Regional mapping of low-affinity kainate receptors in mouse brain using [(3)H](2S,4R)-4-methylglutamate autoradiography

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.

Kainate Receptors Play a Role in Modulating Synaptic Transmission in the Olfactory Bulb

Glutamate is the neurotransmitter used at most excitatory synapses in the mammalian brain, including those in the olfactory bulb (OB). There, ionotropic glutamate receptors including N-methyl-d-aspartate receptors (NMDARs) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) play a role in processes such as reciprocal inhibition and glomerular synchronization. Kainate receptors (KARs) represent another type of ionotropic glutamate receptor, which are composed of five (GluK1-GluK5) subunits. Whereas KARs appear to be heterogeneously expressed in the OB, evidence as to whether these KARs are functional, found at synapses, or modify synaptic transmission is limited. In the present study, coapplication of KAR agonists (kainate, SYM 2081) and AMPAR antagonists (GYKI 52466, SYM 2206) demonstrated that functional KARs are expressed by OB neurons, with a subset of receptors located at synapses. Application of kainate and the GluK1-selective agonist ATPA had modulatory effects on excitatory postsynaptic currents (EPSCs) evoked by stimulation of the olfactory nerve layer. Application of kainate and ATPA also had modulatory effects on reciprocal inhibitory postsynaptic currents (IPSCs) evoked using a protocol that evokes dendrodendritic inhibition. The latter finding suggests that KARs, with relatively slow kinetics, may play a role in circuits in which the relatively brief duration of AMPAR-mediated currents limits the role of AMPARs in synaptic transmission (e.g., reciprocal inhibition at dendrodendritic synapses). Collectively, our findings suggest that KARs, including those containing the GluK1 subunit, modulate excitatory and inhibitory transmission in the OB. These data further suggest that KARs participate in the regulation of synaptic circuits that encode odor information.

Impaired hippocampus-dependent spatial flexibility and sociability represent autism-like phenotypes in GluK2 mice

Autism is a complex neurodevelopmental disorder with high heritability. grik2 (which encodes the GluK2 subunit of kainate receptors) has been identified as a susceptibility gene in Autism Spectrum Disorders (ASD), but its role in the core and associated symptoms of ASD still remains elusive. We used mice lacking GluK2 (GluK2 KO) to examine their endophenotype with a view to modeling aspects of autism, including social deficits, stereotyped and repetitive behavior and decreased cognitive abilities. Anxiety was recorded in the elevated plus maze, social behavior in a three-chamber apparatus, and cognition in different water maze protocols. Deletion of the GluK2 gene reduced locomotor activity and sociability as indicated by the social interaction task. In addition, GluK2 KO mice learnt to locate a hidden platform in a water maze surrounded by a curtain with hanging cues faster than wild-type mice. They maintained a bias toward the target quadrant when some of these cues were removed, at which point wild-types orthogonalized the behavior and showed no memory. However, GluK2 KO mice were impaired in spatial reversal learning. These behavioral data together with previously published electrophysiology showing severe anomalies in CA3 network activity, suggest a computational shift in this network for enhanced propensity of pattern completion that would explain the loss of behavioral flexibility in GluK2 KO mice. Although a single mutation cannot recapitulate the entire core symptoms of ASD, our data provide evidence for glutamatergic dysfunction underlying a number of social- and cognition-related phenotypes relevant to ASD.

Use of confocal microscopy in the study of ischemia-induced hippocampal neuronal damage

The present study was undertaken to reveal by means of confocal laser microscopy the cytoarchitecture of hippocampal CA3 neurons in Mongolian gerbils before and after cerebral ischemia of different duration. The common carotid arteries of gerbils were occluded for 5, 10, or 15 min. On the 4th, 14th and 28th day after reperfusion, neuronal damage was examined by laser scanning confocal microscopy in the CA3 region of hippocampus (30 ?m slices). Slices were stained with fluorescent Nissl staining and fluorescent membrane tracer DiI. Increased duration of cerebral ischemia resulted in a progressive loss of hippocampal CA3 neurons. Four days after the ischemic insult, neuronal damage in the hippocampal CA3 region was mild but visible. On the 28th day after reperfusion, neuronal damage in the observed brain structure was most severe. These results demonstrate the temporal profile of neuronal damage after an ischemic insult as observed using confocal microscopy.

Neuroprotectionby MK-801 following cerebral ischemia in Mongolian gerbils

Global cerebral ischemia in Mongolian gerbils is an established model in experimental research on cerebral ischemia, which is characterized morphologically by selective neuronal damage in the hippocampus, striatum, and cortex. Elevated glutamate levels are thought to be a primary cause of neuronal death after global cerebral ischemia. The purpose of this study was to investigate the potential neuroprotective effects of dizocilpine malate (MK-801), a non-competitive glutamate antagonist, in the model of 10-min gerbil cerebral ischemia. Gerbils were given MK-801(3 mg/kg i.p.)or saline immediately after the occlusion. On day 4 after reperfusion, neuronal damage was examined in the hippocampus (30 ?m)and striatum slices (5 ?m)stained with hematoxylin/eosin, fluorescent Nissl staining and membrane tracer DiI. The striatum and C3 regions of the hippocampus were analyzed by confocal microscopy. Neuroprotection was determined by quantifying the degree of cell loss, reduction of morphologically damaged cells, and the degree of preservation of recog?nizable neuroanatomical pathways after the ischemic insult. Our results demonstrate that the neuronal damage induced by sustained ischemia is related to abnormalities in glutamatergic function associated with NMDA receptors. MK-801significantly prevented neuronal loss in the tested brain structures. All of this contributes to a better understanding of the given pathophysiological process causing ischemic neuronal damage. <br><br><font color="red"><b> This article has been corrected. Link to the correction <u><a href="http://dx.doi.org/10.2298/ABS160524054E">10.2298/ABS160524054E</a><u></b></font>

Effect of 7-nitroindazoleon superoxide production and MnSOD activity in threat brain following kainate-induced neurotoxicity

We investigated the effect of 7-nitroindazole (7-NI), a selective neuronal nitric oxide synthase inhibitor in vivo, on superoxide concentration as well its influence on mitochondrial MnSOD activity since this activity is associated with the production of reactive oxygen species after kainate-induced neurotoxicity. The time course of in vivo oxidative damage in different brain regions was investigated. Measurements were performed at different times (5 min, 15 min, 2 h, 48 h, and 7 days) in the ipsi- and contralateral hippocampus, forebrain cortex, striatum, and cerebellum homogenates. Our results indicated that 7-NI had no statistically significant influence on superoxide concentrations in the tested brain structures compared to the control values. However, superoxide concentrations after kainate-induced neurotoxicity returned to the control values after pretreatment with 7-NI in all tested brain structures. Regarding the activity of MnSOD, our results demonstrated statistically significant increase 7 days after intrahippocampal KA treatment in all tested brain structures after pretreatment with 7-NI. The obtained results suggest that neuronal NO synthase inhibitors may be useful in the treatment of neurological diseases in which excitotoxic mechanisms play a role. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150330033E">10.2298/ABS150330033E</a><u></b></font>

Expression and function of kainate receptors in the rat olfactory bulb

Although recent results suggest roles for NMDA and AMPA receptors in odor encoding, little is known about kainate receptors (KARs) in the olfactory bulb (OB). Molecular, immunological, and electrophysiological techniques were used to provide a functional analysis of KARs in the OB. Reverse transcriptase-polymerase chain reaction revealed that the relative level of expression of KAR subunits was GluR5 approximately GluR6 approximately KA2 > KA1 >> GluR7. In situ hybridization data imply that mitral/tufted cells express mostly GluR5 and KA2, whereas interneurons express mostly GluR6 and KA2. Immunohistochemical double-labeling experiments (GluR5/6/7 or GluR5 + synapsin) suggest that KARs are expressed at both synaptic and extrasynaptic loci. This heterogeneous expression of KAR subunits suggests that KARs may play a multitude of roles in odor processing, each tailored to the function of specific OB circuits. A functional analysis, using whole-cell electrophysiology, suggests that one such role is to increase the frequency of glutamate transmission while attenuating the amplitude of individual events, likely via a presynaptic depolarizing mechanism. Such effects would be important to odor processing particularly by OB glomeruli. In these highly compartmentalized structures, an increase in the frequency of glutamate release and the high density of extrasynaptic KARs, activated by spillover, could enhance glomerular synchronization and thus the transfer of more specific sensory information to cortical structures.

Effect of glutamate antagonists on nitric oxide production in rat brain following intrahippocampal injection

Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. The involvement of ionotropic glutamate NMDA and AMPA/kainate receptors in induction of NO production in the rat brain was examined after injection of kainate, a non-NMDA receptor agonist; kainate plus 6-cyano- 7-nitroquinoxaline-2,3-dione (CNQX), a selective AMPA/kainate receptor antagonist; or kainate plus 2-amino-5-phosphonopentanoic acid (APV), a selective NMDA receptor antagonist. Competitive glutamate receptor antagonists were injected with kainate unilaterally into the CA3 region of the rat hippocampus. The accumulation of nitrite, the stable metabolite of NO, was measured by the Griess reaction at different times (5 min, 15 min, 2 h, 48 h, and 7 days) in hippocampus, forebrain cortex, striatum, and cerebellum homogenates. The used glutamate antagonists APV and CNQX both provided sufficient neuroprotection in the sense of reducing nitrite concentrations, but with different mechanisms and time dynamics. Our findings suggest that NMDA and AMPA/kainate receptors are differentially involved in nitric oxide production. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150319031E">10.2298/ABS150319031E</a><u></b></font>

Abnormal synaptic plasticity in the Ts1Cje segmental trisomy 16 mouse model of Down syndrome

Due to the homology between human chromosome 21 and mouse chromosome 16, trisomy 16 mice are considered animal models of Down syndrome (DS). Abnormal hippocampal synaptic plasticity and behavior have been reported in the segmental trisomy 16 Ts65Dn mouse. In the Ts1Cje DS mouse model, which has a shorter triplicated chromosomal segment than Ts65Dn, more subtle hippocampal behavioral deficits have been reported. In this study, we investigated CA1 hippocampal synaptic plasticity, long-term potentiation (LTP) and depression (LTD) in the Ts1Cje mouse. Field excitatory postsynaptic potentials (fEPSPs) were recorded from the CA1 area of in vitro hippocampal slices from the Ts1Cje mouse and diploid controls, LTP was induced by a single tetanizing train pulse (1 s) at 100 Hz and LTD by a 900-pulse train at 1 Hz. We report for the first time that compared to diploid controls, the hippocampus from the Ts1Cje mouse had a smaller LTP and an increased LTD. The changes are less dramatic than had been reported previously for the Ts65Dn mouse. Furthermore, in the Ts1Cje mouse trains of pulses at both 20 Hz and 100 Hz produced a decrease in the evoked fEPSPs over the length of the train in comparison to diploid fEPSPs. These findings suggest that genes from Ts1Cje chromosome, including GIRK2 potassium channel, contribute to abnormal short- and long-term plasticity.

Differential effects of NMDA and AMPA/kainate receptor antagonists on nitric oxide production in rat brain following intrahippocampal injection

Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. The involvement of ionotropic glutamate NMDA and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainite (KA) receptors in the induction of NO production in the rat brain was examined after injection of kainate, non-NMDA receptor agonist, KA+6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), selective AMPA/KA receptor antagonist, or KA+2-amino-5-phosphonopentanoic acid (APV), selective NMDA receptor antagonist. Competitive glutamate receptor antagonists were injected with KA unilaterally into the CA3 region of the rat hippocampus. The accumulation of nitrite, the stable metabolite of NO, was measured by the Griess reaction at different times (5 min, 15 min, 2 h, 48 h and 7 days) in the ipsi- and contralateral hippocampus, forebrain cortex, striatum and cerebellum homogenates. The detected increase of NO production in distinct brain regions, which are functionally connected via afferents and efferents, suggests that these regions are affected by the injury. The effect of KA on nitrite production was blocked by the glutamate antagonists. Intrahippocampal KA+CNQX injection resulted in decrease of nitrite production, around control levels, in all tested brain structures. Significant decrease in nitrite levels was found only in comparison to KA-treated animals, i.e. the overall effect of selective AMPA/KA receptor antagonist was a decrease of KA-induced excitotoxicity. The accent effect of intrahippocampal KA+APV injection resulted, also, in decrease of nitrite production. However, this effect was detected after 5 min from the injection indicating the existence of an NMDA receptor-mediated component of basal nitrite production in physiological conditions and difference in mechanisms and time dynamics between CNQX and APV. The used antagonists showed same pattern in all tested brain structures. APV and CNQX both expressed sufficient neuroprotection in sense of reducing nitrite concentrations, but with differential effect in mechanisms and time dynamics. Our findings suggest that NMDA and AMPA/KA receptors are differentially involved in NO production.

7-nitroindazole, a selective neuronal nitric oxide synthase inhibitore in vivo, prevents kainate-induced intrahippocampal neurotoxicity

We investigated the effects of 7-nitroindazole (7-NI), a selective neuronal nitric oxide synthase inhibitor in vivo, on nitrite concentration after kainic acid injection unilaterally into the CA3 region of the rat hippocampus. The accumulation of nitrite, the stable metabolite of NO, was measured by the Griess reaction at different times in hippocampus, forebrain cortex, striatum, and cerebellum homogenates. 7-nitroindazole can effectively inhibit NO synthesis in rat brain after kainate-induced neurotoxicity and suppressed nitrite accumulation. The present results suggest that neuronal NO synthase inhibitors may be useful in the treatment of neurological diseases in which excitotoxic mechanisms play a role. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS160412036E">10.2298/ABS160412036E</a><u></b></font>

Kainate-induced oxidative stress and neurotoxicity in the rat brain

We investigated superoxide production and MnSOD activity after kainate injection into the CA3 region of the rat hippocampus. The measurements took place at different times in hippocampus, forebrain cortex, striatum, and cerebellum homogenates. Free radicals including superoxide are responsible for post-lesional cytotoxicity. Neuronal cells responded to oxidative stress in kainate-induced neurotoxicity and caused the protective mechanism to increase MnSOD levels. The increase of MnSOD in distinct brain regions functionally connected via afferents and efferent suggests that these regions are affected by the injury. It implies that MnSOD protects the cells in these regions from superoxide-induced damage and therefore may limit the retrograde and anterograde spread of neurotoxicity. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150318027E">10.2298/ABS150318027E</a><u></b></font>

Mitochondrial superoxide production and MnSOD activity following exposure to an agonist and antagonists of ionotropic receptors in rat brain

The involvement of NMDA and AMPA/kainate receptors in the induction of superoxide production in the rat brain was examined after intrahippocampal injection of kainate, a non-NMDA receptor agonist; kainate plus CNQX, a selective AMPA/kainate receptor antagonist; or kainate plus APV, a selective NMDA receptor antagonist. The measurements took place at different times in the ipsi- and contralateral hippocampus, forebrain cortex, striatum, and cerebellum homogenates. The used glutamate antagonists both ensured sufficient neuroprotection in the sense of lowering superoxide production and raising MnSOD levels, but in the mechanisms and time dynamics of their effects were different. Our findings suggest that NMDA and AMPA/kainate receptors are differentially involved in superoxide production. <br><br><font color="red"><b> This article has been retracted. Link to the retraction <u><a href="http://dx.doi.org/10.2298/ABS150318026E">10.2298/ABS150318026E</a><u></b></font>

Differential effects of NMDA and AMPA/kainate receptor antagonists on superoxide production and MnSOD activity in rat brain following intrahippocampal injection

The involvement of NMDA and AMPA/kainate receptors in the induction of superoxide radical production in the rat brain was examined after injection of kainate, non-NMDA receptor agonist, kainate plus 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), selective AMPA/kainate receptor antagonist, or kainate plus 2-amino-5-phosphonopentanoic acid (APV), selective NMDA receptor antagonist. Competitive glutamate receptor antagonists were injected with kainate unilaterally into the CA3 region of the rat hippocampus. We investigated superoxide production and mitochondrial MnSOD activity after injection. The measurements took place at different times (5, 15 min, 2, 48 h and 7 days) in the ipsi- and contralateral hippocampus, forebrain cortex, striatum, and cerebellum homogenates. Used glutamate antagonists APV and CNQX both expressed sufficient neuroprotection in sense of decreasing superoxide production and increasing MnSOD levels, but with differential effect in mechanisms and time dynamics. Our findings suggest that NMDA and AMPA/kainate receptors are differentially involved in superoxide production. Following intrahippocampal antagonists injection they, also, interpose different neuroprotection effect on the induction of MnSOD activity in distinct brain regions affected by the injury, which are functionally connected via afferents and efferents. It suggests that MnSOD protects the cells in these regions from superoxide-induced damage and therefore may limit the retrograde and anterograde spread of neurotoxicity.

Corticotrophin releasing factor-induced synaptic plasticity in the amygdala translates stress into emotional disorders

The amygdala is involved in the associative processes for both appetitive and aversive emotions, and its function is modulated by stress hormones. The neuropeptide corticotrophin releasing factor (CRF) is released during stress and has been linked to many stress-related behavioral, autonomic, and endocrine responses. In the present study, nonanxiety-inducing doses of a potent CRF type 1 and 2 receptor agonist, urocortin (Ucn), was infused locally into the basolateral amygdala (BLA) of rats. After 5 daily injections of Ucn, the animals developed anxiety-like responses in behavioral tests. Intravenous administration of the anxiogenic agent sodium lactate elicited robust increases in blood pressure, respiratory rate, and heart rate. Furthermore, in the absence of any additional Ucn treatment, these behavioral and autonomic responses persisted for >30 d. Whole-cell patch-clamp recordings from BLA neurons of these hyper-reactive animals revealed a pronounced reduction in both spontaneous and stimulation-evoked IPSPs, leading to a hyperexcitability of the BLA network. This Ucn-induced plasticity appears to be dependent on NMDA receptor and subsequent calcium-calmodulin-dependent protein kinase II (CaMKII) activation, because it is blocked by pretreatment with NMDA receptor antagonists and by coadministration of CaMKII inhibitors. Our results show for the first time a stress peptide-induced behavioral syndrome that can be correlated with cellular mechanisms of neural plasticity, a novel mechanism that may explain the etiological role of stress in several chronic psychiatric and medical disorders.

Microarray-based discovery of highly expressed olfactory mucosal genes: potential roles in the various functions of the olfactory system

We sought to gain a global view of tissue-specific gene expression in the olfactory mucosa (OM), the major site of neurogenesis and neuroregeneration in adult vertebrates, by examination of its overexpressed genes relative to that in 81 other developing and adult mouse tissues. We used a combination of statistical and fold-difference criteria to identify the top 269 cloned cDNAs from an array of 8,734 mouse cDNA elements on the Incyte Mouse GEM1 array. These clones, representing known and poorly characterized gene transcripts, were grouped according to their relative expression patterns across the other tissues and then further examined with respect to gene ontology categories. Approximately one-third of the 269 genes were also highly expressed in developing and/or adult central nervous system tissues. Several of these have been suggested or demonstrated to play roles in neurogenesis, neuronal differentiation, and/or neuronal migration, further suggesting that many of the unknown genes that share this expression pattern may play similar roles. Highly OM-specific genes included a palate, lung, and nasal epithelium carcinoma-associated gene (Plunc); sphingosine phosphate lyase (Sgpl1), and paraoxonase 1 (Pon1). Cell-type-specific expression within OM was established using in situ hybridization for several representative expression pattern clusters. Using the ENSEMBL-assembled mouse genome and comparative genomics analyses to the human genome, we assigned many of the unknown expressed sequence tags (ESTs) and poorly characterized genes to either novel or known gene products and provided predictive classification. Further exploration of this database will provide additional insights into genes and pathways critical for olfactory neurogenesis, neuronal differentiation, olfaction, and mucosal defense.

Localization and photoaffinity labelling of the levetiracetam binding site in rat brain and certain cell lines

Levetiracetam (2S-(2-oxo-1-pyrrolidinyl)butanamide, KEPPRA, a novel antiepileptic drug, has been shown to bind to a specific binding site located in the brain (Eur. J. Pharmacol. 286 (1995) 137). To identify the protein constituent of the levetiracetam binding site in situ, we synthesized the photoaffinity label [3H]ucb 30889 ((2S)-2-[4-(3-azidophenyl)-2-oxopyrrolidin-1-yl]butanamide), a levetiracetam analog with higher affinity for the levetiracetam binding site. This radioligand was used to map the levetiracetam binding site within the brain and to study its cellular and subcellular distribution. Autoradiography experiments using [3H]ucb 30889 in rat brain revealed a unique distribution profile that did not match that of classical receptors known to be involved in the generation of epileptic seizures. There was a high level of binding in the dentate gyrus, the superior colliculus, several thalamic nuclei, the molecular layer of the cerebellum and to a lesser extent in the cerebral cortex, the striatum and the hypothalamus. The levetiracetam binding site was restricted to neuronal cell types, undifferentiated PC12 cells and was highly enriched in synaptic vesicles. [3H]ucb 30889 was also used in photoaffinity labelling studies and shown to bind covalently to a membrane protein with a molecular weight of approximately 90 kDa.

Assembly and cell surface expression of KA-2 subunit-containing kainate receptors

Kainate receptors (KARs) modulate synaptic transmission at both pre-synaptic and post-synaptic sites. The overlap in the distribution of KA-2 and GluR6/7 subunits in several brain regions suggests the co-assembly of these subunits in native KARs. The molecular mechanisms that control the assembly and surface expression of KARs are unknown. Unlike GluR5-7, the KA-2 subunit is unable to form functional homomeric KAR channels. We expressed the KA-2 subunit alone or in combination with other KAR subunits in HEK-293 cells. The cell surface expression of the KAR subunit homo- and heteromers were analysed using biotinylation and agonist-stimulated cobalt uptake. While GluR6 or GluR7 homomers were expressed on the cell surface, KA-2 alone was retained within the endoplasmic reticulum. We found that the cell surface expression of KA-2 was dramatically increased by co-expression with either of the low-affinity KAR subunits GluR5-7. However, co-expression with other related ionotropic glutamate receptor subunits (GluR1 and NR1) does not facilitate the cell surface expression of KA-2. The analysis of subcellular fractions of neocortex revealed that synaptic KARs have a relatively high KA-2 content compared to microsomal ones. Thus, KA-2 is likely to contain an endoplasmic reticulum retention signal that is shielded on assembly with other KAR subunits.

7-nitroindazole reduces nitrite concentration in rat brain after intrahippocampal kainate-induced seizure

Kainic acid is an endogenous excitotoxin acting on glutamate receptors, that leads to neurotoxic damage resembling the alterations observed in some neurological disorders. Stimulation of glutamate receptors induces neuronal nitric oxide (NO) release, which in turn modulates glutamate transmission. NO may be a key mediator of excitotoxic neuronal injury in the central nervous system. We investigated the effects of 7-nitroindazole, a selective neuronal nitric oxide synthase inhibitor in vivo, on nitrite concentration after kainic acid injection (0.6 mg/ml, pH 7.2) unilaterally into the CA3 region of the rat hippocampus. The accumulation of nitrite, the stable metabolite of NO, was measured by the Griess reaction at different times (5 min, 15 min, 2 h, 48 h and 7 days) following kainate injection in the ipsilateral and contralateral hippocampus, forebrain cortex, striatum and cerebellum homogenates. 7-Nitroindazole (100 microM) can effectively inhibit NO synthesis in rat brain after kainate-induced intrahippocampal neurotoxicity and suppressed nitrite accumulation. The present results suggest that neuronal NO synthase inhibitors may be useful in the treatment of neurological diseases where excitotoxic mechanisms play a role.

SYM 2081, an agonist that desensitizes kainate receptors, attenuates capsaicin and inflammatory hyperalgesia

Excitatory amino acids acting at non-NMDA receptors contribute to transmission of nociceptive information. SYM 2081 ((2S,4R)-4-methyl glutamic acid) desensitizes kainate receptors, one subtype of non-NMDA receptors, to subsequent release of excitatory amino acids and thus may attenuate transmission of nociceptive information. To determine if SYM 2081 can prevent development of hyperalgesia, SYM 2081 (10, 50 or 100 mg/kg, i.p.) was administered prior to injection of capsaicin into the hindpaw of rats, which produces mechanical and heat hyperalgesia. To determine if SYM 2081 can reduce ongoing inflammatory hyperalgesia, SYM 2081 (10 or 100 mg/kg, i.p.) was administered after development of carrageenan-evoked hyperalgesia. Intraplantar injection of capsaicin produced an increase in hindpaw withdrawal frequency to mechanical stimuli (from 4+/-2 to 41+/-7%; mean+/-S.E.M.) and a decrease in withdrawal latency to heat (from 12.3+/-0.3 to 5.9+/-0.4 s) in rats that received vehicle. In contrast, rats that received SYM 2081 (100 mg/kg) prior to injection of capsaicin exhibited a lower hindpaw withdrawal frequency (18+/-4%) and a longer withdrawal latency (7.7+/-0.5 s). Intrathecal (1-100 microg/5 microl), but not intraplantar (10 or 100 microg/50 microl), injection of SYM 2081 attenuated the development of capsaicin-evoked heat hyperalgesia suggesting that SYM 2081's antihyperalgesic effects were due to its central effects. Furthermore, SYM 2081 completely reversed ongoing carrageenan-evoked mechanical hyperalgesia and partially (approximately 50%) reversed ongoing heat hyperalgesia. The present study demonstrates that administration of a high-potency ligand that selectively desensitizes kainate receptors attenuates the development of mechanical and heat hyperalgesia and attenuates ongoing inflammatory hyperalgesia.

Low-affinity kainate receptor-mediated events reduce the protective activity of phenobarbital and diphenylhydantoin against maximal electroshock in mice

(2S,2R)-4-Methylglutamic acid (SYM 2081), a potent selective agonist of GluR5 and GluR6 kainate receptor subtypes, applied at the dose of 15.5 mg/kg, equal to its CD(16) value (i.e., a dose required to induce convulsions in 16% of mice), significantly decreased the electroconvulsive threshold from 7.0 to 5.8 mA. When administered at the dose of 11.5 mg/kg, equal to 75% of its CD(16), it markedly attenuated the protective activity of phenobarbital and diphenylhydantoin, but not that of valproate, carbamazepine, or diazepam against maximal electroshock-induced seizures in mice. The respective ED(50) values were increased from 18.5 to 23.8 mg/kg for phenobarbital, and from 11.7 to 14.7 mg/kg for diphenylhydantoin. Since the free plasma levels of both antiepileptic drugs were not influenced by SYM 2081, the pharmacokinetic interaction does not seem to be involved in the observed results. In conclusion, low-affinity kainate receptor-mediated events might be a factor reducing the protective efficacy of some antiepileptic drugs. Furthermore, the activation of GluR5 and GluR6 kainate receptor subtypes by endogenous glutamate during seizures may be associated with the drug-resistance phenomenon.