受体、突触和记忆

大脑中神经元突触间的信号传递是由许多神经递质受体介导的。在过去,Richard L．Huganir实验室一直致力于神经递质受体功能调节的分子机制。而最近,该实验室又聚焦到大脑中一种最主要的兴奋性受体的研究——谷氨酸受体。谷氨酸受体主要可以分为两大类：AMPA受体和NMDA受体。AMPA受体主要介导了快速的兴奋性突触传递;而NMDA受体则在神经可塑性和发育中起到重要作用。实验发现,AMPA受体和NMDA受体都可以被一系列的蛋白激酶磷酸化,而磷酸化的水平则直接影响了这些受体的功能特性,包括通道电导和受体膜定位等。AMPA受体磷酸化的水平同时还在学习和记忆的细胞模型中发生改变,如长时程增强（LTP）和长时程抑制（LTD）。此外,AMPA受体中GluR1亚单位的磷酸化对于各种形式的可塑性以及空间记忆的维持有重要的作用。实验室主要研究突触部位谷氨酸受体在亚细胞水平的定位和聚集的分子机制。最近,一系列可以直接或间接与AMPA和NMDA受体相互作用的蛋白质得以发现,其中包括一个新发现的蛋白家族GRIPs（glutamate receptor interacting proteins）。GRIPs可以直接和AMPA受体的GluR2／3亚单位的C端结合。GRIPs包含7个PDZ结构域,可以介导蛋白与蛋白直接的相互连接,从而把各个AMPA受体交互连接在一起并与其他蛋白相连。另外,GluR2亚单位的c端还可以和兴奋性突触中的蛋白激酶C结合蛋白（PICK1）的PDZ结构域相互作用。另外,GluR2亚单位的C端也可以与一种参与膜融合的蛋白NSF相互作用。这些与AMPA受体相互作用的蛋白质对于受体在膜上的运输以及定位有至关重要的作用。同时,受体与PICK1和GRIP的结合对于小脑运动学习中的LTD有重要作用。总体上说,该实验室发现了一系列可以调节神经递质受体功能的分子机制,这些工作提示受体功能的调节可能是?     

Diffusional trapping of GluR1 AMPA receptors by input-specific synaptic activity

Synaptic activity regulates the postsynaptic accumulation of AMPA receptors over timescales ranging from minutes to days. Indeed, the regulated trafficking and mobility of GluR1 AMPA receptors underlies many forms of synaptic potentiation at glutamatergic synapses throughout the brain. However, the basis for synapse-specific accumulation of GluR1 is unknown. Here we report that synaptic activity locally immobilizes GluR1 AMPA receptors at individual synapses. Using single-molecule tracking together with the silencing of individual presynaptic boutons, we demonstrate that local synaptic activity reduces diffusional exchange of GluR1 between synaptic and extraynaptic domains, resulting in postsynaptic accumulation of GluR1. At neighboring inactive synapses, GluR1 is highly mobile with individual receptors frequently escaping the synapse. Within the synapse, spontaneous activity confines the diffusional movement of GluR1 to restricted subregions of the postsynaptic membrane. Thus, local activity restricts GluR1 mobility on a submicron scale, defining an input-specific mechanism for regulating AMPA receptor composition and abundance.     

Synaptic incorporation of AMPA receptors during LTP is controlled by a PKC phosphorylation site on GluR1

Incorporation of GluR1-containing AMPA receptors into synapses is essential to several forms of neural plasticity, including long-term potentiation (LTP). Numerous signaling pathways that trigger this process have been identified, but the direct modifications of GluR1 that control its incorporation into synapses are unclear. Here, we show that phosphorylation of GluR1 by PKC at a highly conserved serine 818 residue is increased during LTP and critical for LTP expression. GluR1 is phosphorylated by PKC at this site in vitro and in vivo. In addition, acute phosphorylation at GluR1 S818 by PKC, as well as a phosphomimetic mutation, promotes GluR1 synaptic incorporation. Conversely, preventing GluR1 S818 phosphorylation reduces LTP and blocks PKC-driven synaptic incorporation of GluR1. We conclude that the phosphorylation of GluR1 S818 by PKC is a critical event in the plasticity-driven synaptic incorporation of AMPA receptors.     

Learning-induced glutamate receptor phosphorylation resembles that induced by long term potentiation

Long term potentiation and long term depression of synaptic responses in the hippocampus are thought to be critical for certain forms of learning and memory, although until recently it has been difficult to demonstrate that long term potentiation or long term depression occurs during hippocampus-dependent learning. Induction of long term potentiation or long term depression in hippocampal slices in vitro modulates phosphorylation of the alpha-amino-3-hydrozy-5-methylisoxazole-4-propionic acid subtype of glutamate receptor subunit GluR1 at distinct phosphorylation sites. In long term potentiation, GluR1 phosphorylation is increased at the Ca2+/calmodulin-dependent protein kinase and protein kinase C site serine 831, whereas in long term depression, phosphorylation of the protein kinase A site serine 845 is decreased. Indeed, phosphorylation of one or both of these sites is required for long term synaptic plasticity and for certain forms of learning and memory. Here we demonstrate that training in a hippocampus-dependent learning task, contextual fear conditioning is associated with increased phosphorylation of GluR1 at serine 831 in the hippocampal formation. This increased phosphorylation is specific to learning, has a similar time course to that in long term potentiation, and like memory and long term potentiation, is dependent on N-methyl-D-aspartate receptor activation during training. Furthermore, the learning-induced increase in serine 831 phosphorylation is present at synapses and is in heteromeric complexes with the glutamate receptor subunit GluR2. These data indicate that a biochemical correlate of long term potentiation occurs at synapses in receptor complexes in a final, downstream, postsynaptic effector of long term potentiation during learning in vivo, further strengthening the link between long term potentiation and memory.     

Differential Roles of Phosphorylated AMPA Receptor GluR1 Subunits at Serine-831 and Serine-845 Sites in Spinal Cord Dorsal Horn in a Rat Model of Post-Operative Pain

Previous studies have demonstrated that the enhanced levels of phosphorylated α-amino-3-hydroxy-5-methy-4-isoxazole propionate (AMPA) receptor GluR1 subunits at Serine-831 (pGluR1-Ser-831) and Serine-845 (pGluR1-Ser-845) in the spinal cord dorsal horn are involved in central sensitization of inflammatory pain. However, whether the phosphorylatory regulation of AMPA receptor GluR1 subunits is implicated in the development and maintenance of post-operative pain remains unclear. The current study aims to examine the functional regulation of AMPA receptor GluR1 subunit through its phosphorylation mechanism during the period of post-operative painful events in rats. Our data indicated that the expression of pGluR1-Ser-831 in ipsilateral spinal cord dorsal horn increased significantly at 3 h after incision, then decreased gradually, and returned to the normal level 3 day post-incision. Meanwhile, the expression of pGluR1-Ser-845 and GluR1 in ipsilateral spinal cord dorsal horn remained unchanged. The cumulative pain scores increased at 3 h after incision, gradually decreased afterwards and returned to the baseline values at 4 day after incision and the trend was almost parallel to the expression changes of pGluR1-Ser-831 in spinal dorsal horn. Intrathecal injection of a calcium-dependent protein kinase (PKC) inhibitor, Gö6983 (10 μM), significantly reversed the incision-mediated over-expression of pGluR1-Ser-831 in spinal dorsal horn at 3 h after incision and decreased the cumulative pain scores as well. These results indicate that the phosphorylation of GluR1 subunits at Serine-831 and Serine-845 sites might be differentially regulated following surgical procedures and support a neurobiological mechanism of post-operative pain involved in phosphorylation of AMPA subunits GluR1-Ser-831, but not pGluR1-Ser-845. Our study suggests that the therapeutic targeting the phosphorylation regulation of AMPA receptor GluR1 subunit at Serine-831 site would be potentially significant for treating postoperative pain.

     

Regulation of AMPA Receptor Trafficking by O-Glycosylation

The present study investigated the role of O-linked β-N-acetylglucosamine (O-GlcNAc) glycosylation (O-GlcNAcylation) in AMPA receptor trafficking. Alloxan, an inhibitor of O-GlcNAc transferase, potentiated responses of AMPA receptors composed of the GluR1 subunit expressed in Xenopus oocytes. No potentiating effect of alloxan was obtained with mutant GluR1 (S831A) receptor lacking CaMKII phosphorylation site. Alloxan facilitated basal synaptic transmission to approximately 120% of basal levels and enhanced Schaffer collateral-CA1 long-term potentiation (LTP) in rat hippocampal slices, especially in the late phase of the LTP. Alloxan stimulated translocation of the GluR1 and GluR2 subunit from the cytosol towards the plasma membrane in rat hippocampal slices with the LTP, although it had no effect on subcellular distribution of the NR1 subunit. Taken together, the results of the present study show that alloxan regulates AMPA receptor trafficking by inhibiting O-GlcNAcylation, to modulate hippocampal synaptic transmission and synaptic plasticity.     

Regulation of phosphorylation of the GluR1 AMPA receptor by dopamine D2 receptors

In the striatum, stimulation of dopamine D2 receptors results in attenuation of glutamate responses. This effect is exerted in large part via negative regulation of AMPA glutamate receptors. Phosphorylation of the GluR1 subunit of the AMPA receptor has been proposed to play a critical role in the modulation of glutamate transmission, in striatal medium spiny neurons. Here, we have examined the effects of blockade of dopamine D2-like receptors on the phosphorylation of GluR1 at the cAMP-dependent protein kinase (PKA) site, Ser845, and at the protein kinase C and calcium/calmodulin-dependent protein kinase II site, Ser831. Administration of haloperidol, an antipsychotic drug with dopamine D2 receptor antagonistic properties, increases the phosphorylation of GluR1 at Ser845, without affecting phosphorylation at Ser831. The same effect is observed using eticlopride, a selective dopamine D2 receptor antagonist. In contrast, administration of the dopamine D2-like agonist, quinpirole, decreases GluR1 phosphorylation at Ser845. The increase in Ser845 phosphorylation produced by haloperidol is abolished in dopamine- and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) knockout mice, or in mice in which the PKA phosphorylation site on DARPP-32 (i.e. Thr34) has been mutated (Thr34-->Ala mutant mice), and requires tonic activation of adenosine A2A receptors. These results demonstrate that dopamine D2 antagonists increase GluR1 phosphorylation at Ser845 by removing the inhibitory tone exerted by dopamine D2 receptors on the PKA/DARPP-32 cascade.     

Incorporation of inwardly rectifying AMPA receptors at silent synapses during hippocampal long-term potentiation

Despite decades of study, the mechanisms by which synapses express the increase in strength during long-term potentiation (LTP) remain an area of intense interest. Here, we have studied how AMPA receptor subunit composition changes during the early phases of hippocampal LTP in CA1 pyramidal neurons. We studied LTP at silent synapses that initially lack AMPA receptors, but contain NMDA receptors. We show that strongly inwardly rectifying AMPA receptors are initially incorporated at silent synapses during LTP and are then subsequently replaced by non-rectifying AMPA receptors. These findings suggest that silent synapses initially incorporate GluA2-lacking, calcium-permeable AMPA receptors during LTP that are then replaced by GluA2-containing calcium-impermeable receptors. We also show that LTP consolidation at CA1 synapses requires a rise in intracellular calcium concentration during the early phase of expression, indicating that calcium influx through the GluA2-lacking AMPA receptors drives their replacement by GluA2-containing receptors during LTP consolidation. Taken together with previous studies in hippocampus and in other brain regions, these findings suggest that a common mechanism for the expression of activity-dependent glutamatergic synaptic plasticity involves the regulation of GluA2-subunit composition and highlights a critical role for silent synapses in this process.     

Persistent Inflammation-induced Up-regulation of Brain-derived Neurotrophic Factor (BDNF) Promotes Synaptic Delivery of α-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic Acid Receptor GluA1 Subunits in Descending Pain Modulatory Circuits

The enhanced AMPA receptor phosphorylation at GluA1 serine 831 sites in the central pain-modulating system plays a pivotal role in descending pain facilitation after inflammation, but the underlying mechanisms remain unclear. We show here that, in the rat brain stem, in the nucleus raphe magnus, which is a critical relay in the descending pain-modulating system of the brain, persistent inflammatory pain induced by complete Freund adjuvant (CFA) can enhance AMPA receptor-mediated excitatory postsynaptic currents and the GluA2-lacking AMPA receptor-mediated rectification index. Western blot analysis showed an increase in GluA1 phosphorylation at Ser-831 but not at Ser-845. This was accompanied by an increase in distribution of the synaptic GluA1 subunit. In parallel, the level of histone H3 acetylation at bdnf gene promoter regions was reduced significantly 3 days after CFA injection, as indicated by ChIP assays. This was correlated with an increase in BDNF mRNA levels and BDNF protein levels. Sequestering endogenous extracellular BDNF with TrkB-IgG in the nucleus raphe magnus decreased AMPA receptor-mediated synaptic transmission and GluA1 phosphorylation at Ser-831 3 days after CFA injection. Under the same conditions, blockade of TrkB receptor functions, phospholipase C, or PKC impaired GluA1 phosphorylation at Ser-831 and decreased excitatory postsynaptic currents mediated by GluA2-lacking AMPA receptors. Taken together, these results suggest that epigenetic up-regulation of BDNF by peripheral inflammation induces GluR1 phosphorylation at Ser-831 sites through activation of the phospholipase C-PKC signaling cascade, leading to the trafficking of GluA1 to pain-modulating neuronal synapses.     

Extrasynaptic membrane trafficking regulated by GluR1 serine 845 phosphorylation primes AMPA receptors for long-term potentiation

Enhancement of synaptic transmission, as occurs in long-term potentiation (LTP), can result from several mechanisms that are regulated by phosphorylation of the AMPA-type glutamate receptor (AMPAR). Using a quantitative assay of net serine 845 (Ser-845) phosphorylation in the GluR1 subunit of AMPARs, we investigated the relationship between phospho-Ser-845, GluR1 surface expression, and synaptic strength in hippocampal neurons. About 15% of surface AMPARs in cultured neurons were phosphorylated at Ser-845 basally, whereas chemical potentiation (forskolin/rolipram treatment) persistently increased this to 60% and chemical depression (N-methyl-D-aspartate treatment) decreased it to 10%. These changes in Ser-845 phosphorylation were paralleled by corresponding changes in the surface expression of AMPARs in both cultured neurons and hippocampal slices. For every 1% increase in net phospho-Ser-845, there was 0.75% increase in the surface fraction of GluR1. Phosphorylation of Ser-845 correlated with a selective delivery of AMPARs to extrasynaptic sites, and their synaptic localization required coincident synaptic activity. Furthermore, increasing the extrasynaptic pool of AMPA receptors resulted in stronger theta burst LTP. Our results support a two-step model for delivery of GluR1-containing AMPARs to synapses during activity-dependent LTP, where Ser-845 phosphorylation can traffic AMPARs to extrasynaptic sites for subsequent delivery to synapses during LTP.     

Leptin reverses long-term potentiation at hippocampal CA1 synapses

The hormone leptin crosses the blood brain barrier and regulates numerous neuronal functions, including hippocampal synaptic plasticity. Here we show that application of leptin resulted in the reversal of long-term potentiation (LTP) at hippocampal CA1 synapses. The ability of leptin to depotentiate CA1 synapses was concentration-dependent and it displayed a distinct temporal profile. Leptin-induced depotentiation was not associated with any change in the paired pulse facilitation ratio or the coefficient of variance, indicating a post-synaptic locus of expression. Moreover, the synaptic activation of NMDA receptors was required for leptin-induced depotentiation as the effects of leptin were blocked by the competitive NMDA receptor antagonist, D-aminophosphovaleric acid (D-AP5). The signaling mechanisms underlying leptin-induced depotentiation involved activation of the calcium/calmodulin-dependent protein phosphatase, calcineurin, but were independent of c- jun NH₂ terminal kinase. Furthermore, leptin-induced depotentiation was accompanied by a reduction in α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor rectification indicating that loss of glutamate receptor 2 (GluR2)-lacking AMPA receptors underlies this process. These data indicate that leptin reverses hippocampal LTP via a process involving calcineurin-dependent internalization of GluR2-lacking AMPA receptors which further highlights the key role for this hormone in regulating hippocampal synaptic plasticity and neuronal development.     

Bidirectional synaptic plasticity regulated by phosphorylation of stargazin-like TARPs

Synaptic plasticity involves protein phosphorylation cascades that alter the density of AMPA-type glutamate receptors at excitatory synapses; however, the crucial phosphorylated substrates remain uncertain. Here, we show that the AMPA receptor-associated protein stargazin is quantitatively phosphorylated and that stargazin phosphorylation promotes synaptic trafficking of AMPA receptors. Synaptic NMDA receptor activity can induce both stargazin phosphorylation, via activation of CaMKII and PKC, and stargazin dephosphorylation, by activation of PP1 downstream of PP2B. At hippocampal synapses, long-term potentiation and long-term depression require stargazin phosphorylation and dephosphorylation, respectively. These results establish stargazin as a critical substrate in the bidirectional control of synaptic strength, which is thought to underlie aspects of learning and memory.     

Regulation of AMPA receptor-mediated synaptic transmission by clathrin-dependent receptor internalization

Redistribution of postsynaptic AMPA- (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid-) subtype glutamate receptors may regulate synaptic strength at glutamatergic synapses, but the mediation of the redistribution is poorly understood. We show that AMPA receptors underwent clathrin-dependent endocytosis, which was accelerated by insulin in a GluR2 subunit-dependent manner. Insulin-stimulated endocytosis rapidly decreased AMPA receptor numbers in the plasma membrane, resulting in long-term depression (LTD) of AMPA receptor-mediated synaptic transmission in hippocampal CA1 neurons. Moreover, insulin-induced LTD and low-frequency stimulation-(LFS-) induced homosynaptic CA1 LTD were found to be mutually occlusive and were both blocked by inhibiting postsynaptic clathrin-mediated endocytosis. Thus, controlling postsynaptic receptor numbers through endocytosis may be an important mechanism underlying synaptic plasticity in the mammalian CNS.     

NMDA receptor-dependent activation of the small GTPase Rab5 drives the removal of synaptic AMPA receptors during hippocampal LTD

The activity-dependent removal of AMPA receptors from synapses underlies long-term depression in hippocampal excitatory synapses. In this study, we have investigated the role of the small GTPase Rab5 during this process. We propose that Rab5 is a critical link between the signaling cascades triggered by LTD induction and the machinery that executes the activity-dependent removal of AMPA receptors. We have found that Rab5 activation drives the specific internalization of synaptic AMPA receptors in a clathrin-dependent manner and that this activity is required for LTD. Interestingly, Rab5 does not participate in the constitutive cycling of AMPA receptors. Rab5 is able to remove both GluR1 and GluR2 AMPA receptor subunits, leading to GluR1 dephosphorylation. Importantly, NMDA receptor-dependent LTD induction produces a rapid and transient increase of active (GTP bound) Rab5. We propose a model in which synaptic activity leads to Rab5 activation, which in turn drives the removal of AMPA receptors from synapses.     

Age-dependent requirement of AKAP150-anchored PKA and GluR2-lacking AMPA receptors in LTP

Association of PKA with the AMPA receptor GluR1 subunit via the A kinase anchor protein AKAP150 is crucial for GluR1 phosphorylation. Mutating the AKAP150 gene to specifically prevent PKA binding reduced PKA within postsynaptic densities (>70%). It abolished hippocampal LTP in 7-12 but not 4-week-old mice. Inhibitors of PKA and of GluR2-lacking AMPA receptors blocked single tetanus LTP in hippocampal slices of 8 but not 4-week-old WT mice. Inhibitors of GluR2-lacking AMPA receptors also prevented LTP in 2 but not 3-week-old mice. Other studies demonstrate that GluR1 homomeric AMPA receptors are the main GluR2-lacking AMPA receptors in adult hippocampus and require PKA for their functional postsynaptic expression during potentiation. AKAP150-anchored PKA might thus critically contribute to LTP in adult hippocampus in part by phosphorylating GluR1 to foster postsynaptic accumulation of homomeric GluR1 AMPA receptors during initial LTP in 8-week-old mice.     

Metabotropic glutamate and dopamine receptors co-regulate AMPA receptor activity through PKA in cultured chick retinal neurones: effect on GluR4 phosphorylation and surface expression

Glutamate receptor phosphorylation has been implicated in several forms of modulation of synaptic transmission. It has been reported that protein kinase A (PKA) can phosphorylate the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunit GluR4 on Ser842, both in vitro and in vivo. Here, we studied the regulation of GluR4 phosphorylation and intracellular trafficking by PKA and by metabotropic receptors coupled to adenylyl cyclase (AC), in cultured chick retinal amacrine-like neurones, which are enriched in GluR4. The regulation of AMPA receptor activity by PKA and by metabotropic AC-coupled receptors was also investigated by measuring the [Ca2+]i response to kainate in Na(+)-free medium. Stimulation of AC with forskolin (FSK), or using the selective agonist of dopamine D1 receptors (+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol (SKF38393), increased the [Ca2+]i response to kainate, GluR4 phosphorylation at Ser842 and GluR4 surface expression. Pre-incubation of the cells with (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV), an agonist of group II metabotropic glutamate receptors (mGluR), which are coupled to inhibition of AC, inhibited the effect of FSK and of SKF38393 on AMPA receptor activity, GluR4 phosphorylation and expression at the plasma membrane. These results indicate that there is a functional cross-talk between dopamine D1 receptors and group II mGluR in the regulation of GluR4 phosphorylation and AMPA receptor activity. Our data show that GluR4 phosphorylation at Ser842 by PKA, and its recruitment to the plasma membrane upon phosphorylation, is regulated by metabotropic receptors.     

Targeting inhibition of GluR1 Ser845 phosphorylation with an RNA aptamer that blocks AMPA receptor trafficking

Phosphorylation at glutamate receptor subunit 1(GluR1) Ser845 residue has been widely accepted to involve in GluR1-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor trafficking, but the in vivo evidence has not yet been established. One of the main obstacles is the lack of effective methodologies to selectively target phosphorylation at single amino acid residue. In this study, the Escherichia  coli -expressed glutathione- S -transferase-tagged intracellular carboxyl-terminal domain of GluR1 (cGluR1) was phosphorylated by protein kinase A for in vitro selection. We have successfully selected aptamers which effectively bind to phospho-Ser845 cGluR1 protein, but without binding to phospho-Ser831 cGluR1 protein. Moreover, pre-binding of the unphospho-cGluR1 protein with these aptamers inhibits protein kinase A-mediated phosphorylation at Ser845 residue. In contrast, the pre-binding of aptamer A2 has no effect on protein kinase C-mediated phosphorylation at Ser831 residue. Importantly, the representative aptamer A2 can effectively bind the mammalian GluR1 that inhibited GluR1/GluR1-containing AMPA receptor trafficking to the cell surface and abrogated forskolin-stimulated phosphorylation at GluR1 Ser845 in both green fluorescent protein–GluR1-transfected human embryonic kidney cells and cultured rat cortical neurons. The strategy to use aptamer to modify single-residue phosphorylation is expected to facilitate evaluation of the potential role of AMPA receptors in various forms of synaptic plasticity including that underlying psychostimulant abuse.     

Phosphorylation of the AMPA receptor GluR1 subunit is required for synaptic plasticity and retention of spatial memory

Plasticity of the nervous system is dependent on mechanisms that regulate the strength of synaptic transmission. Excitatory synapses in the brain undergo long-term potentiation (LTP) and long-term depression (LTD), cellular models of learning and memory. Protein phosphorylation is required for the induction of many forms of synaptic plasticity, including LTP and LTD. However, the critical kinase substrates that mediate plasticity have not been identified. We previously reported that phosphorylation of the GluR1 subunit of AMPA receptors, which mediate rapid excitatory transmission in the brain, is modulated during LTP and LTD. To test if GluR1 phosphorylation is necessary for plasticity and learning and memory, we generated mice with knockin mutations in the GluR1 phosphorylation sites. The phosphomutant mice show deficits in LTD and LTP and have memory defects in spatial learning tasks. These results demonstrate that phosphorylation of GluR1 is critical for LTD and LTP expression and the retention of memories.     

Changes in Subcellular Distribution and Phosphorylation of GluR1 in Lesioned Striatum of 6-Hydroxydopamine-Lesioned and l-dopa-Treated Rats

Recent evidence has linked striatal amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptor function to the adverse effects of long-term dopaminergic treatment in Parkinson’s disease. The phosphorylation of AMPA subunit, GluR1, reflects AMPA receptor activity. To determine whether serine phosphorylation of GluR1 subunit by activation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) contributes to the process, we examined the effects of unilateral nigrostriatal depletion with 6-hydroxydopamine and subsequent l-dopa treatment on motor responses and phosphorylation states. Three weeks of l-dopa administration to rats shortened the duration of the rotational response. We found a significant reduction in the abundance of both phosphorylated GluR1 at serine-831 site (pGluR1S831) and GluR1 in the cell plasma membrane of lesioned striatum. Chronic treatment of lesioned rats with l-dopa markedly upregulated the phosphorylation of GluR1 in lesioned striatum with a concomitant normalization of the plasma membrane GluR1 abundance, which lasted at least 1 day after withdrawal of chronic l-dopa treatment. Our immunostaining data showed that these changes were confined to parvalbumin-positive neurons where GluR1 subunits are exclusively expressed. Both the altered motor response duration and the degree of pGluR1S831 were attenuated by the intrastriatal administration of CaMKII inhibitor KN-93. These findings suggest that activation of CaMKII contributes to both development and maintenance of motor response duration alterations, through a mechanism that involves an increase in pGluR1S831 within parvalbumin-positive neurons.Maowen Ba and Min Kong are contributed equally to this work.