Pertussis toxin prevents presynaptic inhibition by kainate receptors of rat hippocampal [(3)H]GABA release

Kainate receptors are ionotropic receptors, also reported to couple to G(i)/G(o) proteins, increasing neuronal excitability through disinhibition of neuronal circuits. We directly tested in hippocampal synaptosomes if kainate receptor-mediated inhibition of GABA release involved a metabotropic action. The kainate analogue, domoate (3 microM), inhibited by 24% [(3)H]GABA-evoked release, an effect reduced by 76% in synaptosomes pre-treated with pertussis toxin. Protein kinase C inhibition attenuated by 82% domoate-induced inhibition of GABA release whereas protein kinase C activation did not change kainate receptor binding. Thus, domoate inhibition of GABA release recruits G(i)/G(o) proteins and a protein kinase C pathway.     

Synaptic kainate receptors

Kainate receptors are a family of ionotropic glutamate receptors with poorly understood functions. Recent evidence firmly establishes kainate receptors as postsynaptic mediators of synaptic transmission. A second, presynaptic, modulatory role of kainate receptors has also been suggested, although the mechanism(s) involved remain controversial.     

The physiological role of kainate receptors in the amygdala

The kainate subtype of glutamate receptors has received considerable attention in recent years, and a wealth of knowledge has been obtained regarding the function of these receptors. Kainate receptors have been shown to mediate synaptic transmission in some brain regions, modulate presynaptic release of glutamate and gamma-aminobutyric acid (GABA), and mediate synaptic plasticity or the development of seizure activity. This article focuses on the function of kainate receptors in the amygdala, a brain region that plays a central role in emotional behavior and certain psychiatric illnesses. Evidence is reviewed indicating that postsynaptic kainate receptors containing the glutamate receptor 5 kainate receptor (GLUk5) subunit are present on interneurons and pyramidal cells in the basolateral amygdala and mediate a component of the synaptic responses of these neurons to glutamatergic input. In addition, GLUk5-containing kainate receptors are present on presynaptic terminals of GABAergic neurons, where they modulate the release of GABA in an agonist concentration-dependent, bidirectional manner. GLUk5-containing kainate receptors also mediate a longlasting synaptic facilitation induced by low-frequency stimulation in the external capsule to the basolateral nucleus pathway, and they appear to be partly responsible for the susceptibility of the amygdala to epileptogenesis. Taken together, these findings have suggested a prominent role of GLUk5-containing kainate receptors in the regulation of neuronal excitability in the amygdala.     

The activation of glutamate receptors by kainic acid and domoic acid

The neurotoxins kainic acid and domoic acid are potent agonists at the kainate and alphaamino-5-methyl-3-hydroxyisoxazolone-4-propionate (AMPA) subclasses of ionotropic glutamate receptors. Although it is well established that AMPA receptors mediate fast excitatory synaptic transmission at most excitatory synapses in the central nervous system, the role of the high affinity kainate receptors in synaptic transmission and neurotoxicity is not entirely clear. Kainate and domoate differ from the natural transmitter, L-glutamate, in their mode of activation of glutamate receptors; glutamate elicits rapidly desensitizing responses while the two neurotoxins elicit non-desensitizing or slowly desensitizing responses at AMPA receptors and some kainate receptors. The inability to produce desensitizing currents and the high affinity for AMPA and kainate receptors are undoubtedly important factors in kainate and domoate-mediated neurotoxicity. Mutagenesis studies on cloned glutamate receptors have provided insight into the molecular mechanisms responsible for these unique properties of kainate and domoate.     

Autoreceptor Regulation of Glutamate and Aspartate Release from Slices of the Hippocampal CA1 Area

Slices of hippocampal area CA1 were employed to test the hypothesis that the release of glutamate and aspartate is regulated by the activation of excitatory amino acid autoreceptors. In the absence of added Mg2+, N-methyl-D-aspartate (NMDA)-receptor antagonists depressed the release of glutamate, aspartate, and gamma-aminobutyrate evoked by 50 mM K+. Conversely, the agonist NMDA selectively enhanced the release of aspartate. The latter action was observed, however, only when the K+ stimulus was reduced to 30 mM. Actions of the competitive antagonists 3-[(+/- )-2-carboxypiperazin-4-yl]-propyl-l-phosphonic acid (CPP) and D-2-amino-5-phosphonovalerate (D-AP5) differed, in that the addition of either 1.2 mM Mg2+ or 0.1 microM tetrodotoxin to the superfusion medium abolished the depressant effect of CPP without diminishing the effect of D-AP5. These results suggest that the activation of NMDA receptors by endogenous glutamate and aspartate enhances the subsequent release of these amino acids. The cellular mechanism may involve Ca2+ influx through presynaptic NMDA receptor channels or liberation of a diffusible neuromodulator linked to the activation of postsynaptic NMDA receptors. (RS)-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, a selective quisqualate receptor agonist, and kainate, an agonist active at both kainate and quisqualate receptors, selectively depressed the K(+)-evoked release of aspartate. Conversely, 6-cyano-7-nitro-quinoxaline-2,3-dione, an antagonist active at both quisqualate and kainate receptors, selectively enhanced aspartate release. These results suggest that glutamate can negatively modulate the release of aspartate by activating autoreceptors of the quisqualate, and possibly also of the kainate, type. Thus, the activation of excitatory amino acid receptors has both presynaptic and postsynaptic effects.     

Presynaptic kainate receptor-mediated bidirectional modulatory actions: Mechanisms

Kainate receptors (KARs) are members of the glutamate receptor family, which also includes two other ionotropic subtypes, i.e. NMDA- and AMPA-type receptors, and types I, II and III metabotropic glutamate receptors. KARs mediate synaptic transmission postynaptically through their ionotropic capacity, while presynaptically, they modulate the release of both GABA and glutamate through operationally diverse modus operandi. At hippocampal mossy fiber (MF)-CA3 synapses, KARs have a biphasic effect on glutamate release, such that, depending on the extent of their activation, a facilitation or depression of glutamate release can be observed. This modulation is posited to contribute to important roles of KARs in short- and long-term plasticity. Elucidation of the modes of action of KARs in their depression and facilitation of glutamate release is beginning to gather impetus. Here we will focus on the cellular mechanisms involved in the modulation of glutamate release by presynaptic KAR activation at MF-CA3 synapses, a field that has seen significant progress in recent years.     

Synaptic activation of presynaptic kainate receptors on hippocampal mossy fiber synapses

Kainate receptors (KARs) are a poorly understood family of ionotropic glutamate receptors. A role for these receptors in the presynaptic control of transmitter release has been proposed but remains controversial. Here, KAR agonists are shown to enhance fiber excitability, and a number of experiments show that this is a direct effect of KARs on the presynaptic fibers. In addition, KAR activation inhibits evoked transmitter release from mossy fiber synapses. Synaptic release of glutamate from either neighboring mossy fiber synapses or associational/commisural (A/C) synapses results in the activation of these presynaptic ionotropic KARs. These results, along with previous studies, indicate that KARs, through the endogenous release of glutamate, mediate excitatory postsynaptic potentials (EPSPs), alter presynaptic excitability, and modulate transmitter release.     

Nitric oxide-evoked glutamate release and cGMP production in cerebellar slices: control by presynaptic 5-HT1D receptors

We previously reported that pre- and postsynaptic 5-hydroxytryptamine (5-HT) receptors effectively control glutamatergic transmission in adult rat cerebellum. To investigate where 5-HT acts in the glutamate ionotropic receptors/nitric oxide/guanosine 3',5'-cyclic monophosphate (cGMP) pathway, in the present study 5-HT modulation of the cGMP response to the nitric oxide donor S-nitroso-penicillamine (SNAP) was studied in adult rat cerebellar slices. While cGMP elevation produced by high-micromolar SNAP was insensitive to 5-HT, 1 microM SNAP, expected to release nitric oxide in the low-nanomolar concentration range, elicited cGMP production and endogenous glutamate release both of which could be prevented by activating presynaptic 5-HT1D receptors. Released nitric oxide appeared responsible for cGMP production and glutamate release evoked by 1 microM SNAP, as both the effects were mimicked by the structurally unrelated nitric oxide donor 2-(N,N-diethylamino)-diazenolate-2-oxide (0.1 microM). Dependency of the 1 microM SNAP-evoked release of glutamate on external Ca2+, sensitivity to presynaptic release-regulating receptors and dependency on ionotropic glutamate receptor functioning, suggest that nitric oxide stimulates exocytotic-like, activity-dependent glutamate release. Activation of ionotropic glutamate receptors/nitric oxide synthase/guanylyl cyclase pathway by endogenously released glutamate was involved in the cGMP response to 1 microM SNAP, as blockade of NMDA/non-NMDA receptors, nitric oxide synthase or guanylyl cyclase, abolished the cGMP response. To conclude, in adult rat cerebellar slices low-nanomolar exogenous nitric oxide could facilitate glutamate exocytotic-like release possibly from parallel fibers that subsequently activated the glutamate ionotropic receptors/nitric oxide/cGMP pathway. Presynaptic 5-HT1D receptors could regulate the nitric oxide-evoked release of glutamate and subsequent cGMP production.     

Effects of blockers of voltage-operated potassium channels on an NMDA component of excitatory synaptic transmission in theCA1 subfield of the rat hippocampus

The effects of voltage-operated potassium channel blockers on evoked excitatory synaptic transmission were studied in theCA1 subfield of rat hippocampal slices. Incubation with 50 μM 4-aminopyridine (n=27), 300 nM α-dendrotoxin (n=3), or 5 to 25 mM tetraethylammonium (n=7) resulted in an enhancement of the peak amplitude of excitatory postsynaptic currents (EPSC) and significant prolongation of their decay at strong stimuli, due to an increased contribution of NMDA receptors into EPSC. In five experiments, the presence of an AMPA receptor antagonist, 4-aminopyridine, led to the appearance of NMDA receptor-mediated field excitatory postsynaptic potentials (fEPSP). It is suggested that various modulations increasing presynaptic Ca²⁺ entry and, consequently, glutamate release may increase an NMDA component of synaptic transmission via excitation of polysynaptic excitatory pathways and/or due to glutamate spillover to distant extrasynaptic NMDA receptors.     

A glutamate receptor channel with high affinity for domoate and kainate.

The non-NMDA family of glutamate receptors comprises a growing number of structurally related subunits (GluR-A to -D or -1 to -4; GluR-5, -6; KA-1). GluR-A to -D appear to constitute the major AMPA receptor subtypes but the functional and pharmacological characteristics of the other subunits are unresolved. Using a mammalian expression system we demonstrate here that homomeric GluR-5 receptors exhibit properties of a high affinity domoate (KD approximately 2 nM) and kainate (KD approximately 70 nM) binding site. For these receptors, the rank order of ligands competing with [3H]kainate binding was domoate much greater than quisqualate approximately glutamate much greater than AMPA approximately CNQX. The respective receptor channels were gated in decreasing order of sensitivity by domoate, kainate, glutamate and AMPA. In contrast to recombinantly expressed GluR-A to -D channels, currents elicited at GluR-5 receptor desensitize channels to all agonists. This property is characteristic of currents in peripheral neurons on sensory ganglia. These findings suggest the existence of at least two distinct types of non-NMDA receptor channels, both gated by AMPA and kainate, but differing in pharmacology and current properties.     

Presynaptic kainate receptors in the hippocampus: slowly emerging from obscurity.

Kainate receptor agonists depress transmitter release at several synapses in the hippocampus. Distinct mechanisms appear to underlie this phenomenon at different synapses. Recently, it has emerged that presynaptic kainate receptors can also potentiate the release of both GABA and glutamate and that axonal kainate receptors can trigger ectopic action potentials in interneurons. Because synaptically released glutamate mimics many of the actions of exogenous agonists, presynaptic kainate receptors potentially play an extensive role in hippocampal signaling.     

Modulation of ATP-induced LTP by cannabinoid receptors in rat hippocampus

Cannabinoids exert powerful action on various forms of synaptic plasticity. These retrograde messengers modulate GABA and glutamate release from presynaptic terminals by acting on presynaptic CB1 receptors. In particular, they inhibit long-term potentiation (LTP) elicited by electrical stimulation of excitatory pathways in rat hippocampus. Recently, LTP of the field excitatory postsynaptic potential (fEPSP) induced by exogenous ATP has been thoroughly explored. The present study demonstrates that cannabinoids inhibit ATP-induced LTP in hippocampal slices of rat. Administration of 10 μM of ATP led to strong inhibition of fEPSPs in CA1/CA3 hippocampal synapses. Within 40 min after ATP removal from bath solution, robust LTP was observed (fEPSP amplitude comprised 130.1 ± 3.8% of control, n = 10). This LTP never appeared when ATP was applied in addition to cannabinoid receptor agonist WIN55,212-2 (100 nM). Selective CB1 receptor antagonist, AM251 (500 nM), completely abolished this effect of WIN55,212-2. Our data indicate that like canonical LTP elicited by electrical stimulation, ATP-induced LTP is under control of CB1 receptors.

Electronic supplementary material

The online version of this article (doi:10.1007/s11302-012-9296-5) contains supplementary material, which is available to authorized users.     

A critical role of a facilitatory presynaptic kainate receptor in mossy fiber LTP.

The mechanisms involved in mossy fiber LTP in the hippocampus are not well established. In the present study, we show that the kainate receptor antagonist LY382884 (10 microM) is selective for presynaptic kainate receptors in the CA3 region of the hippocampus. At a concentration at which it blocks mossy fiber LTP, LY382884 selectively blocks the synaptic activation of a presynaptic kainate receptor that facilitates AMPA receptor-mediated synaptic transmission. Following the induction of mossy fiber LTP, there is a complete loss of the presynaptic kainate receptor-mediated facilitation of synaptic transmission. These results identify a central role for the presynaptic kainate receptor in the induction of mossy fiber LTP. In addition, these results suggest that the pathway by which kainate receptors facilitate glutamate release is utilized for the expression of mossy fiber LTP.     

Molecular Cloning, Expression, and Pharmacological Characterization of humEAA1, a Human Kainate Receptor Subunit

Abstract: Kainate is a potent neuroexcitatory agent; its neurotoxicity is thought to be mediated by an ionotropic receptor with a nanomolar affinity for kainate. In this report, we describe the cloning of a cDNA encoding a human glutamate ionotropic receptor subunit protein from a human hippocampal library. This cDNA, termed humEAA1, is most closely related to rat and human cDNAs for kainate receptor proteins and, when expressed in COS or Chinese hamster ovary cells, is associated with high-affinity kainate receptor binding. We have successfully established cell lines stably expressing humEAA1. This is the first report of establishment of stable cell lines expressing a glutamate receptor subunit. The relative potency of compounds for displacing [³H] kainate binding of humEAA1 receptors expressed in these stable cell lines was kainate > quisqualate > domoate > L-glutamate > ( RS )-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid > dihydro-kainate > 6, 7-dinitroquinoxaline-2, 3-dione > 6-cyano-7-nitroquinoxaline-2, 3-dione. Homooligomeric expression of humEAA1 does not appear to elicit ligand-gated ion channel activity. Nevertheless, the molecular structure and pharmacological characterization of high-affinity kainate binding of the humEAA1 expressed in the stable cell line (ppEAA1–16) suggest that the humEAA1 is a subunit protein of a human kainate receptor complex.     

Glutamate Inhibits Adenylate Cyclase Activity in Dispersed Rat Hippocampal Cells Directly via an N-Methyl-d-Aspartate-Like Metabotropic Receptor

Three major subtypes of glutamate receptors that are coupled to cation channels--N-methyl-D-aspartate (NMDA), kainate, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors--are known as ionotropic receptors in the mammalian CNS. Recently, an additional subtype that is coupled to GTP binding proteins and stimulates (or inhibits) metabolism of phosphoinositides has been proposed as a metabotropic receptor. Incubation of dispersed hippocampal cells from adult rats with glutamate or NMDA decreased forskolin-stimulated cyclic AMP (cAMP) accumulation; half-maximal effects were obtained with 5.6 +/- 2.2 and 6.4 +/- 2.3 microM, respectively. Kainate and quisqualate were less potent. The effect of glutamate was antagonized by 2,3-diaminopropionate and 2-amino-5-phosphonovalerate, NMDA/glutamate receptor antagonists, but not by 0.5 microM Joro spider toxin, a specific blocker of the AMPA receptor. The inhibitory effect of glutamate on cAMP formation was not blocked by 2 microM tetrodotoxin or by the absence of Ca2+. In hippocampal membranes, glutamate, similar to carbachol, inhibited adenylate cyclase activity in a GTP-dependent manner. These findings suggest that the glutamate inhibition of adenylate cyclase is direct and is not due to a result of the release of other neurotransmitters. The effect of glutamate on cAMP accumulation was observed in an assay medium containing 0.7 mM MgCl2, which is known to inhibit both ionotropic NMDA receptor/channels in the hippocampus and metabotropic NMDA receptors in the cerebellum. The inhibitory effect of glutamate was abolished by pertussis toxin treatment.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ionotropic glutamate receptors mediate juvenile hormone synthesis in the cockroach,Diploptera punctata

By monitoring changes in the cytosolic [Ca2+](i) and rates of juvenile hormone (JH) synthesis in response to L-glutamate agonists and antagonists, we identified and characterized glutamate receptor subtypes in corpus allatum (CA) cells of the cockroach, Diploptera punctata. During the first ovarian cycle, corpora allata exhibited a cycle of changes in sensitivity to L-glutamate correlated to cyclic changes in rates of JH synthesis. When exposed to 60 microM L-glutamate in vitro, the active corpora allata of day-4 mated females produced 60% more JH, while inactive corpora allata at other ages showed 10-20% stimulatory response. Pharmacological characterization using various L-glutamate receptor agonists and antagonists indicated that several ionotropic subtypes of L-glutamate receptors were present in the CA. The CA showed an increase in rates of JH synthesis in response to NMDA, kainate, and quisqualate, but not to AMPA in both L-15 medium and minimum incubation medium. In contrast, applications of the metabotropic receptor-specific agonist trans-ACPD failed to elicit a change in the cytosolic [Ca2+](i) and JH production.An elevation of cytosolic calcium concentration, followed by 20-30% rise in JH production, was observed when active CA cells were exposed to 10-40 microM kainate. Kainate had no stimulatory effect on JH synthesis in calcium-free medium. The kainate-induced JH synthesis was blocked by 20 microM CNQX but was not affected by 20 microM NBQX. Kainate-stimulated JH production was not suppressed by MK-801 (a specific blocker of NMDA-receptor channel), nor was NMDA-stimulated JH production affected by CNQX (a specific antagonist of kainate receptor). These data suggest that active CA cells are stimulated to synthesize more JH by a glutamate-induced calcium rise via NMDA-, kainate- and/or quisqualate-sensitive subtypes of ionotropic L-glutamate receptors. The metabotropic-subtype and ionotropic AMPA-subtype L-glutamate receptors are unlikely to be present on active CA cells.     

Kainic acid and synaptic transmission in the stellate ganglion of the squid.

Kainate, a conformational analogue of glutamate, blocks synaptic transmission across the giant synapse of the squid. In the presence of blocking doses of kainate, impulses continue to propagate into the nerve terminal, but action potentials are slightly reduced in size and the subsequent hyperpolarization is greatly diminished. Kainate depolarizes the postsynaptic axon. Since the depolarizing action of kainate is confined to the postsynaptic membrane, it appears that kainate can combine with the receptors which are normally activated by the transmitter. This results in a diminished effect of the transmitter released by a presynaptic nerve impulse.     

Potentiation of glutamatergic synaptic input to supraoptic neurons by presynaptic nicotinic receptors

The release of vasopressin and oxytocin from the supraoptic nucleus (SON) neurons is tonically regulated by excitatory glutamatergic and inhibitory GABAergic synaptic inputs. Acetylcholine is known to excite SON neurons and to elicit vasopressin release. Cholinergic receptors are located pre- and postsynaptically in the SON, but their functional significance in the regulation of SON neurons is not fully understood. In this study, we determined the role of presynaptic cholinergic receptors in regulation of the excitatory glutamatergic inputs to the SON neurons. The magnocellular neurons in the rat hypothalamic slices were identified microscopically, and the spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded using the whole cell voltage-clamp technique. The mEPSCs were abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). Acetylcholine (100 microM) significantly increased the frequency of mEPSCs of 38 SON neurons from 1.87 +/- 0.36 to 3.42 +/- 0.54 Hz but did not alter the amplitude (from 19.61 +/- 0.90 to 19.34 +/- 0.84 pA) and the decay time constant of mEPSCs. Furthermore, the nicotinic receptor antagonist mecamylamine (10 microM, n = 16), but not the muscarinic receptor antagonist atropine (100 microM, n = 12), abolished the excitatory effect of acetylcholine on the frequency of mEPSCs. These data provide new information that the excitatory effect of acetylcholine on the SON neurons is mediated, at least in part, by its effect on presynaptic glutamate release. Activation of presynaptic nicotinic, but not muscarinic, receptors located in the glutamatergic terminals increases the excitatory synaptic input to the SON neurons of the hypothalamus.     

The metabotropic glutamate receptor (MGR): pharmacology and subcellular location.

A pharmacological characterization of the metabotropic glutamate receptor (MGR) was performed in striatal neurons. Among the excitatory amino acid receptor antagonists tested, only D, L-2-amino-3-phosphonopropionate (D, L-AP3) inhibited QA-induced inositol phosphate (InsP) formation in a competitive manner (mean pKi = 4.45 +/- 0.43, n = 4). However, this drug was a partial agonist of MGR since it stimulated the inositol-phosphate formation. We found that D, L-AP3 also inhibited NMDA-induced calcium increase, in a competitive manner (mean pIC50 = 4.34 +/- 0.22, n = 8, and mean pKi = 3.7 +/- 0.11 n = 5). 1 mM of the ionotropic agonists alpha-amino-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), kainate (KA) or domoate (DO) (100 microM or higher) induced a significant InsP formation in striatal neurons. The InsP responses induced by all these agonists were totally blocked by the phorbol ester phorbol-12,13-dibutyrate (PdBu), but not by atropine or prazosin. Agonist-induced increases of intracellular calcium concentrations ([Ca2+]i) were insensitive to PdBu, suggesting that all these substances were able to stimulate the MGR in striatal neurons. Trans-1-amino-cyclopentyl-1,3-dicarboxylate (trans-ACPD) evoked dose-dependent inositol phosphate formations with an EC50 of 29 microM but had no significant effect on NMDA or AMPA receptors, as measured by the patch clamp technique. In the presence of 30 microM of AMPA, trans-ACPD induced a significant release of arachidonic acid (AA) in striatal neurons. No important AA release was observed by any of these agonists alone. 56 mM K+ did not mimic AMPA in this associative ionotropic/metabotropic effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.