Ruiz Arnaud, Sachidhanandam Shankar, Utvik Jo Kristian, Coussen Françoise, Mulle Christophe
Laboratoire de Physiologie Cellulaire de la Synapse, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5091, Université Bordeaux 2, 33077 Bordeaux, France.
J Neurosci. 2005 Dec 14;25(50):11710-8. doi: 10.1523/JNEUROSCI.4041-05.2005.
Heteromeric kainate receptors (KARs) containing both glutamate receptor 6 (GluR6) and KA2 subunits are involved in KAR-mediated EPSCs at mossy fiber synapses in CA3 pyramidal cells. We report that endogenous glutamate, by activating KARs, reversibly inhibits the slow Ca2+-activated K+ current I(sAHP) and increases neuronal excitability through a G-protein-coupled mechanism. Using KAR knockout mice, we show that KA2 is essential for the inhibition of I(sAHP) in CA3 pyramidal cells by low nanomolar concentrations of kainate, in addition to GluR6. In GluR6(-/-) mice, both ionotropic synaptic transmission and inhibition of I(sAHP) by endogenous glutamate released from mossy fibers was lost. In contrast, inhibition of I(sAHP) was absent in KA2(-/-) mice despite the preservation of KAR-mediated EPSCs. These data indicate that the metabotropic action of KARs did not rely on the activation of a KAR-mediated inward current. Biochemical analysis of knock-out mice revealed that KA2 was required for the interaction of KARs with Galpha(q/11)-proteins known to be involved in I(sAHP) modulation. Finally, the ionotropic and metabotropic actions of KARs at mossy fiber synapses were differentially sensitive to the competitive glutamate receptor ligands kainate (5 nM) and kynurenate (1 mM). We propose a model in which KARs could operate in two modes at mossy fiber synapses: through a direct ionotropic action of GluR6, and through an indirect G-protein-coupled mechanism requiring the binding of glutamate to KA2.
包含谷氨酸受体6（GluR6）和KA2亚基的异聚海人藻酸受体（KARs）参与CA3锥体细胞苔藓纤维突触处KAR介导的兴奋性突触后电流（EPSCs）。我们报告称，内源性谷氨酸通过激活KARs，可逆性抑制缓慢的钙激活钾电流I（sAHP），并通过G蛋白偶联机制增加神经元兴奋性。利用KAR基因敲除小鼠，我们发现除了GluR6外，KA2对于低纳摩尔浓度的海人藻酸抑制CA3锥体细胞中的I（sAHP）至关重要。在GluR6基因敲除小鼠中，离子型突触传递以及苔藓纤维释放的内源性谷氨酸对I（sAHP）的抑制作用均丧失。相比之下，尽管保留了KAR介导的EPSCs，但KA2基因敲除小鼠中不存在I（sAHP）的抑制作用。这些数据表明，KARs的促代谢作用并不依赖于KAR介导的内向电流的激活。对基因敲除小鼠的生化分析表明，KA2是KARs与已知参与I（sAHP）调节的Gαq/11蛋白相互作用所必需的。最后，KARs在苔藓纤维突触处的离子型和促代谢作用对竞争性谷氨酸受体配体海人藻酸（5 nM）和犬尿氨酸（1 mM）的敏感性不同。我们提出了一个模型，其中KARs可以在苔藓纤维突触处以两种模式发挥作用：通过GluR6的直接离子型作用，以及通过需要谷氨酸与KA2结合的间接G蛋白偶联机制。
