Metabotropic Actions of Kainate Receptors in the Regulation of IsAHP and Excitability in CA1 Pyramidal Cells

Reversing Aging: Decline in Complex Olfactory Learning Can be Rectified by Restoring Intrinsic Plasticity of Hippocampal CA1 Pyramidal Neurons

The acquisition of complex rules requires modifications in intrinsic plasticity of excitatory neurons within relevant brain areas. Olfactory discrimination (OD) rule learning occludes slow calcium-dependent potassium current (sIAHP ) in piriform cortex (PC) pyramidal neurons, which increases their intrinsic neuronal excitability. Similar learning-induced sIAHP changes are demonstrated in hippocampal CA1. The shutdown of sIAHP is mediated by the metabotropic activation of the kainate subtype glutamatergic receptor, GluK2. Here, the duration of training required for OD rule learning increased significantly as the mice matured and aged is first shown, which appears earlier in 5xFAD mice. At the cellular biophysical level, aging is accompanied by reduction in the post-burst AHP in these neurons, while neuronal excitability remains stable. This is in contrast to aging CA1 neurons that exhibit enhanced post-burst AHPs in previous reports. Kainate reduces post-burst AHP in adults, but not in aged PC neurons, whereas it reduces post-burst AHPs in hippocampal CA1 pyramidal neurons of both young and aged mice. Overexpression of GluK2 in CA1 neurons restores OD learning capabilities in aged wild-type and 5xFAD mice, to a level comparable to young adults. Activation of GluK2 receptors in selectively vulnerable neurons can prevent aging-related cognitive decline is suggested.

GluK2 Q/R editing regulates kainate receptor signaling and long-term potentiation of AMPA receptors

Q/R editing of the kainate receptor (KAR) subunit GluK2 radically alters recombinant KAR properties, but the effects on endogenous KARs in vivo remain largely unexplored. Here, we compared GluK2 editing-deficient mice that express ∼95% unedited GluK2(Q) to wild-type counterparts that express ∼85% edited GluK2(R). At mossy fiber-CA3 (MF-CA3) synapses GluK2(Q) mice displayed increased postsynaptic KAR function and KAR-mediated presynaptic facilitation, demonstrating enhanced ionotropic function. Conversely, GluK2(Q) mice exhibited reduced metabotropic KAR function, assessed by KAR-mediated inhibition of slow after-hyperpolarization currents (ISAHP). GluK2(Q) mice also had fewer GluA1-and GluA3-containing AMPA receptors (AMPARs) and reduced postsynaptic AMPAR currents at both MF-CA3 and CA1-Schaffer collateral synapses. Moreover, long-term potentiation of AMPAR-mediated transmission at CA1-Schaffer collateral synapses was reduced in GluK2(Q) mice. These findings suggest that GluK2 Q/R editing influences ionotropic/metabotropic balance of KAR signaling to regulate synaptic expression of AMPARs and plasticity.

Olfactory rule learning-induced enhancement in intrinsic neuronal excitability is maintained by shutdown of the cholinergic M-current

Training rats in a particularly difficult olfactory discrimination task initiates a period of accelerated learning, manifested as a dramatic increase in the rats' capacity to discriminate between pairs of odors once they have learned the discrimination task, implying that rule learning has taken place. At the cellular biophysical level, rule learning is maintained by reduction in the conductance of the slow current (sI_AHP) simultaneously in most piriform cortex layer II pyramidal neurons. Such sI_AHP reduction is expressed in attenuation of the post-burst afterhyperpolarization (AHP) potential and thus in enhanced repetitive action potential firing. Previous studies have shown that a causal relationship exists between long-lasting post-burst AHP reduction and rule learning. A specific channel through which the sI_AHP flows has not been identified. The sI_AHP in pyramidal cells is critically dependent on membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)]. PtdIns(4,5)P(2) regulates the calcium sensitivity of the sI_AHP by acting downstream from the rise in intracellular calcium. These findings led to the interesting hypothesis that PtdIns(4,5)P(2) activates a variety of potassium channels. Thus, the sI_AHP would not represent a unitary ionic current but the embodiment of a generalized potassium channel gating mechanism. We thus hypothesized that the learning-induced increase in intrinsic excitability is mediated by reduced conductance of one or more of the currents that contribute to the sI_AHP. Here we first show, using current-clamp recordings, that the post-burst AHP in piriform cortex pyramidal neurons is also mediated by the I_h, and the contribution of this current to the post-burst AHP is also affected by learning. We also show, using whole-cell patch-clamp recordings, that the sI_AHP in neurons from trained rats is not sensitive to blocking membrane phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)], and to the blocking of the current mediated by the cholinergic muscarinic acetylcholine receptor (M-current). Further current-clamp recordings also show that blocking PtdIns(4,5)P(2) synthesis and application of a specific IKCa blocker have no effect on the post-burst AHP in neurons from trained as well as control rats. Taken together with results from our previous studies, these data suggest that rule learning-induced long-lasting enhancement in intrinsic neuronal excitability results from reduced conductance of the M-current and thus the slow potassium currents, which control repetitive spike firing.

Sustained postsynaptic kainate receptor activation downregulates AMPA receptor surface expression and induces hippocampal LTD

It is well established that long-term depression (LTD) can be initiated by either NMDA or mGluR activation. Here we report that sustained activation of GluK2 subunit-containing kainate receptors (KARs) leads to α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) endocytosis and induces LTD of AMPARs (KAR-LTD_AMPAR) in hippocampal neurons. The KAR-evoked loss of surface AMPARs is blocked by the ionotropic KAR inhibitor UBP 310 indicating that KAR-LTD_AMPAR requires KAR channel activity. Interestingly, however, blockade of PKC or PKA also reduces GluA2 surface expression and occludes the effect of KAR activation. In acute hippocampal slices, kainate application caused a significant loss of GluA2-containing AMPARs from synapses and long-lasting depression of AMPAR excitatory postsynaptic currents in CA1. These data, together with our previously reported KAR-LTP_AMPAR, demonstrate that KARs can bidirectionally regulate synaptic AMPARs and synaptic plasticity via different signaling pathways.

Kainate and AMPA receptors in epilepsy: Cell biology, signalling pathways and possible crosstalk

Epilepsy is caused when rhythmic neuronal network activity escapes normal control mechanisms, resulting in seizures. There is an extensive and growing body of evidence that the onset and maintenance of epilepsy involves alterations in the trafficking, synaptic surface expression and signalling of kainate and AMPA receptors (KARs and AMPARs). The KAR subunit GluK2 and AMPAR subunit GluA2 are key determinants of the properties of their respective assembled receptors. Both subunits are subject to extensive protein interactions, RNA editing and post-translational modifications. In this review we focus on the cell biology of GluK2-containing KARs and GluA2-containing AMPARs and outline how their regulation and dysregulation is implicated in, and affected by, seizure activity. Further, we discuss role of KARs in regulating AMPAR surface expression and plasticity, and the relevance of this to epilepsy. This article is part of the special issue on 'Glutamate Receptors - Kainate receptors'.

A Cellular Mechanism Underlying Enhanced Capability for Complex Olfactory Discrimination Learning

The biological mechanisms underlying complex forms of learning requiring the understanding of rules based on previous experience are not yet known. Previous studies have raised the intriguing possibility that improvement in complex learning tasks requires the long-term modulation of intrinsic neuronal excitability, induced by reducing the conductance of the slow calcium-dependent potassium current (sIAHP) simultaneously in most neurons in the relevant neuronal networks in several key brain areas. Such sIAHP reduction is expressed in attenuation of the postburst afterhyperpolarization (AHP) potential, and thus in enhanced repetitive action potential firing. Using complex olfactory discrimination (OD) learning as a model for complex learning, we show that brief activation of the GluK2 subtype glutamate receptor results in long-lasting enhancement of neuronal excitability in neurons from controls, but not from trained rats. Such an effect can be obtained by a brief tetanic synaptic stimulation or by direct application of kainate, both of which reduce the postburst AHP in pyramidal neurons. Induction of long-lasting enhancement of neuronal excitability is mediated via a metabotropic process that requires PKC and ERK activation. Intrinsic neuronal excitability cannot be modulated by synaptic activation in neurons from GluK2 knock-out mice. Accordingly, these mice are incapable of learning the complex OD task. Moreover, viral-induced overexpression of Gluk2 in piriform cortex pyramidal neurons results in remarkable enhancement of complex OD learning. Thus, signaling via kainate receptors has a central functional role in higher cognitive abilities.

Potentiation of tonic GABAergic inhibition by activation of postsynaptic kainate receptors

Presynaptic kainate-type glutamate ionotropic receptors (KARs) that mediate either the depression or the facilitation of GABA release have been intensively studied. Little attention has been given to the modulation of GABAA receptors (GABAARs) by postsynaptic KARs. Recent studies suggest that two GABAAR populations, synaptic (sGABAAR) and extrasynaptic (eGABAAR) GABAARs, mediate phasic and tonic forms of inhibition, respectively. Tonic inhibition plays an important role in the excitability of neuronal circuits and the occurrence of epileptic seizures. For this study, we are the first to report that the activation of postsynaptic KARs by the KAR agonist, Kainic acid (KA, 5 μM), enhanced tonic inhibition by potentiating eGABAARs. KA enhanced THIP-induced eGABAAR currents and prolonged the rise and decay time of muscimol-induced sGABAAR/eGABAAR currents, but also depressed the amplitude of evoked inhibitory postsynaptic currents (IPSCs), unitary IPSCs (uIPSCs), and muscimol-induced sGABAAR/eGABAAR currents. The PKC inhibitor, staurosporine (1 μM), in the patch pipette solution fully blocked the KA-induced potentiation of tonic inhibition, suggesting the involvement of an intracellular PKC pathway. Our study suggests that the activation of postsynaptic KARs potentiates eGABAARs but depresses sGABAARs. By activating postsynaptic KARs, synaptically released glutamate depresses phasic inhibition to facilitate neuronal plasticity, but potentiates tonic inhibition to protect neurons from over-excitation.

ATPA induced GluR5-containing kainite receptor S-nitrosylation via activation of GluR5-Gq-PLC-IP(3)R pathway and signalling module GluR5·PSD-95·nNOS

GluR5-containing kainite receptor (GluR5-KAR) plays an important role in the pathophysiology of nervous system diseases, while S-nitrosylation exerts a variety of effects on biological systems. However, the mechanism of GluR5-KAR S-nitrosylation is still unclear up to now. Here our researches found that GluR5-KAR selective agonist ATPA stimulation activated the nonclassical GluR5-KAR-Gq-PLC-IP(3)R pathway and the signalling module GluR5·PSD-95·nNOS (the former is more important), led to Ca(2+) release from intracellular calcium stores endoplasmic reticulum (ER) to cytoplasm and extracellular calcium indrawal, respectively, which further resulted in nNOS activation and GluR5-KAR S-nitrosylation, and then inhibited GluR5-mediated whole-cell current attenuation and induced apoptosis in primary cultured hippocampal neurons. Clarification of the primary mechanisms of GluR5-KAR S-nitrosylation induced by ATPA and identification of critical cysteine for GluR5-2a S-nitrosylation (Cys231 and Cys804) open up a brand-new field for revealing downstream signalling pathway of GluR5-KAR and its molecular characteristics, exploring the pathogenesis of neurological diseases and searching for promising therapies.