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...

Insulin receptors are highly enriched at neuronal synapses, but whose function remains unclear. Here we present evidence that brief incubations of rat hippocampal slices with insulin resulted in an increased protein expression of dendritic scaffolding protein postsynaptic density-95 (PSD-95) in area CA1. This insulin-induced increase in the PSD-95 protein expression was inhibited by the tyrosine kinase inhibitor, AG1024, phosphatidylinositol 3-kinase (PI3K) inhibitors, LY294002 and wortmannin, translational inhibitors, anisomycin and rapamycin, but not by LY303511 (an inactive analogue of LY294002), and transcriptional inhibitor, actinomycin D, suggesting that insulin regulates the translation of PSD-95 by activating the receptor tyrosine kinase-PI3K-mammalian target of rapamycin (mTOR) signaling pathway. A similar insulin-induced increase in the PSD-95 protein expression was detected after stimulation of the synaptic fractions isolated from the hippocampal neurons. Furthermore, insulin treatment did not affect the PSD-95 mRNA levels. In agreement, insulin rapidly induced the phosphorylation of 3-phosphoinositidedependent protein kinase-1 (PDK1), protein kinase B (Akt), and mTOR, effects that were prevented by the AG1024 and LY294002. We also show that insulin stimulated the phosphorylation of 4E-binding protein 1 (4E-BP1) and p70S6 kinase (p70S6K) in a mTOR-dependent manner. Finally, we demonstrate the constitutive expression of PSD-95 mRNA in the synaptic fractions isolated from hippocampal neurons. Taken together, these findings suggest that activation of the PI3K-Akt-mTOR signaling pathway is essential for the insulin-induced up-regulation of local PSD-95 protein synthesis in neuronal dendrites and indicate a new molecular mechanism that may contribute to the modulation of synaptic function by insulin in hippocampal area CA1.Insulin and its receptor are widely dispersed throughout the brain with the highest density located in the olfactory bulb, cerebral cortex, hypothalamus, and hippocampus, where they are thought to subserve a number of functions including regulation of glucose metabolism, food intake and body weight, fertility and reproduction, learning, memory, and attention (1-4). Brain insulin receptors are present in particularly high concentrations in neurons, and in much lower levels in glia (5). Although the mRNA of insulin receptors is largely localized in neuronal somata, abundant insulin receptors are found in both cell bodies and synapses (5-7). However, very little is known about the functional significance of synaptic insulin receptors in the neurons. Recently, several studies have drawn links between insulin signaling and intracellular trafficking and plasma membrane expression of ion channels and neurotransmitter receptors at the central nervous system synapses. For example, it has been shown that insulin rapidly recruits functional GABA A receptors to postsynaptic domains in hippocampal neurons, resulting in a long-lasting enhancement of GABA A receptor-mediated synaptic transmission (8). ...

(ϩ)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine. Furthermore, the levels of brain-derived neurotrophic factor (BDNF) protein in the NAc shell increased progressively after cocaine withdrawal, and the impairment of DHPG-LTD in the NAc shell was not found in slices from BDNF-knock-out mice after cocaine withdrawal. These results suggest that withdrawal from repeated cocaine exposure may result in increased BDNF levels in the NAc shell, which leads to a selective downregulation of mGluR5 and thereby impairs the induction of mGluR-dependent LTD.

Neural adaptations in the medial prefrontal cortex (mPFC) are thought to be crucial in the development and maintenance of addictive behaviors. The mPFC receives a dense serotonergic (5-hydroxytryptamine, 5-HT) innervation from raphe nuclei and 5-HT exerts complex actions on mPFC pyramidal neurons. The present study, using a rat model of behavioral sensitization to cocaine, was designed to determine whether repeated cocaine exposure in vivo is capable of altering 5-HT-induced regulation of glutamatergic transmission in the mPFC. In layer V pyramidal neurons of the mPFC, application of 5-HT, through activation of 5-HT 2A receptors, induced a massive enhancement of spontaneous excitatory postsynaptic currents (sEPSCs). Repeated cocaine administration for 5 days resulted in an attenuation in the ability of 5-HT to enhance sEPSCs. This effect was prevented when cocaine was co-administered with the selective 5-HT 2A receptor antagonist ketanserin and was mimicked by repeated 5-HT 2A receptor agonist (À)4-iodo-2,5-dimethoxyphenylisopropylamine administration. Repeated cocaine administration is not associated with any changes in the levels of 5-HT 2A receptors or regulator of GTP-binding protein signaling 4. These results suggest that cocaine-induced inhibition of 5-HT 2A receptor-mediated enhancement of glutamatergic transmission in the mPFC may be caused, at least in part, by the impairment of coupling of 5-HT 2A receptors with GTP-binding proteins during cocaine withdrawal. These alterations in 5-HT 2A receptor responsiveness in the mPFC may be relevant to the development of behavioral sensitization and withdrawal effects following repeated cocaine administration.

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-HT1B 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-HT1B 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-HT1B receptors and thereby impairs the induction of LTD by 5-HT.