Kainate is a potent neurotoxin known to induce acute seizures. However, whether kainate receptors (KARs) play any role in the pathophysiology of temporal lobe epilepsy (TLE) is not known. In TLE, recurrent mossy fiber (rMF) axons form abnormal excitatory synapses onto other dentate granule cells that operate via KARs. The present study explores the pathophysiological implications of KARs in generating recurrent seizures in chronic epilepsy. In an in vitro model of TLE, seizure-like activity was minimized in mice lacking the GluK2 subunit, which is a main component of aberrant synaptic KARs at rMF synapses. In vivo, the frequency of interictal spikes and ictal discharges was strongly reduced in GluK2(-/-) mice or in the presence of a GluK2/GluK5 receptor antagonist. Our data show that aberrant GluK2-containing KARs play a major role in the chronic seizures that characterize TLE and thus constitute a promising antiepileptic target.

B-ephrin - EphB receptor signaling modulates NMDA receptors by inducing tyrosine phosphorylation of NR2 subunits. Ephrins and EphB RTKs are localized to postsynaptic compartments in the CA1, and therefore potentially interact in a non-canonical cis-configuration. However, it is not known whether cis- configured receptor-ligand signaling is utilized by this class of RTKs, and whether this might influence excitatory synapses. We found that ablation of ephrin-B3 results in an enhancement of the NMDA receptor component of synaptic transmission relative to the AMPA receptor component in CA1 synapses. Synaptic AMPA receptor expression is reduced in ephrin-B3 knockout mice, and there is a marked enhancement of tyrosine phosphorylation of the NR2B receptor subunit. In a reduced system co-expression of ephrin-B3 attenuated EphB2-mediated NR2B tyrosine phosphorylation. Moreover, phosphorylation of EphB2 was elevated in the hippocampus of ephrin-B3 knockout mice, suggesting that regulation of EphB2 activity is lost in these mice. Direct activation of EphB RTKs resulted in phosphorylation of NR2B and a potential signaling partner, the non-receptor tyrosine kinase Pyk2. Our data suggests that ephrin-B3 limits EphB RTK-mediated phosphorylation of the NR2B subunit through an inhibitory cis- interaction which is required for the correct function of glutamatergic CA1 synapses.

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AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate) receptors desensitize rapidly and completely in the continued presence of their endogenous ligand glutamate; however, it is not clear what role AMPA receptor desensitization plays in the brain. We generated a knock-in mouse in which a single amino acid residue, which controls desensitization, was mutated in the GluA2 (GluR2) receptor subunit (GluA2 L483Y ). This mutation was homozygous lethal. However, mice carrying a single mutated allele, GluA2 L483Y/wt , survived past birth, but displayed severe and progressive neurological deficits including seizures and, ultimately, increased mortality. The expression of the AMPA receptor subunits GluA1 and GluA2 was decreased, whereas NMDA receptor protein expression was increased in GluA2 L483Y/wt mice. Despite this, basal synaptic transmission and plasticity in the hippocampus were largely unaffected, suggesting that neurons preferentially target receptors to synapses to normalize synaptic weight. We found no gross neuroanatomical alterations in GluA2 L483Y/wt mice. Moreover, there was no accumulation of AMPA receptor subunits in intracellular compartments, suggesting that folding and assembly of AMPA receptors are not affected by this mutation. Interestingly, EPSC paired pulse ratios in the CA1 were enhanced without a change in synaptic release probability, demonstrating that postsynaptic receptor properties can contribute to facilitation. The dramatic phenotype observed in this study by the introduction of a single amino acid change demonstrates an essential role in vivo for AMPA receptor desensitization.MPA receptors are tetramers assembled from the four receptor subunits GluA1-GluA4 (GluR1-4) (1). These receptors are activated by their endogenous ligand glutamate, and rapidly undergo desensitization within milliseconds of glutamate binding. Desensitization involves a conformational change of the receptor complex that allows closure of the channel gate while glutamate remains bound to the receptor (2). Synaptic currents are predominantly mediated by AMPA receptors at most excitatory synapses; therefore there has been interest in the development of pharmacological agents that enhance AMPA receptor function by limiting receptor deactivation and desensitization (3). There are many clear examples of synapses at which postsynaptic receptor desensitization plays a major role in synaptic depression (4-6). Many of these synapses are specialized structures in which glutamate remains in the synaptic cleft for prolonged periods of time during normal operation of the synapse (7). In contrast, at synapses where cleft glutamate is cleared rapidly or where AMPA receptor stoichiometry has become specialized to support high frequency transmission, there is little evidence that synaptic receptor desensitization has much influence on shaping the kinetics of transmission, and it is likely that receptor deactivation is the primary determinant of EPSC time-course (8-10).To determine the importance of AMPA receptor desensitizatio...

1 Group I metabotropic glutamate receptors (mGluRs) are thought to be important modulators of neuronal function in the superior colliculus (SC). Here, we investigated the pharmacology and signalling mechanisms underlying group I mGluR-mediated inhibition of neuronal excitability and synaptic transmission in the rat SC slice. 2 The group I agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) potently depressed synaptically evoked excitatory postsynaptic potentials (EPSPs), currents (EPSCs), and action potentials in a dosedependent manner (IC 50 : 6.3 mM). This was strongly reduced by the broad-spectrum antagonist ( þ )-alpha-methyl-4-carboxyphenylglycine (MCPG, 1 mM, B95% reduction), by the mGluR1 antagonist LY367385 (100 mM, B80% reduction) but not by the mGluR5 antagonist 6-methyl-2-(phenylethynyl)-pyridine (MPEP, 1 -100 mM). 3 The putative mGluR5-specific agonist (RS)-2-chloro-5-hydroxyphenylglycine (CHPG, 500 mM) also inhibited EPSPs. Interestingly, CHPG's actions were not blocked by MPEP, but LY367385 (100 mM) reduced the effect of CHPG by 50%. 4 Inhibition induced by DHPG was independent of phospholipase C (PLC)/protein kinase C pathways, and did not require intact intracellular Ca 2 þ stores. It was not abolished but enhanced by the GABA A antagonist bicuculline (5 mM), suggesting that DHPG's action was not due to facilitated inhibition or changes in neuronal network activity. 5 The K þ channel antagonist 4-aminopyridine (4-AP, 50 -100 mM) converted the inhibitory effect of DHPG into facilitation. Paired-pulse depression was strongly reduced by DHPG, an effect that was also prevented by 4-AP. 6 Our data indicate that group I agonists regulate transmitter release, presumably via an autoreceptor in the SC. This receptor may be involved in adaptation to repetitive stimulation via a non-PLC mediated pathway.

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