Inhibition of willardiine-induced currents through rat GluR6/KA-2 kainate receptor channels by Zinc and other divalent cations

mRNA editing of kainate receptor subunits: what do we know so far?

Kainate receptors (KARs) are considered one of the key modulators of synaptic activity in the mammalian central nervous system. These receptors were discovered more than 30 years ago, but their role in brain functioning remains unclear due to some peculiarities. One such feature of these receptors is the editing of pre-mRNAs encoding GluK1 and GluK2 subunits. Despite the long history of studying this phenomenon, numerous questions remain unanswered. This review summarizes the current data about the mechanism and role of pre-mRNA editing of KAR subunits in the mammalian brain and proposes a perspective of future investigations.

Structure, Function, and Pharmacology of Glutamate Receptor Ion Channels

Many physiologic effects of l-glutamate, the major excitatory neurotransmitter in the mammalian central nervous system, are mediated via signaling by ionotropic glutamate receptors (iGluRs). These ligand-gated ion channels are critical to brain function and are centrally implicated in numerous psychiatric and neurologic disorders. There are different classes of iGluRs with a variety of receptor subtypes in each class that play distinct roles in neuronal functions. The diversity in iGluR subtypes, with their unique functional properties and physiologic roles, has motivated a large number of studies. Our understanding of receptor subtypes has advanced considerably since the first iGluR subunit gene was cloned in 1989, and the research focus has expanded to encompass facets of biology that have been recently discovered and to exploit experimental paradigms made possible by technological advances. Here, we review insights from more than 3 decades of iGluR studies with an emphasis on the progress that has occurred in the past decade. We cover structure, function, pharmacology, roles in neurophysiology, and therapeutic implications for all classes of receptors assembled from the subunits encoded by the 18 ionotropic glutamate receptor genes. SIGNIFICANCE STATEMENT: Glutamate receptors play important roles in virtually all aspects of brain function and are either involved in mediating some clinical features of neurological disease or represent a therapeutic target for treatment. Therefore, understanding the structure, function, and pharmacology of this class of receptors will advance our understanding of many aspects of brain function at molecular, cellular, and system levels and provide new opportunities to treat patients.

Zinc Modulates Olfactory Bulb Kainate Receptors

Kainate receptors (KARs) are glutamate receptors with ionotropic and metabotropic activity composed of the GluK1-GluK5 subunits. We previously reported that KARs modulate excitatory and inhibitory transmission in the olfactory bulb (OB). Zinc, which is highly concentrated in the OB, also appears to modulate OB synaptic transmission via actions at other ionotropic glutamate receptors (i.e., AMPA, NMDA). However, few reports of effects of zinc on recombinant and/or native KARs exist and none have involved the OB. In the present study, we investigated the effects of exogenously applied zinc on OB KARs expressed by mitral/tufted (M/T) cells. We found that 100 µM zinc inhibits currents evoked by various combinations of KAR agonists (kainate or SYM 2081) and the AMPA receptor antagonist SYM 2206. The greatest degree of zinc-mediated inhibition was observed with coapplication of zinc with the GluK1- and GluK2-preferring agonist SYM 2081 plus SYM 2206. This finding is consistent with prior reports of zinc's inhibitory effects on some recombinant (homomeric GluK1 and GluK2 and heteromeric GluK2/GluK4 and GluK2/GluK5) KARs, although potentiation of other (GluK3, GluK2/3) KARs has also been described. It is also of potential importance given our previously reported molecular data suggesting that OB neurons express relatively high levels of GluK1 and GluK2. Our present findings suggest that a physiologically relevant concentration of zinc modulates KARs expressed by M/T cells. As M/T cells are targets of zinc-containing olfactory sensory neurons, synaptically released zinc may influence odor information-encoding synaptic circuits in the OB via actions at KARs.

Glutamate receptor ion channels: structure, regulation, and function

The mammalian ionotropic glutamate receptor family encodes 18 gene products that coassemble to form ligand-gated ion channels containing an agonist recognition site, a transmembrane ion permeation pathway, and gating elements that couple agonist-induced conformational changes to the opening or closing of the permeation pore. Glutamate receptors mediate fast excitatory synaptic transmission in the central nervous system and are localized on neuronal and non-neuronal cells. These receptors regulate a broad spectrum of processes in the brain, spinal cord, retina, and peripheral nervous system. Glutamate receptors are postulated to play important roles in numerous neurological diseases and have attracted intense scrutiny. The description of glutamate receptor structure, including its transmembrane elements, reveals a complex assembly of multiple semiautonomous extracellular domains linked to a pore-forming element with striking resemblance to an inverted potassium channel. In this review we discuss International Union of Basic and Clinical Pharmacology glutamate receptor nomenclature, structure, assembly, accessory subunits, interacting proteins, gene expression and translation, post-translational modifications, agonist and antagonist pharmacology, allosteric modulation, mechanisms of gating and permeation, roles in normal physiological function, as well as the potential therapeutic use of pharmacological agents acting at glutamate receptors.

pH-dependent inhibition of kainate receptors by zinc

Kainate receptors contribute to synaptic plasticity and rhythmic oscillatory firing of neurons in corticolimbic circuits including hippocampal area CA3. We use zinc chelators and mice deficient in zinc transporters to show that synaptically released zinc inhibits postsynaptic kainate receptors at mossy fiber synapses and limits frequency facilitation of kainate, but not AMPA EPSCs during theta-pattern stimulation. Exogenous zinc also inhibits the facilitatory modulation of mossy fiber axon excitability by kainate but does not suppress the depressive effect of kainate on CA3 axons. Recombinant kainate receptors are inhibited in a subunit-dependent manner by physiologically relevant concentrations of zinc, with receptors containing the KA1 subunit being sensitive to submicromolar concentrations of zinc. Zinc inhibition does not alter receptor desensitization nor apparent agonist affinity and is only weakly voltage dependent, which points to an allosteric mechanism. Zinc inhibition is reduced at acidic pH. Thus, in the presence of zinc, a fall in pH potentiates kainate receptors by relieving zinc inhibition. Acidification of the extracellular space, as occurs during repetitive activity, may therefore serve to unmask kainate receptor neurotransmission. We conclude that zinc modulation of kainate receptors serves an important role in shaping kainate neurotransmission in the CA3 region.

Zinc at glutamatergic synapses

It has long been known that the mammalian forebrain contains a subset of glutamatergic neurons that sequester zinc in their synaptic vesicles. This zinc may be released into the synaptic cleft upon neuronal activity. Extracellular zinc has the potential to interact with and modulate many different synaptic targets, including glutamate receptors and transporters. Among these targets, NMDA receptors appear particularly interesting because certain NMDA receptor subtypes (those containing the NR2A subunit) contain allosteric sites exquisitely sensitive to extracellular zinc. The existence of these high-affinity zinc binding sites raises the possibility that zinc may act both in a phasic and tonic mode. Changes in zinc concentration and subcellular zinc distribution have also been described in several pathological conditions linked to glutamatergic transmission dysfunctions. However, despite intense investigation, the functional significance of vesicular zinc remains largely a mystery. In this review, we present the anatomy and the physiology of the glutamatergic zinc-containing synapse. Particular emphasis is put on the molecular and cellular mechanisms underlying the putative roles of zinc as a messenger involved in excitatory synaptic transmission and plasticity. We also highlight the many controversial issues and unanswered questions. Finally, we present and compare two widely used zinc chelators, CaEDTA and tricine, and show why tricine should be preferred to CaEDTA when studying fast transient zinc elevations as may occur during synaptic activity.

Syntaxin 1A occludes GABA(B) receptor-induced inhibition of exocytosis downstream of Ca(2+) entry in mouse hippocampal neurons

The mechanisms underlying gamma-amino butyric acid (GABA(B)) receptor-mediated inhibition of exocytosis have been characterized in a variety of synapses. Using patch-clamp recording methods, we attempted to clarify the intracellular mechanisms underlying presynaptic inhibition in autaptic synapses of isolated mouse hippocampal neurons. Baclofen, a selective GABA(B) receptor agonist, decreased the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) without changing their amplitude in Ca(2+)-free extracellular solution, suggesting that baclofen inhibits exocytosis downstream of Ca(2+) entry. Syntaxin 1A is known to modulate exocytosis and suppress neuronal sprouting. Antisense oligonucleotide-mediated knockdown of syntaxin 1A increased the frequency of mEPSCs under Ca(2+)-free condition. Estimation of the number of functional release sites by staining with FM1-43 indicated that the increased frequency of mEPSCs was induced by facilitation of exocytosis at each site, rather than by an increased number of release sites due to neuronal sprouting. Baclofen reduced mEPSC frequency in syntaxin 1A-knockdown neurons to the same level as that in nonsense oligonucleotide transfected neurons under Ca(2+)-free condition. These results suggest that the GABA(B) receptor- and syntaxin 1A-induced inhibitions of exocytosis occlude one another and that the GABA(B) receptor shares a common intracellular pathway with syntaxin 1A in inhibiting transmitter release downstream of Ca(2+) entry.

Kainate receptors

Kainate receptors form a family of ionotropic glutamate receptors that appear to play a special role in the regulation of the activity of synaptic networks. This review first describes briefly the molecular and pharmacological properties of native and recombinant kainate receptors. It then attempts to outline the general principles that appear to govern the function of kainate receptors in the activity of synaptic networks under physiological conditions. It subsequently describes the way that kainate receptors are involved in synaptic integration, synaptic plasticity, the regulation of neurotransmitter release and the control of neuronal excitability, and the manner in which they might play an important role in synaptogenesis and synaptic maturation. These functions require the proper subcellular localization of kainate receptors in specific functional domains of the neuron, necessitating complex cellular and molecular trafficking events. We show that our comprehension of these mechanisms is just starting to emerge. Finally, this review presents evidence that implicates kainate receptors in pathophysiological conditions such as epilepsy, excitotoxicity and pain, and that shows that these receptors represent promising therapeutic targets.