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 ␤␥-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.Long term depression (LTD) 1 is a persistent activity-dependent decrease of synaptic efficacy that together with the converse process, long term potentiation, has been considered to be crucial for information storage in the brain (1, 2). In the hippocampus, LTD is divided into three categories: homosynaptic, heterosynaptic, and associative LTD (3). The bestcharacterized form of homosynaptic LTD is induced in the CA1 region of the hippocampus by prolonged low frequency synaptic stimulation via a NMDA receptor-dependent rise in postsynaptic [Ca 2ϩ ] i and the activation of serine/threonine protein phosphatases (4). Recent work has shown that mechanistically distinct type of LTD can be induced in the CA1 region by other types of synaptic stimulation or brief pharmacological treatments. For example, a prolonged period of paired-pulse stimulation or a direct application of the selective group I mGluR agonists, such as DHPG, can induce a robust mGluR-dependent form of LTD that is independent of NMDA receptor activation (5). In contrast to the mechanisms of NMDA receptor-dependent LTD, which are fairly well established, the mechanisms of induction and the site of expression of mGluR-dependent LTD are still a matter of some considerable debate. Current studies have reported that the induction of mGluR-dependent LTD does not require extracellular Ca 2ϩ (6), Ca 2ϩ release from intracellular stores (7), activation of Ca 2ϩ /calmodulin-dependent protein kinase II (8), protein kinase A or protein kinase C (7, 9), or serine/ threonine protein phosphatases (9). However, this form of LTD requires activation of G q -type G proteins (10), a local translation of dendritic mRNA (5, 11), a long-lasting loss of postsynaptic AMPA receptors (12, 13), and activation of protein tyrosine phosphatases (14). In addition, more recent studies suggest that p38 MAPK signaling also serves as a signal mediator in the induction of mGluR-depende...

Oxytocin (OXT) receptors (OXTRs) are prominently expressed in hippocampal CA2 and CA3 pyramidal neurons, but little is known about its physiological function. As the functional necessity of hippocampal CA2 for social memory processing, we tested whether CA2 OXTRs may contribute to long-term social recognition memory (SRM) formation. Here, we found that conditional deletion of from forebrain () or CA2/CA3a-restricted excitatory neurons in adult male mice impaired the persistence of long-term SRM but had no effect on sociability and preference for social novelty. Conditional deletion of CA2/CA3a showed no changes in anxiety-like behavior assessed using the open-field, elevated plus maze and novelty-suppressed feeding tests. Application of a highly selective OXTR agonist [Thr,Gly]-OXT to hippocampal slices resulted in an acute and lasting potentiation of excitatory synaptic responses in CA2 pyramidal neurons that relied on -methyl-d-aspartate receptor activation and calcium/calmodulin-dependent protein kinase II activity. In addition, mice displayed a defect in the induction of long-term potentiation, but not long-term depression, at the synapses between the entorhinal cortex and CA2 pyramidal neurons. Furthermore, deletion led to a reduced complexity of basal dendritic arbors of CA2 pyramidal neurons, but caused no alteration in the density of apical dendritic spines. Considering that the methodologies we have used to delete do not rule out targeting the neighboring CA3a region, these findings suggest that OXTR signaling in the CA2/CA3a is crucial for the persistence of long-term SRM. Oxytocin receptors (OXTRs) are abundantly expressed in hippocampal CA2 and CA3 regions, but there are little known about their physiological function. Taking advantage of the conditional knock-out mice, the present study highlights the importance of OXTR signaling in the induction of long-term potentiation at the synapses between the entorhinal cortex and CA2 pyramidal neurons and the persistence of long-term social recognition memory. Thus, OXTRs in the CA2/CA3a may provide a new target for therapeutic approaches to the treatment of social cognition deficits, which are often observed in patients with neuropsychiatric disorders.

The induction of hippocampal long-term synaptic plasticity is exquisitely sensitive to behavioral stress, but the underlying mechanisms are still unclear. We report here that hippocampal slices prepared from adult rats that had experienced unpredictable and inescapable restraint tail-shock stress showed marked impairments of long-term potentiation (LTP) in the CA1 region. The same stress promoted the induction of long-term depression (LTD). These effects were prevented when the animals were given the glucocorticoid receptor antagonist 11␤,17␤-11[4-(dimethylamino)phenyl]-17-hydroxy-17-(1-propynyl)-estra-4-9-dien-3-one before the stress. Immunoblotting analyses revealed that stress induced a profound and prolonged extracellular signal-regulated kinase/mitogen-activated protein kinase (ERK1/2 MAPK) hyperphosphorylation through small GTPase Ras, Raf-1, and MAPK kinase 1/2 (MEK1/2). Furthermore, the stress effects were obviated by the intrahippocampal injection of specific inhibitors of MEK1/2 (U0126), protein kinase C (bisindolylmaleimide I), tyrosine kinase (K252a), and BDNF antisense oligonucleotides. These results suggest that the effects of stress on LTP and LTD originate from the corticosterone-induced sustained activation of ERK1/2-coupled signaling cascades.

The involvement of adenosine on the development of time-dependent reversal of long-term potentiation (LTP) by low-frequency stimulation (LFS) was investigated at Schaffer collateral-CA1 synapses of rat hippocampal slices. A train of LFS (2 Hz, 10 min, 1200 pulses) had no long-term effects on synaptic transmission but produced lasting depression of previously potentiated responses. This reversal of LTP (depotentiation) was observed when the stimulus was delivered

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