateral horn has been implicated in innate responses to repellent odorants, we propose that MB-V2 neurons recruit the olfactory pathway involved in innate odor avoidance during memory retrieval.

The role of growth retardation in lasting effects of neonatal dexamethasone treatment on hippocampal synaptic function

BACKGROUND

Dexamethasone (DEX), a synthetic glucocorticoid, is commonly used to prevent or lessen the morbidity of chronic lung disease in preterm infants. However, evidence is now increasing that this clinical practice negatively affects somatic growth and may result in long-lasting neurodevelopmental deficits. We therefore hypothesized that supporting normal somatic growth may overcome the lasting adverse effects of neonatal DEX treatment on hippocampal function.

METHODOLOGY/PRINCIPAL FINDINGS

To test this hypothesis, we developed a rat model using a schedule of tapering doses of DEX similar to that used in premature infants and examined whether the lasting influence of neonatal DEX treatment on hippocampal synaptic plasticity and memory performance are correlated with the deficits in somatic growth. We confirmed that neonatal DEX treatment switched the direction of synaptic plasticity in hippocampal CA1 region, favoring low-frequency stimulation- and group I metabotropic glutamate receptor agonist (S)-3,5,-dihydroxyphenylglycine-induced long-term depression (LTD), and opposing the induction of long-term potentiation (LTP) by high-frequency stimulation in the adolescent period. The effects of DEX on LTP and LTD were correlated with an increase in the autophosphorylation of Ca(2+)/calmodulin-dependent protein kinase II at threonine-286 and a decrease in the protein phosphatase 1 expression. Neonatal DEX treatment resulted in a disruption of memory retention subjected to object recognition task and passive avoidance learning. The adverse effects of neonatal DEX treatment on hippocampal synaptic plasticity and memory performance of the animals from litters culled to 4 pups were significantly less than those for the 8-pup litters. However, there was no significant difference in maternal care between groups.

CONCLUSION/SIGNIFICANCE

Our results demonstrate that growth retardation plays a crucial role in DEX-induced long-lasting influence of hippocampal function. Our findings suggest that therapeutic strategies designed to support normal development and somatic growth may exert beneficial effects to reduce lasting adverse effects following neonatal DEX treatment.

Metabotropic glutamate receptor-mediated long-term depression: molecular mechanisms

The ability to modify synaptic transmission between neurons is a fundamental process of the nervous system that is involved in development, learning, and disease. Thus, synaptic plasticity is the ability to bidirectionally modify transmission, where long-term potentiation and long-term depression (LTD) represent the best characterized forms of plasticity. In the hippocampus, two main forms of LTD coexist that are mediated by activation of either N-methyl-d-aspartic acid receptors (NMDARs) or metabotropic glutamate receptors (mGluRs). Compared with NMDAR-LTD, mGluR-LTD is less well understood, but recent advances have started to delineate the underlying mechanisms. mGluR-LTD at CA3:CA1 synapses in the hippocampus can be induced either by synaptic stimulation or by bath application of the group I selective agonist (R,S)-3,5-dihydroxyphenylglycine. Multiple signaling mechanisms have been implicated in mGluR-LTD, illustrating the complexity of this form of plasticity. This review provides an overview of recent studies investigating the molecular mechanisms underlying hippocampal mGluR-LTD. It highlights the role of key molecular components and signaling pathways that are involved in the induction and expression of mGluR-LTD and considers how the different signaling pathways may work together to elicit a persistent reduction in synaptic transmission.

Hook-up of GluA2, GRIP and liprin-alpha for cholinergic muscarinic receptor-dependent LTD in the hippocampus

The molecular mechanism underlying muscarinic acetylcholine receptor-dependent LTD (mAChR-LTD) in the hippocampus is less studied. In a recent study, a novel mechanism is described. The induction of mAChR-LTD required the activation of protein tyrosine phosphatase (PTP), and the expression was mediated by AMPA receptor endocytosis via interactions between GluA2, GRIP and liprin-alpha. The hook-up of these proteins may result in the recruitment of leukocyte common antigen-related receptor (LAR), a PTP that is known to be involved in AMPA receptor trafficking. Interestingly, the similar molecular interaction cannot be applied to mGluR-LTD, despite the fact that the same G-protein involved in LTD is activated by both mAChR and mGluR. This discovery provides key molecular insights for cholinergic dependent cognitive function, and mAChR-LTD can serve as a useful cellular model for studying the roles of cholinergic mechanism in learning and memory.

Muscarinic acetylcholine receptors and voltage-gated calcium channels contribute to bidirectional synaptic plasticity at CA1-subiculum synapses

Hippocampal output is mediated via the subiculum, which is the principal target of CA1 pyramidal cells, and which sends projections to a variety of cortical and subcortical regions. Pyramidal cells in the subiculum display two different firing modes and are classified as being burst-spiking or regular-spiking. In a previous study, we found that low-frequency stimulation induces an NMDA receptor-dependent long-term depression (LTD) in burst-spiking cells and a metabotropic glutamate receptor-dependent long-term potentiation (LTP) in regular-spiking cells [P. Fidzinski, O. Shor, J. Behr, Target-cell-specific bidirectional synaptic plasticity at hippocampal output synapses, Eur. J. Neurosci., 27 (2008) 1111-1118]. Here, we present evidence that this bidirectional plasticity relies upon the co-activation of muscarinic acetylcholine receptors, as scopolamine blocks synaptic plasticity in both cell types. In addition, we demonstrate that the L-type calcium channel inhibitor nifedipine converts LTD to LTP in burst-spiking cells and LTP to LTD in regular-spiking cells, indicating that the polarity of synaptic plasticity is modulated by voltage-gated calcium channels. Bidirectional synaptic plasticity in subicular cells therefore appears to be governed by a complex signaling system, involving cell-specific recruitment of ligand and voltage-gated ion channels as well as metabotropic receptors. This complex regulation might be necessary for fine-tuning of synaptic efficacy at hippocampal output synapses.

Paradoxical facilitation of object recognition memory after infusion of scopolamine into perirhinal cortex: implications for cholinergic system function

The cholinergic system has long been implicated in learning and memory, yet its specific function remains unclear. In the present study, we investigated the role of cortical acetylcholine in a rodent model of declarative memory by infusing the cholinergic muscarinic receptor antagonist scopolamine into the rat perirhinal cortex during different stages (encoding, storage/consolidation, and retrieval) of the spontaneous object recognition task. Presample infusions of scopolamine significantly impaired object recognition compared with performance of the same group of rats on saline trials; this result is consistent with previous reports supporting a role for perirhinal acetylcholine in object information acquisition. Scopolamine infusions directly before the retrieval stage had no discernible effect on object recognition. However, postsample infusions of scopolamine with sample-to-infusion delays of up to 20 h significantly facilitated performance relative to postsample saline infusion trials. Additional analysis suggested that the infusion episode could cause retroactive or proactive interference with the sample object trace and that scopolamine blocked the acquisition of this interfering information, thereby facilitating recognition memory. This is, to our knowledge, the first example of improved recognition memory after administration of scopolamine. The overall pattern of results is inconsistent with a direct role for cortical acetylcholine in declarative memory consolidation or retrieval. Rather, the cholinergic input to the perirhinal cortex may facilitate acquisition by enhancing the cortical processing of incoming stimulus information.

Physiological role for casein kinase 1 in glutamatergic synaptic transmission

Casein kinase 1 (CK1) is a highly conserved serine/threonine kinase, present in virtually all cell types, in which it phosphorylates a wide variety of substrates. So far, no role has been found for this ubiquitous protein kinase in the physiology of nerve cells. In the present study, we show that CK1 regulates fast synaptic transmission mediated by glutamate, the major excitatory neurotransmitter in the brain. Through the use of CK1 inhibitors, we present evidence that activation of CK1 decreases NMDA receptor activity in the striatum via a mechanism that involves activation by this kinase of protein phosphatase 1 and/or 2A and resultant increased dephosphorylation of NMDA receptors. Indeed, inhibition of CK1 increases NMDA-mediated EPSCs in medium spiny striatal neurons. This effect is associated with an increased phosphorylation of the NR1 and NR2B subunits of the NMDA receptor and is occluded by the phosphatase inhibitor okadaic acid. The mGluR1, but not mGluR5, subclass of metabotropic glutamate receptors uses CK1 to inhibit NMDA-mediated synaptic currents. These results provide the first evidence for a role of CK1 in the regulation of synaptic transmission in the brain.

Sustained activation of metabotropic glutamate receptor 5 and protein tyrosine phosphatases mediate the expression of (S)-3,5-dihydroxyphenylglycine-induced long-term depression in the hippocampal CA1 region

Previous studies have shown that brief application of group I metabotropic glutamate receptor (mGluR) agonist (S)-3, 5-dihydroxyphenylglycine (DHPG) to hippocampal slices can induce a chemical form of long-term depression (DHPG-LTD) in the hippocampal CA1 region; however, the expression mechanisms of this LTD remain unclear. We show here that the expression of DHPG-LTD can be specifically reversed by application of the broad-spectrum mGluR antagonists, (S)-alpha-methyl-4-carboxyphenylglycine (MCPG) and LY341495, and mGluR5 antagonist, 2-methyl-6-(phenylethyl)pyridine, but not by NMDA receptor antagonist, D-2-amino-5-phosphonopentanoic acid, mGluR1 antagonist, LY367385, group II mGluR antagonist, (2S)-alpha-ethylglutamic acid, or group III mGluR antagonist, (S)-2-amino-2-methyl-4-phosphonobutanic acid (MAP4). In addition, the ability of MCPG to reverse DHPG-LTD was mimicked by the protein tyrosine phosphatase inhibitors, phenylarsine oxide and orthovanadate, but not phospholipase C inhibitor, U73122, protein kinase C inhibitor, bisindolylmaleimide 1, p38 mitogen-activated protein kinase inhibitor, SB203580, or protein phosphatases 1/2 A inhibitor, okadaic acid. Moreover, MCPG reversed the DHPG-LTD without affecting the paired-pulse facilitation. The expression of DHPG-LTD was associated with the reduction of both tyrosine phosphorylation and surface expression of AMPA receptor GluR2 subunits. Together, these results suggest that sustained activation of mGluR5 and in turn triggering a protein tyrosine phosphatase-dependent regulation of postsynaptic expression of AMPA receptors may contribute to the expression of DHPG-LTD.

Long-term depression in rat CA1-subicular synapses depends on the G-protein coupled mACh receptors

The subiculum, which is the primary target of CA1 pyramidal neurons and sending efferent fibres to many brain regions, serves as a hippocampal interface in the neural information processes between hippocampal formation and neocortex. Long-term depression (LTD) is extensively studied in the hippocampus, but not at the CA1-subicular synaptic transmission. Using whole-cell EPSC recordings in the brain slices of young rats, we demonstrated that the pairing protocols of low frequency stimulation (LFS) at 3 Hz and postsynaptic depolarization of -50 mV elicited a reliable LTD in the subiculum. The LTD did not cause the changes of the paired-pulse ratio of EPSC. Furthermore, it did not depend on either NMDA receptors or voltage-gated calcium channels (VGCCs). Bath application of the G-protein coupled muscarinic acetylcholine receptors (mAChRs) antagonists, atropine or scopolamine, blocked the LTD, suggesting that mAChRs are involved in the LTD. It was also completely blocked by either the Ca2+ chelator BAPTA or the G-protein inhibitor GDP-beta-S in the intracellular solution. This type of LTD in the subiculum may play a particular role in the neural information processing between the hippocampus and neocortex.

Sub-neurotoxic neonatal anoxia induces subtle behavioural changes and specific abnormalities in brain group-I metabotropic glutamate receptors in rats

Anoxia in the first week of life can induce neuronal death in vulnerable brain regions usually associated with an impairment of cognitive function that can be detected later in life. We set-up a model of subneurotoxic anoxia based on repeated exposures to 100% nitrogen during the first 7 days of post-natal life. This mild post-natal exposure to anoxia specifically modified the behaviour of the male adult rats, which showed an attention deficit and an increase in anxiety, without any impairment in spatial learning and any detectable brain damage (magnetic resonance imaging and histological analysis). Post-anoxic rats showed a reduction in the expression of group-I metabotropic glutamate receptors (i.e. mGlu1 and mGlu5 receptors) in the hippocampus and cerebral cortex, whereas expression of the mGlu 2/3 receptors, the NR1 subunit of NMDA receptors, and the GluR1 subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors was unchanged. mGlu1 and mGlu5 receptor signalling was also impaired in postanoxic rats, as revealed by a reduced efficacy of the agonist (1S,3R)-1-Aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) to stimulate polyphosphoinositide hydrolysis in hippocampal slices. We conclude that rats subjected to subneurotoxic doses of anoxia during the early post-natal life develop behavioural symptoms that are frequently encountered in the inattentive subtype of the attention deficit hyperactivity disorder, and that group-I mGlu receptors may be involved in the pathophysiology of these symptoms.

Removal of cholinergic input to perirhinal cortex disrupts object recognition but not spatial working memory in the rat

The perirhinal cortex of the temporal lobe has a crucial role in object recognition memory. Cholinergic transmission within perirhinal cortex also seems to be important for this function, as the muscarinic receptor antagonist scopolamine disrupts object recognition performance when administered systemically or directly into perirhinal cortex. In the present study, we directly assessed the contribution of cholinergic basal forebrain input to perirhinal cortex in object recognition. Selective bilateral removal of the cholinergic basal forebrain inputs to perirhinal cortex was accomplished by injecting the immunotoxin 192 IgG-saporin directly into perirhinal cortex in rats. These animals were significantly impaired relative to vehicle-injected controls in a spontaneous object recognition task despite intact spatial alternation performance. These results are consistent with recent reports of object recognition impairment following acute cholinergic receptor blockade and extend these findings by demonstrating that chronic removal of cholinergic basal forebrain input to an otherwise intact perirhinal cortex causes a severe object recognition deficit similar to that associated with more extensive cell body lesions of perirhinal cortex.

Metabotropic glutamate receptor signalling in perirhinal cortical neurons

Long-term depression (LTD) induction relies upon receptor cross-talk between group I and group II metabotropic glutamate receptors (mGluRs) in perirhinal cortex. The molecular mechanism of this mGluR interplay is not clear. Here, we show that the mGluR subtypes postulated to be involved in this mechanism are developmentally regulated and mGluR2 has a preferential role over mGluR3 in the synergistic interaction with mGluR5. We have identified a >70% reduction in basal cAMP levels following mGluR2 stimulation, which could lead to increased mGluR5 function via reduced PKA mediated phosphorylation and decreased desensitisation of mGluR5. To further investigate the roles of mGluRs in downstream intracellular signalling, we have examined the effects of mGluRs on the phosphorylation state of cAMP response element-binding protein (CREB). Both group I and group II agonists increased the phosphorylation of CREB, which indicates a cAMP- and PKA-independent signalling mechanism. These results suggest a convergence of signalling mechanisms from surface mGluRs to CREB-mediated transcription.

Both mGluR1 and mGluR5 mediate Ca2+ release and inward currents in hippocampal CA1 pyramidal neurons

Using combined whole-cell voltage-clamp recording and Ca2+ imaging we have investigated further the characteristics and pharmacology of group I metabotropic l-glutamate receptor (mGluR)-mediated responses in CA1 pyramidal neurons of the rat hippocampus. The selective group I mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG), evoked a transient increase in intracellular Ca2+ levels ([Ca2+]i), within neuronal somas and apical dendrites, together with a relatively long lasting inward current (I(DHPG)). Both types of response were enhanced by depolarisation (-30 mV), and this condition was used for their characterisation. The DHPG-induced [Ca2+]i rise was much more sensitive to manipulations of Ca2+ homeostasis, such as using the Ca2+ store depleting agent, cyclopiazonic acid (50-100 microM), the fast Ca2+ buffer, BAPTA (intracellular; 20-40 mM) and Ca(2+)-free/EGTA (1 mM) bath solution, than I(DHPG), suggesting that these responses are, in the main part, mediated by distinct processes. The selective mGluR1 and mGluR5 antagonists, (S)-(+)-alpha-amino-a-methylbenzeneacetic acid (LY367385; 100 microM) and 2-methyl-6-(phenylethynyl)-pyridine (MPEP;10 microM), respectively, markedly inhibited both I(DHPG) and the DHPG-evoked increase in [Ca2+]i. Moreover, these antagonists inhibited the Ca2+ response by more than 50% suggesting a synergistic interaction between mGluR1 and mGluR5. This study demonstrates that in CA1 pyramidal neurons group I mGluR-mediated inward currents and Ca2+ release from intracellular stores are enhanced under depolarising conditions and that mGluR1 and mGluR5 both contribute to these phenomena.

Hypoxia increases calcium flux through cortical neuron glutamate receptors via protein kinase C

The effects of 30 s to 10 min hypoxia (PO2-10 mmHg) on glutamate receptor activity were studied in murine cortical neurons. Receptor activity was assessed as a rise in intracellular calcium concentration ([Ca2+]i) following a 10 s application of 1 mm glutamate or 100 micro mN-methy-d-aspartate (NMDA) in the presence of 0.1 mm Mg2+ and 10 micro m glycine. Change in [Ca2+]i elicited by glutamate increased 26% (n = 192, p < 0.001) and that to NMDA by 74% (n = 9, p < 0.01) during a 100-s period of hypoxia. After 10 min hypoxia, responses to glutamate were 62% smaller than those in normoxia, with increased basal intracellular [Ca2+]i predicting reduced receptor activity. When neurons were exposed to NMDA after 10 min of hypoxia, [Ca2+]i increases were 12% smaller than after 100 s hypoxia, but still 53% larger than in oxygenated neurons (n = 9, p = 0.01). Neurons expressed relatively similar amounts of NR2A, -B, -C, and -D subunits. The phosphorylation of NMDA NR1 subunits increased during hypoxia. Pre-treatment of neurons with a protein kinase C (PKC) inhibitor (chelerythrine, 10 micro m) prevented increases in N-methy-d-aspartate receptor (NMDAR) activity during hypoxia and reduced the phosphorylation of NR1 subunits. These results suggest that enhancement of glutamate receptor activity during the first minutes of hypoxia is mediated by phosphorylation of NMDARs by PKC and that other mechanisms, possibly involving intracellular calcium, limit glutamate receptor-mediated calcium influx during longer periods of hypoxia.

Rap1-induced p38 Mitogen-activated Protein Kinase Activation Facilitates AMPA Receptor Trafficking via the GDI·Rab5 Complex

Recent evidence has emphasized the importance of p38 mitogen-activated protein kinase (MAPK) in the induction of metabotropic glutamate receptor (mGluR)-dependent long term depression (LTD) at hippocampal CA3-CA1 synapses. However, the cascade responsible of mGluR to activate p38 MAPK and the signaling pathway immediately downstream from it to induce synaptic depression is poorly understood. Here, we show that transient activation of group I mGluR with the selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) activates p38 MAPK through G protein betagamma-subunit, small GTPase Rap1, and MAPK kinase 3/6 (MKK3/6), thus resulting in mGluR5-dependent LTD. Furthermore, our data clearly show that an accelerating AMPA receptor endocytosis by stimulating the formation of guanyl nucleotide dissociation inhibitor-Rab5 complex is a potential downstream processing of p38 MAPK activation to mediate DHPG-LTD. These results suggest an important role for Rap1-MKK3/6-p38 MAPK pathway in the induction of mGluR-dependent LTD by directly coupling to receptor trafficking machineries to facilitate the loss of synaptic AMPA receptors.